Software Communication 3-34
Set Commands (Velocity and Acceleration)
Set commands are used to control the speed of the plunger. Plunger movement is
structured into three phases:
Ramping Up. Plunger movement begins with the start velocity and accelerates
with the programmed slope to the constant or top speed.
Constant or Top Speed. The plunger is moved at the constant or top speed.
Plunger speed or velocity can be programmed in Hz (half-steps/second) or in
preprogrammed Set Speeds. The actual time the plunger travels is dependent on the
ramping up and down. If the plunger move is short, it may never reach top speed.
Ramping Down. The plunger will decelerate based on the programmed slope. To
enhance fluid breakoff, the Cutoff command ([c]) can be used to define the end
velocity of the plunger just before it stops.
NOTE The Cutoff command is only active in a dispense move. During aspiration the
move will end at the start velocity [v].
For each plunger move, the firmware calculates how many steps the plunger must travel
during each phase in order to move the total number of steps commanded. If the plunger
is moving less than 1024 Hz, the pump automatically microsteps to reduce the pulsation.
The top velocity can be changed on-the-fly (while the plunger is moving) providing the
initial speed is less than the start velocity. Ramps are not included in on-the-fly speed
changes; therefore, large speed changes (100 Hz to 1000 Hz) are not recommended.
NOTE Unless the top speed is less than the start or cutoff velocity, always program
the pump in order of the move: start velocity [v], top velocity [V], cutoff
velocity [c].
K <n> BACKLASH STEPS
The [K] command sets the number of backlash steps. The syntax for this command is:
[K<n>], where <n> 0..31 (0 is the default)
When the syringe drive motor reverses direction, the carriage will not move until the
backlash due to mechanical play within the system is compensated. To provide this
compensation, during aspiration, the plunger moves down additional steps, then backs up
the set number of backlash steps. This ensures that the plunger is in the correct position to
begin a dispense move. Note that a small volume of fluid flows out the “input” side of the
valve during this operation.
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