Making good use of IO-Link

28 June 2022

Rolf Horn discusses a variety of applications which benefit from the use of the IO-Link communication protocol.

With the advent of Industry 4.0, intelligent automation has been defined by advanced controls, monitoring, and diagnostics. However, such capabilities are only possible through industrial connectivity  which can unify controls and machine devices on a single platform –such as IO-Link – for continual data exchange.

The key enabling technologies underpinning industrial connectivity are standardised networks and devices with onboard communications features. Protocols abound for these functions. However, not all industrial protocols satisfy the data-exchange and intelligence requirements required by today’s automation, whereas IO-Link was created to satisfy a wide array of these modern applications.

IO-Link is a wired point-to-point communication protocol that facilitates smart bidirectional data communication between devices. Typically local controllers will have several IO-Link ports into which various IO-Link devices can independently plug. These node-to-node endpoint connections are what render IO-Link a point-to-point communication protocol. The harmonised IO-Link interface is defined by the IEC 61131-9 standard .

Modes correlate to uses
The IO-Link communication protocol renders each connector port on an IO-Link controller capable of four communication modes. These include a fully deactivated mode as well as IO-Link, digital input (DI), and digital output (DQ) operating modes. 

The IO-Link operating mode supports bidirectional data communications with field devices and is typically used during data collection for monitoring, testing, and diagnostics. A local controllers port in DI mode accepts digital inputs and works when the port is connected to sensors — in this context, acting as input devices. In contrast, a port in DQ mode acts as a digital output, typically when the port is connected to an actuator (in this context, effectively an output device) or when a system PLC is set up to directly send instructions to another IO-Link device.

It is worth noting that the ports on an IO-Link primary can readily switch between modes. For example, a controller’s port connected to a sensor can run in DI mode — and then switch to IO-Link communication mode when diagnostics and monitoring data from the sensor is requested by the controller.

Actionable status communications
Machine monitoring is possible with IO-Link devices set up to report status that can, in turn, inform the system of necessary adjustments and corrections. Consider one use in the machine-tool industry – that of IO-Link pressure sensors which verify workpieces are clamped with a pressure appropriate for damage-free yet secure holding during material-removal operations. Here, IO-Link sensors essentially support the optimisation of machine tasks for fewer rejected workpieces.

IO-Link devices can also make actionable status communications to support enhanced maintenance routines for minimised downtime. For example, IO-Link position sensors on an assembly machine might continually report the locations of end effectors to ensure none are out of range or alignment.

By analysing diagnostics data provided by IO-Link devices, it is possible to predict and correct errors and potential breakdowns before they happen. Weak links in a machine or plant can also be identified to inform enterprise-level operational changes, purchasing decisions, and captive machine designs in the future.

Advanced control and automation
Control and automation are other application functions supported by IO-Link. Where an IO-Link installation supports functions that run without human intervention, the IO-Link controller often connects to a host system or higher-level PLC that processes received data and then directly or indirectly commands actuators in the design to the appropriate coordinated responses. Such automated control requires that the IO-Link system connect to a higher-level controller via standardised fieldbus or Ethernet protocols and cabling. In fact, most IO-Link controllers have fieldbus or Ethernet ports for such connections.

Devices in advanced control applications, involving IO-Link systems, integrate in one of three ways:

• They directly connect to the host computer or PLC.
• They connect to an IO-Link primary and communicate via the IO-Link protocol.
• They use IO-Link compatible communications and connect to an IO-Link primary via an IO-Link hub.

The latter essentially acts as an intermediary to connect non-IO-Link devices to the primary.

An added benefit of IO-Link systems having fieldbus and Ethernet-communications connectivity is that long-distance connections are allowable — which in turn enables installers to locate IO-Link controllers in a control cabinet or at the outermost machine reaches if that makes the most sense for a given application.

IO-Link controllers can benefit advanced assembly applications by serving as low-level controllers capable of processing both digital and analogue signals. Here, they might:

• Accept the data generated by IO-Link linear encoders on the axes of an XY stage.
• Process that data as a gateway.
• Submit that processed IO-Link field-device data to the PLC or other system controller.

Device intelligence
The third application of IO-Link is to render devices smart. Especially common in sensor designs that resemble legacy sensor options with no (or more modest) programming, these IO-Link-enabled devices can receive instructions, monitor, and execute self-testing routines and generate data. Because IO-Link also lets devices provide more than basic two-value (yes-no or pass-fail) data, the reporting of precise values is also possible. For example, process-automation tasks benefit from IO-Link temperature sensors that go beyond reporting high or low temperature status by continually reporting the exact temperature value of a monitored zone or volume.

Another benefit of IO-Link for smart field devices is the compactness of its physical connections when compared with the physical connections of fieldbus and Ethernet interfaces, which can sometimes be too big to fit on field microdevices.

IO-Link smart components can also be precisely controlled. For example, instead of basic off-and-on controls, an actuator can be commanded to turn off once a scenario satisfies a set of conditions.

There are, however, some caveats to the use of IO-Link for smart-device applications. Though there is a wireless form of IO-Link under development, it is still a wired communication protocol — so it is still subject to all the limitations of hard wiring. To maintain data integrity, IO-Link primary-to-device cabling must not exceed 20m. Plus, because the IO-Link protocol can only transmit up to 32 bytes of data per cycle, it is insufficient for use with field devices such as cameras, which can generate many MB of data per minute.

Uses for IO-Link systems abound to complement existing protocols underpinning virtually limitless controls and data-collection systems. Spurring adoption has been the simplicity of IO-Link systems —comprising only an IO-Link primary and its devices and their connectorised three or five-wire cables. Plug-and-play installation and cost-effectiveness are other IO-Link benefits.

Efforts by the IO-Link consortium of member companies have ensured wide compatibility between controllers, devices, and actuators from various manufacturers, which has given the widest selection of equipment for specific engineering use cases.

Rolf Horn is an applications engineer at Digi-Key.

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