Stepper motors are easier to control than servos, but they typically produce lower power. A number of people have built stepper motor run CNC projects, so for small or mid-size CNC control they work.<\/p>\n
I have three bipolar stepper motors, but seven 6-wire unipolar steppers plus other unipolar steppers with other wire counts so it makes sense to design a unipolar stepper controller.<\/p>\n
A unipolar stepper has three coils all of which get driven in the same direction by PWM signals amplified and isolated by a driver circuit. Because the drive current is always in the same direction an H-bridge is not required, rather a simpler driver can be used. <\/p>\n
The simplest way to drive the stepper is to activate each coil in turn slowly enough that the motor has time to move a step before the next step is given. A smoother and more power efficient method called microstepping uses PWM to transition gradually from one step to the next (Microchip App Note<\/a>). This requires four signals from the PIC to the driver. The PIC18F4x31 chips can output 8 PWM signals and so could drive two motors.<\/p>\n An intelligent driver with integrated stepping like the Allegro SLA7070MR<\/a> ($7) at only requires a direction and step signal. A PIC16F88<\/a> has 16 IO lines, enough to drive 4+ unipolar steppers, and PIC18 or PIC30 can run 4+ unipolar steppers plus has PWM lines to drive additional hobby servos. The PIC18 can have a USB interface and so could run a CNC by itself. <\/p>\n A more robust stepper controller interfaced to an SLA70xx driver would have a RESET pin, a common one running to all the drivers, and M0<\/sub> line monitoring the each stepper, and shared M1<\/sub> and M2<\/sub> lines running to all the drivers, so three common lines, and three per driver, so fifteen for four motors. Then figure each axis would have a pair of end stop switches, so four input lines for those, and one input line for a big STOP button. So altogether 20 IO lines for four axis control.<\/p>\n