Advice for using servos
This summer I worked on a technical solution for Jan Barcentewicz, a vehicle designer who wanted a unique way of displaying his work at the RCA show. His model vehicle would rotate in time to an animation being overlaid with a projector. I want to share good news about how that went, but it was kind of stressful and sadly things broke after the first day on display. If you want to see a happy story of technical excellence, read this thing about a watch I made. In this post I thought I’d share some things I learned about working with servo motors.
Let's twist, like we didn't do this summer
When a new servo motor arrives in the post, grab the arm and gently rotate it to see its full moment of rotation.People will tell you not to do this. They will tell you it’s unsafe and that it could break the motor. But a motor that doesn’t do what you thought it did is as good as broken - just do it.
Servo motors don’t make full rotations - they turn about 120 degrees, give or take. You need to know the full range of movement the motor has before you start programming for it.
Buy a sail winch servo
Sail winch servos are designed for remote-controlled model boats. They can turn more than 360 degrees - often somewhere upwards of 900 degrees in fact. They are beautiful little machines, and they come in at the high end of hobby servo strength because they have to work against the wind. Pulling in a sail is hard work! I love sail winch servos so much after this summer's project.Buy lots of sail winch servos
I did a damn good job with this project. My programming was accurate, my code was clean, and after some painful wrangling, I got the right servo doing the right job. I got a phone call from Jan after he had finally finished assembling the whole model - he was delighted. It worked. It worked perfectly.The next day, it didn’t work anymore. The program hadn’t changed. The wiring hadn’t changed. Everything was exactly as it had been before. But after 24 hours of whirring away happily, the servo had made its last rotation. It was now spinning in robot heaven. And we had nothing to replace it with.
A model that is going to run for days and days at a time will eat through servos like a beast. Buy replacements ahead of time.
Motors come in two sizes
This isn't true of course. They come in lots of sizes. But those sizes come in two major groups at opposite ends of a spectrum - little and big. Little ones are friendly, affordable, and easy to program for. Big ones are incredibly expensive and scare the shit out of me. There is a huge deadspace in-between - load weights for which you cannot buy the perfect servo.Designers like to make last-minute changes to things. As an art technologist, probably what has to happen is that you buy the strongest hobby servo, and tell them ‘x is your maximum weight. I know you’re going to make last-minute iterations without telling me first, but these changes cannot cause the load on the motor to exceed this weight.’