This week, together with the Machine Design week, aims to teach us the basics of designing, building, operating, automating, and troubleshooting our own machines. Given the fairly general machine inventory of a basic Fab Lab, it is possible to construct more specialized tools, which expand the range of available manufacturing processes, making it possible to create even more refined machines and projects.

However, designing a useful machine is hard, and there were not many restrictions on this weeks assignment, so we decided to build a motorized ball game: A ring with a light sensor is attached to four ropes, which can be lengthened and shortened to move the ring around, much like the famous sky cam (but a little less sophisticated). The goal is to throw a ball through that ring to score points; once a hit is detected, the ring is moved to another position. To get more details on the machine and a higher overview of what we built, visit our group assignment page. My contribution is a motor controller shield for an Arduino.

The Motorshield

Our machine requires four motors, one attached to each of the four ropes holding the ring in the middle. We used stepper motors, since they are comparatively easy to control and have a high torque and precision. I designed a shield for the Arduino Uno which features a power indicator, a connection to an external power source, and space for up to four stepper driver boards of type A4988. The step and direction pin of each driver is connected to an I/O pin on the Arduino. Microstepping mode can be selected via hardware jumpers. All drivers are always enabled. Because space is really tight with four drivers, the board is double sided.

The bottom side is milled first.

After turning the copper board. The row of small vias at the bottom side is used to fine tune the alignment before milling the top.

First try after turning, no adjustments. Note that the via is not milled properly.

Second try. Better.

The milled board, top side. I tried a couple more times to adjust the position, as can be seen on the bottom.

I solder the vias with a single thread of wire that connects top and bottom.

All vias soldered. Threading the wire first and then soldering all at once saves time.

Top side is finished.

The finished motor shield.

Bottom side.

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I had to hand-solder the vias with thin wire and solder, so I tried to design the board in a way that minimizes the via count while making the necessesary vias easily accessible. This way of making vias works, but is cumbersome to solder a lot of them, and they cannot be put underneath SMD components. I would really like a way to get plated through holes and vias like it's done on a professional level.