The properties of IIoT (Industrial Internet of
Things) communications differ significantly
from more familiar control protocols, such
as EtherNet/IP, Profinet, and EtherCAT. So, how
can manufacturers assess the numerous communication
methods available to them to take advantage
of the IIoT?
The first step is realizing that IIoT and Industry
4.0-based solutions require detailed data. IO-Link
is a standardized sensor interface, which provides
access to detailed sensor/actuator data from the
lowest field level, meets this requirement. While
digital switching sensors provide only individual
bits, IO-Link provides access to detailed identification,
diagnostic, and parameter data from a
sensor or actuator.
Next, take note of the fact that most automation
systems are based on a PLC that contains
the logic of the application. For applications such
as controlling a robot arm, sensor data needs to
be highly accurate and reliable. In a few milliseconds,
the PLC calculates the output signals and
transfers them to actuators, such as valves and
motors. Control-based Ethernet protocols, such
as Profinet, EtherNet/IP, and EtherCAT, meet
these requirements.
Although these protocols are essentially based
on the Ethernet standard, specific properties have
been changed to achieve the high degree of timing
accuracy and quick millisecond cycle times required
in industrial applications. The data carried by these
protocols can only be exchanged using special hardware,
such as a PLC, and processed using specific
software offered by the control system manufacturers.
But transferring data from the field level to
higher-tier systems, such as cloud-based systems,
is a basic requirement for IIoT.
OPC UA, MQTT,
and REST APIs
This is where IIoT communications such as OPC
UA, MQTT, and REST APIs come in. These communication
methods rely less on real time data in
the millisecond range and more on end-to-end
data availability across different systems from
different manufacturers. IIoT is not about the
individual process data of sensors and actuators,
but rather about the overall picture. The most
important process may be collecting condition
data to avoid any future failures or detecting
correlations in process parameters that have an
influence on product quality. In these operations,
real time is not a top priority.
OPC UA is an entire framework with sophisticated
security mechanisms. A key advantage
of OPC UA is that no specific device description
files are required. Each individual device has all
the necessary data, such as its own data structure,
in a format that can be read by both people and
machines. OPC UA is especially suitable for larger
IIoT projects, where devices from different manufacturers
must be combined but the network can
still be dimensioned accordingly.
While OPC UA largely operates using client/server connections, MQTT is based on the publish/subscribe mechanism (note: OPC UA also
now supports publish/subscribe, though most
existing installations use
the traditional client/
server method). In publish/
subscribe, a publisher
(data provider) provides its
data to a centrally located
server (often referred to as
a broker) on the network.
Subscribers (data consumers)
can flexibly subscribe
to all the publisher’s data
or to individually selected
topics. Since there is no
permanent connection
between the publisher
and each subscriber, the
data overhead for MQTT
is significantly lower. This
makes MQTT especially
suitable in networks with limited availability or
when information must be transmitted to several
consumers at the same time.
An API (application programming interface)
is a programming interface provided on a device.
REST stands for Representational State Transfer
and outlines the conditions for designing the API.
A REST API allows customers to create their
own applications based on the device data, where
basic conditions are defined without obligation.
The APIs for an organization’s own devices are
standardized within that organization. This makes
REST APIs especially suitable for applications
where many different devices from the same
manufacturer are used.
The key difference between IIoT communications
and control network protocols is that IIoT
communication methods were designed to allow
for an end-to-end and transparent data flow from
the sensor to the cloud, enabling the huge potential
of Industry 4.0 and IIoT.
