In a perfect world, all you would need to do is install your servo motor, turn it on, and everything works as expected – perfectly!
Unfortunately, this is not that world and once your new servo motor receives its first command, it will rarely end up in the position you want it to be at.
So, read on for tips on how to tune a servo motor.
What is servo motor tuning?
Servo motors rely on the continuous current to reach the specified torque, velocity, or position. The precise amount of current required is determined by a servo controller depending on the actual torque, speed, or position condition of the motor it receives from an encoder.
The controller relays commands to the servo drive, providing the motor with the right amount of current required to adjust the differences between command values and actual values.
The servo motor tuning procedure, using tools like Elmo Composer software, adjusts gains to reduce the error between the commanded speed, torque, or position and the actual value achieved by the motor.
Types of servo motor
Servo motors are used in various ways: from running DVD players to driving MAVEN in outer space. As you learn how to tune a servo motor, here are the most common types:
- DC servo – A servo motor with decoupled flux and torque for fast torque responses. This allows a small armature current or voltage change to activate a significant shift in the speed or position of the rotor.
- AC servo – An AC servo motor uses high voltages for greater torque achievement and has an encoder incorporated for providing closed-loop control and feedback when used with controllers.
- Positional rotation servo – These have a 180-degree rotating output shaft with physical stops built into the gear mechanism to protect the rotational sensor.
- Continuous rotation servo – They can act on a range of commands that cause the servo to spin counterclockwise or clockwise and at different speeds depending on the commanding signal.
- Linear servo – This motor is like a positional rotation motor, but has additional pinion and rack gears that change the circular output to back and forth.
What is a servo loop?
A servo loop is the feedback system where a controlled variable is a mechanical position. The servo loop is used as part of determining a motor’s command output. It is used to match as close as possible the commanded position and the actual position of the motor. The three types of servo loops are current, velocity, and position.
How to manually tune a servo motor
While there are several methods for manually tuning a servo motor, the most popular is the PID algorithm. It generates the level of command sent to the servo drive, effectively correcting the factor between the output of the servo controller and the performance of the motor and load. A discrepancy triggers the PID algorithm to generate an error correction command.
The PID algorithm has these three components:
- P for Proportional Gain (Kp) related to system stiffness and level determines command voltage required to correct the position error
- I for Integral Gain (Ki) provides the restoration force required to push the axis to a zero-error position
- D for Derivative Gain (Kd) acts as the shock absorber for reducing oscillations and overshoot
Each of the PID values acts independently, but outputs are summed to create the PID output signal. Manual tuning involves manually configuring the loop of each required parameter using the software.
How to autotune a servo motor
Servo drives now have functions that allow automatic tuning of the system. Faster computing power and even more complex algorithms give auto-tuning functions enough power to run complex systems, with little effort or input from a user.
Within the firmware in servo drives are algorithms that run the command and feedback process automatically, tuning the system in case of inertial mismatches. Huge load disparities cause the system to be tuned more than once and parameters stored. So, you have another set of parameters stored and activated automatically due to the action of the operator or load.
Manual servo drive tuning took several days of intensive work, and autotuning now reduces it to a few minutes.
Finally, servo tuning is not an exact science, so you will never find a set of perfect tuning parameters, no matter the application. As you dive into how to tune a servo motor, keep in mind that the best tools are closely observing your system as you adjust the parameters, experience, and patience to carry out a trial-and-error approach.