In SX Microcontrollers, SX/B Compiler and SX-Key Tool, tim292stro wrote:

The biggest concerns with mechanical joysticks are:

1) power supply stability for the analog supply (what you power the potentiometer with)
2) life-cycle of the potentiometer, especially when feathering (if you move the potentiometer in the same place, over time the wiper will literally grind away the resistor material)
3) Mechanical tolerances (how well the joystick handle is attached to the potentiometers, and how rigid the base is attached to the vehicle)

What you might consider, instead of potentiometers, is hall-effect sensors with digital outputs.  Hall effect sensors come in various forms - the most useful to you would be one that detects the absolute rotational angle of a magnet and reports it with a quadrature or serial output.  This way you take out of the reliability and consistency equation, the analog power supply, the potentiometer life cycle, and a A-to-D conversion that is probably temperature sensitive.  The absolute position can be loaded at boot-up as some Hall sensors allow for an SPI absolute reading.

Take a look at the [url=http://www.melexis.com/Prodmain.aspx?nID=566]Melexis MLX90316[/url].  This is a rotary sensor that is available with an analog radiometric, PWM, or SPI output which you can [url=http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=MLX90316KDC-SPI-ND]get from Digikey for about $14US[/url].


As for the mixing, try this (assume 8-bit variables, "turn" input is assumed centered at 127):

TurnLeft = TurnInput - 127
TurnRight = 127 - TurnInput
SpeedLeft = SpeedInput + TurnLeft
SpeedRight = SpeedInput + TurnRight

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