September
1997, Issue 86
PC/104
Quarter:
Motion Control with PC/104
IMMEDIATE
MOVE
An
immediate move starts an axis moving similar to a jog
routine. Velocity and acceleration are downloaded to
the node along with the immediate move byte.
The
velocity and acceleration information can be modal,
which means it stays in effect until changed, so future
immediate move commands comprise only the command and
direction bytes.
POINT-TO-POINT
MOVE
The
point-to-point move is used for simple axis motions.
It's well-suited for operations like peck drilling,
which uses a mechanical drill bit or laser device.
Repeatable
step motions enable this operation to be accomplished
quickly since the distance to move can be relative motion.
In this situation, the move, along with the velocity
and acceleration, is modal.
TIME
MOVE
A
time move moves the axis to a specified position in
a specified time. The operator enters the acceleration,
but the velocity to achieve the position in time is
calculated by the node.
Time
move can also be modal in context, enabling simple repeatable
motion using only a command-byte transfer.
SPLINE
MOVE
For
any controller--especially for embedded controllers--spline
move is the most flexible form of motion. It enables
the host to send down complicated motion profiles with
a minimum of required commands.
Target
projection using cubic polynomials enables the software
to produce continuous or discontinuous motion (e.g.,
sharp corners). With spline motion, the cubic spline
algorithm can be the sole device producing straight
lines, circles, or any other coordinated motion.
The
cubic spline algorithm can be given in the form <Point><Point><Time>,
so the trajectory can be calculated by a polynomial
in the form:
where
k is the sample increment since the start of the move,
k = 0 is the time at the beginning of the move, x(kt)
is the target velocity at each sample time t, and c1
and c2 are calculated terms based on known starting
and transmitted ending values.
