Qualitative stability of large space structures with non-colocated actuators and sensors

The use of non-colocated sensors and actuators for controlling a large space structure introduces a serious difficulty. Because of the phase shift introduced by the sign reversal when the sensor is on one side of a mode and the actuator is on the other, the mode is easily driven unstable. These characteristics can be deduced easily by looking at the mode shapes and the locations of the actuator and sensor for that particular mode. Thus a qualitative stability of the mode is given by a cursory inspection. These are defined for the modes as stably interacting when the mode phase shift is zero and unstably interacting when the mode phase shift is 180??.