伺服运动控制基础-国外经典技术手册
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2024-03-17
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Parker Hannifin – Electromechanical Automation Div. / 800-358-9070 / www.parkermotion.com
Fundamentals of Servo Motion Control
The fundamental concepts of servo motion control have not changed significantly in the last 50
years. The basic reasons for using servo systems in contrast to open loop systems include the need
to improve transient response times, reduce the steady state errors and reduce the sensitivity to
load parameters.
Improving the transient response time generally means increasing the system bandwidth. Faster
response times mean quicker settling allowing for higher machine throughput. Reducing the steady
state errors relates to servo system accuracy. Finally, reducing the sensitivity to load parameters
means the servo system can tolerate fluctuations in both input and output parameters. An example
of an input parameter fluctuation is the incoming power line voltage. Examples of output
parameter fluctuations include a real time change in load inertia or mass and unexpected shaft
torque disturbances.
Servo control in general can be broken into two fundamental classes of problems. The first class
deals with command tracking. It addresses the question of how well does the actual motion follow
what is being commanded. The typical commands in rotary motion control are position, velocity,
acceleration and torque. For linear motion, force is used instead of torque. The part of servo
control that directly deals with this is often referred to as “Feedforward” control. It can be thought
of as what internal commands are needed such that the user’s motion commands are followed
without any error, assuming of course a sufficiently accurate model of both the motor and load is
known.
The second general class of servo control addresses the disturbance rejection characteristics of the
system. Disturbances can be anything from torque disturbances on the motor shaft to incorrect
motor parameter estimations used in the feedforward control. The familiar “P.I.D.” (Proportional
Integral and Derivative position loop) and “P.I.V.” (Proportional position loop Integral and
proportional Velocity loop) controls are used to combat these types of problems. In contrast to
feedforward control, which predicts the needed internal commands for zero following error,
disturbance rejection control reacts to unknown disturbances and modeling errors. Complete servo
control systems combine both these types of servo control to provide the best overall performance.
We will examine the two most common forms of disturbance rejection servo control, P.I.D. and
P.I.V. After understanding the differences between these two topologies, we will then investigate
the additional use of a simple feedforward controller for an elementary trapezoidal velocity move
profile.
P.I.D. Control
The basic components of a typical servo motion system are depicted in Fig.1 using standard
LaPlace notation. In this figure, the servo drive closes a current loop and is modeled simply as a
linear transfer function G(s). Of course, the servo drive will have peak current limits, so this linear
model is not entirely accurate; however, it does provide a reasonable representation for our
analysis. In their most basic form, servo drives receive a voltage command that represents a
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