Types of Sensors used in Measurement and Process Control

A sensor is an element in a measurement system that detects the magnitude of a physical variable or parameter and changes it into a signal that can be processed by the system. The active element of a sensor is often referred to as a transducer.

The monitoring and control systems require sensors to measure physical quantities such as force, distance, strain, temperature, vibration, acceleration, position, etc. In this article we discuss various sensors commonly used in mechatronics, instrumentation and industrial control.

Position and Speed Measurement

Position is one of the most commonly measured quantity in mechatronics and process control systems besides voltage, current and resistance. Most often we need know where various parts of a system are in order to control the system. Some of the sensors used in position measurement are:

• Proximity sensors and limit sensors
• Potentiometer
• Linear variable differential transformer
• Digital encoder

Since most applications involve measuring and controlling rotation (for example in robot joints, motors, generators, numerically controlled mill axes, and lathe machines); rotary position sensors are more common than linear sensors. In addition, linear motion can often be easily converted to rotary motion (e.g. with a belt gear or wheel mechanism) allowing the use of rotary position sensors in linear motion applications.

Proximity sensors and Limit switches

Proximity sensors and limit switches are position sensors that detect whether or not something is close or has reached a limit of travel.

A proximity sensor consists of an element that changes either its state or an analog signal, when it is close to but often not actually touching an object.

Proximity sensors are designed using various principles:

• Capacitance
• Magnetism
• Inductance
• Eddy currents
• Photo/Optical detection

Optical proximity sensors

Optical proximity sensors, sometimes called interrupters use a light source and a photo sensor that are mounted in such a way that the object to be detected cuts the light path.

In a photoemitter-detector pair, the emitter can be a laser or focused LED and the detection is usually a phototransistor or photodiode.

We have 4 types of photodetectors that are generally used with proximity sensors:

• Photoresistor
• Photodiode
• Phototransistor
• Photovoltaic cell

A photoresistor which is made out of material such as cadmium sulfide (Cds) has the property that its resistance decreases when the light level increases. It is expensive and quite sensitive i.e. the resistance can change by a factor of 100 or more when exposed to light and dark. The Figure (b) below shows a typical interface circuit of a Photoresistor, as the light increases R decreases and V_out increases.

A photodiode is a light-sensitive diode. A little window allows light to fall directly on the PN junction where it has the effect of increasing reverse-leakage currents as shown in Figure (c) below. Note that the photodiode is reverse-biased and that the small reverse-leakage current is converted into an amplified voltage by the op-amp.

A phototransistor has no base lead; instead, the light effectively creates a base current by generating electron-hole pairs in the (B junction); the more the light, the more the transistor turns on. This illustrated in figure below:

The photovoltaic cell is different from the other three photosensors we have mentioned above, because it creates electrical power from the light, the more the light, the higher the voltage is produced. A solar cell is a photovoltaic cell, when used as a sensor; the small voltage output must be amplified as illustrated in figure (e):

Optical sensors enjoy the advantage that neither the light source, the object to be detected nor the detector have to be near each other. This property can be of great importance especially in cases where we have limited mounting surfaces near the part to be measured.

Hall-effect proximity sensors

This is based on a property of copper and semiconductors that produce voltage in the presence of a magnetic field. This is called hall-effect and is employed in proximity sensor design.

The hall-effect sensor outputs a voltage when the magnetic field in which it finds itself increases. This is done either by moving a magnet or by changing the magnetic field path.

Operation of a Hall-effect sensor

First an external voltage source is used to establish current (I) in the semiconductor crystal. The output voltage (V) is detected across the sides of the crystal, perpendicular to the current direction. When a magnetic field is brought near, the negative chares are deflected to one side producing voltage:

Where V = hall-effect voltage

K = constant (dependent on material)

I = current from external source

B = Magnetic flux density

D = thickness constant

Hall-effect sensors are employed in many applications e.g. computer keyboard switches and proximity sensors in machines. In addition, they are also used as sensors in the toothed-rotor tachometers.

Limit sensors

A limit switch is an example of a proximity sensor. A limit switch is a mechanical push-button switch that is mounted in such a way that is actuated when a mechanical part or lever arm gets to the end of its intended travel. An example is a car door switch, which senses whether or not the door is closed. Limit switches can detect these two conditions. Although they work well in many applications, limit switches have 2 drawbacks:

• Being mechanical devices, they eventually wear out.
• They require a certain amount of physical force to activate them.

Switches are characterized by number of poles (p) and throws (T) and whether connections are normally open (NO) or normally closed (NC). A pole is a moving element in the switch that makes or breaks connections. A throw is a contact point for a pole.

A SPST switch is a single pole (SP), single-throw (ST) device that opens or closes a single connection. A SPDT switch changes the pole between two different throw positions. We also have DPDT switch mechanism which has two poles switching between two different throw positions.

Potentiometer

A potentiometer can be used to convert rotary or linear displacement to a voltage. The potentiometer itself gives resistance but this resistance value can be easily converted to a voltage.

A rotary potentiometer is a variable resistance device that can be used to measure angular position. It consists of a wiper at point (B) that makes contact with a resistive element terminating at contacts A and C. This material has a uniform resistivity so that the ohm-per-inch value a long its length is a constant.

The circuit symbol for this potentiometer is shown below:

Linear variable differential transformers (LVDT)

The linear variable differential transformer (LVDT) is a high resolution position sensor that outputs an AC voltage with a magnetic proportion to linear position. Let’s consider the LVDT illustrated below:

It consists of three windings and a movable iron core. The center winding or primary winding is connected to AC reference voltage. The outer two windings called secondary windings are wired to be out of phase with each other and are connected in series. If the iron core is exactly in the center, the voltages induced in the secondary windings by the primary will be equal and opposite giving a net output (V_net) of 0 V. If the core is moved a little to the right, there will be more coupling to secondary 2 so its voltage is higher, while secondary 1 is lower. If this core is moved a little left of center, the secondary 1 has he greater voltage than secondary 2 producing a net output that is in phase with secondary 1.

In summary, the output of the LVDT is an AC voltage with magnitude and phase angle. The magnitude represents the distance that core is off center, and the phase represents the direction of the core (left or right).

LVDT interface circuit

Let’s consider the circuit below:

An oscillator provides the AC reference voltage to the primary typically 50 Hz to 10 kHz at 10 V or less. The output of the LVDT goes first to a phase-sensitive rectifier. This circuit compares the phase of LVDT output with reference voltage. If they are in phase, the rectifier outputs only the positive part of the signal. If they are out phase, the rectifier outputs only the negative part. Next, a low-pass filter smooth out the rectified signal to produce DC output.

Lastly, an amplifier adjusts the gain to the desired level. An integrated circuit like AD 698 (Universal LVDT signal conditioner) combines all these functions indicated in the interface circuit above on a single chip.

Commercial LVDTs are available in cylindrical forms with different diameters, lengths, and strokes. Often they include internal circuitry that provides a DC voltage proportional to displacement.

Advantages of LVDTs

• Good accuracy over the linear range.
• They produce an analog output that may not require amplification.
• They are less sensitive to wide ranges in temperature than other position transducers like potentiometers, encoders, etc.

Disadvantages of LVDTs

• They have limited range of motion.
• They have limited frequency response. The overall frequency response is limited by inertial effects associated with the core’s mass and the choice of the primary excitation frequency and the filter cut off frequency.

Digital Optical Encoder

A digital optical encoder is a device that converts motion into a sequence of digital pulses. By counting a single bit or decoding a set of bits, the pulses can be converted to relative or absolute position measurements. Encoders have both linear and rotary configurations, but the most common type is rotary.

Rotary encoders are manufactured into two basic forms: the absolute encoder where a unique digital word corresponds to each rotation position of the shaft and the incremental encoder which produces digital pulses as the shaft rotate allowing measurement of relative displacement of the shaft.

The concept of position measurement is illustrated in the figure below:

The slotted disk is attached to a shaft. A light source and photocell arrangement are mounted so that the slots pass the light beams as the disk rotates. The angle of the shaft is deduced from the output of the photocell. The linear patterns and associated timing diagrams are what photodetectors sense as the code disk circular tracks rotate with the shaft.

Stress and Strain Measurement

Bonded Strain Gauge

This is the commonly used transducer for experimentally measuring strain in a mechanical component. It consists of a thin foil of metal, usually constantan deposited as a grid pattern onto a thin plastic backing material usually polyimide. The foil pattern is terminated at both ends with large metallic pads that allow lead wires to be easily attached with solder. The entire gauge is usually very small typically 5 to 15 mm long.

The gauge is oriented so that the wires lie in the same direction as the expected deformation. The entire strain gauge is securely bonded to some structural object and will detect any deformation that may take place.

Principle of operation of Bonded Strain Gauge

To measure strain on the surface of a machine component or structural member, the gauge is adhesively bonded directly to the component usually with epoxy or cyanoacrylate. The backing makes the bonded wire gauge easy to handle and provides a good bonding surface that also electrically insulates the metal foil from the component. Lead wires are then soldered to the solder tabs on the gauge. When the component is loaded, the metal foil deforms, and the resistance changes. If the resistance change is measured accurately, the strain on the surface of the component can be determined.

We know that:

Where R = resistance of a length of wire (at 20°C)

ρ= resistivity (constant dependent on the material)

L = length of wire

A = cross sectional area of wire

The change in the resistance of the strain gauge can be used to calculate the elongation of the strain gauge (and the object to which it is cemented). If you know the elongation and the spring constant of the supporting member, then the principle of Hooke’s law can be used to calculate the force being applied.

The resistance change in strain gauge is small. Typically, it is only a few percent, which may be less than an ohm. Measuring such small resistances usually requires a bridge circuit with this circuit, a small change in one resistor can cause a relatively large percentage change in the voltage across the bridge. Initially the bridge is balanced by adjusting the resistances so that V₁ = V₂, then when the gauge resistance changes, the voltage difference (V₁ – V₂) changes. The bridge also allows us to cancel out variation due to temperature by connecting a compensating gauge (known as the dummy) as one of the bridge resistors as illustrated below:

The actual compensation gauge is placed physically near the active gauge so as to receive the same temperature, but it is oriented perpendicularly from the active gauge so that the force will not elongate the wires.

To analyze the bridge circuit we first calculate the individual voltages V₁ and V₂ using the voltage divider rule.

We can simplify the analysis by specifying that all the resistors in the bridge including (R_G and R_D) have same value (R) when it is balanced, then when the bridge is stretched, R_G will increase a little to become R + ΔR (where ΔR is how many ohms R_G increased because of stretching). Using these conditions, the equation above becomes:

Looking at the denominator, we notice that the sum of 4R and 2ΔR, but in all practical situations 4R will be much greater than 2ΔR, so we could say that 4R + 2ΔR = 4R. With this assumption and using algebra to arrange the equation, we arrive at the equation that we can use to calculate the change in the strain gauge.

That is, the resistance on the basis of measured voltage across the bridge.

Where ΔR = change in the strain gauge resistance

R = nominal value of all bridge resistors

ΔV = voltage detected across the bridge

V_s = source voltage applied to the bridge

As the strain gauge is stretched its resistance rises. The precise relationship between elongation and resistance can be computed by the following equation:

Where ε = elongation of the object per unit length (ΔL/L called strain)

R = strain gauge resistance

ΔR = change in strain gauge resistance due to force

GF = gauge factor, a constant supplied by the manufacturer. GF is the ratio

Relationship between Stress and Strain

Stress is the force per cross-sectional area while strain is the amount of length (per unit length) that the object stretches as a result of being subjected to stress. Stress and strain are related by a constant called Young’s modulus is also called modulus of elasticity. Young’s modulus (E) is a measure of how a stiff material is and could be thought of as a kind of spring constant.

Where E = Young’s modulus (a constant for each material)

ρ = stress (force per cross-sectional area)

ε = strain (elongation per unit length)

Load Sensors

Load sensors measure mechanical force. The forces can be large or small. In most cases it is the slight deformation caused by the force that the sensor measures and not the force directly. Typically this deformation is quite small. Once the amount of tension (stretching) or compression (squeezing) displacement has been measured, the force that must have caused it can be calculated using mechanical parameters of the system. The ratio of the force to deformation is a constant for each material as defined by Hooke’s law:

F = kx

Where F = applied force

k =spring constant of the material

x = Extension or compression as a result of force

Strain gauge force transducers (called load cells) are available as self-contained units that can be mounted anywhere in the system. A load cell may contain two strain gauges (active and compensating) and a bridge. A typical application for load cells is monitoring the weight of a tank. The tank would be sitting on 3 -4 load cells; so the weight of the tank is the sum of the outputs of the load cells.

Semiconductor Strain Gauge

Another type of force sensors uses the piezoelectric effect of silicon. These units change resistance when force is applied and they are 25 -100 times more sensitive than the bonded-wire strain gauge. A semiconductor strain gauge is a single strip of silicon material that is bonded to the structure. When the structure stretches, the silicon is elongated and the resistance from end to end increases however the resistance change is nonlinear.

Temperature Measurement

Temperature sensors give an output proportional to temperature. Most temperature sensors have a positive temperature coefficient which means that the sensor output goes up as the temperature goes up but some sensors have a negative temperature coefficient, which means the output goes down as the temperature goes up.

We discuss several types of temperature sensors commonly used in industrial measurement applications.

Liquid in Glass Thermometers

This is a simple nonelectrical temperature measuring device. It typically uses mercury or alcohol as the working fluid, which expands and contracts relative to the glass container. The upper range is normally on the order of 600 °F. When making measurements in a liquid, the depth of immersion is important, as it can result in different measurements. Also because readings are made visually, and there can be a meniscus at the top of the working fluid, measurements must be made carefully and consistently.

Bimetallic Temperature Sensors

This is another nonelectrical temperature measuring device used in simple measurement and control applications. It consists of two or more metal layers having different coefficients of thermal expansion. The bimetallic strip can be straight or coiled into a spiral for a more compact design. Because these layers are permanently bonded together, the structure will deform when the temperature changes. This is due to the difference in the thermal expansions of the two metal layers. For example,

In the illustration above, the metal on the inside expands more than the one inside. When temperature increases, the tube containing the liquid mercury rotates clockwise. When the tube rotates past the horizontal, the mercury runs down to the right and completes the electrical connection between the electrodes. Presently most mercury switches have been replaced by contact-type switches.

Bimetallic strip sensors are typically used for on-off control e.g. in a household thermostat and industrial thermostats where the mechanical motion of the strip makes or breaks an electrical contact and turn heating or cooling systems on or off.

One distinct advantage of a bimetallic strip temperature measurement system is that the output from the switch can be used directly without further signal conditioning.

Thermocouples

If two dissimilar metals are joined together and one junction is maintained at a high temperature with respect to the other, a current will flow which is a function of the two temperatures. This current is known as the Peltier effect, and forms the basis of a temperature sensor called a thermocouple. In practice, it is more appropriate to measure the voltage difference between the two wires rather than the current. The voltage, typically a few mV is again a function of the temperature at the meter and the measuring junction.

We can think of the junctions at each end of the dissimilar metal wires producing a voltage, so the net voltage (V_net) is actually the difference between the junction voltages. One junction is on the probe and is called the hot junction. The other junction is kept at some known reference temperature and is called the cold junction or reference junction. The output voltage from this system can be expressed as:

V_net = V_hot -V_cold

Because of the indication is a function of the temperature at both ends of the cable, correction must be made for the local meter temperature. A common method used is called cold junction compensation which measures the local temperature by some other method such as RTD and adds in the correction as illustrated below:

Commercial thermocouples are available with different temperature ranges and sensitivity (sensitivity being a measure of volts/degree).

The figure below shows the volts versus temperature curves of the major classes of thermocouples.

You can notice from the figure above, type J (Iron-Constantan) has the highest sensitivity but the lowest temperature range, type K (Chromel-Alumel) has a higher temperature range but a lower sensitivity and type R (Platinum-Rhodium) has an even lower sensitivity but can work at higher temperatures.

Thermocouples are simple and rugged but require extra electronics to handle low-sensitivity and cold junction problems. Nevertheless, since they are linear (over a limited range), reliable and stable, they are extensively used in measuring high temperatures in furnaces and ovens.

Advantages of Thermocouples

• They are very small in size.
• Have low time response compared to other sensing temperature sensing elements.
• They have wide operating temperature range.
• They can convert temperature directly into electrical units.

Disadvantages of Thermocouples

• Thermocouples require a reference.
• They have low signal amplitude

Electrical Resistance Thermometer

The resistance temperature detector (RTD) is a temperature sensor based on the fact that metals increase in resistance as temperature rises. The resistance temperature relationship is usually approximated by the following linear expression:

R = R₀[1 + α(T –T₀)]

Where T₀ is a reference temperature, R₀ is the resistance at the reference temperature and α is a calibration constant. The sensitivity ) is R₀α. The reference temperature is usually at the ice point of water (°C). The most common metal used in RTDs is platinum because of its high melting point, resistance to oxidation, predictable temperature characteristics and stable calibration values. The operating range for typical platinum RTD is -220 °C to 750 °C. We have lower cost nickel and copper types but they have narrow operating ranges.

Thermistor

A thermistor is a semiconductor device available in different shapes and sizes, whose resistance changes exponentially with temperature. Its resistance, temperature relationship is normally expressed in the form:

Where T₀ is a reference temperature

R₀ is the resistance of the reference temperature

β is a calibration called characteristic temperature of the material

A well calibrated thermistor can be accurate within 0.01 °C or better, which is better than typical RTD accuracies. But thermistors have much narrower operating ranges than RTDs. Note that a thermistor resistance actually decreases with increasing temperature. This is very different from metal conductors (used in other RTDs) that experience increasing resistances with increasing temperature.

Flow Measurement

Types of flow sensors commonly used in Industrial flow applications

Types of flow sensors commonly used in Industrial flow applications

Flow sensors measure the quantity of fluid material passing by a point in a certain time. Normally the material is a gas or liquid flowing in pipe or open channel.

There are many techniques for measuring gas and liquid flow rates. A pitot tube measures the difference between total and static pressure of a moving fluid. Venturi and orifice meters are based on measuring pressure drops across obstructions to flow (Differential pressure principle). Turbine flow meters detect the rate of flow by measuring the rate of rotation of an impeller in the flow. Coriolis flow meters measure the mass flow rate through a U tube or straight tube in rotational vibration. Hot-wire anemometers sense the resistance changes in a hot current- carrying wire. The temperature and resistance of the wire depend on the amount of heat transferred to the moving fluid. The heat coefficient is a function of the flow rate. Positive displacement measures flow by displacing a precisely defined (positive) quantity of fluid, so that a count of the number of mechanism cycles yields a precise quantity for the total fluid volume passed through the meter. Ultrasonic flowmeters measure the velocity of a flowing medium by monitoring the interaction between the flow stream and an ultrasonic sound wave transmitted through it.

We also have electromagnetic flowmeters based on Faraday’s law of electromagnetic induction for conductive liquids only. Vortex flow meters whose functioning is based principle of formation of vortices on a bluff body placed in the fluid flow and the variable area flowmeter where a fluid flows through a tapered tube with a float or tapered plug.

Pressure Measurement

Pressure is defined as the force per unit area that one material exerts on another.

Pressure sensors usually consist of two parts: The first part converts pressure to a force or displacement and the second converts the force of displacement to an electrical signal. Pressure measurements are made only for gases and liquids. The simplest pressure measurement yields gauge pressure, which is the difference between the measured pressure and ambient pressure. A little complicated pressure sensor can measure differential pressure i.e. the difference in pressure between two places where neither pressure is necessarily atmospheric. The third type of pressure sensor measures absolute pressure, which is measured with a differential pressure sensor where one side is reference at 0 psi (close to a total vacuum).

Examples of pressure sensors are:

• Bourdon tubes
• Manometers
• Piezoelectric pressure transducer
• Elastic diaphragm, bellows or tube

Bourdon Tube

A bourdon tube is a short bent tube, closed at one end. When the tube is pressurized it tends to straighten out. This motion is proportional to the applied pressure.

The displacement of the bourdon tube sensor can be either linear or angular. A positioner sensor such as potentiometer or linear variable differential transformer (LVDT) can convert the displacement into an electrical signal. Bourdon tube sensors are available in pressure ranges from 30 to 100,000 psi. Typical uses of bourdon tubes are steam and water pressure gauges.

Manometers

Manometers measure static pressure or pressure difference by detecting fluid displacement in a gravitational field. They are not widely used in the industry.

Principle of Operation of a Manometer

If a U-tube is partly filled with liquid and differing pressures applied to both legs the liquid will fall on high pressure side and rise on the low pressure side until the head pressure of liquid matches the pressure difference. This is illustrated below:

If the two levels are separated by a height h then

h = (P₁ –P₂)ρg

Where P₁ and P₂ are pressures in Pascals, ρ is density of the liquid, g is the acceleration due to the gravity.

Piezoelectric Pressure Transducer

This type of pressure transducer measures dynamic pressure when the piezoelectric crystal deforms in response to the applied pressure. The piezoelectric effect occurs in Quartz crystals. An electric charge appears on the faces when the force is applied. This charge is directly proportional to the applied force. The force can be related to pressure with a suitable diaphragm. Note piezoelectric transducers are unsuitable for measuring static pressures but they have fast response and are ideal for measuring fast dynamic pressure changes.

Bellows Pressure sensor

This pressure sensor uses small metal bellows to convert pressure into linear motion as illustrated below:

As the pressure inside increases, the bellows expand against the resistance of a spring (the spring is often the bellow itself). This motion is detected with a position sensor such as potentiometer. Bellows are more sensitive to measurement than bourdon tube in the lower-pressure range of 0 to 30 psi.

Level Measurement

Liquid level sensors which measure the height of liquid in a container can be classified into two categories:

• Discrete level detection
• Continuous level detection

Discrete level detection

Discrete level sensors determine when a liquid has reached a certain level. The simplest type of level detector employs a float and a limit switch. Another type of level detector is based on a photocell; when the liquid level submerges the light path the photodetector signal changes thus indicating the presence of the liquid. Conductive liquids i.e. water based liquids like alcohol; weak acids etc. can use an electric probe as a level detector. When the liquid reaches the probe, the resistance in the circuit abruptly decreases.  A common application of this type of sensor is an automotive low-coolant sensor. Another good example of discrete level detector is Tuning fork level switch that uses a metal tuning fork to detect the presence of a liquid or in some cases solid (powder or granules).

Continuous level detectors

Continuous level detectors provide a signal that is proportional to the liquid level. Continuous level measurement can be accomplished by sensing the pressure head at the bottom of the vessel (tank). This method is based on the fact that the pressure at the bottom of the tank (called head) is directly proportional to the level.

Employing load sensors which monitor the weight of the liquid to determine the liquid level; the level can then be calculated knowing the diameter and weight of the tank when empty and density of the fluid.

One simple and direct method of continuous level measurement is the use of a float connected to a position sensor. This float moves with the fluid level in the vessel. You can also detect level directly using two vertical electrodes mounted inside a tank. The output of the device which is processed or amplified is either resistance or capacitance and is proportional to level.

 An ultrasonic level detector mounted over the tank provides an affordable method of measuring level.

Velocity, Vibration and Acceleration Measurement

Tachometer

Angular velocity sensors i.e. tachometers are devices that give an output proportional to angular velocity. These sensors are employed widely in motor-speed control systems. They are also used in position systems to improve their performance.

Accelerometer

Accelerometer is a sensor designed to measure acceleration or rate of change of speed, due to motion (e.g. in a video game controller), vibration (e.g. from rotating equipment), and impact events e.g. to deploy an automobile airbag.

Accelerometer is usually bonded to an object or structure for which acceleration is to be measured. The accelerator detects acceleration along one axis and is insensitive to motion in orthogonal directions. Strain gauges or piezoelectric elements constitute the sensing element of an accelerometer converting acceleration into a voltage signal.