## ASSIGNMENT PROBLEMS

1. Convert the following timing diagram to ladder logic. It should begin when input ‘A’ becomes true.

2. Use the timing diagram below to design ladder logic. The sequence should start when input X turns on. X may only be on momentarily, but the sequence should continue to execute until it ends at 26 seconds.

3. Use the timing diagram below to design ladder logic. The sequence should start when input X turns on. X may only be on momentarily, but the sequence should execute anyway.

4. Write a program that will execute the following steps. When in steps b) or d), output C will be true. Output X will be true when in step c).

a) Start in an idle state. If input G becomes true go to b)

b) Wait until P becomes true before going to step c).

c) Wait for 3 seconds then go to step d).

d) Wait for P to become false, and then go to step b).

5. Write a program that will execute the following steps. When in steps b) or d), output C will be true. Output X will be true when in step c).

a) Start in an idle state. If input G becomes true go to b)

b) Wait until P becomes true before going to step c). If input S becomes true then go to step a).

c) Wait for 3 seconds then go to step d).

d) Wait for P to become false, and then go to step b).

6. A PLC is to control an amusement park water ride. The ride will fill a tank of water and splash a tour group. 10 seconds later a water jet will be ejected at another point. Develop ladder logic for the process that follows the steps listed below.

1. The process starts in ‘idle’.

2. The ‘cart_detect’ opens the ‘filling’ valve.

3. After a delay of 30 seconds from the start of the filling of the tank the tank ‘outlet’ valve opens. When the tank is ‘full’ the ‘filling’ valve closes.

4. When the tank is empty the ‘outlet’ valve is closed.

5. After a 10 second delay, from the tank outlet valve opening, a water ‘jet’ is opened.

6. After ‘2’ seconds the water ‘jet’ is closed and the process returns to the ‘idle state.

7. Write a ladder logic program to extend and retract a cylinder after a start button is pushed. There are limit switches at the ends of travel. If the cylinder is extending if more than 5 seconds the machine should shut down and turn on a fault light. If it is retracting for more than 3 seconds it should also shut down and turn on the fault light. It can be reset with a reset button.

8. Design a program with sequence bits for a hydraulic press that will advance when two palm buttons are pushed. Top and bottom limit switches are used to reverse the advance and stop after a retract. At any time the hands removed from the palm button will stop an advance and retract the press. Include start and stop buttons to put the press in and out of an active mode.

9. A machine has been built for filling barrels. Use process sequence bits to design ladder logic for the sequential process as described below.

1. The process begins in an idle state.

2. If the ‘fluid_pressure’ and ‘barrel_present’ inputs are on, the system will open a flow valve for 2 seconds with output ‘flow’.

3. The ‘flow’ valve will then be turned off for 10 seconds.

4. The ‘flow’ valve will then be turned on until the ‘full’ sensor indicates the barrel is full.

5. The system will wait until the ‘barrel_present’ sensor goes off before going to the idle state.

10. Design ladder logic for an oven using process sequence bits. (Note: the solution will only be graded if the process sequence bit method is used.) The operations are as listed below.

1. The oven begins in an IDLE state.

2. An operator presses a start button and an ALARM output is turned on for 1 minute.

3. The ALARM output is turned off and the HEAT is turned on for 3 minutes to allow the temperature to rise to the acceptable range.

4. The CONVEYOR output is turned on.

5. If the STOP input is activated (turned off) the HEAT will be turned off, but the CONVEYOR output will be kept on for two minutes. After this the oven returns to IDLE.

11. We are developing a safety system (using a PLC-5) for a large industrial press. The press is activated by turning on the compressor power relay (R, connected to O:013/05). After R has been on for 30 seconds the press can be activated to move (P connected to O:013/06). The delay is needed for pressure to build up. After the press has been activated (with P for 1.0 seconds) the system must be shut down (R and P off), and then the cycle may begin again. For safety, there is a sensor that detects when a worker is inside the press (S, connected to I:011/02), which must be off before the press can be activated. There is also a button that must be pushed 5 times (B, connected to I:011/01) before the press cycle can begin. If at any time the worker enters the press (and S becomes active) the press will be shut down (P and R turned off). Develop the process sequence and sequence bits, and then ladder logic for the states. State all assumptions, and show all work.

12. A machine is being designed to wrap boxes of chocolate. The boxes arrive at the machine on a conveyor belt. The list below shows the process steps in sequence.

1. The box arrives and is detected by an optical sensor (P), after this the conveyor is stopped (C) and the box is clamped in place (H).

2. A wrapping mechanism (W) is turned on for 2 seconds.

3. A sticker cylinder (S) is turned on for 1 second to put consumer labelling on the box.

4. The clamp (H) is turned off and the conveyor (C) is turned on.

5. After the box leaves the system returns to an idle state.

Develop ladder logic programs for the system using the following methods. Don’t forget to include regular start and stop inputs.

i) a timing diagram

ii) process sequence bits