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Issue with limiter pot inline with joystick pot

Gary E. Terry

Jan 10, 2015
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Jan 10, 2015
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I am working on a project to limit the inputs on a joystick on a RC quadcopter transmitter.

Simple enough to add a 10k linear dual gang rotary potentiometer inline with the wires coming off of the joystick pot.

It works fairly well, except for one small issue that I've not figured a way to solve yet.

When my limiter pot is at about 50% travel the joystick pot gets thrown off center by a very small amount but only in one direction. . I can see it in the application that calibrates the joystick and when the quadcopter is flying it'll start a very slow turn. The limiter is on the yaw.

Does anyone have any thoughts about how I might be able to elimitate what I think might be a bit of feedback in my circuit?
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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Nov 28, 2011
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Hi Gary and welcome to Electronics Point :)

Yes, that's going to happen. Think about it like this.

Say your joystick potentiometer's resistance is 50 kΩ (and it has a linear law). Therefore at half position, its resistance will be 25 kΩ. Your controller will regard a 25 kΩ resistance as being centred.

Now you're adding another resistance (your limiting trimpot) in series with the joystick to limit the maximum distance in one direction. Let's say that you've set this resistance to 5 kΩ because that gives you the limiting behaviour you want.

Now when your joystick is at mid-position, the total resistance seen by the controller will be 30 kΩ and it will think the joystick is slightly off-centre.

A simple, non-ideal but probably adequate solution is to add a high-resistance trimpot in parallel with the joystick, so when it's at mid-position, its resistance is reduced to 20 kΩ and the controller sees a total resistance of 25 kΩ as it should do.

Resistances in parallel follow the formula RTOTAL = (RA × RB) / (RA + RB), which can also be written as 1 / RTOTAL = (1 / RA) + (1 / RB).

So for my example numbers, a resistance of about 100 kΩ in parallel with the joystick will reduce the centre resistance from 25 kΩ to 20 kΩ. You should make some measurements and work out what value to use.

Use a trimpot because adding a resistance in parallel with the joystick will also affect its maximum resistance, and you'll have to adjust the limiting trimpot to compensate. These two trimpots will interact with each other and you'll have to do some experimenting to get settings that will work.

I said this is not the ideal solution because adding a resistance in parallel with the joystick will affect the linearity of the relationship between the joystick position and the combined resistance, so it will affect the relationship between the joystick position and the value being sent to whatever the joystick is controlling.

The smaller the resistance of the limiting trimpot in comparison with the joystick resistance, the higher the parallel trimpot's resistance, and the less effect the change will have on the linearity.
 
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