1 rotation per 12 hours high torque

Hello all,

I am looking for some general advice on the approach/feasibility of my project.

I am working on a bespoke clock with a single hand that rotates continuously, making one rotation every 12 hours. The hand will be 500mm long (from centre of rotation to tip) and counter balanced, and weigh about 1.4kg. In contrast to your average clock hand, this clock hand will be pulling something along the vertical clock face as it rotates, so there will be more friction than usual.

I was originally looking into the option of a geared ac synchronous motor, but they seem to be very big and expensive. My current thinking is that controlling the speed of a stepper motor with arduino would be more logical than gearing down an AC motor.

I have been looking into the stepper motor/arduino combination but am unsure if:

  • They are suited for continuous operation for a clock, and would have a long life span (10 years?)
  • The movement of the steps will be visible, and if so if there is any way to make the movement appear more continuous.
  • Whether they can cope with the continuous friction and the load of the arm

Any suggestions/pointers are much appreciated.

Thank you for looking.

Steppers are ideal for precision position control at low speeds.

  • They are suited for continuous operation for a clock, and would have a long life span (10 years?)

I would say yes. A quality motor well chosen for the task should last.

  • The movement of the steps will be visible, and if so if there is any way to make the movement appear more continuous.

a modern stepper driven with a modern driver can be "microstepped", A 200 pulse per revolution motor driven at X32 microstepping would take 6400 steps to make 1 revolution or about .06 degrees per step.

  • Whether they can cope with the continuous friction and the load of the arm

Again, a properly chosen motor would. You will need to characterize the "load and friction" to get a number for the required torque. Then choose a stepper that will provide the torque with some overhead. Once the motor is chosen, a compatible driver can be obtained. Gearing could help to make more torque and make the step size even smaller.

You may need a real time clock (RTC) to track the time as the timebase of Arduinos is not real accurate.

groundFungus:
.... A 200 pulse per revolution motor driven at X32 microstepping would take 6400 steps to make 1 revolution or about .06 degrees per step.....

When microstepping the motor will not have the same torque during the microsteps as at the full step position. Gearing might be the best option.

yamacui:
I have been looking into the stepper motor/arduino combination but am unsure if:

  • They are suited for continuous operation for a clock, and would have a long life span (10 years?)

It's the mechanical parts that wear out (the stepper motor, the clock mechanism); not the Arduino. That will last just about forever.

  • Whether they can cope with the continuous friction and the load of the arm

This is something you will have to look into. Friction means stuff rubbing against each other - and they have to do that for 10 years, moving a distance of nearly 6 km (tip of the arm). I would really want to try to get rid of the friction as much as possible, to lessen the wear and tear, and lessen the stresses in the system.

The stepper may move at tiny steps, if there's significant friction to be overcome the hand of the clock is likely to start moving in larger steps whenever the strain is big enough to overcome the friction.

I agree with ardly, hence the (ambiguous) "some overhead". Gearing does not have to be gears. The popularity of 3d printers (and other small CNC machines) has made the tooted belt and pulleys readily available and reasonably priced.

Just using a stepper motor might end up expensive, I reckon you'd need at least 350Nm torque.

On the other hand, a cheap MEMA 17 stepper with a reduction gearbox, worm drive or reduction belt drive would be cheaper overall.

Another consideration may be that with, say, 5:1 reduction you would not need to microstep; using full steps gives you the option of turning the power off between steps.

Yours,
TonyWilk

Is a stepper motor the right motor for this job?

The OP says he wants a single 0.5m hand that "rotates continuously". I take that to mean that he does not want to see the hand 'tick'.

With 5:1 gearing the 200 full steps/rev of the stepper becomes 1000 steps/rev of the hand. However the hand is only making 1 rev/12H which is roughly 3 steps every 2 minutes , each step being 0.36deg.

That will look like a tick, though I suppose it depends on the distance you look at the hand from. Would a DC motor a much higher gearing not be better?

What's stopping you from increasing the gear ratio on the stepper? Make it 500:1 and you have 100,000 steps per revolution. Going to be hard to see those steps, as you're making five tiny steps every second. It should also solve the torque issue.

For a constantly turning DC motor you're going to need a lot more gearing than that - at 500:1 the stepper would do 0.7 rpm. DC motors usually do at least a couple hundred rpm. Control also becomes harder as you'l have to start measuring the actual rpm of the DC motor and adjust that to keep the clock on time.

wvmarle:
What's stopping you from increasing the gear ratio on the stepper? Make it 500:1 and you have 100,000 steps per revolution. Going to be hard to see those steps, as you're making five tiny steps every second. It should also solve the torque issue.

For a constantly turning DC motor you're going to need a lot more gearing than that - at 500:1 the stepper would do 0.7 rpm. DC motors usually do at least a couple hundred rpm. Control also becomes harder as you'l have to start measuring the actual rpm of the DC motor and adjust that to keep the clock on time.

Sounds reasonable and I suppose the slower things run the better for wear and tear.

The normal assumption about a clock is that it's driven from the center. Could it not work as well to paint the hand on a disc and have the whole face turned by a driving gear on the outer edge?

Something like:

In contrast to your average clock hand, this clock hand will be pulling something along the vertical clock face as it rotates

I don't know quite what that means. If the clock face is vertical I would have expected the 'thing' to just hang from the hand. It also suggests that the face has to be a lot bigger than the hand otherwise the 'thing' would be dangling in space when the hand is at 6 o'clock.

You could just paint a small hand on a big face and pin the 'thing' to the end of the hand, I suppose the effect would be the same?

Thanks for being so generous with all the replies and suggestions. Its amazingly helpful. A geared stepper sounds like it good be a good option.

To clarify re the hand - the something its pulling is a wire brush which is fixed to the hand. An extension of the hand rather than a hanging object. The brush would be in constant contact with the surface, hence the additional friction. Sorry if this is wasn't clear.

I did consider the geared face but the hand/brush would need to be a separate element from the face so it can be brushed.

Thanks again for the replies

Curious. What is it that you are trying to do? What is the wire brush for?