Intelligent sensors connect with IO Link

01 June 2006

A new sensor communication scheme promises to reach ‘the last metre of the process.’

Fieldbuses such as Profibus are extensive factory communications systems, but their reach stops at the I/O modules. Sensors are still connected to digital I/O channels through interfaces that have been more or less the same for the last 25 years. These represent a real ‘language barrier’ for sensor communications
because diagnostic data and parameters either cannot be transferred to the control system or if so, only with

For example, photoelectric sensors are typically connected to digital input modules of a PLC. The sensor’s
digital switching signal occupies one input channel. A second sensor switching output is needed to signal diagnostic events. With potentially hundreds of sensors in an area, this results in additional costs as twice the number of input channels are needed if diagnostics are to be included.

It is even more difficult to remotely parameterise optical sensors. If special settings are required for range, sensitivity, or hysteresis, this must either be done manually at each sensor, or devices must be used with external teach input. However, this requires yet another PLC output channel.

All of the sensor parameters, no matter how they are set, are saved in the sensor itself. It is not possible to
document and save the settings, for instance, for quality assurance purposes. When a failed sensor is replaced, all of the settings must again be manually taught into the new device, a time consuming operation.

IQ Sense
In the early 2000s Siemens A&D addressed these problems with the creation of IQ-Sense, a system of
sensors and special interface modules that allows controllers to communicate more directly with sensors, giving them, for example, parameterisation information.

Many sensor manufacturers and end users were interested in these concepts, and a group of them, led by SICK, proceeded to develop more open specifications that could be adopted as industry standards. Working with the Profibus organisation their goal is to develop a specification for a fieldbusindependent
communication interface for intelligent sensors and actuators. The scope of the specification work focuses on the transmission physics (electrical and mechanical properties, topology), sensor/actuator communication with the IO assemblies, and a neutral data interface to the higher-level communication system and engineering.

IO Link
IO Link maintains the classic point-to-point wiring; it does not involve a new bus system. The sensor power supply is provided through the two-wire cable. A simple telegram superimposed on the power supply voltage permits bidirectional data transfer between the sensor and control system.

An IO Link connection module is required for a fieldbus connection (see Fig 1). This permits connection of up to four sensors via unscreened, three-pin standard cables. The concept is compatible with current technology so that non-IO Link sensors can be connected, though these cannot exploit the advantages offered by IO Link.

IO Link should not be thought of as a classical bus system, but a fieldbus-neutral point-to-point connection for dialogue between sensor and controller. The physical coupling of one or more sensors to a fieldbus takes place with the help of an IO Link connection module. IO Link allows the controller to communicate with the sensor, to parameterise it, or even to download entire parameter sets, such as when there is a batch
change. The substantial supplementary information that can also be transmitted includes, among other things, precontamination warnings, the presence of sources of interference in the sensor surroundings, the quality of the switching signal, the current scanning distance, or the sensor’s serial number.

IO Link will have positive effects on the design and control technology of machines and plants in the future. Supporters say within 5 to 10 years, all factory sensors will be IO Link enabled and the additional price for the sensor will be small, in the range of two euros.

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