ABB ACS860, Firmware Manual

Page: 30 / 438

30 Program features
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Master/follower functionality
General
The master/follower functionality can be used to link several drives together so that
the load can be evenly distributed between the drives. This is ideal in applications
where the motors are coupled to each other via gearing, chain, belt, etc.
The external control signals are typically connected to one drive only which acts as
the master. The master controls up to 10 followers by sending broadcast messages
over an electrical cable.
The master drive is typically speed-controlled and the other drives follow its torque. In
general, a follower should be
• torque-controlled when the motor shafts of the master and the follower are rigidly
coupled by gearing, chain etc. so that no speed difference between the drives is
possible
With torque control, follower parameter 26.15 Load share can be used to scale the
incoming torque reference for optimal load sharing between the master and the
follower. Some torque-controlled follower applications, eg. where the torque is very
low, or very low speed operation is required, may require encoder feedback.
If a drive needs to quickly switch between master and follower statuses, one user
parameter set (see page
74 ) can be saved with the master settings, another with the
follower settings. The suitable settings can then be activated using eg. digital inputs.
Communication
A master/follower link can be built by wiring together the XD2D connectors of the
drives. The medium is selected by parameter
60.01 M/F communication port .
Parameter 60.03 M/F mode defines whether the drive is the master or a follower on
the communication link. Typically, the speed-controlled process master drive is also
configured as the master in the communication.
The communication on the master/follower link is based on the DDCS protocol, which
employs data sets (specifically, data set 41). One data set contains three 16-bit
words. The contents of the data set are freely configurable using parameters
61.01 … 61.03 . The data set broadcast by the master typically contains the control
word, speed reference and torque reference, while the followers return a status word
with two actual values.
The default setting of parameter 61.01 M/F data 1 selection is Follower CW . With this
setting in the master, a word consisting of bits 0…11 of 06.01 Main control word and
four bits selected by parameters 06.45 … 06.48 is broadcast to the followers.
However, bit 3 of the follower control word is modified so that it remains on as long as
the master is modulating, and its switching to 0 causes the follower to coast to a stop.
This is to synchronize the stopping of both master and follower.