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[Keywords] Time difference comparison compensation method (TDCC method), Two-degrees-of freedom control, Desired-value's filter, LQ optimal control, Dead beat control, PID controller, PI controller
1. Introduction
PID (Proportional, integral and differential) control occupies 80 percent of the feedback control in an actual feedback system even at present.
If the time difference comparative compensation is introduced in the PID controller, two degrees of freedom control will become possible.
Moreover, when this compensation is applied to digital control, the dead beat control also will become possible without excessive manipulation variable.
2. What is the Time Difference Comparison Compensation method (TDCC method) ?
We propose that the time-difference comparison between a desired value and a controlled variable is introduced in the error detector in order to come over the various difficulties brought by the time delay or the phase delay generated in the open-loop transfer function of a feedback system.
Such composed compensation method (TDCC method) can be applied in two ways. One is used in (1) a two-degrees-of-freedom control, and another is used for (2) the simultaneous effects of dead-beat control and suppression of the peak-value of input variables in LQ optimal control in digital system.
In the case of (1) two-degrees-of-freedom control, the TDCC method is used for the improvement of the desired-value's response, after the feedback control on robustness and suppression of response in disturbance, etc. is adjusted by the ordinary controller such as PID/PI controller.
In the case of (2) in the LQ optimal control in digital system, the dead-beat effect and the suppression of the peak-value of input variables can be achieved simultaneously with the use of TDCC method.
(1) Two-degrees-of-freedom control is continued to 
(2) Dead beat effect is continued to
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