Turntable Traction Drive Safety and Liability

As it's intended use involves live actors on a rotating stage, the safety of the Turntable Traction Drive must be considered seriously. But before we dig into a risk assesment understand this:

I am not an engineer. I write software for a living and am not formally trained to assess safety risks. As such, any risk assesment made here should not be considered comprehensive.

In undertaking this project, and making it publicly available, it behooves me to draw attention to this fact, as well as to to state clearly that I don't guarantee that this design works as intended, and I am not liable for any harm resulting from your use of this design.

Liability

This design is available for use under the Creative Commons Attribution 4.0 International license. By using this design you agree to the terms of this license.

The license includes the following language, which you should read:

Section 5 -- Disclaimer of Warranties and Limitation of Liability.

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     EXTENT POSSIBLE, THE LICENSOR OFFERS THE LICENSED MATERIAL AS-IS
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In plain language: I don't guarantee that this design works as intended, and I am not liable for any harm resulting from your use of this design.

Limitations

The Turntable Traction Drive has the following known limitations/deficiencies:

• No Brake: The Turntable Traction Drive does not provide built-in braking. (This may be remedied by using a motor configuration with a power-off brake.) In practice this means that when the drive is not rotating the turntable is only kept in place by two things: The inertia of the turntable itself, and the friction in the chain and gearbox system of the Turntable Traction Drive. For massive (high-mass) turntables with substantial loss due to friction this is not a problem, as the turntable will tend to stay put. For low-mass or low-friction turntables it may be neccessary to use an external braking mechanism to keep the turntable from free-rotation when the drive is not rotating.
• Insufficient Current Limiting Capabilities: Ideally we would like to set a current limit at various stages of operation such that the inverter trips when there is an unexpected spike in current. The Hitachi L100-007NFU does not have sufficiently granular current-limit controls.
• Open Loop: The Turntable Traction Drive is a machine for rotating a turntable. It is not a machine for accurately "positioning" a turntable. As such this machine's utility in precise motion control applications may be limited.

Key Safety Principals

The Turntable Traction Drive is a powerful machine. Using it should not be taken lightly. Caution and care must be taken in every integration to ensure that safe operating parameters are configured.

A turntable may pose a hazard on set even when operated manually by backstage technicians, however, unlike backstage technicians this machine has no brain. It doesn't know to stop because a piece of scenery is late during a scene-shift. It is not context-aware. Also, unlike a backstage technician, under the right circumstances this machine can summon inhuman amounts of torque.

Due to its power and remoteness this machine is not designed as part of an automation system. Rather, it is intended to be manually operated by a user with a clear view of the turntable.

The Turntable Traction Drive has exposed moving parts and therefore poses some first-hand risk of injury to operators. However, when attached to a turntable new hazards may appear.

A scenic turntable creates risks of injury independant of the drive mechanism. Take care to evaluate the risks not just from the Turntable Traction Drive, but from the system as a whole.

Risk Management

As some risk is unavoidable, my analysis will focus on mitigation strategies. Any requirement layed out in this section must be specifically required in either the Integration Checklist or Operations Checklist.

Risk	Mitigation Strategy
Risk of electrical shock	Requirements: Ground VFD appropriately to input mains. Ground motor appropriately to VFD. Ground chassis appropriately to the motor chassis.
Pinching hazards from exposed rotating machine parts.	Requirement: Turntable Traction Drive must be in a fully enclosed secure area during operation.
Pinching/Crushing hazards between traction wheels and turntable edge.	Requirement: Interface between the turntable and the traction drive must be fully enclosed during operation.
Risk of unexpected start	Requirement: Enable Unattended Start Protection in VFD by setting c04 to 13 and short P24 to PIN 4 Disconnect power from the system prior to maintenance.
Risk of fall during normal operation	Requirements: Rehearse with onstage personel in order to find safe speeds and acceleration values. Do not deviate from these values without notice.
Risk of fall due to sudden acceleration	Requirement: Use conservative acceleration/deceleration values when configuring VFD. (For Hitachi L-100 see settings f01, f02, and f03)
Risk of fall due to overspeed	Requirement: Set a conservative hard frequency limit in your VFD configuration. (For Hitachi L-100 see settting a61)
Risk of crushing	Requirement Where possible tune-narrowly the current limiting features of your VFD. (For the Hitachi L-100 this configuration may be limited.)
Risk of crushing because of external scenery-based crush points.	Requirements: Turntables must maintain a raidal safety margin sufficient to prevent a person from being caught between a rotating piece of scenery and any adjacent piece of scenery. Emergency Stop button must located near the turntable. Cast and crew must be trained on the location of the Emergency Stop button.
Risk of entanglement	Requirements: Operator must have a clear view of the turntable. Emergency Stop button must located near the turntable. Cast and crew must be trained on the location of the Emergency Stop button.
Risk of slipping on a free-rotating turntable	Requirement A parking brake (or other disablement device) must be employed to prevent the turntable from turning when the Turntable Traction Drive is not in use. All on-stage personel must be instructed on the free-rotating-nature of the turntable.
Risk of operator/integrator ignorance	Requirements: All safety requirements and warnings must be clearly printed on a label applied to the machine. Documentation must be made available.
Risk of structural collapse on turntable	Requirement: Ensure that all scenery is constructed such that it can absorb tortional load such as might result from a sudden change in rotational speed.