The vehicle offers four axis degrees of freedom in the x,y and z planes and also yaw rotation. Pitch is stabilized to the horizontal and roll is righting moment stabilized. Five Proportional Integral Derivative (PID) feedback controllers were used for the control system in the degrees of freedom.
To obtain stable vehicle dynamics, the PID controllers were designed with the following considerations:
- Derivative component low pass filtered to reduce the exponential effects of sensor noise
- Set point weighting to reduce transient effects in set point changes
- Integrator windup protection when actuators are unable to fulfill the requirement of the PID Controller
- Time sampling with variable period for more accurate integral and differential component computation during discretisation due to task periods not being guaranteed in a non real-time system.
Each feedback controller was also band limited to a certain range in limit the controller output to be within the limitations of the physical actuator model. Bumblebee’s control loop operates at a 20Hz cycle which proved to be sufficient for us to maintain optimal vehicle dynamics.