Originally it was: potentiometer ->(analogue voltage) -> arduino ->(i2c) -> Servo control pic -> (pwm) -> servo. This works fine in the garage but when an engine is running it frequently cuts out. In some bench testing we found the pic was sensitive to noise (even the noise generated by servos was enough for it to lock up sometimes). Adding extra capacitance on the power lines appeared to cure that.
We're now going to try: potentiometer ->(analogue voltage) -> arduino -> (pwm) -> servo. to reduce the variables.

I think bolt lightning was using an Arduino to control there engines, so I think that should be tough enough to survive working near an engine. I'm not sure if they were using servos or something else though.