Just another fieldbus?

IO-Link has been around now for quite some time, but it only gained momentum in the last years. First, I thought it was just another fieldbus that was entering the scene. But I was not in need of another fieldbus. In contrary there are too many already. For instruments, vendor A offers fieldbus option x and vendor B offers fieldbus option Y and of course a proprietary RS485 interface.

IO-Link is NOT a fieldbus. It is the bridge from any fieldbus to the field device. IO-Link is point to point which might sound like a restriction, but computer networks are built this way for a reason. You do not daisy-chain your computers. If your network link breaks it usually will not affect any other PC in the network. The switch in the computer network is called Master for IO-Link. It comes with similar high-level features. You can tell the master to only accept a certain device on a certain port, as you can do with a managed switch. If the maintenance team reconnects the cables after repair of the machine it gets instant feedback in the field from the master that the connections are correct or that something is wrong, if they swapped two cables by mistake that look too similar.

Cable Engineering

IO-Link uses STANDARD CABLES. How long does it take to figure out which diagram is applicable to your specific version of the device you selected? How should we handle the cable shield? Why are all cables different? Cable engineering can be time consuming. Not for IO-Link. The only question you need to ask is: What is the cable length and is a straight or a 90° connector better suited from mechanical point of view? Unless the field device needs a separate power supply (most do not), this is all you need. It is the same for the simple proximity sensor as for the multi-function feedback and control unit. One Port – on cable – on device, that is it! That is why it is called the USB of OT.

IO-Link uses CONNECTORS ON BOTH SIDES. Yes, there is an option for panel mount terminal-based master modules, but then you are giving away a key benefit. For IO-Link you do not need to think about wire colors and connector pinouts. On the wiring diagram you show the port of the master and the port of the field-device and assign a cable number, like you do it for an ethernet connection. Nobody shows the RJ45 pinout because it is standardized and you use prefabricated cable.

Smart Sensors

What makes a sensor smart? Basically, most sensors can be called “Smart” today, because they are using built in controllers that handle the sensor signals and use them to calculate the main process value, detect faults and convert it to something that can be used by the PLC or displayed locally.

But most of the PLC systems I saw so far just didn’t use those abilities. They used the 4-20mA signal that can either show the main value or indicate an error. Additional values were ignored. Sometimes HART was used to get diagnostics or additional values out of the sensors, but most of the time it was not used.

IO-Link uses a digital protocol to transfer all process values. It uses smart sensor profiles that unify how this data is presented. The device manufacturer uses these profiles instead of a proprietary format. So, you can reuse your PLC code for different products. A pressure sensor can use the same profile as a distance sensor.

For setup and diagnostic data IO-Link uses asynchronous communication. When an error is detected, the program can request the error code. When the System starts up, the setup data can be written. This data is only transferred on demand.

Multifunction Devices

Most sensors are simple and stand-alone. Proximity sensors and pressure sensors fall into this category. Some sensors combine several values. An electromagnetic flowmeter also detects the conductivity of the liquid and the temperature and sometimes also the pressure, not only the flow.

Some new devices combine sensors and actors. A smart feedback unit for pneumatic actuators connected to one single IO-Link channel with one single cable, delivers end-position signals and the actual position and switches the coil for the pilot valve on and off. Instead of one multicore cable for feedback and one cable for the coil and 3 IO points, you will have one IO-Link cable on one IO-Link port and a short jumper cable to the pilot valve. A flow control valve can also be connected to a single IO-link channel, which is handling the setpoint, the feedback and whatever other data is needed. Especially when the devices are small, the connector size is a challenge for traditional designs and often nonstandard connectors are used that are difficult to deal with. Not so with IO-Link.

Signal quality

The digital signal of IO-Link is not as sensitive against external influences as the 4-20mA signal. One reason is the type of signal, and the other is the cable length. The signal is collected in the field, close to the field device and not in the central Panel or the RIO box.

But there is more. In the 4-20mA world the sensor value is first processed in the sensor controller in digital form. Then it is converted into an analog signal and transferred to the IO-card. In the IO-card it is converted back into digital format. Every conversion is adding some error. And then you need to set up what 4-20mA means, both in the sensor and in the PLC. This can be tricky when replacing a field sensor. In the IO-link world the smart sensor is calculating the process value in the target unit. Pressure sensors use bar. Distance sensors use meters. This value is transferred without conversion into the PLC. No conversion error, no setup error, no need for the PLC to convert the data. Yes, this is a benefit that IO-Link shares with many fieldbus-connected sensors but at a lower price, with lower complexity and with standardized smart sensor profiles.

Industry 4.0

This buzzword is around now for some time. In short, the idea is to join IT and OT to get better results in shorter time. Some approaches tried to collect data from PLCs, but the minimalist data approach in the PLC world doesn’t help here. IO-Link offers a new path here. Skipping the PLC by accessing the IO-Link masters directly to collect data that the PLC doesn’t need, but is required by quality management, maintenance, or any other analytics. This data can even be sent on a separate physical network to keep the control system safe. That is why IO-Link is advertised as a door opener for Industry 4.0. But who knows if Industry 4.0 is holding what it promises. At least some areas will benefit.

Outlook

More and more companies develop IO-Link devices with smart capabilities that are not locked-in in the device. I hope to see this technology on dosing pumps, large flowmeters, analytical instruments, and other complex devices that are used in industrial plants in the near future.

Conclusion

IO-Link is a bridge technology that is beneficial in many parts of the project lifecycle. Standardization is saving time in engineering, in purchasing, in programming, in installation, in testing and in commissioning. IO-Link is often advertised as Industry 4.0 technology, but it offers much more for the EPC business.