Gripper sensor

[PeeWee]

Hi all,

I work in a nutritional powder factory and we have an automated bag filling machine that takes the bag from a stack moves it down a conveyor where it is then picked up and placed in two grippers that open the bag ready for the powder to be dispatched into it.
My question is this.....there are two plunger type switches,one on each gripper that are then wired to the plc.
Occasionally the plunger type switch will stick due to the nature of the powder and if the bag is not properly held in the gripper the powder will still be despatched because the plc thinks it is in place due to the switches being made.
How can I rectify this issue by fitting an added safety feature?
We have tried changing the switches but the problem still persists!
Thanks in advance.

 

[Yellowhammer]

Material dependent could you use an inductive sensor rather than a plunger to detect the gripper closed state?

Or less fail safe, twin plungers with the PLC set to raise a flag if the logic state of each differs. This would just be an alert rather than solving the issue, and not failsafe as both plungers may stick!

 

[James]

Try these people.

KEYENCE (UK) Ltd.

Sensors, machine vision systems, measuring instruments, barcode readers, PLCs and other factory automation sensor products. KEYENCE UK & Ireland.

www.keyence.co.uk

 

[PeeWee]

Thanks for the replies guys!
I did think of an inductive sensor but due to the nature of the mechanism and it's movement I don't think it would work.
The double plunger on each side though could potentially work if there is room unless you can buy a unit with a double plunger?
I'll email Keyence to see if they've come across this issue before and have any product to remedy it?
Thanks again

 

[PeeWee]

Pics of switch and circuit

 

[plugsandsparks]

Is the issue, that the plunger switch does not return to the off state because it is stuck ?

If so there are a couple of ways to solve this, either by the PLC detecting the off-state (i.e. because it is not in the on-state), this can be done a number of ways depending on plc type and programmer preference but essentially the off-state resets a virtual output which needs to be made prior to a signal to close the jaws again.

Or a dual switch to detect both clamped state and un-clamped state which can be used by the plc to ensure before the jaws are clamped that the un-clamp state is detected.

Or use a different detector as previously mentioned to detect the state of the jaws.

 

[PeeWee]

Is the issue, that the plunger switch does not return to the off state because it is stuck ?

If so there are a couple of ways to solve this, either by the PLC detecting the off-state (i.e. because it is not in the on-state), this can be done a number of ways depending on plc type and programmer preference but essentially the off-state resets a virtual output which needs to be made prior to a signal to close the jaws again.

Or a dual switch to detect both clamped state and un-clamped state which can be used by the plc to ensure before the jaws are clamped that the un-clamp state is detected.

Or use a different detector as previously mentioned to detect the state of the jaws.

Yes that's right,the plunger switch remains in the on position.
I think the first option you mentioned may be the best as it will be a programming upgrade on the plc that will take away the ineffective mechanical set up that's constantly exposed to the sticky powder.
I have limited knowledge on PLC's so will have to seek advice from a third party.
Thanks for your advice and suggestions,much appreciated

 

[James]

What make is the plc?

 

[baldelectrician]

Just thinking about this simply would be to wire both microswitches in series then any one opened would cause an open circuit.
The open switches could be used (as others have suggested) to prevent restarting of the process

 

[PeeWee]

What make is the plc?

Hi James,plc is a Siemens ET200S.

 

[PeeWee]

I think maybe it can be done with an arrangement of relays but need to think about the wiring ?