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  #16   Spotlight this post!  
Unread 01-29-2018, 10:00 PM
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Re: Robot curving with motors drawing different amounts of voltage.

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Originally Posted by Thorobotics6166 View Post
I tested the resistance and it's fine.
What resistance did you test? How did you test it? What value(s) did you get?

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Unread 01-30-2018, 07:44 AM
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Re: Robot curving with motors drawing different amounts of voltage.

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Originally Posted by Thorobotics6166 View Post

We are setting the robot to drive forward at 0.5 with turn power at 0.0, using DifferentialDrive. However, when we drive forward, it curves left and even more so when going backwards.
When you say "curves to the left" along with "even more so ... backward", does this indicate the robot rotates counter-clockwise when going forward, and clockwise when going backward (all viewed top-down)?

Assuming so, a few things to check:
--Does the left side of the drivetrain have more frictional losses than the right? This is a common cause of drift to one side or the other when commanding the same voltage to both sides.
--Have you calibrated all motor controllers? Most have a cal procedure to ensure the min/max range output from the RIO actually causes the full range of motor voltage outputs. If one side is calibrated differently, it could also cause the symptoms you mentioned.

I too am interested what voltage measurement was done when you mention there was a 0.2V difference. This could be very real, and indicative of a potential problem. However, the voltage at the motor terminals (for example) is a fairly deeply-buried variable in the whole transfer function of software-command to robot-motion - That is to say, it might give you some info, but is dependent on many many other things.... Folks usually look at motor speed or current over time to better understand behavior.

--With that in mind, is the wiring between both sides symmetrical? Is there any major difference between how the left and right sides are wired?

Edit: Missed the portion about taking off the gearboxes, so gearbox/chain/wheel axle friction is not in the loop for the particular problem you're seeing.

If you swap sides with the gearboxes (aka current right gearbox hooked into left motor controllers), does the 0.2V difference move with the gearboxes? This can help you determine if it's a genuine difference between your motors on the left/right side, or a problem further upstream (motor controller, wiring, etc).

Last edited by gerthworm : 01-30-2018 at 07:48 AM.
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Unread 01-30-2018, 08:38 AM
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Re: Robot curving with motors drawing different amounts of voltage.

Here's how we measured voltage:

We used a multi-meter to measure the "voltage out" from the Sparks. Each of the Sparks are brand new this year. When we measured the voltage out on the right drive train Sparks, they both read 1.9V. When we read each Spark on the left, it was 1.7V for both. The voltage in on each was what the battery was outputting (via what the driver station was reading (around 12.5V)).

We switched the motors:Sparks. We put the left front motor with the front right motor controller, and the back left with the back right motor controller. When we ran it, the right motors were now lower voltage.

Also, we tested the resistance by putting the robot up on blocks (it has been this entire time) and spinning the wheels. No resistance difference between the trains that each of the 9 members that tested, could feel.

Next, we took the motors off and let them spin without the gearboxes, still had the same problem.

What is this motor controller calibration that is being mentioned? How do I do it?

Thanks!
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Unread 01-30-2018, 09:28 AM
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Re: Robot curving with motors drawing different amounts of voltage.

http://www.revrobotics.com/content/d...11-1200-UM.pdf, section 2.5.2 will have the calibration procedure.

Also, thanks for the info on resistance. I think we may have been assuming it was electrical resistance (V/I sorta stuff). Makes sense.
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Unread 01-30-2018, 09:53 AM
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Re: Robot curving with motors drawing different amounts of voltage.

If you're actually giving the motors 50% of max power, there's no way those voltage numbers are correct; you should see on the order of 6v.

I'll reiterate that the voltage across the motors is not determined by the motors. There is nothing you can do with a motor to change that voltage. If you unplug the motor entirely, the voltage drop across the motor controller output will still remain the same. It is determined entirely by the circuitry in the motor controller.
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Unread 01-30-2018, 09:58 AM
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Re: Robot curving with motors drawing different amounts of voltage.

Right. The only reason why we are running it at 50% is because that is when the curve becomes noticeable.

Also, I'm not trying to say the motors are determining that, I know they don't. I am just reporting what the multi-meter is reading. 1.7V or 1.9V (depending on the drive side) for what is being outputted on the motor controller.
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Unread 01-30-2018, 10:02 AM
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Re: Robot curving with motors drawing different amounts of voltage.

Quote:
Originally Posted by Thorobotics6166 View Post
Right. The only reason why we are running it at 50% is because that is when the curve becomes noticeable.

Also, I'm not trying to say the motors are determining that, I know they don't. I am just reporting what the multi-meter is reading. 1.7V or 1.9V (depending on the drive side) for what is being outputted on the motor controller.
If you are commanding an output of 50% in code and measuring a voltage of ~2V at the motor controller outputs, there's a much bigger problem somewhere than the two sides not outputting equal voltages.
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Unread 01-30-2018, 10:06 AM
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Re: Robot curving with motors drawing different amounts of voltage.

We'll run the motors at 100% and report back what we get.
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Unread 01-30-2018, 10:07 AM
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Re: Robot curving with motors drawing different amounts of voltage.

Oblarg, would you suggest we rewire the robot? It is just the motors, PDP, controllers, RIO, and motors, so wouldn't be a problem. Also, we looked at the DS and there is a power fault. Comms says "13".
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Unread 01-30-2018, 10:23 AM
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Re: Robot curving with motors drawing different amounts of voltage.

Yes, I'd suggest you re-check all of your wiring; there is likely a serious problem somewhere.
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Unread 01-30-2018, 11:52 AM
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Re: Robot curving with motors drawing different amounts of voltage.

If you have error-prone wiring practices, re-wiring your robot will likely just create a different set of wiring errors than you have now.

I would suggest you check each and every wire. Use a flashlight if some of them are "buried". Do not stop just because you found one wiring error since you may have multiple errors. By finding your wiring errors, you can see what wiring practices your team needs to improve.

It may be good to get a servo tester such as this one from AndyMark and connect it to one motor controller at a time. If you set the servo tester to 50% or 100% and measure the output voltage, you should get the same voltage on all the motor controllers. Make sure you are measuring between the "+" and the "-" terminals of the motor controller.

If you find that the output voltage is very different between setting the servo tester to 100% and when your software commands 100%, you might want to go through your code line-by-line to see what the code does (not what you want it to do).
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Unread 01-30-2018, 12:05 PM
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Re: Robot curving with motors drawing different amounts of voltage.

Quote:
Originally Posted by Thorobotics6166 View Post
I am just reporting what the multi-meter is reading. 1.7V or 1.9V (depending on the drive side) for what is being outputted on the motor controller.
How are you running the motors to 50%? If you are using tank or arcade drive methods in the DifferentialDrive class then by default, the inputs would be squared, making a 50% "joystick" signal into a 25% output of 3v. While that's still not quite as low as the voltages you're reporting, that may be part of the cause.
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Unread 01-30-2018, 04:01 PM
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Re: Robot curving with motors drawing different amounts of voltage.

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Originally Posted by Oblarg View Post
Yes, I'd suggest you re-check all of your wiring; there is likely a serious problem somewhere.
Concur. Here's what I'd do: Check the voltages at many points:

Battery + to Battery -
Main circuit breaker input to Battery -
Main circuit breaker input to Battery -
PDP + input to PDP - input
PDP + output for controller to PDP - output for controller
Spark + power input to Spark - power input
Spark + motor output to Spark - motor output
(if possible) Motor input + terminal to Motor input - terminal

Figure out one point where the voltage is ~12V as expected, but the next point is much lower than expected. The issue likely resides between those two points.

I'm assuming if you command full power in software, the blinky lights on the spark change color appropriately (I think they're supposed to go solid at the full-on/full-off case, but I don't recall...)?

Note if some component is in fact soaking up lots of power and pulling the voltage down like that, you may have something warm or have some magic smoke if you run at full power for long. Keep an eye (and nose) out for it, and be safe...


Also, +1 for the servo tester to possibly take the roboRIO and the software out of the test loop.

Last edited by gerthworm : 01-30-2018 at 04:04 PM.
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Unread 01-30-2018, 07:38 PM
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Re: Robot curving with motors drawing different amounts of voltage.

Let me add a few items to this discussion...
1. You cannot measure the output voltage of any speed controller at less than full throttle. The output is a PWM signal that drives the measurement electronics in the meter crazy.
2. Most DC motors are would with a slight directional bias. So two motors running in opposite directions will run at different speeds. Large CIM motors are the least biased but they still are different. Software modification is needed to make them run at the same speed. Rotation sensors and software on each transmission makes this less of an issue.
3. Different lengths of wire to each side will cause the motors to run at different speeds. Long wire length on one side and short wire length on the other will cause the longer length motors to run slower.
4. Accuracy in the assembly of the transmission, alignment of the drive parts and misalignment of wheel hardware cause significant friction. If you lift the robot and drive the wheels with no load they need to run at near the same speed to guarantee that friction is at a minimum.

I see many robots each year that do not balance the electrical system or mis-assemble drive systems. The robot is talking to you, you need to listen.
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Unread 01-31-2018, 11:17 AM
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Re: Robot curving with motors drawing different amounts of voltage.

We tore off the old electronics last night. I hooked up only one motor controller.

I only plugged 1 motor and motor controller in with the robot up on blocks.

Here's what we found:

- When the motor controller is plugging into port 1 (on the RoboRIO) the motor that is plugged in runs at 1.7V at 50%. Code: Spark frontLeft = new Spark(0);
- When I plug the motor controller into port 3 on the RoboRIO (the port for the back right motor) without editing the code, the motor now runs at 1.9V at 50%.
- I changed the motors to the right side (with the motor controller plugged into one motor in the right drivetrain into port 3 on the RoboRIO) and did the same. Now, with the motor 3 hooked up into port 3, the motor was getting 1.9V, and then 1.7V at port 0 (also, 1.9V at port 2, 1.7V at port 1) (all running at 50%).
- I then changed the spark. No change in the outcome for either side.

Note: I also tested this with motors 0 and 2 with no outcome change.
Note 2: Our former electrical captain was there last night and watched over us wire the robot up (graduated last year).
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