OMS MAXnet, User Manual

Page: 17 / 67

TUNE THE SYSTEM GETTING STARTED
MAXnet User’s Manual 2-11
2.8. TUNE THE SYSTEM
2.8.1. INTRODUCTION
The following is an introduction to the basics of tuning a servo motor. Tuning a servo system is the
process of balancing three primary gain values Proportional, Integral, and Derivative in order to achieve
optimum system performance.
In a closed loop system, an error signal is derived from the command position and actual position,
amplified, and then supplied to the motor to correct any error. If a system is to compensate for infinitely
small errors, the gain of the amplifier needs to be infinite. Real world amplifiers do not possess infinite
gain; therefore, there is some minimal error which cannot be corrected.
The three primary gain values used in servo systems are
P
(proportional),
I
(integral) and
D
(derivative).
The "P" term is used as a straight gain factor to get the system response "in the ballpark." The "I" term
defines how quickly the system will respond to change. The "D" term is a dampening term. This term
defines how quickly the system settles at its desired position without oscillating.
The effects of these parameters can be seen when looking at the system’s response to a step change at
the input. The shape of the step response falls into one of three categories: under damped, critically
damped or over damped. Over damped systems are slow to reach their final value and produce little or
no oscillation. Critically damped systems reach final value quickly, without overshoot. Under damped
systems reach final value quickly, but have various degrees of “ringing” or oscillation, that decay to zero
over time. Ideally, a system should be critically damped, allowing for the fastest response time with the
least amount of oscillation.
2.8.2. TUNING ASSISTANT
MAXTune.exe is a tuning assistant utility that is provided to assist the user in finding the right combination
of parameters. This utility plots the motor’s response. The user can analyze this data to arrive at the
right servo parameters for their servo system. The application and documentation can be found on the
.
2.8.3. MANUAL TUNING
In most motion control applications the optimum tuning of the servo system is achieved through a manual
tuning process. Auto-tuning algorithms typically can only get the system parameters close and require
manual steps to fine tune the parameters. An empirical trial and error approach will be discussed first.
There are some system parameters that need to be determined before attempting to tune a motor. The
encoder resolution (counts per revolution) is one element to be determined. Another is the system's
maximum velocity. Note that a motor should never exceed 90% of the motor’s maximum rate rpm. If the
system requirement is for a velocity higher than 90% of the motors top rpm, then another motor with
higher rpm capability should be used.
The system’s maximum acceleration is determined several different ways. The best method is to
determine the system time constant, which includes “hitting” or “bumping” the motor under system load
and measuring the time from 0 rpm to maximum rpm and divide this value by 5. The maximum
acceleration is either 2.5 times this value or is based on the system requirements for handling the load as
defined in the operating specifications of the system. This value is always lower than the calculated value
and if this acceleration value is not high enough then a different motor/amplifier with more power or
bandwidth should be utilized.
The MAXnet can control either current mode or voltage mode amplifiers. The servo update rate of the
MAXnet is user selectable: 976.6

s, 488.3

s, 244.1

s, 122.1

s. High "Following Error" can be