Connections

A stepper motor normally requires that each coil gets powered in "direct" and "reverse" mode. As EZ-Red has fixed polarity on its outputs, I took a unipolar motor which, for each coil, has 3 wires: by using the middle one as common return, it is possible to reverse the polarity when needed (losing some power, but this is just an experiment). The connection scheme I used is the following:

Power supply and resistors must be chosen depending on the motor: as a starting point, 16 volts and 200 ohms / 2 watts are safe values.

PLC Cycle

The cycle is designed around the idea that the correct sequence for energizing the coils is like:

• Coil 1 direct
• Coil 2 direct
• Coil 1 inverse
• Coil 2 inverse

...and this sequence repeats forever. After every new output configuration, some delay is needed - this time determines the angular speed of the rotor. The motor used in this case has 200 steps per turn, so a delay of 5 ms per step (200 steps a second) generates a full rotation every second. These concepts expressed in EZ-Red language lead to a simple task like this:

To rotate the motor in the opposite direction, simply use this sequence, but reversed. If, after each step, a variable "posmot" is incremented (or decremented), it is possible to know (and hence, control) completely the motor. So, the PLC cycle contains two tasks (clockwise and counter-clockwise) to make the motor turn: by starting (and stopping) one of the two tasks, and monitoring the rotor position, we can completely control the motor. By writing the STPDELAY variable, even the speed is controlled. The main task reads the potentiometer and decides whether to change the rotor position and, if yes, in which direction. The difference between the potentiometer position and the motor position is also examined in order to choose the motor speed.

The full program listing (which is not a perfect one) is the following: