Automationtechies Technical Assessment
1) Examine the following PLC Ladder Logic program. (Note: The answer in each row is the status of the bits after scanning both rungs. Assume this is a PLC that scans the logic from left to right and from top to bottom, like Allen-Bradley/Rockwell Automation PLCs)
Scan0 has just completed. Fill in the scan state table below for each of the successive scans listed in the table below. Fill in the states for the addresses AT THE END OF EACH SCAN.
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SCAN
|
Input
Address "A"
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Address "B"
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Address "C"
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Address "D"
|
|
Scan0
|
0
|
0
|
0
|
0
|
|
Scan1
|
1
|
|
|
|
|
Scan2
|
1
|
|
|
|
|
Scan3
|
0
|
|
|
|
|
Scan4
|
0
|
|
|
|
|
Scan5
|
1
|
|
|
|
|
Scan6
|
1
|
|
|
|
|
Scan7
|
0
|
0
|
0
|
0
|
The two rungs of ladder logic shown above provide a Flip-Flop or alternating function upon output address "D" on each false to true transition of address "A. The correct Scan7 data is provided. If your answer does not match, please review your work.
2) Consider the following application scenario. You need to add logic to an existing PLC system to control the coil of a normally open relay contact connected to a pump system. Both switches are normally closed with no liquid in the tank, and open when the liquid rises to the level of the switch. When the liquid product level is below the Low Level switch LS-2, the relay connected to output "O4" shall be energized. The relay shall remain energized until the liquid product level reaches the High Level switch LS-1.
Using keyboard characters (example: |--------] [-----]/[-----------( )-------| ) draw the ladder logic below that will meet the requirements of this application scenario. Use "I1" as the High Level Switch address, use "I2" as the Low Level Switch address, and use "O4" as the Pump Relay address.
3) A positive displacement pump is to be used to fill a liquid into a package. Test with the pump reveal that it dispenses 128 milliliters of liquid for each revolution of the pump. An encoder is attached to the shaft of the pump. The encoder generates 1024 pulse for each revolution. The pump runs at 25 revolutions per minute. The encoder counts will be run into a control system. Based on this information, how many counts will the control system see if 2 liters are dispensed from the pump? How many revolutions of the pump will occur to dispense that 2 liter amount? How many seconds would it take the pump to dispense the 2 liters?
4) Continuous process control systems typically use PID control to hold processes at a given set point. For the example process shown below, a proportional gain of 1 is used. After running the process for a time, it is shown that the system Process Variable overshoots and oscillates about the set point. What could be done to the gain setting to achieve more stable performance? Would there be any other effect on the system in making the change you propose?