Servo - November 2008

Current (i)

Rated coil current

Ideal
Simple driver
Current regulated driver
Simple driver with double voltage

Time (t)

Figure 5: Current transient curves showing the benefits of a regulated current driver

limiting with a transistor. Initially, the transistor allows full supply voltage across the coil. Then, as current through the coil increases, the transistor partially turns off to limit current to the rated value.

The down side to this analog approach is that significant energy is wasted as heat in the transistor. A more efficient method is to switch the large supply voltage all the way on or all the way off. The on time is adjusted so that the average coil current is appropriately limited to the rated value. This technique requires a few additional

components to sense and switch the supply current, but it is one of the best ways to noticeably increase motor torque and top cutoff speed. And conveniently, we already have a microcontroller available to monitor the current and control the switching.

Conclusion

There are several methods of fine-tuning the control system to enhance a stepper motor’s performance. Fortunately, by using a microcontroller like the Propeller and a few external components, we have tremendous flexibility to create a control system as simple or complex as necessary for our application.

Next month, Chris Savage will be back with the final article in this series to discuss the use of PWM for DC motor control. Be sure to check out the Parallax web link for schematics, Spin code, and photos of this project. And as always, good luck and have fun! SV