Kollmorgen AKD PDMM series, User Manual

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AKD PDMM User Guide | 12.3.2 Determining Tuning Criteria
The AKD PDMM works in three major operation modes: torque, velocity, and position operation mode. No
servo loop tuning is required for torque mode. Velocity loop and position loop tuning are covered below.
The AKD PDMM has an auto tuner that will provide the tuning that many applications will need. This sec-
tion describes the tuning process and how to tune the AKD PDMM, specifically for cases where the user
does not want to use the auto tuner.
Tuning in this section will focus on tuning in the time domain. This means that we will look at the velocity
or position response vs. time as the criteria we use to decide how well tuned a control loop is tuned.
12.3.2 Determining Tuning Criteria
Choosing the proper specifications for a machine is a prerequisite for tuning. Unless you have 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.
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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 concern
yourself 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.
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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.
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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:
a. +/– x position error counts during the entire motion.
b. Settling /- x position error counts, within y milliseconds.
c. Velocity tolerance of x% measured over y samples.
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DO NOT pick criteria based on what is the most popular technique of the day. It is important to focus
on the things that will get the machine into production with reliable performance, based on fundamental
understanding of the system.
After you have constructed a detailed servo performance specification, you are now ready to start tuning
your system.
12.3.3 Before You Tune
In the worst case, if something goes wrong during tuning, the servo can run away violently. You need to
make sure that the system is capable of safely dealing with a servo run away. The drive has several fea-
tures that can make a servo run away safer:
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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.
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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.
12.3.4 Closed Loop Tuning Methods
The closed loop control loop is responsible for the desired position and / or velocity (trajectory) of the
motor and commanding the appropriate current to the motor to achieve that trajectory. The challenge in
closed loop control loops is to make a system that not only follows the desired trajectory, but also is sta-
ble in all conditions and resist external forces, and do all of this at the same time.
Kollmorgen™ | December 2012 88