Home Search; Figure 6.4-3 - Origin Switch And Encoder Index Pulse - Newport ESP6000 User Manual

Section 6 — Motion Control Tutorial
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6.4.3 Home 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 stage very accurately and repeatably. The need for
this absolute reference point is twofold. First, in many applications it is
important to know the exact position in space, even after a power-off cycle.
Secondly, to protect the stage from hitting a travel obstruction set by the
application (or its own travel limits), the controller uses programmable
software limits. To be efficient though, the software limits must be defined
accurately in space before running the application.
To achieve this precise position referencing, the ESP6000 motion control
system executes a unique sequence of moves.
First, let's look at the hardware required to determine the position of a
motion device. The most common (and the one supported by the ESP6000
controller card) are incremental encoders. By definition, these are encod-
ers that can track only relative movements, 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 or origin switch. An
important prerequisite 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 a 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 stage is unique but not accurate
(repeatable) enough. The solution is to use both, following a search algo-
rithm. An origin switch (Figure 6.4-3) separates the entire travel in two
areas: one for which it has a high level and one for which it is low.
origin switch
encoder
index pulse
Figure 6.4-3 — Origin Switch and Encoder Index Pulse
The most important part of it is the transition between the two areas. Also,
looking at the origin switch level, the controller knows on which side of the
transition it currently is and which way to move to find it.
The task of the home search routine is to identify one unique index pulse as
the absolute position reference. This is done by first finding the origin
switch transition and then the very first index pulse (Figure 6.4-4).
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