Problem 19.2 Write a short ladder logic program that will turn a light (LA) on 10 seconds after a button (PB) is pushed. A second light (LB) should be on while the button is pushed and then turn off 10 seconds after it has been released.
• NOTE: these are structured design methods: although many of the design activities are obvious, by following the same steps, others can understand what you have done, and others will get similar results when they solve a similar problem. This is a simple start, and we will advance to significant design techniques later.
19.2.1 Process Sequence Bits
Problem 19.5 Design a flag raiser that will go up when an up button is pushed, and down when a down button is pushed, both pushbuttons are momentary. There are limit switches at the top and bottom to stop the flag pole. When turned on at first the flag should be lowered until it is at the bottom of the pole.
19.2.2 Timing Diagrams
Problem 19.6 A handicap door opener has a button that will open two doors. When the button is pushed (momentarily) the first door will start to open immediately, the second door will start to open 2 seconds later. The first door power will stay open for a total of 10 seconds, and the second door power will stay on for 14 seconds. Use a timing diagram to design the ladder logic.
19.3.1 Counters And Timers
19.3.2 More Timers And Counters
Develop a program that will latch on an output ‘B’ 20 seconds after input A has been turned on. After ‘A’ is pushed, there will be a 10 second delay until ‘A’ can have any effect again. After ‘A’ has been pushed 3 times, ‘B’ will be turned off.
19.3.4 More Timers
19.3.5 Cascaded Timers
19.3.6 Deadman Switch
A motor will be controlled by two switches. The Go switch will start the motor and the Stop switch will stop it. 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 active a light should be turned on. The Stop switch will be wired as normally closed.
A conveyor is run by switching on or off a motor. We are positioning parts on the conveyor with an optical detector. When the optical sensor goes on, we want to wait 1.5 seconds, and then stop the conveyor. After a delay of 2 seconds the conveyor will start again. We need to use a start and stop button: a light should be on when the system is active.
19.3.8 Accept/Reject Sorting
For the conveyor in the last case we will add a sorting system. Gages have been attached that indicate good or bad. If the part is good, it continues on. If the part is bad, we do not want to delay for 2 seconds, but instead actuate a pneumatic cylinder.
19.3.9 Shear Press
2. While the stamping solenoid is energized, it must remain energized until a limit switch (LS2) is activated. This second limit switch indicates the end of a stroke. At this point the solenoid should be de-energized, thus retracting the cylinder.
5. A safety check should be included. If the cylinder solenoid has been on for more than 5 seconds, it suggests that the cylinder is jammed or the machine has a fault. If this is the case, the machine should be shut down and a maintenance light turned on.
19.3.10 Actuator Failure
Problem 19.10 We are using a pneumatic cylinder in a process. The cylinder can become stuck, and we need to detect this. Proximity sensors are added to both endpoints of the cylinders travel to indicate when it has reached the end of motion. If the cylinder takes more than 2 seconds to complete a motion this will indicate a problem. When this occurs the machine should be shut down and a light turned on. Develop ladder logic that will cycle the cylinder in and out repeatedly, and watch for failure.
19.3.11 Palm Button Detection
Problem 19.11 In dangerous processes it is common to use two palm buttons that require a operator to use both hands to start a process (this keeps hands out of presses, etc.). To develop this there are two inputs that must be turned on within 0.25s of each other before a machine cycle may begin.
Problem 19.14 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.
Problem 19.16 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).
Problem 19.17 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.
Answer 19.17 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.
Problem 19.20 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.