The tuning procedure consists of the following steps:
1) Set or measure the motor supply voltage. To measure the supply voltage, press the Detect button and open the VDC Detection Test.
2) Tuning is done in the order: first the current controller, then the speed controller (if present), and finally the position controller.
3) For each controller press the Tune & Test button to open the tuning dialog. The dialog has 2 tabs:
— the first tab, Test Parameters, allows you to set the parameters of the test ;
— the second tab, Test, executes the test and displays the results.
The parameters of the controller can be changed from the Test tab. If the test result is not satisfactory, you can either adjust the Passband and press the Tune button or adjust directly the controller parameters. This operation can be done for the speed and position controllers even when the test is running.
Remark: The value displayed in the Passband field represents the controller transfer function pass band. This parameter is used to compute a set of controller gains (Proportional, Integral and Derivative) and represents the design criteria for the PI / PID response. The gains are computed so that the system will have the imposed passband (at least from the theoretical point of view). The higher the value of the passband the higher the resulting PID gains and as such the system will be more responsive / aggressive.
Too big gain will result in motor oscillating / vibrating during movement or during standing still while too low gains will result in the motor not following the imposed reference too well.
Tuning tips:
1) Current controller:
– first use the Tune button with default settings for Passband then test the result. The parameters are computed using the motor parameters (resistance and inductance) and the motor supply voltage.
– if the motor current rising time is short, double Kp and test again. Repeat this until the answer starts to oscillate, then reduce Kp by 30-50%
– if the answer has oscillations during rising time (e.g., before reaching the reference), increase Ki, until the oscillations are eliminated
– make sure you get a fast response from the current controller (even if this implies a certain overshoot level), as you need a sharply tuned current controller in case your application has a high static torque.
2) Speed controller:
– first use the Tune button. Set the Pass band around 150 rad/s and test the result.
– to further improve the response time and stiffness, repeat the Tune procedure with a higher Passband. Try until the motor speed starts to oscillate. Then set the bandwidth 20-30% below the oscillation value.
– if the answer presents a steady-state error, increase the integral limit. The integral limit must be kept as small as possible, just enough to eliminate the steady state error. A too big value may lead to motor oscillations.
Remark: the speed controller should be used together with the position controller in applications requiring on-line change between speed and position control modes, and where the positioning movements do not require a high bandwidth. For fast, short moves, we recommend disabling the speed controller. Hence, the position controller will give directly the current command
3) Position controller (without speed controller):
– first use the Tune button. Set the Passband around 80-150 rad/s and test the result.
– to further increase stiffness, repeat the Tune procedure with a higher Pass band. Try until the motor starts to oscillate. Then set the bandwidth 20-30% below the oscillation value.
– if the answer presents a steady-state error, increase the integral limit. Try, for example, with 1-10%. The integral limit must be kept as small as possible, just enough to eliminate the steady state error. A too big value may lead to motor oscillations.
– if you can’t zero steady state error without oscillations, try to increase the D part (Kd coefficient)
As a general rule, when referring to the response characteristics like overshoot, rise time and steady-state error, the controller coefficients have the following effects:
- Increasing Kp will result in increased overshoot but with reduced rise time and steady-state error
- Increasing Ki will result in increased overshoot and slightly increased rise time but will drastically reduce the steady-state error (in an ideal system, where the output voltage will never saturate, the Ki will eliminate the position error)
- Increasing Kd will result in increased rise time but with reduced overshoot (the steady-state error is not affected by Kd).