Using sensor output via relay to trigger a solenoid

[connell.richard]

A little bit more complicated than the title - here are the complications.

The System involves a cylinder extending a metal target holding process parts into a hot zone for a defined period of time. This movement is triggered by 2 hand control system which is monitored by Safety relays until the cylinder stroke is completed backwards. It is then retracted to the operator zone. A solenoid then "blasts" the hot part with a cooling cycle to allow an operator to handle it safely. The Hot and Cold cycle lengths and automated cycle are controlled by a PLC but sometimes the operator takes their hands off early and the PLC program kills the power to the air blowing heater modules but before they can react it is possible that the process part is very hot and the system may not run the cooling cycle as it resets itself. - We are in a Medical device regulated industry so a simple SW update to the PLC ladder logic is not possible - I need to implement a Hardware driven solution to force a cooling cycle in parallel to the automated one.

Sensor to detect the cylinder retracted position is already wired to a PLC input. I want to take this signal and use it to trigger a stand alone timer, which will generate a pulse whose length will be defined by the timer pots - say 10seconds for now.

I want the cooling cycle to come on if either the normal automode signal from the PLC OR the timed pulse from in previous paragraph nut I want this to be external to the PLC and use DIN rail mountable components.

I have an idea but I want some confirmation to ease my mind. I want to safely tie 2 independent outputs together to fire a single input using discrete Din rail relays.

 

[connell.richard]

Here is a little bit more of my idea.

Black is original and RED are my add ons

R1 and R2 and T1 are added.

I can swap over the 2 signals to give priority to the current auto signal as this comes on first theoretically - If it does not come on because of fault then the second one will kick in and trigger the cooling cycle.

 

[Harald Kapp]

We are in a Medical device regulated industry so a simple SW update to the PLC ladder logic is not possible

I wonder whether a change (upgrade) to the existing hardware is allowed under these regulations?

Anyway: Something like one of these safety devices with monoflop function might be what you are looking for. The monoflop could be triggered by the same sensor that detects the cylinder position.

 

[Tha fios agaibh]

Connell.richard:
Nice, but it doesn't look quite right. The Timer output goes to a NO contact of R1 so it'll do nothing. Even if you change R1 to NC contacts, it will switch on R2 but the solenoid relay won't energize. You'd need separate contacts on R2 to power the sol relay during your fault cycle.

 

[Tha fios agaibh]

Something as simple as this might work.

 

[connell.richard]

Ta fios agaibh -

Would this work - Only 2 scenarios for the cooling to come on -

Normal Auto Mode - PLC O/P A goes ON, energizes A1/A2 on R1 - CAUSES 24VDS on NO CONTACT to be sent to the coil on K6 and trigger the cooling cycle
Recovery Mode - PLC O/P A Does NOT go ON but PLC O/P B does go on, this triggers a 10sec timer output pulse which energizes A1/A2 on Relay R2 which passes 24VDC through its NO contact to the NC contact on R1, this will then be passed through R1 as A1/A2 on R1 is not energized because PLC O/P A is off.

Does this make sense

 

[connell.richard]

I wonder whether a change (upgrade) to the existing hardware is allowed under these regulations?

Anyway: Something like one of these safety devices with monoflop function might be what you are looking for. The monoflop could be triggered by the same sensor that detects the cylinder position.

Thanks Harald - The SW update drives a SW validation requirement which would prove to be cumbersome. The HW change will be documented and tested and the paperwork is less painful then the SW alternative. I will look into the monoflop as an option as well (EHS might not like to see a Safety relay mentioned though as it drives its own connotations.

 

[Tha fios agaibh]

connell.richard,
Yes, that should work but you may be able to eliminate one or even both relays if the timer has relay contacts build into it.

You can use the NC contact of the existing K6 relay to directly send 120v via timer contacts itself, or the external relay associated with it.
I suggest adding a varistor (mov) across the solenoid coil to mitigate inductive spikes due to back emf in order to save relay contacts.