Choice of an elictric motor for a "rotisserie"

[Lillo86]

Hello everyone. I am a young mechanical engineer at his first job and, not being supported by any tutor, I really need to get advice from some more experienced colleagues.

In particular, I would like to ask you for advice on choosing an electric motor. I should rotate a sort of "rotisserie" with an eccentric center of gravity with respect to the rotation axis (photo attached). The whole will weigh about 500 kg. The rotation can also take place slowly as long as the four desired positions with an increase of rotation of 90 ° are extremely accurate as the piece must be machined by a laser in all four positions that will follow. Consequently, the service will be of an intermittent type with running phases (rotation of 90 °) and stop which are repeated alternately.
Given the high precision required and the weight of the object, I thought of orienting myself on a brushless servo going to compensate for the low rotation speeds (which could annoy this type of motor) with the insertion of a gearbox.
A possible alternative could be an asynchronous motor with feedback control. It should be less expensive than a brushless and easier to implement at the control logic level. In this case, however, I am concerned by the fact that the torque to be supplied by the motor will not be constant but, since the center of gravity is eccentric with respect to the axis, it will vary according to the position of the latter.
Finally, there is the possibility of the stepper. Even in this case, however, one with closed-chain control would be needed to avoid the risk of losing the step. But I don't know if it could be suitable with the weights involved.
In short, as you can see, I am a bit confused about what to do and would really appreciate the opinion of some colleagues

 

[tygerdawg]

As in most cases of motion control, it all depends is the answer.

You make no mention of how fast the 90° rotation must occur. You need to work on that.
Whatever it is, it will determine the rotational acceleration required to start & stop the rotary mass.
The acceleration will determine the torque required.
The torque will specify the motor.
Ultimately you will need T = Jα.

If really slow, maybe you get away with a mechanical or pneumatic actuator that hits some sort of hard-stop for accuracy.

If speed not too critical, and your positioning will never change from 4x90° positions, and running trouble-free for a kazillion cycles is important to you, then you may consider a Commercial Off The Shelf cam indexer that is sized appropriately.

Fast rotation, dynamic positioning with variable programming capability will require some sort of motor, controller, and reduction system: gear head, timing belts, etc., programmable controller, properly-sized motor of some sort.
Reliable accuracy of dynamic motion will require position feedback.
Extremely accurate is not a useful description of positional tolerance required, so you need to work on that.
That positional tolerance spec will determine reduction system, feedback components, and programming.


TygerDawg
Blue Technik LLC
Virtuoso Robotics Engineering

http://www.bluetechnik.com

 

[Lillo86]

Thank you so much for your reply!

I didn't mention the rotational speed and acceleration because they aren't important for this specific application (or so I've been told). The rotation can also take place very slowly.
Regarding positional accuracy, the specification requires centesimal precision. To achieve such accuracy what reduction system and feedback components do you suggest?

 

[BrianE22]

Have you looked at a servo controlled rotary table typically used in CNC mills? Hass, Hardinge and many others make them. With the control you can probably get a new one for less than $15k.

 

[Lillo86]

Thanks for the reply!
It might be a good solution. Do you think it would save money compared to make a homemade product with the same features? $15k seems to be a lot of money

 

[mfgenggear]

years ago I looking at buying a headstock that could index at .0001 for a custom gear gear grinder. I was looking at equipment that Brian recommended.

 

[BrianE22]

For cost you would have to look at the motor and encoder, gears, housings, couplings, controller, cables and ???.

I'd look on Ebay myself for a used one.

 

[mfgenggear]

Brian
back then for that precision it was the whole works, 30K pops in my head. ultimately boss did not want to spend the money,

 

[rc0213]

I am starting to design a rotisserie for our company, also.

So, I'd like to know what you guys come up with.

We are making service truck bodies, so we don't have an easy CG to use to rotate about.

As for the motor, just remember that your assembly is on a rotisserie. And, with proper size bearings to rotate around with, you really just need enough force to start rotating the part. If you estimate that your part is 500kg, you just need know how much force is need to get it started. Just think of it like this. Remember those shows where a person uses their teeth to pull a big truck? HOW can they do it?! It's not that complicated. The truck is on wheels, just like the rotisserie is basically on wheels. So, the person just needs to pull the truck with just enough force to get it started rolling. Once you get it started rolling, the momentum helps you. It's the stopping that now becomes the issue.

Once you get it started, you don't want it to go fast, with that kind of weight, so I've heard some companies use a gear to control the rotation. Because with the gear you do two things, use the ratio to use less force to start and stop the rotation. So, let's say you use a 10:1 gear, you might only need a servo with, as an example, 100 lb force to move your part.

This is about as far as I got in developing a rotisserie for our company.

I hope that some of this info helps.

And, if there is anyone who has a CAD design of a rotisseries they would like to share with me, I'd appreciate it!

 

[Jessica Walliams]

A 240v rotisserie motor is recommended for most rotisserie users who will be having a spit roast at home or where 240v power mains are available. The weight capacity varies widely with 240 volt motors able to handle anywhere from 20 to 100kg depending on the make and model.