3. Develop a program that will latch on an output B (201), 20 seconds after input A (001) has been turned on. The timer will continue to cycle up to 20 seconds, and reset itself, until input A has been turned off. After the third time the timer has timed to 20 seconds, the output B will be unlatched.
5. A motor will be connected to a PLC and controlled by two switches. The GO switch will start the motor, and the STOP switch will stop it. If the motor is going, and the GO switch is thrown, this will also stop the motor. If the STOP switch was used to stop the motor, the GO switch must be thrown twice to start the motor. When the motor is running, a light should be turned on (a small lamp will be provided).
8. For the ladder diagram below, what does the program do? (Note: the keep is equivalent to an S-R flip-flop, in other words S will latch the value on, and R will unlatch the value). Note that 200 and 210 are lights and 202 is a motor. 100, 102 and 104 are input push buttons.
(ans. this is a motor controller. 100 is a start button, and 102 is a stop button. Light 200 will indicate when the motor is on. Light 210 will indicate when either start or stop is pushed, or the emergency button is not pushed. The motor 202 will run if the start button has been pushed and the normally closed stop switch is not pushed).
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 O013/05). After R has been on for 30 seconds the press can be activated to move (P connected to O013/06). The delay is needed for pressure to build up. After the press has been activated (with P) 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 I011/02), which must be off before the press can be activated. There is also a button that must be pushed 5 times (B, connected to I011/01) 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 state transition diagram, and then ladder logic for the states. State all assumptions, and show all work.