4.2.1.2.3 Tuning Concept in Brief
Tuning steps decribed for cascaded controller should be applied for tuning the gain scheduling controller. First
the controller should be tuned for high velocities and then for low ones. In the following section, the explained
tuning concept is divided in separate systematic steps.
4.2.1.2.4 Tuning Steps
Step 1.
Set the reference step signal to a higher value, which will cause the axis to move with high velocity. Set the PID
constants of both controllers equal to 0. By default, the integral limit of the velocity controller should be set to
motor maximum torque in [mNm], and the integral limit of position controller should be set to motor
maximum velocity in [rpm].
Important
from this step forth, the step response of position controller should always be evaluated.
Step 2.
In this step, the k constant of the velocity controller should be tuned. Increase the k of both position and
velocity controllers with a ratio of 10 (k = 10 * k ). Once the real position starts to follow the
reference position, stop increasing the k and only focus on increasing the k . Increase (sharpen)
the velocity controller as much as possible. As a rule of thumb, increase k until you get close to the
instability margin (at this margin, you will feel a vibration effect and some acoustic noise which is because of
controller sharpness). Now, you can reduce k to 90% of its value to increase the stability margin, and
remove vibration noise.
Step 3.
Now that the velocity controller is tuned, it is time to tune the parameters of the PID position controller. Start
with k , and increase it until you again get close to the instability margin, and then reduce k to
its 90%. So far, the proportional parts of both inner velocity loop and outer position loop are tuned.
Step 4.
In this step, increase k to eliminate the steady state error. As a suggestion start with k equal to 0.01, and in
each step, increase it with a factor of 2. Increase k step by step until the following two conditions are met at
the same time:
the overshoot is in its acceptable range
the steady-state error is eliminated in a short enough period of time
Step 5.
Repeat steps 1 - 4 for a small step reference signal (couple of thousands of ticks).
Step 6.
At this stage gains of the controller are determined. Beside gains user needs to determine the limit for low and
high velocity dynamics. Recommendation is to observe the value of velocity during the tuning process. This
should give a good basis for selection of those limits.
P P
P_velocity P_position
P_position P_velocity
P_velocity
P_velocity
P_position P_position
I I
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