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I'm building something that I want to control from mcus, and while I'm fine with the MCU part, I'm in the dark about motors and want to avoid buying too many of the wrong ones while I figure it out. The requirements:

  • the motor will turn a wooden plank, not sure about the size yet, but probably on the order of a couple of feet wide, maybe up to four feet long, half inch thick. Imagine it like a miniature desk that can rotate to tip on it's side I guess. Stuff will be rolled on and off it.
  • need to be able to accurately position the plank in terms of time and degree position (for the rolling stuff)
  • it ought to be able to turn 90 degrees in as little as about 0.25 to 0.5sec, though if this is a pipe dream I could live with slower, say 1 to 2 seconds
  • sometimes things will be on this plank, or hitting the plank (balls and getting bounced or rolled off it). I don't know enough to know if this complicates things by adding variable torque requirements or something

I'm guessing this is not a cheap device, so if anyone has pointers on what I need for such a task, I'd love to hear them. Both in terms of type and model. thanks!

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    \$\begingroup\$ Let's see if I've got this straight. You have a 2ft x 4 ft x 1/2 in board, right? And the rotational axis runs crosswise parallel to the 2 ft edges but in the middle, right? And how much does the board weigh? \$\endgroup\$
    – WhatRoughBeast
    Commented Mar 1, 2016 at 1:16
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    \$\begingroup\$ That's going to take quite a motor. For starters, I'd be looking more at steppers used in CNC machning rather than the "small" ones found more commonly in consumer electronic devices. \$\endgroup\$
    – rdtsc
    Commented Mar 1, 2016 at 5:01
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    \$\begingroup\$ One of the specifications I usually see for stepper motors is torque. If you review your basic physics, and make an assumption about density of wood, you can probably figure out how much torque you need to move your board in 0.25 seconds. Note that the inertia will be very substantial, so you should be thinking in terms of fixed acceleration for roughly 45 degrees followed by fixed deceleration for roughly 45 degrees. You are not going to stop all at once (If you try, the rotor will continue moving for many steps beyond the intended stopping point). \$\endgroup\$
    – user57037
    Commented Mar 1, 2016 at 5:30
  • \$\begingroup\$ thanks everyone, I don't have specs yet as I'm brainstorming currently about what I could build, and what kind of object manipulation I could do once it's built. I'm a juggler with an interest in robotics and music, trying to find ways of combining them. So yeah, I don't know anything about the design yet! \$\endgroup\$
    – Iain Duncan
    Commented Mar 2, 2016 at 2:28

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I've asked about the exact geometry of the system and have not received an answer. So I'll make some assumptions. I'll assume the plank is 2 feet by 4 feet by 3/4 inch plywood. 1/2 inch strikes me as being pretty weak. I assume that the stepper is attached at the midpoint of one 4 foot edge, and the axis of rotation is parallel to the 2 foot edges. This plank will consist of 1/4 sheet of a standard 4 x 8 sheet of plywood. Assuming 40 lbs per sheet the piece will weigh 10 pounds.

As mkeith has pointed out, minimum rotation time for a given motor torque and rotation of 90 degrees corresponds to acceleration for .25 seconds followed by deceleration for .25 seconds, with the rotation reaching 45 degrees at the end of the acceleration. In other words, $$\theta = \frac{\alpha t^2}{2} $$ and $$\alpha = \frac{T}{I} $$ where T is the motor torque. For a thin rectangular slab with radius r from the axis, $$I = \frac{m r^2}{3} $$ Combining these, $$T = \frac{2\theta I}{t^2} = \frac{2\theta mr^2}{3 t^2} = \frac{2\times (\frac{\pi}{4}\times 10\times 2^2 )}{3\times (.25)^2} = 134 \text{ ft-lb} = 182\text{ Nm}$$

I'm not sure this is possible, see here, for instance for a 38 Nm motor but the motor will be large (NEMA 66 for the example) and require a lot of current. The example motor weighs 53 lb and draws 10 or 20 amps depending on model.

So I'd recommend a motor with some sort of gear reduction.

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  • \$\begingroup\$ and then there's things rolling on and off it too! This would be easier with a drawing/diagram of the problem. \$\endgroup\$
    – AlexanderBrevig
    Commented Mar 1, 2016 at 23:30
  • \$\begingroup\$ thanks, this gives me lots to think about and a lot of help so I'll accept it as correct even though I haven't spec'd out the problem properly. The issue here is that the whole apparatus is very much still at the brainstorming stage, so I'm trying to figure out if it's possible/feasible/practical with various sizes. It's for a juggling act, so the things will vary depending on what I figure out I can do! =) \$\endgroup\$
    – Iain Duncan
    Commented Mar 2, 2016 at 2:26

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