User's Manual

PMAC User Manual
Basic Motor Moves 129
Trigger Signals and Edges
Once the set of flags for the motor have been specified with Ix25, use Encoder/Flag I-variable 2 (I902,
I907, etc.) to tell PMAC whether to use a flag, the index channel, or both, as the capture trigger, and
which edge of the flag and/or the index channel to use.
Next, use Encoder/Flag I-variable 3 (I903, I908, etc.) to specify which of the four flags (HMFLn, +LIMn,
-LIMn, FAULTn) is to use for the capture. If using a limit or a fault flag for home capture, disable the
normal function of that input by setting high bits of Ix25, at least for the duration of the homing search
move.
Torque-Mode Triggering
Normally, the trigger condition for homing search moves, jog-until-trigger moves, and motion program
move-until-trigger moves is an input flag signal transition. Sometimes it is desired that a trigger occur
when an obstruction such as a hard-stop is encountered. To support this type of functionality, PMAC
permits triggering on a warning following error condition instead of an input flag. This is sometimes
called torque-mode triggering, because it effectively triggers on a torque level (except for velocity-mode
amplifiers) because output torque command is proportional to following error. It is also called a torque-
limited mode, because it provides an easy way to create moves that are limited in torque, and that stop
when the torque limit is reached.
To enable this torque-mode triggering, set bit 17 of the position-loop feedback address I-variable Ix03 to
1. In addition, Bit 16 of Ix03 should be set to 1 to tell PMAC to use the software-read position on a
capture instead of the hardware-latched position, because there is no input signal to latch the position in
this mode. Bits 0-15 contain the actual address of the feedback. For example, the default value of I103 is
$0720, specifying the address of the first entry in the encoder conversion table, and specifying signal-
based triggering. If I103 is changed to $30720, the same register is used for feedback, but torque-mode
triggering is specified.
In this mode, the trigger for a homing search move or a move-until-trigger is a true state of the warning
following error status bit for the motor. The warning following error magnitude for the motor is set by
Ix12, with units of 1/16 of a count. When PMAC detects this transition, it will read the present feedback
position as the trigger position, then move relative to this position. In a homing search move, the relative
distance is specified by Ix26, in units of 1/16 count. In a jog-until-trigger, the distance is specified by the
second value in the jog command — the value after the ^ arrow — in units of counts. In a motion
program move-until-trigger, the distance is specified by a second value in the axis command (the value
after the ^ arrow) in user axis units.
In many cases in these types of moves, the Ix69 command output should be set to a lower value
representing the torque or force limit to ensure that this limit is not exceeded at any time during the move,
before or after the trigger.
Notice that if the warning following error status bit is true at the start of the move, the trigger will occur
almost immediately.
Merits of Dual Trigger
It is common practice to use a combination of a homing switch and the index channel as the home trigger
condition. The index channel of an encoder, while precise and repeatable, is not unique in most
applications, because the motor can travel more than one revolution. Typically, the homing switch, while
unique, is not extremely precise or repeatable. By using a logical combination of the two, uniqueness
from the switch and precision and repeatability from the index channel can be achieved. In this scheme,
the homing switch is effectively used to select which index channel pulse is used as the home trigger.