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4QD controllers
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Conventionally, joysticks are mounted (and designed) so that movement front to back (axis A on diagram 1) operates one pot (P1) and movement sideways operates the second pot, P2. Whatever logic is required is done in the controller, so that P1 causes both channels to react equally while P2 causes one channel to increase speed and channel B to decrease speed. This differential logic is done by 4QD's dual-channel joystick interface, or within the radio control interface.
However, with a suitable joystick the sum and difference control can be done mechanically, by mounting the joystick at 45°
Motion along axis A affects P1 and P2 equally. Motion on axis B turns P1 one way and P2 the other way giving differential action.
The advantage of doing this differential action electronically is that the 'steering' gain can be altered to give appropriate differential action to suit the vehicle mechanical design. Using the mechanical method, the differential gain is fixed as the controller operating on, for instance, P1 cannot know whether the changed input voltage is due to movement on axis A, axis B or on both.
If you are going to experiment with this, you need to source a joystick that has no 'feel': many joysticks (especially the simpler, resistive ones) have mechanisms that give a definite feel to straight movement on the A and B axes in figure 1. These would hardly feel right mounted at 45°. More up-market joysticks tend to use inductive technology rather than resistive and often the 'straight' axes in these have no 'feel' at all.