Delta RMC151 - Manual (Page 128)

RMC70/150 and RMCTools User Manual
The Direct Output (9) and Open Loop Rate (10) commands work like on a linear axis,
except that the positions will wrap as usual for a rotary axis.
The Open Loop Absolute (11) and Open Loop Relative (12) commands ramp the Control
Output from it's current value to the requested Output. This ramping is linear based on
distance (not time) from the current Actual Position to the requested Position or
requested Displacement command parameters.
For both these commands, the ramping range is calculated in the same manner as the
Command Position is calculated for the point-to-point moves described above; for the
Open Loop Absolute command, the range of the ramping is determined by the Direction
parameter, and for the Open Loop Relative command, the range of the ramping is
determined by the sign of the requested Displacement.
3.6.3. Velocity Control
The RMC fully supports velocity control. Velocity control can be achieved on either a position
axis or a velocity axis.
On a Position Axis:
On a position axis (an axis that has a position feedback, such as an encoder), the RMC
can control velocity in several ways:
• With Position Moves
The most basic velocity control on a position axis is with standard position moves in
Position PID mode (the standard control mode). For example, when issuing a Move
Absolute (20) command, the speed must be specified in the command. The axis will
move at the specified speed until it reaches the requested position.
• Continuous Position Control
Velocity can also be controlled on a position axis with the Move Velocity (37) command
in Position PID mode. In this mode, the Move Velocity command does not actually
generate a Target Velocity. Instead, it generates a moving Target Position. Therefore,
if the axis falls behind, it will not try to catch up to the Target Velocity; it will actually
attempt to catch up to the Target Position. This may cause the axis to move
considerably faster than the Target Velocity while it catches up.
The Move Velocity command works well on axes where position is important.
• True Velocity Control
True velocity control can be achieved on a position axis with the Move Velocity (37)
command in Velocity PID mode. In this mode, the Move Velocity command generates
a Target Velocity. A Target Position is not calculated; it is set to the same as the
Actual Position. Velocity I-PD mode can also be used for true velocity control.
Velocity control on a position axis is often done with a rotary encoder. See the Using
Rotary Motion topic for details on using rotary motion.
On a Velocity Axis:
On a velocity axis (an axis that has a velocity feedback, such as a tachometer), the RMC
can control true velocity. Notice that in velocity control on a velocity axis, it is impossible
to achieve exactly zero speed. This is because velocity transducers report the speed in
voltage, which can never be exactly zero due to its analog nature and noise.
The standard command for velocity control is the Move Velocity (37) command. The
Velocity PID mode is typically used for velocity control on a velocity axis. See the Velocity
PID topic for more details, such as commands, etc.
For advanced applications, the Velocity I-PD control mode may be used.
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