Parker Hannifin
Chapter 5 Tuning 79
Servo Tuning Overview
The drive uses a digital control algorithm to control and maintain the position
and velocity. The digital control algorithm consists of a set of numerical
equations used to periodically (once every servo sampling period) calculate
the value of the control output.
The numerical terms of the equations consist of the current commanded and
actual position values (including a few from the previous sampling period),
and a set of control parameters. Each control parameter, commonly called a
gain, has a specific function (for more information, see “Servo System Gains”
on page 83). In addition, tuning is the process of selecting and adjusting
gains to achieve optimal servo performance.
When using the control algorithm described above, the whole servo system
acts as a closed-loop system (see Figure 37). It is called closed loop
because the control algorithm accounts for both the command (position,
velocity, tension, etc.) and the feedback data from the encoder. Therefore, it
forms a closed loop of information. When all gains are set to zero, the digital
control algorithm is disabled.
Figure 37 Closed Loop and Open Loop System Comparison
To command a drive, controllers can provide ±10V analog output or step and
direction signals. Once the digital control algorithm has calculated the digital
control signal, the resultant digital value is sent out from the DSP (digital
signal processor) to the DAC (Digital-to-Analog Converter). The DAC has an
analog output range of -10V to +10V.
It is possible for the digital control signal, calculated by the control algorithm,
to exceed the DACs output range. If this occurs, the resulting analog output
becomes saturated—where the analog output signal remains at the limit until
the position error changes such that the control algorithm calculates a control
signal less than the limit.
The phenomenon of reaching the output limit is called controller output
saturation. When saturation occurs, increasing the gains does not help
improve performance because the DAC is already operating at its maximum
level.