ing to its program. These conditions typically
require the operator to press a start button.
The controller is likely to be programmed
to move the axis both forward and reverse.
What is considered forward on the machine
may turn out to be reverse for the servo. The
servo amplifier will have a parameter to invert
the direction. It is important to match the
servo direction with the machine direction not
only for programming the controller, but also
for any other features in the servo amplifier
that are sensitive to direction. For example,
the amplifier may be set to limit torque in the
motor in one direction but not the other.
Another direction-sensitive feature found in
amplifiers is the over-travel, also known as the
limit switch. Over-travel means that two external sensors are placed near the two extremes
of the range of motion where they can sense
the proximity of the load. These sensors are
connected to the amplifier so it can quickly
stop the motor and avoid a machine collision.
This situation can arise if there is a controller programming error or human mistake
during setup and installation. The positive
over-travel sensor is connected to the amplifier input that stops motion in the forward
direction, and the negative over-travel sensor
is connected to the amplifier input that stops
motion in the reverse direction. So you can
see that if you get these sensors backwards
or if the direction of the servo doesn’t match
the direction of the machine, then the motor
won’t stop at the over-travel sensor.
One detail to keep in mind is that an
over-travel stop is not an e-stop. It’s more
like a quick way to stop motion in one direction. The controller can be programmed to
recover and the servo can still be moved in
the opposite direction.
Distance and Position
While the servo amplifier and servomotor
are the core of the motion control sys-
tem, the required movement originates
with the controller. Controller program-
ming is a huge topic, but one funda-
mental concept for controller program-
ming is that of distance and position.
The program may call for a relative or
incremental move a certain distance
from the current position.
This lends itself well when programming
the same move to repeat several times, but
when the repetitions are complete, the
programmer may want to move back to the
starting position, or maybe the program
uses a preset table of positions to which the
machine is to move. In this case, an absolute
positioning move to the exact position makes
it a lot easier to program the controller.
Using absolute moves requires one important step: defining where the servo is when it
is at position zero. So, where is position zero?
Finding that starting position in the controller
is called homing. It’s possible that an axis was
moved while power is off, so the controller
has to home the axis every time the machine
powers up. A very simple way to accomplish
homing is to prompt for manual operation of
the axis to move it to the zero position.
Most machines rely on a fully automated
homing sequence. A typical homing routine in
a controller first moves the axis
slowly in reverse until a dedicat-
ed proximity sensor is detected.
After this home sensor is detect-
ed, the controller will move the
axis in the opposite direction
an offset distance. This home
offset distance can be adjusted
to calibrate the home position of
a particular machine. The final
stopping point is then taken as position zero for
the controller’s coordinate positioning system.
As long as the motor and sensor remain in
place, this homing routine will always put the
axis back to the same starting point.
Different types of machines require differ-
ent types of homing routines. Programming a
successful homing routine in the controller for
non-standard machine configurations can be
one of the challenges for the controls engineer.
Linear actuators are common and the
most simple to understand, but there are
many types of mechanisms in the industry.
Most can be classified as either producing
linear motion or rotary motion. For linear,
axis movement is measured in inches or mm.
For rotary, it is measured in degrees.
roll can pull inches of material virtually forever. A rotating fixture with cables has a fixed
distance it can move or else the cables can
twist and break, but a rotary table can keep
spinning in the same direction.
The variety of machine types and possibilities for infinite movement further contribute
to the challenges faced by control engineers
and many times the solutions are a tribute to
human ingenuity. PDD