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What is meant by actuator stroke time (tt)?

The actuator stroke time is a variable provided by the actuator drive and is therefore only relevant for modulating controllers or proportional (continuous) controllers with integral actuator driver.
The time that the actuator drive takes to travel once across the full usable manipulation range is set under actuator stroke time.
The actuator stroke time cannot be determined by self-optimization (autotuning). It must always be set before the optimization.
The actuator stroke time provides the controller with information about the effect of the actuating pulses. At an actuator stroke time of 20 seconds, for example, the percentage change in manipulating variable, at the same actuating pulse, is significantly larger than for an actuator with 100 seconds stroke time, for example.
When selecting or dimensioning actuator drives, it must be taken into account that a short stroke time of, say, less than 10 seconds will result in large steps of the manipulating variable, and consequently to a reduced control accuracy. If, for example, we assume that 0.5 seconds is the shortest actuating pulse time, a stroke time of 10 seconds would result in only 20 actuating steps. This would mean that the manipulating variable can only be changed in 5 % steps.
Actuator drives with a very long stroke time can, however, be disadvantageous as far as the dynamics is concerned, because the manipulating variable can only be changed relatively slowly by the control action. In actual operation, however, problems arising from stroke times that are too short occur more frequently than those caused by stroke times that are too long.