The first PC-based controller in 1986 was
already high-performance because, thanks to fast PC processors, it was already
significantly more powerful and faster than the so-called hardware controllers
at that time. Even then it was a challenge to bring this computing power “down to the ground,” i.e. to the sensors and
actuators of the plant to be controlled. Although plug-in I/O cards were a
solution, they coupled the maximum number of inputs and outputs to the number of
free card slots and led by nature to the multi-wire, inflexible wiring of all
peripheral devices up to the PC in the central control cabinet. As a result, a
serial fieldbus was required – and there was still no
convincing system at the close of the 1980s: Interbus, SERCOS and co. were
still in their infancy and initially supported either I/Os or drives. PROFIBUS
DP and CANopen hadn’t even been invented yet.
Beckhoff made a virtue out of necessity and developed the Lightbus protocol
that was the basis for the current real-time Ethernet fieldbus: EtherCAT.
The operational principle of Lightbus of
telegram processing on the fly, coupled with priority-controlled, logical
communication channels, had already led in 1989 to impressive performance
figures: While over 1000 distributed I/Os were addressed every millisecond,
some fast drive controllers could be updated at the same time every 100 μs with the Lightbus. Lightbus was thus even more powerful than the
fast PC CPUs of the time, laying the foundations for a long life of this
fiber-optic-based fieldbus technology.
Over 20 years after its development, the
Lightbus system is still faster than practically all other fieldbuses and also
faster than the vast majority of the Industrial Ethernet variants, which
represent the latest fieldbus generation. But even the Lightbus is no longer
fast enough to completely map the computing power of today’s PC CPUs in the plant peripherals.
Therefore, Beckhoff developed EtherCAT, in
a manner of speaking the next Lightbus generation. EtherCAT also uses the
principle of processing on the fly and supports an arbitrary number of logical
communication channels – but on a medium that is 40
times faster, on 100 Mbit/s Fast Ethernet. EtherCAT can communicate, for
example, with 100 servo axes every 100 μs. However,
even the fastest Industrial PC cannot calculate the control algorithms of this
number of axes every 100 μs. So Beckhoff has once again
developed a bus system that is “future-proof” and will not represent a bottleneck in the control system in the
foreseeable future.
Fieldbus becomes the bottleneck for
PC-based control
Classic fieldbuses cannot match the performance
of PC-based control. Therefore, despite a high-performance controller,
additional local special controllers and regulators are frequently used for
local pre-processing of the data when a slower fieldbus or Industrial Ethernet
system is chosen.
These controllers must be programmed and
configured separately. Examples of this are special hydraulic controllers,
integrated drive positioning controllers or even the mass flow controllers in
semiconductor manufacturing plants. The variety of hardware and tools required
for this forces up the costs of acquisition, engineering and maintenance.
Distributed local controllers may be a tried-and-tested means in modular
machines, but in many applications they are quite simply a necessity born of
the communication bottleneck.
Using EtherCAT and TwinCAT, very fast
control loops can also be closed over the bus: complex special controllers and
their proprietary programming tools become redundant and can be replaced by
fast I/O terminals and a TwinCAT controller function block. That is not only
less expensive; it also opens the “black box” of the special controller: if necessary, the machine manufacturer
or system integrator can optimize the controller software for its application
or even replace it entirely using proprietary control algorithms, with which it
sets itself apart from the competition.
With TwinCAT 3, users can also elegantly
develop these control algorithms and debug them online with Matlab®/Simulink® or C/C++.
EtherCAT architecture makes compact IPC designs
possible
The extraordinary performance of EtherCAT
leads not only to simplified control architectures, but also to simplified
hardware structures in the IPC itself: Fieldbus interfaces – which are still necessary in many plants for reasons of investment
protection and the elegant migration to EtherCAT or for connectivity to
neighboring systems – can be implemented as EtherCAT
terminals and thus connected locally. The number of card slots on the local PC
bus no longer determines the shape and the expandability of the system. IPCs
become more compact. Fieldbus cables become shorter and thus make higher baud
rates possible – a further gain in performance.
However, it is not only applications with
closed control loops that benefit from the combination of a high-performance
controller and high-performance communication: all applications with
event-dependent control are also accelerated. Very frequently the next work
step is dependent on the occurrence of an event. It continues as soon as the
work piece has arrived, the pneumatic cylinder has reached its target position,
the part to be mounted is actually in the gripper or the desired pressure is
available.
To this end, sensors are continuously and
cyclically scanned, wherein the time taken to respond to the occurrence of the
event is directly dependent on the scanning frequency and the communication
efficiency. With an ultrafast bus such as EtherCAT, fast I/O modules, TwinCAT
and the PC controller, these small waiting periods are shortened significantly.
The result is a noticeable increase in the efficiency of the plant compared to
conventional control technology. In the case of an assembly plant with, for
example, two such events per second, just converting from “a PLC with a traditional fieldbus” to “a software PLC with EtherCAT” can achieve
approx. 3 % more throughput.
EtherCAT – fast,
flexible, inexpensive
EtherCAT is the fastest fieldbus and is
therefore often reduced to “highperformance.” That, however, is putting it
too briefly – just as “PC-based”
with TwinCAT means not only ultrafast, but also scalable,
flexible, highly integrated, user-friendly and inexpensive. Many users
implement EtherCAT even though they do not need the performance advantages at
all. What counts for them is the inexpensive costs, the flexible topology without
active infrastructure components, the simple configuration,
thanks to automatic address assignment, the
excellent diagnostic characteristics with fault location or the global
acceptance and large selection of EtherCAT devices. Or simply the advantage
that the EtherCAT master makes do entirely without auxiliary hardware; the
Ethernet interface, which is present on the PC anyway, is enough. That alone
makes EtherCAT the natural partner of PC-based control technology – and both technologies are indeed “high-performance” in any case.
www.ethercat.org
