The course of converting an analog quantity to a digital quantity is referred to as data sampling. The lower limit of the sampling frequency fsl=2·fc, which is the Nyquist sampling theorem, is provided by Nyquist in 1924. However, a system, which is stable under an analog controller becomes a digital control system after sampling. Said system the system would no longer be stable even if it complies with the sampling theorem. Parameter identification utilizing the data sampling rarely succeeded.
For the unsuccessful cases, it is normally deemed that the faster sampling frequency, the better the performance is, according to the sampling theorem. So, a technical person usually would choose to increase the sampling frequency. However, it is proved by the practical situation that even if the fastest sampling frequency is used, the problem cannot be solved. Therefore, attempts for constantly modifying, debugging, re-modifying and re-debugging the digital controlling and parameter identification apparatus are needed.