pic: Reverse Drive

[ttldomination]

Originally Posted by Aren_Hill

1. I believe its a method to overcome possible direction bias of cims, enabling both sides to spin in the same direction by adding one more stage of 1:1 gearing.

Hm... I'm curious as to why that works? It would seem like it wouldn't make that much of a difference...

[kevin.li.rit]

As far as I can tell the direction bias of the CIMS is less than 100 rpm.

But why add an extra gearing stage when you could simply mount the gearbox in the same direction.

[kgzak]

Originally Posted by Coffeeism

As far as I can tell the direction bias of the CIMS is less than 100 rpm.

But why add an extra gearing stage when you could simply mount the gearbox in the same direction.

I was just gonna suggest this. Also, the only problem we had with turning was with our open loop autonomous when we went straight we turned slightly. this was probably cause by our electric board and battery being on the same side and nothing on the other. We tool a Tachometer and measured the speed of the motors with no load and they were going the same speed. Are you sure this wasn't your problem? or maybe they have a direction basis while under load?

[kevin.li.rit]

That's a good point, There are also differences motor to motor especially for older motors.

[EricH]

Using the 100 RPM bias figure that Coffeeism mentioned (note--I'm not sure how accurate that is; someone may want to figure it out), and the free speed of a CIM at 5500 RPM, that's a 1.8% difference from switching directions. Pretty small. When you gear down a CIM, you multiply that a bit--but it's still pretty small. For a free-speed motor, it's entirely possible that one motor and its otherwise identical twin have a larger free-speed difference than a motor and its bias.

Would it work? Probably. Are there other, simpler, and possibly more elegant, ways of doing the same thing? Yep; I can think of 2-3 off the top of my head, ranging from ignoring automode entirely to having a gyro and a pair of encoders and some programming to ensure that the robot maintains heading to within 1/1000 of a degree or some other ridiculous degree.

I think it's a new solution to a much-solved problem; while I can appreciate the engineering, I don't see why it's better than the other solutions.

[Alan Anderson]

It's not motor bias tripping you up. The problem is the default Victor calibration, which has its neutral point a little higher than it should. If you calibrate your Victors, or if you tell WPIlib that you're using factory-calibrated Victors, or if you switch to Jaguars, there should be no difference in forward vs. reverse speed.

[kevin.li.rit]

I searched for it and someone else has measured it on this chief delphi post.

http://www.chiefdelphi.com/forums/sh...t=41255&page=2

[Chris is me]

With a 1.5% difference in free speed, assuming it is not an issue of Victor calibration (much lighter of a fix than a second gear ), at 12.75:1, one side of your drive will spin freely at about 417 RPM, and a theoretically slower CIM will spin at 410 RPM. This assumes you have only 2 CIMs in your drivetrain; I'd gander that 2 more CIMs would "average out" this effect to make it even smaller.

With 6" direct driven wheels, and an assumed 81% efficiency, that's about 8.84 feet per second on one side and 8.69 feet per second on the other. That's a difference of about 2 inches per second. Turning scrub would probably stop your drive from tracking two inches in one direction over 9 feet.

[Hawiian Cadder]

does anyone know why the Cims have a bias, i noticed that the older the cim is the worse it gets, i would assume it is something to do with the rotor becoming magnetized or the permanent magnets starting to mess up.

[Chris is me]

Originally Posted by Hawiian Cadder

does anyone know why the Cims have a bias, i noticed that the older the cim is the worse it gets, i would assume it is something to do with the rotor becoming magnetized or the permanent magnets starting to mess up.

Who says the CIMs have a significant bias?

[Ether]

Originally Posted by Hawiian Cadder

does anyone know why the Cims have a bias,

If a brushed DC motor has a bias, it's often due to the commutation timing being slightly biased in one direction or the other.

[joeweber]

I do not know if the CIM’s have or have not bias direction but our robots for the last 6 years have all traveled to the left over a 20 ft autonomous mode. Tele-op is not a problem and the thought of setting up the drives motors and gear boxes off to one side so they are going the same direction is not always an easy placement with the space available. We also have used Fisher Price motors on a home made Segway and it always drove to the left. Yes we can program it out but some times it becomes a difficult task when you are dealing with gyros, rotation sensors, accelerometers and a camera to keep it on track. We just thought the addition of a small idle gear, short axel, and two bearings would be an easy fix. The weight is minimal and the time saved in programming can make a large difference when you only have six weeks to make everything work. We have a small team with one programer that has other things to do durring build season.

[RyanCahoon]

Originally Posted by Alan Anderson

The problem is the default Victor calibration, which has its neutral point a little higher than it should.

Side note: Another way to fix this that's always worked for me is wiring one of the motors backwards. (wire the Victor's output backwards, not the input)

[AdamHeard]

Originally Posted by RyanCahoon

Side note: Another way to fix this that's always worked for me is wiring one of the motors backwards. (wire the Victor's output backwards, not the input)

I'd learn to calibrate the victors, it's very easy to do, and should be done to all victors each year.

[skimoose]

Originally Posted by Chris is me

That being said, I've been told many times before that CIMs have no significant motor bias whatsoever. I have no data, but my team has also never had this problem so I'm inclined to think for my purposes, it's not a big deal. Your team and setup could be completely different, particularly if you have a 2WD robot which has no turning scrub.

We ran tests in 2007 using victors to control the CIMs and there was some bias, and it was different for each motor tested, it was most apparent when ramping speed up or down. The response curves varied, not significantly but enough to be a problem occasionally, because they were not linear. We wrote code that year to flatten out the response curves to be more linear and that helped.

We haven't done the same testing with Jaguars yet, might be a good exercise for this season.

Drive line friction would probably cause greater issues with a robot driving straight, than motor bias which is fairly consistent throughout the motor's life. No matter what the source of driving error, I'd be looking to sensors and code to fix those issues if you want to achieve truly accurate autonomous navigation.