Measurements, Conversion, and Control

1. Chapter 21 Measurements, Conversion, and Control

2. Objectives Discuss the operation of three types of temperature-measuring circuits Describe methods of measuring strain, pressure, and flow rate Describe methods of measuring motion Explain the operation of a sample-and-hold circuit Explain the operation of an analog-to-digital converter Describe how power to a load is controlled

3. Temperature Measurement A thermocouple is formed by joining two dissimilar metals A small voltage, called the Seebeck voltage is produced across the junction of the two metals when heated The amount of voltage produced is dependent on the types of metals and is directly proportional to the temperature of the junction The voltage produced by the junction is generally much less than 100 mV

4. Temperature Measurement When a thermocouple is connected to a signal-conditioning circuit, an unwanted thermocouple is effectively created at the point(s) where the thermocouple wires connect to the circuit terminals (made of dissimilar metal) The unwanted thermocouple junction is sometimes referred to as a cold junction These unwanted thermocouples can have an unpredictable effect on the overall voltage

5. Temperature Measurement One method for eliminating an unwanted thermocouple effect is to add a reference thermocouple at a constant known temperature (usually 0�C) It is bulky and expensive to maintain a reference thermocouple at a fixed temperature (usually in an ice bath) Another approach is to compensate for the unwanted thermocouple effect by adding a compensation circuit consisting of a resistor and an integrated circuit temperature sensor

6. Temperature Measurement A second major type of temperature transducer is the RTD (resistance temperature detector) The RTD is a resistive device in which the resistance changes directly with temperature The RTD is more nearly linear than the thermocouple RTD temperature range is more limited than that of a thermocouple RTDs are constructed in either a wire-wound configuration or by a metal-film technique The most common RTDs are made of either platinum, nickel, or nickel alloys

7. Temperature Measurement A third major type of temperature transducer is the thermistor, which is a resistive device made from a semiconductive material such as nickel oxide or cobalt oxide The resistance of a thermistor changes inversely with temperature The temperature characteristic for thermistors is more nonlinear than that for thermocouples or RTDs The temperature range of a thermistor is more limited than that of a thermocouple Thermistors have greater sensitivity than RTDS or thermocouples

8. Strain, Pressure, and Flow Rate Measurements Strain is the deformation, either expansion or compression, of a material due to a force acting on it If a metal plate is bent, there is an expansion on the upper surface, called tensile strain, and a compression on the lower surface, called compressive strain A strain gauge is basically a long very thin strip of resistive material that is bonded to the surface of an object on which strain is to be measured

9. Strain, Pressure, and Flow Rate Measurements When a force acts on the object to cause a deformation, the strain gauge is also deformed proportionally and its resistance changes Gauge Factor (GF) is the ratio of fractional change in resistance to fractional change in length along the axis of the gauge

10. Strain, Pressure, and Flow Rate Measurements The fractional change in length (?L/L) is designated strain (?) and is usually expressed in parts per million, called microstrain (??) GF = (?R/R) / (?L/L) Strain gauges exhibit a resistance change when deformed, therefore, strain gauges are usually applied in bridge circuits or in constant-current-driven circuits

11. Strain, Pressure, and Flow Rate Measurements Pressure transducers are devices that exhibit a change in resistance proportional to a change in pressure Pressure sensing is accomplished using a strain gauge bonded to a flexible diaphragm

12. Strain, Pressure, and Flow Rate Measurements An absolute pressure transducer measures applied pressure relative to a vacuum A gauge pressure transducer measures applied pressure relative to the pressure of the surroundings (ambient pressure) A differential pressure transducer measures one applied pressure relative to another applied pressure A bridge circuit is usually used to measure pressure transducers

13. Strain, Pressure, and Flow Rate Measurements Flow rate of a fluid through a pipe may be measured using differential pressure A flow restriction is placed in the flow stream The volume of fluid through the restriction remains constant, so the velocity and pressure increase

14. Motion Measurement A contacting type of displacement sensor that relates a change in inductance to displacement is the linear variable differential transformer (LVDT) A sensing shaft is connected to a moving magnetic core inside a specially wound transformer The primary winding is excited with ac When the core is centered, the voltage induced in each secondary is equal As the core moves off center, the voltage in one secondary will be greater than the other

15. Motion Measurement

16. Motion Measurement Nonconducting displacement transducers include optical and capacitive transducers Photocells can be arranged to observe light through holes in an encoding disk Capacitive sensors can be made into very sensitive displacement and proximity transducers; capacitance is varied by moving one of the plates with respect to the other The moving plate can be any metallic surface such as the diaphragm of a capacitive microphone

17. Motion Measurement Velocity Transducers Direct measurement of linear velocity may be done by placing a permanent magnet inside a concentric coil; the relative motion will generate an emf proportional to the velocity Direct measurement of angular velocity may be done by rotating a coil in a magnetic field, and measure the voltage produced Acceleration transducers Relative displacement of a mass is measured

18. Sample-and-Hold Circuits A sample-and-hold circuit samples an analog input voltage at a certain point in time and holds the sampled voltage for an extended time after the sample is taken A sample-and-hold will keep the sampled analog voltage constant for the length of time necessary to allow an analog-to-digital converter (ADC) to convert the voltage to a digital form

19. Sample-and-Hold Circuits A basic sample-and-hold circuit consists of an analog switch, a capacitor, and input and output amplifiers

20. Sample-and-Hold Circuits Sample-and-Hold Specifications Aperture time - the time for the analog switch to fully open after the control voltage switches from its sample level to its 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 of the hold capacitor

21. Sample-and-Hold Circuits Feedthrough - the component of the output voltage that follows the input signal after the analog switch is opened. The inherent capacitance from the input to the output of the switch causes feedthrough

22. Analog-to-Digital Conversion Analog-to-digital conversion is the process of converting the output of a sample-and-hold circuit to a series of binary codes that represent the amplitude of the analog input at each of the sample times The sample-and-hold keeps the analog input constant for the period required to do the conversion

23. Analog-to-Digital Conversion Conversion Parameters Resolution - the number of bits (binary digits) used to represent each value of the analog signal Conversion Time - the time required for conversion of a sampled analog value into a digital value Quantization Error - the change in value of an analog signal during the conversion time At the Nyquist rate, an analog signal is sampled and converted two times per cycle The higher the sample rate, the more accurately an analog signal can be represented

24. Analog-to-Digital Conversion The flash method of conversion uses several comparators to compare reference voltages with the analog input voltage Simultaneous approximation is another method

25. Power-Control Circuits A thyristor is a semiconductor switch composed of four or more layers of alternating pnpn material An SCR (silicon-controlled rectifier) is a form of gated diode as shown at right

26. Power-Control Circuits The SCR is one of the most important devices in the thyristor family, because it acts like a diode that can be turned on when required There are four regions of the characteristic curve Reverse blocking and reverse avalanche characteristics are the same as a normal diode The first forward region is the forward-blocking region where the SCR is basically off with a very high resistance between anode and cathode

27. Power-Control Circuits The second forward region is the forward-conduction region, where anode current occurs as in a normal diode To move an SCR into the forward-conducting region, the forward-breakover voltage, VBR(F), must be exceeded

28. Power-Control Circuits There are two ways to turn on an SCR (forward-conduction region) Anode to cathode must be forward-biased (anode must be positive with respect to cathode) The first method requires a forward voltage that exceeds the breakover voltage VBR(F) The second method requires a positive pulse of current (trigger) on the gate, which reduces the forward-breakover voltage (the greater the gate current, the lower the value of VBR(F))

29. Power-Control Circuits Once the SCR is turned on, the gate loses control The SCR is latched as long as anode current is maintained When the anode current drops below the holding current (IH), the SCR will drop out of conduction If the anode current is dropped to zero, the SCR will turn off; this is most often achieved by the negative cycle of the ac waveform

30. Power-Control Circuits The triac is a thyristor with the ability to pass current bidirectionally and is therefore an ac power control device Its performance is equivalent to two SCRs connected in parallel in opposite directions but with a common gate

31. Power-Control Circuits The triac is usually triggered on with a gate current Once conduction begins, the triac will conduct on with either polarity Conduction ceases when anode current is removed, usually on zero-crossing of the ac wave The triac can be triggered on such that ac power is supplied to the load for a portion of the ac cycle

32. Power-Control Circuits When thyristors are switched on during the ac cycle, RFI (radio frequency interference) is generated due to switching transients If a thyristor is switched on near the peak of the ac cycle, the sudden transition of voltage or current generates many high-frequency components which are radiated, and may affect sensitive electronic circuits Zero-voltage switching will turn the thyristor on at, or near, the ac zero crossing, avoiding the sudden surge of current associated with a mid-cycle trigger

33. Power-Control Circuits Microcontrollers (a special class of computer) find application in some of our most basic machines A microcontroller is a single integrated circuit with all of the basic features found in a microprocessor with special input/output circuits, ADCs, counters, timers, oscillators, memory, and other features Microcontrollers can also be used to trigger thyristors, and hence control motor speed or torque, as well as sequencing and motor protection

34. Summary A thermocouple is a type of temperature transducer formed by the junction of two dissimilar metals When the thermocouple junction is heated, a voltage is generated across the junction that is proportional to the temperature Thermocouples can be used to measure very high temperatures The resistance temperature detector (RTD) is a temperature transducer in which the resistance changes directly with temperature. It has a positive temperature coefficient

35. Summary RDTs are typically used in bridge circuits or in constant-current circuits to measure temperature The thermistor is a temperature transducer in which the resistance changes inversely with the temperature. It has a negative temperature coefficient Thermistors are more sensitive than RTDs or thermocouples, but their temperature range is more limited

36. Summary The strain gauge is based on the fact that the resistance of a material increases when its length increases The gauge factor of a strain gauge is the fractional change in resistance to the fractional change in length Pressure transducers are constructed with strain gauges bonded to a flexible diaphragm An absolute pressure transducer measures pressure relative to a vacuum

37. Summary A gauge pressure transducer measures pressure relative to ambient pressure A differential pressure transducer measures one pressure relative to another pressure The flow rate of a liquid can be measured using a differential pressure gauge Motion-measuring circuits include LVDT displacement transducers, velocity transducers, and accelerometers

38. Summary A sample-and-hold circuit produces an essentially constant output voltage equal to the instantaneous value of an input signal at a given point in time The output of a sample-and-hold circuit goes to an analog-to-digital converter (ADC) The sample-and-hole process minimizes quantization error An analog-to-digital converter in conjunction with a sample-and-hold circuit converts an analog signal into a series of digital codes

39. Summary A minimum of two samples per period of the input signal is necessary to represent an analog waveform. This minimum rate is known as the Nyquist rate The higher the sample rate, the more accurately an analog signal can be represented One type of ADC is the flash or simultaneous converter, another type is the successive approximation converter

40. Summary The thyristor is a category of electronic switches The SCR (silicon-controlled rectifier) and triac are two types of thyristors used in power control A zero-voltage switch generates pulses at the zero crossings of an ac voltage for triggering a thyristor