Installation guide

Mint™ v4 Advanced Programming Guide
134 MN1270 02.2002
Attributes:
Controller Read Write Command Multi-
Axis
Scaled Default Range
All
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Description:
Reads the current instantaneous demand velocity as generated by the profiler.
Example:
dim currentVel=0
currentVel = VELDEMAND.0
This get the current demand velocity for axis zero
See Also:
POSDEMAND
VELENCODER/VEN
Purpose:
To select the source of the velocity signal used in dual encoder feedback systems.
Controllers Supported:
NextMove PCI NextMove PC NextMove BX MintDrive ServoNode 51
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Format:
VELENCODER.axis = <expression>
value = VELENCODER.axis
Dot Parameters:
axis – the axis number.
Attributes:
Controller Read Write Command Multi-
Axis
Scaled Default Range
NextMove
  
0 – 3 (0 – 7 on PCI
with expansion card)
Description:
Dual encoder feedback is a control technique where two encoders are used to control a single axis.
Generally one of the encoders is attached to the load and the other to the motor. The encoder attached
to the load gives high positional accuracy and the encoder attached to the motor provides stability.
The implementation of Dual Encoder Feedback uses the two encoders to close two control loops,
position and velocity. The encoder attached to the load is used to close the positional loop and the
encoder attached to the motor shaft is used to close the velocity loop. This enables high positional
accuracy to be achieved via the encoder attached to the load, yet the stability problems associated with
coupling imperfections (compliance and backlash) are eliminated as these are now inside the velocity
loop.
The
VELENCODER keyword is used to specify the encoder channel that the axis will use for velocity
feedback. The encoder can be changed at any time though it is recommended that the axis is disabled
when this is done. The velocity used in the servo to be multiplied by the
KVEL term comes from the
velocity encoder. The
VEL keyword and ‘measured speed’ in data capture are from the velocity
encoder. Positional information is read from the standard axis encoder.
The velocity encoder is specified as a channel number in the range 0 – 3. On NextMove PCI, if an
expansion card is present, then range increases to 0-7. Auxiliary encoder channels may also be
specified as shown in the following table: