IEEE Robotics & Automation Magazine - March 2018 - 24

or more Ethernet frames. This is the so-called summation frame
principle, as opposed to the individual frame approach in
which every frame carries process data for only one device [8].
With EtherCAT, the standard Ethernet packet is no longer
received, interpreted, and copied at every node. Instead, slave
devices process frames on the fly, reading and inserting data
while the frame is passing through the device. This is handled
by hardware-integrated EtherCAT slave controllers. The process data in industrial networks are relatively small in quantity
(only a few bytes) so that the summation frame method, combined with the processing-on-the-fly feature of the EtherCAT
slaves, offers strong system performance [8], [22]. Moreover,
network topology plays an important role when the performance of a system is evaluated [22]. Crucial aspects are not
only cycle time or efficiency but also cabling effort, diagnostic features, redundancy options, and plug-and-play features.
EtherCAT networks have no practical limitations regarding
the topology, line, star, tree, ring, and all those combined with
up to 65,535 nodes per segment. Then, for synchronization,
EtherCAT relies on a clock synchronization mechanism that
is known as a distributed clock (DC). Essentially, all the DCenabled slaves in the network are synchronized with a common timing reference under direct control of the master [7].
Despite being simple and straightforward, the DC mechanism enables accurate synchronization (in small-to-medium
systems, clock deviations are well below 1 µs).
Ethernet-Based Solutions
There are more than 25 Ethernet-based industrial protocols
on the market, but the list of protocols that have a considerable impact on industry is much shorter [23]. They are

Figure 2. A Kollmorgen RBE-type 12- pole frameless motor
as used in the test setup. The sensors in the motor include
hall sensors for the commutation of the motor and an optical
incremental encoder for its position. A small ring is clamped to
the main axle to be able to turn the motor axle manually.

IEEE ROBOTICS & AUTOMATION MAGAZINE

March 2018

EtherCAT
Powerlink
● Ethernet/Internet Protocol (IP)
● Modbus/Transmission Control Protocol (TCP)
● ProfiNet.
A multitude of performance evaluations of the remaining systems are reported in the literature. The conclusions
of these studies are that EtherCAT is an overall highly performing real-time protocol when compared to the aforementioned protocols [24], [25], [8]. ProfiNet has advantages over EtherCAT in specific conditions, such as efficiency
in asynchronous communication [26]. Neither Ethernet/IP
nor Modbus/TCP are deterministic and, by consequence,
not suited for hard real-time control. The major advantages of EtherCAT over ProfiNet and Powerlink are the
costs of implementation and the commercial diffusion of
the technology [27]. Studies also predict the future pervasiveness of EtherCAT in the industrial automation and
robotics fields [23] . This suggests that EtherCAT is the
leading communication protocol for these applications.
Another major communication protocol that is not part of
Ethernet-based systems are serial protocols, of which CAN
is widely used in the robotics field.
●
●

CAN
CAN is still an adequate choice for low-cost industrial
embedded networking. However, Ethernet-based protocols
are now able to overcome the shortcomings of CAN, such as
limited baud rate and limited network length (1 Mb/s at
120 ft). Although the very low-cost implementation as well as
the low resource requirements of CAN protocols still make it
an adequate choice in certain applications (such as the automotive industry, small embedded solutions, and aerospace),
the overall advantages of EtherCAT over CAN are
● the data throughput (currently 100 times higher)
● the unlimited network length
● increased system performance
● the use of established hardware components.
EtherCAT Deployment
An EtherCAT master runs the EtherCAT network and communicates with all slaves. This master needs to be implemented on a real-time operating system. For this case, several
solutions have been developed. The one demonstrated in this
tutorial is the Windows control and automation technology
(TwinCAT). Beckhoff provides the TwinCAT program,
whose essential functionality is to reserve a number of physical cores on a user's personal Windows computer and run the
EtherCAT network from these cores. The Windows operating
system does not run on these cores anymore and only operates on the cores specified by the user. As an example, in this
tutorial, a quad-core laptop was used in which two cores are
reserved for the EtherCAT protocol and two cores are used
for running Windows.
The driver running on the EtherCAT reserved cores is a
compiled version of a program that can be written in either

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - March 2018