Newport MM4006, User Manual

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MM4006 M o t i o n Control Tutorial
4.4.3 J o g
When setting up an application, it is often necessary to move a devices
manually while observing motion. The easy way to do this without resort-
ing to specialized input devices such as joysticks or track-wheels is to use
simple push-button switches. This type of motion is called a jog. When a
jog button is pressed the selected axis starts moving with a predefined
velocity. The motion continues only while the button is pressed and stops
immediately after its release.
The MM4006 offers two jog speeds. The high speed is programable and the
low speed is ten times smaller. The jog acceleration is also ten times small-
er than the programed maximum acceleration values.
4.4.4 H o m e Search
Home search is a specific motion routine that is useful for most types of
applications. Its goal is to find a specific point in travel relative to the
mounting base of the motion device very accurately and repeatably. The
need for this absolute reference point is twofold. First, in many applica-
tions i t is important t o know the exact position in space, even after a
power-off cycle. Secondly, to protect the motion device from hitting a trav-
el obstruction set by the application (or its own travel limits), the con-
troller uses programable software limits. To be efficient though, the
software limits must be placed accurately in space before running the
application.
To achieve this precise position referencing, the MM4006 motion control
system executes a unique sequence of moves.
First, lets look at the hardware required to determine the position of a
motion device. The most common (and the one supported by the MM4006)
are incremental encoders. By definition, these are encoders that can tell
only relative moves, not absolute position. The controller keeps track of
position by incrementing or decrementing a dedicated counter according
to the information received from the encoder. Since there is no absolute
position information, position "zero" is where the controller was powered
on (and the position counter reset).
To determine an absolute position, the controller must find a "switch" that
is unique to the entire travel, called a home switch o r origin switch. An
important requisition is that this switch must be located with the same
accuracy as the encoder pulses. If the motion device is using a linear scale
as position encoder, the home switch is usually placed on the same scale
and read with the same accuracy.
If, on the other hand, a rotary encoder is used, the problem becomes more
complicated. To have the same accuracy, a mark on the encoder disk could
be used (called index pulse) but because it repeats itself every revolution,
it does not define a unique point over the entire travel. An origin switch, on
the other hand, placed in the travel of the motion device is unique but not
accurate (repeatable) enough. The solution is to use both, following a
search algorithm.
Origin Switch
Encoder Index
Pulse
Fig. 4.22 — Origin switch and encoder index pulse.
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