Hello everyone, I'll do my best to be as thorough as possible in posing my questions:

My plant is a single hydraulic actuator that is controlled using a servovalve and Temposonic position feedback. I've identified the open loop characteristics by using the method of least squares to fit response data to a z-domain model that has second order dynamics, one zero, and eight extra time delays (8ms delay). The data was sampled at

1kHz.I'm designing a pole placement controller, with a 1kHz control loop frequency, using Karl Astom's method presented in his "Computer Controlled Systems" book. The desired system model I'm using keeps the same process zero and order of time delay that was identified in the open loop model, and includes second order dynamics for defining the desired response. I'm finding that as I push the natural frequency of the desired system model higher, I begin to create unstable poles in my closed loop system. (To a lesser extent, I see this same effect occur when I increase the desired model's damping ratio). I'm not 100% sure, but I speculate that the instability is a result of trying to create a closed loop system with a bandwidth that is higher than that of the open loop system. Any input with respect to this proposition would be appreciated.

Assuming that my speculation is true, I am left to determine the bandwidth of the open loop system in order to design the closed loop system with a bandwidth that will give me an acceptable margin of stability. (Does that make sense?) My difficulty here is due to the fact that the open loop system takes a velocity input (servovalve position = flow), and gives a position output, so I'm not really sure how the bandwidth is defined. The magnitude of the frequency response plot is pretty much just a straight line. If I differentiate the output to get velocity, does the bandwidth of the velocity transfer function still correlate in some way to the achievable bandwidth of the closed position loop?

Thanks in advance for your help!