User's Manual

PMAC User Manual
Closing the Servo Loop 111
Lead-Lag
The notch filter can be used as a lead-lag filter if the roots are real rather than imaginary. A lead-lag filter
is similar in performance to a PID filter; it is useful when filter settings are determined analytically rather
than experimentally. When a basic lead-lag servo filter is desired, all servo gains Ix31 to Ix35 should be
set to zero; Ix30 is used as the generalized gain term.
Low-Pass Filter
It is also possible to use this filter component as a low-pass filter if reducing roughness of operation is
more important than high system bandwidth. This can be accomplished by setting the D1 term (Ix38) to a
negative value between -1.0 and 0.0. The larger the value, the lower the cutoff frequency of the filter.
The DC gain of this filter is 1/(1+D1), so the proportional gain must be increased by a factor of (1+D1) to
keep the overall loop gain the same.
Extended (Pole-Placement) Servo Filter
For systems with more difficult dynamics, such as multiple resonances and low-frequency resonances, the
extended servo algorithm purchased with PMAC Option 6 can be used instead of the PID filter. When
Option 6 is purchased, all motors on the PMAC must use the extended algorithm instead of the PID; no
mixing is possible. With Option 6, the meanings of I-variables Ix30-Ix69 are different from the standard
PMAC; refer to the manual for the extended servo algorithm for these meanings.
Because the extended algorithm is expressed in pole/zero form instead of gain form, it is not an intuitively
tunable filter as the PID is. For this reason, usually the Acc-25 Servo Evaluation Package must be used to
tune this filter properly.
Figure 20 Extended Control Alogorithm Block Diagram
User-Written Servo Filter
For the sophisticated user with very unusual and/or difficult dynamics, PMAC provides the hooks for
custom user-written servo algorithms. These routines are to be written in Motorola 56000 assembly
language code, usually on a PC or compatible, and cross assembled for the 56000. Delta Tau provides the
information about where to pick up the needed information, where to leave the output commands, and
where to store the algorithm itself.
Note:
This is not a task for the inexperienced user. To attempt this, the user should be
well acquainted with both servo theory and assembly language coding.