Designing an Arduino Control System for a LEGO Watt Balance

The physical apparatus of our LEGO Watt balance is based on a paper by L. S. Chao et al. (American Journal of Physics, Nov 2015). We replaced the LabVIEW interface with an Arduino Uno as the control system because there is no license fee, the software is widely available, and it is more instructive for undergraduates. Our goal was to construct an Arduino-controlled LEGO Watt balance to 1% precision, matching the precision in the paper. In force mode, we used a PID system. This computes the position and velocity of the lever arm, and based on those values, it adjusts an output to tune the balance to a goal height. We implemented a 12-bit DAC for more precise control over the balance. In velocity mode, we sent a varying pulse-width modulation signal from the Arduino through a low-pass filter to create a sinusoidal voltage, driving the coil vertically. We successfully operated the LEGO Watt Balance with 2.6% precision. Force mode dominates the uncertainty, but is insufficient to explain all the inaccuracy. The error is likely from the fulcrum block slipping because the lever arm does not consistently return the coil to the same height. To improve our precision, we can try filing down the fulcrum block to a knife-edge or limiting the range of motion of the balance.