Professor Charlton S. InaoDefence University

College of EngineeringBishoftu, Ethiopia

PE-4030Chapter 2/a

Sensors

Sensor is a device that produces an output signal for the purpose of sensing of a physical

phenomenon

Sensor is used for an input device that provides a usable output in response to a specified physical input. For example, a thermocouple is a sensor that converts a temperature difference into an electrical output.

Transducer

The term transducer is generally used to refer to a device that converts a signal from one form to a different physical form. Thus sensors are often transducers, but also other devices can be transducers, such as a motor that converts an electrical input into rotation

TransducersTransducer is often used in place of the term sensor. They are elements that when subject to some physical change experience a related change.

Sensors are transducers.

A measurement may use transducers, in addition to the sensor, in other parts of the system to convert signals in one form to another form.

Transducers

A transducer is a device that converts one type of energy to another. Energy types include (but are not limited to) electrical, mechanical, electromagnetic (including light), chemical, acoustic or thermal energy.

While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a transducer. Transducers are widely used in measuring instruments.

Transduce means converts.

Transducers

SensitivityResolutionAccuracyPrecisionRepeatability/ReproducibilityLinearity

Quality Parameters of an Instrumentation System

Terminologies

Signal Conditioning - a front-end preprocessing, which generally includes functions such as signal amplification, filtering, electrical isolation, and multiplexing.

In addition, many transducers require excitation currents or voltages, bridge completion, linearization, or high amplification for proper and accurate operation.

Signal Conditioning Devices

Charge amplifiers, lock-in amplifiers, power amplifiers, switching amplifiers, linear amplifiers, tracking filters, low-pass filters, high-pass filters, and notch filters are some of the signal-conditioning devices used in mechatronic systems.

• Range- it is the limits between which the input can vary. eg – load cell for the measurement of forces might have a range of 0-50 kN.

• Error- the difference between the result of the measurement and the true value of the quantity being measured.

Error= measured value- true value

Terminology• Accuracy- is the extent to which the value

indicated by a measurement system might be wrong. It is the summation of all possible errors that are likely to occur, as well as the accuracy to which the transducer has been calibrated.

• Accuracy is often expressed as the full range output or full scale deflection. Eg. A sensor might be specified as having an accuracy of +

5% of full range output. Given: Temp Range 0-200 deg Centigrade. Reading could be within +or -10 deg centigrade of the true reading.

• Sensitivity- the sensitivity is the relationship indicating how much output you get per unit input, ie. Output/input.

Example: A resistance thermometer may have sensitivity of 0.5 ohms/deg Centigrade.

-A transducer for the measurement of pressure might be quoted as having a temperature sensitivity of + 0.1% of the reading per degree Centigrade change in temperature.

• The sensitivity is defined as the ratio between output signal and measured property. For example, if a sensor measures temperature and has a voltage output, the sensitivity is a constant with the unit [V/K]; this sensor is linear because the ratio is constant at all points of measurement.

Sensitivity is the ability of the measuring instrument to respond to changes in the measured quantity.

It is also the ration of the change of output to change of input.

• Hysteresis error- Transducers can give different outputs from the same value of quantity being measured according to whether that value has been reached by a continuously increasing change or a continuously decreasing change. The hysteresis error is the maximum difference in output for increasing and decreasing values.

Hysteresis

• Hysteresis is an error caused by when the measured property reverses direction, but there is some finite lag in time for the sensor to respond, creating a different offset error in one direction than in the other.

Hysteresis Error

Nonlinearity and hysteresis

• The error of a measurement is the difference between the result of the measurement and the true value of the quantity being measured.

• Nonlinearity error is used to describe the error that occurs as a result of assuming a linear relationship between the input and

output over the working range, that is, a graph of output plotted against input is assumed to give a straight line.

• Non linearity error- The error associated in the deviation from linearity between the input and the output. The error is quoted as the percentage of the full range output.

• Three methods: 1) Draw the straight line joining the output values at the end points of the range 2) Find the straight line by using the method of least squares to determine the best fit line. 3) Find the straight line by using the method of least squares to determine the best fit line which passes through zero point.

Non Linearity Error

• Repeatability- The repeatabiity of the transducer is its ability to give the same output for repeated applications of the same input value. Example: Angular velocity => repeatability + 0.01% of the full range at a particular angular velocity.

• Reproducibility- the ability to give the same output when used to measure a constant and is measured on a number of occasions.

• Between the measurements the transducer is disconnected and reinstalled. The error is usually expressed as a percentage of the full range output.

• Stability- The stability of a transducer is its ability to give the same output when used to measure a constant input over a period of time. The term drift is often used to describe the change in output that occurs over time.

• The drift may be expressed a s a percentage of the full range output.

• The term zero drift is used for the changes that occur in output when there is zero input.

• Deadband- the deadband or dead space of a transducer is the range of input values for which there is no output. For example bearing friction in a flowmeter using a rotor might mean that there is no output until the input has reached a particular velocity threshold.

• Resolution- The resolution of a sensor is the smallest change it can detect in the quantity that it is measuring.

• Often in a digital display, the least significant digit will fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the precision with which the measurement is made.

• For example, a scanning tunneling probe (a fine tip near a surface collects an electron tunnelling current) can resolve atoms and molecules.

Example: Strain Gauge Transducer

•Thermal Sensitivity :0.03 % full range output /deg Centigrade

Interpretation for Strain Gauge Transducer Specs

• The range indicates that the transducer can be used to measure pressures between 70 and 1000 kPa or 2000 and 70000kPa.

• It requires a supply of 10 Vdc or ac rms for its operation

• It will give an output of 40mV when the pressure on the lower range is 1000 kPa and on the upper range of 70 000kPa

Interpretation for Strain Gauge Transducer Specs

• Nonlinearity and hysteresis will lead to errors of + .5% of 1000, i.e + 5kPa on the lower range and + .5% of 70 000 , that is i.e + 350kPa on the upper range.

• The transducer can be used between the temperature range -54 deg C and +120 deg C.

• When the temperature changes by 1 deg C the output of the transducer for zero input will change by 0.03% of 1000=0.3kPa on the lower range and 0.03% of 70 000=21 kPa on the upper range.

Interpretation for Strain Gauge Transducer Specs

• When the temperature changes by 1deg C, the sensitivity of the transducer will change by 0.3 kPa on the lower range and 21kPa on the upper range.

• This means that readings will change by these amounts when such temperature change occurs.

Example : MX100AP Pressure Sensor Specs

• Supply voltage: 3 V (6 V max)• Supply current: 6 mA• Full-scale span: 60 mV• Range: 0 to 100 kPa• Sensitivity: 0.6 mV/kPa• Nonlinearity error: 0.05% of full

range• Temperature hysteresis: 0.5% of full

scale• Input resistance: 400 to 550 O• Response time: 1 ms (10% to 90%)

Displacement, Position and Proximity Sensors

Displacement/Position Sensors

The term position sensor is used for a sensor that gives a measure of the distance between a reference point and the current location of the target, while a displacement sensor gives a measure of the distance between the present position of the target and the previously recorded position.

POSITION SENSORSTypes of Position Sensors

Every commonly applied position sensing technology has its own characteristic benefits and limitations. some of these technologies provide a better fit than others in different applications. The goal is to find the most cost-effective solution for the performance parameters that are important in your specific application and environment.

The types of position sensors include:

■ Contact devices• Limit switches• Resistive position transducers

■ Non-contact devices• Magnetic sensors, including Hall effect and magneto-resistive sensors• Ultrasonic sensors• Proximity sensors• Photoelectric sensors

Limit SwitchesLimit switches are electromechanical contact

devices. Easy to understand and apply,they are the cost-effective switches of choice for

detecting objects that can be touched. These rugged, dependable switches are offered in a variety of sizes with different seals, enclosures, actuators, circuitries and electrical ratings.

CONTACT TYPE

Various limit switches provide years of reliable operation even in the most demanding environmental conditions. They are appropriate for: ■ Material handling

■ Breweries■ Packaging machinery■ Wood products■ Special machinery■ Garbage compactors/trucks■ Valves■ Foundry equipment

Resistive Position SensorsResistive position sensors, also called potentiometers or simply position transducers, were originally developed for military applications. They were widely used as pane lmounted adjustment knobs on radios and televisions in the years before pushbuttons and remote controls. Today, potentiometers are most commonly found in industrial applications that range from forklift throttles to machine slide sensing. Potentiometers are passive devices, meaning they require no power supply or additional circuitry to perform their basic linear or rotary position sensing function. They are typically operated in one of two basic modes: rheostat and voltage divider (true potentiometric operation). As resistance varies with motion, rheostat applications make use of the varying resistance between a fixed terminal and the sliding contact wiper. In voltage divider applications, a reference voltage signal is applied across the resistive element track so that the voltage “picked up” by the movable contact wiper can be used to determine the wiper’s position.

NON-CONTACT TYPE

Applications

Ultrasonic Position Sensors

Application

Proximity Sensors

Capacitive Sensors

Inductive Sensors

Position Sensors• Two Groups

– Linear– Angular Linear displacement sensors might be used to

monitor the thickness or other dimensions of sheet materials, separation of rollers, the position or presence of a part, the size of a part.

Angular displacement methods might be used to monitor the angular displacement of shafts.

Application: Location and position of object on a conveyor

• Displacement Sensors– Potentiometer– Strain Gauge element– Capacitive Element– Differential Transformer– Optical Encoders

• Absolute • Incremental

Potentiometer

• A potentiometer consists of a resistance element with a sliding contact which can be moved over the length of the element. Such element can be used for linear or rotary displacements.

Potentiometer

Strain Gauge• The electrical resistance strain gauge is a metal wire,

metal foil strip, or a strip of semiconductor material which is waferlike and can be stuck into surfaces like a postage stamp.

• When subject to strain, its resistance R changes, the fractional change in resistance delta R/R being proportional to the strain E, that is delta R/R=GE

Where G, the constant of proportionality, is termed a s the gauged factor. The resistance change of a strain gauge is a measurement of the change in length of the element to which the strain gauge is attached

Example: An electrical resistance of 100 ohms and a gauge factor of 2.0. What is the change of resistance ΔR of the gauge when it is subject to a strain of 0.001.

Answer=Fractional Change in R= G x strain x R=2.0X0.001x100=0.2 ohms

Linear Variable Differential Transformer.

Linear variable differential transformer is a mechanical displacement transducer.

It gives an a.c. voltage output proportional to the distance of the transformer core to the windings. The LVDT is a mutual-inductance device with three coils and a core An external a.c. power source energizes the central coil and the two- identical secondary coils connected in seriesin such a way that their outputs oppose each other.

The net result is zero output.A magnetic core is moved through the central tube as a result of

displacement being monitored. However when the core is displaced from the central position there is a greater amount of magnetic core in one coil than the other. A greater displacement means even more core in one coil than the other, the output, the difference between the emf increases, the greater the displacement being monitored.

LVDT

A greater displacement means even more core in one coil than the other, the output, the difference between the emf increases, the greater the displacement being monitored.

LVDT

Optical Encoder

• An encoder is a device that provides digital output as a result of linear and angular displacement. Position encoders are of two types: Incremental and absolute. Incremental encoders detect changes in rotation from some datum while the absolute encoders give the actual angular position.

Absolute encoder

Incremental encoder

Optical Encoder

Optical Encoder

1.2 Proximity SensorsProximity switches are used to detect the presence of an item

without making contact with it.

Proximity Sensors -eddy

-reed -capacitive -inductive

There are a number of forms of such switches, some being suitable only for metallic objects.

The eddy current type of proximity switch has a coil that is energized by a constantalternating current and produces a constant alternating magnetic field. When a

metallic objectis close to it, eddy currents are induced in it .

Proximity Sensors

Photoelectric Sensors

CapacitiveA proximity switch that can be used with metallic

and nonmetallic objects is the capacitive proximity switch. The capacitance of a pair of plates separated by some distance depends onthe separation; the smaller the separation, the higher the capacitance. The sensor of ecapacitive proximity switch is just one of the plates of the capacitor, the other plate being themetal object for which the proximity is to be detected Thus the proximity of the object is detected by a change in capacitance.

Inductive• The inductive proximity switch, consists of a coil wound a

round a ferrous metallic core. When one end of this core is placed near a ferrous metal object, there is effectively a change in the amount of metallic core associated with the coil and so a change in its inductance.

• This change can be monitored using a resonant circuit, the presence of the ferrous metal object thus changing the current in that circuit.

• The current can be used to activate an electronic switch circuit and so create an on/off device. The range over which such objects can be detected is typically about 2 mm to 15 mm. An example of the use of such a sensor is to detect whether bottles passing along a conveyor belt have metal caps on.

Velocity and Motion Sensors

Example Velocity Sensor Specs

Velocity Sensor• Incremental Encoder-This can be used for measuring

angular velocity, number of pulses produced per second being determined.

• Tachogenerator -Used to measure angular velocity. It is essentially a small electric generator, consisting of coil mounted in magnetic field .when the coil rotates an alternating emf is induced in the coil, the size of the maximum emf being a measure of the angular velocity. when used with a commutator a dc output can be aobtained which is a measure of the angular velocity.

Motion SensorBy motion, we mean the four kinematic variables:• Displacement (including position, distance, proximity,

and size or gage)• Velocity• Acceleration• Jerk

Note that each variable is the time derivative of the preceding one. Motion measurements are

extremely useful in controlling mechanical responses and interactions in mechatronic systems.

The rotating speed of a work piece and the feed rate of a tool are measured in controlling machining operations.

Displacements and speeds (both angular and translatory) at joints (revolute and prismatic) of robotic manipulators or kinematic linkages are used in controlling manipulator trajectory In high-speed ground transit vehicles, acceleration and jerk measurements can be used for active suspension control to obtain improved ride

quality.

• Angular speed is a crucial measurement that is used in the control of rotating machinery, such as turbines, pumps, compressors, motors, and generators in power-generating plants. Proximity sensors (to measure displacement) and accelerometers (to measure acceleration) are the two most common types of measuring devices used in machine protection systems for condition monitoring, fault detection, diagnostic, and on-line (often real-time) control of large and complex machinery .

• The accelerometer is often the only measuring device used in controlling dynamic test rigs.

• Displacement measurements are used for valve control in process applications. Plate thickness (or gage) is continuously monitored by

the automatic gage control (AGC) system in steel rolling mills.

Force and Fluid Pressure

Sensors

Force, Load and Weight Sensors

• While other technologies exist, the most commonly used sensors are generally based on either piezoelectric quartz crystal or strain gage sensing elements

• Force: The measurement of the interaction between bodies.

• Load: The measurement of the force exerted on a body.

• Weight: The measurement of gravitational forces acting on a body.

Quartz Sensors

• Technology Fundamentals

Quartz force sensors are ideally designed and suited for the measurement of dynamic oscillating forces, impact, or high speed compression/tension forces. The basic design utilizes the piezoelectric principle, where applied mechanical stresses are converted into an electrostatic charge that accumulates on the surface of the crystal.

The quartz crystals of a piezoelectric force sensor generate an electrostaticcharge only when force is applied to or removed from them. In other words, if you apply a static force to a piezoelectric forcesensor, the electrostatic charge output initially generated will eventually leak away and the sensor output ultimately will return to zero.

Discharge Time Constant (DTC).

the charge signal decays according to the equation:

q = Qe–t/RC

where:q = instantaneous charge (Coulomb)Q = initial quantity of charge (Coulomb)R = resistance prior to amplifier (ohm)C = total capacitance prior to amplifier (Farad)e = base of natural log (2.718)t = time elapsed after time zero (Second)

Piezoelectric Force Sensor Construction

The basic mechanical construction of general purpose quartz forces sensors consistof thin quartz discs that are “sandwiched” between upper and lower base plates. Arelatively elastic, beryllium-copper stud (or sometimes a sleeve) holds the upper andlower plates together and preloads the crystals. Preloading of the crystals is required to assure that the upper and lower plates are in intimate contact with the quartz crystals, ensuring good linearity and the capability for tension as well as compression measurements. This “sensing element” configuration is then packaged into a rigid, stainless steel housing and welded to provide hermetic sealing of the internal components against contamination. The Figure depicts thetypical construction of a general-purpose quartz force sensor.

Type of Piezoelectric Force Sensor

Applicable Standards

The basic design of quartz-based force sensors is not governed by a specific standard.

However, applicable standards do exist for calibration and certification. Most manufacturers comply or conform to standards such as ISO 10012-1 (former MIL– STD-45662A), ISO 9001 and ISO/IEC 17025.

Major Manufacturers

PCB Piezotronics, Inc. – 3425 Walden Avenue, Depew, NY 14043

Kistler Instrument Corporation – 75 John Glenn Drive, Amherst, NY 14228

Dytran Instruments, Inc. – 21592 Marilla St., Chatsworth, CA 91311

Endevco Corporation – 30700 Rancho Viejo Rd., San Juan Capistrano, CA 926

Strain Gauge Force SensorTechnology Fundamentals

Sensors based on foil strain gage technology are ideally designed for the precise measurement of a static weight or a quasi-dynamic load or force. The design of strain gage-based sensors consists of specially designed structures that perform in a predictable and repeatable manner when a force, load or weight is applied. The applied input is translated into a voltage by the resistance change in the strain gages, which are intimately bonded to the transducer structure. The amount of change in resistance indicates the magnitude of deformation in the transducer structure and hence the load that is applied.

The strain gages are connected in a four-arm Wheatstone bridge configuration, which acts as an adding and subtracting electrical network and allows for compensation of temperature effects as well as cancellation of signals caused by extraneous forces.A regulated 5 to 20 volt DC or AC rms excitation is required and is applied between A and D of the bridge. When a force is applied to the transducer structure, the Wheatstone Bridge is unbalanced, causing an output voltage between B and C proportional to the applied load. Most load cells follow a wiring code established by the Western Regional Strain GageCommittee as revised in May 1960. The code is as follows:

Sensor Types

Sensor TypesThe most critical mechanical component in any strain gage-based sensor is the “spring element.” In general terms, the spring element serves as the reaction mechanism to the applied force, load or weight. It also focuses it into a uniform, calculated strain path for precise measurement by the bonded strain gage.

3 Common Designs of Strain Gauge

• Three common structure designs used in the industry are

1) bending beam, 2) column and3) shear.

ClassificationGeneral PurposeGeneral-purpose load cells are designed for a multitude of applications in the automotive, aerospace, and industrial markets. The general-purpose load cell, as the name implies, is designed to be utilitarian in nature. Within the general-purpose load cell market there are several distinct categories: precision, universal, weigh scale, and special application. Universal load cells are the most common in industry.

Fatigue RatedFatigue rated load cells are specially designed and manufactured to withstand millions of cycles. Many manufacturers utilize premium fatigue resistant steel and special processing to insure mechanical and electrical integrity as well as accuracy. Fatigue rated load cells typically are guaranteed to last 100 million fully reversed cycles (full tension through zero to full compression). An added benefit of fatigue rated load cells is that they are extremely resistant to extraneous bending and side loading forces

Special ApplicationSpecial application load cells are load cells that have been designed for a specific unique force measurement task. Special application load cells can be single axis or multiple axes. They include but are not limited to:

• Pedal Effort • Seat Belt • Steering Column• Crash Barrier • Hand Brake • Road Simulator• Tow Ball • Femur • Skid Trailer• Bumper Impact • Tire Test • Gear Shift

• Strain Gauge Load Cell –use of electrical resistance to monitor the strain produced in some member when stretched, compressed or bent.

Strain Gauge

Strain Gauges a)metal wire b)Metal foil c)semiconductor

Strain Gauge

Sample Calculations –Strain GaugeWheatstone bridge. Calculate the value of the output

voltage(Vo) of the Wheatstone bridge below if the value of R1 varies from 10 kΩ to 15 kΩ and the Value of Supply Voltage, Vs= 1 V

Vo=R1/(R1 + R2) – R3/(R3 + R4)

R1 R1/(R1 + R2) R3/(R3 + R4) Output Voltage(Vo)

10kΩ 10/(10 + 10) 10/(10 + 10) 0 V

11kΩ12kΩ13kΩ14kΩ15kΩ

Pressure SensorsCommon methods of pressure sensing are the following:

1.Balance the pressure with an opposing force (or head) and measure this force. Examples are liquid manometers and pistons.

2. Subject the pressure to a flexible front-end (auxiliary) member and measure the resulting deflection.Examples are Bourdon tube, bellows, and helical tube.

3. Subject the pressure to a front-end auxiliary member and measure the resulting strain (or stress). Examples are diaphragms and capsules.

The liquid column of height h and density r provides a counterbalancing pressure head to support the measured pressure p with respect to the reference (ambient) pressure pref

The pressure is determined by measuring F using a force sensor. The Bourdon tube shown in Figure 6.77(c) deflects with a straightening motion as a result of internal pressure. This deflection can be measured using a

displacement sensor (typically, arotatory sensor) or indicated by a moving pointer.

The bellows deflect with internal pressure, causing a linear motion, as shown in Figure 6.77(d). The deflection can be measured using a sensor such as LVDT or a capacitive sensor, and can be calibrated to indicate pressure.

The helical tube shown in Figure 6.77(e) undergoes a twisting (rotational) motion when deflected by internal pressure. This deflection

can be measured by an angular displacement sensor (RVDT, resolver, potentiometer, etc.), to provide pressure reading through proper calibration.

Figure 6.77(f) illustrates the use of a diaphragm to measure pressure. The membrane (typically metal) will be strained due to pressure. The pressure can be measured by means of strain gauges (piezoresistive sensors) mounted on the diaphragm. MEMS pressure sensors that use this principle are available. In one such device, the diaphragm has a silicon wafer substrate integral with it.

Through proper doping (using boron, phosphorous, etc.) a microminiature semiconductor strain gauge can be formed. In fact more than one piezoresistive sensor can be etched on the diaphragm, and used in a bridge circuit to provide the pressure reading, through proper calibration. The most sensitive locations for the piezoresitive sensors are closer to the edge of the diaphragm, where the strains reach the maximum.

Fluid Pressure Sensors

Bellows and Orifice

The End