From drive to motion control node
At the end of the 1980s when the first digital ac drives were launched,
it ushered in an age not only of improved motor control, but also of
improved application control. Eik Sefeldt Moeller explains
The trend in drives was, and still largely is, to become physically ever
more compact. Even more so, the drives business has become highly
competitive, with many ac drives seen as commodities, largely
undifferentiated, and selling on price. Along with that, the evolution of
'vector' control has propelled the ac drive into application areas some
predicted but many doubted. Now standard ac drives offer high dynamic
performance right up into servo levels, with the control of permanent
magnet motors as well as standard cage machines.
Each of these factors limited the development of application control
within the drive. Terminals are a case in point. The greater the level of
external control to be provided, the greater the number of terminations
required. This has space and cost implications and delayed matters. The
rising performance complexity, too, meant that there was a greater and
greater demand for processing power to handle the vector control
functions, soaking up microprocessor capacity and minimising the PLC
possibilities. Any reasonable level of PLC control also made demands on
programmability and troubleshooting. This, in turn, meant a requirement
for a more complex, multi-lingual LCD control display. All of these
factors had severe cost implications, so integrated PLC control was slow
in evolving.
The first true control function to evolve as a standard feature within
the drive was PID control. This made little demand on termination space,
required modest processing capacity and was a frequent demand in many
drive applications. The set points could also be programmed on basic LED
displays and since analogue PID control had been a standard function on
analogue drives, it was equally so on the earliest digital drives.
Embedding PID control within the drive offers significant benefit beyond
that of simply saving the cost and space of a dedicated controller.
Having the control function integral within the drive controller ensures
faster, smoother response since there is no 'filter' that would otherwise
have existed between the drive and any external controller.
Flux vector control ensured high dynamic performance and accurate speed
holding, so applications requiring synchronisation between individual
drive sections - normally the province of dc drives - were opened up to
ac drives, and this led to the development of 'option modules' that
effectively tailor the drive to specific applications. Vector control
modes were becoming increasingly accurate, thanks to advances in digital
processors. Indeed, the installed computing power of the so-called
'intelligent' drive was rapidly exceeding that required for speed and
torque control, allowing control functionality previously provided by
external devices to be embedded within the drive. The net result is that
today, truly intelligent drives can be regarded as distributed logic
'nodes' possessing some of the functionality of external PLCs or other
control units. The physical limitations of terminations still limit the
drive's I/O capacity, but this problem can be resolved to some extent by
adding option modules.
This transformation of the drive into a decentralised control node offers
significant benefits. The greatest advantage is the speed of response,
which is particularly important when synchronising the shaft position of
two or more motors, for example. An internal control architecture that
eliminates long wiring runs between control elements significantly boosts
response speed. There is also a trend to decentralise the drives
themselves, moving away from multiple drives within a control cabinet,
located some distance from the driven machinery - possibly in an offsite
control room. With the benefits of fieldbus control, the intelligent
drive can be mounted adjacent to the point of application and genuinely
regarded as a distributed logic node, offering faster more accurate
response to control demands while still remaining perfectly synchronised
to other control elements within the application.
This all makes the modern intelligent drive ideal for complex
applications requiring high dynamic performance and accurate
synchronisation, such as machine tool axis control, circular knitting
machines, flying shears, colour printing machines and the complete
spectrum of packaging and labelling machinery. The speed accuracy, torque
control and rate of response take modern drives into servo control
territory, serving drives applications right through from simple fan and
pump drives to demanding machine tool positioning drives. The latest
technology drives will also drive permanent magnet motors and offer
resolver and SinCos inputs for ultimate accuracy. This, allied with the
integral PLC functionality and a variety of high-speed fieldbus options,
has turned today's digital drive into a powerful stand-alone motion
controller.
Eik Sefeldt Moeller is product manager for the Danfoss FC30X drives
series. He is based at Graasten in Denmark
The latest generation of drives is typified by the Danfoss
AutomationDrive FC300 series, of which the FC302 and FC301 are most
recent additions. Increased computing power in the shape of
multiprocessor configurations have now been added to the drive, not just
to meet the primary drive functions but to expand and enhance the
intelligent control functions. Although a powerful DSP is utilised to
provide accurate motor control, an additional ATAC-D microprocessor,
communicating directly with the DSP via an internal high-speed bus, now
handles the external application control.
In effect, the expanded application control has been split out of the DSP
and is now the role of the ATAC-D chip. This means that the FC300 series
can - as standard - offer a wide range of application control features,
including integrated PID loops, brake control, self-initialising
adjustable S- ramps, multiple set-ups, precise start/stop features,
counters, timers and even PLC functionality. Danfoss prefers to use the
term 'Smart Logic Control' (SLC) to describe its integrated logic
control. This is similar to IEC 61131/3 Sequential Function Chart, in the
form of seven digital I/O, two of which are configurable, plus two relay
outputs rated for 240V and 400V and a 1ms scan time. Option modules,
integral within the drive, extend the available I/O and processing power
for more demanding applications.
For example, the successor to Danfoss' SyncPos motion control module, the
MCO 305, offers ten digital inputs, eight digital outputs, and two
encoder inputs accepting 5V incremental, SSI absolute, or SinCos encoder
signals. An extension of the MC0 305 (MCO 310) is due shortly and will be
fully IEC 61131/3 compliant, supporting all five programming standards.
Programming and interrogation of the drive is another area that has been
extended to support the SLC functionality. Of late, complex LCD display
screen have become much more available and cost effective than in the
past. Their cost inhibited a number of drives companies from adopting
them and there is little possibility of programming logic control
functions with an eight-digit LED display. Today, it is possible to
support the programmable logic function with a comprehensive
multi-lingual display in clear text. This opens up a new level of user
friendliness in HMI design - so much so that the graphical LCD (LCP 102)
control panel for the FC300 won the prestigious iF Design Award 2004.