However, if you know that the response decays sharply over the frequency range,
consider decreasing the amplitude of the lower-frequency inputs and increasing the
amplitude of the higher-frequency inputs. It is numerically better for the estimation
experiment when all the plant responses have comparable magnitudes.The perturbation amplitudes must be:- Large enough that the perturbation overcomes any deadband in the plant actuator
and generates a response above the noise level - Small enough to keep the plant running within the approximately linear region
near the nominal operating point, and to avoid saturating the plant input or
output
In the experiment, the sinusoidal signals are superimposed (with the step
perturbation, if any, in the case of open-loop tuning). Thus, the perturbation can be at
least as large as the sum of all amplitudes. Therefore, to obtain appropriate values for
the amplitudes, consider:- Actuator limits. Make sure that the largest possible perturbation is within the
range of your plant actuator. Saturating the actuator can introduce errors into the
estimated frequency response. - How much the plant response changes in response to a given actuator input at the
nominal operating point for tuning. For instance, suppose that you are tuning a
PID controller used in engine-speed control. You have determined that at
frequencies around the target bandwidth, a 1° change in throttle angle causes a
change of about 200 rpm in the engine speed. Suppose further that to preserve
linear performance the speed must not deviate by more than 100 rpm from the
nominal operating point. In this case, choose amplitudes to ensure that the
perturbation signal is no greater than 0.5 (assuming that value is within actuator
limits).
If your plant is asymptotically stable, specify amplitude of the step perturbation in
the Step amplitudes (Astep) field. The considerations for choosing a step amplitude
are the same as the considerations for specifying the step amplitudes.Configure Design Goals
In the Design Specifications section of the dialog box, you specify your goals for PID
tuning.
Frequency-Response Based Tuning