Self-balancing robot version 3

Version 1 was made out of Lego. It keeps breaking into pieces, the motors were not secured and it also contained the unnecessary Kitronic servo:lite board. So, I rebuilt it.

Parts:

• BBC micro:bit
• 2x Feetech FS90R 360 degrees servo motors
• InvenSense MPU-9255 9-axis gyroscope, accelerometer, compass.
• Pololu Wheel for FEETECH FS90R/FT90R Micro Servo 60x8mm
• 2x AAA battery holder to power the micro:bit
• 2x AA or 4x AA battery holder to power the motors
• 3D printed frame
• In place of wires, a little custom perfboard with connectors
• Bolts, spacers

The code for this version is identical to the code for version 1 except the tweek for a different orientation of the MPU-9255. With 2x AA batteries, identical PID parameters will work. With 4x AA batteries, the parameters have to be retuned.

But, alas, it still doesn't balance well on smooth hard floors!

It works fine on carpet or cushion even though it is only half the height of the version 1 robot. It just won't work on wood or marble floors!

After much tuning, I decided to record the motor response. I added two more wheels and turned the bot into a "car". Then I recorded the horizontal acceleration using the accelerometer in response to various motor control like a 1 second 66% power pulse. Here is a typical result with 2x AA batteries:

Why is the acceleration oscillating around zero? The amplitudes of the oscillations are much worse with 4x AA batteries.

Oh, the motor controller in the servo is a closed loop controller!

The closed-loop controller inside the FS90R is probably a variant of the standard PID controller. The oscillations are caused by the controller trying to match the output voltage to the pulse duration.

The accelerometer and gyroscope are picking up physical feedback from the controller's actions. On soft floor like carpet, the feedback is dampened physically. On hard floors, the sensors pick up random spikes that throws the PID algorithm off.

On top of that, the accelerometer only picks up the increase in motor speed after about 60ms!

In short, don't build a self-balancing robot using servo motors. Just use DC motors.

Update: I've fixed the problem and got it to work on hard floors too!