IMT 223
Instrumentation and Controls
Final Exam
This exam is closed book and open note. You may use your notes, worksheets and old quizzes to help you with
problems on this exam. Show all of your work and explain your answers. I want to see your thought process. If
you are doing calculations it is sufficient to show the steps of your calculation. Take your time, and draw
pictures, schematics, and make notes. It will help.
Number of Questions: 18 (this does not include all the subparts)
Time Limit: 3 hours
Total Points: 310
Farrell
3/20/2019
1
IMT 223
1.
Instrumentation and Controls
(10 points) The definition of a ”setpoint” is:
A.
B.
C.
D.
2.
Final Exam
The position of the control valve in an automatic control system
The target value for a control system to maintain the process variable
The amount of milliamps (signal) output by the 4-20 mA analog transmitter
The process variable value (limit) at which a warning alarm will activate
(10 points) The definition of a “process variable” is:
A. The position of the control valve in an automatic control system B. The
target value for a control system to maintain its setpoint at
C. The physical quantity measured (and controlled) by an instrument system
D. The amount that the process is allowed to vary (change) over time
3.
(10 points) When a NO switch is not activated, does it look like a short circuit or an open circuit?
4.
(10 points) Activated NC switches are short circuits or open circuits?
5.
(25 points)Given the following circuit:
Switch actuates
when object
approaches this
end!
+
-
+
−
The circuit is functioning properly. In its current state the light is on, but there are no objects close to the
switch. Based on this information answer the following questions:
(a) (10 points) Is this a sinking or sourcing sensor? Explain how you know this.
(b) (5 points) Do you think the sensor is more likely to contain a NPN or PNP transistor?
(c) (10 points) Is the sensor’s switch a NO or NC switch? Explain how you know this.
Farrell
3/20/2019
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IMT 223
6.
Instrumentation and Controls
Final Exam
(30 points)Suppose an electronic pressure transmitter has an input range of 0 to 15 PSI and an output
range of 4 to 20 mA. When subjected to a series of known pressures, it responds as such:
Applied Pressure Output Signal
(PSI)
(mA)
0
4.0
5
8.1
10
15.0
15
19.8
(a) (15 points) Find the ideal output signal values in mA for each of the shown pressure inputs.
Applied Pressure Output Signal
(PSI)
(mA)
0
5
10
15
(b) (10 points) If you detected 0mA from the transmitter, what would that tell you about your system?
7.
(15 points)A loop-powered 4-20mA pressure transmitter is connected to a data logger as seen in the
schematic below:
+
−
Out
24V
Pressure Transmitter
4-20mA
250 Ω
Channel 1
1-5 VDC
Datalogger
Farrell
3/20/2019
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IMT 223
Instrumentation and Controls
Final Exam
(a)
(10 points) This is not the correct way to hook up a loop powered transmitter. Redraw this
schematic showing how to correctly hookup the transmitter. Hint: All the parts on the given
schematic are used and you don’t need any other elements
(b)
8.
(5 points) What does the 250Ω resistor do? i.e. Why is it necessary for this circuit? Hint: Look
at the labels on the schematic
(20 points)In this process the controller (LIC) is attempting to maintain the level of water in the filtering
media of the vat. It uses an ultrasonic level sensor (LT) to measure the distance from the sensor to the
water, and thus computes the water level.
Setpoint
LIC
M
LT
Ultrasonic
Influent
LIR
Filter
H
L
Filtering
media
Effluent
(a) (15 points) Describe what the controller should do if the level sensor detects that the water level is
above the setpoint.
(b) (5 points) The LT is calibrated so that it reads 2” too high, i.e.: it reports to the controller that the water
level is 2” higher than it actually is. What affect will this have on the operation of the control loop?
Farrell
3/20/2019
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IMT 223
9.
Instrumentation and Controls
Final Exam
(20 points)Use the diagrams below to answer these questions:
(A)
(B)
Proximity switch
Proximity switch
+V
+V
LED
Output
Output
+ 24VDC
−
Sensor
circuit
Ground
LED
Sensor
circuit
+ 24VDC
−
Ground
(C)
(D)
Proximity switch
+V
Proximity switch
+V
Sensor
circuit
Output LED
+ 24VDC
−
Sensor
circuit
Output LED
Ground
+ 24VDC
−
Ground
(a) (10 points) Which picture above shows a properly connected sourcing switch?
(b) (10 points) Which picture above shows a properly connected sinking switch?
10. (25 points)Use the ladder diagram provided to answer this question.
L1
L2
A
CR1
B
CR1
G
CR1
R
Farrell
3/20/2019
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IMT 223
Instrumentation and Controls
Final Exam
(a) (15 points) Complete the following truth table indicating when the Green (G) and Red (R) lightswill
be on and off.
A
B
G R
Not Pushed Not Pushed
Not Pushed
Pushed
Pushed
Not Pushed
Pushed
Pushed
(b) (5 points) In your own words describe how one should operate both switches to turn the green light
on?
(c) (5 points) In your own words describe how one should operate both switches to turn the red light
on?
11. (20 points)Use this ladder diagram to answer these questions.
L1
L2
o
75 C
TS1
o
100 C
TS2
Element
(a) (15 points) Describe how this heater control works. Make sure you explain the conditions necessary
to turn on and turn off the heater.
(b) (5 points) How would this circuit operate if the settings on the temperature switches were
accidentally reversed (TS1 set to 100 and TS2 set to 75)? Be sure to justify your answer.
Farrell
3/20/2019
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IMT 223
Instrumentation and Controls
Final Exam
12. (20 points)This pressure alarm circuit is designed to energize a warning light if the process pressure
sensed by the pressure switch ever crosses a certain threshold. The X1 and X2 terminals of the
transformer provide the power for the circuit. That is the connection for your voltage source.
Hint: draw the circuit in a ladder diagram to solve
2006 Kuphaldt. Reproduced with permission
(a) (15 points) Is this a ”Low-pressure” or ”High-Pressure” alarm? (i.e. will the light bulb energize for
abnormally low pressure or abnormally high pressure? Explain how you came up with your answer.
(b) (5 points) If the wire to terminal 2 of the relay is not connected, what effect will this have on the
warning light?
Farrell
3/20/2019
7
IMT 223
Instrumentation and Controls
Final Exam
13. (15 points) Sketch the necessary wiring to make this float-type level switch control a pump and a lamp in
the following manner:
• High liquid level: pump on and lamp off
• Low liquid level: pump off and lamp on
Farrell
3/20/2019
8
IMT 223
14.
Instrumentation and Controls
Final Exam
Using the following data points (x, y):
(2, 8)
(5, 14)
(a)
(b)
(c)
(d)
Farrell
(15 points) Solve for the slope, m
(10 points) Solve for the y intercept
(15 points) Graph the line
(5 points) If x = 2, what does y equal?
3/20/2019
9
IMT 223
Instrumentation and Controls
Final Exam
15.
(10 points) Why use a 4-20 ma sensor instead of a 0-20 ma sensor?
A
It saves power.
B
It’s more precise
C
If the current drops below 4 ma then it provides fault detection.
D
There is less signal loss due to wire resistance.
16.
(10 points) What is the operating principle of a thermocouple (TC)?
A
A galvanic cell creates a voltage proportional to the temperature.
B
The chemical properties of dissimilar metals creates a magnetic field which induces a current.
C
Bi-metallic coils measure temperature.
D
A thermocouple is the same as a resistive thermal device.
17.
(10 points) What is the primary reason for using 24 V for control circuits?
A
The current is lower than 120 V circuits.
B
It’s safer for the technician/ Repair-person to work with.
C
24 V creates less signal noise than 120 V.
D
24 V has a lower galvanic potential which prohibits wire oxidation.
18. (10 points) How does a GFCI (ground fault circuit interrupter) work?
A
The device relies on a voltage change to trip
B
The device sees a temperature increase when there is a short to ground
C
A magnetic field is induced into a coil when there is an imbalance between the hot and neutral, causing
a trip.
D
The current no longer cancels between hot and neutral when the ground becomes disconnected,
causing a trip.
Farrell
3/20/2019
10
IMT 223
Farrell
Instrumentation and Controls
3/20/2019
Final Exam
11