Determining Tuning Criteria

Choosing the proper specifications for a machine is a prerequisite for tuning. Unless there is a clear understanding of the type of performance needed to push the machine into production, the tuning process will cause more problems and headaches than it solves. Take time to layout ALL the requirements of the machine—nothing is too trivial to consider.

  • Determine what the most important criteria are. The machine was likely designed and developed with a certain performance in mind. Include ALL performance criteria in the specification. Do not be concerned with whether or not the criteria sound scientific. (i.e. If the motion needs to visibly look smooth, put it in the specification. If it can't have any noise, put it in the specification.) At the end of the development phase, the machine's performance should match the performance previously set in the specification. This will ensure that the machine meets its performance goals and that it is ready for production.
  • Test the machine with realistic motion. Do not simply tune the machine to make short linear motion, when it will make long, s-curve motions in the real world. Unless you test the machine with realistic motion, there is no way to determine if it is ready for production.
  • Determine some specific, quantitative criteria for identifying unacceptable motion. It's better to be able to tell when a motion is unacceptable than to try and figure out the exact point where acceptable motion becomes unacceptable. Here are some examples of motion criteria:
  1. +/– x position error counts during the entire motion.
  2. Settling within +/- x position error counts, within y milliseconds.
  3. Velocity tolerance of x% measured over y samples.
  • It is important to focus on the things that will get the machine into production with reliable performance, based on a fundamental understanding of the system.

After a detailed servo performance specification is constructed, the system tuning may be started.

Before Tuning is Started

In the worst case, if something goes wrong during tuning, the servo can run away violently. Make sure that the system is capable of safely dealing with a servo run away. The drive has several features that can make a servo run away safer:

  • Make sure that the limit switches turn the drive off when tripped. If a complete run away occurs, the motor can move to a limit switch very quickly.
  • Make sure the max motor speed is set accurately. If a complete run away occurs, the motor can reach max speed quickly and the drive will then disable.