Micrologix or timing relay


Thread Starter


Momentary pushbutton inputs to control PC digital IO. Pushbutton A is used now to control machine to start machine when hit once, and then continue next operation when hit again. But if Pushbutton A is double hit, then the machine just starts and goes right into the next operation and the operator did not think they hit it two times. Which is a super safety hazard!

What I want to do is to have two buttons to control this operation without a PLC. Pushbutton A will start the process, and with an added pushbutton b will continue the process. But the catch is once pushed, button A is triggered to start then it is out of the equation until pushbutton b is triggered to continue the operation. Can you help me?

This machine has an old motion PC program with digital PC IO connected to servo drives, and mostly everything is in German. So it makes it hard to change anything on this machine. Is it possible to connect two NO momentary pushbuttons to one input so I can trick the machine without have to mess with anything else?

Michael Griffin

First I would suggest checking if the input to the machine has a "one shot" in software. Do a test where when you start the machine you press the button and hold it down. If the machine continues to the next step just by holding the button down (without releasing it), then the real problem is there is no "one shot".

Suggestion A) If the machine has a "one shot" input and you just want to enforce a time delay between button presses, you might consider connecting the existing push button to a timing relay with an off-delay output. Connect the output of the timing relay to the input of the control system. When you press the push button, the timing relay will turn on. When you release the button the timing relay will time (set this to some suitably long delay) and then eventually turn off. If the operator presses the button twice before the timing relay has turned off, there will be only one pulse so you won't get an accidental repeat. This method uses just one button.

Suggestion B) If the machine does not has a "one shot" input, or if you are sure you need the two button start sequence, then I would suggest that you use a "programmable relay", such as the Siemens Logo (many other companies have similar products). You could wire the two buttons to the inputs of the programmable relay and the output of the "relay" to the input of the control system. You would need to put a small program in the programmable relay, but many of these can be programmed through some buttons and a small display on the front of them. These typically have 6 inputs and 4 outputs. A small PLC can do the same thing, but would take up a bit more space. The sequence would be something like the following:

0) The second button is disabled. Pressing it has no effect at this time.

1) Press the first button. This sends a short timed pulse from the relay to start the machine. A second output on the relay could (if desired) turn on a pilot light to give feedback to the operator.

2) Pressing the first button again has no effect.

3) After a suitable time delay (or you could monitor the state of the machine with another input) the second push button is enabled.

4) When the second push button is pressed, another short timed pulse from the relay is sent to the machine.

5) After a suitable time delay (or you could monitor the state of the machine with another input) the program is reset to the beginning of sequence to enable the next start.

You could do the above with just relay logic (normal relays), but getting that working right would be more complicated and less reliable than using a programmable relay.
Not too sure if I've understood what you require, but to avoid any software changes you could install an on-delay timer and wire the pushbutton through a N/C contact of the timer so that when you hit the pushbutton it breaks the circuit from the PB to the Input & activates & latches the timer, then after your set time get the timer to energise another "Next Process Enable" relay and wire a second N/O contact of the PB through a N/O contact of that relay onto the Input.

Hope this helps.

Bob Peterson

If it is a true safety hazard, controlling it through anything other than a safety rated device would seem inappropriate.

Michael Griffin

In reply to Tom Chubb: I think you may have a race condition with that configuration. The relay may pick up and break the circuit before the control system scans the input for the first push button.