SEP718 Assignment 1 (10%)
Instructions for problems requiring the writing of ladder logic:
1. You can use your handwriting to write your ladder logic and submit your handwritten solutions.
2. You can write your ladder logic in Studio 5000 software, print it as pdf and submit it.
3. In both of the above cases, you will still need to implement your code in Studio 5000 software
and submit program files (.ACD).
4. In you Ladder Logic, all tags must be properly described.
5. If the I/O assignment is not given, feel free to do your I/O assignment.
Problem 1 [1]
Assuming the hardwired circuit below is to be implemented using a PLC program,
a. identify all input field devices
b. identify all output field devices
c. identify all devices that could be programmed using internal relay instructions
d. develop an equivalent PLC ladder logic program
Problem 2 [1]
Write the ladder logic program needed to implement each of the following (assume inputs A, B, and C
are all normally open toggle switches):
a. When input A is closed, turn ON and hold ON outputs X and Y until A opens.
b. When input A is closed and either input B or C is open, turn ON output Y; otherwise, it should be OFF.
c. When input A is closed or open, turn ON output Y.
d. When input A is closed, turn ON output X and turn OFF output Y.
Problem 3 [1]
A temperature control system consists of four thermostats controlling three heating units. The
thermostat contacts are set to close at 50°, 60°, 70°, and 80°F, respectively. The PLC ladder logic
program is to be designed so that at a temperature below 50°F, three heaters are to be ON. From 50° to
60°F, two heaters are to be ON. For 60° to 70°F, one heater is to be ON. Above 80°F, there is a safety
shutoff for all three heaters in case one stays on because of a malfunction. A master switch is to be used
to turn the system ON and OFF. Prepare a typical PLC program for this control process.
Problem 4 [1]
A pump is to be used to fill two storage tanks. The pump is manually started by the operator from a
start/stop station. When the first tank is full, the control logic must be able to automatically stop flow to
the first tank and direct flow to the second tank through the use of sensors and electric solenoid valves.
When the second tank is full, the pump must shut down automatically. Indicator lamps are to be
included to signal when each tank is full.
a. Draw a sketch of the process.
b. Prepare a typical PLC program for this control process.
Problem 5 [1]
Study the ladder logic program below and answer the questions that follow:
a. What type of timer has been programmed?
b. What is the length of the time-delay period?
c. What is the value of the accumulated time when power is first applied?
d. When does the timer start timing?
e. When does the timer stop timing and reset itself?
f. When input LS1 is first closed, which rungs are true and which are false?
g. When input LS1 is first closed, state the status (on or off) of each output.
h. When the timer’s accumulated value equals the preset value, which rungs are true and which are
false?
i. When the timer’s accumulated value equals the preset value, state the status (on or off) of each
output.
j. Suppose that rung 1 is true for 5 s and then power is lost. What will the accumulated value of the
counter be when power is restored?
Problem 6 [1]
Study the ladder logic program below and answer the questions that follow:
a. What type of timer has been programmed?
b. What is the length of the time-delay period?
c. What is the value of the accumulated time when power is first applied?
d. When does the timer start timing?
e. When does the timer stop timing and reset itself?
f. When input LS1 is first closed, which rungs are true and which are false?
g. When input LS1 is first closed, state the status (on or off) of each output.
h. When the timer’s accumulated value equals the preset value, which rungs are true and which are
false?
i. When the timer’s accumulated value equals the preset value, state the status (on or off) of each
output.
j. Suppose that rung 1 is true for 5 s and then power is lost. What will the accumulated value of the
counter be when power is restored?
Problem 7 [1]
Study the ladder logic program below and answer the questions that follow:
a. What is the purpose of interconnecting the two timers?
b. How much time must elapse before output PL is energized?
c. What two conditions must be satisfied for timer T4:2 to start timing?
d. Assume that output PL is on and power to the system is lost. When power is restored, what will the
status of this output be?
e. When input PB2 is on, what will happen?
f. When input PB1 is on, how much accumulated time must elapse before rung 3 will be true?
Problem 8 [2]
In a processing plant, jars on a conveyer belt are cleaned out with a high-pressure air jet just prior to
being filled. When a jar approaches the cleaning station, it activates a switch (with both NO and NC
contacts). The conveyer belt stops for 10 s while the air jet is on; then the conveyer belt starts, and the
jar moves along. Draw a ladder logic diagram to control this process.
Problem 9 [2]
A PLC is to control the solar heating system shown below. The system has two interrelated parts: (a) A
solar thermostat turns on and off the solar heater if the sun sensor says the sun is shining, and (b) a
backup thermostat turns on and off a conventional furnace if the solar energy is insufficient. Both
heating systems share the same ductwork, so if the backup thermostat closes, the PLC must turn on the
backup furnace (and turn off the solar heater if it’s on). Draw a PLC ladder diagram for this system.
Problem 10 [2]
A batch process—which involves filling a vat with a liquid, mixing the liquid, and draining the vat—is
automated with a PLC. The Figure below shows the hardware. The specific sequence of events is as
follows: When the start button near the process is pushed:
1. A fill valve opens and lets a liquid into a vat until it’s full.
2. The liquid in the vat is mixed for 3 min.
3. A drain valve opens and drains the tank.
Design the PLC ladder logic program
Problem 11: Lubricating Drilling Machine [3]
The figure below shows a drill press with a coolant pump. After the metal stock is secured in a holding
fixture, the coolant pump is started by a push button. To ensure that the coolant is flowing where the
drill bit makes contact with the metal part, a pressure switch detects the coolant flow.
The drilling process begins by pressing a second push button. The drill bit then turns and a solenoid is
energized, which causes a pneumatic cylinder to extend. The extension of the cylinder causes the drill
bit to advance toward the stock. As the drill body approaches the stock, it makes contact with a limit
switch that activates a 5-second timer. During the 5 seconds, the drill bit bores a hole into the stock.
After the timer has timed out, the solenoid de-energizes, causing the spring-return cylinder to retract
and remove the drill bit from the stock.
An orange light turns on when the coolant pump is running, a green light is on during the drill-and-feed
sequence, and a red light turns on when the equipment is turned off. A stop button turns off the coolant
motor, or shuts off the equipment in an emergency situation.
Problem 12: Electric ladder control [4]
The figure below shows an escalator that can operate only when a person is approaching (and not
permanently, as happens sometimes). The detection of the approaching person at the up-escalator is
performed via a photocell. At the entrance and exit of the ladder there are, at appropriate points, two
emergency STOP buttons for use by the public, so if any accident occurs during the operation, anyone
can stop the movement of the ladder. In the space of the escalator machinery, there is a control panel
with buttons and indicators for controlling the electric ladder during maintenance periods. Specifically,
the operating specifications of the above installation are as follows:
1. By instantaneous pressing of the “Stand-by ON” button, the automation system is powered, and the
escalator is ready to operate if someone approaches it.
2. The readiness and hence the operation capability is removed by pressing the “Stand-by OFF” button.
3. For the “Stand-by ON” status, a corresponding light indicator is activated.
4. If the electric ladder is operating due to human presence, it stops immediately if the STOP button on
the control panel is pressed, if one of the two emergency buttons at the entrance and exit of the ladder
is pressed, or if the thermal overload switch of the motor is activated.
5. If the escalator is in “Stand-by ON” status and a person is approaching, the activation of the photocell
will cause the ladder to operate for 40 s, after which it will automatically stop.
6. Successive arrivals of people will cause a reset to the 40-s operating time.
7. During the maintenance period, it is necessary to operate the ladder manually. This is accomplished
with the “momentary operation” button, which means that the ladder operates as long as the button is
pressed, regardless of whether the system is in “Stand-by ON” status or not.
References
1
Petruzella, F.: ‘Programmable Logic Controllers’ (McGraw-Hill Education, 2016)
2
Kilian, C.: ‘Modern Control Technology’ (Delmar Cengage Learning, 2005)
3
Bartelt, T.: ‘Industrial Automated Systems’ (Nelson Education, 2010)
4
Manesis, S., Nikolakopoulos, G.: ‘Introduction to Industrial Automation’ (CRC Press, 2018)