PMAC Product Guide Next use Encoder/Flag I-Variable 3 (I903, I908, etc.) to specify which of the four flags (HMFLn, +LIMn, -LIMn, FAULTn) is to be used 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 (see example below). 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.
Bit 16 of Ix03 should also 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 now 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, and 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.
When using these types of moves, set the Ix69 command output 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. If the warning following error status bit is true at the start of the move, the trigger will occur almost immediately. Note: 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 can be reached from the switch and precision and repeatability from the index channel. In this scheme, the homing switch is effectively used to select which index channel pulse is used as the home trigger. Although the homing switch does not need to be placed extremely accurately in this type of application, it is important that its triggering edge remain safely between the same two index channel pulses. In addition, the homing switch pulse must be wide enough to always contain at least one index channel pulse. Programming PMAC 59