Kollmorgen BDS5, Installation And Setup Manual, Page: 71 / 150

BDS5 C
HAPTER
3 - O
PERATION
3-9
3.8 PROBLEMS
Some times there are problems tuning. Usually the
TUNE command will provide you with a tuning that
is either acceptable or close to acceptable. If not, you
can tune the system yourself. Sometimes there are
physical factors that prevent you from attaining the
performance you need. These problems fall into four
categories:
1. Overloading the Motor
2. Compliance
3. Resonance
4. Changing Load Inertia or Reflected Inertia
3.8.1 Overloading the Motor
Overloading the motor is the most common problem
for positioning systems (that is, systems with PL on).
If you overload the system, the position error can
grow to very large values. When the command stops,
the motor "reels in" the following error and can
overshoot excessively. It looks like a tuning problem,
but it is actually caused by the motor being
undersized, ACC or DEC being set too high, or ILIM
being set too low.
When a motor is overloaded, it has the following
characteristics:
•
The system overshoots, sometimes excessively,
but does not ring or oscillate.
•
Reducing ACC and DEC eliminates the
problem.
•
Turning off PL eliminates the problem.
•
The motor current is near or at saturation
during a large part of the move. Use the BDS5
RECORD function to record ICMD.
If ICMD is equal to ILIM for more than a few
milliseconds, then your system is saturated.
Overloading the motor can be corrected by the
following actions:
•
Reducing ACC and DEC.
•
Reducing the load on the motor.
•
Increasing ILIM (if it is less than IMAX).
•
Using a BDS5 with a higher current rating.
•
Using a motor with more peak stall torque.
3.8.2 Compliance
In compliant systems, the load is not tightly coupled
to the motor shaft. If you move the load by hand, you
can feel springiness. Compliant systems often are
very stable when you tune with lower target
bandwidths. However, they oscillate vigorously at
low frequencies when you try to tune them for higher
bandwidths.
When a system is compliant, it has the following
characteristics:
•
There is springiness between the motor and the
load or at the motor mounting plate.
•
The TUNE command calculates tuning
variables that cause the system to oscillate.
•
The frequency of oscillation is less than
100 Hz.
Compliance can be corrected by the following
actions:
•
Reduce the bandwidth of the system.
•
Stiffen the machine so the load is not springy.
3.8.3 Non-Linear Mechanics
BDS5 tuning is based on linear control theory. The
most important requirement of a linear motor
controller is that the total reflected inertia should not
change substantially during operation. Load inertia
includes all the inertia reflected to the motor, such as
inertia through gearboxes and leadscrews. Inertia can
change in ways that are easy to understand, such as
the inertia of a spool of cable decreasing when the
cable is unrolled. It can also change in less intuitive