VMC 186/40 Motion Control Module How It Works
24
How It Works
Initialization
As the motion controller starts up, the
Scale
and
Position Offset
will be set to the default values. The
Actual
Position
is then calculated based on the default values. The
Requested Position
,
Command Position
and
Target
Position
are set to the calculated position. The
Extend Limit
and
Retract Limit
are also set to the actual
position. These values keep a 'G' command from moving the axis until the axis parameters are initialized. This
puts the responsibility of opening up the limits on the VME Controller by using the 'P' command to change the
parameters. After a 'P' command is issued, a new actual position is computed and the
Target Position, Actual
Position
,
Requested Position
and
Command Position
and are updated with the new actual position.
Once the initialization parameters are set, moving an axis can be accomplished by setting the
Acceleration
and
Deceleration
distance or rate,
Requested Speed
, and
Requested Position
to the desired values and writing a 'G'
in the
Command
parameter. The VMC 186/40 will move the axis to the new position using the most recently
programmed parameters. These same parameters can be changed while the axis is moving, but only if a new
Requested Position
and
Command
are issued for each new move.
Position Sensing
The position sensing of the VMC 186/40 is done by sending an interrogation pulse to the axis transducer and
measuring the time between the interrogation pulse and returned pulse by the transducer. The polarity of the
interrogation pulse can be set by the proper selection of jumpers P4-P7 (see page 32). The return pulse must be
a positive pulse of at least 1.5 volts. The time resolution is determined by the frequency of the counter crystal
which is 55.5 MHz. The actual resolution is determined by the number of interrogation pulses used for a single
position measurement. Each pulse is called a recirculation. On the VMC 186/40, the number of recirculations
can be selected in hardware by the proper selection of jumpers P8-P11. Normally, two recirculations are used.
See page 33 for more information on recirculations.
Drive Output
The drive output is ±100 mA at full drive. Motor drives and some proportional valves require voltage outputs.
If a voltage is needed, use a voltage divider across the output to develop the appropriate voltage range (install
100 ohm resistors in sockets R1-R4 if
±
10 volts is needed). Use as much of the full swing in the output as
possible for best results. Servo valves usually use a current source since the output drive will not be affected by
changes in coil resistance. Some servo valves have two 40 milliamp coils which will draw a total of 80
milliamps when wired in parallel. This means that full drive will occur at only 80% of the drive.
If so, the
Overdrive
status bit will not be set since the output drive never exceeds 100%
.
Closing the Loop
The VMC 186/40 CPU uses the information from the position sensing to correct for differences between the
actual position and target position. A positioner will tend to drift away from the target position and the change
in location is detected by the position transducer. The VMC 186/40 finds the difference between where it is
and where it should be. It then changes the drive output so the actual position will move back to where it
should be. This is called "closing the loop." The same thing occurs when an axis is moving, except the target
position is also moving. If the actual position is lagging behind the target position, the VMC 186/40 will
increase the drive to help the actual position catch up to the target position. If the actual position is ahead of the
target position or leading, the VMC 186/40 will reduce the drive output to slow the axis down. The amount of
drive for a given error in position is determined by three different proportional error gains:
Static Gain
,
Extend
Gain
and
Retract Gain
.
The ability to independently adjust static, extend and retract gains provides a means to compensate for
differences in system dynamics such as the difference in force and velocity constants of a hydraulic cylinder
