MECHATRONIC SYSTEM DESIGN ENT 473 LECTURE 2 SENSORS

MECHATRONIC SYSTEM DESIGN [ENT 473] LECTURE #2 SENSORS AND TRANSDUCERS HASIMAH ALI Programme of Mechatronic Engineering, . EXT: 5205, hashimah@unimap. edu. my

Outlines q q q Sensors Performance Terminology Displacement, position and proximity Velocity and Motion Force Fluid Pressure Liquid Flow Liquid level Temperature Light Selection of sensors

Sensors q Sensors is a device that when exposed to a physical phenomenon (temperature, displacement, force, etc) produces a proportional output signal (electrical, mechanical, magnetic, etc. ) q The term transducer is synonymously with sensors. often used q However, ideally, a sensor is a devices that responds to a change in the physical phenomenon.

Sensors

Sensors q On the other hand, a transducer is a device that converts one form of energy into another form of energy q Sensors are transducers when they sense one form of energy input and output in a different form of energy. For example, a thermocouple responds to a temperature change (thermal energy) and outputs a proportional change in electromotive force (electrical energy).

Sensors q Therefore, a thermocouple can be called a sensor or transducer. q In virtually every engineering application there is the need to measure some physical quantities such as displacements, speeds, forces, pressures, temperatures, stresses, flows and so on. q These measurement are performed using a physical devices called sensors, which are capable of converting a physical quantity to a more readily manipulated electrical quantity.

Sensors q The key issues in the selection of sensors are: (a) the field of view and range, (b) accuracy; (c ) repeatability and resolution (d) responsiveness in the target-domain; (e) power consumption; (f) hardware reliability; (g)size; (h)interpretation reliability. q Often the active element of a sensor is referred to as a transducer. q Most sensors, therefore, convert the change of a physical quantity (e. g. pressure, temp) to a corresponding and usually proportional change in an electrical quantity (e. g voltage or current).

Sensors Performance Terminology • Range and span • Hysteresis error • Error • Backlash • Sensitivity • Repeatability • Resolution • Linearity • Accuracy • Stability • Precision • Resolution

Performance Terminology Range and Span q The range of a transducer defines the limits between which the input can vary. q The span is the maximum value of the input minus the minimum value Example: A load cell for the measurement of forces might have a range of 0 to 50 k. N and a span of 50 k. N.

Performance Terminology Error is the difference between the result of the measurement and the true value of the quantity being measured. Error = measured value - true value

Performance Terminology Example If a measurement system gives a temperature reading of 25°C when the actual temperature is 24"C, then the error is +l°C. If the actual temperature had been 26 OC then the error would have been -1°C. A sensor might give a resistance change of 10. 2 1(2 when the true change should have been 10. 5 Q. The error is -0. 3 Q.

Performance Terminology Accuracy q Accuracy is a measure of the difference between the measured value and actual value. q Accuracy is often expressed as a percentage of the full range output or full-scale deflection.

Performance Terminology Example A sensor might, for example, be specified as having an accuracy of ± 5% of full range output. Thus if the range of the sensor was, say, 0 to 200°C, then the reading given can be expected to be within + or 10°C of the true reading.

Performance Terminology Sensitivity q The sensitivity is the relationship indicating how much output you get per unit input, i. e. output/input. q Where S is the sensitivity, ∆O is the change in output, and ∆ I is the change in input.

Performance Terminology Example In a n electrical measuring instrument, if a movement of 0. 001 mm causes an output voltage change of 0. 02 V, the sensitivity of the measuring instrument is

Performance Terminology 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. This effect is called hysteresis

Performance Terminology Repeatability/ Reproducibility q The terms repeatability and reproducibility of a transducer are used to describe its ability to give the same output for repeated applications of the same input value. q Usually expressed as a percentage of the fill range output. Repeatability = max. - min. values given x 100 full range

Performance Terminology Stability q 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. q The term drif is often used to describe the change in output that occurs over time. q The term zero drift is used for the changes that occur in output when there is zero input.

Performance Terminology Dead band/time q The dead band or dead space of a transducer is the range of input values for which there is no output. q The dead time is the length of time from the application of an input until the output begins to respond and change. Resolution The resolution is the smallest change in the input value that will produce an observable change in the output.

Performance Terminology Static and Dynamic Characteristics q The static characteristics are the values given when steady-state conditions occur, i. e. the values given when the transducer has settled down after having received some input. q The dynamic characteristics refer to the behaviour between the time that the input value changes and the time that the value given by the transducer settles down to the steady-state value

Performance Terminology Static and Dynamic Characteristics Dynamic characteristics are stated in terms of the response of the transducer to inputs in particular forms. • • Response time Time constant: 63% response time Rise time: 10% to 90% of steady state value Settling time: time taken for the output to settle down.

Performance Terminology q In this chapter, we classify sensors into distance, movement, proximity, stress/ strain/ force and temperature. q There are many commercially available sensors but we have picked on the ones that are frequently used in mechatronics applications.

Displacement, Position, Proximity 1. Displacement (or distance) sensors: • are concerned with the measurement of the amount by which some object has been moved. 2. Position sensors : • are concerned with the determination of the position of some object with reference to some reference point. 3. Proximity sensors • are a form of position sensor and are wed to determine when an object has moved to within some particular critical distance of the sensor

Displacement, Position, Proximity Displacement and position sensors can be grouped into two basic types: q Contact sensors in which the measured object comes into mechanical contact with the sensor or q Non-contacting where there is no physical contact between the measured object and the sensor

Displacement, Position, Proximity Potentiometer

Displacement, Position, Proximity Potentiometer q Potentiometer are variable resistance devices. q A change in the linear or angular displacement of a potentiometer varies the effective length of conductor, and therefore the resistance of the device. This change in resistance can be related to the displacement through a change in output voltage. q Potentiometers have tendency for non-linearity and care must be taken when a high degree of accuracy is required.

Displacement, Position, Proximity Potentiometer

Displacement, Position, Proximity Strain-gauged element § Strain is the ratio (change in length/ original length) § When subject to strain the element’s resistance R changes: ∆R/R = Gє § G= gauge factor strain § Є = strain § G ≈ 2. 0 for metal wire and metal foil gauges § G=+100 for p-type Si § G=-100 for n-type Si

Displacement, Position, Proximity Strain-gauged element § The strain gauge are attached of flexible elements in form of § Cantilevers § Rings or U-Shapes § Flexible element bent or deformed =>R of the strain gauge changes § Change in R is a measure of displacement or deformation § Used for linear displacement of the order 1 mm to 30 mm § Non-linearity error = +-1%

Displacement, Position, Proximity Capacitive element • The capacitance of a parallel plate capacitor is C = εoεr. A/d • When there is a change in displacement x, there will be a change in capacitance ∆C. • Non-linear relationship between ∆ C and x

Displacement, Position, Proximity Capacitive element • Push pull displacement sensor provides a linear dependency between capacitance and displacement. • Range: few mm to few cm • Non-linearity and hysteresis =± 0. 01 of full range • Capacitive proximity sensor • One capacitor probe plate • Object acting as the other plate

Displacement, Position, Proximity Linear Variable Differential Transformer q The linear variable differential transformer is a mechanical displacement transducer. q It gives an a. c voltage output proportional to the distance of the transformer core to the windings. q The LVDT is a mutual-inductance device with three coils and a core (see Figure).

Displacement (or Distance) Linear Variable Differential Transformer

Displacement (or Distance) Linear Variable Differential Transformer q An external a. c power source energizes the central coil and the two-phase opposite end coils are used as pickup coils. q The output amplitude and phase are dependent on the relative position of the two pickup coils and the power coil. q Theoretically there is null or zero position between the two end coils, although in practice this is difficult to obtain perfectly.

Displacement (or Distance) Linear Variable Differential Transformer q The sensitivity of an LVDT can be determined by the following equation: Sensivity =(Output. X input)/(excitation voltage X displacement) q Typical range for LVDT sensitivity is, 0. 4 -2. 0 m. V/V x 10~3 cm. q LVDTs are typically used in force, displacement and pressure measurement. They offer the advantages of being relatively insensitive to temp change, and provide high outputs without intermediate amplification.

Displacement, Position, Proximity Eddy current proximity Sensor • A reference coil supplied with AC current generates and alternating magnetic field • Eddy currents are induced in a metal object that is close to this field. • Eddy currents produce a magnetic field that impacts the impedance of the sensor coil • • The amplitude of the AC current changes A switch can be activated.

Displacement, Position, Proximity Inductive Proximity switch • This consists of a coil wound round a core: • When the end of the coil is close to a metal object its inductance changes • This change can be monitored by its effects on a resonant circuit and the change can be used to trigger a switch. • Works only for detection of metal objects

Displacement, Position, Proximity Optical encoders

Displacement, Position, Proximity Optical encoders • An encoder is a device that provides a digital output as a result of a linear or angular displacement. • Types of position encoders • Incremental: detect changes in rotation • Absolute: give the actual angular position • Give an example of a functionality of the incremental encoder

Displacement, Position, Proximity Pneumatic Sensors § Involve the use of compressed air § Displacement or proximity of an object being transformed into a change in air pressure § Range : 3 mm to 12 mm § Q: Give an example of its use?

Displacement, Position, Proximity Switch § Can be activated by the present of an object § Micro-switch is a small electrical switch which requires physical contact and a small operating force to close contacts.

Displacement, Position, Proximity Hall Effect Sensor § The Hall effect applies to a current flowing in a conductor placed in a magnetic field. § The moving electrons are deflected to one side § A transverse potential difference is creates § Thus if a constant current source is used with a Hall sensor a Hall voltage is a measure of the magnetic flux.

Velocity and Motion q Can be used to monitor linear or angular velocities and detection motion. q Motion sensors are widely used in security systems and interactive toys. q Examples v Incremental encoder v Described as a displacement sensor, can be used also, to measure velocity by determining the number of pulses per second. Ø Tachogenerator

Velocity and Motion Tachogenerator q Measure angular velocity q Types of tachogenerators v Variable reluctance v A magnetic circuit with an air gap which periodically changes q Speed could be determined by measuring the maximum value of the induced or by counting pulses q AC generator v The amplitude or frequency of the alternating emf can be used to measure speed

Velocity and Motion Incremental Encoder

Force Sensors q By the use of electrical strain gauge –load cells v When force applied to the cylinder resistance change q Sensor range: up to 10 MN q Non-linearity: ± 0. 03% of full range (FR). q Hysteresis error: ± 0. 02% FR q Repeatability error: ± 0. 02% FR q Load cells based on bending of a strain gauged metal element are used for smaller forces.

Fluid Pressure Sensors q By monitoring of the elastic deformation of diaphragms, capsules, bellows and tubes. q Types of pressure: v Absolute, differential, etc. q Diaphragms detect difference in pressure that is monitored by displacement sensors q Types of diaphragms: v Flat, corrugated, silicon, etc

Fluid Pressure Piezoelectric Sensors q Piezoelectric materials when stretched or compressed generate electric charges q The voltage is proportional to the applied pressure q What is the use of this type of sensor?

Fluid Pressure Tactile Sensor q Applications: v Fingertips of robot arms v Touch display screen q Functionality: v AC voltage to lower layer v AC voltage generated by upper layer v Pressure affects the upper AC voltage.

Liquid Flow Sensor q Orifice plate: v Based on Bernoulli’s equation v Liquid flow Q (in a pipe) is proportional to √(P 1 -P 2) v Accuracy: ± 1. 5%FR q Turbine meter: v Multi-bladed rotor placed in the pipe in the flow v Flow results in rotor rotation v Rotor rotation can be measured using a magnet pick-up.

Temperature Sensor q Bimetallic strips: v Two different metal strips are bonded together v The metals have different coefficient of expansion q Thermistors v All small pieces of material made fr mixtures of metal oxides v Thermistors come in various forms v Non-linear relationship between R and T v Advantages v Rugged; can be small; fast response time; large resistance changes/ degree q Resistance q Thermodiodes and transistors temperature detectors v When the temp. of doped (RTDs) : semiconductors changes, the mobility of their carrier changes v Use the linear and affects the rate of electrons change of resistance and holes diffusion across p-n. with temperature.

Temperature Sensor q Thermocouples: v Circuit composed of a hot junction and reference junction. v Junction: two different metals joined together V across the junction v V depends on t and the type of metals

Temperature Sensor q A compensation circuit can be used to provide an emf that varies with the temperature of the cld junction that compensates for the cases when the cold junction is not kept at constant 0 C q Thermopiles: v Series connection of more than one thermocouples

Light Sensor q Photodiodes: can be used as a variable resistance controlled by light. v When the light fall on the diode the resistances decreases q Phototransistors: have a light sensitive collector base p-n junction. v No light very small current v Light a base current is produced. q Photoresistor: has a resistance that depends on the intensity of light. v Cadmium sulphide is the most responsive photoresistor. q Array of light sensors is required to measure the variation of light in a space.

Parameter for Sensor Selection q The size q Whether the displacement is linear or angular q The resolution required q The accuracy required q What material is the measured object made of q The cost

Problems q A copper-constantan thermocouple is to be used to measure temperature between 0 and 200 o. C. The emf at 0 o. C is 0 m. V, at 100 o. C is 4. 277 m. V and at 200 o. C is 9. 286 m. V. What will be the non-linearity error at 100 o. C. q A float sensor for the determination of the level of water in a vessel has a cylindrical float of mass 2. 0 kg, cross sectional area 20 cm 2 and the length of 1. 5 m. It floats vertically in the water and presses upwards against a beam attached to its upward end. q What will be the min and max up thrust forces exerted by the float on the beam? q Suggest a means by which the deformation of the beams under the action of the up thrust force could be monitored.

Further Readings 1. Bolton, W. , Mechatronics: Electronic Control Systems in Mechanical and Electrical Engineering • Chapter 2 2. D. Shetty and R. A. Kolk, Mechatronics System Design • Chapter 3 3. D. G. Alciatore and M. B. Histand, "Introduction to Mechatronics and Measurement Systems • Chapter 9