What If We Don't Maintain The 1s And 0s?

What If We Don't Maintain The 1s And 0s?

CX 91 Staging What If

Most of us are well aware of the need to maintain the nuts and bolts (and ropes and shackles and pulleys) of a piece of machinery so that it keeps working reliably, day in and day out, but what about the less visible parts of the system that are just as essential for the continuing safe operation of stage machinery?

It’s fairly self evident that gearboxes and rotating shafts need to be cleaned and lubricated, that moving parts need to adjusted, checked and replaced as they wear (and wear out), and the riggers amongst us will be well versed in the practice of regularly checking and servicing ropes, shackles and slings.

It would seem though that when it comes to testing, checking, verifying and servicing Stage Machinery, that Control Systems are often neglected, or they get at best, lip service.

We will occasionally clean the dust filters and (as many manuals tell us) we will “clean the screen with a damp soft cloth”, but how systematically and diligently do we check that the control system for our stage machine is working as it should be, and, most importantly, that the Safety Related Control Functions (SRCFs – don’t you just love acronym-speak?) are still set up correctly, programmed correctly and vigilantly standing-by ready to do their Safety Related thing when needed?

The degree of “onerous” that such routine testing requires depends largely on the complexity of the control system itself, but the mental approach taken to the testing doesn’t really change, whether you’re checking a motorised curtain track or a multi user, high precision computer controlled power flying system.

In the “control system” for a simple motorised curtain track (a fixed speed motorised track that uses Open/Close pushbuttons to run a curtain to positions set by limit switches) you would check that “open” and “close” are operating in the correct direction, that the curtain stops at its limits and that movement stops when you release the open/close button (in a dead man controller) or press “Stop” (in a latching controller). You’d also check that pressing any E-Stop control causes the curtain to stop immediately.

The same logic applies equally to complex multi axis computerised control systems - it’s just that the number of combinations and permutations of direction, speed and position is higher.

Many of these systems use software positions for operating limit positions, with hard switches only used for emergency limits (when the software limits don’t operate as expected). In these cases, the only time we know that the hard limit switch is working correctly is when something goes wrong with the software positioning – this could mean that the hard limit switches are not operated for many years at a time if we don’t test them periodically.

Once you have “soft” positioning available you can program a system to help avoid collisions by having the soft limits of a machine change, depending on the relative positions of other machines. While this is a great development, it can lead to operator complacency, with “magic” control systems replacing good old fashioned “pay attention” as the preferred method of ensuring safety.

We need to make sure that the “magic” system is put through its paces on a regular basis to give us a high degree of confidence that it will perform as intended when it is really needed.

This brings us back, as it so often does, to the basics of Risk Assessment. Assuming that a (magic) control function was put into the control system to help mitigate an identified hazard, we need to verify (validate) that the control system is still doing this particular part of its job properly.

Identify – Mitigate – Validate.


Jands Staging
provide articles to the "Staging What If" section in CX Magazine. If you have any questions regarding this article then please comment below or email info@jands.com.au.

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