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Miniature stepper motor

11/02/2021 7:37 AM

I need to rotate a gear in steps of 120 degrees. The torque is much less around 8N.mm and motor size shouldn't be more than 18mm in diameter. I am looking for cheaper options. Is stepper motor the only option? Any product links??

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#1

Re: Miniature stepper motor

11/02/2021 8:31 AM

Perhaps a drone motor.

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#2

Re: Miniature stepper motor

11/02/2021 8:40 AM
  • A solenoid and a ratchet would work.
  • A pneumatic cylinder and a ratchet would work.
  • A lever and a ratchet would work.
  • A handle with three dog ports <...120 degrees...> apart from each other would work.
  • Three switches at <...120 degrees...>, a handful of relays and an ordinary motor would work.

There are dozens of other possibilities.

<...cheaper options...>

If enough Meccano is to hand then why would the matter of expenditure arise?

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#3

Re: Miniature stepper motor

11/02/2021 10:25 AM

You should be able to do this with a rotating permanent magnet and three small electromagnets.

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#4

Re: Miniature stepper motor

11/02/2021 10:27 AM

A stepper motor has the advantage that it can be operated in open-loop mode. In other words, no added mechanism and/or circuitry is needed to identify where anything resides. The disadvantage of using open-loop mode is the controller does not know if things moved properly.

A simple DC motor with a Geneva drive could produce these 120° incremental steps. If the motor must stop between steps then a simple set of detecting switches can detect the motor cam position.

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#8
In reply to #4

Re: Miniature stepper motor

11/03/2021 8:11 AM

120o is that maximum single step angle that can be delivered by a Geneva mechanism. I tried to find a supplier but failed. Check out the web site of "europeana.eu" and search on it for a "Three-slot inverse geneva wheel mechanism". The site gives all the math formula needed to design and build your own including how to size a motor, the running to stopped ratio, the accelerations, ect. There are several styles of geneva that all achieve the same motion but the inverse is the simplest to machine and assemble, so should be the cheapest. Making three segments and fixing them to a back plate will be easier to make than machining the slots but if you have access to a CNC mill there won't be much cost difference. 8N mm is about 3 oz inch so making in a self lubricating plastic like nylon 6 is a low cost option.

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#9
In reply to #8

Re: Miniature stepper motor

11/05/2021 4:21 AM

I liked this 120° design.

Since the desired torque is so small, this might be something to 3-D print in plastic.

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#11
In reply to #4

Re: Miniature stepper motor

01/23/2022 3:44 AM

can you provide some links to detecting switches? are they cheap?

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#12
In reply to #11

Re: Miniature stepper motor

01/23/2022 8:10 AM

Depending on the environment, longevity, mounting options, and other aspects of the mechanical and electrical design, limit switches or switches, in general, can be expensive or cheap. Deciding on how much to spend on what is one of the most fundamental engineering questions.

I would expect the geneva drive mechanism parts or the electric motor will be your major expense but so many critical details are unknown to me.

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#5

Re: Miniature stepper motor

11/02/2021 12:06 PM
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#6
In reply to #5

Re: Miniature stepper motor

11/02/2021 12:57 PM
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#7

Re: Miniature stepper motor

11/02/2021 11:00 PM

I don't have any product links, retired 20 years ago. What your question brought to mind was a brushless 3 phase dc motor with permanent magnet rotor. Experimented with a large version and depending on which coil was energized with DC it would rotate the required 120 degrees. Simple 3 bit counter could be driver source. Stop clocking counter if hold in position required, allow to run for continuous rotation. Some what like a stepper but a lot in common with 3 phase motor.

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#10

Re: Miniature stepper motor

11/06/2021 5:22 PM

I once used some synchro-resolvers, which I just happened to have in my junk box, for stepper motors. There are 3 field coils and one rotor coil, and energizing the rotor coil and each of the three field coils in sequence allowed it to step 120 degrees.

It's not what it is designed for, but it got the job done.

https://www.analog.com/media/en/training-seminars/design-handbooks/synchro-resolver-conversion/Chapter1.pdf

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