A flight command mixer for delta and V-tail radio-controlled model planes

Exactly what made the author start out on this PIC-based project is unknown to him, since he has the good fortune of owning a radio-controlled (RC) transceiver with a programmable mixer. He thinks it was probably because he was just looking for something to program and tinker around with!

There were one or two challenges on the way that he thought were insurmountable (having only limited programming knowledge). These applied mainly to the resolution of the acquired input pulses and the generation of the mixed output pulses. The challenge was to achieve a resolution of one microsecond, giving a thousand steps over the servo’s range – the resolution which top-notch RC radio sets offer.

With the PIC running at 4MHz, this means a resolution of one program instruction cycle. PIC programs run at the oscillator rate divided by four, fosc/4, over a range of ten bits. This 10-bit requirement ruled out the use of the PIC’s TMR0 timer, which only has an 8-bit capability. The function of the mixer is just to crossmix two channels for delta or V-tail configurations. In such a configuration, one servo operates one control surface.

However, taking elevator/aileron mixing as an example, each of the servos must respond to both aileron and elevator commands for the required action to be performed. The servos must receive the signal to move the flaps in the right way. The mixer thus takes the signals as they come from the receiver, does the calculations, and generates new signals to move the servos correctly.