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Old June 26th, 2017, 01:06 PM   #1
theColonel26
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Muting a light curtain when machine is home

We have a request to mute a light curtain when the machine is in it's home position. So that air is not dumped from the lines while the operator loads parts.

We have 4 pneumatic cylinders with a prox at their home position.

Our customers general machine specifications states,

Quote:
MUTING
1. Muting is defined by ANSI B11.1-1988: Muting (bypassing of the protective function) of the device
shall be permitted after the hazardous portion of the point of operation (or press stroke) is clear of any
dangerous motion.
2. Muting the light curtain system requires that the motion be complete in order for the associate to enter
the point of operation.
3. The muting relay shall be of the captive type and accomplished in such a way that no single
component failure shall prevent the normal stop command, but shall prevent subsequent cycles or
press strokes until the failure is corrected. Refer to APPENDIX C: SAMPLE ELECTRICAL
DRAWINGS for interface muting examples. Must meet ANSI B11.1-1988 Standards
4. Where muting is required for associate loading/unloading of the work pieces, there shall be a
hardwired relay and a PLC output relay for redundancy to meet ANSI B11.1-1988, 6.3.2 (3).
Quote:
REDUNDANCY
No Single point failure circuits shall be permitted within a safety circuit as defined above. Redundancy is
required, so that if any single component were to fail, there would be a second (redundant) component to
apply stopping action. The requirements of “control reliability” must be implemented to a safe switching point.
The safe switching point is typically the machine control interface panel. Faults such as short circuits across
the output of the safety device and open circuits between the safety device and the machine control must not
prevent stopping action from being applied, but must be detected and all further machine cycles prevented.
Safe failures are failures that are indicated by the machine not operating. Any interface to the presence
sensing device requiring additional relays shall be of the captive type, see APPENDIX A: STANDARD
COMPONENTS Section A.1.1 EQUIPMENT FOR POWER, ELECTRICAL SOURCE CIRCUITS for part
number and manufacturer.
I would think this would mean I would need to have 2 prox switches for ever home position on each cylinder for a total of 8. correct?

See my schematic for what I have so far.

ETA:

Yes the light curtain Controller has a built in muting function with 2 muting inputs
Attached Images
File Type: jpg Muting.jpg (33.1 KB, 69 views)

Last edited by theColonel26; June 26th, 2017 at 01:48 PM.
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Old June 26th, 2017, 01:25 PM   #2
CapinWinky
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You could get away with program redundancy. The PLC generating a muting signal when the program thinks it is safe and also use the home proxes to release that signal. That way the program and a sensor would both need to fail.
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Old June 26th, 2017, 01:35 PM   #3
dmargineau
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You will have to implement muting within the Safety circuitry, where, I presume, the Light Curtains are being used as to interdict any machine operation if they are not clear.

Moreover, the Light Curtains themselves will need to support the 'muting' feature; most major manufacturers provide at least one model which could be muted if and only safety circuitry connected sensing devices are being 'triggered' and 'released' in a specific sequence.

In your case, most likely, you will need to use two pairs of photoeyes which will need to be 'masked' and 'unmasked' in a sequence that the loading operator will need to follow in order for the safety circuitry to remain functional (Reset) when the machine is being fed parts.

The safety redundancy will be implicitly implemented by a properly installed and configured Safety System.
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