An introduction to IO-Link for sensors and actuators

With the aim of improving the total cost of ownership of plant and machinery, manufacturers and operators are continually on the look-out for optimisation potential. In the automation technology sector, networking is one possibility.

Since functions are becoming more complex in all levels of automation technology and the density of information provided is continually on the rise, the communication requirement is also increasing. In a search for ways of achieving an improvement, IO-Link has come to the fore as a system that aims to revolutionise the sensor and actuator interface.

The performance of components and, therefore, the need to exchange information (the degree of networking) are increasing in applications ranging from quality assurance and goods tracking, through to automated manufacturing processes. This trend is also reflected in the communication structures used in automation; for example, peripheral assemblies have predominantly decentralised connections using fieldbus systems, and control units communicate with the corporate networks to archive production data.

However, within this development, sensors and actuators have generally been connected to the peripheral assemblies of the control units via digital and analogue voltage or current signals. While many other connections today allow the most complex of information to be transmitted, thereby allowing for an almost limitless transparency in the automation network, at the field level an individual interface must still be provided for each individual signal, one which only allows information to be transmitted in one direction.

This type of interface can no longer cope with the functions of modern sensors and actuators. For some time now, modern field devices have been unable to provide pure measuring or switching units. Data such as parameters or diagnostics functions, feedback signals from actuators, or validation information in sensors has been supported by the intelligence provided by today's field devices. Unfortunately this data is not accessible through the conventional interface or, if it is, then only with great difficulty: parameters must be set using local operating units, mostly buttons or potentiometers; diagnostic information is read by observing individual warning signals on the device; and once a faulty unit is replaced, the parameters need to be reset.

The reason for this is that the individual signal wiring can no longer handle the increase in the variety of field component functions available. IO-Link's aim is to to eliminate this final bottleneck in automated manufacturing.

IO-Link members

25 renowned automation technology manufacturers joined together behind the doors of Profibus and Profinet International (PI) to form a working group aimed at defining a new standardised interface.

The process signal connection should therefore become more intelligent without needing to change the topology and wiring technology involved. Furthermore, this interface should be able to connect to all common fieldbus systems. Preserving the topology also means that, as a rule, individual distances of up to 20m must be bridged to enable the decentralised connection of process signals using conventional, unshielded signal cables and plugging and clamping technologies. In addition, the fact that conventional connections are now so prevalent means that existing investments must be protected. Hence what is required is a new interface with extended functionality, one that is simultaneously compatible with the most widespread form of signal connection, namely the sensor interface.

IO-Link combines both requirements by enabling the system to set up a point-to-point connection via familiar three-wire sensor connections. However it uses this, as needed, to either transmit a simple switching signal or for bi-directional serial communication. The serial communication is based on digital, bi-directional, asynchronous data transmission. Here, the voltage step between the 0 and 1 signal is 24V. The technological compatibility requires an intelligent switch between the two transfer modes. When the interface is started up, the switched components initially try to set up the serial communication indicated in the IO-Link specifications. If one of the two interfaces does not support IO-Link, the intelligent partner automatically switches over to the so-called SIO (or standard input/output) mode, the simple switching mode.

Further opportunities

The ability of the interfaces to switch provides other opportunities: sensors and actuators can be deliberately developed so that IO-Link communication is only activated if needed. To transfer a process switching signal, the interfaces are switched over to switching mode. On the other hand, communication can be performed cyclically with a typical cycle time of 2ms for the process-based transmission of measured values.

With this, project managers, commissioning engineers and plant operators finally have the chance to have something which was previously only supported by devices on the fieldbus: all of the parameters, diagnostics data, check-back signals and other events on the field device are provided on the entire plant with IO-Link. This also allows data to be stored in a central position, for example, and projects to be managed with transparency up to the field level.

In practice, this means for example:

• Smooth parameter changes during batch change-overs
• Continuous diagnosis of error warnings
• The exchange of field devices without having to adjust the parameters

These characteristics pay off particularly for field devices due to the fact that, due to their exposed position in the field, they require a great degree of maintenance and commissioning work. The parameterisation of some sensors, for example, has been complex up to now because it can only be performed on the device itself. This means that these sensors need to be parameterised individually on site. On the machines, sensors are often difficult to access as well. Frequently, the same parameters are even often used in entire sensor groups. Prior to the advent of IO-Link it was not possible to reproduce the set parameters.

However, with its central parameter storage, IO-Link offers an effective multiplication of parameters sets that have been created just once instead of having to set each individual parameter for each individual device.

During plant operation, IO-Link field devices ensure that faults are quickly eliminated due to the fact that the device diagnoses are also available at a central position and the plant no longer needs to be scanned for warning signals. The replacement of faulty devices is accelerated: a parameter set is automatically reloaded, the sensor or actuator does not need to be re-adjusted.

Thanks to the flexibility offered by this system, the range of process data can be adapted to suit the functionality of the field device. If, for example, both the measured value and switching information need to be transmitted, this is possible due to IO-Link's flexible protocol structure. Up to 32 bytes of process data can be transmitted between the processing unit and the control level in this way.

Of course, this also means that different process data fields and structures will need to be continually reproduced for the manufacturer and field device in question. As desirable as this flexibility may be for complex field devices, when it comes to simple sensors, for example, the ability to exchange field devices - which is something engineers are accustomed to - becomes more difficult due to the fact that, by changing the hardware, the software implementation also needs to be adapted. Simplifying this is a task performed by a uniform description language that provides the functionality and the process data image for the control level. The description file provided by the device manufacturer in accordance with this description language is loaded into the engineering environment. The functionality of the device is then made available in plain language in the configuration user interface of the engineering tool.

IO-Link device availability

This part of the specifications has been completed and the first devices with the suitable description files are already available. In addition to this, PI is currently working on test specifications and specifications for a superordinate certification of IO-Link components that should guarantee that the most varied IO-Link components can work together smoothly. Then, intuitively speaking, it should be possible to implement IO-Link in engineering systems.

With IO-Link, all of the sensor and actuator data is available throughout the entire plant. Supporters therefore believe that IO-Link closes the final gap in automation communication technology, and the field device level is connected to the line network. The advantages are obvious: faster and more transparent project management, commissioning and maintenance, and savings in time and costs during each phase of the plant lifecycle.

The IO-Link website is at www.io-link.com.