plc question

shakerbess · Nov 15, 2014

hi I was hoping to get some help with some college work the question I am on is as follows:

A machine starter motor is only allowed to run if a start signal has been received and emergency stop signal has not been received. Also there must be a signal detecting that the machines safety guard is closed and a safety feature in front of the machines working area has not detected any human presence

I was hoping to get some help with how this would be setup and the equipment used

I hope some one the time to point me in the right direction thankyou

Darkwood · Nov 15, 2014

Too many variables, each machine has a different risk assessment and criteria for safety, you plc is a second level control usually and your safety relay is primary... using an E-stop input to a plc is normally a secondary signal and not part of the safety system.

In a control system you normally have a primary safety system, then a secondary control system, your E-Stop system (primary) is usually independent but some more expensive set-ups can be used with E-stop signals directly safely shutting down control equipment like motor VSD's etc but these are more expensive normally to fit .

I don't think your question even warrants a plc with the info given, just a basic safety circuit but with no info I cant tell you anymore.

Knobhead · Nov 15, 2014

One of the trick questions with PLC’s is the inputs for stops. I think this is what they may be asking for. Otherwise as DW said a PLC would be a total waste of resources.

Without giving you the answer, you have to think the opposite of a normal stop circuit.

shakerbess · Nov 15, 2014

Thankyou for all reply so kind

I was hoping to keep it simple lol for my novice level
I was hoping for this plc what input devices to be used

Knobhead · Nov 15, 2014

What switching devices? There’s far too many to choose from to give a definitive answer.

Have a rummage through the RS catalogue for each of the devices you mentioned. It will have data sheets giving the connections needed.

Robotstar5 · Nov 15, 2014

If it's simply a PLC exercise with no real world application you just have 4 contacts (switches) in series operating an output.
Inputs would be, start button, E stop, guard switch and presence detector, use N/C contacts for the last 3 so they would fail safe and N/O contact for the start button, the output operates the motor.
That's as simply as it get's for a classroom exercise, but as stated above it would be a LOT more complex in a real application.

Knobhead · Nov 15, 2014

Robotstar5 said:
If it's simply a PLC exercise with no real world application you just have 4 contacts (switches) in series operating an output.
Inputs would be, start button, E stop, guard switch and presence detector, use N/C contacts for the last 3 so they would fail safe and N/O contact for the start button, the output operates the motor.
That's as simply as it get's for a classroom exercise, but as stated above it would be a LOT more complex in a real application.

This is why I mentioned the “standard trick question”.