Question about RMP 400 traction

RMP 50, 50XL, 100, 200, 400
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yunyijia
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Joined: Wed Oct 10, 2012 11:32 am
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Question about RMP 400 traction

Post by yunyijia » Wed Oct 10, 2012 11:41 am

Hello,

We are doing experiment with our RMP 400 and found the following problem. We checked the manual but are not sure about the reason. So could you please help us to check the problem? The problem is as following:

After we charge the battery, the voltage is around 76 V. At this time, when we send command, like "turn = 3 and vel = 0", the robot can turn (at the same location). However, after we operate the robot for half an hour, the voltage is around 75.6 V. Then if we send the same command "turn = 3 and vel = 0", the robot is trying to but cannot turn although the command is kept sending. We have to increase the turn command to 3.3 to make it turn at this time.

We don't know if the system has a regulator for the traction power which could keep the voltage to be 76V or other constant volt although the battery voltage is below it. If it does, then the problem we met shouldn't happen.

If not, could you please tell us how the traction power system works and why the problem we met happens? And do you have any suggestion to cope with this problem? What we want is that the robot can move with the velocity we send although the battery voltage decreases.

Thank you very much!

--
Best regards,

Yunyi Jia
Robotics and Automation Lab
Michigan State University

Craig
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Joined: Thu Jan 26, 2012 3:02 pm
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Re: Question about RMP 400 traction

Post by Craig » Wed Oct 10, 2012 11:50 am

Hi,

Your observation about battery voltage is somewhat un-related to your problem. On high traction surfaces(asphalt or carpet) the current demanded by the motors for scrub steering is quite high. As the system heats up it limits the maximum current and hence the maximum torque which can be applied through the motors.

One test you can do to see if this is your problem is try the same procedure on a smooth surface like concrete with high tire pressure (9 psi).

How much mass do you have on top of the platform?

There are limits in the 400 for how large the integration command can get for yaw. Without additional command its possible that the system cannot drive the required current to get the system to turn. This is especially true for very small yaw commands.

/Craig
Craig Shaffer
Principal Engineer

STANLEY INNOVATION, INC
www.stanleyinnovation.com

yunyijia
Posts: 3
Joined: Wed Oct 10, 2012 11:32 am
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Re: Question about RMP 400 traction

Post by yunyijia » Wed Oct 10, 2012 2:38 pm

Hi, Craig,

Thank you very much for your reply.
(1) We tested it on flat concrete surface with tire pressure more than 9 PSI. The problem is the same. And the system didn't not heat up obviously.
(2) Our first RMP 400 has a payload of around 185 lbs and needs at least 3 deg/s turning speed to turn. Our second one has a payload of around 140 lbs and needs at lease 2.2 deg/s to turn.

We think the problem may be because of the large payload we add on the robot. I checked the interface manual and found that we should set the configuration as "Controller Gain Schedule 2 – Heavy" for both our RMP400s. Is that correct?

Except this setting, are there any other configuration settings or commands we should send?

Thanks!
Yunyi

Craig
Posts: 58
Joined: Thu Jan 26, 2012 3:02 pm
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Re: Question about RMP 400 traction

Post by Craig » Wed Oct 10, 2012 5:47 pm

The gain scheduling is more for the balancing platforms than the non-balancing ones. I don't think that will help.

185lbs on the RMP400 is really not a problem with properly inflated tires.

I think if you use higher commands you will have behavior more like you are looking for; 2.2 deg/s would result in a full revolution in about 2.7 minutes. This is a very slow turn rate. There is a drain on the integrator which prevents the machine from continuously fighting small disturbances and this drain is likely reducing your command to zero.

The RMP wasn't really designed to turn so slowly. You may have to externally close the yaw loop if you want to control the system to turn so slowly.
I don't have an answer why it seemed to work better at higher battery voltages. Perhaps something else changed? From a propulsion standpoint battery voltage really doesn't affect motor current (torque) until the voltage gets down below 62V.

/Craig
Craig Shaffer
Principal Engineer

STANLEY INNOVATION, INC
www.stanleyinnovation.com

yunyijia
Posts: 3
Joined: Wed Oct 10, 2012 11:32 am
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Re: Question about RMP 400 traction

Post by yunyijia » Mon Nov 05, 2012 4:43 pm

Thank you for your reply. Then we have two questions:
(1) How to externally close the yaw loop? We do need to turn the robot with a low speed.
(2) What could we do to make the robot turn with a low speed if we add some payload more than 200 pounds?

Thanks!
Yunyi

Craig
Posts: 58
Joined: Thu Jan 26, 2012 3:02 pm
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Re: Question about RMP 400 traction

Post by Craig » Tue Nov 06, 2012 11:11 am

To externally close the yaw loop another sensor is required. This could be a magnetometer ( electronic compass). An external controller would then ramp the command up until the platform turns the desired amount.

The deuce board architecture platforms are limited in the yaw stiffness we can set. The X440 system will allow smaller commands, however, even in that system yaw rate does not directly correlate to body yaw rate due to the scrub steering nature of the platform.
Craig Shaffer
Principal Engineer

STANLEY INNOVATION, INC
www.stanleyinnovation.com

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