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Moog Animatics SmartMotor™ Developer's Guide, Rev. L
Page 235 of 909
set so that a surge of power will not be delivered if the mechanism were to jam or reach one
of the physical (hard stop) limits.
Setting EL=formula
Set Maximum Position Error
The difference between where the motor shaft is supposed to be and where it is actually
positioned is called the "position error". The magnitude and sign of the error are delivered to
the motor in the form of torque after it is put through the PID control. As the error increases,
the motor becomes more uncontrolled. Therefore, it is useful to put a limit on the allowable
error, which will turn the motor off.
The EL command serves that purpose. It defaults to 1,000, but it can be set from 0 to 262,143.
You can view the current value with the REL (report EL) command.
For more details, see Part 2: SmartMotor Command Reference on page 238.
Other PID Tuning Parameters
There are additional parameters that can be used to reduce the position error of a dynamic
application. Most of the forces that aggravate a PID loop through the execution of a motion
trajectory are unpredictable. However, there are some that can be predicted and eliminated.
KG=formula
Set KG, Gravitational Offset
The simplest force to eliminate is gravity. When power is off, if motion would occur due to
gravity, a constant offset can be incorporated into the PID control to balance the system. The
KG (gravitational offset) provides this offset. The value for KG can range from -16777216 to
16777215.
To set KG, use the RKG (report KG) command to view the current value, and then make
changes to KG. Remember to use the F command to update the new value. Continue adjusting
KG until the load equally favors upward and downward motion.
KV=formula
Set KV, Velocity Feed Forward
Another predictable cause of position error is the natural latency of the PID loop itself. At
higher speeds, because the calculation takes a finite amount of time, the result is somewhat
delayed — the higher the speed, the more the actual motor position will slightly lag the
trajectory-calculated position. This can be programmed out with the KV (velocity feed
forward) parameter. KV can range from zero to 65535; typical values range in the low
hundreds.
To tune KV, use the RKV command to view the current value, and then make changes to KV
while running the motor at a constant speed if the application will allow. Remember to use the
F command to update the new value. Continue increasing the value of KV until the error is
reduced to near zero and stays there. The error can be seen in real time by activating the
Motor View window in the SMI software. For details, see Motor View on page 68.
Part 1: Programming: Setting EL=formula
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