User's Manual

PMAC User Manual
60 Setting Up a Motor
Parallel Absolute Feedback
When using an absolute encoder as the feedback device, the data is presented to PMAC in parallel form.
All lines must be presented together; no high-word low-word select schemes are permitted. With the
absolute nature of the device, the power-on/reset position is not automatically zero. For this type of
device, PMAC can use the Ix10 parameter to read the absolute power-on/reset position up to a width of
48 bits. If Ix10 is set to 0, the absolute power-on/reset position read function is disabled, and the power-
on/reset position is set to zero, regardless of the setting of the sensor, and subsequent position readings are
incrementally referenced to this zero position. For more information, refer to the Absolute Power-Up
Position section in this manual and to the Ix10 description in the Software Reference manual.
Sensor Rollover
If the overall travel for the axis is more than the range of the absolute device, PMAC will automatically
extend the position in software to handle rollover. In this case, however, the device should be considered
a parallel incremental device (see next section). A device can be considered absolute for commutation
purposes, so no power-on phasing search is required (set Ix81=0 if PMAC is doing the commutation), but
still incremental for overall machine positioning functions. In most systems, single-turn resolvers and
absolute encoders have this functionality. Refer to the Phasing Referenced to an Absolute Sensor in
Setting Up PMAC Commutation section in this manual for more information on this type of setup.
It is important with this type of feedback device to perform a PMATCH (position-match) function before
the first programmed move after power-up/reset. Usually this is done automatically by having I14 equal
to 1. If this is not done, PMAC will calculate the first move for the motor assuming a starting point of
zero, instead of the true position, leading to unexpected performance on the first move.
Parallel Incremental Feedback
A device such as a laser interferometer often provides parallel feedback data, but the device is
fundamentally incremental, so it does not know where it is on power-on/reset. The PMAC setup to accept
this type of feedback is the same as for an absolute parallel device, but recognize that a homing procedure
is necessary. For this type of device, leave Ix10 equal to zero, so that PMAC does not perform an
absolute power-on/reset position read.
However, since the position information is not absolute, and since PMAC has the ability to extend
position range in software, it is not necessary to bring all the lines of a device to PMAC. This can save
money on interfacing costs. All that is needed is enough lines (starting from the LSB) so that half the
range of those lines will not be covered in a single servo cycle. For instance, a typical interferometer
interface has 32 bits of parallel data. Even with a servo cycle of 1 msec, which is slow for PMAC, wiring
only the low 16 lines to PMAC is sufficient as long as the maximum speed is less than 32,768 (2
15
)
counts/msec, or 32,768,000 counts/sec.
Software Capture on Homing
The motor using this device for position feedback must be programmed to do a software position capture
on a homing search move, instead of the hardware position capture performed with incremental encoders.
This is done by setting bit 16 of variable Ix03 for the motor to 1 (if Ix03 were $0720, it would become
$10720). The delay in software capture can be a few milliseconds; the speed of the homing search move
may need to be limited for high homing accuracy.
Linear Displacement Transducer Feedback
PMAC can accept feedback from a linear displacement transducer (LDT) through its Acc-29 interface
board. (The best-known brand name for this type of device is Temposonics from MTS Corp.) This type
of device operates much like sonar, and what is being measured is the time before the echo is returned.
Acc-29 uses the timer registers of its on-board DSPGATE ICs to record this information, therefore, the
larger the time, the longer the distance.