OT - Safety Relay - Relay Logic Question

corkers

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Jan 2004
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OK here we go

Client received a new 3 colour printing press from India. Suffice to say it needs some work on the control side of things... Estops work in control voltage only – machine restart when Estop released, no earths (grounding) anywhere the list goes on... Its completely relay logic no PLC (just to be clear)

The client is having guarding built and has given me the following design brief

Estops as per Australian Standards (close enough to IEC standards for the purpose of this) let’s say cat3 for the sake of the argument.

Guarding to have appropriate solenoid operated latches, coded micro switches as deemed appropriate.

Machine not to operate in the event of estop activation (obviously?!?!?) – achieved by dual safety contactors in series with the supply – the only thing left operating will be the supply to safety relay and indicator lights.

Machine not to operate in the event of guarding removal – except in jog mode with the use of a safety pendant control – achieved by dual safety contactors in series with the supply – the only thing left operating will be the supply to safety relay and indicator lights as well as to the system for operating in jog mode see below

Complications

Variable Speed Drive (control Techniques) controls drive motor of press – I don’t want to trust this to give a low speed only so I was thinking of simply pulsing a DOL starter i.e. contactor only energises for a very brief time 0.5 sec (for example).

To achieve this I envisage I would need to remove to VSD from the circuit with a bypass contactor scenario. I would use the removal of the pendant from a tongue interlock hold to control this switching.

How can I safely achieve this low speed jog how can i get the coil to energise for a brief time only? I wouldn’t be comfortable using a normal timer for obvious reasons.

By my count Ill end up with 1 safety relay for the estops, 1 for the guarding, 1 for the enable switch and associated contactors etc. is this over the top. at this stage AB or telemecanique gear.


Am I coming at this from the wrong angle?
 
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Why dont you want to trust the VSD? Use a logic input from the pendant to the drive and use the VSDs jog function on that input.
What brand of Vsd is it? The E-stop will shut off control power in any case so the pendant wont work while the E-stop is in.
 
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What you need is a Pilz PNOZ programmeable safety relay with the additional speed module option. The one safety relay can control the 2 safety contactors and if it sees over speed (detected by sfety rated sensors) it will drop the safety contactors. ie still use the VFD for speed control.
If you need more info, please ask.
Regards Alan Case
 
Like Alan Case, we also use the Pilz PNOZ m1p Programmable safety relay. They work quite well. (as does the Pilz PSS)

As for the VSD, some of them have built in Cat3 safety inputs. Control Techniques, while I don't use them, are generally well up there with advanced VSD technology, so it may have it. If not, it may be cheaper (as in cost of design and commissioning man hours is less than hardware) to use an SEW drive with the Cat 3 safety inputs.
 

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