Using MAX to Tune a Servo Motor

Complete the following steps to tune a servo motor in MAX:

1. Launch MAX.
2. Expand Devices and Interfaces.
3. Expand NI–Motion Devices, and then expand the item for the appropriate motion controller.
4. Expand Calibration in the configuration tree, and select Servo Tune.
5. Select the Control Loop tab.
6. Configure the PID Gain and Velocity Feedback Gain settings as appropriate.
   • Proportional Gain (Kp) is the system stiffness. It determines the contribution of restoring force directly proportional to the position error. Restoring force is comparable to a spring in a mechanical system.
     A high proportional gain gives a stiff responsive system but can cause instability from overshoot and oscillation.
   • Derivative Gain (Kd) is the damping effects on the system. It determines the contribution of restoring force proportional to the rate of change, or derivative, of position error. This force is much like viscous damping in a damped spring and mass mechanical system, such as a shock absorber.
     Increasing derivative gain reduces oscillation at the commanded position. However, increasing Kd results in high acceleration, which can cause a ringing sound.
   • Integral Gain (Ki) is the static torque load on the system. It determines the contribution of restoring force that increases with time, ensuring that the static position error in the servo loop is forced to 0. This restoring force works against constant torque loads to help achieve zero position error when an axis is stopped.
     Integral gain improves positional accuracy. High static torque loads need integral gains to minimize position error when stopped.
7. Click the Step Response tab to view the system step response.
   • Settling Time is the time required by the response curve to reach and stay within a range that is approximately the final value of size. This value is specified by the absolute percentage of the final value, which is 2% to 5%.
   • Rise Time is the time required by the response to rise from 10% to 90% of its final value. The faster the response time of the system, the faster the rise time.
   • Peak Time is the time required for a response to reach the first peak of the overshoot.
   • Maximum Overshoot is the maximum peak value of the response curve measured from the specified position. The maximum overshoot directly indicates the relative stability of the system.
   Use the Step Response tab to plot the transient response of your system. Typically, the transient response is measured by first commanding a step, and then measuring how quickly a system reaches a steady state. Using the transient response, you can calculate the maximum overshoot, rise time, peak time, and settling time of your system.
   You also use this tab to determine the relative stability of the system. A system is considered stable if the actual position is finite when the commanded position is finite. Similarly, a system is stable if a commanded position results in the motor coming to rest at a single position.
   

   Tip  A system is considered unstable when any commanded position typically results in an exponential increase in position error, or the oscillations never dampen.

8. Analyze the step response plot.
   • Over-damped system produces a smoother, slower step response. An over-damped system is characterized by no overshoot and long rise and settling times.
   • Under-damped system produces a slight oscillatory response that eventually dampens out. An under-damped system is characterized by a large overshoot, a long settling time, and short peak and rise times.
   • Critically damped system response provides a balanced medium between over- and under-damping. This type of system response balances response time and damping effects. A critically damped system is characterized by low overshoot, shorter rise and settling times when compared to over-damped systems, and longer peak times when compared to under-damped systems.
   • Oscillatory system response produces a constant-amplitude, continuous-position oscillation. An oscillatory system is characterized by infinite settling times.
   • Chattering system response is an oscillatory response characterized by high-frequency, low-amplitude oscillations that are audible.