Pressure Sensors

The $2.5 Billion rover of NASA, Curiosity is all set to take the tiniest detail of the planet Mars. The rover is designed to be highly precise in observing its ambient conditions including atmospheric pressure and the gravitational force. For this purpose, it’s Instrumental Control Unit or the ICU has been equipped with pressure sensors. These sensors will help in detecting the Dust Devils that are a characteristic of the planet and will also detail with the concentration of various gases in the atmosphere at various points. Let’s have a look on what makes a pressure sensor capable enough to perform such specific tasks and where else it can be used.

A pressure sensor is a device which senses pressure and converts it into an analog electric signal whose magnitude depends upon the pressure applied. Since they convert pressure into an electrical signal, they are also termed as pressure transducers.

Need for Pressure Sensors

Since a long time, pressure sensors have been widely used in fields like automobile, manufacturing, aviation, bio medical measurements, air conditioning, hydraulic measurements etc. A few prominent areas where the use of pressure sensors is inevitable are:

1. Touch Screen Devices: The computer devices and smart phones that have touch screen displays come with pressure sensors. Whenever slight pressure is applied on the touch screen through a finger or the stylus, the sensor determines where it has been applied and accordingly generates an electric signal that informs the processor. Usually, these sensors are located at the corners of the screen. So when the pressure is applied, usually two or more such sensors act to give precise location information of the location.

2. Automotive Industry: In automotive industry, pressure sensors form an integral part of the engine and its safety. In the engine, these sensors monitor the oil and coolant pressure and regulate the power that the engine should deliver to achieve suitable speeds whenever accelerator is pressed or the brakes are applied to the car.

For the purpose of safety, pressure sensors constitute an important part of anti-lock braking system (ABS). This system adapts to the road terrain and makes sure that in case of braking at high speeds, the tires don’t lock and the vehicle doesn’t skid. Pressure sensors in the ABS detail the processor with the conditions of the road as well as the speed with which the vehicle is moving.

Air bag systems also use pressure sensors so that the bags get activated to ensure the safety of the passengers whenever high amount of pressure is experienced by the vehicle.

3. Bio Medical Instrumentation:  In instruments like digital blood pressure monitors and ventilators, pressure sensors are needed to optimize them according to patient’s health and his requirements.

 4. Industrial Uses: Pressure sensors are used to monitor gases and their partial pressures in industrial units so that the large chemical reactions take place in precisely controlled environmental conditions. In oil industry, sensors detail with the depth that the oil rig has reached while exploring.

5. Aviation: In the airplanes, these sensors are needed to maintain a balance between the atmospheric pressure and the control systems of the airplanes. This not only protects the circuitry and various internal components of the airplane but also gives exact data to the system about the external environment. Also, particular levels of air pressure need to be maintained in the cockpit and the passengers lobby to provide nominal ground like breathing conditions.

6. Marine Industry: For ships and submarines, pressure sensors are needed to estimate the depth at which they are operating and for detailing the marine conditions so that the electronic systems can remain safe. Oxygen requirements of under water projects are also regulated by the pressure sensors.

Types of Pressure Measurements

Pressure measurement can either be relative to a reference value or on an absolute scale.

1.   Absolute Pressure Measurement:  Pressure measured relative to perfect vacuum is termed

as absolute pressure. Perfect vacuum is a condition where there is no matter present in the atmosphere and hence, nil air pressure exists in that region. Absolute pressure sensors have limited usage because it is impossible to attain a state of perfect vacuum. Hence, sensors based on absolute pressure measurement require strict specifications for precise outputs. Sensors based on this type of measurement are used in barometric or altitude related pressure measurements.

2.   Differential Pressure Measurement:  In differential pressure measurement, pressures of two distinct positions are compared.  For example, pressure difference calculated by measuring it at different floors of a tall building will give us differential pressure. Differential pressure measurements, typically taken in pound per square inch differential (psid), are applied when high amount of pressure is to be measured. These types of measurements are used for feed pressure monitoring purposes where the pressure with which the fluid is flowing in a medium is monitored, so that homogeneity in the flow can be maintained.

Differential pressure measurements find an important application in monitoring filters in various types of purification systems. They take the reference of the normal pressure with which the filters clean the fluid. Whenever the filters face the problem of clogging due to contaminants, these pressure sensors give a reading relative to the normal pressure. This helps in keeping the filter clean and operational.

3.   Gauge Pressure Measurement: It can be defined as a subtype of differential pressure measurement where we compare pressure at any point to the current atmospheric pressure. Gauge pressure measurement is used in applications like tire pressure or blood pressure measurement. There is no consistency in gauge pressure measurements because atmospheric pressure does vary with altitude and hence its applications are limited to non-critical measurements.

Types of Pressure Sensors

Based on the type of applications they are used in, pressure sensors can be categorized into many types. However, following are most common types of pressure sensors that have been widely used:

      1.      Strain Gauge Type: These sensors are similar to a wheat stone bridge in their working. In wheat stone bridge, the ratio of resistances of two adjacent arms connected to one end of the battery should be equal to that of other two arms connected to another end of battery.  When the two ratios are equal, no output is generated from the wheat stone bridge. In the case of a strain gauge, one arm of the wheat stone bridge is connected to a diaphragm. The diaphragm compresses and expands due to the pressure applied. This variation in the diaphragm causes the output in the bridge to vary.  A voltage would be generated proportional to every deviation from the normal balanced condition, so every single compression or expansion movement of the diaphragm will produce an output indicating a change in pressure conditions. Since resistance change is the main cause for potential difference, these sensors are also termed as piezo-resistive type of pressure sensors.

       2.      Capacitive Pressure Sensor: A capacitor has two metal plates and a dielectric sandwiched between them.  In capacitive pressure sensor, one of these metal plates is permitted to move in and out so that the capacitance between them changes due to varying distance between the plates. The movable plate is connected to a diaphragm which senses the pressure and then expands or compresses accordingly.  The movement of the diaphragm would affect the attached metal plate’s position and capacitance would vary.

These sensors, though much ineffective at high temperatures, are widely used at ambient temperature range due to their linear output.

3.      Piezoelectric Pressure Sensor: Piezoelectric crystals develop a potential difference (i.e. voltage is induced across the surfaces) whenever they are subjected to any mechanical pressure.   These sensors have the crystal mounted on a dielectric base so that there is no current leakage. Attached to the crystal is a horizontal shaft to which a diaphragm is connected.  Whenever the diaphragm senses pressure, it pushes the shaft down which pressurizes the crystal and voltage is produced.

Pressure Sensor Specifications

Since pressure sensors have diverse applications, it has certain specifications that are adopted to make them work optimally in a given environment. A few of the major configurations are listed as under:

1.      Measuring Range: This defines the minimum and maximum pressure between which the sensor is designed to operate without getting damaged. This criterion is more essential for differential and gauge sensors as their measurements are relative and if the reference pressure’s magnitude is beyond their range, they will not work.

2.      Operating Temperature: It is the range of temperature under which the sensor works optimally. It is always required to make the sensor work in the defined temperature range so that the output is consistent. In the ambient conditions are extremely hot or cold, the sensors may not work properly. This is applied specially in the case of electronic pressure sensors which are used in touch screen computers and other devices.

3.      Dimensions of the sensor: Based on the application, the size of the sensor would vary according to the type of area where pressure needs to be sensed. Hence, dimensions of the sensor are an important consideration while sensors design. Usually, sensors which are small in size are preferred as they can be easily installed at difficult places such as air filters.

4.      Measurement Type: It is also important for the user to know which type of pressure measurement is been made by the sensor: absolute, gauge or differential. This is because different measurement techniques are followed by different processing methods and hence the outputs will vary accordingly.

5.      Accuracy: Differential pressure measurements are the best way to make a sensor as accurate as possible. This is because the reference pressure is more under the control of the user than the atmosphere which is the case in gauge pressure measurement.

6.      Repeatability: This can be defined as the ability of the sensor to produce the same result when a specific amount of pressure is applied on it again and again. Repeatability forms one of the most crucial specifications of a sensor. Since sensors are range specific, the probability that they will be calibrated at the same pressure is high, hence results should be reproduced by the sensor for the same amount of pressure time and again.

7.     Type of Output Generated:  The electrical output generated by the sensor can be of various types depending on its design and what the ultimate output device is. Some known formats in which output is being generated are analog voltage; analog current, digital signal (TTL), RS 232 interface and frequency shift keying based HART protocol.

8.      Response Time: This denotes the time spent between the inputs applied and the output achieved. Pressure sensors are expected to have a small response time so that instant outputs can be generated and in the case of quick pressure variations, the system can respond swiftly too.

9.      Offset Voltage:  Offset voltage can be termed as the output voltage generated when no input is applied. In the case of differential sensors, offset voltages are generated when no reference pressure is there and in case of gauge pressure, it is generated when ambient pressure is applied to the sensor. Offset voltages are needed to reduce the error in the output and final outputs are calculated after subtracting offset voltages from them.

Limitations and Challenges:

 Pressure sensors have several limitations that restrict their use in several areas. High temperature dependency, hysteresis, inability to deduce quick and dynamic pressure variations, sensitivity to the external vibrations, irreparability of the electronic board assembly, sensitivity to electric, magnetic and RF fields, incompatibilities with external devices are few challenges that a general pressure sensor faces. In some applications, pressure sensors have limited accuracy. For example, in a touch screen, the sensitivity of the sensor when multiple touches are made is affected while in the aviation sector high pressure can limit the working of the sensor.

Pressure sensors are essentially required to make a device respond to its ambient conditions in an optimized manner. Their types and uses are plenty and will continue to evolve as the sensor technology continues to mature. Extensive use of pressure sensors such as in touch screens or automobiles degrades their efficiency quite soon and hence the ruggedness of the pressure sensors is also a growing priority of industrial research. Nevertheless, pressure sensors tools, and as it goes with every other tool, these are to be used carefully as well as checked constantly to ensure quality results.

Hall Effect Sensors

Magnetic sensors are solid state devices that are becoming more and more popular because they can be used in many different types of application such as sensing position, velocity or directional movement. They are also a popular choice of sensor for the electronics designer due to their non-contact wear free operation, their low maintenance, robust design and as sealed hall effect devices are immune to vibration, dust and water.

One of the main uses of Magnetic Sensors is in automotive systems for the sensing of position, distance and speed. For example, the angular position of the crank shaft for the firing angle of the spark plugs, the position of the car seats and seat belts for air-bag control or wheel speed detection for the anti-lock braking system, (ABS).

Magnetic sensors are designed to respond to a wide range of positive and negative magnetic fields in a variety of different applications and one type of magnet sensor whose output signal is a function of magnetic field density around it is called the Hall Effect Sensor.

Hall Effect Sensors are devices which are activated by an external magnetic field. We know that a magnetic field has two important characteristics flux density, (B) and polarity (North and South Poles). The output signal from a Hall effect sensor is the function of magnetic field density around the device. When the magnetic flux density around the sensor exceeds a certain pre-set threshold, the sensor detects it and generates an output voltage called the Hall Voltage, V_H. Consider the diagram below.

Hall Effect Sensor Principals

Hall Effect Sensors consist basically of a thin piece of rectangular p-type semiconductor material such as gallium arsenide (GaAs), indium antimonide (InSb) or indium arsenide (InAs) passing a continuous current through itself. When the device is placed within a magnetic field, the magnetic flux lines exert a force on the semiconductor material which deflects the charge carriers, electrons and holes, to either side of the semiconductor slab. This movement of charge carriers is a result of the magnetic force they experience passing through the semiconductor material.

As these electrons and holes move side wards a potential difference is produced between the two sides of the semiconductor material by the build-up of these charge carriers. Then the movement of electrons through the semiconductor material is affected by the presence of an external magnetic field which is at right angles to it and this effect is greater in a flat rectangular shaped material.

The effect of generating a measurable voltage by using a magnetic field is called the Hall Effectafter Edwin Hall who discovered it back in the 1870’s with the basic physical principle underlying the Hall effect being Lorentz force. To generate a potential difference across the device the magnetic flux lines must be perpendicular, (90^o) to the flow of current and be of the correct polarity, generally a south pole.

The Hall effect provides information regarding the type of magnetic pole and magnitude of the magnetic field. For example, a south pole would cause the device to produce a voltage output while a north pole would have no effect. Generally, Hall Effect sensors and switches are designed to be in the “OFF”, (open circuit condition) when there is no magnetic field present. They only turn “ON”, (closed circuit condition) when subjected to a magnetic field of sufficient strength and polarity.

Hall Effect Magnetic Sensor

The output voltage, called the Hall voltage, (V_H) of the basic Hall Element is directly proportional to the strength of the magnetic field passing through the semiconductor material (output ∝ H). This output voltage can be quite small, only a few microvolts even when subjected to strong magnetic fields so most commercially available Hall effect devices are manufactured with built-in DC amplifiers, logic switching circuits and voltage regulators to improve the sensors sensitivity, hysteresis and output voltage. This also allows the Hall effect sensor to operate over a wider range of power supplies and magnetic field conditions.

The Hall Effect Sensor

Hall Effect Sensors are available with either linear or digital outputs. The output signal for linear (analogue) sensors is taken directly from the output of the operational amplifier with the output voltage being directly proportional to the magnetic field passing through the Hall sensor. This output Hall voltage is given as:

Where: VH is the Hall Voltage in volts RH is the Hall Effect co-efficient I is the current flow through the sensor in amps t is the thickness of the sensor in mm B is the Magnetic Flux density in Teslas

Linear or analogue sensors give a continuous voltage output that increases with a strong magnetic field and decreases with a weak magnetic field. In linear output Hall effect sensors, as the strength of the magnetic field increases the output signal from the amplifier will also increase until it begins to saturate by the limits imposed on it by the power supply. Any additional increase in the magnetic field will have no effect on the output but drive it more into saturation.

Digital output sensors on the other hand have a Schmitt-trigger with built in hysteresis connected to the op-amp. When the magnetic flux passing through the Hall sensor exceeds a pre-set value the output from the device switches quickly between its “OFF” condition to an “ON” condition without any type of contact bounce. This built-in hysteresis eliminates any oscillation of the output signal as the sensor moves in and out of the magnetic field. Then digital output sensors have just two states, “ON” and “OFF”.

There are two basic types of digital Hall effect sensor, Bipolar and Unipolar. Bipolar sensors require a positive magnetic field (south pole) to operate them and a negative field (north pole) to release them while unipolar sensors require only a single magnetic south pole to both operate and release them as they move in and out of the magnetic field.

Most Hall effect devices can not directly switch large electrical loads as their output drive capabilities are very small around 10 to 20mA. For large current loads an open-collector (current sinking) NPN Transistor is added to the output.

This transistor operates in its saturated region as a NPN sink switch which shorts the output terminal to ground whenever the applied flux density is higher than that of the “ON” pre-set point.

The output switching transistor can be either an open emitter transistor, open collector transistor configuration or both providing a push-pull output type configuration that can sink enough current to directly drive many loads, including relays, motors, LEDs, and lamps.

Hall Effect Applications

Hall effect sensors are activated by a magnetic field and in many applications the device can be operated by a single permanent magnet attached to a moving shaft or device. There are many different types of magnet movements, such as “Head-on”, “Sideways”, “Push-pull” or “Push-push” etc sensing movements. Which every type of configuration is used, to ensure maximum sensitivity the magnetic lines of flux must always be perpendicular to the sensing area of the device and must be of the correct polarity.

Also to ensure linearity, high field strength magnets are required that produce a large change in field strength for the required movement. There are several possible paths of motion for detecting a magnetic field, and below are two of the more common sensing configurations using a single magnet: Head-on Detection and Sideways Detection.

Head-on Detection

As its name implies, “head-on detection” requires that the magnetic field is perpendicular to the hall effect sensing device and that for detection, it approaches the sensor straight on towards the active face. A sort of “head-on” approach.

This head-on approach generates an output signal,V_H which in the linear devices represents the strength of the magnetic field, the magnetic flux density, as a function of distance away from the hall effect sensor. The nearer and therefore the stronger the magnetic field, the greater the output voltage and vice versa.

Linear devices can also differentiate between positive and negative magnetic fields. Non-linear devices can be made to trigger the output “ON” at a pre-set air gap distance away from the magnet for indicating positional detection.

Sideways Detection

The second sensing configuration is “sideways detection”. This requires moving the magnet across the face of the Hall effect element in a sideways motion.

Sideways or slide-by detection is useful for detecting the presence of a magnetic field as it moves across the face of the Hall element within a fixed air gap distance for example, counting rotational magnets or the speed of rotation of motors.

Depending upon the position of the magnetic field as it passes by the zero field centre line of the sensor, a linear output voltage representing both a positive and a negative output can be produced. This allows for directional movement detection which can be vertical as well as horizontal.

There are many different applications for Hall Effect Sensors especially as proximity sensors. They can be used instead of optical and light sensors were the environmental conditions consist of water, vibration, dirt or oil such as in automotive applications. Hall effect devices can also be used for current sensing.

We know from the previous tutorials that when a current passes through a conductor, a circular electromagnetic field is produced around it. By placing the Hall sensor next to the conductor, electrical currents from a few milliamps into thousands of amperes can be measured from the generated magnetic field without the need of large or expensive transformers and coils.

As well as detecting the presence or absence of magnets and magnetic fields, Hall effect sensors can also be used to detect ferromagnetic materials such as iron and steel by placing a small permanent “biasing” magnet behind the active area of the device. The sensor now sits in a permanent and static magnetic field, and any change or disturbance to this magnetic field by the introduction of a ferrous material will be detected with sensitivities as low as mV/G possible.

There are many different ways to interface Hall effect sensors to electrical and electronic circuits depending upon the type of device, whether digital or linear. One very simple and easy to construct example is using a Light Emitting Diode as shown below.

Positional Detector

This head-on positional detector will be “OFF” when there is no magnetic field present, (0 gauss). When the permanent magnets south pole (positive gauss) is moved perpendicular towards the active area of the Hall effect sensor the device turns “ON” and lights the LED. Once switched “ON” the Hall effect sensor stays “ON”.

To turn the device and therefore the LED “OFF” the magnetic field must be reduced to below the release point for unipolar sensors or exposed to a magnetic north pole (negative gauss) for bipolar sensors. The LED can be replaced with a larger power transistor if the output of the Hall Effect Sensor is required to switch larger current loads.

Alcohol Gas Sensor

MQ3 Gas Sensor
This is an alcohol sensor from futurlec, named MQ-3, which detects ethanol in the air. It is one of the straightforward gas sensors so it works almost the same way with other gas sensors. Typically, it is used as part of the breathalyzers or breath testers for the detection of ethanol in the human breath.

Datasheet
Here is a datasheet, only 2 pages. It shows features, applications, specifications and configurations etc. It is a pretty simple datasheet. Since this datasheet was not prepared in English, the translation is not very accurate.

How it looks like :
Basically, it has 6pins, the cover and the body. Even though it has 6 pins, you can use only 4 of them. Two of them are for the heating system, which I call H and the other 2 are for connecting power and ground, which I called A and B.

If you look at the inside of the sensor, you will find the little tube. Basically, this tube is a heating system that is made of aluminum oxide and tin dioxide and inside of it there are heater coils, which practically produce the heat. And you can also find 6 pins. 2 pins that I called Pin H are connected to the heater coils and the other ones are connected to the tube.

How it works :
How does it work? The core system is the cube. As you can see in this cross-sectional view, basically, it is an Alumina tube cover by SnO2, which is tin dioxide. And between them there is an Aurum electrode, the black one. And also you can see how the wires are connected. So, why do we need them? Basically, the alumina tube and the coils are the heating system, the yellow, brown parts and the coils in the picture.
5. Working Process :
If the coil is heated up,

SnO2 ceramics will become the semi – conductor, so there are more movable electrons, which means that it is ready tomake more current flow.

Then, when the alcohol molecules in the air meet the electrode that is between alumina and tin dioxide, ethanol burns into acetic acid then more current is produced. So the more alcohol molecules there are, the more current we will get. Because of this current change, we get the different values from the sensor.
Microcontroller Connections
Here is the schematic. It is pretty simple. First, you can use 5v. And as you can see one of H pins goes to the power and the other one is connected to the ground. And the pin A is connected between the power and the pin H and the pin B is goes to the microcontroller. Also between the ground and the Arduino, you need the resistor. Before you connect the resistor if you use the pot, you can tune the resistor for getting more accurate values. In the datasheet they say you can used 100k om to 470k om.

Typical Behavior
If you blow, it will react. Depending on the environment, it gives you little bit of different values. But in my case, it gives me 200 as the lowest value and 1000 as the highest value. And when it detects the alcohol in the air, actually it is pretty sensitive, the value gets higher very quickly but you have to wait for about 1 to 5 minutes to reset it. So that means getting values is fast but resetting is so slow. And the sensitivity of this sensor is affected by time span.

Ultrasonic Sensors

Bats are wonderful creatures. Blind from the eyes and yet a vision so precise that could distinguish between a moth and a broken leaf even when flying at full speed. No doubt the vision is sharper than ours and is much beyond human capabilities of seeing, but is certainly not beyond our understanding. Ultrasonic ranging is the technique used by bats and many other creatures of the animal kingdom for navigational purposes. In a bid to imitate the ways of nature to obtain an edge over everything, we humans have not only understood it but have successfully imitated some of these manifestations and harnessed their potential to the greatest extent.

History

The history dates back to 1790, when Lazzaro Spallanzani first discovered that bats maneuvered in flight using their hearing rather than sight. Jean-Daniel Colladon in 1826 discovered sonography using an underwater bell, successfully and accurately determining the speed of sound in water. Thereafter, the study and research work in this field went on slowly until 1881 when Pierre Curie’s discovery set the stage for modern ultrasound transducers. He found out the relationship between electrical voltage and pressure on crystalline material. The unfortunate Titanic accident spurred rigorous interest into this field as a result of which Paul Langevin invented the hydrophone to detect icebergs. It was the first ultrasonic transducer. The hydrophone could send and receive low frequency sound waves and was later used in the detection of submarines in the World War 1.

On a note parallel to the SONAR, medical research also started taking interest in ultrasonics. In late 1930’s Dr. Karl Dussik used a technique called hyperphonography which recorded echoes of ultrasonic waves on a sensitive paper. This technique was used to produce ultrasound pictures of the brain to help detect tumors and marked the birth of ultrasound imaging. After that, many scientists like Ian Donald, Douglas Howry, Joseph Holmes, John Wild and John Reid improved upon the various aspects of ultrasonic sensors in the medical field which enabled diagnosis of stomach cancers, ovarian cysts, detection of twin pregnancies, tumors etc. Industry too did not waste time in jumping on to the bandwagon and soon developed techniques like ultrasonic welding and non destructive testing at the outset of the 1960s

How Ultrasonic Sensors work?

Ultrasonic sensors are devices that use electrical–mechanical energy transformation, the mechanical energy being in the form of ultrasonic waves, to measure distance from the sensor to the target object. Ultrasonic waves are longitudinal mechanical waves which travel as a succession of compressions and rarefactions along the direction of wave propagation through the medium. Any sound wave above the human auditory range of 20,000 Hz is called ultrasound. Depending on the type of application, the range of frequencies has been broadly categorized as shown in the figure below:

When ultrasonic waves are incident on an object, diffused reflection of the energy takes place over a wide solid angle which might be as high as 180 degrees. Thus some fraction of the incident energy is reflected back to the transducer in the form of echoes and is detected. The distance to the object (L) can then be calculated through the speed of ultrasonic waves (v) in the medium by the relation

Where‘t’ is the time taken by the wave to reach back to the sensor and ‘’ is the angle between the horizontal and the path taken as shown in the figure. If the object is in motion, instruments based on Doppler shift are used.

Generating Ultrasonic Waves

For the generation of such mechanical waves, movement of some surface like a diaphragm is required which can then induce the motion to the medium in front of it in the form of compression and rarefaction. Piezoelectric materials operating in the motor mode and magnetostrictive materials have been widely employed in the generation of ultrasonic waves at frequency ranges of 1-20 MHz and 20-40 kHz respectively. The sensors employ piezoelectric ceramic transducers which flex when an electric signal is applied to them. These are connected to an electronic oscillator whose output generates the oscillating voltages at the required frequency. Materials like Lead Zirconate Titanate are popular piezoelectric materials used in medical ultrasound imaging. For best results, the frequency of the applied oscillations must be equal to the natural frequency of the ceramic, which produces oscillations readily through resonance. It offers maximum sensitivity and efficiency when operated at resonance.

Piezoelectricity being a reversible phenomenon produces electrical voltages when ultrasonic waves reflect back from the target and impinge upon the ceramic structure. In this way, a transducer may work both as a transmitter and a receiver in pulsed mode. When continuous measurement of distances is required, separate transducers may be used for transmission and reception. The sensors when used in industry are generally employed in arrays which may be mechanical arrays consisting of oscillating or rotating sensors, or electronic arrays which may be linear, curved or phased. To visualize the output of an ultrasonic sensor, displays of different kind are used whose shape depends on the type of transducer array used and the function. A sectored Field of View is produced by mechanical arrays and curved and phased electronic arrays, while a linear field is generated by linear arrays. The display modes may be linear graphical plotting with amplitude on y-axis and time on x-axis called Amplitude mode or A-mode, or intensity modulated B-scans where the brightness of a spot indicates the amplitude of reflected waves. Other modes include M-mode, Doppler (D) Mode etc.

The parameterization of these sensors is generally done by monitoring the reflected and transmitted signals from the lateral an axial motion of transducer while keeping the target fixed in a specific medium (water in general). The sound beam diverges rapidly, hence care is taken that the transducer produces the smallest possible beams. The narrower the beam pattern, the more sensitive the sensor is. However, the angle possible between the transducer and the surface increases with the beam width. The beam patterns of the kind shown below are observed:

 Axial and Cross Sectional beam profiles

The parameters on which the performance of an ultrasonic sensor is measured include bandwidth, attenuation, dynamic range and resolution like grayscale, axial and lateral resolution. Other parameters are Nominal Frequency, Peak Frequency, Bandwidth center Frequency, Pulse Width, sensitivity and Signal to Noise Ratio (SNR).

Importance of Ultrasonic Sensors

There are a variety of sensors based on other physical transduction principles like the optical range finding sensors and the microwave based devices too. Then why should one use ultrasonic transducers in the first place, given that the speed of sound is very slow than the speed of electromagnetic waves? The answer lies in the question itself. Because the EM waves based devices are too fast. Being slower that the EM waves, the time taken by ultrasonic waves is much longer than that taken by the latter and hence its measurement can be done more easily and less expensively. Because these are based on sound waves rather than EM waves, these would work in places where the latter would not.

For example, in the case of clear object detection and measurement of liquid levels or high glare environments, light based sensors would suffer greatly because of the transmittance of the target or the translucence of the propagating media. Ultrasonic devices being based upon sound propagation would remain practically unaffected. These also function well in wet environments where optical beams may suffer from refraction from the water droplets in the environment. On account of range and accuracy, the ultrasonic sensors may lie in between two EM wave based sensors, the Infrared rangefinders on the lower end and the LIDARs on the upper end. Not as accurate or long distance as the LIDARs, the Ultrasonic rangefinders fare better than the IR rangefinders which are highly susceptible to ambient conditions and require recalibration when environment changes. Further these devices offer advantage in medical imaging as compared to MRI or X-Ray scans due to inexpensiveness and portability. No harmful effects of ultrasonic waves at the intensity levels used have been detected in contrast to X-rays or radioactivity based methods and is particularly suited for imaging soft tissues.

Problems & Concerns

However, Ultrasonic sensors too aren’t free of all the problems. The speed of sound in a medium increases as the temperature of the medium increases. Thus even when the target has remained in the same place, it may now seem that it has shifted to a place closer to the sensor. Air currents due to varied reasons may disturb the path of the wave which could lead to ‘Missed Detection’ or a wrong measurement.

Acoustic noise like high pitched sounds created due to whistling or hissing of valves and pneumatic devices at the frequency close to the operating frequency may interfere with the output of the sensor. Electrical noise also affects the performance of the sensor. These may generate artifacts which are not a true representation of the imaged object. Just like the vision starts to blur when the distance of the object from the eye gets too small for the eyes to see it, ultrasonic devices also have a ‘dead zone’ where the sensor cannot reliably make measurements. This happens due to a phenomenon called ringing which is the continuous vibration of the transducer after emitting the pulse. Thus when the distance is too small, the transducer has not yet come to rest to be able to differentiate between the vibration due to the incident radiation or the oscillation from the electrical excitation. The dangers of Ultrasonic waves are also well founded. If the intensity is too high, it can cause human tissues to heat and may cause ruptures in people exposed to it. Ethical issues like fetus identification and resulting abortions in medical field are also a widespread concern.

Applications

The applications of ultrasonic sensors can be classified on the basis of the property that they exploit. These can be summarized as:

Domain	Parameter	Applications
Time	Tile-of-Flight, Velocity	Density, Thickness, Flaw Detection, Anisotropy, Robotics, Remote Sensing etc.
Attenuation	Fluctuations in reflected and Transmitted Signals	Defect characterization, microstructures, interface analysis
Frequency	Ultrasonic Spectroscopy	Microstructure, grain size, porosity, phase analysis.
Image	Time-of-Flight, velocity, attenuation mapping in Raster C-Scan or SARs	Surface and internal Defect imaging, density, velocity, 2D and 3D imaging.

Research has been going on to overcome the problems of ultrasonic sensors, particularly in medical imaging where it is known as ultrasound. The artifacts of ultrasonic sensors like Acoustic shadowing and Acoustic Enhancement are being exploited to characterize tissues which allow the differentiation between solid and cystic tissues. The industry too has reaped the benefits from ultrasonic sensors in applications like plastic welding, jewelry cleaning, remote sensing and telemetry, assisted parking systems etc. Robotics has been known to use ultrasonic rangefinders as a favorite tool for distance ranging and mapping. Even the fashion industry is using ultrasonic sensors in hair styling like hair extension implants.

                                               Flaw Detection Using Ultrasonic Sensors

Future

Non destructive testing and flaw detection uses ultrasonic waves in various modes like the Longitudinal (L-wave) mode and the Shear (S-wave) mode to detect flaws in materials. With the advances in Science, new materials offering increased performance at lower voltages like the capacitive micromachined ultrasonic transducers (CMUTs) are being developed which are expected to have higher bandwidth and greater potential for integration with electronic circuits.

These devices provide non-invasive measures for the detection of problems in all kinds of materials, be it a living tissue or non-living manufactured goods. With a healthy history of being able to detect many problems which otherwise left the doctors dazed and the problem untreated, ultrasonic sensors do offer a lot of promise even in the coming times. The environmental and psychological effects of exposure to EM-radiation being rigorously being put under the scanner, ultrasonic applications are expected to thrive and offer a substantial alternative to the contemporary technologies.

Light Sensors

A Light Sensor generates an output signal indicating the intensity of light by measuring the radiant energy that exists in a very narrow range of frequencies basically called “light”, and which ranges in frequency from “Infra-red” to “Visible” up to “Ultraviolet” light spectrum.

The Light Sensor is a passive devices that convert this “light energy” whether visible or in the infra-red parts of the spectrum into an electrical signal output. Light sensors are more commonly known as “Photoelectric Devices” or “Photo Sensors” because the convert light energy (photons) into electricity (electrons).

Photoelectric devices can be grouped into two main categories, those which generate electricity when illuminated, such as Photo-voltaics or Photo-emissives etc, and those which change their electrical properties in some way such as Photo-resistors or Photo-conductors. This leads to the following classification of devices.

• • Photo-emissive Cells – These are photodevices which release free electrons from a light sensitive material such as caesium when struck by a photon of sufficient energy. The amount of energy the photons have depends on the frequency of the light and the higher the frequency, the more energy the photons have converting light energy into electrical energy.
• • Photo-conductive Cells – These photodevices vary their electrical resistance when subjected to light. Photoconductivity results from light hitting a semiconductor material which controls the current flow through it. Thus, more light increase the current for a given applied voltage. The most common photoconductive material is Cadmium Sulphide used in LDR photocells.
• • Photo-voltaic Cells – These photodevices generate an emf in proportion to the radiant light energy received and is similar in effect to photoconductivity. Light energy falls on to two semiconductor materials sandwiched together creating a voltage of approximately 0.5V. The most common photovoltaic material is Selenium used in solar cells.
• • Photo-junction Devices – These photodevices are mainly true semiconductor devices such as the photodiode or phototransistor which use light to control the flow of electrons and holes across their PN-junction. Photojunction devices are specifically designed for detector application and light penetration with their spectral response tuned to the wavelength of incident light.

The Photoconductive Cell

A Photoconductive light sensor does not produce electricity but simply changes its physical properties when subjected to light energy. The most common type of photoconductive device is thePhotoresistor which changes its electrical resistance in response to changes in the light intensity.

Photoresistors are Semiconductor devices that use light energy to control the flow of electrons, and hence the current flowing through them. The commonly used Photoconductive Cell is called the Light Dependent Resistor or LDR.

The Light Dependent Resistor

Typical LDR

As its name implies, the Light Dependent Resistor (LDR) is made from a piece of exposed semiconductor material such as cadmium sulphide that changes its electrical resistance from several thousand Ohms in the dark to only a few hundred Ohms when light falls upon it by creating hole-electron pairs in the material.

The net effect is an improvement in its conductivity with a decrease in resistance for an increase in illumination. Also, photoresistive cells have a long response time requiring many seconds to respond to a change in the light intensity.

Materials used as the semiconductor substrate include, lead sulphide (PbS), lead selenide (PbSe), indium antimonide (InSb) which detect light in the infra-red range with the most commonly used of all photoresistive light sensors being Cadmium Sulphide (Cds).

Cadmium sulphide is used in the manufacture of photoconductive cells because its spectral response curve closely matches that of the human eye and can even be controlled using a simple torch as a light source. Typically then, it has a peak sensitivity wavelength (λp) of about 560nm to 600nm in the visible spectral range.

The Light Dependent Resistor Cell

The most commonly used photoresistive light sensor is the ORP12 Cadmium Sulphide photoconductive cell. This light dependent resistor has a spectral response of about 610nm in the yellow to orange region of light. The resistance of the cell when unilluminated (dark resistance) is very high at about 10MΩ’s which falls to about 100Ω’s when fully illuminated (lit resistance).

To increase the dark resistance and therefore reduce the dark current, the resistive path forms a zigzag pattern across the ceramic substrate. The CdS photocell is a very low cost device often used in auto dimming, darkness or twilight detection for turning the street lights “ON” and “OFF”, and for photographic exposure meter type applications.

Connecting a light dependant resistor in series with a standard resistor like this across a single DC supply voltage has one major advantage, a different voltage will appear at their junction for different levels of light.

The amount of voltage drop across series resistor,R₂ is determined by the resistive value of the light dependant resistor, R_LDR. This ability to generate different voltages produces a very handy circuit called a “Potential Divider” or Voltage Divider Network.

As we know, the current through a series circuit is common and as the LDR changes its resistive value due to the light intensity, the voltage present at V_OUT will be determined by the voltage divider formula. An LDR’s resistance, R_LDR can vary from about 100Ω’s in the sun light, to over 10MΩ’s in absolute darkness with this variation of resistance being converted into a voltage variation at V_OUTas shown.

One simple use of a Light Dependent Resistor, is as a light sensitive switch as shown below.

LDR Switch

This basic light sensor circuit is of a relay output light activated switch. A potential divider circuit is formed between the photoresistor, LDR and the resistor R1. When no light is present ie in darkness, the resistance of the LDR is very high in the Megaohms (MΩ’s) range so zero base bias is applied to the transistor TR1 and the relay is de-energised or “OFF”.

As the light level increases the resistance of the LDRstarts to decrease causing the base bias voltage at V1 to rise. At some point determined by the potential divider network formed with resistor R1, the base bias voltage is high enough to turn the transistor TR1 “ON” and thus activate the relay which in turn is used to control some external circuitry. As the light level falls back to darkness again the resistance of the LDR increases causing the base voltage of the transistor to decrease, turning the transistor and relay “OFF” at a fixed light level determined again by the potential divider network.

By replacing the fixed resistor R1 with a potentiometer VR1, the point at which the relay turns “ON” or “OFF” can be pre-set to a particular light level. This type of simple circuit shown above has a fairly low sensitivity and its switching point may not be consistent due to variations in either temperature or the supply voltage. A more sensitive precision light activated circuit can be easily made by incorporating the LDR into a “Wheatstone Bridge” arrangement and replacing the transistor with anOperational Amplifier as shown.

Light Level Sensing Circuit

In this basic dark sensing circuit, the light dependent resistor LDR1 and the potentiometer VR1 form one adjustable arm of a simple resistance bridge network, also known commonly as a Wheatstone bridge, while the two fixed resistors R1 and R2 form the other arm. Both sides of the bridge form potential divider networks across the supply voltage whose outputs V1 and V2 are connected to the non-inverting and inverting voltage inputs respectively of the operational amplifier.

The operational amplifier is configured as a Differential Amplifier also known as a voltage comparator with feedback whose output voltage condition is determined by the difference between the two input signals or voltages, V1 and V2. The resistor combination R1 and R2 form a fixed voltage reference at input V2, set by the ratio of the two resistors. The LDR – VR1 combination provides a variable voltage input V1 proportional to the light level being detected by the photoresistor.

As with the previous circuit the output from the operational amplifier is used to control a relay, which is protected by a free wheel diode, D1. When the light level sensed by the LDR and its output voltage falls below the reference voltage set at V2 the output from the op-amp changes state activating the relay and switching the connected load.

Likewise as the light level increases the output will switch back turning “OFF” the relay. The hysteresis of the two switching points is set by the feedback resistor Rf can be chosen to give any suitable voltage gain of the amplifier.

The operation of this type of light sensor circuit can also be reversed to switch the relay “ON” when the light level exceeds the reference voltage level and vice versa by reversing the positions of the light sensor LDR and the potentiometer VR1. The potentiometer can be used to “pre-set” the switching point of the differential amplifier to any particular light level making it ideal as a simple light sensor project circuit.

Photojunction Devices

Photojunction Devices are basically PN-Junction light sensors or detectors made from silicon semiconductor PN-junctions which are sensitive to light and which can detect both visible light and infra-red light levels. Photo-junction devices are specifically made for sensing light and this class of photoelectric light sensors include the Photodiode and the Phototransistor.

The Photodiode.

Photo-diode

The construction of the Photodiode light sensor is similar to that of a conventional PN-junction diode except that the diodes outer casing is either transparent or has a clear lens to focus the light onto the PN junction for increased sensitivity. The junction will respond to light particularly longer wavelengths such as red and infra-red rather than visible light.

This characteristic can be a problem for diodes with transparent or glass bead bodies such as the 1N4148 signal diode. LED’s can also be used as photodiodes as they can both emit and detect light from their junction. All PN-junctions are light sensitive and can be used in a photo-conductive unbiased voltage mode with the PN-junction of the photodiode always “Reverse Biased” so that only the diodes leakage or dark current can flow.

The current-voltage characteristic (I/V Curves) of a photodiode with no light on its junction (dark mode) is very similar to a normal signal or rectifying diode. When the photodiode is forward biased, there is an exponential increase in the current, the same as for a normal diode. When a reverse bias is applied, a small reverse saturation current appears which causes an increase of the depletion region, which is the sensitive part of the junction. Photodiodes can also be connected in a current mode using a fixed bias voltage across the junction. The current mode is very linear over a wide range.

Photo-diode Construction and Characteristics

When used as a light sensor, a photodiodes dark current (0 lux) is about 10uA for geranium and 1uA for silicon type diodes. When light falls upon the junction more hole/electron pairs are formed and the leakage current increases. This leakage current increases as the illumination of the junction increases.

Thus, the photodiodes current is directly proportional to light intensity falling onto the PN-junction. One main advantage of photodiodes when used as light sensors is their fast response to changes in the light levels, but one disadvantage of this type of photodevice is the relatively small current flow even when fully lit.

The following circuit shows a photo-current-to-voltage converter circuit using an operational amplifier as the amplifying device. The output voltage (Vout) is given as Vout = Ip × Rf and which is proportional to the light intensity characteristics of the photodiode.

This type of circuit also utilizes the characteristics of an operational amplifier with two input terminals at about zero voltage to operate the photodiode without bias. This zero-bias op-amp configuration gives a high impedance loading to the photodiode resulting in less influence by dark current and a wider linear range of the photocurrent relative to the radiant light intensity. CapacitorC_f is used to prevent oscillation or gain peaking and to set the output bandwidth (1/2πRC).

Photo-diode Amplifier Circuit

Photodiodes are very versatile light sensors that can turn its current flow both “ON” and “OFF” in nanoseconds and are commonly used in cameras, light meters, CD and DVD-ROM drives, TV remote controls, scanners, fax machines and copiers etc, and when integrated into operational amplifier circuits as infrared spectrum detectors for fibre optic communications, burglar alarm motion detection circuits and numerous imaging, laser scanning and positioning systems etc.

The Phototransistor

Photo-transistor

An alternative photo-junction device to the photodiode is the Phototransistor which is basically a photodiode with amplification. The Phototransistor light sensor has its collector-base PN-junction reverse biased exposing it to the radiant light source.

Phototransistors operate the same as the photodiode except that they can provide current gain and are much more sensitive than the photodiode with currents are 50 to 100 times greater than that of the standard photodiode and any normal transistor can be easily converted into a phototransistor light sensor by connecting a photodiode between the collector and base.

Phototransistors consist mainly of a bipolar NPN Transistor with its large base region electrically unconnected, although some phototransistors allow a base connection to control the sensitivity, and which uses photons of light to generate a base current which in turn causes a collector to emitter current to flow. Most phototransistors are NPN types whose outer casing is either transparent or has a clear lens to focus the light onto the base junction for increased sensitivity.

Photo-transistor Construction and Characteristics

In the NPN transistor the collector is biased positively with respect to the emitter so that the base/collector junction is reverse biased. therefore, with no light on the junction normal leakage or dark current flows which is very small. When light falls on the base more electron/hole pairs are formed in this region and the current produced by this action is amplified by the transistor.

Usually the sensitivity of a phototransistor is a function of the DC current gain of the transistor. Therefore, the overall sensitivity is a function of collector current and can be controlled by connecting a resistance between the base and the emitter but for very high sensitivity optocoupler type applications, Darlington phototransistors are generally used.

Photo-darlington

Photodarlington transistors use a second bipolar NPN transistor to provide additional amplification or when higher sensitivity of a photodetector is required due to low light levels or selective sensitivity, but its response is slower than that of an ordinary NPN phototransistor.

Photo darlington devices consist of a normal phototransistor whose emitter output is coupled to the base of a larger bipolar NPN transistor. Because a darlington transistor configuration gives a current gain equal to a product of the current gains of two individual transistors, a photodarlington device produces a very sensitive detector.

Typical applications of Phototransistors light sensors are in opto-isolators, slotted opto switches, light beam sensors, fibre optics and TV type remote controls, etc. Infrared filters are sometimes required when detecting visible light.

Another type of photojunction semiconductor light sensor worth a mention is the Photo-thyristor. This is a light activated thyristor or Silicon Controlled Rectifier, SCR that can be used as a light activated switch in AC applications. However their sensitivity is usually very low compared to equivalent photodiodes or phototransistors.

To help increase their sensitivity to light, photo-thyristors are made thinner around the gate junction. The downside to this process is that it limits the amount of anode current that they can switch. Then for higher current AC applications they are used as pilot devices in opto-couplers to switch larger more conventional thyristors.

Photovoltaic Cells.

The most common type of photovoltaic light sensor is the Solar Cell. Solar cells convert light energy directly into DC electrical energy in the form of a voltage or current to a power a resistive load such as a light, battery or motor. Then photovoltaic cells are similar in many ways to a battery because they supply DC power.

However, unlike the other photo devices we have looked at above which use light intensity even from a torch to operate, photovoltaic solar cells work best using the suns radiant energy.

Solar cells are used in many different types of applications to offer an alternative power source from conventional batteries, such as in calculators, satellites and now in homes offering a form of renewable power.

Photovoltaic Cell

Photovoltaic cells are made from single crystal silicon PN junctions, the same as photodiodes with a very large light sensitive region but are used without the reverse bias. They have the same characteristics as a very large photodiode when in the dark.

When illuminated the light energy causes electrons to flow through the PN junction and an individual solar cell can generate an open circuit voltage of about 0.58v (580mV). Solar cells have a “Positive” and a “Negative” side just like a battery.

Individual solar cells can be connected together in series to form solar panels which increases the output voltage or connected together in parallel to increase the available current. Commercially available solar panels are rated in Watts, which is the product of the output voltage and current (Volts times Amps) when fully lit.

Characteristics of a typical Photovoltaic Solar Cell.

The amount of available current from a solar cell depends upon the light intensity, the size of the cell and its efficiency which is generally very low at around 15 to 20%. To increase the overall efficiency of the cell commercially available solar cells use polycrystalline silicon or amorphous silicon, which have no crystalline structure, and can generate currents of between 20 to 40mA per cm².

Other materials used in the construction of photovoltaic cells include Gallium Arsenide, Copper Indium Diselenide and Cadmium Telluride. These different materials each have a different spectrum band response, and so can be “tuned” to produce an output voltage at different wavelengths of light.

In this tutorial about Light Sensors, we have looked at several examples of devices that are classed as Light Sensors. This includes those with and those without PN-junctions that can be used to measure the intensity of light.

In the next tutorial we will look at output devices called Actuators. Actuators convert an electrical signal into a corresponding physical quantity such as movement, force, or sound. One such commonly used output device is the Electromagnetic Relay.

Accelerometer

Body in motion usually experience vibration as well as shock. When a mobile falls on a floor, it is subjected to shock. When a vehicle moves on a bumpy road, it experiences vibrations. Likewise, there are many situations, where an object encounters shock and vibrations. Sometimes, they survive and at times, they get damaged. When delicate items like glass, crockery, etc. are packaged properly, they can withstand severe shock and vibrations. Whether a system will survive or not, how do we know this a priori? While some vibrations are desirable, some may be disturbing or even destructive. Hence, often a need is felt to understand the causes of vibrations and to develop methods to measure and prevent them.

An ability of a system to withstand vibrations and shock depends upon the ‘g’ level the system can withstand. To measure these ‘g’ levels, a sensor – accelerometer is used.

An accelerometer is a sensor that measures the physical acceleration experienced by an object due to inertial forces or due to mechanical excitation. Acceleration is defined as rate of change of velocity with respect to time. It is a measure of how fast speed changes. It is a vector quantity having both magnitude and direction. As a speedometer is a meter to measures speed, an accelerometer is a meter to measure acceleration. An ability of an accelerometer to sense acceleration can be put to use to measure a variety of things like tilt, vibration, rotation, collision, gravity, etc. Accelerometers measure in terms of ‘g’ (‘g’ is acceleration measurement for gravity which is equal to 9.81m/s²). Accelerometers are made using tilt sensors.

THEORY OF ACCELEROMETERS – WHAT IS AN ACCELEROMETER?

The term ‘Accelerometers’ refer to the transducers which comprises of mechanical sensing element and a mechanism which converts the mechanical motion into an electrical output.

Theory behind working of accelerometers can be understood from the mechanical model of accelerometer, using Newtonian mechanics. The sensing element essentially is a proof mass (also known as seismic mass).  The proof mass is attached to spring which in turn is connected to its casing.  In addition, a dashpot is also included in a system to provide desirable damping effect; otherwise system may oscillate at its natural frequency. The dashpot is attached (in parallel or in series) between the mass and the casing. The unit is rigidly mounted on the body whose acceleration is of interest.

When the system is subjected to linear acceleration, a force (= mass * acceleration) acts on the proof-mass. This causes it to deflect; the deflection is sensed by a suitable means and is converted into an equivalent electrical signal.

When force is applied on the body, proof mass moves. Its movement is countered by spring and damper.

Therefore, if    m = proof mass of the body

x  = relative movement of the proof-mass with respect to the frame

c  = damping coefficient

k  = spring stiffness

then

Thus, with the knowledge of damping coefficient(c ), spring stiffness (k), and proof mass (m), for a useful acceleration sensor, it is sufficient to provide a component that can move relative to sensors housing and a means to sense the movement.

Displacement and acceleration are related by fundamental scaling law. A higher resonant frequency implies less displacement or low sensitivity.

TYPES OF ACCELEROMETERS

As movement of the proof mass is sufficient for an accelerometer, accelerometers are designed using various sensing principles.

·         Potentiometric

One of the simplest accelerometer type – it measures motion of the proof mass motion by attaching the spring mass to the wiper arm of a potentiometer. Thus position of the mass and thereby, changing acceleration is translated to changing resistance.

The natural frequency of these devices is generally less than 30 Hz, limiting their application to low frequency vibration measurements. Dynamic range is also limited. But they can measure down to 0 Hz (DC response).

·         Capacitive accelerometers

Capacitive accelerometers sense a change in electrical capacitance, with respect to acceleration. Single capacitor or differential capacitors can be used; differential ones being more common

In these accelerometers, a diaphragm acting as a mass moves in the presence of acceleration. The diaphragm is sandwiched between the two fixed plates creating two capacitors; each with an individual fixed plate and each sharing the diaphragm as a movable plate. Movement of the diaphragm causes a capacitance shift by altering the distance between two parallel plates, the diaphragm itself being one of the plates.

The two capacitors form the two arms of the bridge; the output of the bridge varies with the acceleration.

Capacitive sensing is most commonly used in MEMS accelerometers. Like potentiometric accelerometers, capacitive accelerometers have true DC response but limited frequency range and limited dynamic range.

·         Piezoelectric accelerometers

Piezoelectric accelerometers employ piezoelectric effect. When piezoelectric materials are stressed, they are deformed and an electric charge is generated on the piezoelectric materials.

In piezoelectric accelerometers, piezoelectric material is used as an active element. One side of the piezoelectric material is connected to rigid base. Seismic or proof mass is attached to the other side. When force (generated due to acceleration) is applied, piezoelectric material deforms to generate the charge. This charge is proportional to the applied force or in other words, proportional to acceleration (as mass is constant). The charge is converted to voltage using charge amplifiers and associated signal conditioning circuit.

Compared to other type of accelerometers, piezoelectric accelerometers offer unique advantages –

– Wide dynamic range

– Excellent linearity

– Wide frequency range

– No wear and tear due to absence of moving parts

– No external power requirement

However, alternating acceleration only can be measured with piezoelectric accelerometers. These accelerometers are not capable of measuring DC response.

·         Piezo-resistive accelerometers

Piezo-resistive accelerometers use piezo-resistive materials, i.e., strain gauges. On application of the force (due to acceleration), resistance of these strain gages changes. The change in resistance is monitored to measure the acceleration.

Piezo-resistive elements are typically used in micro-machined structures. They have true DC response. They can be designed to measure upto ±1000 g.

·         Variable inductance accelerometers

Using the concept very similar to the one used in LVDTs, variable inductance accelerometers can be designed.  In these accelerometers, proof mass is made of ferromagnetic materials.  The proof mass is designed in the form of core which can move in or out of the coil.

When the body is accelerated, the proof mass moves. In other words, portion of the core inside the coil changes and so the coil impedance. Thus, the coil impedance is a function of the applied acceleration.

·         Hall Effect accelerometers

Hall Effect accelerometers measure voltage variations resulting from a change in the magnetic field.

If a magnet is mounted/ integrated on a proof mass, the output of the hall element will vary according to the applied force due to the variation of the magnetic field sensed by the Hall element. Hall voltage is calibrated in terms of acceleration.

·         Magnetoresistive accelerometers

Magnetoresistive accelerometers employ magnetoresistive effect. Resistance of magnetic materials changes when exposed to varying magnetic field. These accelerometers are similar to Hall Effect accelerometers; the only difference is the use of magnetoresistive material instead of Hall element. Hence, the change in resistance due to the applied acceleration is measured.

·         FBG Based accelerometers

A fiber Bragg grating (FBG) is a type of distributed Bragg reflector fabricated in a short section of optical fiber that reflects specific wavelengths of light and transmits all others. When a broad-spectrum light is transmitted through the fiber, and the transmitted beam impinges on the grating, a part of the signal is transmitted through, and another part is reflected off. The reflected signal is centered at Bragg wavelengths. Any change in the grating pitch of the fiber caused by strain or temperature results in a shift of Bragg wavelength.  This is the property used for sensing of movement of mass in the accelerometers.

In FBG sensor based accelerometers, the acceleration is coupled to a mechanical load on the FBG. Due to the strain experienced by the FBGs (as a result of applied acceleration), there is a shift in the reflected Bragg wavelengths. Shift in the wavelengths is then calibrated to the level of acceleration.

·         Heated Gas accelerometers

Heat Gas accelerometers measure internal changes in heat transfer due to acceleration. These accelerometers use gas as a proof mass.

Gas is enclosed in a cavity and a heat source is suspended at the center. Two (or more) thermistors are placed at equal distances from the suspended heat source.

Under rest condition (or zero acceleration), the gas is heated to an equilibrium temperature, the heat gradient is symmetrical, and hence two thermistors are at same temperature.

Under acceleration, the heat gradient become asymmetrical due to convective heat transfer, the gas shifts to the direction opposite the motion (the gas is the inertial mass) causing a temperature gradient. The temperature gradient is calibrated in terms of acceleration.

·         MEMS-Based Accelerometers

MEMS is an enabling technology which allows miniaturization of existing devices, to offer solutions which cannot be attained by macro-machined products. MEMS allows the complex electromechanical systems to be manufactured using batch fabrication techniques, decreasing the cost and increasing the reliability.  It allows integrated systems, viz., sensors, actuators, circuits, etc. in a single package and offers advantages of reliability, performance, cost, ease of use, etc.  This technology is being utilized widely to manufacture state of the art MEMS-Based Accelerometers.

First MEMS accelerometers used piezoresistors. However, piezoresistors are less sensitive than capacitive detection. Most of the MEMS accelerometer use capacitive sensing principle. Typical MEMS accelerometer is composed of movable proof mass with plates that is attached through a mechanical suspension system to a reference frame. Movable plates (part of the proof mass) and ?xed outer plates form differential capacitor. Due to application of the force, proof mass deflects; the deflection is measured in terms of capacitance change.

SEM photograph of MEMS 3D accelerometer is shown below

UNDERSTANDING OF SPECIFICATIONS

Often user fails to match the required test specifications with the available accelerometer models. Selection of a sensor requires proper understanding of the specifications. The specifications of an accelerometer include dynamic specifications, electrical specifications and mechanical specifications. Some of the important specifications of an accelerometer are as follows:

Sensitive Axis	Accelerometers are designed to detect inputs in reference to an axis; single-axis accelerometers can detect inputs only along one plane. Triaxial accelerometers can detect inputs in any plane.
Dynamic Range	Dynamic range refers to the maximum amplitude vibration that can be measured by an accelerometer before distortion occurs in the amplifier. It is normally specified in ‘g’s.
Sensitivity	Sensitivity refers to the ability of an accelerometer to detect motion. Sometimes referred to as the “scale factor” of the accelerometer, it is the ratio of the sensor’s electrical output to mechanical input. It is typically specified in terms of mV/g and it is valid only at one frequency (usually 100 Hz) and at particular temperature (25° C). This indicates the voltage output per g of acceleration
Frequency Response	The frequency response specification shows the maximum deviation of sensitivity over a frequency range. More appropriately known as amplitude response, it is the sensitivity specified over the transducer’s entire frequency range. The frequency response is specified over a tolerance band; they are specified in percentage and/or dBs, typical bands being ±10%, ±1 dB or ±3 dB. Upper frequency limit is typically governed primarily by the mechanical resonance of the sensor. Lower frequency limit appears because of “high pass” filtering used for reduction of the low frequency amplifier noise.
Mounted Resonance Frequency	This is the primary (largest) mechanical resonance of the sensor when mounted on the structure. At this frequency, accelerometer shows maximum sensitivity.
Transverse Sensitivity	Transverse sensitivity is the sensitivity of the accelerometer at 90 degrees to the sensitive axis of the sensor. Also referred to as cross-axis sensitivity, it is expressed as a percentage of the axial sensitivity. Ideally, it should be zero, but can be as much as 5%.
Amplitude linearity	Often referred to as amplitude non-linearity, amplitude linearity is a measure of how linear the output of an accelerometer is over its specified amplitude range. Amplitude linearity specifies the limits to how far the accelerometer’s output will differ from the perfect linearity. Again, amplitude linearity is only valid at a (usually undisclosed) single frequency. It is specified as percentage of reading; sometimes expressed in a piecewise manner also.
Output polarity	Output polarity describes the direction of the accelerometer’s output signal (whether it is positive or negative going), given a particular direction of the input acceleration.
Electronic Noise	This is the electronic noise generated by the amplifier circuit. Noise is specified as either “broadband”, or “spectral”. The broadband measurement is a measurement of the total noise energy over a specified bandwidth. Spectral noise is the noise measured at a specific frequency.
Size and Mass	Size and mass of an accelerometer can change the characteristics of the object being tested. The mass of the accelerometers should be significantly smaller than the mass of the system on which measurement is to be done.

Though accelerometers based on different sensing principles were discussed in the previous sections, MEMS based accelerometers share the major market today, due to their obvious advantages.

METHODS OF CALIBRATION

Calibration of an accelerometer is to accurately determine its sensitivity at various frequencies of interest. Methods commonly employed to calibrate the accelerometers are:

1.      Gravity Test

The accelerometers having true DC response can be calibrated using this method.

In this method, an accelerometer is placed with its sensitive axis (+ and -) along the direction of gravity and the outputs are noted. Difference between the two readings corresponds to 2 g difference. From this scale factor can be computed.

2.      Back-to-back Accelerometer Calibration

This technique is arguably the most convenient method for accelerometer calibration.

Back-to-back calibration involves coupling the test accelerometer directly to a (NIST) traceable double-ended calibration standard accelerometer and driving the coupled pair with a vibration exciter at various frequencies and acceleration (g) levels. Since the accelerometers are tightly coupled together, both experience exactly the same motion, thus the calibration of the back-to-back standard accelerometer can be precisely “transferred” to the test accelerometer.

APPLICATIONS OF ACCELEROMETERS

Accelerometers are one of those sensors which find numerous applications in academia as well as in large number of industries. These applications range from airbag sensor in automotive applications to monitoring vibrations on a bridge and in many military and space systems. There are a number of practical applications for accelerometers; accelerometers are used to measure static acceleration (gravity), tilt of an object, dynamic acceleration, shock to an object, velocity, and the vibration of an object. Accelerometers are being used nowadays in mobile phones, laptops, washing machines, etc.