User's Manual

PMAC User Manual
Setting Up PMAC Commutation 87
SETTING UP PMAC COMMUTATION
Introduction
This section explains how to set up the commutation scheme if PMAC is performing the commutation for a
motor. If not using PMAC to perform the commutation on any of the motors, skip this section. Simply
make sure that Ix01 is set to zero for all of the activated motors, so PMAC will not try to commutate them.
If using PMAC to commutate a motor, tell PMAC how to perform the commutation. This is done by
proper setup of I-variables to Ix83 for the motor. This section explains how to set these variables. Once
this setup is done the commutation operation proceeds automatically and invisibly.
PMAC has sophisticated on-board commutation features for DC brushless, variable (switched) reluctance,
AC induction, and stepper motors. These algorithms allow PMAC to drive the phases of the motor
directly, requiring only simple current-loop bridges for the amplifier. As its commutation feedback
device, PMAC can utilize the same feedback device that is used for servo position feedback (an encoder
or resolver).
Incremental Encoder Feedback Requirement
The ongoing commutation position information for a motor must come through an incremental encoder
counter. If the commutation feedback device is an absolute encoder or resolver the power-up information
can be received straight from the device, but an incremental signal must be derived from the absolute
position information for the ongoing commutation. The PMAC Acc-8D Option 7 resolver-to-digital
converter board does this automatically for resolvers, and Option 6 on an Acc-14 does this for absolute
encoders.
Specify the address of the register holding the ongoing commutation position feedback information with
Ix83. Usually (except for PMAC microstepping), this is the phase position register for an encoder in the
DSPGATE (addresses X:$C001, X:$C005, X:$C009, etc.).
Phase Referencing
When commutating a synchronous motor — permanent magnet brushless or switched reluctance — it is
necessary to reference the phasing cycle to physical features of the motor. Contrary to much that is
written, this does not require an absolute sensor. An incremental sensor can be used if a reliable phasing
search can be performed on power-up. PMAC is capable of performing such a phasing search. If an
absolute sensor is used, the phase referencing is performed only once on assembly of the system. Both
methods of phase referencing are covered in this section.
Two-Analog-Output Requirement
If PMAC is commutating a motor, two analog output channels are required per motor; the third — and
fourth, if necessary — phases are generated by balance loops in the amplifier. (Remember that if a
multiphase motor is commutated inside the amplifier, only one PMAC analog output is required.)
As mentioned previously in the Selecting the Output(s), for a PMAC-commutated motor, specify the
lower address of the pair of adjacent DAC registers that will be used to output the phase commands with
Ix02. The legitimate values of Ix02 are $C002 (DAC 1 and 2), $C00A (DAC 3 and 4), $C012 (DAC 5
and 6), $C01A (DAC7 and 8), $C022 (DAC 9 and 10), $C02A (DAC 11 and 12), $C032 (DAC 13 and
14), and $C03A (DAC 15 and 16).