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Unformatted text preview: Name Real Time Computing Systems EE 4770 Midterm Examination
15 March 1995, 8:40-9:30 CST Problem 1 Problem 2 Problem 3 Alias Good Luck! 34 pts 33 pts 33 pts 100 pts Exam Total Problem 1: Finish the design of a circuit and interface routine to convert process variable x 2 x 250 K; 350 K , the temperature in a system, to H x = C , to be written in variable tee. The value should have a precision of no less than 0:05 C. Take into account the non-linear response of the thermistor. 34 pts Use a thermistor and thermistor model Ht2 x = R0 e =x where = 2500 K and R0 = 0:03 . Use an HADC7 V;b analog-to-digital converter with the minimum number of bits, b, necessary to meet the precision requirement. Be sure to take into account the non-linear response of the thermistor when computing b.* The part of the design completed makes full use of the ADC's dynamic range.
C vC RE RD
v B + RB RA
+ v o The following values have already been found: vB = 5 V, RD = 500 k , RE = Ht2 x, RA = 1 k , RC = 2:64 M , vC = 1 V, and RB = 1:12 M . The output of the gain o set circuit is then A5 RE , O5 , where A5 = 11:24 mA and O5 = 37:95 . * This sentence was not included in the original exam. 2 light source, to a voltage, vo = H x = x mW V. The circuit is to use two photodiodes, one placed 30 cm from the light source, the other placed 70 cm from the light source. Design the circuit so that the output is based on the average radiant intensity seen by the two photodiodes. The photodiodes 2 have sensitivity Ks = 5 A cm . The light source radiates uniformly in all directions. 33 pts mW Problem 2: Design a circuit to convert process variable x 2 0; 10 mW , the radiant intensity of a sr sr 3 Problem 3: Answer each question below. Brief answers will be appreciated. Please do not give multiple-choice answers. a A relative coded displacement transducer has the following disadvantage: the counter value may be incorrect from the time the power is turned on until the index mark passes under the mark reader. Since there is only one index mark, this may take a while. Modify the two-way relative CDT shown below by adding three index marks for a total of four, so that the counter can be properly set at four di erent positions. Leave the other tracks unchanged. Explain how the counter would be reset to one of the four correct values given only the output of the three mark-reading transducers. 15 pts 4 b How does a magnetic reluctance proximity transducer work? Could it be used to measure displacement? Explain. 9 pts c Why are new practical temperature scales de ned every few decades? Why does the thermodynamic temperature scale stay the same? 9 pts 5 ...
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