We are considering using the Denso Window Motor and the globe motor together on the same chain. One has a free RPM of 85 and the other 82.
Will this cause problems? With the close RPM, I don’t think it should create a huge issue. Of course we were worried about one trying to drive the other - but we’re going to use a y-PWM cable with victors for each.
I suspect it won’t have a problem with the match lengths we’re talking about.
Has anyone out there done something similar?
The globe says it has a 150 in-lb of torque which is 16.9 Nm. The window motor has 12 Nm.
We are only going to turn them through 1/2 a rotation.
You may have a problem due to the fact that the worm reduction in the window motor is not backdriveable. What will happen is the Globe motor will try to “push” the window motor to meet it’s speed, but the worm gearing will prevent this.
I would not use the setup you are describing. Many people may be able to suggest alternate designs if you give some more info about the application.
A rule of thumb that I like when combining 2 motors is that they should both be able to be backdriven. I would also not combine motors of different types unless I had to.
Along with Rob’s concerns, I will also point out that the globe motor has a tendency to “overdrive” so that even though you cut power to it, it still rotates as it slows down. We’ve always had difficulty with it when trying to move in exactly 90 or 180 degree increments. The window motor will stop quickly, which will lead to the same problems that Rob described above.
I 2nd his concerns, and would not use these two motors in tandem.
The concerns expressed are valid, but one needs to have only one motor that can be backdriven, not both.
Our first arm joint on out 2005 robot has a setup similar to what you describe - the van door motor (for power, but backdriveable) and the denso window motor (weak but not backdriveable) on the same shaft, driving a 12 or 15 tooth sprocket to a 50 or 60 tooth sprocket at the arm joint, #35 chain.
We did experience one failure of the plastic shaft coupler, and one failure of the window lifter motor, during the NJ regional. Neither failed again at Nationals, but we took a more defensive stance there.
I do agree it is not an ideal situation for longevity, but it’s not a guaranteed failure either.
I think that it would work ok if your design allowed some sort of slack in the system, since you won’t be rotating much I wouldn’t be too worried about it, but as long as one motor can turn free of the other for a bit of the turn, the issues of different speeds should be dealt with.
It looks like from what you’re telling me, one motor is faster and lower torque, and one motor is slower and higher torque. Gear the slower one up, gear the faster one down, so that they both have roughly the same torque and the same free speed.
As for the backdriving problem, perhaps you could gear the window motor up so that it’s slightly faster than the globe motor. This would make it so that the globe motor won’t try to overdrive the window motor. Another solution could be found in software - make sure you don’t power the globe motor faster than the window motor. This might require more than just a linear scaling function.
They’re right, though, you’re better off using 2 identical or similar motors.
We used the Keyang and the Globe directly coupled and it was the best part of that years robot. I did post an inventor file for the part we had built.
Good luck
Their characteristics are so similar I couldn’t believe it would cause a problem. You give me hope that our arm may not fall apart within 30 seconds of the first bell
Tom,
By now you have gotten the feeling that this is generally a bad idea. However, two motors who have a close free speed do not run close to the same speed under load. That being said, two motors running on the same shaft(shafts coupled together) will obviously both turn at the same speed. What likely will develop with your setup is neither motor will be running near efficient speeds and will develop some heat. What you do have to remember that before the transmission, each motor is running very different speeds due to the different gear ratios of each motor. That is why the Globes tend to run on after removing the drive signal. The armature is spinning (and early stages of the planetary gears) but has very little drag to slow it down. It might be a better idea to feed the motors with separate Victors and match the loaded speed of each one in software.
I had a feeling it wasn’t a good idea from the start. However, at this stage in the game, where we have a real minimum of time, the Thursday before competition doesn’t really afford us the chance to reengineer the whole thing. We’re wedged into a corner as far as size constraints go, and we’ve used one of every powerful/geared motor on the robot already.
So if we want extra power for the arm, and we do, then we need to try to match uneven motors.
Like I said in my original post, we’re talking about 1/2 of a revolution of the output shaft. Total. As long as we don’t overheat the motors and we turn on the brakes in the victors to minimize the globe overspinning, we should be good. We’ll check the keyang with the brake off - who knows maybe they’ll be close to matching.
We’ll check it out on Thursday running each motor seperately to make sure globe does stop quickly enough. Seperate victors are a good idea - but truthfully I’m not sure how we could do a quick and dirty measurement of the motor speed under load.
Thanks for all your input. This is where we sit back and do a little seat-of-the-pants engineering and hope it holds together :ahh:
Just to let everyone know - using the two motors in tandem worked out far better than we ever expected.
In brake mode, they both stop almost instantly when power is cut. We tested without the chain off and saw more of a difference in free speed that was listed in the specs (hmm…) but on the chain together they both run well and neither is heating up. We’ve been checking the window motor for any gear issues and I’m happy to say it is not getting torn up either.
Oh, and Al, we have a 100 lb gas shock to assist the lifting. The problem was our overall speed was just too slow with a single motor. This fixed it beautifully.
We went through the Detroit regional without an issue related to the dual motor drive on the arm. Though the increased speed did wear out the key we used on the arm shaft
