Mobility Scooter Potentiometer

Thread Starter

BCFD546

Joined Jun 19, 2017
7
Okay guys,
I'm working on an Invacare Buzz HMV mobility scooter and found it to have a bad 5k throttle pot. I replaced it with another 5k pot and now it works. Unfortunately, the throttle wigwag only travels about 25 degrees and the pot has a 300 degree mechanical travel. Because of this, I have to manually turn the pot to get any speed. If I attach the wigwag, it doesn't turn far enough to really get any power due to the limited travel distance of the wigwag. Is there a way to tweak the new pot or is there a different type of pot or rheostat I need to replace it with? I can't find the old pot that I removed, so I don't know the model number of it. Any help would be greatly appreciated.
Thanks!
 

djsfantasi

Joined Apr 11, 2010
9,188
Someone else had a similar problem. So the "throttle wigwag mechanically restricts the rotation to 365°? I'm at the dentist, so if you can measure the rotation of your pot, thanks. It will help me answer your question.
 

Thread Starter

BCFD546

Joined Jun 19, 2017
7
Unfortunately I'm at work right now so I can't measure. The wigwag has around a 25-30 degree rotation before it hits the physical stops on the scooter, yet the pot has a 300 degree travel. The pot works fine but has to be turned much further than the wigwag can travel before there's any real speed. I haven't been able to find a pot with a smaller mechanical travel (degrees). I'm at a loss for ideas other than tearing the pot open and drawing a trace with a conductive pen.
 

#12

Joined Nov 30, 2010
18,224
I started running the numbers and hit a limit: Is the machine looking for +/- 2.5K or is it looking for the voltage on the wiper at the alleged ends of the wiper rotation?

It's simple enough to play with some resistor values to get +/- 2.5K, but if the right answer is a voltage, that gets into a DC amplifier.
 

Thread Starter

BCFD546

Joined Jun 19, 2017
7
Ooh. I'm not sure. With the pot in the middle position, it outputs about 2.5 v to the controller at 2.5k. A little to the negative side and we get speed in reverse, increasing to the full -5k gets us full speed in reverse. Opposite applies for forward. So in order to get full speed, the wiper had to be at its travel limit. Does this make sense?
 

djsfantasi

Joined Apr 11, 2010
9,188
I started running the numbers and hit a limit: Is the machine looking for +/- 2.5K or is it looking for the voltage on the wiper at the alleged ends of the wiper rotation?

It's simple enough to play with some resistor values to get +/- 2.5K, but if the right answer is a voltage, that gets into a DC amplifier.
If the answer is voltage, it depends on how the pot is wired into the circuit. If wired as a variable resistor, a standard pot can be used. If wired as a voltage divider, then I agree with you.
 

djsfantasi

Joined Apr 11, 2010
9,188
I think I have confused the issue with imprecise terms. If the "brain" needs a single input from 0-5V, then a variable resistor is all that's needed.

The schematic would be useful.
 

Thread Starter

BCFD546

Joined Jun 19, 2017
7
I've been looking at that throttle pot. The weird thing is, that's a completely different throttle wigwag than the Buzz has. I'm thinking the pot from it may work though.
 

Alec_t

Joined Sep 17, 2013
14,411
You could perhaps use a (good quality) standard pot plus a DC amplifier (as #12 mentioned).
Something like this should get you a 0V-5V output swing with the wiper moving over a range from 40% to 60% of its full travel V (i.e. ~30° either side of centre).
WigWag.PNG

Edit: R1 and R2 should be 10k.
 
Last edited:

Alec_t

Joined Sep 17, 2013
14,411
If you do give it a shot, the opamp needs to have a rail-to-rail output capability and be able to run from a single 5V supply. I assume the pot in the existing wigwag has 5V across its end terminals?
 

#12

Joined Nov 30, 2010
18,224
only travels about 25 degrees and the pot has a 300 degree mechanical travel.
a range from 40% to 60% of its full travel
the usual answer is that the input to the brain is a voltage between zero and five.
There are still some uncertainties.
a) +/- 25 degrees is 1/6th of a rotation, not 1/4th. Easily solved by changing R4 in the circuit of post #15.
b) potentiometers are not perfect in their centering and range. "Zero" is rarely zero and "all" is rarely what the label on the pot says, +/- 20%
c) Automotive processors discover the actual voltages of the throttle position sensor and adjust accordingly, thus rendering a and b moot, if a battery operated scooter has that kind of programming and processing power.
 

Thread Starter

BCFD546

Joined Jun 19, 2017
7
If you do give it a shot, the opamp needs to have a rail-to-rail output capability and be able to run from a single 5V supply. I assume the pot in the existing wigwag has 5V across its end terminals?
I'll look into it further but I do believe it's 5v across the end terminals.
 
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