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Lecture 11 Resistive Transducer 1 Lecture11:ResistiveTransducer Resistive transducer The resistance of a transducer varies as the physical quantity varies (e.g. temperature or displacement) As values of R varies, value of V and i also varies Two basic devices for measurement of temperatures are RTD and thermistor 2 Lecture11:ResistiveTransducer Resistive Transducer a)Thermistor • Semiconductor device • The resistance value of the thermistor changes according to temperature • Increase in temperature causes a decrease in resistance 3 Lecture11:ResistiveTransducer • The relation between the temperature and the resistance 1 1 RT RT 1 exp( ( )) T T1 • RT: The resistance value at the temperature T • T: The temperature [K] • R1: The resistance value at the reference temperature • T1: The reference temperature [K] typically, 25°C is used • : The coefficient of thermistor. 4 Lecture11:ResistiveTransducer FIGURE 4.5 Thermistor resistance versus temperature is highly nonlinear and usually has a negativeslope. Curtis Johnson Lecture11:ResistiveTransducer 5 Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 Thermistor Characteristics Sensitivity – change in resistance 10% per 0C, for 6 nominal resistance of 10k may change 1 k for 10C Construction – semiconductor in various forms discs, beads, rods Range - -200C to 1000C Response time – depends on quality of material Signal conditioning - bridge Lecture11:ResistiveTransducer Thermistor: Construction and symbols 7 Lecture11:ResistiveTransducer Advantages: • Low cost, small size • High output voltage • Fast response Disadvantages: • Highly nonlinear • Restricted to relatively low temperature 8 Lecture11:ResistiveTransducer b) Resistive Temperature Detector (RTD) • • • • Electrical resistance is a function of metal temperature As temperature increases, the resistance increases Resistance temperature relationship: R = R0(1+ T ) with R = resistance of the conductor at temperature t0C R0 = ambience resistance (at reference point) = temperature coefficients of resistance T = difference between temperature at t and ambience 9 Lecture11:ResistiveTransducer FIGURE 4.2 Metal resistance increases almost linearly with temperature. Curtis Johnson Process Control Instrumentation Technology, 8e] 10 Lecture11:ResistiveTransducer Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Common Resistance Materials for RTDs: • Platinum (most popular and accurate) • Nickel • Copper • Balco (rare) • Tungsten (rare) 11 Lecture11:ResistiveTransducer Sensitivity An estimate of RTD sensitivity is noted by value of Platinum – 0.004/0C Nickel – 0.005/0C For 100 platinum RTD, a change of 0.4 if temperature is changed by 10C Range Platinum RTD –100 to 6500C Nickel RTD – 180 to 3000C Response time 0.5 to 5 s or more, slowness due to thermal conductivity 12 Lecture11:ResistiveTransducer 13 Lecture11:ResistiveTransducer 14 Lecture11:ResistiveTransducer • Temperature range (from -200 to 8500 C) • Advantages: • relatively immune to electrical noise and therefore well suited for temperature measurement in industrial environments More stable, have an output response that is more linear, more accurate • Disadvantages: Expensive • Very small fractional changes of resistance with temperature, bridge circuit is needed 15 Lecture11:ResistiveTransducer FIGURE 4.4:Note the compensation lines in this typical RTD signalconditioning circuit. Curtis Johnson Lecture11:ResistiveTransducer 16 Process Control Instrumentation Technology, 8e] Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 Signal conditioning Bridge circuit Compensation line in R3 leg is required Same resistance change due to RTD leg cause no net shift in the bridge null 17 Lecture11:ResistiveTransducer Dissipation Constant RTD is a resistance, there is an I2R power dissipated by the 18 device cause a slight heating effect, called self-heating Cause erroneous reading, therefore current of RTD must be kept low and constant to avoid self-heating Dissipation constant or PD is usually in the specs of RTD It relates power required to raise RTD 100 C For PD = 25mW/0C: If I2R power loses in RTD equal 25 mW, RTD will be heated 10C Lecture11:ResistiveTransducer Dissipation constant (cont.) Dissipation constant is specified under 2 conditions: free air and well-stirred oil bath Difference in capacity of medium to carry heat away from device The self-heating temperature rise can be found: P T PD T = temp rise of self-heating P = power dissipated by RTD from circuit in W PD = dissipation constant of RTD in W/0C Lecture11:ResistiveTransducer 19 Example 4.7 An RTD has 0=0.005/0C, R = 500 , and a dissipation constant of PD = 30mW/0C at 200C. The RTD is used in a bridge circuit such as that in previous figure, with R1 = R2 = 500 and R3 a variable resister to null the bridge. If the supply is 10 V and RTD is placed in a bath at 00C, find the value of R3 to null the bridge 20 Lecture11:ResistiveTransducer Solution Find RTD resistance at 00C without dissipation effect R = R0(1+ T ) =500(1+ 0.005(0-20)) RRTD = 450 Without considering self heating, for the bridge to null R3 = 450 (from R1R4 = R2R3) Self-heating effects?? P = I2R Calculate the current I from bridge Power dissipated from RTD 21 Lecture11:ResistiveTransducer Voltage supply V = 10V, R1 = R2 = 500 and R3 = a variable resistor to null the bridge Current I is calculated: I I 22 Lecture11:ResistiveTransducer 10 0.011A (500 450) Solution (cont.) Therefore power dissipated in RTD: P = (0.01)2(450) = 0.054 W Find the temperature rise P = TPD Temperature rise: 0.054 T 1.80 C 0.030 Thus, RTD is not actually at bath temperature of 00C but at 1.80C Resistance of RTD R = R0(1+ T ) =500(1+ 0.005(1.8-20)) RRTD = 454.5 Therefore, bridge will null with R3 = 454.5 Lecture11:ResistiveTransducer 23 c)Potentiometer • Displacement sensor – converts linear or angular motion into a changing in resistor • Simple potentiometric displacement sensor • Voltage divider: RTH VD 10V (3.5k RTH ) 24 Lecture11:ResistiveTransducer FIGURE 5.1 Potentiometric displacement sensor. Curtis Johnson Process Control Instrumentation Technology, 8e] 25 Lecture11:ResistiveTransducer Copyright ©2006 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage E is applied to resistor with length L Measure displacement, generate output e (Ohm’s Law) x eE L 26 Lecture11:ResistiveTransducer Resistive Sensors - Potentiometers Translational and Rotational Potentiometers Translational or angular displacement is proportional to resistance. 27 Lecture11:ResistiveTransducer Taken from www.fyslab.hut.fi/kurssit/Tfy-3.441/ luennot/Luento3.pdf 28 Lecture11:ResistiveTransducer 29 Lecture11:ResistiveTransducer Advantages: Cheap, easy to use, adjustable Problem: Mechanical wear, friction in wiper, high electronic noise 30 Lecture11:ResistiveTransducer Example: The value of R is 100k and the maximum displacement is 2.0cm. If E = 9V and x is 1.5 cm, determine the value of output voltage e 31 Lecture11:ResistiveTransducer Solution x eE L The output voltage e = 9V(1.5/2) = 6.75 V 32 Lecture11:ResistiveTransducer Example 4.8: A thermistor is to monitor room temperature. It has a resistance of 3.5k at 20C with a slope of -10%/C. It is proposed to use the thermistor in the divider of Figure below to provide a voltage of 5.0 V at 20C. Evaluate the effects of self-heating 33 Lecture11:ResistiveTransducer More on potentiometer 34 Lecture11:ResistiveTransducer Potentiometer 35 Lecture11:ResistiveTransducer 36 The potentiometer on the MCBXC866 board connects to port 2, pin 6 (P2.6) for generating analog voltage to the on-chip ADC. The analog input is AIN6 and the voltage range is 0-5.0 VDC. Lecture11:ResistiveTransducer Rotary Potentiometer 37 Lecture11:ResistiveTransducer 100 K Potentiometer 38 Lecture11:ResistiveTransducer Potentiometer Foot Paddle 39 Lecture11:ResistiveTransducer The slide potentiometer changes its resistance linearly with position. The slide potentiometer has about 60 mm (2.3 inches) of travel, and a nominal resistance of 10k ohms ± 20%. 40 Lecture11:ResistiveTransducer System Components: PIC Microcontroller: Potentiometer: the potentiometer will control the rpm of the stepper motor. This setting will be read by the A-to-D on the PIC. Stepper Motor: Stepper Motor Controller: 41 Lecture11:ResistiveTransducer Motor Potentiometer Assemblies Motor Potentiometer Assemblies have become extremely popular with system designers. Today, Betatronix can supply the complete motor-pot assembly or mount the potentiometer to the motor at our facility. 42 Lecture11:ResistiveTransducer End of Lecture 11 43 Lecture11:ResistiveTransducer