# How to size a motor for DIY roman shade?

I’m a novice at DIY electronics at best but I am learning and I’m not afraid to learn. So here is the problem I have right now. I have an unusually large and heavy roman shade (also a DIY project) that is about six feet long and four feet tall. Almost all of the tutorials out there that I see are using stepper motors that appear they might be too weak to raise this shade.

I can easily measure the weight of the fabric and from there probably calculate the force necessary to raise or lower them, but I don’t know how to convert that into a number that I can use to choose a properly powerful stepper motor.

Everything else I think I can manage. And I can probably rig a counter weight if I just blindly choose a motor and discover it’s not strong enough. But I’d rather save that effort and choose the right motor for the job in the first place.

I have search but my google-fu does not seem up to this task or, more likely, I don’t know enough to know what to search for and understand the pages that come back to me.

Thanks!

Rich,

The power that you need is the torque (Newton.meter) times the angular velocity (radians/s = 2.pi.rotations per second). Plus you have to assume a efficiency factor of the motor and mechanics.
I hope this helps.

Regards Herman

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It helps but what units is the result of that calculation? Clearly I’ve never had opportunity to take high school physics but I did get enough match to know about that formula.

But is the weight of the roman shade the inertia and so long as the result of the power calculation which you provided is greater than the max weight of the roman shade (when raising a roman shade the weight will increase in discrete amounts) I’m OK (assuming I get the units right (NM, ft-lb etc)? I don’t remember enough to take the result of the calculation and apply it to my specific situation (i.e. raising 10 lbs four feet).

And what about holding torque? How do I do the calculation since the angular velocity is zero?

If I see a chart like this:

Which field is the angular velocity? I have a step angle but that only tells me how far it moves per step, not the velocity.

Or this chart:

which doesn’t including positioning torque. How do I know it can hold up the shade?

Or should I not be relying on the motor to hold up the shade and need some other mechanism to keep it in place?

I really would not, pure instinct says relying on an active drive to suspend something is bad news. Apart from not very green. Lastly, bear in mind steppers can buzz or squeal when supporting against a steady load.

I don’t know what he mechanical solution is, but even involving a spring or gravity counterbalance is iffy given the variable “weight”.

Simplest solution I’d guess at a worm drive from a motor. Most worm drives are non-reversible - the gear ratio is very high so that any reasonable force applied to the “output” wheel is insufficient to rotate motor against simple friction.
In other words, it moves when motor runs but is effectively braked when motor off.

Such mechanisms can be supplemented with an actual motor brake e.g. on goods roller doors but I doubt thats needed for blinds.

Worm drives are bigly torque multipliers; a small motor will do, so long as you are not in a hurry.

Have you any ideas on mechanical limits? Limit switchs or physical stops? You don’t need switches if using a stepper drive, so long as you safely run to the extermes and keep driving without damage, you can software calibrate stepper position.

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I’m guessing this commercial product (can be battery operated!) uses a DC motor and a high reduction epicyclic gearbox - compact arrangement giving same effect as worm drive i.e. small motor giving lots of torque and also resists reverse loads.

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I’ve seen that product but at 150 euro is beyond what I’m willing to spend in this. I like the solar power option though.

I’ll look into worm drives. I was looking at stepper motors because I could keep track of how open the shade is. Because the weight changes as it goes up or down I didn’t feel I could tell in timing.

In case having the actual specs for the share are of interest:

70" x 59" (177.8cm x 149.86cm)

Max Weight: 10.8 lbs (4.9 Kg)

It’s triple ply with an insulating layer of felt in the middle which adds to the weight considerably.

It’s 100% custom made so I have the ability to modify it should the need arise.

I’ve also toyed with the idea of retrofitting one of the Ikea automated blinds motors but I’m worried about them being strong enough.

My roman blinds have a shaft of 1cm which they spool the lifting thread on.
Conveniently enough kg.cm is a torque measurement unit. 1kg.cm = 9.81N.cm (gravitational acceleration anybody?)
So you need a holding torque of 5kg.cm that is a 50N.cm stepper (rounding to a tired mind numbers involved)
If the spooling radius is not 1cm then multiply/divide the needed torque. The product of radius*kg.cm is constant, so for a 2cm spool you need twice the force. For 0.5cm spool you need half the force.
Knowing the slow speed needed and that we don’t need exact positioning and can go full steps we can assume all the torque of the stepper is available.
A Nema 17, 17hs8401 shoud do it, or a Nema 23 like 23hs0405 if there is space for it.
Then there is the detent torque needed, which is the torque needed to rotate the shaft while the stepper is not energised, unfortunately a 17hs8401 stepper has only around 2 N.cm of detent torque so the stepper not energised can not hold the romans, or they might drop if bumped around (wind) BUT a stepper with it’s coils short circuited has a HUGE detent, almost it’s holding torque, but I don’t know if that value is specified anywhere in the datasheet. I would use a relay with NC to hold the coils short-circuited while unenergised, and activate the relay just before commanding the stepper driver.

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