CIM speed in reverse?

Technical Motors
#13

Wow, lots of disinformation in this thread. Let me give you some facts.

  1. 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.

  2. 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:

  1. 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.

  2. 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.

-Paul

#14

Sounds like there is an answer somewhere in here.

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.

#15

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.

:slight_smile: Thanks Again :slight_smile:

#16

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.

#17

Al,

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

#18

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.

#19

Al,

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.

-Paul

#20

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 :smiley: , but thanks guys, it always does turn out to be something silly

#21

Sorry about the delay in reply. This is really an unusual failure for a Victor. Can you detail how you measured the current? Were you tripping breakers? Do you still have the controller or did you return it to IFI? I would like the chance to check it out.

#22

Al is right!
I’ve never heard of a transistor jumping to saturation before. have you calibrated that victor lately? Al, if you get any info, tell me please!!!

PS: Read my signature. Amazing as it sounds, many electrical failures were because of it, and I have seen some weird things when I fix stuff.

Sparks

#23

1.) We measured the current by propping the bot up on boxes and running it forward, then we hooked up a micrometer to the 2 screws that went to the motor and it was reading 0 volts. We checked the code and everything was in place.

2.) I really doubt we were tripping the breakers, there was no resistance since it was propped on boxes. Plus when we installed the new Victor it worked, so that was the problem.

3.) Yes we still have the Victor, actually we put it back in the working victor box for unknown reasons. If you would like to check it out, PM me details to send it to you.

#24

The Bobcats have encountered something similar. We had a few issues where a Victor gave us no output. We would see a flashing LED (as if the controller was not powered).

We replaced the Victor and everything was OK. However, we checked the “bad” Victor and it would check out OK… Further checks and we found that the PWM cable was not seated fully down in some of the Victors.

This was NOT a bad cable problem. We would install a 1’ PWM extension cable at the Victor end and the problem would go away.

Does this sound familiar to your problem?

#25

Not really, although I am not 100% sure, I think that the LED lights were lighting correctly according to the joystick movement.

#26

Colt,
A multimeter might not measure any output voltage on the Victors until they are at full throttle. (Indicated by a steady RED or Green LED) Depending on the meter you use it may not be able to measure the pulsing AC waveform sent to the motor. If you were at full throttle then a DC meter should indicate approx. 12 volts. An oscilloscope would indicate whether pulsed output was available but be careful to not ground the output of the Victor when using a scope. Connect the ground lead to the “-” screw on the input to the Victor and then probe either of the output terminals for a display. Only one will show output this way for forward and the opposite one for reverse.

#27

Ah, yes, I forgot to mention that we pushed the joystick forward full throttle with the LEDs solid colors when we were testing the volts, which read 0v.

#28

Al,
I think the victors put out DC.
They take in PWM and DC, and the motors are DC, so I don’t think they’d have AC coming to the motor. I think it’s simply a steady variable voltage going to the motor. Pulsing a motor will shorten its life considerably.

Sparks

#29

The speed controllers do not have the room for a smoothing inductor capable of such amperage. The inductance of the motor provides for a rather smooth DC current but the DC voltage is chopped at a fairly high frequency.

Almost all variable speed motors (DC or AC) use chopped (PWM) waveforms.

#30

Sparks,
The controllers are capable of pulsing battery voltage to the motor at full level with a PWM type signal. As the length of the pulse gets longer the average current in the motor goes up. At full throttle (when the LED is “on” steady) is the only time that the controller puts out a pure DC signal. If the controllers were to put out a varying DC voltage they would have to dissipate the difference in voltage between what the battery puts out and what you want to send to the motor. At low throttle near stall, that could be considerable heat to get rid of. One of the nice effects of this operation is that the motors are pulsed at full voltage and therefore easily overcome starting frictions and torques. I can go into a more in depth desciption if you would like. They are really pretty cool little animals.

#31

I might as well add in something about variable speeds through variable voltage…some folks in R/C car racing have dealt with a Manual Speed Controller (MSC). Essentially, it’s a servo with an arm that feeds power to the motor and a set of resistors, doing exactly what Al described (giving X amount of voltage to the motor, then the rest is dissipated through resistors that create heat.

I’ve personally never used one (the ones I dealt with seem to have a tendency to release the magic smoke, according to my sources), but the fact that you’re feeding a fair amount of your battery power through resistors simply to make that excess power into heat is grossly inefficient. Dangerous, in some cases. (That, and having taken one out of a truck I bought off eBay, they’re obscenely bulky. Seriously–if weight is the enemy, these things have pictures in all the post offices.)

Hence roughly 99.999% of hobby-level R/C cars (and all of the racing versions) use Electronic Speed Controllers.

Just figured I’d give a practical example–the tip jar is at the door. :smiley:

#32

Al,
Thanks for the offer, but I know how they work. I got bored one day and looked them up.
Furthermore: You are correct in saying that they don’t put out DC, persay, but I wouldn’t say they put out AC either. AC as most think of it is a voltage travelling from a plus voltage to a minus voltage, and back, all at a frequency. If you hook that up to a DC motor, sparks will fly. Guaranteed. In conclusion, AC motors are hard to vary speed to (they use different windings for different speeds.) Victors do put out PWM, or, as I like to think of it, modulated DC, and I am sorry for any confusion I may have caused. Also, barring the fact that there isn’t much in the way of alternatives, I think it’s a really stupid way to vary motor speeds. Like I said before, it can shorten motor life considerably.

Sparks