Open Source Pan/Tilt Head: Part Two - Motors and Motion
Drivetrain
In Part One I indicated that to reduce the footprint of the bracket I wanted to place the motors in-line with the horizontal and vertical beams of the yoke. This meant that direct drive for each axis was not going to be an option. This was fine since I could incorporate additional speed reduction within the drivetrain. From there the two options seemed to be gear or belts and pulleys. Gears presented two issues; the tolerances required to eliminate backlash in the drive are quite small, and the size and number of gears needed to get the motors in the right place would be excessive. A belt drive seemed like the best choice, and again ServoCity carried timing belts and pulleys compatible with the Actobotics platform I was using.
Here is a better idea of what I mean when I say the motors will be in-line with the beams of the yoke:
The drivetrain ratios are easy to calculate with timing belts because the number of teeth are directly proportional to the circumference of the pulley, just like gears. I am using a 10T pulley on the motor, and a 60T pulley on the shafts, giving me a 6:1 ratio. With a 20 RPM motor output, the final speed of each axis is 3.33 RPM or 20 degrees/second. While the motor shaft has a little backlash, this extra reduction divides it by 6, so each axis remains very accurate.
In CAD, I calculated the circumference of the belt running between the small drive pulley on the motor and the large pulley mounted on the shaft. Each time I did this I ended up ordering the wrong size belt. I still haven’t figured out where my calculations went wrong. If you’re going to order belts, I suggest ordering a few sizes in the range of what you need. ServoCity carries them in circumference increments of 10mm.
Motors
There were three major options for motors that I considered, each with unique pros and cons:
DC brushed: Inexpensive, easy to find, many sizes available. Many options for gearing, including spur or planetary gearboxes. Easy to drive, but potentially low torque at low speeds which can mostly be solved by tuning the motor driver electronics (not that either axis needs much torque, anyway). Brushes can add noise.
Brushless: Standalone motors are more expensive than brushed motors. Geared brushless motors are hard to find in smaller form factors. Great torque performance at all speeds and are quiet, but use a lot of energy. Driving and speed control requires more complex and more expensive controllers than brushed motors.
Stepper motors: Many sizes available and varying prices. Torque performance is great, but they use a lot of energy. Great positional accuracy. Microstepping can be pretty smooth, but requires special drivers and programming. Noisy, unless you use special and somewhat complex drivers compared to brushed motor drivers. Available with gearboxes at the cost of a fairly large assembly.
After considering each of these, I chose brushed motors specifically for the ease of finding options to fit the project and the simplicity of getting them running. Knowing that noise will be an issue, I started researching ‘precision’ motor/gearbox packages. My hope was that Swiss or German manufactured motors would have such tight tolerances that the noise would be reduced especially at low RPMs.
Two notable manufacturers of these precision motors are Maxon and Faulhaber. Each has an extensive product line that varies by physical size, torque specifications, motor RPM and voltages, and gearbox ratios. The cost of these motors new from the manufacturer is outside the budget of this project, but eBay unsurprisingly has a large selection of surplus new or used models.
Faulhaber caught my attention due to the sheer abundance of options, and so after figuring out the part numbering system, I found a set of 5 new motors, model 2025U012S + 20/1 989.1 + X0758B. The first set of numbers is the motor P/N, importantly: 20 is the diameter of the motor in mm; 25 is the series the model belongs to, and 12 is the nominal voltage in volts. The second set of numbers is the gearbox P/N, importantly: 20 is the diameter of the gearbox in mm, and 981.1 is the ratio of input revolutions to 1 output revolution.
Normally I would go into more detail about how I was going to use these motors, but instead I’ll just include this:
An absolute disaster! This is the motor simply being powered by a DC supply, varying the voltage. Way too noisy!!!
So back to the drawing board, I reached out to ServoCity and another supplier and asked if they had any reference on how quiet their quietest motors were, and although ServoCity didn’t have a number they could give me, they recommended I try out one of their precision spur gearmotors. Now, who knows their definition of ‘precision’ as these aren’t made in Switzerland or Germany, but I ordered one and gave it a try. Listen to the difference between the tilt (Faulhaber) and pan (ServoCity) motors! (The background noise is a computer power supply…)
So until I find an even better option, the ServoCity motors will work great, and they are readily available in multiple output speeds. The current design is using the 20 RPM motor.