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  #11  
Old 02-21-2012, 05:41 AM
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The VEX Raptors The VEX Raptors is offline
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Re: Lift Electrical Issues

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.
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  #12  
Old 02-21-2012, 07:40 AM
jgraber jgraber is offline
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Re: Lift Electrical Issues

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?

Quote:
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.

Last edited by jgraber; 02-21-2012 at 07:55 AM. Reason: answer question
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  #13  
Old 02-21-2012, 12:30 PM
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jpearman jpearman is online now
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Re: Lift Electrical Issues

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

See the wiki for details.
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  #14  
Old 02-21-2012, 02:43 PM
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Re: Lift Electrical Issues

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.
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  #15  
Old 02-21-2012, 03:25 PM
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Re: Lift Electrical Issues

Quote:
Originally Posted by The VEX Raptors View Post
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?
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  #16  
Old 02-21-2012, 03:35 PM
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Re: Lift Electrical Issues

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.
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  #17  
Old 02-22-2012, 09:57 PM
jgraber jgraber is offline
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Re: Lift Electrical Issues

Quote:
Originally Posted by The VEX Raptors View Post
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.
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  #18  
Old 02-23-2012, 07:22 PM
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The VEX Raptors The VEX Raptors is offline
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Re: Lift Electrical Issues

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.
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  #19  
Old 02-24-2012, 11:04 AM
jgraber jgraber is offline
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Re: Lift Electrical Issues

emphasis added by jgraber
Quote:
Originally Posted by The VEX Raptors View Post
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.
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?
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  #20  
Old 02-24-2012, 08:04 PM
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Re: Lift Electrical Issues

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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