John Pritchard explains how to get the best performance from distributed
digital servo drives using the Sercos networking protocol over fibre
optic media
What is Sercos?
Sercos communicates information between the controller and servo drive
via a real time, deterministic fibre optic network. The controller sends
a coarse position command to the drive using a master data telegram
(MDT), and the drive responds by adjusting its motor velocity, sending
axis position information using an amplifier telegram (AT) from drive to
controller.
This process repeats at regular intervals and is synchronised by master
synchronisation telegrams (MST). The drive uses a process known as
micro-interpolation to determine what its position should be between
telegrams, and Sercos uses a deterministic protocol, so the drive remains
safely in control of all axis movements at all times.
Physically, Sercos uses a fibre optic ring configuration (see diagram
below), with a maximum length of 800m node-to-node and 200km plus total
length using glass fibre. There can be up to 254 nodes on a ring and
multiple rings are allowed.
All data, including coarse position command, set-up and diagnostic
information, passes between controller and drive over the Sercos network.
In fact, more than 400 parameters are included in the Sercos
specification (IEC 61491/EN61491), which avoids having to programme or
retrieve diagnostic information at the drive - an option that is
available for users wishing to 'pre-commission' drives before connecting
them to the controller.
Modern controllers and servo drives are now almost entirely digital, but
the analogue interface between them remains. A digital controller
calculates a commanded speed, converts it into an analogue signal, and
transmits it to the servo drive, which promptly converts it back into a
digital command. Compared with analogue devices, digital control offers
faster response time, better control while contouring and virtually no
adverse reactions to extreme ambient temperature changes.
elegant solution - we don't do it with sound recordings, so why with
servos? Enter the next major improvement for general motion control
applications that need distributed control. Its name is Sercos.
Originated as a solution for the machine tool industry, its benefits are
now being realised in general motion control.
With Sercos, command signal and all position feedback information is
routed over a fibre optic network, reducing wiring requirements
significantly. For added ease of set-up, digital drive configuration
values, such as the type of operation (fine position, velocity or torque
command), can be the same for the majority of applications. In addition,
digital drives can now gather many of the configuration values that are
unique to the user, including motor current used in motion profiles, bus
voltage, over/under voltage and power efficiency characteristics.
Smart devices, such as motors with on-board memory, can communicate their
identity to the drive, transparently to the user, while an advanced
auto-tune feature automatically determines the ideal response
characteristics of the system. Easy-to-use configuration wizards guide
the customer through those set-up of values that cannot be pre-determined
or read during the power-up sequence. In all, the user ends up with
simplified set-up requirements and a shorter commissioning time.
In the packaging industry, for example, where individual products have a
low per unit value, users place emphasis on high speed operation. When
the motion control system has been designed and installed, the next
logical step is to streamline the process. Digital drive systems with
power sharing can provide the tools to optimise the motion processes and
get products flowing faster.
To do this, the drive system can collect data about the operation and
transmit it to