Les Hunt went behind the scenes of a totally new digital dc drive
development that took user suggestions and observations very much to heart
There are few manufacturers that specialise in just one type of drive.
Most operators in this business spread their options to cover ac, dc and
servo products (dc to a lesser extent these days), but one relatively
young company based in West Sussex has countered the tide and spent the
last 14 or so years concentrating on just one - dc drives. Sprint
Electric introduced a single-phase analogue dc range back in the late
1980s, moving into three-phase analogue by the early 1990s. These
products are still manufactured today and are well regarded by their
users, but towards the end of the 1990s, Sprint's vision was to develop
its first digital three-phase product, and in 1997 Aris Potamianos joined
the company from Eurotherm to provide the necessary injection of
expertise.
That was four years ago. With the development and launch stages behind
them, Sprint now has a successful range of three-phase digital dc drives
called PL/X. While the move to digital was an inevitable one, given the
market forces, this was no 'me too' exercise on Sprint's part. Mr
Potamianos had been instrumental in the development of Eurotherm's 590
digital dc drive, which, arguably, became something of an industry
benchmark. What he identified when agreeing to join Sprint and help
develop the PL/X were opportunities for a 'clean slate' approach to this
development; PL/X was to be a totally new product and, as such, would be
unfettered by the 'baggage' of previous models and entrenched design
methodologies.
Given this clean slate, the first thing the company set about doing was
to find out what the customer wanted from a modern dc drive. During a
lengthy market research project a number of key pointers were identified
that were later to influence the design specification. Surprisingly,
product documentation drew the largest number of complaints from those
surveyed. Sprint addressed this by using block diagrams and pictorial
representations of parameters, repeating parameter information throughout
the manual to avoid sifting back and forth through the pages. The manual
flows in the same sense as the product block diagram.
All users that were surveyed wanted a large back-lit display with full
language parameter descriptions. Instead of numeric codes, they wanted to
scroll through a list of target connections, each with an unambiguous
language description. Even the 'feel' and travel of the programming keys
attracted attention. The I/O, too, came in for a bit of stick. With all
the software blocks available to them, many users were experiencing a
lack of available I/O in the products they were using. Sprint believes it
has overcome this in the PL/X by giving many terminals the ability to be
universal inputs or outputs. There are 17 digital inputs, eight analogue
inputs (adjustable for +/- 5/10/20/30V), seven digital outputs sustaining
loads of 350mA, and five control inputs.
Users complained that some products require extra cost option boards to
accommodate the three main sources of speed feedback: armature voltage,
tachogenerator and incremental encoder. The PL/X not only accepts all
three as standard, but is also able to run them in combinational modes -
armature voltage with low resolution encoder feedback, for example,
providing very low cost feedback with very high steady state accuracy.
As well as controlling the speed of a motor, the drive is usually
combined with many other devices that control related functions within
the machine, and all this adds to costs. With the help of users in the
survey, Sprint analysed a very large number of systems to identify these
peripheral application blocks and then incorporate them as standard
within the PL/X. According to Mr Potamianos, Sprint has around 90% of all
functions required