Servo - March 2008

Building a

STEPPER MOTOR

CONTROLLER Part 2

by Fred Eady

Now that I have a Minebea 29SM-K series 1.8°-per-step stepper motor shaft spinning back and forth on the bench, I figured it might be a good time to sit down and tell you how I persuaded an STMicroelectronics L6208PD to push current through the Minebea’s stepper motor coils. Unfortunately, I can’t provide a visual play-by-play of my Minebea’s alternately spinning motor shaft. However, I can give you a description of the hardware and software stuff you will need to make your motor spin under the

control of an STMicroelectronics L6208PD and a PIC18F2620 microcontroller.

Over the years, I’ve read thousands of datasheets and application notes. To this day, it is still a thrill to translate the words of the manufacturers into a functional device. In the previous SERVO L6208PD installment, we walked around the L6208PD with the intent of putting the L6208PD to work with a stepper motor and PIC microcontroller. All of the preflight work we expended last month will be applied to our goal this month of creating controlled rotational movement

of a stepper motor shaft.

Spinning the Printed Circuit Board

SCREENSHOT 1. The idea is to provide as much heatsink area as possible for the L6208PD. We also have to adhere to the L6208PD printed circuit board guidelines to keep electrical noise to a minimum.

The beginnings of our L6208PD Motor Driver Board can be seen in Screenshot 1. I’ve used ExpressPCB as the printed circuit board design and layout program so that you can either copy my printed circuit board layout or use my original printed circuit board pattern as a basis for your own L6208PD printed circuit board (PCB) design.

Note that the L6208PD Motor Driver Board design provides a relatively large heatsink area for the L6208PD on the top side of the PCB. It’s a good idea to physically bond the L6208PD’s heatsink slug to the heatsink pad area. So, as you can see in Screenshot 1, I’ve included some top-to-bottom thru holes under the L6208PD’s heatsink lug to provide solder access once the L6208PD has been mounted on its pads.

Before mounting the L6208PD, I applied some solder paste to the L6208PD’s heatsink lug. After completely soldering down all of the L6208PD pins, I used a high-wattage soldering iron applied to the bottom side heatsink pad to flow the solder paste on the L6208PD heatsink lug. I can tell you from practical experience that this method of mounting the L6208PD is sound.