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8/10/2019 2011 Final MECH 368
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Name Student #
MECH 368 Measurement and Instrumentation
Final Exam
Spring 2011
Notes:
There are 16 pages in total
Partial credit is given only when intermediate steps are shown
Problem Grade Maximum
#1 10
#2 6
#3 12
#4 12
#5 12
#6 10
#7 12
#8 6
Total 80
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1. Explain the following using clearand simple language. Use illustrations to aid your explanations
where appropriate.
1.1Provide three reasons why you may want to automate a measurement process. (3)
1.2An LED has a Vd= 2V. Why is the following not a good circuit for illuminating this LED? (3)
2V Vd=2V
1.3What is a free-wheeling diode? Why is it necessary? (4)
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2. Design an op amp filter that contains a high-pass stage with fc= 100 Hz, a low-pass stage with fc= 1
kHz, and Gain = -10. Use realistic component values, i.e. capacitors ranging from 100 pF to 1 F, and
resistors ranging from 1 k to 10 M. (6)
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3. For the circuit below, derive a general (symbolic) expression for Vout(j)/Vin(j) in terms of R1, R2,
R3, and C. Determine a numerical expression for Vout(j)/Vin(j) using the given R and C values. Draw
the Bode plot for Vout(j)/Vin(j) based on your numerical expression. Label all axes, as well as any
corner frequencies. (Hint: use Thevenin-Norton equivalents to simplify your expression) (12)
R1
20k
R2
10k
Vout
R3
20k
C
1nFVin
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4. Determine the current through each LED for all four switch states. If the LED is not turned on then
write 0 for the current. Show work to justify each value. (12)
S1 S2 LED1 current D2 current LED3 current
OFF OFFOFF ON
ON OFF
ON ON
LED1
Vd= 2V
5V
R1
100
S1
D2
Vd= 0.7V
LED3
Vd= 2V
5V
R2
200
S2
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5. For the Vin waveform shown in the graph, draw the resulting Vout waveform from t=0 to t=130ms.
Write the expression for Vout(t) for regions (1) and region (2) on the graph. Determine the value of the
time constant in both cases. (12)
C1
1 F
R2
30k
Vout
R1
10k
Vin
VF= 2V
LED1
Vd= 0.7V
D1
0ms
1V
2V
3V
4V
5V
10ms 20ms 30ms 40ms 50ms 60ms 70ms
0V
-1V
80ms 90ms 100ms 110ms 120ms 130ms
Vin
Region (1) Region (2)
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6.The photodiode circuit below is used to detect signals from a 5 kHz sinusoidal light source. The
sinusoidal light source generates a photodiode current with amplitude 0.2 A. The ambient light level
generates a photodiode current of 1 A. Draw the voltage signal you expect to see at V1, V2, V3, and
Vout on the graph below. Clearly label each signal. Fluorescent lighting in the room produces an
interference signal at 100 Hz with 0.01 A amplitude. How will this signal impact Vout? (10)
R4
1M
5V
5V
C1
0.1 F
R1
1.6k
5V
Vout
R61M
C2
0.1 F
5V
5V
10k
10k
V1
V2
V3
5V
10k
10k
R2
16kIin
0ms
1V
2V
3V
4V
5V
0.1ms 0.2ms 0.3ms
0V
-1V
0.4ms 0.5ms 0.6ms
Expected impact on Vout from fluorescent lighting:
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Use this page for extra work if necessary.
0ms
1V
2V
3V
4V
5V
0.1ms 0.2ms 0.3ms
0V
-1V
0.4ms 0.5ms 0.6ms
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7. You are designing a cooling system for an internal combustion engine where it is necessary to
measure coolant temperatures ranging from -50C to 200C. You decide to use an RTD sensor with
characteristics listed in the table. (12)
(1) The circuit below is used to generate a constant current for the RTD. What is IOUTas a function of
RREFand V3?
(2) What should be the value of RREFfor IOUTto be 1 mA?
(3)For temperatures ranging between -50C and 200C, what is the expected range of (V1- V2)?
(4)For this temperature range of interest, design an op amp circuit to interface the voltage across the
RTD to a 0 to 5V analog to digital converter. Assume only a single 5V power supply is available.
V3
R1
5V
R1
R1
R1
V2
RREF
RRTD
RW
RW
V1
IOUT
Temp (C) RRTD()-50 90
-25 95
0 100
25 105
50 110
75 115
100 120
125 125
150 130
175 135200 140
*Maximum RTD current = 1 mA
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8. The circuit you designed in the previous problem gave you a reading from 0 to 5V for temperatures
ranging from -50C to 200C. After testing this sensor, you realize that its output reading fluctuates by
2C at a constant temperature. Design a comparator circuit to generate the signal to activate the
cooling system at 75C with a hysteresis band equivalent to the expected fluctuations. Your answer for
Vth+and Vth-should be within 10 mV of the expected fluctuation levels. The cooling system is de-
activated with a 5V signal and is activated with a 0V output. Assume the output reading scales linearly
with the temperature change. Label all component values in your circuit. (6)