An extremely low cost, but high performance distributed motion control system is presented. During the development of automation equipment, rapid prototyping of multi-axis designs is often required. One can use centralized controls with PC motion control cards, external amplifiers, and a tangle of wires, or utilize existing high cost distributed motion control system with RS485, Ethernet, or one of the fieldbuses (DeviceNet, CANOpen, Profibus, etc.). With the use of highly integrated microcontrollers, such as the PIC18FXX8, and power devices like the Allegro A3977/A3959, a very small and inexpensive system was developed.

The main goals of the system were as follows:

The system consists of two units (I/O expansion in the future); a USB-CAN converter and a dual axis controller. The USB-CAN converter uses the PIC18F258 and a FTDI FT245BM USB interface chip to translate messages between the USB and CAN buses.

The PIC18F458 is kept busy implementing the following servo/stepper trajectory and PID calculations. A new calculation is made every 500 usec:

V(k) = V(k-1) + A - current commanded velocity

P(k) = P(k-1) + V(k) + A/2 - current commanded position (maintained as 48 bit integer 32.16)

E(k) = P(m) – P(k) - error = measured position – calculated position

Output(k) = min ( KpE(k) + KdV(k) + Ki£ Ej + Kvff * V(k) /4+ Kaff * A * 8 + Bias, max_output) – Output DAC value

The stepper step rate is calculated as follows:

V(k) = V(k-1) + A

Timer3Reload = 65535 – 40960000 / V(k) – Timer 3 setup as 800nsec per tick (Fosc/8).

Most of the on chip resources are utilized including all four timer/counters, hardware SPI, CAN, UART, ADC, EE, program updates of flash, digital I/O, interrupts, ICD, RAM, and watchdog timer.