What's your first choice when it comes to implementing motion or position
control? A variable speed drive or a servo drive? Some might think the
choice is obviously a servo drive, in which case they have probably lost
track of developments on the variable speed drive front. Les Hunt talks
to Rockwell Automation's Mark Daniels and Michael Spence who see the
perceived barrier between these two types of hardware coming down
TFinding the right tool for the job is arguably the most fundamental of
all engineering principles. Driven by the demand for increasingly
cost-efficient production processes, the products that power today's
industrial machines are constantly evolving. But what constitutes the
right tool in a world of changing requirements and priorities? Take
Allen-Bradley's variable speed and servo drive products as an example.
These are now challenging conventional wisdom about the best solutions in
speed or positional controlled applications. But, for Rockwell
Automation, making sure customers get the right solution from its
Allen-Bradley range is as much about changing perceptions as it is about
developing hardware.
Mark Daniels, commercial manager for Rockwell Automation's drive products
believes there are perceptual barriers about the market positions that
variable speed drives and motion controllers occupy. But these are
becoming less meaningful. It's getting harder and harder to pigeon-hole
roles like speed or positional control as being exclusively the domain of
one or the other as we can now achieve positional control with variable
speed drives and speed control with servos, he says. Michael Spence,
commercial manager for Rockwell Automation's motion control products,
agrees. The boundary between what is possible using one solution or
another has certainly moved on. Consequently, the concept of best
practice has got to move on too.
At either end of the spectrum of industry's motion requirements, the
status quo between variable speed drives and servo based solutions
remains unchanged. However, there are a growing number of applications
where alternatives now exist. In high-power drive or synchronised
multi-axis applications, the choice of solution is invariably going to
remain straightforward, asserts Mark Daniels. But advances in
technology and a narrowing price differential have given engineers the
opportunity to step back and re-evaluate the requirements of each
application. We think it's important that customers are aware of what can
now be achieved.
Michael Spence believes customers require flexibility, efficiency and
scalability. Developments in variable speed drive capability or less
expensive motion control solutions may well tip the balance one way or
another in particular situations, he says. But the real market drivers
are customer's needs for solutions that are easy to implement and manage,
make the most efficient use of plant and resources, and above all,
deliver the scalability and flexibility to cope with the changing
commercial environment.
The driving philosophy behind our product range is to be able to deliver
the optimum solution for each situation - as distinct from allowing
outdated concepts to dictate to the customer the route they should adopt.
Compared with servo systems, variable speed drives can still be the most
cost-effective way of achieving movement in many industrial applications
Drives have become even better value and ensured continuing demand in
general variable speed applications. But it is the development factors
such as improvements in accuracy and better integration with encoders and
PLCs that have moved the performance boundaries to the extent that
variable speed drives can now be considered as an option in some
positional control applications.
The most notable advances in recent times have been in the area of
positional accuracy, says Mark Daniels, who goes on to relate the
example of a palletising application from a few years ago: At that time,
even with our best resolution sensors, we were unable to achieve accuracy
and repeatability better than 1mm. But with drives like the PowerFlex
700S in a closed loop application, accuracy to within a fraction of a
millimetre is now the norm. We now specify PowerFlex 700S in a
significant number of single-axis applications that would have previously
required a motion control solution.
The option of using products like PowerFlex allows OEMs to stay
competitive, whilst still achieving high standards of performance. Faced
with the challenge of upgrading the performance of a packaging machine
within a tight budget, a Rockwell Automation OEM recently achieved a
practical yet cost-effective solution using an inverter drive in a closed
control loop with Allen-Bradley high-speed proximity sensors. Achieving
the critical balance of cost versus accuracy was important to this OEM,
so a full motion solution would just not have been practical in this
instance, recalls Mark Daniels. But by combining the latest generation
drives with Allen-Bradley high-speed proximity sensors and a PLC, we were
able to exceed the required standard of performance for much less than
the cost of going the full motion control route.
But while inverter drives have become more sophisticated and inexpensive,
servo drives are also becoming cleverer and cheaper, and powerful enough
to cope with much more demanding applications.
Ironically, one of the strong growth areas for the new generation of
Allen-Bradley servo drives like Kinetix 6000 and Ultra 3000 is in the
replacement of variable speed drives used in brake/clutch systems,
pneumatic rod-less cylinders and cam boxes employed in cutting, indexing,
filling and sealing applications. The cost of motion solutions has
dropped, while the power capability and technology has developed to the
point where a servo solution is not only a practical possibility, but
also the most sensible one, says Michael Spence. There may still be a
price differential between a motion controller and an inverter solution,
but as Michael Spence points out, just looking at the top-line cost can
be misleading. Once improvements in production capacity, maintenance
costs, efficiency and life span are factored in, the servo option, in
many cases, becomes the more attractive, he says.
A typical example is that of a Rockwell Automation customer using a
variable speed drive and brake/clutch driven rotary knife to cut food
products before cooking. The main feed axis carrying the products was
fixed speed, with the knife being triggered on timing, not position of
the product. While this provided an adequate solution initially, when the
customer's needs changed, it proved impossible to vary axis speed and
accurately match this with the cutting action of the knife. Rockwell
Automation engineers replaced the inverter drive with a servo drive
solution using an Allen-Bradley Ultra 3000.
Both mechanically and from a control perspective, this is a far more
efficient set-up, says Michael Spence. The feedback loop is closed
within the drive, meaning that the whole system can run faster, yet with
greater efficiency and accuracy. Moreover, the control is less complex
and easier to set-up, requiring less code to be written. The net result
is a more profitable operation with the flexibility to accommodate future
requirements.
In this example, the existing PLC start command was simply re-used with
the new servo drive, but in a Rockwell Automation Integrated Architecture
solution, this is not the only option. Rockwell Software's Logix
software, for example, uses the same universal built-in motion commands
for PLCs, variable speed drives like Powerflex700S, and servo drive
solutions such as ControlLogix. Everything works together so we are not
tied to one particular technology or solution, says Mark Daniels. The
whole concept behind the Rockwell Automation Integrated Architecture is
that the technology is flexible, open and adaptable.
Those same attitudes are important when working with customers, says
Michael Spence. Each solution has its own strengths and weaknesses and
we recognise that these days there is frequently more than one answer to
any given problem.
But before a customer can choose the right solution, its crucial to have
asked the right questions: Not being limited by a product range biased
one way or the other allows Rockwell Automation to adopt a solution,
rather than product-focused approach.