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Transcript
Today • Course overview and information 09/16/2010 © 2010 NTUST Temperature Scales In industrial applications many physical quantities must be measured and controlled. For many processes, temperature measurement is critical. F You are probably familiar with the two principle temperature scales in use – Fahrenheit and Celsius. Recall that… On the Fahrenheit scale, water freezes at 32o; on the Celsius scale, it freezes at 0o. On the Fahrenheit scale, water boils at 212o; on the Celsius scale, it boils at 100o. C 212 100 32 0 Temperature Scales Most industrial applications use the Celsius scale, which is more common in most countries. The conversion formulas between Fahrenheit and Celsius are: 9 F C 32 5 and C 5 F 32 9 Temperature Measurement - Thermocouple There are a number of temperature sensors available. The choice of which one to use depends on the specific requirements (range, threshold, accuracy, resolution, cost, etc.) A common industrial temperature sensor with a very large range is the thermocouple. In general, thermocouples can be used from about 250oC (418oF) to 2000oC (3632oF) but any given thermocouple has a smaller range. Metal A Metal B Heat + VAB Temperature Measurement - Thermocouple A thermocouple measurement may use a reference thermocouple that is kept at a known temperature. This avoids errors due to unwanted thermocouple junctions. Measuring thermocouple + Copper (Cu) VT Constantan Cu Cu Av VREF + Reference thermocouple in controlled environment Av(VT – VREF) Temperature Measurement - Thermocouple Another method to avoid unwanted thermocouple junctions is to use a compensation circuit as shown in the green box. The output is only a function of VT. The unwanted thermocouple voltage is canceled by Vc. Copper (Cu) + Measuring thermocouple VT Constantan Cu + + Vc Cu Av Ic An unwanted thermocouple voltage is created between the Constantan and the Copper junction. +V The current source generates a voltage in R that matches the unwanted thermocouple junction. AvVT Temperature Measurement - Thermocouple An RTD is a resistance temperature device in which the resistance increases with temperature. RTDs tend to be expensive, but are more accurate than other sensors. A three-wire bridge connection cancels wire resistance of the leads by putting each lead from the RTD in a different leg of the bridge. Wire resistance R1 + R3 VOUT R2 Upper leg RA The wire resistance of the third wire has little effect on VOUT because the current in this lead is almost zero. RTD RB Lower leg Temperature Measurement - Thermocouple Thermistors are formed from certain metal oxides with a large negative resistance coefficient. Although they are not as accurate as RTDs, they are small and inexpensive, so have application in thermostats and other non-critical systems. A simple thermistor may be formed as a small bead, such as the ones illustrated here. Because of their small size, they can respond quickly to a temperature change. Temperature Measurement - IC IC temperature sensors are based on the changes in breakdown voltage that occur in zener diodes with temperature. They have limited range, but are small, linear, and inexpensive. A typical IC temperature sensor is the LM135, LM235, and LM335. The LM335 can measure temperatures from 40oC to 100oC. It is available in a surface-mounted package or the TO-92 package, common with small signal transistors. TO-92 package Strain Measurements Strain is the deformation of a material due to a force acting on it. Strain can be induced along a material when a force is applied to it. The basis of most electronic scales is measuring the strain due to compressing or stretching a F solid material. F When a force is applied as shown, the material compresses a small amount. This can change the resistance of a strain gauge, which is bonded to the material. The resistance change is extremely small, but can be measured with sensitive instruments. Strain gauge resistance changes with the deformation. DL L Strain Measurements The strain gauge is constructed from a resistive wire (typically about 350 W), which changes by a few mW when it is stretched or compressed. The basis of the resistance change is the wire resistance equation you studied in Chapter 2: L R A Compressing the strain gauge causes the length to decrease and the area to increase, thus making the resistance smaller. terminals Resistive material Strain Measurements The gauge factor for a strain gauge is the ratio of the change in resistance to the nominal resistance divided by the change in length divided by the nominal length. Gauge factors for metallic gauges are typically about 2. DR R GF DL L The fractional change in length (DL/L) is called strain (e). Strain is such a small quantity it is usually expressed in parts per million (designated as me). Strain Measurements The most common method for measuring the resistance change of a strain gauge is using a Wheatstone bridge because of its great sensitivity. A three wire bridge circuit +V is illustrated. R1 In automated scales, the bridge circuit is generally automated, so that the readout is directly in weight units. Frequently, in this application, all four legs of the bridge contain active gauges. R3 R2 Strain gauge RG + Pressure Measurements Pressure is force per area. The defining equation for pressure is F P A In the English system, the typical pressure unit is the psi (pound per square inch). In the metric system, pressure is measured in newtons per square meter, which defines the pascal (Pa). The Pa is a very small unit, so kilo and mega prefixes are common. Pressure Measurements There are three ways to specify a pressure measurement. These are Absolute pressure: pressure measured with respect to a vacuum. Gauge pressure: pressure measured with respect to the atmosphere. Differential pressure: the difference between two pressures. vacuum atmosphere open pressure pressure pressure-1 pressure-2 Pressure Measurements When you have a flat tire, the pressure is said to be zero. What type of pressure measurement is this? This is gauge pressure because the remaining air in the tire has the same pressure as the atmosphere. Pressure Measurements There are many types of pressure transducers. One type uses a diaphragm which is instrumented with a strain gauge. When pressure is applied to the gauge, it expands and elongates the strain gauge, causing the resistance to increase. pressure Motion Displacement transducers can detect the position of an object using either contacting or noncontacting methods. A widely used contacting transducer is the LVDT (linear variable differential transformer). The output of the LVDT is proportional to the position of a movable core that is inside a differential transformer. When the core is centered, the output is zero. The output is positive or negative when the core moves. Moveable core ac Demodulator Vout Motion Velocity is the rate of change of displacement, so velocity measurements can use an LVDT to find the position as a function of time and calculate velocity. Another measurement of motion is acceleration. Acceleration is the rate of change of velocity. Acceleration measurements are important in fields such as destructive testing or vibration analysis. A basic accelerometer is shown. V mass Vout dashpot Sample and Hold Circuit A sample-and-hold circuit is used in instrumentation applications to retain a voltage from a transducer to allow time for conversion to digital form. A basic sample-and hold circuit consists of an analog switch, a storage capacitor and a high impedance amplifier that prevents the capacitor from discharging too quickly. JFET switch + + RG CH Control input Holding capacitor Sample and Hold Circuit The output of the sample-and-hold circuit tracks the input when the control input is high, and holds the input voltage when the control input is low. For the input and control waveforms shown, sketch the output. S H S H S H S H S H S H Sample and Hold Circuit Specifications for sample-and-hold circuits: Aperture time – the time for the analog switch to fully open after the control voltage switches from the sample level to the hold level. Aperture jitter – the uncertainty in the aperture time. Acquisition time – the time required for the device to reach its final value when the control voltage switches from its hold level to its sample level. Droop – the change in voltage from the sampled value during the hold interval because of charge leaking off the hold capacitor. Feedthrough – the component of the output voltage that follows the output voltage after the analog switch is opened. Analog-to-digital Conversion Analog-to-digital conversion is the process of converting the output of the sample-and-hold circuit to a series of binary codes that represent the amplitude of the analog input at each of the sample time. The output of the sampleand-hold circuit is a series of pulses that represent the original input. The ADC converts each pulse into a group of binary pulses that represent a binary number. S/H output ADC output 0100 0101 ………. 1100 1010 … Analog-to-digital Conversion Three specifications for analog to digital conversion circuits are: Resolution – the number of bits involved in an analog to digital conversion. Conversion time – the time it takes for an ADC to convert an analog voltage sample to a digital code. Quantization error – the change in value of an analog signal during the conversion process as shown in the figure. Notice that the analog voltage changes during the conversion process leading to the error. V Value that is actually converted Value that should have been converted DV (Quantization error) t Start End Nyquist Rate Harry Nyquist, working for AT and T, studied information theory and published an important paper entitled Certain topics in Telegraph Transmission Theory. In his paper he showed that an analog signal should be sampled at a frequency that is at least twice the highest frequency of the signal in order to be able to adequately reconstruct the signal. This rate is known as the Nyquist rate or Nyquist frequency. The human ear can detect frequencies as high as 20 kHz. What is the Nyquist frequency for this upper limit? 40 kHz (Actual digitizing would be done a little faster than this – about 44 kHz.) Power Control Electronic control of power is generally accomplished using a class of devices known as thyristors. A thyristor is a semiconductor switch composed of layers of alternating pnpn material. (A) Anode (A) One important type of thyristor is the SCR, which acts like a diode that is turned on by a Gate (G) control signal. The control signal is applied to the gate. p n p n Cathode (K) (G) (K) The SCR Normally the SCR conducts by forward-biasing the anode to cathode and applying a positive pulse of current to the gate. The gate then loses control. IF Conduction continues as long Forwardconduction as the anode to cathode region remains forward-biased. The characteristic curve IG2IG1> >IG1IG0IG = 0 changes when a current pulse IH2 is applied to the gate. As gate VR VF V V V current increases, the forward BR(F2)BR(F1)BR(F0) breakover voltage decreases, Reverseblocking which triggers the SCR. I region The holding current, IH, is the minimum anode current to keep the SCR in conduction. R Selected Key Terms Thermocouple Transducer RTD Thermistor A type of temperature transducer formed by the junction of two dissimilar metals that produces a voltage proportional to temperature. A device that converts a physical parameter into an electrical quantity. Resistance temperature detector. A type of temperature transducer in which resistance is inversely proportional to temperature. Selected Key Terms Strain gauge A transducer formed by a resistive material in which lengthening or shortening due to stress produces a proportional change in resistance. Quantization The determination of a value for an analog quantity. Thyristor SCR A class of four-layer (pnpn) semiconductor switching devices. Silicon controlled rectifier; a type of three terminal thyristor. Quiz 1. The number of Fahrenheit degrees between the normal freezing point and boiling point of water is a. 100. b. 180. c. 212. d. 473. Quiz 2. A thermocouple is a temperature transducer that a. has a resistance change that is proportional to temperature. b. has a resistance change that is inversely proportional to temperature. c. is constructed from two dissimilar metals connected together. d. is constructed from a semiconductor material. Quiz 3. A sensitive and inexpensive temperature transducer that is a good choice for a home thermostat is a. an RTD. b. a thermocouple. c. a strain gauge. d. a thermistor. Quiz 4. The response of a strain gauge to a strain (DL/L) is a. a small resistance change. b. a voltage that is proportional to the strain. c. a voltage that is inversely proportional to the strain. d. none of the above. Quiz 5. Assume a pressure gauge reads the pressure of a gas relative to the atmosphere. This is an example of a. absolute pressure. b. gauge pressure. c. differential pressure. d. none of the above. Quiz 6. An example of a transducer that has an output voltage that is proportional to displacement is a. an LVDT. b. an RTD. c. a Wheatstone bridge. d. a strain gauge. Quiz 7. The circuit illustrated below is a a. demodulator. b. analog-to-digital converter. c. power control circuit. d. sample-and-hold circuit. JFET switch + + RG CH Control input Holding capacitor Quiz 8. The term high resolution with an ADC means that it a. is very fast. b. is able to digitize with very small errors. c. uses a large number of bits. d. all of the above. Quiz 9. Assume a measuring system is required to measure mechanical vibrations that are no greater than 500 Hz. The Nyquist frequency is a. 250 Hz. b. 500 Hz. c. 750 Hz. d. 1.0 kHz. Quiz 10. Once an SCR is conducting, it continues until the a. gate signal is zero. b. gate signal is made negative. c. holding current is exceeded. d. none of the above. Quiz Answers: 1. b 6. a 2. c 7. d 3. d 8. c 4. a 9. d 5. b 10. d