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.