Manufacturer-independent IO-Link interface for intelligent process automation

Industry 4.0 is the fusion of information technology (IT) and telecommunication technology to form ITC. The basis is the networking of sensors, actuators and data processing for consistent communication right down to the field device level. The vision of Industry 4.0 includes the digitalization, automation and networking of all applications for controlling the overall process for all functions, areas and segments of the manufacturing industry, right through to economic activity. This transformation is primarily being driven by increasing customer requirements and the need for manufacturing companies to be able to fulfil ever faster, more dynamic and individualized customer wishes. This requires a transition from rigid, centralized production control systems to decentralized intelligence down to field device level.

The introduction of Industry 4.0 in production processes is intended to achieve adaptive manufacturing and optimization of individual processes in real-time operation. The materials and components should be selected independently in accordance with the defined production and process technologies and it should be possible to adjust and readjust in real time with the motto "The product controls the plant". The advantages are increased efficiency and flexibility due to a faster response to a higher number of variants, shorter changeover cycles for complex products, production of a variety of series or individual products on the same production line, tailored products for the customers and the production of small and very small series at competitive costs.

Industry 4.0 pursues the idea to veer away from preventive maintenance and repair of plants towards predictive diagnostics and remote maintenance even beyond plant borders and sites. All this requires access to connected intelligent data sources such as sensors or actuators.

One shortfall of industry 4.0 is the often missing standard and the inaccurate definition of the term. System-wide consistency and worldwide suitability needs a uniform framework for technologies, systems and processes on the basis of international standards. Combined with fundamental standardizations of structuring principles, interfaces and data formats.

Traditional networks and field bus systems were developed by leading PLC manufactures supporting a system-specific technology that optimally go in line with their programming and configuration tools. The market knows several competing systems, such as Profibus/ProfiNet (Siemens), DeviceNet and ControlNet (Rockwell Automation), Modbus and CANopen (Schneider Electric), Interbus (Phoenix Contact) or CC-Link (Mitsubishi Electric). The PLC system used dictates which field bus will be employed. Significant technical differences are given for the cable length, the number of data bits and the scope of functions. Functions beyond this scope, such as diagnostics, non-cyclical transmission of demand data, alarm handling and slave-to-slave communications between the individual bus participants are not supported by a field bus system.

If we take the classical automation technology, the communication usually ends at the lowest field bus level, i.e. at the sensors and actuators (Figure 1). Modules limited to mere analogue or switching input and output signals, incapable to communicate, are often used there. The sensors and actuators with a digital interface available on the market are not standardised but use company-specific hardware and software for communication. Depending on the sensor, special and mostly expensive modules have to be installed in the control system. Heterogeneous wiring with most different wire types and pin assignments entail high installation costs. Comprehensive shielding measures are necessary to provide immunity to interference both of the analogue and digital signals. In practice, it is again and again visible that interferences in signal transmission can often be traced back to faulty or insufficient shielding. Networking and integrating the various interfaces and transmission protocols is also complex and error-prone. When converting a machine, replacing or checking the devices, the parameters must be set manually on the device or directly for each sensor and actuator using a separate tool. As has been shown time and again, this is a major source of errors and manipulation for the safe operation of the plants. As there is no continuous communication from the field device level to the higher levels, diagnostic data from the sensors and actuators is not available. However, it is often precisely these assembly groups that are responsible for system downtimes due to their positioning in the plant and use under difficult industrial production conditions such as heat, cold, vibration, dirt and moisture. Without diagnostic data, troubleshooting and rectification is often difficult and time-consuming. Preventive maintenance to avoid unplanned downtimes is certainly out of the question.

Such a wide range of bus systems and the lack of standards is a major disadvantage for the development of automation technology. Manufacturers of automation products have also had to recognize this. The leading suppliers have formed a consortium with the aim of developing a generally applicable and globally standardized I/O interface technology for the communication of sensors and actuators. The result is the IO-Link concept for the standardized, fieldbus-independent and manufacturer-independent connection of switching devices and sensors to the control level by means of a cost-effective point-to-point connection. This communication standard is defined in the IEC 61131-9 standard. IO-Link devices create transparency and continuous communication from the field device level right up to the highest automation level (Fig. 2). As an open interface, IO-Link can be integrated into all common fieldbus and automation systems. With IO-Link, only digital transmission will ultimately be used instead of the previous parallel use of analogue, switching and digital signals. IO-Link offers the possibility of central fault diagnosis and localization right down to actuator/sensor level. Thanks to the option of dynamic parameterization of the sensors from the plant control system, the field devices can be adjusted to the respective production requirements during operation. Field devices with an IO-Link interface therefore form the basis for implementing Industry 4.0.

For certain, the IO-Link interface is rightly regarded as the USB interface of automation technology. Both are inexpensive serial point-to-point connections for signal transmission and are suitable for plug-and-play operation. A key feature is the very simple wiring with standardized cables with screw connectors. In addition to huge time savings during wiring, since terminal strips are not needed, the solution with plugs prevents incorrect and improper connections. The elimination of separate multipole cables for analogue signal transmission, switching contact and external parameter setting, time and also space is saved in the switch cabinet, as there is no more need to connect each device separately to the central periphery. Manufacturer-independent standardization reduces the variety of interfaces for sensors and IO modules as well as the different connecting wires. 

Sensors with an IO-Link interface offer a reliable diagnostic option. Diagnostic messages, especially preventive status messages, can be forwarded including the description and displayed on the HMI (Human Machine Interface). This makes it possible to react quickly in the event of a sensor failure, contamination of optical sensors, an impermissible operating temperature, wire breakage or a short circuit, thus avoiding long downtimes. 

However, if a sensor needs to be replaced, a major source of error is the correct parameterization or even the use of the wrong sensor. With IO-Link devices, the parameters are saved in the IO-Link master. With IO-Link, devices are identified by unique serial numbers, vendor and device IDs, which prevents devices from being mixed up. When a device is replaced, the parameters are also automatically transferred to the sensor. This prevents incorrect operation or even manipulation. In addition, the parameter changes can be documented and thus tracked later. 

IO-Link data transmission is based on a 24 V signal and is therefore particularly insensitive to electromagnetic interference. As the signal transmission is purely digital and secured by means of checksums, faulty transmissions and inaccuracies due to signal conversions as with analogue signals are ruled out. Shielded cables and separate earthing measures are generally not required.

An IO-Link system consists of IO-Link masters as a gateway between the higher-level communication systems such as Profinet, Ethernet/IP and the IO-Link devices. The IO-Link devices are the communication-capable field devices such as sensors, switching devices, valves or signal lights.

Data transmission via IO-Link always takes place between an IO-Link master and the IO-Link device as a slave. Both fieldbus interface modules and PLC interface modules are available as IO-Link masters. Switching devices can either be operated as before like a switching input or switching output, or the switching status can be transmitted digitally in IO-Link mode. As both signals are transmitted via the same pin 4, parallel operation is impossible. In an IO-Link system, components with and without IO-Link can be combined as required and operated in parallel. Non-IO-Link-capable standard devices can be connected either via special standard IO ports or via the compatible IO-Link ports of the master. Binary or analogue sensors can thus be linked to the fieldbus level via the master. The downward compatibility of the IO-Link ports is ensured by the IO-Link interface module through two different operating modes, the IO-Link mode and the standard IO mode (SIO). IO-Link sensors can be operated like a binary device. This means that an IO-Link switch sensor can also be integrated into classic automation solutions. During initialization, the IO-Link master automatically establishes communication. Mixed operation of standard sensors and IO-Link sensors is supported by the IO-Link standard.

With IO-Link, the line for the switching signal is also used for serial communication. Technically, this is a half-duplex interface in which data is sent and received one after the other. M12 connectors are used as standard. The maximum cable length to the IO-Link master is 20 metres. 

In the first concept phase for the specification of the IO-Link interface, the focus was on switching sensors and actuators. It has now been recognized that the use of the IO-Link interface also makes sense for measuring devices. More and more sensor manufacturers are already offering devices for various physical measured variables. In the IO-Link specification, only pins 1, 3 and 4 are permanently defined in accordance with the Port Class A pin assignment. Pins 2 and 5, which are used for an additional power supply in the event of increased current requirements, can alternatively be used for the analogue output 0/4-20 mA or for a second switching output on the measuring devices (Fig. 3).

As long as users do not want to completely dispense with the analogue output, parallel operation of the analogue output, switching output and digital interface opens up interesting possibilities for external parameterization, evaluation of fault messages and diagnostic signal functions. If the control system is later converted to purely digital measured value transmission, the effort involved is limited to changing the configuration of the control software. Measuring devices such as infrared thermometers for non-contact temperature measurement must process the smallest signals in the pico-ampere range. This requires a high level of internal interference protection measures as well as external measures such as the use of a shielded cable. The IO-Link consortium advertises the fact that no shielded cable is required to connect IO-Link devices, as digital signals cannot be interfered with. With the introduction of the IO-Link interface in the measuring devices, certain restrictions are unavoidable here. The market has already reacted to this and offers prefabricated cables with shielding.

IO-Link communication supports the transmission of cyclical and acyclical data. Process data and status information on the validity of the process data are transmitted cyclically. Device data such as identification data, parameters and diagnostic information are exchanged acyclically at the request of the IO-Link master. Furthermore, events such as error messages (short-circuit, interruption) or warning messages (contamination, overheating) are signalled to the master by a device.

Device profiles are defined for IO-Link in order to standardize the access of the user program of the control system to the devices. This defines the data structure, the data content and the basic functionality. This ensures identical programme access to the control unit. The "Smart Sensor Profile" device profile is defined for IO-Link.

Part of an IO-Link device is the IODD (IO Device Description), i.e. a device description file. The structure of the IODD is the same for all devices from all manufacturers. This guarantees the same handling for all IO-Link devices, regardless of the manufacturer. This contains all the information and descriptive texts for identification, the device parameters with value ranges, the error messages, the process and diagnostic data and the communication properties (Fig. 4). The texts can be stored in several languages. The ports of the connected devices are assigned in the IO-Link master (Fig. 5). The IO-Link master is then usually connected to the control system as a fieldbus slave. 

Parameterization and diagnostics are carried out automatically by a function module in the machine control system. During parameterization, the functional block first queries the identification parameters of the connected devices via IO-Link. A database comparison is then used to check whether these sensors are approved for the machines. In the positive case, the function module also finds the configuration parameters associated with the sensors in the database. These are then automatically written to the respective sensors via IO-Link as required. For example, the emissivity, the switching points and function of the switching contact, the scaling of the analogue output and the peak picker can be parameterized for the pyrometer (Fig. 6).

Temperature simulation, a self-test and resetting to factory settings are also possible as command functions (Fig. 7). Errors in the hardware or software, maintenance requests or operation of the device outside the specification can be analyzed using the diagnostic function, among other things. Integration into the control system also makes the sensors accessible for remote maintenance.

The user-specific parameterization of an IO-Link device can be carried out externally in three ways: via a PC with a USB IO-Link master, via a software tool in the PLC controller or program-controlled by function blocks in the system controller.

An old-established commissioning engineer will certainly argue that it used to be much easier to check an analogue sensor using a current measuring device. The parameterization could be set via buttons or switches on the device. However, if you then have to forego the other advantages of digital communication, the question arises as to whether this can really still be a decisive purchasing argument in today's international competition to optimize production costs. 

IO-Link USB masters are available for service applications (Figure 8). This allows an IO-Link device to be operated via a PC using a USB interface. Special IO-Link adapters can be looped into the supply line in order to access and record the data without feedback, either wired or wirelessly via Bluetooth. Adapters are also available for cloning the device parameters.

It is not possible to predict how quickly the switch to purely digital signalling communication will take place and is certainly closely dependent on the degree of automation of the machines, the industry and the applications. As modern sensors with an IO-Link interface and analogue output are often offered at no extra cost, it is advisable to use these devices in advance when replacing or expanding a system, or even when installing a new system. This makes subsequent conversion extremely simple and possible without any conversion costs for the sensors and wiring. 

Over 3000 IO-Link products are now available. IO-Link masters are now available for 16 fieldbus systems. In addition, 8 control system manufacturers already offer centralized masters. Moreover, there are numerous manufacturers of sensors for a wide range of measured variables, for object detection or for position detection, as well as actuators such as signal lights, valves, power contactors or frequency converters. Various companies now also offer the technology for device design and technical support. The certification requirement and the use of accredited test tools ensure that all products available on the market fulfil the IO-Link standard.