1. 4 years ago

    The VEX Raptors

    20 Feb 2012 East Lansing, Michigan 7581H

    We ran into an odd problem at a scrimmage on Saturday. Our scissor lift, which uses 4 269s at a 1:21 torque gear ratio with elastic, would spontaneously stop. It happened during several matches and our first programming skills attempt. We know we are not overworking our motors, because our gear ratio is huge, and because this design has worked faithfully for 3 full tournaments, the most recent of which we won.

    Back when all of our wheel motors were in the power expander and frequently tripped the circuit breaker, we programmed button 7U on our controller to hold all of our motors at 0 for 5 seconds to reset the circuit breakers. We found that after using this reset, the lift would function normally. Because of this, we think there is some electrical problem. Here is our wiring:

    Port 1: Intake 1 (269)
    Port 2: Left front wheel motor (393) *plugged into power expander*
    Port 3: Left rear wheel motor (393)
    Port 4: Lift motor 1 (269)
    Port 5: Lift motor 2 (269) *plugged into power expander*
    Port 6: Lift motor 3 (269) *plugged into power expander*
    Port 7: Lift motor 4 (269)
    Port 8: Right rear wheel motor (393)
    Port 9: Right front wheel motor (393) *plugged into power expander*
    Port 10: Intake 2 (269)

    Could the scissor lift motors be tripping a circuit breaker? We don't think so, but I wanted to make sure.

    If it is not a circuit breaker, what could it be? We know that whatever it is it resets after 5 seconds.

  2. murdomeek

    20 Feb 2012 SFU (Ex 1107B)

    when everything "stops" is it just your scissor lift?
    or does everything (drive, intake, lift) stop?
    if thats the case, then you are having connection issues
    did you try changing to a set of keys that you know works?

  3. The VEX Raptors

    20 Feb 2012 East Lansing, Michigan 7581H

    No, it's just the scissor lift. Before we figured out that the 5-second-reset worked, we drove our robot into others for defensive strategy and then tried the corner goals. Once it stopped in the up position, and the intake still worked so we used match loads.

  4. jgraber

    20 Feb 2012 Dallas Texas metroplex

    Your problem text is a mix of
    "this is currently what happens"
    "we used to have a different motor port assignment and had a similar problem and fixed it with a button for 5 seconds drive 0"

    I can't tell what your current situation is because of this mixed description.
    Did the previous solution help your current situation?

    We know that whatever it is it resets after 5 seconds.

    This last line is not in the previous described evidence for your current situation.

    Have you verbally completely explained your problem to a teddy bear?
    They are known to have 50% success rate and can respond in real-time, if given a sufficiently detailed description.

    If it is not a circuit breaker, what could it be?

    These are the standard hypotheses:

    • broken motor internal gears
    • jammed mechanisms in the lift
    • bad programming code
    • pwm cables frayed by rubbing at joints
    • overworked motors
    • old elastic (originally installed 3 tournaments ago) has lost its tension

    Or if it is a circuit breaker, which one could it be?

    • circuit breaker in some motors tripping
    • 4A circuit breaker in power expander tripping
    • 4A circuit breaker in cortex motor port 1-5 tripping
    • 4A circuit breaker in cortex motor port 6-10 tripping

    What evidence do you have to support or unsupport each hypothesis?

    A $4 (on sale) DVM from Harbor Freight, in the 10A series current measurement mode, in series with each battery, can provide a lot of debugging information.

  5. Rick TYler

    20 Feb 2012 Teachers/Coaches, Event Partner Redmond, Washington None

    @jgraber Have you verbally completely explained your problem to a teddy bear? They are known to have 50% success rate and can respond in real-time, if given a sufficiently detailed description.

    First: :):):):):)

    Second: When I was in high school and trying to get my ancient and malformed car to start my father told me that the best thing to do was to wash the car's windows inside and out. This worked about 90% of the time.

  6. The VEX Raptors

    20 Feb 2012 East Lansing, Michigan 7581H

    The description of the past events is to explain why we programmed button 7U to hold the motors at 0 for 5 seconds. We have not needed to use it much for the previous problem, which has its own thread that you can find if you use the forum's search features that you are so fond of, but when this button is pressed the scissor lift will start working after the 5 seconds of waiting are done. That is how we know that whatever is stopping the lift motors resets after 5 seconds. It probably is not a jammed mechanism because when the lift stops, the motors are not trying. During a practice, when it stopped, we tried helping it up manually, but the motors weren't trying.

    The elastic has been replaced periodically, however it does not reset its tension after 5 seconds of waiting and as such is not the root of the problem.

    The wiring listed is our current wiring.

    Indeed I have explained my situation to a teddy bear. He, however, was fairly unintelligent and suggested posting to the forum.

    If it is a circuit breaker, it is most likely the power expander, as this (shown on the list of wires) contains the greatest concentration of scissor lift motors.

    Because of our extreme torque ratio, it is unlikely that the motors are being overworked.

    Our programming code is as it has been for the entire season, and has been checked over by MSU's software mentors.

  7. jgraber

    20 Feb 2012 Dallas Texas metroplex

    Do the intakes work when the lift is frozen?

  8. The VEX Raptors

    21 Feb 2012 East Lansing, Michigan 7581H

    The intakes do work when the lift freezes.

  9. jgraber

    21 Feb 2012 Dallas Texas metroplex

    That rules out

    - 4A circuit breaker in cortex motor port 1-5 tripping
    - 4A circuit breaker in cortex motor port 6-10 tripping

    Does it still drive when the lift is frozen?

  10. Oliver W

    21 Feb 2012 Auckland, New Zealand AURA

    This a slightly out of date picture and description of the robot:

    @The VEX Raptors Our scissor lift is 3-stack so that our manipulator can hang over the 30" goal and block other teams' scoring attempts while we're scoring or defending (it works very well in the autonomous mode; all other teams that try to score in the same goal as us in the autonomous period fail to score due to our blockage). I didn't know that the double ratio multiplied the torque. Perhaps we can change down the 12:84 to 12:60 for more speed.

    Sorry jgraber that is a typo. I meant to say we use four 269s. We have 24 strains of elastic on the lift. Here is a picture:
    Note that the picture is slightly out of date. The pneumatics are no longer present.
    -image-

    My guess is that the scissor lift is jamming. Scissor lifts in general (at least in Vex) are not very robust mechanisms, especially if the weight they carry is off centre like it is on this one. The fact that it resets after 5 seconds could be because there is a large amount of friction in the lift but the motors are still able to overcome it just because 1:21 is such a ridiculously torquey gear ratio. The friction might be overworking them while they do it though so they're tripping either the circuit breakers or the temperature sensors.

    Something to try would be to take the axles out that are connected to the motors and try to manually raise and lower the scissor lift by manipulating only the bottom sections. If you can't, or it gets stuck, then you'll know why the motors aren't able to.

  11. The VEX Raptors

    21 Feb 2012 East Lansing, Michigan 7581H

    It does drive when the lift has frozen, but we have found that the wheel base can function moderately well with only two motors running (we tried it with the power expander unplugged).

    It's important to note that our lift motors use clutches. If the motors are being overworked by friction, wouldn't the clutches strip?

    We will try moving the lift without the motors on. However we have found that if you pull up on the top of the scissor lift, the 2nd and 3rd stacks move freely until they start pulling on the 1st stack, which has the motors on it.

  12. jgraber

    21 Feb 2012 Dallas Texas metroplex

    1:21 for torque on a 3stage scissor lift,
    is like 1:7 on a short armed 4 bar, with 3x normal friction.
    That is not outstandingly high gear ratio.

    The Y cable test would be helpful to show if power expander is tripping breaker.
    Another method is to swap motor assignment ports in your program,
    and swap matching wires,
    so that one intake motor is on the power expander.
    Then check if that intake runs while scissor is frozen.

    That tests between breaker trip in the individual lift motors, and breaker trip in the power expander.

    If you show that the power expander breaker is tripping, then either

    • that is only the extremely normal sign of overstress motors
    • a sign that your power expander PTC fuse is worn

    Does the lift usually freeze in the same position?

    If the motors are being overworked by friction, wouldn't the clutches strip?

    Interesting point: Not necessarily. Friction just pushes current higher and speed lower for the same motor speed setting. If the friction is below the clutch slip torque, the clutches wont slip.
    Thought (or real) Experiment: motor through clutch into locking bar.
    at speed 0, no current, no clutch slip
    at speed 20, 20% duty cycle of stall current, no clutch slip
    at speed 127, 100% duty cycle of stall current, clutch slips(?)

    Looking back at your motor port assignments, you have 2x 393 drive motors on the power expander, as well as half the lift. If it is the power expander tripping, it could just as well be from the drive motors as from the lift.
    You just don't notice it because (as you said) you can drive with half motors, but probably you can't lift with half motors.

  13. jpearman

    21 Feb 2012 Moderator, ROBOTC Tech Support Rockstar, Los Angeles 8888

    If the PTC in the power expander is tripping then it's LED should indicate this.

    See the wiki for details.

  14. The VEX Raptors

    21 Feb 2012 East Lansing, Michigan 7581H

    Usually the lift freezes in the down position. We use it for part of a match, and then we bring it down in the middle of the match to pick up a ball or barrel, and then sometimes will not rise again until we use the reset button.

    Our power expander PTC may very well be worn; it tripped a lot earlier in the season when all 4 drive motors were in it.

    If the drive motors are still tripping the power expander PTC, how can we change our wiring to avoid it?

    We must remember that the structure of the scissor lift hasn't changed since the three tournaments in which it functioned. The joints of the scissor lift are all screws with locking nuts, loosened to the point where the joint can move freely (the only exception is the center of the bottom stack, which joins with a shaft). As shown in the picture we have nylon spacers between the two bars of each joint to prevent wobbling. The center joint of each stack has bearings to further reduce wobbling and friction. At the top and bottom we have linear slides with grease in them to ease friction.

  15. jpearman

    21 Feb 2012 Moderator, ROBOTC Tech Support Rockstar, Los Angeles 8888

    @The VEX Raptors Our power expander PTC may very well be worn; it tripped a lot earlier in the season when all 4 drive motors were in it.

    PTC's do have a limited lifetime, I have heard perhaps 100 trips but it will depend on the exact device. If the PTC trips on the power expander then you will get a flashing LED (green and flashing if the battery is good), does the LED flash on the power expander when the lift is in trouble?

  16. The VEX Raptors

    21 Feb 2012 East Lansing, Michigan 7581H

    Since the lift usually froze in the down position, the LED was very difficult to see (especially since we were in a match whenever it happened). At our meeting tomorrow we will try to recreate the circumstances and then we will check the light.

  17. jgraber

    23 Feb 2012 Dallas Texas metroplex

    @The VEX Raptors
    If the drive motors are still tripping the power expander PTC, how can we change our wiring to avoid it?

    Your post1 port list was this:
    Port 1: Intake 1 (269)
    Port 2: Left front wheel motor (393) *plugged into power expander*
    Port 3: Left rear wheel motor (393)
    Port 4: Lift motor 1 (269)
    Port 5: Lift motor 2 (269) *plugged into power expander*
    Port 6: Lift motor 3 (269) *plugged into power expander*
    Port 7: Lift motor 4 (269)
    Port 8: Right rear wheel motor (393)
    Port 9: Right front wheel motor (393) *plugged into power expander*
    Port 10: Intake 2 (269)

    One thought would be like this:
    Port 1: Right rear wheel motor (393)
    Port 2: Left front wheel motor (393)
    Port 3: Intake 2 (269) *plugged into power expander*
    Port 4: Lift motor 1 (269)
    Port 5: Lift motor 2 (269) *plugged into power expander*
    Port 6: Lift motor 3 (269) *plugged into power expander*
    Port 7: Lift motor 4 (269)
    Port 8: Intake 1 (269) *plugged into power expander*
    Port 9: Right front wheel motor (393)
    Port 10: Left rear wheel motor (393)

    • * moved 2x393 drive motors from expanders to ports 1,10
    • * split right/left wheel motors across hi/lo ports, so if hi or lo trip, you can do more than move in circles.

    If you post a picture with your lift in the down position,
    it will be easier to see that the elastic has near-zero transmission angle there.

  18. The VEX Raptors

    24 Feb 2012 East Lansing, Michigan 7581H

    We found at the meeting that the lift motors are tripping their own internal temperature sensors, not the power expander's circuit breaker. We tried some friction reduction on the sliders at the bottom of the lift, but after a few minutes of driving the motors still overheat. We think that we may have worn out the motors a bit, since our present motors have been on the lift for two months. That is sixteen meetings, a scrimmage, and two tournaments.

    I don't have access to my pictures now, but later I can post a few.

  19. jgraber

    24 Feb 2012 Dallas Texas metroplex

    emphasis added by jgraber

    @The VEX Raptors We found at the meeting that the[COLOR="Red"] lift motors [/COLOR]are tripping their own internal temperature sensors, not the power expander's circuit breaker. We tried some friction reduction on the sliders at the bottom of the lift, but after a[COLOR="Purple"] few minutes of driving [/COLOR]the motors still overheat.

    For the benefit of others,
    how did you determine that the (lift) motors are tripping their own internal temperature sensors?

    Why would driving (the wheel motors) overheat the lift motors?

  20. The VEX Raptors

    25 Feb 2012 East Lansing, Michigan 7581H

    By driving I meant executing the procedures that we use in a match (picking up balls and barrels, raising them to the appropriate height, moving the robot to a goal and then dropping the balls and barrels into the goal, and then repeating the process). This uses all of the motors.

    We discovered the motors were tripping their own sensors when the power expander's LED did not flash, and yet the lift was still frozen. Also the motors were hot (we did not know that earlier because all of the previous occurrences of this problem happened in matches at a scrimmage, when one is not allowed to touch their robot and feel its motors).

    We think that the motors may have worn a bit. We replaced them with new motors as a quick fix for our tournament tomorrow, and the lift works very nicely now. We discovered one slightly loose wire connection while replacing the motors. While that may have contributed to the problem, though, there must have been other issues because just one motor stopping would cause the lift to raise in an extremely lopsided fashion, not the full stop that we observed.

 

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