Selecting a Motor

[HandsFreeCookieDunk]

Hi all,

I've been reading this forum since last season and this is my first post because I've been stumped when it came to shopping for this event. With all of the recent talk about microcontrollers, I think the very heart of the event has more or less been overlooked: the motor. While browsing Ebay, Amazon, and even more specialized robotics sites, I haven't really found a motor that I've liked. The time that has been suggested for a good speed is 2 seconds, meaning that the vehicle will be travelling at 4m/s. Assuming wheels with a diameter of 3 inches (The largest Banebot T40s are 2-7/8), your motor will need to spin at about 1000rpm in order to achieve this speed. Unfortunately All motors I have found are either ungeared and significantly above this speed or geared and below it (The Tamiya motor-gearbox combos that have been suggested fall into this category. Additionally, the motors that come with them are only rated for 4.5V). What's more, I have been unsuccessful in finding gearboxes that come separate from a motor. If anyone could offer suggestions on where to find a motor/gearbox that meets these specifications or tell me where I may have gone wrong in my considerations, that would be great.

Thanks,
HFCD

[Bazinga+]

Its practically impossible to get a good time with regular DC motors. If you want a time of less than 3 seconds, use a brushless motor. Unfortunately brushless motors are a bit harder to use with microcontrollers. I've been experimenting with brushless motors and i recently got a 30 A esc and a 25 turn brushless motor, but I've had problems with controlling it from the arduino. For one, the esc calibration is sketchy since it sometimes makes me calibrate it when I turn it on, and some times it is ready to go. Also, the motor twitches back and forth making a loud squeak and sometimes starts spinning super fast (when the vehicle wheels are freely rotating), but the motor has almost no torque and can't move the vehicle. Have any of you had any success with brushless motors with arduino? If so could you let me know how you got it to work and which motor/esc/battery you use. Thanks.

[windu34]

Ya brushless is the way to go if you have the money and coding expertise. I will not say how I did it since those rights belong to my team as well as my partner, but I will go as far as to say it is possible to do so effectively and the top teams and nationals will most certainly do it. Also a good motor/ESC combo (castle, traxxas, etc) will cost you at least $80 and the rest of the parts will be ~$50 (transmission, etc) so before going for the brushless option, Id advise totaling your costs and making sure you have the money to spend.

[Bazinga+]

Thanks a lot. I've been trying out crappy 10$ to 20$ brushless motors since I've been afraid the more expensive ones might be too heavy, but if someone got it to work then I guess its the way to go. Just wondering, are you using a quadrature encoder for measuring distance? Cause I would expect greater accuracy than ~20 cm from an encoder.

[windu34]

Thanks a lot. I've been trying out crappy 10$ to 20$ brushless motors since I've been afraid the more expensive ones might be too heavy, but if someone got it to work then I guess its the way to go. Just wondering, are you using a quadrature encoder for measuring distance? Cause I would expect greater accuracy than ~20 cm from an encoder.

Right now, I have an electronic brake coded to run at the 8.5m mark to slow the vehicle and a wingnut brake for the final brake to hot the precise spot. I may try an encoder, but it wouldn't help a whole lot because the reason for our pack of precision is the skid in the wheels during our deceleration.

Weight of the motor won't become a problem once you go brushless because the motors are ridiculously torqued with high rpms

[Bazinga+]

Well in principle an encoder is basically the same thing as a wingnut break, except for the encoder you need a separate break. Does the brushless motor's brake not brake fast enough? If so, have you tried reversing the direction of the brushless motor at the end?

[windu34]

Well in principle an encoder is basically the same thing as a wingnut break, except for the encoder you need a separate break. Does the brushless motor's brake not brake fast enough? If so, have you tried reversing the direction of the brushless motor at the end?

No, the wingnut is definetly not the same imo. The wingnut brake allows me to adjust the cars distance physically so I don't have to bring a laptop to competition at all. If a wingnut brake is done correctly, it is essentially just as precise as a encoder-induced electronic brake (just look at scrambler). I can brake the brushless fast enough, but if I go too fast, the vehicle will just skid so that's not an option (plus once you skid, the vehicle often turns and slips sideways a bit)

[Bazinga+]

Well you could use a PID algorithm to make it slow down gradually after 8/8.5 meters, but its probably not necessary. I think if you put it to medium power for the first .25 meters and then transition to full power by .5 meters that will enable you to avoid sliding in the beginning, and then transition to a very low speed(where it just barely move ~.4 m/sec, and then have it travel at that speed after 8.5 meters, and it should stop consistently. Also note that the encoder must be on a separate axle than the motor.

[windu34]

Well you could use a PID algorithm to make it slow down gradually after 8/8.5 meters, but its probably not necessary. I think if you put it to medium power for the first .25 meters and then transition to full power by .5 meters that will enable you to avoid sliding in the beginning, and then transition to a very low speed(where it just barely move ~.4 m/sec, and then have it travel at that speed after 8.5 meters, and it should stop consistently. Also note that the encoder must be on a separate axle than the motor

That could work, but not for the speeds that I am going. My motor turns at over 100,000 rpm and is geared to about 10,000.I need a good 1.5-2m to accelerate to top speed without skidding

[Bazinga+]

Oh I see. 100,000 rpm is insane. The motor probably has very few turns so it can't provide enough torque. Hearing it is an option but I think a ~2000-3000 kV motor should enable you to get up to top speed much quicker.