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I am trying to understand the kinematics of a machine that uses a parallel indexing cam. As you can see, this mechanism has a dwell phase where there is minimal load on the cam, and a motion phase where the follower is quickly accelerated and decelerated to an indexed position by the cam. The machine is driven by a small 12V hobby motor. I've plotted the angular velocity of the cam and follower relative to the angle of the cam based on experimental results. The blue plot is the cam velocity and the red plot is the follower velocity.

enter image description here

As you can see here, the RPM of the motor drops as the follower is accelerated. From what I understand, the RPM is linearly proportional to the load, so this makes sense. What I don't understand is the second drop in the deceleration phase of the the motion. At this point, there is a torque exerted on the motor in the direction of rotation. This is where my limited understanding of DC motors fails me. Would it ever make sense for the RPM of the motor to drop in this case?

One theory I have here is that during this rapid deceleration, the pressure exerted by the bearings on the cam increases, which increases friction in the system. Is this a reasonable explanation, or could something else be going on here?

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    \$\begingroup\$ It is possible that your index shaft/assembly is not perfectly balanced. This can cause an imbalance which could result in the shaft wanting to return to a lower gravitational potential. Alternatively, it could be the bearings you're using not rolling smoothly and bumping as if it wants to "click" into a new position. Since this is more of a Dynamics question than an electrical one, I suggest asking on the physics or mechanical stack exchange as opposed to here. \$\endgroup\$
    – Pxl
    Nov 23, 2023 at 19:52
  • \$\begingroup\$ Yeah, I was planning to go there next. Just wanted to rule out anything that might be going on with the motor itself. I think I should be able to test your theory by remounting the cam on the motor shaft at a different angle. Thanks \$\endgroup\$ Nov 23, 2023 at 20:17

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It looks like it's a friction thing. Especially if the follower does not have a bearing and is just two parts sliding against each other. If that's the case, lubrication may have a significant effect.

Keep in mind that the roughly inverse linear relationship of RPM to torque of a DC motor (between no-load or zero torque RPM and zero RPM stall torque) is only true in the steady state. Inertia is a factor in all the parts. You can see that when it has accelerated the follower the motor begins to recover RPM (but does not get there in the available time).

In steady state, if the no-load RPM is n0 and the stall torque is Mh then the RPM with a torque M is just n0(1 - M/Mh). Image from here

enter image description here

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  • \$\begingroup\$ Thanks, this helps. My take away here is that I can’t necessarily draw a vertical line on my chart and look for exactly what might be causing the issue in the cam because the relationship isn’t exact and immediate. After reading your post, one thing I realized is the apparent impulse at the peak velocity of the follower. I’m wondering if that may be causing a delayed response in the motor. Anyways, you’ve given me something to consider. I’ll go ahead and mark this as the answer. \$\endgroup\$ Nov 24, 2023 at 13:39

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