PLC Chapter 2 Answer of the review questions 1. Draw the input wiring diagram for a PLC system using a 24 VDC input unit with the following inputs: •
IN1 - Normally open pushbutton ON switch
•
IN2 - Normally open pushbutton OFF switch
•
IN3 - Normally closed selector switch labeled STEP 1 / STEP 2
•
IN4 - Normally open footswitch
Show all switches using the correct drawing symbol and show all devices including the power supply. The PLC being used does not have an internal 24VDC power supply. Solution
IN1
NO Pushbutton
IN2 NC Pushbutton
C L P
Selector switch OFF S1
IN3
S2 IN4
Foot Switch
COM
2. If one terminal of a lamp is connected directly to the negative terminal of the power supply, what kind of transistor output unit will be required (sourcing or sinking) to allow the PLC to light the lamp? 1/8
Solution:
Sourcing output (PNP) 3. The negative lead of a power supply is connected directly to one lead of a coil. Draw the wiring diagram for the proper connection of the coil to a transistor output unit. Also, show the spike protection diode across the coil.
Solution:
4. Using a relay output unit with one FORM C contact driven from OUTPUT 1, draw the wiring diagram for a system with two AC powered lamps, one that will light when OUTPUT 1 is ON and one that will light when OUTPUT 1 is OFF. Solution:
2/8
5. Draw the wiring diagram for a system combining the requirements of problems 1, 2 and 3 above utilizing one power source which is not part of the PLC. Solution:
6. What is the voltage resolution of a 10-bit unipolar 5 volt analog input? Solution: 10
Resolution = 5/(2 -1)= 5/ 1023 = 0.004888 V 7. How many bits would be needed for an analog output if, after applying the 25% rule of thumb, we need a resolution of 4.8 millivolts for a signal that has a range of 0 to +10 volts? Solution:
The 10 V volt range will be divided into 10 V / 4.8 mV = 2083 steps. To find the bit resolution we find the smallest value of n that solves the inequality 2
n
≥ 2083. n
The smallest value of n that will satisfy this inequality is n=12, where 2 = 4096. Therefore, we would need a 12-bit A/D C. 8. An 8-bit bipolar 5 volt analog input has an input of -3.29 volts. What will be the decimal value of the number the converter sends to t he CPU in the PLC? Solution:
Find the voltage resolution of the converter. Since the span is 10 volts, the resolution is 10 / 28 = 0.0390625 = 39.1 mV.
Divide the required Volt by the resolution to get (- 3.29 V) /(39.1 mV ) = - 84.224.
Convert the output decimal number to binary , (84.224)10 = (01010100)2
To get the negative, add logic 1 the binary is (01010101) 2 3/8
9. You program a PLC to output the binary number 10110101 to its analog output. The analog output is 8-bits, 10 volts, bipolar. What DC voltage do you expect to see on the output? Solution:
Find the voltage resolution of the converter. Since the span is 10 volts, the 8
resolution is 20 / 2 = 78.2 mV.
Convert the output binary number to decimal (10110101) 2 = 181 and
Multiply the result by the resolution to get 20 / 2
Finally, since the converter uses offset binary, we subtract 10 volts from the result
8
× 181
= 14.14 V.
to get 14.14 V -10 V = 4.14 V 10. An AC motor controller has a frequency control input of 0-10 volts DC which varies the output frequency from 0-60Hz. It drives a 3-phase induction motor that is rated at 1750 RPM at 60 Hz. The DC input to the motor controller is provided by an analog output from a PLC. The analog output is unipolar, 10 volts, 10 bits. What binary number must you program into the PLC to cause it to output the appropriate voltage to run the motor at 1000 RPM (assume for the motor that the relationship between frequency and speed is a simple ratio)? Solution:
Find the voltage resolution of the converter. Since the span is 10 volts, the resolution is 10 / 210 = 5.714 mV/RPM.
Since the converter uses 10 bits Voltage resolution = 10 V / 210= 0.009766
Using 25% rule the voltage resolution is = 0.009766 /25% = 0.039063
The required voltage corresponding to 1000 RPM, = 5.714 mV/RPM × 1000 RPM= 5.714 V
Divide the required Volt by the resolution to get (5.714 V) /(39.1 mV ) = 146.2784
Convert the output decimal number to binary , (146.2784)10 = (10010010)2
11. A pressure sensor is rated at 0-500 psi and has an output range of 0-10 volts DC (10 volts corresponds to 500 psi). It is connected to a PLC’s analog input that is 10 bits, 10 volts, unipolar. If the PLC reads the analog input as (8B3) 16, what is the pressure in psi? Solution:
Find the voltage resolution of the converter. Since the span is 10 volts, the resolution 10
is 10 / 2 = 0.0390625 = 5.714 mV/psi.
10
Since the converter uses 10 bits Voltage resolution = 10 V / 2 = 0.009766 4/8
Using 25% rule the voltage resolution is = 0.009766 /25% = 0.039063
Convert the analog input (8B3) 16 into decimal = 2227
Find the corresponding voltage, Multiply the analog input signal level, 2227, by the resolution to get the required voltage 10 / 210 * 2227 = 21.74805 V
Since 21.74805 V > Max. Voltage (10 V), the converter will clip it and read as 10 V which is corresponding to 500 psi
12. Can a PLC input switch a relay coil to control a motor? Ans: No - a plc OUTPUT can switch a relay 13. How do input and output cards act as an interface between the PLC and external devices? Ans: input cards are connected to sensors to determine the state of the system. Output cards are connected to actuators that can drive t he process. 14. What is the difference between wiring a sourcing and sinking output? ANS: sourcing outputs supply current that will pass through an electrical load to ground. Sinking inputs allow current to flow from the electrical load, to the common. 15. Is AC or DC easier to interrupt? ANS: AC is easier, it has a zero crossing. 16. What can happen if the rated voltage on a device is exceeded? ANS: it will lead to premature failure 17. What are the benefits of input/output modules? ANS: by using separate modules, a PLC can be customized for different applications. If a single module fails, it can be replaced quickly, without having to replace the entire controller. 18. Explain the operation of AC input and output conditioning circuits. ANS: AC input conditioning circuits will rectify an AC input to a DC waveform with a ripple. This will be smoothed, and reduced to a reasonable voltage level to drive a n optocoupler. An AC output circuit will switch an AC output with a triac, or a relay. 19. What will happen if a DC output is switched by an AC output? ANS: An AC output is a triac. When a triac output is turned off, it will not actually turn off until the AC voltage goes to 0V. Because DC voltages don’t go to 0V, it will never turn off. 20. We have a PLC rack with a 24 VDC input card in slot 3, and a 120VAC output card in slot 2. The inputs are to be connected to 4 push buttons. The outputs are to drive a 120VAC light bulb, a 240VAC motor, and a 24VDC operated hydraulic valve. Draw 5/8
the electrical connections for the inputs and outputs. Show all other power supplies and other equipment/components required. Solution:
21. You are planning a project that will be controlled by a PLC. Before ordering parts you decide to plan the basic wiring and select appropriate input and output cards. The devices that we will use for inputs are 2 limit switches, a push button and a thermal switch. The output will be for a 24Vdc solenoid valve, an 110Vac light bulb, and a 220Vac 50HP motor. Sketch the basic wiring below including PLC cards. Solution:
22. Add three push buttons as inputs to the figure below. You must also select a power supply, and show all necessary wiring.
6/8
Solution:
23. Sketch the wiring for PLC outputs that are listed below. -
a double acting hydraulic solenoid valve (with two c oils).
-
a 24Vdc lamp.
-
a 120 Vac high current lamp.
-
a low current 12Vdc motor.
Solution:
7/8
8/8
PLC Chapter 3 Answer of the review questions 1. Decide whether each of these statements is True (T) or False (F). the following figure shows a ladder diagram rung for which:
(i) The input contacts are normally open. (ii) There is an output when there is an input to the contacts. A) (i) T (ii) T B) (i) T (ii) F C) (i) F (ii) T D) (i) F (ii) F 2. Decide whether each of these statements is True (T) or False (F). the following figure shows a ladder diagram rung for which:
(i) The input contacts are normally open. (ii) There is an output when there is an input to the contacts. A) (i) T (ii) T B) (i) T (ii) F C) (i) F (ii) T D) (i) F (ii) F
3. Decide whether each of these statements is True (T) or False (F). the following figure shows a ladder diagram rung for which:
(i) When only input 1 contacts are activated, there is an output. (ii) When only input 2 contacts are activated, there is an output. A) (i) T (ii) T B) (i) T (ii) F
C) (i) F (ii) T D) (i) F (ii) F 4. Decide whether each of these statements is True (T) or False (F). the following figure shows a ladder diagram rung for which there is an output when:
(i) Inputs 1 and 2 are both activated. (ii) Either one of inputs 1 and 2 is not activated. A) (i) T (ii) T B) (i) T (ii) F
C) (i) F (ii) T D) (i) F (ii) F 5. Decide whether each of these statements is True (T) or False (F). the following figure shows a ladder diagram rung with an output when:
(i) Inputs 1 and 2 are both activated. (ii) Input 1 or 2 is activated. A) (i) T (ii) T B) (i) T (ii) F
C) (i) F (ii) T D) (i) F (ii) F 6. Decide whether each of these statements is True (T) or False (F). the following figure shows a ladder diagram rung for which there is an output when:
(i) Input 1 is momentarily activated before reverting to its normally open state. (ii) Input 2 is activated. A) (i) T (ii) T B) (i) T (ii) F
C) (i) F (ii) T D) (i) F (ii) F 7. Which form of logic gate system is given by a ladder diagram with a rung having two normally open sets of contacts in parallel? OR gate 8. Which form of logic gate system is given by a ladder diagram with a rung having two normally closed gates in parallel? NAND gate 9. Which form of logic gate system is given by a ladder diagram with a rung having two normally closed gates in series?
NOR gate 10. Which form of logic gate system is given by a ladder diagram with a rung having two normally open gates in series? AND gate
A) Input A is in series with input B, both inputs being normally off. B) Input A is in parallel with input B, both inputs being normally off. C) Input A, normally off, is in series with input B which is normally on. D) Input A is in parallel with input B, both inputs being normally on. 11. Which arrangement of inputs is described by the Boolean relationship A .B? Input A is in series with input B, both inputs being normally off. 12. Which arrangement of inputs is described by the Boolean relationship A + B? Input A is in parallel with input B, both inputs being normally off. 13. Which arrangement of inputs is described by the Boolean relationship
A + B ?
Input A is in parallel with input B, both inputs being normally on. 14. Which arrangement of inputs is described by the Boolean relationship A
B ?
i
Input A, normally off, is in series with input B which is normally on. 15. The arrangement of inputs in the following figure is described by the Boolean expression:
A) A . B . C B) (A + C) . B
C) (A + B) . C D) A . C + B 16. Decide whether each of these statements is True (T) or False (F). For the function block diagram shown in the following figure, there is an output:
(i) When A is 1. (ii) When B is 1. A) (i) T (ii) T
B) (i) T (ii) F C) (i) F (ii) T D) (i) F (ii) F 17. Decide whether each of these statements is True (T) or False (F). For the function block diagram shown in the following figure, there is an output:
(i) When A is 1.
(ii) When B is 1. A) (i) T (ii) T B) (i) T (ii) F C) (i) F (ii) T
D) (i) F (ii) F 18. Decide whether each of these statements is True (T) or False (F). For the function block diagram shown in the following figure, there is an output:
(i) When A is 1, B is 0 and C is 0. (ii) When A is 0, B is 1 and C is 1. A) (i) T (ii) T B) (i) T (ii) F C) (i) F (ii) T
D) (i) F (ii) F 19. Decide whether each of these statements is True (T) or False (F). For the function block diagram shown in the following figure, with A being a steady input condition and B a momentary input:
(i) When A is 1 and B is 0. (ii) When A is 0 and B is 1. A) (i) T (ii) T B) (i) T (ii) F C) (i) F (ii) T D) (i) F (ii) F
20. Figure 1(a) shows a ladder diagram. Which of the function block diagrams in Figure 1(b) is its equivalent?
Figure 1: Problem 20.
C
21. Figure 2(a) shows a ladder diagram. Which of the function block diagrams in Figure 2(b) is its equivalent?
Figure 2: Problem 21.
A 22. Draw the ladder rungs to represent: A) Two switches are normally open and both have to be closed for a motor to operate.
B) Either of two, normally open, switches has to be closed for a coil to be energized and operate an actuator.
C) A motor is switched on by pressing a spring-return push button start switch, and the motor remains on until another spring-return push button stop switch is pressed.
D) A lamp is to be switched on if there is an input from sensor A or sensor B.
E) A light is to come on if there is no input to a sensor.
F) A solenoid valve is to be activated if sensor A gives an input. A
23. Suppose we were given the task of designing a flame detection circuit for a toxic waste incinerator. The intense heat of the fire is intended to neutralize the toxicity of the waste introduced into the incinerator. Such combustion-based techniques are commonly used to neutralize medical waste, which may be infected with deadly viruses or bacteria: •
So long as a flame is maintained in the incinerator, it is safe to inject waste into it to be neutralized. If the flame were to be extinguished, however, it would be unsafe to continue to inject waste into the combustion chamber, as it would exit the exhaust un-neutralized, and pose a health threat to anyone in close proximity to the exhaust. What we need in this system is a sure way of detecting the presence of a flame, and permitting waste to be injected only if a flame is "proven" by the flame detection system.
•
Several different flame-detection technologies exist: optical (detection of light), thermal (detection of high temperature), and electrical conduction (detection of ionized particles in the flame path), each one with its unique advantages and disadvantages. Suppose that due to the high degree of hazard involved with potentially passing un-neutralized waste out the exhaust of this incinerator, it is decided that the flame detection system be made redundant (multiple sensors), so that failure of a single sensor does not lead to an emission of toxins out the exhaust. Each sensor comes equipped with a normally-open contact (open if no flame, closed if flame detected) which we will use to activate the inputs of a logic system as shown in Figure 3.
Figure 3: Problem 23.
It is required to design the circuitry of the logic system to open the waste valve if and only if there is good flame proven by the sensors. Ans:
24. Develop a program that will cause output D to go true when switch A and switch B are closed or when switch C is closed. Ans:
25. Develop a program that will cause output D to be on when push button A is on, or either B or C are on. Ans:
26. Develop Ladder Logic for a car door/seat belt safety system. When the car door is open, and the seatbelt is not done up, the ignition power must not be applied. If all is safe then the key will start the engine. Ans:
27. Design a motor controller that has a forward and a reverse button. The motor forward and reverse outputs will only be on when one of the buttons is pushed. When both buttons are pushed the motor will not work. Ans: The following is a possible connection.
Or the following connection using Limit switch.
28. Consider the design of a burglar alarm for a house. When activated an alarm and lights will be activated to encourage the unwanted guest to leave. This alarm is activated if an unauthorized intruder is detected by window sensor and a motion detector. The window sensor is effectively a loop of wire that is a piece of thin metal foil that encircles the window. If the window is broken, the foil breaks breaking the conductor. This behaves like a normally closed switch. The motion sensor is designed so that when a person is detected the output will go on. As with any alarm an activate/deactivate switch is also needed. The inputs and outputs are chosen to be;
A = Alarm and lights switch (1 = on) W = Window/Door sensor (1 = OK) M = Motion Sensor (0 = OK) S = Alarm Active switch (1 = on) The basic operation of the alarm can be described with rules. 1. If alarm is on, check sensors. 2. If window/door sensor is broken (turns off), sound alarm and turn on lights. Ans: In this case the controller has 3 different inputs, and a single output, so a truth table is a reasonable approach to formalizing the system. A Boolean equation can then be written using the truth table
A Boolean Equation and Implementation for the Alarm
Or
The Simplest Circuit and Ladder Diagram 29. Design ladder logic for a car that considers the variables below to control the motor M. Also add a second output that uses any outputs not used for motor control to start an alarm that goes Bing t warm that the key are still in the car. - Doors opened/closed (D) - Keys in ignition (K) - Motor running (M) - Transmission in park (P) - Ignition start (I) The control equation is :
M = (P I) K B = K D
The ladder implementation
30. a) Explain why a stop button must be normally closed and a start button must be normally open. b) Consider a case where an input to a PLC is a normally closed stop button. The contact used in the ladder logic is normally open, as shown below. Why are they both not the same? (i.e., NC or NO)
Ans: a) If a NC stop button is damaged, the machine will act a s if the stop button was pushed and shut down safely. If a NO start button is damaged the machine will not be able to start.) b) For the actual estop which is NC, when all is ok the power to the input is on, when there is a problem the power to the input is off. In the ladder logic an input that is on (indicating all is ok) will allow the rung to turn on the motor, otherwise an input that is off (indicating a stop) will break the rung and cut the power.) 31. Make a simple ladder logic program that will turn on the outputs with the binary patterns when
the corresponding buttons are pushed.
Ans:
32. Convert the following ladder logic to a Boolean equation.
Ans: Y
=
(( A
• B
+ B
• A
)
• D
+
(A
• C
• D
))
• D
+
C
)
33. Develop the Boolean expression for the circuit below.
Ans:
X
=
(
A B C i
i
+
B
) ( i
B
i
A
34. Given a system that is described with the following equation,
A) Simplify the equation using Boolean algebra. B) Implement the original and then the simplified equation with a digital circuit. C) Implement the original and then the simplified equation in ladder logic. Ans: (a)
Ans: (b) Original
Simplified
Ans: (c)
Original
Simplified
35. Convert the following ladder logic to a Boolean equation.
Ans: X
=
( ( C
i
AB
+
END
BA
) D) i
+
D
i
D
