User's Manual

PMAC User Manual
128 Basic Motor Moves
Homing Search Move Control
Homing Acceleration
The acceleration for homing search moves is controlled by the same parameters (Ix19, Ix20, and Ix21) as
for jogging moves. These are described in the previous section.
Homing Speed
Homing speed and direction are specified by Ix23. If Ix23 is greater than zero, the homing search move
will be positive. If it is less than zero, the move will be negative. The magnitude of Ix23 controls the
speed of the move (in counts/msec).
Home Trigger Condition
The PMAC homing search moves utilize the hardware position capture feature built in to the DSPGATE
IC. Because no software action is required to do the actual capture, it is incredibly fast and accurate
(delay less than 100 nsec). This means that the capture is fully accurate regardless of motor speed, so
there is no need to slow down the homing move to get an accurate capture.
Specify Flag Set
In the basic setup of the motor, Ix25 specifies which set of flags (associated with one of the encoder
counters) is used for that motor. It is important that the flag number match the position encoder number
for the motor (e.g. if using ENC1 as the position-loop feedback, use Flags1 — HMFL1, +/-LIM1,
FAULT1 — for the flags, and CHC1 as the encoder index channel) in order to make use of the PMAC
accurate hardware position capture feature.
Software Capture Option
If not using quadrature encoder feedback for the position loop, but a homing search move still needs to be
done, set bit 16 of the position-loop feedback address parameter Ix03 to 1 to tell PMAC that it cannot use
the hardware capture feature, so it must use a software capture technique. For example, if the address for
Ix03 is $0724, Ix03 should be set to $10724 for the software capture of home position.
When software capture is used, there is a potential delay between the actual trigger and the PMAC
position capture of several milliseconds. This can lead to inaccuracies in the captured position; the speed
of the motor at the time of the trigger must be kept low enough to achieve an accurate enough capture. A
two-step procedure with a fast, inaccurate capture followed by a slow, accurate capture is commonly used
in these types of systems.
Ix21
Ix20
Ix21
Ix20
Ix21
Ix21 Ix21 Ix21
Ix20
Ix23
Home Complete=0
Home Search
In Progress=1
Home Complete=1
Home Search in Progress=0
Trigger
Occurs
Net distance from
trigger position
=Ix26
Desired Velocity Zero=1
In Position=1
(when FE in range)
Time
Vel
Rate of acceleration
limited by Ix19 - can override
Ix20 and Ix21
Figure 22 Homing Search Move Trajectory