PID, APC

How to tune servo systems: Force control, Part 4: Questions and answers

Inside Machines: More answers regarding servo system tuning follow the April 15 webcast on force control as it relates to tuning servo systems.

By Joseph Profeta April 23, 2021
Courtesy: Control Engineering, Aerotech

 

Learning Objectives

  • How to tune servo systems: Force control, Part 4 webcast offers more answers to listener questions.
  • Tuning answers cover servo stability, sensors, compensation.
  • Temperature can affect precision motion control.

Tuning the servo system to performance specifications can be among the most troubling tasks in machine building. It’s not always about what three numbers should go into the proportional-integrated-derivative (PID) controller. In an April 15 webcast, “How to Tune Servo Systems: Force Control (Part 4),” Joseph Profeta, Ph.D., director, Control Systems Group, Aerotech, covered how to tune the force loop tools to meet system specifications and create an arbitrary force trajectory, limitations of a force loop around the position loop and current loop, how to command arbitrary force trajectories, and how to minimize bump.

Diagram shows a general control problem, frequency domain, as discussed in a series of webcasts on “How to Tune Servo Systems.” Courtesy: Control Engineering, Aerotech

Diagram shows a general control problem, frequency domain, as discussed in a series of webcasts on “How to Tune Servo Systems.” Courtesy: Control Engineering, Aerotech

Tuning: Servo stability, sensors, compensation

Profeta answered questions in the live webcast about force control in servo system tuning, and more answers from webcast participant questions are provided below.

Q: Why is tuning so difficult to attain smooth performance, linear servo, pick and place, at speed?

A: It is possible with pick and place that the linear servo stability is affected by the load change on the axis. Using gain scheduling for different loads is one strategy to use.

Q: How do I tune a servo motor and troubleshoot?

A: Generally depending on the servo loop structure, starting with low proportional gains is the first step. With a stable system, stability can be assessed with the loop gain frequency response and gains adjusted as needed. A demonstration of this can be seen in the first webinar in this series.

Q: Is there any concern with the ringing in the transients?

A: This is application-dependent. Some force applications cannot tolerate overshoot. In this case, tune the system such that it is critically damped to have no overshoot in a step response. If you are able to contour the force command, this also can reduce the transient oscillations.

Q: What effects might you see with precision rotary stages subject to temperature transients? Can this be mitigated with adding some thermistors to the control loop?

A: Say the environment were to slowly fluctuate between 10 and 35°. The temperature variations will cause the mechanical system to grow or shrink. This can also include the feedback device depending on what feedback is used and how it is mounted. If the feedback is growing and shrinking with the mechanics, the control loop will not be able to correct for temperature variation. As suggested, adding some temperature measurements to the system can be used to add a correction into the system. Some controllers have a linear interpolation algorithm for this purpose.

Diagram shows a model of force control on an object as explained in the webcast “How to Tune Servo Systems: Force Control (Part 4).” Courtesy: Control Engineering, Aerotech

Diagram shows a model of force control on an object as explained in the webcast “How to Tune Servo Systems: Force Control (Part 4).” Courtesy: Control Engineering, Aerotech

Q: How does the I2 gain impact the performance in force control? (Or stated another way: Why do we use two integrals in force loop when the force loop is wrapped around the position loop?)

A: This is used in applications to control the height of an axis above a target when ramp commands are needed. It is not used in the force loop.

Q: Have you considered optical sensor as position feedback in the loop?          

A: Yes. This can work well if the sensor is linear and high resolution enough.

Q: After touch, is there any compensation on force loop?        

A: The webinar showed two modes. Touchdown in position mode where no force compensation takes place until the force loop is turned on and touchdown in force mode where compensation is active during and after touchdown.

Diagram shows a force feedforward diagram and graph of a force feedforward calculation as explained in the webcast “How to Tune Servo Systems: Force Control (Part 4).” Courtesy: Control Engineering, Aerotech

Diagram shows a force feedforward diagram and graph of a force feedforward calculation as explained in the webcast “How to Tune Servo Systems: Force Control (Part 4).” Courtesy: Control Engineering, Aerotech

Edited by Mark T. Hoske, content manager, Control Engineering, CFE Media and Technology, mhoske@cfemedia.com.

KEYWORDS: Motion control, loop tuning, force control

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What other servo tuning questions do you have?


Joseph Profeta
Author Bio: Joseph Profeta, Ph.D., is director, control systems group, Aerotech. Dr. Profeta has more than 30 years of experience in technology-driven companies. He is the director of the controls system group at Aerotech with P&L responsibility for the controls, drives and motors. In this role, he works with companies to design and select control system architectures that result in increased machine performance while minimizing component cost. He has earned a BS, MS, Ph.D., and Executive MBA. He has been awarded five patents and published 43 papers. Dr. Profeta is also an adjunct professor in the electrical and computer engineering department at the University of Pittsburgh where he teaches software engineering.