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Old June 25th, 2019, 01:16 PM   #1
Ken Roach
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OT: Non-electronic cylinder position sensing ?

I have an application where it would be ideal to simplify a pneumatic cylinder that extends and retracts a locking pin mechanism on a movable platform that does not (easily) have its own power supply, because it's moved very infrequently.

The operator always hand-operates the lock and unlock valve. But the mechanism itself is not visible to her and we need a way to indicate that the cylinder has actually reached the full extent of its travel and has not jammed or been blocked. There are two sets of locks, about 50 feet from one another, and the operator is closest to one of them and can't see the "far side" unit.

Hydraulic master/slave are disfavored because of the environment it's in.

All I came up with was a push-pull operation cable with a flag at the operator's console that literally shows the position of the cylinder mechanism. Because she will be operating the valve during the transition, she will be able to verify that it changes positions, so a broken cable will fail to indicate and allow her to investigate the mechanism.

Am I overlooking some other fundamental principle I could use ?
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Old June 25th, 2019, 01:29 PM   #2
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Have the cylinder make one of these; https://www.quinetic.co.uk/
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Old June 25th, 2019, 01:37 PM   #3
dmargineau
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Set of pressure gauges for the two cylinder's piston sides?

In normal operation, once the actuator is fully shifted one gauge should read full system pressure and the other one '0'
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Old June 25th, 2019, 01:40 PM   #4
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If it got stuck at 50% stroke it would still show full pressure... you would need to know the volume somehow. On the same train of thought though you could have a pneumatic limit switch that the cylinder makes back to a gauge.
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Old June 25th, 2019, 01:46 PM   #5
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Is this a some sort of moving cart? if you have pneumatics on the cart why not do it all in pneumatic, with pneumatic limit switches and signal popup indicators

Guessing that the locking pins are when they are loading and unloading the cart, they need to be locked in so the cart does not move and lose the load?

EDIT: something like this https://www.smcpneumatics.com/VR3100-F01G.html using the air and limit switches they can see with the cylinder is in the retracted position
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Old June 25th, 2019, 01:51 PM   #6
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Run a mechanical push-pull mimic cable back to a position indicator visible to the operator.

https://www.cccables.com/

I googled "build to length push pull control cable"




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Old June 25th, 2019, 02:52 PM   #7
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You need to visually indicate that the cylinder has reached its final position? And the indicator must be visible from a distance.

I am thinking about the mechanism in many safety locks where you have a key that actuates a tumbler.
But instead of activating electrical contacts it could activate a flag on an arm.
That would be quite safe if it works the same way as the safety key and tumbler.
So you would have to copy the way that works, but it would not be too difficult.
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Old June 26th, 2019, 10:00 AM   #8
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Is there no power supply in the vicinity at all? Is this on mobile equipment, or for some other reason can't run a two wire cable from the operator console to a prox switch on the far end? If you can, why not just get 120V from the lighting circuit, run through a 2 wire prox, back to an indicator light?
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Old June 30th, 2019, 11:49 AM   #9
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make a pin you can see with a flag or a bowden cable to the operator.
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Old June 30th, 2019, 06:49 PM   #10
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Use a WiFi security camera and display if you can. Walmart?
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Old July 1st, 2019, 01:35 PM   #11
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Thanks for all the input, guys !

It's a platform that will be lifted by a roof-mounted hoist. It will be moved maybe once a day for a couple of months, then disassembled and removed, then re-assembled a few months later.

The problem is that during a lift, there's no electrical power on the platform. Using an electrical sensor would mean a battery box on the platform with a wireless link to the control system and indicators on the operator's wireless pendant (which is necessarily a *different* wireless system).

Wireless is a big challenge in this environment, both for mechanical/electrical reasons and because of extremely strict regulatory and approval reasons.

I didn't realize that "Bowden cable" was the common name for a push-pull control cable; thanks !

I made my presentation to the customer this morning, emphasizing that a mechanical flag was the most reliable and simple and that every actuation was also a functional verification test.

They said "we'll think about it".
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Old July 1st, 2019, 01:59 PM   #12
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They make pneumatic limit switches. Use a couple to sound two different sounding whistles.
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Old July 1st, 2019, 03:27 PM   #13
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I would look at using pneumatic limit switches and visual indicators like this:

https://www.automationdirect.com/adc...icators/pmu14a

If you'd like to get really fancy, you can use pneumatically piloted valves to provide some logic to your device.
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