Hi,
I have built an autonomous vehicle which is working quite well but I would like it to go faster (perhaps 2x current speed).
I am currently using four of the “34:1 Metal Gearmotor 25Dx52L mm HP” motors". My robot weighs just under 8 lbs and I am using Wild Thumper wheels (120mm diameter). I am currently using one 7.2v NiMH battery and one motor controller but am considering doubling up on these. The robot will be running on flat surfaces.
I have used a web site calculator and it tells me I need 7 oz-in torque based on my inputs (which I’m not totally sure are correct) so I’m thinking about purchasing four of the “4.4:1 Metal Gearmotor 25Dx48L mm HP” motors now, but I’d like to get a sanity check on this before I place the order. Any help would be greatly appreciated.
Thanks,
Andy.
Hello, Andy.
It is going to be difficult to nearly double your robot’s current speed without moving to more powerful motors. As you suggested, you could lower the gear ratio, but as the gear ratio goes down, you will find yourself with a lot of top-end speed and almost no acceleration, which will make your robot very challenging to control (e.g. it would be like trying to drive a car in nothing but fifth gear). I think the 4.4:1 versions will not be adequately controllable, and I suspect the same is true for the 9.7:1 versions, but you’d need to try it to be sure. My recommendation would be to try the 20.4:1 versions, and I am not sure how well even those would work. How easily is your robot able to turn with its current 34:1 gearmotors?
You might consider increasing the voltage supplied to your motors to get more power (i.e. speed and torque), though that would wear the brushes down quicker and decrease motor lifetime, and you would need to be extra vigilant about preventing the motors from stalling.
-Jon
Hi Jon,
Thanks very much for replying. Your answer is helping me understand this better.
Here is a video showing my robot in operation to give some perspective (it’s the yellow one, named “G-Force”).
If I want to go for more powerful motors, is there anything suitable that you stock? I looked at the 37D 30:1 gear motor and is has almost twice the torque but the RPM isn’t much higher (310 compared to 285). I’m not understanding torque too well yet. Does that mean it will have faster acceleration but similar top speed?
I am also wondering if using a different battery would help. I am currently using the “Rechargeable NiMH Battery Pack: 7.2 V, 2200 mAh” from Pololu (pololu.com/product/2225) and I cannot find any details on what current it is capable of supplying. Maybe if I went with an R/C type battery than can deliver 20 A current that would help? I was thinking more current would make them faster but in your reply you mentioned that more voltage would do that. Could you please clarify.
If it helps, I am using the “Pololu Qik 2s12v10 Dual Serial Motor Controller” (pololu.com/product/1112) which can handle 13A per channel continuous and 30A peak.
Thanks again for your help!
Andy.
Thanks for posting that video! It looks like your robot can turn fairly easily with the 34:1 motors, so 20.4:1 ones might be a viable way to get a higher speed. You could also switch to 19:1 37D mm motors, which would give you 500 RPM if you power them at 12 V. With the 37D mm motors, you would also have enough power to use bigger wheels, which would further raise your top speed. Doubling the diameter of your wheels will double your top speed and halve the torque.
Torque is the rotational analog of force, and higher torque means higher angular acceleration. You might find this blog post on force and torque helpful. I can try to give you some examples to give you a more intuitive feel for what’s going on. First, you should realize that all of these HP 25D mm motors have approximately the same power output since they all use the same motor. The gearbox acts as a way to trade speed for torque (and keep in mind that your wheel in some sense acts as a final gear reduction). High gear ratios give you a lot of torque and very little speed; low gear ratios do the opposite. This is analogous to the transmission system in your car. If you have ever driven a manual car, you might have noticed what happens if you attempt to floor it while in fifth gear vs flooring it in, say, third gear. Acceleration is very gradual in fifth gear (there isn’t much torque available), and you generally have very poor slow-speed control, but you have a much higher top speed. Another example is shifting gears on a bike. The power you generate is independent of the gear you are in, but depending on the gear you either get a lot of speed and little acceleration or a lot of acceleration and low speed. Unfortunately, you cannot dynamically change gear ratios on your robot the way you can on a bike or in a car.
The faster the motors turn, the lower the current draw since they generate a back EMF that cancels most of the voltage across them, so getting batteries that can deliver more current is not likely to help much with top speed. However, higher capacity batteries will discharge slower and allow you to run faster for longer, since the motor speed will go down as the battery voltage drops. Generally speaking, with DC motors, voltage corresponds to speed and current corresponds to torque.
Motors draw the most current when the acceleration is high or the load is high (i.e. when they are seeing the full battery voltage while stalled or rotating slowly). Note that it is good for your battery to be able to deliver these burst currents without experiencing too much of a voltage drop. With four of the HP 25D mm motors, these current spikes could be as high as 24 A. If your battery cannot handle this, the voltage will drop until the current draw from the motors balances out with what the battery can deliver; this could mean lower initial acceleration, but it might work out okay. The main problem would be if the voltage drops so much that if falls below what the rest of your system needs to operate and things start to reset/brown out. It seems like this has not been a problem for you so far. If you use the qik’s acceleration-limiting feature, you can drastically lower acceleration-based current spikes and more safely get by with lower-capacity batteries.
-Jon
Hi Jon,
Wow. Thanks so much for such an awesome response. I feel like I totally understand this now. 
By the way, I actually timed my robot traveling a fixed distance last night and did the math to confirm that I am getting ~285 RPM from the current motors, so battery definitely isn’t an issue. I should have figured out to do this earlier on to better understand this.
I think upgrading to those 37D motors is going to be the way to go. My next challenge will be to find some compatible wheels since the wild thumper wheels I use now won’t be compatible (different shaft size). I like the idea of doubling the size of the wheels too. Any suggestions on where I find something suitable?
Thanks again for taking the time to help me out with this. This is a new hobby and there is much to learn. The support here is awesome and one of the reasons I keep coming back to Pololu for parts.
Thanks,
Andy.
Thank you for the kind words. Unfortunately, we do not have a nice solution for mounting the Wild Thumper wheels to the output shafts of our gearmotors. However, we have something in the works, though it likely won’t be available for a few months.
ServoCity has some hex adaptors that might be useful for mounting to the output shaft of the 37D mm gearmotors. They also carry Wheels & Tires that are similar in size to the Wild Thumper wheels. For larger diameter wheels, you might try specifically looking for large scale RC wheels.
-Jon
