RC Powerwheel potentiometer tweeks?

[rotarypower101]

So I have a RC powerwheel that I am building, and I need some help with the control systems that I am building for it.

If you are familiar with the RC hobby, then you will probably know what a servo tester is, (it can be directly put in line of a std servo and battery and control the servo via a potentiometer) well I am trying to make these servo testers my manual input for the vehicles steering servo and electronic speed control.

So the control system is both RC and manual based on a remotes 3rd channel toggling between the two modes, for quick recovery of the little tikes from imminent peril.

I have a few problems with the pots that I have come across that I would love to solve.

First a small description of the mode of operation of the servo testers used: they use a small linear 10k ohm pot to control the pulse width that is being sent to the servo 1.0-2.0ms

The issue I am having is that I need to change the operation of these “servo testers” for both the throttle and steering to accomplish my goals and give the correct operation.

For the throttle I would like to change the servo testers operation to only operate in half the normal range either 0-5k (or more precisely 5-0k for correct operation) or 5-10k range of the original full 0-10k pot. I only need half the range because the pot will be connected to a gas pedal and can only utilize the forward direction, there is a stock gear shifter that has reverse on it so this will not be an issue to enable reverse.

For the steering I have technically solved the problems I have had (I needed 1/3 of the sweep ~90* of the full ~270*) by gearing the pot and steering wheel.

But I would still like to find a solution to the question of how to deal with a situation where 90* of operation was required for the full range of a pot to operate correctly? The issue I was having was that I had a 10k ohm pot, and thought since I needed only 90* of motion, I could use a 30k pot and just use 1/3 of the 0-30k sweep to get a 0-10k ohm output, but no dice...

Here is a short clip showing its operation, safe to assume I will never be a cinematographer..
For reference this is the windshield and steering column upside down.

YouTube - Powerwheel steering pot

And here is a pic of what the device looks like
**broken link removed**

 

[Electronworks]

It is true, you are no cinematographer

The issue I was having was that I had a 10k ohm pot, and thought since I needed only 90* of motion, I could use a 30k pot and just use 1/3 of the 0-30k sweep to get a 0-10k ohm output, but no dice...

What went wrong with the 30k pot? Instinctively this would be way I would do it...

 

[BrownOut]

If forward motion is controlled by 5-10k portion, then just replace the 10k pot with a 5k fixed resistor and 5k pot in series. Unless I'm missing something???

 

[rotarypower101]

It is true, you are no cinematographer



What went wrong with the 30k pot? Instinctively this would be way I would do it...

Thanks for chiming in I really need some help understanding this issue.

Well just from empirical testing this did not produce any change in the system,(which believe me through my ideas and grasp of how this seemingly simple device “should” work right out the window)

Would this be a nonstandard utilization of this component,or am I missing something if this is the reaction I get from the device by changing the resistance of the pot?

If I change the “servo tester” pot to any pots I have on hand the percentage of the throw on the given pot is directly proportional to the action I see on the output i.e. Servo.

So I am assuming the “servo tester” reads the total resistance across the pot, and compares this to the middle leg (do pots have a name for the individual terminals? Could not find this through a short search) as opposed to just knowing the pot is = 10k and comparing that to the variable resistance through two consecutive pins.

Does this seem like a sensible conclusion?

 

[rotarypower101]

If forward motion is controlled by 5-10k portion, then just replace the 10k pot with a 5k fixed resistor and 5k pot in series. Unless I'm missing something???

I did try this, but this did not seem to produce the effect I was looking for either, which again surprised me. And thats why I am searching for a little knowledge on the subject.

Logically this should work IMO, but seemed to produce no direct effect on how the servo tester instructed the servo to react. I could have made a mistake, I was fairly tired, and fairly disappointed that my ideas on how this should work had failed my twice on the same subject....

I will give It a more thorough investigation and report back.

 

[rotarypower101]

If forward motion is controlled by 5-10k portion, then just replace the 10k pot with a 5k fixed resistor and 5k pot in series. Unless I'm missing something???

(I tried to post this yesterday running out the door, and i guess it didnt post because of a 30sec restriction on postings..)

I also have some questions about hardware, I have found a few items that may work for my throttle potentiometer, but again I will have to modify how the potentiometer is seen by the servo tester.

Because the gas pedal needs to only work over half the 10k range of the servo testers pot, I was hoping I could simply use a 5k pot and use a resistor in series (so 5k is seen initially, and 0-5k is added to the total, R=5k+pot), but when I was testing that last night is did not seem to work.

This is the item I was hoping to use but I am sure their are better substitutes out there?

Does anyone have a more suitable component that they could comment on, this one seems to fit the bill fairly well, but it is rather expensive, and I am sure there is probably a much more suitable component at a lower cost that could even be better.

BI TECHNOLOGIES / TT ELECTRONICS|404R5KL1.0|POT, LIN MOTION, 5KOHM, 10%, 250mW | Newark.com

**broken link removed**

 

[Electronworks]

So I am assuming the “servo tester” reads the total resistance across the pot, and compares this to the middle leg (do pots have a name for the individual terminals? Could not find this through a short search) as opposed to just knowing the pot is = 10k and comparing that to the variable resistance through two consecutive pins.

This should work. You can use a potentiometer in 2 ways - one is as a variable resistor. The other is as a variable 'divider' that divides the voltage across its 2 end terminals to give you a voltage at the wiper that is dependent on the wiper position.

Therefore, if you put a series resistor in line with the potentiometer, you will be adjusing the wiper voltage over a smaller range.

Looking at the resistor on your video, it looks like a normal standard, carbon tracked potentiometer that you can pick up for <$0.50. Put a 0.5W resistor in series with this, value 5k, and you should notice a difference