Stopping time for a cycloidial dial table

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Discussion Overview

The discussion revolves around the stopping time of a cam-driven dial table indexer, specifically focusing on the implications of its sinusoidal move profile and the associated risks in a manufacturing context. Participants explore the mechanics of the system, the calculations involved in determining stop times, and the potential risks to both operators and equipment.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant notes that the manufacturer claims the indexer will stop in 0.86 seconds, suggesting that stop times may vary depending on the angle of the dial, with maximum times at 90 and 270 degrees.
  • Another participant questions the mathematical approach of using SIN(r)*θ, suggesting instead that SIN(θ)*r would be more dimensionally appropriate.
  • A participant raises concerns about the nature of the risks involved, asking whether they pertain to potential damage to workpieces or operator safety.
  • It is mentioned that the 0.86 seconds may refer to the time taken to disable power to the motor and engage the brake, with additional considerations regarding mechanical friction if no brake is present.
  • One participant explains that the sinusoidal move profile is common in manufacturing, providing smooth acceleration and deceleration, and confirms that the 0.86 seconds likely represents the time from maximum velocity.
  • Another participant elaborates on the various factors contributing to the stopping time, including system response times and regenerative braking during normal operation.
  • A later reply expresses curiosity about the differences in response times between contactors with integral versus external diodes, speculating on the effects of inductance and field collapse.

Areas of Agreement / Disagreement

Participants express various viewpoints on the mechanics and risks associated with the indexer, with no clear consensus on the mathematical approach or the implications of the stopping time. Multiple competing views remain regarding the nature of the risks and the calculations involved.

Contextual Notes

There are unresolved assumptions regarding the specific definitions of stopping time and the conditions under which it is measured. The discussion also highlights potential dependencies on the mechanical design and operational parameters of the indexer.

Jonesy
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I am working on a Risk Assessment for a dial table, the manufacturer states that the indexer will stop in 0.86 sec, worst case. Since the indexer is cam driven the speed varies throughout its motion. That being said I expect stop times to be highest at 90 and 270deg, while lowest at 0 and 180deg. My thinking is to multiply the SIN(r)*θ. Am I on the right path, or off on a tangent.
 
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Welcome to PF.

Jonesy said:
That being said I expect stop times to be highest at 90 and 270deg, while lowest at 0 and 180deg.
Please give a diagram of the cam mechanism. What is the reference direction relative to the cam?

Jonesy said:
My thinking is to multiply the SIN(r)*θ
You cannot take the Sin(r) of a dimensioned radius, only of a dimensionless angle. Sin(θ)*r would make more dimensional sense.
 
Jonesy said:
I am working on a Risk Assessment for a dial table, the manufacturer states that the indexer will stop in 0.86 sec, worst case. Since the indexer is cam driven the speed varies throughout its motion. That being said I expect stop times to be highest at 90 and 270deg, while lowest at 0 and 180deg. My thinking is to multiply the SIN(r)*θ. Am I on the right path, or off on a tangent.

What kind of risks? Risks that the workpiece will be ruined for some reason, or risks that the operator can be hurt?

When you say the indexer will stop in 0.86s worst case, that's 0.86s from what?
 
It sounds like the manufacturer quoted you the time to disable power to the driving motor and engage the motor brake. If no brake, then stoppage occurs due to cam motion friction between the mechanical components.

If they did their engineering sizing calculations properly, the motor is matched to the specified rotating payload mass & inertia on the dial table. So it follows that the brake is properly matched also. On ESTOP, cut power to motor and clamp on the brake...rotation stops in 0.86 seconds. Seems reasonable. But the cost to do that is likely damage to the mechanical components if the rotational mass payload is spinning at max speed.
 
Thank you all for prompt reply.

Baluncore,
The move profile is sinusoidal, this type of indexer is common in the manufacturing industry, it give a nice slow start/stop at the pitch, this system has two positions. Thanks for correcting my math.

Berkeman.

Risks in my business is prioritized to people, tooling, part. I am assuming the .86 represents from max velocity, since the profile is sinusoidal, ithink the velocity varies throughout the index cycle, at this point I assume at 90 deg intervals.

Tigerdawg.

The stop time is based on power disconnection from the motor driving the thing, the time to disable is the system response time. This includes the actuation of the stop signal, fieldbus (Ethernet) transport time, processor input update time, logic execution time, I/O faults within the system, output update time, and contactor opening time (power removal). Under normal stopping conditions the drive (VFD) supplies regenerative braking, when the dial is in dwell. Under the operator protection scenario, the contactor rules.

I am reasonably sure the calculations are correct, this is a common system, and the technology has been around for a long time. Braking is common, and works just like the brakes on your car, no damage unless you hit something.

All,

I am using worst case .86 seconds for stop time, but my intellectual curiosity always overpower me and I have to know more. Interestingly enough I find the response time of a contactor with an integral diode, is faster that one with an external diode. The diodes are used to 'snub' counter EMF. Does anyone have an explanation? Maybe some additional inductance, or slower field collapse?