Robin2:
I wonder if there is any point in calling it more frequently than the input data can change?...R
If the input is theoretically a step-input, then that'd be an infinite slope at the theoretical step. They do have rules of thumb about taking samples at a high enough rate to ensure that the control system is able to behave in the way we want. For some systems, sampling at a rate of 5 times the reciprocal of a first order time-constant might not be enough to make a sampled-data system behave like a continuous time version of the system. A sampling rate of 10 times the reciprocal of the time-constant is usually better (ie. a sampling period 10 times smaller than the time-constant). But you're right..... sampling at some relatively fast rate where system performance doesn't get much better than some minimal optimum rate is probably just waste of resources and time etc.