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December 3, 2009
Kwantlen Polytechnic University
about 3 years ago
The stability of a positional control system is inherently dependent on the nature of the load. Frictional losses will improve stability and inertia will reduce stability. If the control system is proportional only the control signal approaches zero as the system approaches the set point position. If the system has significant inertial it will overshoot and the control signal to drive it back will initially be very small. If velocity feedback is added the control signal can start breaking the movement of the system as the signal proportional to the error becomes small. This helps to avoid overshoot. Qubix mentions that the v2 system does not have sensors for “back EMF” which could be used to provide velocity feedback (the “back EMF” is proportional to the speed of the motor).
The derivative of PD control is somewhat different than velocity feedback in that it is the derivative of the error rather than the speed of the motor. From a math point of view derivative of position is the same as velocity but the derivative of the error also includes changes in setpoint. In practice an actual velocity signal is better than calculating the derivative of position because change in position is small and needs to be amplified resulting in a very noisy sometimes useless signal. The back emf from the motor is large. It can also be affected by noise spikes but spikes contain only very high frequencies that can be filtered from the signal.
The bottom line is that for practical reasons it would be best to use proportional control with velocity feedback (in this case in the form of sensing the back emf). The typical analog RC servo does sense the back emf so if version two does not it is at a significant disadvantage in comparison.
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