In many applications, there are significant benefits to be gained by
using drives which offer integrated PLC functionality, says Robert Brooks
An important trend in the design of modern automation systems is the
widespread adoption of a modular approach. A typical machine may, for
example, have one control system for the in-feed area, another for the
process itself, and yet another to control the out-feed section. These
small control systems will be linked via a fieldbus which, if necessary,
can also provide links to an overall supervisory or data collection
system.
For machine suppliers, this approach simplifies the design process, and
makes it easy for the elements of the machine - the modules - to be mixed
and matched to meet the end user's precise needs. For end users, the
modular approach means easier maintenance and greatly increased
flexibility in the event that it should become necessary to reconfigure
the installation. Modularity is not the only significant automation
system design trend, however; another is the move toward adding
functionality, even in relatively simple applications.
For example, a modern multi-stage pumping system, where several pumps can
be brought on-line as needed to meet demand, is likely to incorporate at
least one variable-speed controlled pump. This means that smooth and
accurate matching can be achieved between pump capacity and demand,
thereby optimising energy requirements. But sequencing and duty rotation
between the pumps is also needed, so the control system requires a degree
of intelligence which, these days, is most easily and cost-effectively
provided by a programmable logic controller (PLC).
Both of these trends clearly mean growing requirements for control
systems which incorporate both variable-speed drives and PLCs. In a
typical system today, the drive and the PLC would be separate devices,
but there are benefits to be gained by building the PLC functionality
into the drive. Before we explore the benefits of this integrated
approach further, however, there is one important caveat: the supplier of
integrated products must have proven experience of both drives and PLCs.
Over the past 20 years, automation suppliers have honed the development
of their PLC and inverter ranges so as to produce the reliable,
easy-to-use and highly functional products we have today. When users
choose one of the new products which combine PLC and drive technologies,
it is important that they do not lose any of the benefits of this
valuable experience. With that said, let's now take a look at the
attractions which drives with integrated PLCs hold for users.
The first is that valuable space is saved in the control panel, and panel
assembly time is reduced, as there is only one component to mount, rather
than two. Arguably a more significant benefit is that there is no longer
any need for external connections between the drive and the PLC. This
simplifies the control panel wiring, cutting panel costs and making
maintenance easier. Data exchange between the drive and an integrated PLC
is also simpler, with control system designers no longer having to worry
about protocol compatibility and similar issues. Further, an integrated
drive/PLC solution means easier programming and set-up since, in a well
implemented product, the software tool used for programming and debugging
the control functions may well be the same as that used for entering and
monitoring the drive parameters.
Another potential benefit is that suppliers of drives with integrated PLC
functions can design them to support powerful macros that satisfy
commonly encountered control requirements. A macro might be offered, for
example, to control a label printing station or a winder. This makes the
control system designer's job easier, and also means that end users
benefit from being supplied with a tried-and-tested solution.
In spite of these impor