Add stepper documentation

implementation/SCARA/05_stepper/implementation.md

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+# Stepper motor
+
+## Motor model
+Two detail steps ago we already looked in to stepper motors.
+I implemented a model that I found on a [Github repository](https://github.com/SuYouge/Stepper-motor-modeling).
+Sy Youge build a model of a stepper motor with a good explaination.
+
+I could easily implement this.
+I expended his bondgraph model with a phase controller.
+I only have to put in the desired angle. However, this phase controller does not have any limiting.
+Therefore, some control in required to make sure the physical part of the motor can keep up with the electrical rotation.
+
+## Motor Control
+This control is a seperate block.
+There are two limitations to the controller.
+First it has a limited speed, second it has a limited torque.
+If you exceed this speed or torque the motor will stop operating.
+
+The torque requires a good feed-forward model of the dynamic system.
+However, I can just assume the max moment of inertia and then convert the torque in to a maximum acceleration.
+This was the easiest option.
+
+The result is that I have a setpoint for the angle. This is the setpoint for the first PD controller.
+This is not a PID as the design of the stepper already solves the steady state error that would have been caused by gravity.
+The first PD will then output the desired velocity. We limit this velocity and feed it in to the second PD controller.
+This will output the desired acceleration. This is also limited. The acceleration is then integrated twice resulting in a position that is velocity and acceleration limited.
+Which is fed in to the stepper motor model.
+
+A nice feature of this setup is that has no feedback outside of the controller.
+Therefore, we do not need any sensors in the system.
+Maybe some initial setpoint.
+