It would be very beneficial to your customers and Pololu if you also offered encoder motors suited for indoor robots. The lowest RPM of encoder motors currently offered is 80 RPM. These are Metal Gearmotor 37Dx57L mm with 64 CPR Encoder(pololu.com/catalog/product/1447/pictures)
For an indoor robot, 80 RPM is very fast. For example, it would translate into about 44 cm/sec on a 6cm radius wheel (at 70 RPM). If you could offer this motor at about 30 RPM, it would be an ideal solution for in indoor robot (about 19cm/sec for 6 cm wheel and 31 cm/sec on a 10 cm wheel).
Please let me know if you can arrange this and the cost & lead time required. Currently I am interested in buying two such motors, but I am sure many indoor robot makers would also love them too.
Thanks for the feedback. We are always looking for more encoder options, and we have some progress toward a solution for the micro metal gearmotors. Unfortunately, I don’t think we can get the line you are talking about in a higher gear ratio. You have not said anything about the torque you need, but the unit you mentioned probably has plenty of torque for small, indoor robots, so you do not necessarily need a higher gear ratio. Your calculation only gives a theoretical top speed; you can always run the motors at less than full power for lower speed. Is there a reason that would not be a good option for you?
The 80 RPM specification is for running the motor at 12 V. You can power the motor at around 6 V and get approximately 40 RPM, or as Jan suggests, use a motor controller at speeds ranging from something like 0 to 50%.
Thank you for the reply. You are correct in saying that the motor has a lot of torque. Torque is not my concern. I am requesting a higher gear reduction not to increase the torque, but primarily to reduce speed. The reason for this are in my next post.
thanks for the reply. In the example above, a robot with a top speed of 19 cm/sec needs a maximum of 30 RPM. And to follow a curved trajectory, lets say one wheel turns at half the top speed i.e. 15 RPM. This corresponds to 18.7% to 37.5% of 80 RPM (top speed of 122:1 37D motors).
I could be wrong and if I am, feel free to correct me. I am of the opinion that using the 80 RPM motor at lower speeds may not work. This is due to the following reasons:
“DC motors can often be effectively operated at voltages above and below their specified rating. If the motor is rated for 12 V, and you run it at 6 V, the odds are the motor will still turn but at reduced speed and torque. Conversely, if the motor is run at 18 to 24 V, the motor will turn faster and will have increased torque. This does not mean that you should intentionally under- or overdrive the motors you use.”
(Source: Page 348 Robot Builder’s Bonanza, Third Edition)
“You’ll find that most motors will refuse to run, or will not run well, at voltages under 50 percent of the specified rating.”
(Source: Page 328 Robot Builder’s Bonanza, Third Edition)
Velocity is very complex when it comes to DC motors. The general rule is, motors run the most efficient when run at the highest possible speeds. Obviously however this is not possible. There are times we want our robot to run slowly. So first you want gearing - this way the motor can run fast, yet you can still get good torque out of it.
(Source: societyofrobots.com/actuators_dcmotors.shtml )
Whether a motor runs on a lower voltage varies a lot. Some good motors, even when rated for something like 24V, run down to a volt or two. In this case, we’re talking about a motor with encoder, so you can also use the feedback to attain the speed you want.
This is not to say we are not looking for higher gear ratio motors; I am just saying that for the particular performance you are describing, these should be fine.