From SeniorDesign

[edit] Introduction

A stepper motor is much like a regular electric DC motor. Like a DC motor, it rotates by energizing electromagnetic coils to apply a magnetic force on permanent magnets which causes the stator to rotate. It differs in the fact that multiple coils are used. This allows the rotation to be very accurately controlled by energizing the coils in a certain pattern.

The stepper motors used are 2 coil. This means there are four leads per motor. By changing the direction of current flow in a specific pattern through the 2 coils each a step of around 1.8 degrees can happen. Other stepper motors using more coils and other methods can be much more accurate.

A stepper motor requires a driver circuit to function correctly. The driver can be programmable or fixed logic. Fixed logic has the advantage of speed, where a programmable driver board has the advantage of customization. The driver board designed here is programmable. There are many methods used to drive stepper motors. The method used here involves using an H-Bridge to handle controlling the current flow across the coils and a programmable integrated circuit to control the sequence.

[edit] H-Bridge

H-Bridge circuits are used in robotics primarily to invert polarities across DC motors. This allows a motor to flip between rotational directions. The IC selected uses digital logic gates to control BJT transistors. The transistors switch between cut-off and forward-active mode to determine the flow of current.

This is a circuit setup to simulate an H-Bridge.

The red waveform is the simulated signal. The blue waveform is the voltage applied to the load. As you can see the h-bridge doesn't just apply Vs to the load and then remove it. The H-Bridge inverts the voltage applied to the load. The frequency used is around the max of what the CNC router will run at. Since the IC selected, ST L298, has limited documentation it is impossible to setup the simulator to run exactly like it will in real life.