Stepper Motor Sizing Help Please

I am trying to get a swing bedroom door opener working dependably. All the online resources I have consulted so far have been so technically above my paygrade as to be of no help to my old brain.

My design is essentially a stepper motor with a 65mm RC car tire mounted on the bottom at the edge and driving the door across the floor thru the opening arc. A couple of limit switches stop the motor when the door is fully open or closed.

The components are an Arduino MKR WiFi 1010, an L298N controller (https://smile.amazon.com/gp/product/B01M29YK5U/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1), a NEMA 17 stepper (https://www.sparkfun.com/products/9238), and a 12v battery (https://smile.amazon.com/gp/product/B016BJCRUO/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1).

The motor skips and cannot open the door when the AC is running as air pressure pushes the door closed. With a digital luggage scale I measured that it takes 3.5 lbs of pull to open the door when the AC fan is running.

Can someone please teach me how to determine the proper torque spec / motor for this application?

Ron

What is the gear ratio between the motor and the part that actually moves the door?

A diagram of the driving mechanism would be useful as I'm not sure I can visualize it properly.

I suspect you measured the 3.5 lbs pull at the door handle. And I suspect the motor is operating at the hinge side of the door where the force needed may be much greater.

If you can increase the gear reduction between the motor and the door actuator you may find that your present motor is sufficient.

Another thing that should improve matters (if you are not already doing it) is to accelerate the motor up to speed.

If you go for a motor with larger torque you will need to use a specialised stepper motor driver rather than the L298 - which is really a DC motor driver, and very old technology as well with a substantial internal voltage drop.

I suggest you study these links before you go any farther Stepper Motor Basics Simple Stepper Code

AccelStepper library

...R

Thanks for your help Robin2!

The motor does not have a gear reduction. The 65mm diameter tire is fastened directly to the motor shaft.

You are correct to guess that the 3.5 pounds was measured at the door handle. However, the motor is attached to the door at the very edge of the door opposite the hinges. So the motor should only be experiencing the 3.5 pounds of resistance.

I was concerned about speeding up the motor as I don’t want the door slamming.

I have a Sparkfun EasyDriver on hand. Would that be a better controller? I thought the motor had enough torque but maybe the control signals I am sending are not appropriate.

I have attached a couple of photos showing my installation.

Thanks again for your help!

Images from Reply #2 so we don't have to download them. See this Simple Image Posting Guide

|500x281

|500x281

...R

Thanks Robin2. I set up the images as you suggested. I am doing your suggested reading now.

Ron

Thanks for the images. That's very different from what I had imagined.

I can see how you need the large wheel to clear the motor body but in effect it is acting to as a gear ratio that significantly reduces the effective torque of the motor.

I suggest you revise the mechanical arrangement so that a small wheel on the motor rotates the larger wheel that contacts the ground. That will multiply the effective torque compared to your present arrangement. It may be that a rubber sleeve on the motor shaft would have sufficient friction in contact with the tire. That might give you a gear reduction of (roughly) 65:6.

Of course a gear reduction will slow things down and I don't know if that would be a problem.

The alternative is to get a motor with considerably more torque.

There is a simple example for measuring torque in Stepper Motor Basics

Assuming your 3.5 lbs is correct (let's call that 1.6kgs for simplicity) then with the wheel radius of 65/2 = 3.25cm the minimum required torque is 1.6 * 3.25 = 5.2 kg.cm (I hope the calculation is correct). The holding torque for your motor is only 2.3kg.cm and the running torque will always be less than the holding torque. And it would be well to have considerably more than the calculated torque to allow for measuring errors, friction etc.

If you could arrange a 4:1 gear reduction the available torque would (in theory) go up close to 9kg.cm

...R

Thank You so much Robin2!

The paragraph with your calculations is so helpful. I can see which torque spec is important to use and your thought process for sizing the mechanism for my project.

My wife, who is now in a wheelchair, will really appreciate being able to use the door without needing to touch it! (Alexa access is working successfully already)

Ron

One more please Robin2:

Since it would be easier for me maintain the current form factor for the motor, what resources/vendors do you use when buying stepper motors with gear reducers? And stepper motors in general?

Thanks,

Ron

youngrh:
Since it would be easier for me maintain the current form factor for the motor, what resources/vendors do you use when buying stepper motors with gear reducers? And stepper motors in general?

The only stepper motors I bought are the same as yours. Others have “arrived” as part of a cheap CNC milling machine.

I was not thinking of a motor that incorporates a gear reducer - though they may be available. I was thinking that you would make your own speed reduction system. It would not necessarily have to use gears.

You can certainly get DC motors with built in gearboxes and one of them might be perfectly suitable. I don’t think you are concerned with the fine precision that a stepper motor is intended for. The Pololu website has a range of motors.

…R

Thanks so much for your time Robin2!

I did indeed find a NEMA 23 stepper on Pololu with 9 kg/cm holding torque that should work well for me.

(After I modify the motor mounting to my project board and figure out how to mount the tire to the 1/4 inch motor shaft that is.)

Ron

youngrh: (After I modify the motor mounting to my project board and figure out how to mount the tire to the 1/4 inch motor shaft that is.)

Ron

You really don't need a tire on the motor shaft. You need to make the shaft turn the door wheel. You can do that by knurling the motor shaft, but you likely don't have the equipment to do that.

The alternative to knurling is to use a triangular file and make a series of sharp teeth around the shaft, so it appears to be gear teeth. These will engage the large rubber tire just fine and you are set to go.

Paul

Paul_KD7HB: You really don't need a tire on the motor shaft.

I think the OP used the word "tire" to mean "wheel". In his pictures the wheel is directly on the motor shaft.

...R

Robin2: I think the OP used the word "tire" to mean "wheel". In his pictures the wheel is directly on the motor shaft.

...R

Yes, but it avoids the need for gear reduction on the motor.

That is how self-propelled rotary lawn mowers are driven.

Paul

Paul_KD7HB: Yes, but it avoids the need for gear reduction on the motor.

Your wording is a bit ambiguous. It could be taken to mean attaching the wheel to the motor shaft avoids the need for gear reduction whereas I presume you meant that driving the surface of the tyre with a knurled motor shaft would provide simple gear reduction.

I suggested a similar approach (but without knurling) in Reply #5.

The obvious attraction of fitting the wheel directly to the motor shaft is that there is no need for an extra shaft and bearings for the wheel nor a mechanism to bring the motor shaft in contact with the surface of the tyre.

...R

youngrh:
I did indeed find a NEMA 23 stepper on Pololu with 9 kg/cm holding torque that should work well for me.

I should have said this earlier …

Please post a link to the datasheet for that motor. It will almost certainly need a specialised stepper motor driver, but without seeing the datasheet it is impossible to say what type of driver is appropriate.

I suspect that connecting it to an L298 will damage the motor or the L298.

…R

Robin2: The obvious attraction of fitting the wheel directly to the motor shaft is that there is no need for an extra shaft and bearings for the wheel nor a mechanism to bring the motor shaft in contact with the surface of the tyre.

...R

Yes, but the whole project screams to be simplified!

Paul

Robin2,

Thanks for the heads-up about the driver. Due to your earlier mention of the L298 controller, I also purchased the TIC T500 controller (https://www.pololu.com/product/3134) when ordering the motor referenced in this datasheet:

https://www.pololu.com/file/0J672/SY57STH56-1006A.pdf.

Are there any pointers for the setup of this controller that I should be aware of?

For mounting the tire, I am going to attempt to drill out the 5mm Flexible Coupling Connector (https://smile.amazon.com/gp/product/B07Z4HB148/ref=ppx_yo_dt_b_asin_title_o08_s00?ie=UTF8&psc=1) to 1/4 inch and see if that works.

Other suggestions would be most appreciated!

Ron

Paul_KD7HB: Yes, but the whole project screams to be simplified!

Paul

Paul, I would really appreciate your thoughts about simplification as this is my first project of this type in 70 years! I mostly do woodworking but the need arose for this after the wife's back surgery.

Of course a career in IT and a life long desire to learn new things came in handy ;>)

Ron

youngrh: Paul, I would really appreciate your thoughts about simplification as this is my first project of this type in 70 years! I mostly do woodworking but the need arose for this after the wife's back surgery.

Of course a career in IT and a life long desire to learn new things came in handy ;>)

Ron

Same here! But I have 10 more years into life than you have!

First, you have the correct device to move something on a hardwood of other smooth floor, the rubber tired wheel. But, as I mentioned, I would have driven the edge of the tire with a regular small DC motor. The motor shaft would be knurled or be fitted with a course sand paper cylinder such as used in Dremel motors and flexible shafts.

The motor is controlled by use of a H-bridge to allow motor to turn either direction and apply power to the rubber tire.

The door closed position is set by a magnet on the door frame and sensed by a reed switch.

The door open position is sensed by a micro switch with a leaf actuator, and that is pressed by a door stop on the wall or on the floor. If that is not possible, a reflective spot on the floor could reflect IR light from a source to a sensor and accomplish the same result.

Your Arduino can control the operation and move the motor to open or close the door and could even delay the closing or wait for some other signal.

Before microcontrollers, all this could be done with relays and switches, but much more convenient to let software control it!

My thoughts.

Paul

Looks like you have a limit switch when the door is open and a Hall effect switch for door closed.

A stepping motor is so wrong for this project.

You should use a simple reversible gear motor, a car widow motor for example.

In fact suggest you take the assemble off the door, move it to the door top and add a pulley or threaded rod system of movement.


That’s an awfully narrow hallway, is it legal ;).


“ wife's back surgery.”

Sympathize with her.