A couple evenings ago it became abundantly clear to me that my use of stepper motors was not going to provide enough power. Which is a little frustrating. I learned something very important that I didn’t know before. Stepper motors are rated by holding torque.
The holding torque on these motors is great and sufficient for what I need. But because of the way stepper motors work, when they are ‘stepping’ to the next position (which can happen thousands of time per second) they have very little moving torque. Certainly not enough to move around a 14 pound robot body inside a sphere.
Mindset of a builder and maker
When I make a mistake like this it helps me to take stock of what I have, what I don’t have, where my next step is and what’s salvageable. This helps me focus on actionable tasks instead of focusing only on the negative. But I also need to know what the negatives are. so we’ll get there.
What I do have
I can control my BB-8 body with my rf remote. It can move in all directions.
What I don’t have
Torque. I simply don’t have enough movement torque to overpower the other forces at work and make the sphere move well. It can move slowly, it’s just not quick to make trajectory changes. I can hear the motors skipping steps so I know it’s the motors.
Where I go from here?
I just need to change motor technology. I like the precision that I can get from steppers, but I simply need more torque. Some precision is important since my motors have to move in relation to each other, but what’s precision without enough torque?
I’ll move forward using DC motors with encoders (pictured above). The motor encoders will allow me to get feedback from the motors as they are turning. I’m going to try setting them up without using feedback. If the speed is pretty consistent between motors, I won’t need to use encoding.
Almost everything. I’ll need to replace the motors and motor controllers. Fortunately, I have written a lot of code that I can keep. The radio control code, Raspberry Pi code, movement math. All of this is reusable.
I’ll need to tweak my vector bot frame to accommodate the different style of motor, but even that should prove to be a relatively easy task. I’ll have to change my gearing, I was stepping down for power, but I’ll need to step up for speed (probably a 1:2 ratio vs 16:1 of the steppers).
What are the negatives?
Time mostly. Being on a pretty tight rolling budget for this build, I can only purchase 1 motor a month if I want to do other work on BB-8 as well. So it will be a bit before I’ll be able to have a ‘working’ body sphere again.
This does give me time to address solving a few other problems and complete the build of the dome.
The positive side of the negatives, is that I want to build a CNC anyway. now I have the steppers and controllers required. I only need a few more parts to build a primitive one.
Now, on to figuring out what I can sell to make all this move faster.