I’ve recently purchased two 29:1 Metal Gearmotors 37Dx52L mm with 64 CPR Encoders (Pololu item 1443) from a Canadian supplier. One of the motors appears to malfunctioning. It’s rotational speed is greatly reduced. It makes more noise than the other, and the shaft resists turning more than the other. I contacted the supplier (www.robotshop.ca) and they suggested I post on the forum to ensure that the motor is faulted and that I should return it.
I plugged them both in to the same battery, and removed all mounting screws (which I determined were not more than 3 mm inside the casing). If I can remove the casing without removing the warranty I could check to see if there was something inside the casing causing the problem. But I’d like have the motor replaced if possible.
Thank you in advance for your time.
I have noticed something new that may be the result of something on my end.
When used with the TREX DMC01 Motor controller. With the power turned on… one motor spins more freely then the other. The good motor when, when spun with my hand seems to move freely (Almost like coasting). When trying to spin the other motor with my hand it spins less freely (almost like breaking). When I’m supplying no power to the motors, they seem to be a lot closer in resistance (when spun with my hand). Does the motor controller have kind of breaking feature that I have active?
I switched the motor connections to the controller to see if it was controller related. They seem to be working a little better now. It looks like the difference might be related to the common difference in between motors from manufacturing. I’m going to try multiplying the output to the weaker motor by 1.1 and seeing if that solves the problem completely.
After further testing, it appears the same motor is still giving me problems. It’s way more obvious at lower rotational speeds. Perhaps static friction related?
Can you quantify how much of a difference you see between the rotational speeds of the two motors?
In general, you should expect some degree of variation even among motors of the same type. To make two motors turn at exactly the same speed, you typically need to use encoders or some other type of sensors to form a closed-loop feedback system. Unless your motors are turning at drastically different rates or drawing very different amounts of current, what you are seeing might just be an expected variation.
You can go ahead and try taking the gearbox apart to look for any problems inside if you feel comfortable doing so.
It is very difficult to quanitify (with the load) without hooking up the encoders. I haven’t played with the encoders yet so that could take me some time (not sure I have it). BUT, I did take measurements using an optical tachometer without the load. I was only getting a 5 rpm difference between wheels. Oddly, the motor I figure is damaged is producing the higher rpm. During the test I did notice in insane different in sound coming from the two motors. One sounds like it’s clicking all the, the other sounds like it’s speeding up and slowing down. both motors are being run from the same signal.
I’m using the TREX DMC01 in serial mode. Do you have an address I can send the video/sound clips I took to compare the sound difference? I haven’t taken he gearbox apart yet… was hoping to figure this out without having to do that… but the sound difference indicates something is wrong (to me). As well, since the motor in question is faster without load, and slower with load… that also indicates something is wrong with the motor (to me). The best I can do other then send you a video, is to say that with low rpm, my balancing robot pivots around one wheel almost on the spot.
I’ve determined based on the operation of the motors without any load that there is something wrong with one of them. Due to my school time constraint, I need to order a new one now (It wont be here until next week now). I’m going to order a new one from Robotshop.ca (since I’m in canada) who informed me that once you guys gave the go ahead and received the damaged motor, they’d give me a refund. I hope that’s okay. Once you send me the go ahead and an email address I’ll send you some videos of what’s going on so you can see and hear it. I think it’s fairly obvious (with my little knowledge of these things) that it’s gotta be the motor.
On another note. would there be any reason the motor controller would suddenly stop communicating while drawing high-ish current? I’ve noticed, with load, that at high rpm the motor controller suddenly stops communicating. I had this happen with the Dual Low Voltage Serial controller as well (used with tamiya gearset).
Do the motors behave the same if you connect them directly to your battery instead of through the motor controller? Please go ahead and send us some videos of the motors. You can either email them to us or upload them on YouTube (or a similar site) and post a link here.
In response to your question, one explanation could be that your motors are drawing enough current for the voltage from your power supply to drop significantly, below the minimum operating voltage of the controller, and causing the controller to reset. Another possible explanation is that electrical noise from the motors is interfering with the communication lines; you can reduce the noise by using capacitors. Measuring the input and motor voltages with an oscilloscope should allow you to determine if one of these is causing the problem.
Thanks for the reply.
I’ll look into these things.
I’m using a 4 cell Lipo batter with 5Ah, 35C cont. I am powering these two motors, from which I can tell will draw at upmost max (stall) 5 A… So I’m probably only using 5 amp since I shouldn’t be anywhere near stall… I have the ability to run some tests. I will wait until the new motor comes in and run them on all of them, so I can compare between the three motors. I do think that the motor controller may be stalling because it’s drawing to much current.
The motor controller puts the motors in parallel right? So my battery is around 14.8 V (I check it frequently)… I have plenty of capacity, and the peak current is 170 A… so that can’t be a problem.
the only thing I can figure is that the damaged motor is drawing more current due to it’s malfunction… but I will have to test this. I’ll re-post next week with some of my results. My new my motor will be in on Monday… so I should have it by the end of the week.
In conclusion to this thread. I ordered a new motor, and it was different then the first two. So I decided that the motors must have a manufacturing issue and there was nothing I could do about it. So I ended up using the encoders try to solve the problem.
It worked, but I still don’t think I should have had do that. Without the encoders, my robot turns on the spot. Anyway, if you’re interested in the project it’s here.
I split durbak’s post into its own topic.
Thanks for the update, Jeremy. I am still not sure there was necessarily a “manufacturing issue” with your motors. Typical variations between individual motors make it hard to run two motors at the exact same speed without any feedback, especially at low voltages or duty cycles. You might just have expected the motors to be more consistent than would be reasonable, although I cannot be sure without seeing the particular details of your setup. (I am not sure I remember seeing any videos from you; did you ever send them to us?)
At any rate, I am glad to hear you were able to come up with a solution using the encoders. Your project site looks like a nice presentation. Were you able to solve the problem you had with your motor controller communication?