Identifying and Correcting Servo Errors in a Robot Cat

Tools, Design, and Process

- By Jose L. Rivero

When a robot cat doesn't move like a real cat, it's a problem. To make his robot move with catlike grace, Jose identified the problem (unstable servo voltage), and designed a solution, made with a Hall effect current sensor, a linear regulator, and a hand-built PCB. In this article he give us details on the final device that enabled the robot to consistently walk like a live cat.

My friends and I were building a Robot Cat, and encountered a problem with the robot walking—the servos were not behaving as expected. The robot would work well, and then would not, with no clear explanation of what was happening.

After a lot of confusion and hair pulling, by examining the signals and supply voltage, we determined that the servo peak current was drawing down the supply voltage, causing the servos to have an unstable voltage and to malfunction. To fix this battery power supply problem, I needed a method to measure both the DC and AC (peak) current used by the servos, so I could design the right power supply solution.

Based on previous experience with servos and other mechanical devices, I knew that figuring out how much current is needed to have a stable project is a real problem. So, after some analysis and a new current sensor design, I determined how to properly measure the current used, identify the power spike problem, and address this problem with an updated power supply design.

In this article, I will describe a good solution for measuring our project currents, but also explain the design process I followed to arrive at this good solution, as an example of how to handle the engineering and design process.

SOLUTIONS AND SHORTCOMINGS