We experience it every morning as we struggle to find the right water temperature in the shower. Time delays are everywhere -- they arise in engineering, biology, physics, economics, and the environment.
As engineering systems become more complex, multiple sensors, actuators, and controllers introduce multiple delays, particularly in interconnected and distributed systems. In a dynamical system, changes cannot be effected instantaneously, and so an otherwise correct control decision applied at the wrong time could result in catastrophe. From our first exposure to feedback control systems, we are taught always to conserve phase. What is the relationship between time delay and phase? How are they related to the stability of a feedback control system? Does time delay always degrade the performance of a feedback control system?
Time delays arise in control systems from delays in the process itself (represented by time constants τ and natural frequencies ωn in transfer functions), from delays in the processing of sensed signals, and from delays in the implementation of a digital control system as a result of sample-and-hold, calculation, and velocity estimation, where the total time delay can be between one and two times the sample period.
The input-output time delay shifts the signal in time and is shown in the figure above. The frequency response of a time delay is exact with a magnitude of one and a phase angle that decreases linearly with frequency. A greater time delay corresponds to a more rapid increase of phase lag with frequency.
When the control loop is some derivative of a PID loop, maintaining phase is very good advice.
When there are unavoidable delays present in the system, your suggestion of some kind of "wait and act" strategy is probably good advice. When you think about it, adding a delay in the control loop is just another way to "conserve phase".
I have been involved in developing, improving and consulting anything related with control system for the last 15 years in electronics component testing and automation system. Time delay is really important and at some cases it is the only mean to solve the problems.
I really like how you emphasize the "judicious introduction" of time delays. I have seen some folks use time delays as a quick fix without carefully evaluating their effect on the entire system, which can either be a great solution or it can introduce other errors in subtle ways.
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