Pololu Robotics & Electronics
Menu
My account Comments or questions? About Pololu Contact Ordering information Distributors

Pololu Forum

Motor Selection Assistance / Low RPM, High Torque

Hello,

I am looking for advice on which type of motor to use in my project. Here are my requirements:

  • Quiet
  • Variable speed via potentiometer
  • Low speed… around 1 - 60rpm; most common usage would probably be 5 - 20 rpm
  • High torque
  • Ability to implement a start / stop toggle switch; preferably this could default to the “stopped” state on power-up
  • Full speed (or close to it) at press of the start button, i.e., it doesn’t have to “ramp up” to rpm set by pot

I’m afraid I don’t have specific figures for torque, but the motor will turn a group of four interlocked 6" diameter, 1" thick Baltic BIrch plywood gears, if that helps.

From what I have read, it sounds like a stepper motor may be the best choice, but hopefully someone can make some specific recommendations. If at all possible, I would like to avoid using a microcontroller or motor driver; I know this is possible with a stepper motor, but I don’t know it’s feasible. For example, could I limit the maximum rpm without a microcontroller? Or implement a start / stop?

If I do need a microcontroller and / or motor driver, could you please make recommends about what would work well with my project? Or perhaps there is another motor that would work better?

Thank you so much for your input!

Greg

Hello, Greg.

“High torque” is relative; a motor considered to be high torque for a mini sumo robot would have little torque compared to motors that might be used to move humans around. Our motors generally have torque curves or stall torque provided in their datasheets and product pages, but if you do not have a good idea for how much torque you need, you might read through the Force and Torque post on our blog. What kind of object are you moving, how fast do you want to move it, and how fast do you want it to accelerate? Is there drag in your movement mechanism between the motor and the object being moved that might require more power?

In general, stepper motors are used for precise position or speed control. Also, they hum at whatever frequency they are being stepped at. Brushed DC motors are often better for faster movement and it is possible to use feedback to precisely control speed. What kind of speed and distance accuracy are you trying to achieve? How much does your mechanical load vary?

If you want to control the speed of the motor with a potentiometer, we have stepper motor controllers and brushed DC motor controllers with analog control options that make it pretty straight forward to do that. These devices all have microcontrollers built in, but they are preprogrammed and only need some initial configuration to work. It should be possible to incorporate a switch into your potentiometer circuit to stop movement without changing the position of the potentiometer with any of those controllers.

-Nathan

Thank you, Nathan! Good point about the torque. Perhaps I should nail that down first.

So my project has four 6" diameter Baltic Birch plywood gears, each with 36 teeth. They are lined up in a row and interlocked, so turning one turns all four gears.

Each gear will carry a small amount of weight, and each of the four gears will have to overcome a small amount of resistance created by a small flexible tine as it rotates (one for each gear).

So, is the procedure I have outlined below correct for determining torque requirements?

  1. Weigh the gears and the amount of weight carried by each gear. Let’s say each gear is 1/2 pound, and each will carry a 1/2 pound, for a total of 4 pounds.

  2. Attach one of the tines to the edge of a table like a little diving board, and tie a string with a weight to it. Add weight until the tine flexes enough to clear the impediment. Repeat for all four tines, and add the weights together. Let’s say each tine requires 1 pound to bend, for an additional 4 pounds. So I need a motor capable of moving 8 pounds. I will arbitrarily add 2 pounds to make sure I have a motor that is powerful enough.

  3. If t = r x F, and I had a single 6" wheel that had to move 10 pounds, I think I would need a motor that had a minimum torque of 3 x 160 = 480 oz-in. Is this correct so far?

  4. Now, here is where I’m a little confused. How do the 4 gears affect this? Since the gears are identical, the gear ratio is 1:1, so there is no increase or decrease in torque. So would I just need a motor capable of producing at least 480 oz-in.? Or do I need to take into consideration the radii of the other 3 gears? It seems like it would be mechanically advantageous to put the motor on one of the two inner gears, but I’m not sure if this is correct. Will the torque be equal across all four gears even if I put the motor on one of the end gears?

Thank you very much for your assistance!

Greg

I’m having a little trouble visualizing your system, but it sounds like your gears aren’t moving any kind of wheel, just turning themselves and pushing against some tines. A load that rotates has an angular momentum and the torque determines how quickly the rotation rate changes.

If you have a gear-train of 4 gears and they are all the same size, the speed and torque at the output will be the same as the input (minus any losses like friction).

-Nathan