My team just finished building and mounting our first attempt at combining the Boshe Drill motors and the CIM aka Chiaphua aka Atwood motors. We matched the free load speeds of both using gears and some chains, and it tested fine while we had the robot propped up on a table, but when we placed it on the floor, one side was completely overpowering the other side and it was impossible to maneuver the robot.
The way our gearbox is set up is that both the Bosh Drill motors turn forward when the robot went forward, but due to lack of space and eagerness to see the robot move under CIM and drill power we had 1 CIM go forward and 1 CIM go back wards, hoping that the RPM difference was not too great in forward or reverse. Can this be the problem that we are facing? That the forward CIM is going so much faster then the back wards one that it is dominating the other side?
From what I can tell in your post, the CIM shouldn’t be causing your problem. Do you have any pictures or CADs of your setup? This might help us try to figure out your problem.
-Bill
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Mike brings up a good point about the possibility of breakers tripping in his post below.
There are any number of possible reasons for this symptom.
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Yes, the CIM is different in forward and reverse. However, due to the magnitude of error you are experiencing, I’m guessing there is something more fundamental involved.
Try and disconnect the power between the drill speed controllers and the drill motors (run just the CIMs). Repeat with just the drills. I’m guessing that this test will provide more illumination.
Also, pay attention to the circuit breakers, one may be tripping on you.
The CIMs are, in theory, pretty symmetrical (no huge angle difference as in the drills), so that should not be your major issue. You even made both drills turn the same way, so your motors are not likely to be the problem. Look for gears misalignment and, above of all, different chain tensioning from side to side.
Other factors might be shaft bending under load (robot OK on the table, bad on the floor) and, less likely, uneven weight distribution between the left and right sides.
Do you identify any of these in your design?
i don’t believe we even need the cim’s but they still want to use them, and sry, but there will be no pictures of our setup, sry for the dispointment, but thats top secret :rolleyes:
hmmm, it could be the chains, but i don’t believe that’s the main problem being it worked fine until we used the cim’s, but tahnks for oyour help guys
Another thing to check, make sure that the drill motors are both shifted into the correct gear tightly. I know this may sound dumb, but I actually had a problem with this and it took me a long time and a few posts on here to figure it out. I’m probably the only one dumb enough to do that but I just wanted to throw the idea out there. Good luck with getting it straightened out.
-Aaron
Here are my teams experimentaly determined numbers for RPM.
It sounds as though we did basically the same thing you are doing last year, and it worked wonderfully for us. I don’t think its possible that your issue is being caused be the CIM motor differences themselves. More likely are uneven chain tensions, bent axles, or a too-tight center distance on one gear set. Before you put the motors in, did one side turn more easily than the other? Is one side now noticably more difficult to backdrive by hand than the other side, when the system is not even turned on? (if you didn’t remove the internal pins in the drills, maybe don’t try that)
I can say that, as a driver, adding the CIM motors to the drill motors made a huge improvement to both the pushing power and manuverability of our six wheel drive robot. I am a big fan of quad motors drives because they are within the reach of almost any team and are still competetive with more complicated shifting drive trains. I would not give up on this experiment, especially given than its the off season.
Check that each individual motor is functioning. We had similar problems and it turned out one of the motors wasn’t running, I think it ended up being a wireing problem.
The free load speed? Err…isn’t that a contradiction. If you meant the speed they spin up when there’s no load (I.E. Free Speed) than that’s why one sides overpowering the other, you need to match their speeds while under load, if you meant you matched them under load, then…I don’t know.
As far as I know, the CIM motor has NO winding-bias, and spins with the SAME characteristics in forward and reverse.
The variance between drive sides probably has something to do with your gearbox. Possibly one has a parasitic load somewhere you don’t know about, which accounts for it running slower.
No you don’t… people match the free speeds all the time. As long as both sides are matched to the free speed, they’ll be moving at the same speed… whether that’s the optimal place to match the speeds or not is a different question.
Could you describe your control system? While I agree that a mechanical problem is the most likely cause, I wouldn’t discount the possibility of a problem with the joystick(s) or even one of the victors.
Have you calibrated the victors? Are the victors all running? Are you sure that your programming is legit? It’s possible that one motor isn’t being powered at all. It’s actually a very easy thing to have happen, and you probably wouldn’t notice with no load on the system. All it takes is one of the breakers to come loose or a PWM cable to not be fully seated or any number of other things.
Just one more thing to check on. As long as all the victors are lighting up green when you have the stick down, then it’s probably not a control system problem.
Wow, lots of disinformation in this thread. Let me give you some facts.
The CIM motor windings are symmetric, so there is no inherent speed difference between forward and reverse. However, there are small differences from motor to motor due to manufacturing tolerances but not enough to notice. We have tested these motors extensively and they have the same behavior in forward and reverse.
Matching motors at free speed is perfectly fine. Matching motors at any other speed is fine, too. Please note that if you match at a speed lower than your top speed, then any speed over that matching speed one of the motors will be doing negative work. What this means is that one of the motors will be acting like a generator and not like a motor. What this also means is that one of the motors will be doing all the work when running at a speed above the matching speed.
Now for some of my opinions on your drive problems:
The fact that your conditions under load are different than sitting on the table points to either your gear box or drive train. Bent shafts, bad bearings, gear or chain misalignment could all be the culprits.
It could also be a bad connection, but usually you can find that problem by just running on the table. To find out if it is a bad motor, systematically disconnect one motor at a time and see if the side you disconnected the one motor from still drives. This happened to us a few times and this method found the problem each time.
Our normal debugging goes like this:
1). Is it software? Always blame the software guys first…
2). Check that each motor is working by itself. (a wiring problem)
3). Make sure that the motor pairs are driving in the same direction. (a wiring polarity problem)
4). While propped up on a stand see if you can see if one side is running faster than the other. (a gearbox or drivetrain problem).
After that, you should have a good idea of where the problem lies.
We have been using the drill motors and CIMs combined for several years now and have not seen a problem like you describe.
First, Thanks for all your help guys. Honestly if you guys did not respond, we probably would have tried to mount the motors so they are both facing forward and thus wasting time because it wasn’t the actual problem.
It was, in fact, a hardware problem. We went though the various ideas that you guys posted and it turns out the victor controlling the right motor was not functioning, so we swapped it out and everything worked like a charm.
If it worked right on the table but not on the floor I think you still have a problem. The dead Victor may be a symptom and not the disease. Anytime a drive works in the air but not on the floor, start looking for misaligned parts in the drive. Bearings that are not parallel ( on two ends of a drive shaft) is often the problem. Those that use chains often put too much tension on the chains. Remember there are very few of the kit motors that can stand any side load and therefore need to be coupled to shafts and bearings that can take the load. i.e. Don’t put a chain sprocket on the shaft of an FP, drill or Chalupa and expect it to last.
In general, you are correct. However, for this particluar case I think the Victor was the problem. They are using the drills and CIM motors together matched at free speed. If only one victor was bad, then one of the paired motors would still work. While in the air (aka almost at free speed), the side with one motor would still look like it was running with two due to the matching. But once under load, the problem becomes clear. This exact method is how we have found bad Victors (or disconnected wires) in the past.
Paul,
Wouldn’t there have been a noticeable difference in speed if one of the motors were not driving, even in no load? I guess the effect would have to depend on which motor was nonfunctional vs. it’s ratio to the final output.
You would think that the one side would move slower due to the fact that one motor had to actually drive the dead one. The fact is, even though it does move slower it is very hard to tell by eye if it is slower. If I understand where you are going with this, you are right that this problem could have been detected on the table.
This next part is to everyone else (Al’s team already does something similar to this and they probably taught us this at one time),
A procedure we do every year is to check the current draw of each motor when the drive base is up in the air. It is rather easy to do. Simply put an ammeter (or multimeter set to current) in series with the motor and measure the current draw of each motor. The CIMs on each side should draw the same current and the Drill motors from each side should draw the same current. You would have found the Victor problem from this test. We do this test to make sure our gearbox is the most efficient it can be and to make sure we do not have a bad motor, bad connection, bad speed controller (Victor).
If the above procedure was done while the robot was in the air, then you would have noticed the problem.
Using a multimeter was how we actually found the problem. The victor was tricky :rolleyes: … light were lighting up right but when we tested the current going to the motor it never changed when we moved the joystick…
i of course blamed the code for it couldn’t be my wiring , but thanks guys, it always does turn out to be something silly
Thanks Again 
, but thanks guys, it always does turn out to be something silly