chairs roll an inch backwards

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chairs roll an inch backwards

Postby biscuit » 19 Jul 2022, 13:45

I have two chairs that both of them roll backwards a couple of cm when I let go of the joystick, while riding up the car ramp. One is a Q500R with VR2 and 75Ah battery, one is a - ahem - Cirrus I think they call it, with a VSI AND 40Ah battery. Both do the same annoying thing and the Q500R is hard to line up in the car's minute parking space because it rolls back after going up the ramp as well, though not when it is on the driveway.

I expect I could programme the joystick/controller better. I do not feel that the roll back settings in the programme refer to this habit. Any suggestions please, for VR2 and VSI programme, but pointers from r-net will be a help even though I don't have that.
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Re: chairs roll an inch backwards

Postby Burgerman » 19 Jul 2022, 14:34

There is a time. A delay. That we cannot adjust. Theres also about 1 inch or so of actual backlash in the gears/shaft/ that cannot ever be controlled. The time the brake is applied (the BRAKE power released so that its spring clamps the brake back on) depends in the firmwares code looking at several things. The adjustments we can change in programming dont really change that. We can only disguise the delay. By increasing or at least setting more precise motor load compensation. And 2ndly by setting the minimum turn acceleration, mininum forwards and reverse acceleration, minimum turn deceleration, to the max 100. Even then if the current sensors detect a motor generated current or a power module generated current it will not set the brake as you may be at some speed decending a steep ramp and just trying to slow. It cannot just go by stick position. And some delay is needed to prevent them from clicking in/out maybe 50 times a second when the right signs are present. To act as an added hysterisis. Or they would apply as you were at speed and cross neutral point when adding some back stick braking to slow. So there will always be some time when the computer reads the zero motor power condition and it actually applying the brake.
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Re: chairs roll an inch backwards

Postby biscuit » 19 Jul 2022, 15:19

Ok many thanks, Burgerman. I will tweak my settings as per your suggestions and as usual I have a better understanding of the machines from visiting this forum. And the hysterisis will not make me hysterical any more.
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Re: chairs roll an inch backwards

Postby Burgerman » 19 Jul 2022, 15:26

It might when you find it still does it!
:clap
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Re: chairs roll an inch backwards

Postby biscuit » 19 Jul 2022, 15:38

No it has lost the power to annoy me, as it is nothing I caused. :joint
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Re: chairs roll an inch backwards

Postby slomobile » 19 Jul 2022, 17:52

This is a common problem across all wheelchair brands when stopping on a ramp, and is unavoidable to some extent.
Rolling backwards happens when the chair transitions from being held in position by motors to being held by brakes.

If you just keep moving the joystick near the center position rather than letting go, it will keep the motors on and prevent the parking brake from engaging. If you have typical programming settings(not aggressive acceleration), It doesn't matter much which direction you move the stick as long as it is constantly changing and near the center it wont move the chair much and just hold it in place.

Of course there are some times when you just have to let go of the stick. So just be prepared for that rollback followed by a sudden stop. And when starting again on the ramp, its common to roll backwards before the controller ramps up enough to start moving forward. Especially with typical low acceleration programming.

You can eliminate rolling back when starting on a hill by just pushing the joystick farther forward initially to ramp up current faster, then quickly lower the joystick to a more reasonable level to prevent popping a wheelie and going over backwards. Its a tricky, risky move, but possible with good hand control.

I can't really speak knowledgeably about how Burgerman's programming would change the way you need to handle the stick. I still haven't got my hands on an Rnet programmer. But I did just get a van w/ ramp, so I'm getting lots of practice trying not to tip over.

In a system like Lenny's where we have fine control over all aspects of programming, this rolling backwards could be tuned out if we provide a means for the controller to know when it is on a ramp. An accelerometer, or wheel encoders, along with a bit of code could keep the motor current flowing until just after the brakes engage, rather than before.
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Re: chairs roll an inch backwards

Postby Burgerman » 19 Jul 2022, 18:23

Not to mention you can set the brake delay time to any value you want in the roboteq's own programming.
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Re: chairs roll an inch backwards

Postby slomobile » 19 Jul 2022, 20:12

That is very handy. What happens if you set brake delay to -0.5 seconds on Roboteq? Do negative numbers actually apply brake before stopping motor current?
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Re: chairs roll an inch backwards

Postby Burgerman » 19 Jul 2022, 21:14

Dont think it allows that. I set mine the ther way. several seconds. Becaue all that clicketey clack bugs me. So it seldom clicks unless you really stop! On sopes/ramps etc I hold the chair with the joystick anyway. Many of my chairs have the brake lever flipped so it never gets applied unless I choose to allow that. And the "fault" detect microswitch on the lever is removed. Or shorted. So it always drives.
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