Page 739 - Mechatronics with Experiments

PROGRAMMABLE MOTION CONTROL SYSTEMS 725
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FIGURE 10.8: HOMEing routine (establishing a position reference) using motor and tool
mounted sensors.

HOME: standard name used in the programmable motion control industry to describe the
motion sequence used to establish a reference position for the motion axis. The objective
of the homing motion is to bring the axis to a known reference position, called the “home”
position. All other positions are referenced to that position for the axis. For instance,
the machine tool axes must be at a pre-defined position before the beginning of a cycle.
Figure 10.8 shows an example of homing motion. If there is an absolute position sensor
for the motion axis, the homing can be performed from the current known position to the
desired position in one move where the distance is known. Quite often, a motion axis uses
an incremental position sensor. The position of the axis is not known at power-up. The
axis must search for a reference position using a combination of sensors that indicate a
known position. Typically the encoder index channel and/or external ON/OFF sensor are
used to establish the reference position. Depending on the accuracy required in starting
at the known reference position, different homing motion sequences can be performed.
Figure 10.8 shows three different types of common homing motion sequences. Others can
be designed to meet the needs of a particular application. The first home sequence simply
moves at a constant speed until a sensor signal changes state (i.e., turns ON) and then stops
the motion at a defined deceleration rate. The second home sequence does the same thing
as the first home sequence, plus it moves backward by a predefined distance after stopping.
The third home sequence adds another predefined motion distance in the forward direction.
Notice the small wait periods between the stop and incremental moves. The purpose there
is to allow the axis motion to settle to a complete stop. Parameters used to define a HOME
motion are home search speed, acceleration and decceleration rates, incremental move
distances, and time periods to complete them.

JOG: standard name used to move at a certain speed while the operator holds an input
switch on. Using the JOG function, the machine axes can be moved around to any position
under the operator control. This function is needed in machine setup and maintenance. JOG
motion refers to a constant speed motion of an axis until a condition is met to stop it. The