introduction to mechatronics Sensors I.pptx

introduction to mechatronics Sensors I.pptx

1. Sensors Dr. Khaldon Meselhy

2. Sensor and transducers The term sensor is used for an element which produces a signal relating to the quantity being measured. Thus in the case of, say, an electrical resistance temperature element, the quantity being measured is temperature and the sensor transforms an input of temperature into a change in resistance. A sensor /transducer is said to be analogue if it changes in a continuous way proportional to the size of the variable being measured. The term digital is used if the systems give outputs which are digital in nature, i.e. a sequence of essentially on/ off signals which spell out a number whose value is related to the size of the variable being measured.

3. Smart sensors Some sensors come combined with their signal conditioning all in the same package. Such an integrated sensor does still, however, require further data processing. However, it is possible to have the sensor and signal conditioning combined with a microprocessor all in the same package. Such an arrangement is termed a smart sensor. A smart sensor is able to have such functions as the ability to compensate for random errors, to adapt to changes in the environment, give an automatic calculation of measurement accuracy, adjust for nonlinearities to give a linear output, self-calibrate and give self-diagnosis of faults.

4. Performance terminology 1 Range and span The range of a transducer defines the limits between which the input can vary. The span is the maximum value of the input minus the minimum value. Thus, for example, a load cell for the measurement of forces might have a range of 0 to 50 kN and a span of 50 kN. 2 Error Error is the difference between the result of the measurement and the true value of the quantity being measured: error = measured value - true value Thus 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°C then the error would have been -1°C. A sensor might give a resistance change of 10.2 when the true change should have been 10.5 . The error is -0.3 .  

5. Performance terminology 3 Accuracy Accuracy is the extent to which the value indicated by a measurement system might be wrong. It is thus the summation of all the possible errors that are likely to occur, as well as the accuracy to which the transducer has been calibrated. A temperature-measuring instrument might, for example, be specified as having an accuracy of ± 2°C. This would mean that the reading given by the instrument can be expected to lie within ± 2°C of the true value. Accuracy is often expressed as a percentage of the full range output or full-scale deflection. The percentage of full-scale deflection term results from when the outputs of measuring systems were displayed almost exclusively on a circular or linear scale. 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 ± 10°C of the true reading. 4 Sensitivity The sensitivity is the relationship indicating how much output there is per unit input, i.e. output/input. For example, a resistance thermometer may have a sensitivity of 0.5 /°C. This term is also frequently used to indicate the sensitivity to inputs other than that being measured, i.e. environmental changes. Thus there can be the sensitivity of the transducer to temperature changes in the environment or perhaps fluctuations in the mains voltage supply. A transducer for the measurement of pressure might be quoted as having a temperature sensitivity of ± 0.1 % of the reading per °C change in temperature.  

6. 5 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. Figure 2.1 shows such an output with the hysteresis error as the maximum difference in output for increasing and decreasing values. 6 Non-linearity error For many transducers a linear relationship between the input and output is assumed over the working range. Few transducers, however, have a truly linear relationship and thus errors occur as a result of the assumption of linearity. The error is defined as the maximum difference from the straight line. The error is generally quoted as a percentage of the full range output. For example, a transducer for the measurement of pressure might be quoted as having a non-linearity error of ± 0.5% of the full range.

7. 7 Repeatability / reproducibility 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. The error resulting from the same output not being given with repeated applications is usually expressed as a percentage of the full range output: A transducer for the measurement of angular velocity typically might be quoted as having a repeatability of ± 0.01 % of the full range at a particular angular velocity. 8 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 as 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. 9 Dead band/time The dead band 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. The dead time is the length of time from the application of an input until the output begins to respond and change.  

8. 10 Resolution When the input varies continuously over the range, the output signals for some sensors may change in small steps. A wire-wound potentiometer is an example of such a sensor, the output going up in steps as the potentiometer slider moves from one wire turn to the next. The resolution is the smallest change in the input value that will produce an observable change in the output. For a wire-wound potentiometer the resolution might be specified as, say, 0.5° or perhaps a percentage of the full-scale deflection. For a sensor giving a digital output the smallest change in output signal is 1 bit. Thus for a sensor giving a data word of N bits, i.e. a total of bits, the resolution is generally expressed as . 11 Output impedance When a sensor giving an electrical output is interfaced with an electronic circuit it is necessary to know the output impedance since this impedance is being connected in either series or parallel with that circuit. The inclusion of the sensor can thus significantly modify the behavior of the system to which it is connected. See Section 6.1.1 for a discussion of loading.  

9. To illustrate the above, consider the significance of the terms in the following specification of a strain gauge pressure transducer: Ranges: 70 to 1000 kPa, 2000 to 70 000 kPa Supply voltage: 10 V d.c. or a.c. r.m.s. Full range output: 4-0 m V Non-linearity and hysteresis: ± 0.5% of full range output Temperature range: - 54°C to + 120°C when operating Thermal zero shift: 0.030% of full range output/°C

10. The range indicates that the transducer can be used to measure pressures between 70 and 1000 kPa or 2000 and 70 000 kPa. It requires a supply of 10 V d.c. or a.c. r . m.s. for its operation and will give an output of 40 m V when the pressure on the lower range is 1000 kPa, and on the upper range 70 000 kPa. Non-linearity and hysteresis will lead to errors of ± 0.5% of 1000, i.e. ± 5 kPa on the lower range and ± 0.5% of 70 000, namely ± 350 kPa, on the upper range. The transducer can be used between the temperatures of -54 and+120°C. When the temperature changes by 1°C the output of the transducer for zero input will change by 0.030% of 1000 = 0.3 kPa on the lower range and 0.030% of 70 000 = 21 kPa on the upper range.

11. Static and dynamic characteristics 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. 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. Dynamic characteristics are stated in terms of the response of the transducer to inputs in particular forms. For example : step input ramp input sinusoidal input of a specified frequency.

12. 1 Response time This is the time which elapses after a constant input, a step input, is applied to the transducer up to the point at which the transducer gives an output corresponding to some specified percentage, e.g. 95%, of the value of the input. For example, if a mercury-in-glass thermometer is put into a hot liquid there can be quite an appreciable time lapse, perhaps as much as 100 s or more, before the thermometer indicates 95% of the actual temperature of the liquid.

13. 2 Time constant This is the 63.2% response time. A thermocouple in air might have a time constant of perhaps 40 to 100 s. The time constant is a measure of the inertia of the sensor and so how fast it will react to changes in its input the bigger the time constant, the slower the reaction to a changing input signal. 3 Rise time This is the time taken for the output to rise to some specified percentage of the steady-state output. Often the rise time refers to the time taken for the output to rise from 10% of the steady-state value to 90 or 95% of the steady-state value. 4 Settling time This is the time taken for the output to settle to within some percentage, e.g. 2%, of the steady-state value.

14. To illustrate the above, consider the graph in Figure 2.4 a thermometer plunged into a liquid at time t = 0. The steady-state value is 55°C and so, since 95% of 55°C is 52.25°C, the 95% response time is about 228 s.

15. The following sections give examples of transducers grouped according to what they are being used to measure. The measurements considered are those frequently encountered in mechatronic engineering, namely displacement, proximity, velocity, force, pressure, fluid flow, liquid level, temperature and light intensity.

16. Displacement, position and proximity Displacement sensors are concerned with the measurement of the amount by which some object has been moved. Position sensors are concerned with the determination of the position of some object in relation to some reference point. Proximity sensors are a form of position sensor and are used to determine when an object has moved to within some particular critical distance of the sensor. They are essentially devices which give on/off outputs. Displacement and position sensors can be grouped into two basic types: contact sensors, in which the measured object comes into mechanical contact with the sensor; non-contacting sensors, where there is no physical contact between the measured object and the sensor. For those linear displacement methods involving contact, there is usually a sensing shaft which is in direct contact with the object being monitored. The displacement of this shaft is then monitored by a sensor. The movement of the shaft may be used to cause changes in electrical voltage, resistance, capacitance or mutual inductance. For angular displacement methods involving mechanical connection, the rotation of a shaft might directly drive, through gears, the rotation of the transducer element. Non-contacting sensors might involve the presence in the vicinity of the measured object causing a change in the air pressure in the sensor, or perhaps a change in inductance or capacitance. The following are examples of commonly used displacement sensors.

17. Potentiometer sensor A potentiometer consists of a resistance element with a sliding contact which can be moved over the length of the element. Such elements can be used for linear or rotary displacements, the displacement being converted into a potential difference. The rotary potentiometer consists of a circular wire-wound track or a film of conductive plastic over which a rotatable sliding contact can be rotated (Figure 2.5). The track may be a single turn or helical. With a constant input voltage between terminals 1 and 3, the output voltage between terminals 2 and 3 is a fraction of the input voltage, the fraction depending on the ratio of the resistance between terminals 2 and 3 compared with the total resistance between terminals 1 and 3, i.e. .  

18. An important effect to be considered with a potentiometer is the effect of a load connected across the output. The potential difference across the load is only directly proportional to if the load resistance is infinite. For finite loads, however, the effect of the load is to transform what was a linear relationship between output voltage and angle into a non-linear relationship. The resistance is in parallel with the fraction of the potentiometer resistance . This combined resistance is . The total resistance across the source voltage is thus total resistance =  

19. The circuit is a potential divider circuit and thus : If the load is of infinite resistance then we have . Thus the error introduced by the load having a finite resistance is: Error= Example: consider the non-linearity error with a potentiometer of resistance 500 when at a displacement of half its maximum slider travel, which results from there being a load of resistance 10 k . The supply voltage is 4 V. find the error error As a percentage of the full range reading, this is 0.625%.  

20. Potentiometers are used as sensors in automobiles, being used for such things as the fuel-level sensor and accelerator pedal sensor. The Figure shows how, for a fuel-level sensor, a float lever system is used to rotate the wiper arm of a potentiometer and give a voltage output which is a measure of the fuel level.

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