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393 motors irratic on RobotC vs 2.31 
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Joined: Sat Mar 19, 2011 2:32 am
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Post 393 motors irratic on RobotC vs 2.31
Installed RobotC vs 2.31 into the new Cortex. Our Program Skills ran perfectly on vs 2.3. The drive drain motors (393 geared for speed) now seem to be very uneven. No longer goes straight on program skills & veers a lot (almost 60 degrees) to the right. Every time the cortex is turned on, the high strength motors give off a loud high pitched hum. Tried to correct for uneven driving using proportional control which was already part of our code. It couldn't compensate for it. On driver control, when both joysticks are pushed forward, it goes straight but when they are released it suddenly veers right like the motors are going off at different times. Both the autonomous & driver portions were working perfectly prior to updating the cortex. Also, would suddenly just lose connection and then reconnect.
1. Tried to repeatedly redownload Master, joystick & RobotC firmware.
2. Finally reinstalled vs 2.3 back into the cortex & joystick, then the humming motors stopped & the robot ran the program & drove perfectly.
3. Tried a new humped Cortex & joystick with vs 2.31 on the robot. The humming came back & so did the veering motors. Plus the battery was draining quickly (could be the reason for losing connection).
4. Tried downloading the sample remote control program. Still humming.
5. Tried downloading the competition template with all the lines in the autonomous & user control sections deleted. Still humming.
6. Disconnected the high strength motors (2 on the right Y connected to port 2 & 2 on the left Y connected to port 9). The humming stopped.
7. Connected the right Y on, it hummed. Took it off & connected the left Y, it still hummed.
8. Connected an extra high strength motor with motor controller into port 2 & 9 with no Y. Humming again. Checked motor power in debug window. Says 0. But they are definitely humming. By the way we recalibrated the joystick each time we downloaded the firmware.
9. Tried connecting just an extra high strength motor into port 1 & 10. No humming.

Looks like there is a bug that is affecting the high strength motors using the motor controller in the 3 wire ports. This is probably why the drive train is veering way off when it is supposed to go straight. By the way the code is written to have the robot go straight by using proportional control by checking the left & right encoders constantly.

Is there a bug in RobotC vs 2.31 that affects the 393 2 wire motors? thanks. jokai


Sat Mar 19, 2011 2:51 am
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Post Re: 393 motors irratic on RobotC vs 2.31
Ho Jokai,

Thanks for your detailed post - posts like these really help us replicate things on our end. There's no known issue in ROBOTC 2.31 regarding the motors, but I will look into it and let you know what I find.

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Jesse Flot
CMU Robotics Academy
ROBOTC Support


Mon Mar 21, 2011 8:37 am
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Post Re: 393 motors irratic on RobotC vs 2.31
Jesse, Here is what my mentor found by putting an oscilloscope on the two wire outputs of the 393 high strength motors when plugged into ports 1 (2 wire) and port 2 (3-wire) of the Cortex using RobotC Version 2.31 (This problem was not evident with version 2.30)

Motor 383 in port 1 with code motor[port1] = 0, the oscilloscope showed 0 volts(no pulses). Turning the wheel attached to the motor moves easily in both direction. With motor[port1]=10, it see about a 50us pulse every 850us and it becomes difficult to move the wheel in one direction which is correct. Using higher motor commands (>30) makes the wheel turn.

Motor 393 with controller in port 2 with code motor[port2]=0, I see a 20 us pulse which I believe should be 0 volts. Turning the wheel is again difficult in one direction only. It is in effect, stalling all 2-wire motors connected to 3-wire ports. This could explain the fast battery drain we sometimes experience.

It appears that the latest version of RobotC is somehow not correctly converting the Pulse Position Modulation - PPM signals going to the 3-wire ports to the Pulse Width Modulation -PWM signal used by the 393 motors. This problem is on at least 4 four of the 3-wire ports we are using.and because we are using these motors for our drive train it severely affects the autonomous program modes.

Could this bug be corrected? Otherwise we will have to take off our 2 wire motors (the same applies to the 296 motors) from the y connectors which would limit us to only 8 motors not 10. Thanks. joan


Mon Mar 21, 2011 1:55 pm
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Post Re: 393 motors irratic on RobotC vs 2.31
A 393 motor will not plug into Cortex motor port 2 without a #29MC dongle, which is where the 3wire to 2wire H-bridge conversion is done. Try swapping dongles, the fault may be there.

This is a generic trim issue. It is a great opportunity to learn about per-port trimming and how to create an programming infrastructure to implement it. If you do that, it will be a great Robotic Notebook entry and impress the judges, as well as any industry recruiters.
In the real world, things like LED lightbulbs and LED signage need per port trimming to create equal brightness across a variation of LEDs.


Wed Mar 23, 2011 2:15 pm
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Post Re: 393 motors irratic on RobotC vs 2.31
We've been looking into this on our end, and it does appear that the motor centering / trim is ever so slightly off. Setting the motors connected to the Motor Controller 29 to a power level of -3 gets rid of the program.

As a short term solution, you should be able to provide the motors with -3 power, when you actually want them to be at power level 0.

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Jesse Flot
CMU Robotics Academy
ROBOTC Support


Wed Mar 23, 2011 2:55 pm
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Post Re: 393 motors irratic on RobotC vs 2.31
Jesse, ours are off by as much as -9.


Thu Mar 24, 2011 11:23 pm
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Post Re: 393 motors irratic on RobotC vs 2.31
We've received a new master firmware. After we do some testing, we'll release a new version of ROBOTC which fixes the issue.

Thanks again,

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Jesse Flot
CMU Robotics Academy
ROBOTC Support


Thu Apr 07, 2011 11:12 am
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Post Re: 393 motors irratic on RobotC vs 2.31
Jokai - thanks for posting the problem. We experienced exactly the same problem this week and have been pulling out our hair trying to solve it. We were convinced it was a friction issue somewhere in the drive train but just couldn't find any problems. :eek:

Thanks also to the RobotC programmers for fixing the problem. It's a week before Worlds and everyone is frantically trying to get their autonomous program working.

Brad V


Thu Apr 07, 2011 9:00 pm
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