■ IO-Link, Rapid Growth as a New Standard for Industrial Sensors

Traditionally, industrial sensors have been, and still are in many cases, analog.

Industrial sensors basically consist of sensing devices and devices for transmitting sensing data to a controller.

Data was unidirectional analog. Then came binary sensors. Binary sensors provided digital on/off signals and included sensing devices such as inductive, capacitive, ultrasonic, and photoelectric, as well as semiconductor switching elements. The output was high-side (HS) switching (PNP), low-side (LS) switching (NPN), or push-pull (PP).

However, data was still limited to one-way communication from sensor to master, error control was impossible, and technicians had to be dispatched to the field for tasks such as manual calibration.

In recent years, advanced solutions have become necessary to meet requirements such as ‘Industry 4.0’, smart sensors and reconfigurable factory equipment.

The IO-Link protocol provides this solution. IO-Link is a relatively new standard for industrial sensors and is growing rapidly.

The IO-Link consortium estimates that there are now more than 16 million IO-Link enabled nodes in use in the field, and that number continues to grow.

IO-Link is standardized in IEC 61131-9 and defines how sensors and actuators interact with controllers in industrial systems.

IO-Link is a point-to-point communication link using standardized connectors, cables, and protocols. An IO-Link system is designed to operate within the industry standard 3-wire sensor and actuator infrastructure and consists of IO-Link master and IO-Link device products.

IO-Link communication takes place between one master and one device (sensor or actuator).

Communication is binary (half-duplex) and limited to a distance of 20 meters using unshielded cable.

A 3-wire interface (L+, C/Q, L-) is required for communication. In an IO-Link system, the power range is 20 to 30 V for the master and 18 to 30 V for the devices (sensors or actuators).

The IO-Link Handbook1 from Analog Devices explains the advantages of IO-Link as follows:

“IO-Link is a technology that can transform conventional binary or analog sensors into intelligent sensors. This allows users to not only collect data, but also remotely change settings based on real-time feedback on the status of other sensors on the line and the required manufacturing process. IO-Link technology enables the exchange of sensors through a common physical interface using a protocol stack and an IO Device Description (IODD) file that allows for the implementation of a configurable sensor port. This supports true plug-and-play and also allows for parameter reconfiguration during operation.”

As shown in Figure 2, in the factory network layer, the IO-Link protocol exists at the edge, mainly consisting of sensors and actuators. In many cases, the edge devices communicate with a gateway, which converts the IO-Link protocol into the selected fieldbus.
For a more in-depth look at how IO-Link enables next-generation manufacturing environments or the Industrial Internet of Things (IIoT), see the articles below. https://www.eletimes.com/io-link-enables-industrial-iot

■ IO-Link sensor design

Industrial sensors must be rugged, compact, and energy efficient to minimize heat generation. Most IO-Link sensors consist of the following components:

- Sensing devices and their associated analog front ends (AFEs)
- Microcontroller (MCU): Processes data and runs a lightweight protocol stack for IO-Link sensors.

- IO-Link transceiver: Belongs to the physical layer.
- Power supply and protection features (TVS diodes to protect against surges, EFT/burst, ESD, etc.)

■ Heating characteristics (power efficiency)

Now that we have looked at the typical components of an IO-Link sensor, let’s consider the power consumption of the sensor. All figures in Figure 3 are rough estimates. The transceiver (output stage) power consumption is the overall system power consumption of the sensor. You can see that it is most important in .

Looking at the graphs from the left side of the figure, the leftmost graph shows the power consumption of the previous generation of IO-Link sensors. As you go to the right graph, you can see how the total system power decreases over time as the MCU and output stage (i.e., transceiver) advance. You can tell it's been lowered.

The first generation of IO-Link transceivers consumed 400 mW or more. The latest generation of low-power IO-Link transceivers from Analog Devices consume less than 100 mW. MCUs are also power-saving. Older MCUs consumed up to 180 mW, but the latest low-power MCUs have reduced this to 50 mW.

Therefore, by combining a low-power MCU with the latest IO-Link transceiver, the total power consumption of the sensor can be reduced to 400-500 mW.

Power consumption is directly related to heat generation. The smaller the sensor size, the more stringent the requirements for heat generation. A sealed cylindrical IO-Link sensor with a diameter of 8 mm (M8) has a maximum power consumption of 400 mW, while a sealed cylindrical IO-Link sensor with a diameter of 12 mm (M12) has a maximum power consumption of 600 mW.

Technology continues to advance. Analog Devices’ latest IO-Link transceiver, the ㎃X14827A, consumes only 70㎽ while driving a 100㎃ load. This is made possible by optimizing the technology to achieve an extremely low on-resistance (RON) of 2.3Ω (typical).
Sensors that use extremely low operating currents of 3–5 mA and require 3.3 V or 5 V power can be supplied with a regulated power supply via an LDO. IO-Link transceivers from Analog Devices integrate an LDO internally.

However, if the current demand increases to, say, 30 mA, this LDO will become the main power consumer/heat generator in the system. When compared to 30mA, the power consumption of LDO can increase up to 600mW.

LDO power dissipation at 30mA = (24-3.3) x 30mA = 621mW

In comparison, a DC-DC buck converter consumes only 90 mW when providing a 3 V output voltage with a 30 mA sensor. If the power conversion efficiency of this converter is 90% (only 9 mA of power loss), then the overall power consumption is 90 + 9 = 99 mW.

The latest IO-Link transceivers from Analog Devices integrate high-efficiency DC-DC regulators internally (Figure 4).
■ IO-Link sensor size

Size is another major concern for all industrial sensors. This is also true for IO-Link sensors. Board space is becoming increasingly critical due to the constant demand for size reduction.

Figure 5 shows the transceiver (WLP package) and DC-DC mounted side by side on a typical PCB with a width of 10.5 mm in a housing with a diameter of 12 mm. There is still space for vias and wires on the same PCB surface. If the sensor housing is 6 mm, the PCB width is reduced to 4.5 mm. Then, even if chips in small WLP packages are used, chips must be mounted on both the top and bottom of the PCB.
To achieve this size reduction, the transceiver must be provided in a WLP (wafer level package). The latest IO-Link transceiver from Analog Devices integrates a DC-DC converter, which is also advantageous in terms of minimizing size.

Industrial sensors must also be designed to operate robustly in harsh environments. This means that they must include protection circuits such as TVS diodes (not shown in Figure 5). In this regard, it is necessary to pay attention to the absolute maximum ratings (mAximum Ratings) of IO-Link transceivers.

Let’s take a closer look. How does the absolute maximum rating of 65 V for IOs allow for a smaller sensor subsystem? Typically, these sensors must be able to withstand surge pulses between four pins (GND, C/Q, DI, DO). IO-Link transceivers from Analog Devices have an absolute maximum rating of 65 V. For example, let's say there is a 1kV/24A surge between C/Q and GND.

Voltage between C/Q and GND = TVS clamp voltage + TVS forward voltage

For higher absolute maximum ratings, design engineers can use smaller TVS diodes such as the S㎃J33, which has a clamp voltage of 60 V at 24 A and a TVS forward voltage of 1 V at 24 A.

Voltage between C/Q and GND = 61V

This value is within the absolute maximum rating specification range for Analog Devices transceivers.

However, if the absolute maximum rating is lower (45 V is typical in the industry), a much larger TVS diode, such as the SMCJ33, is needed to clamp the voltage to an acceptable level. This diode is more than three times larger than the one required by the Analog Devices transceiver.

Therefore, a lower absolute maximum transceiver rating requires a larger TVS diode, which has a significant impact on the overall sensor design size. Table 1 shows the difference in PCB area, assuming the sensor is required to withstand a high surge of 1 kV/24 A.
The next generation of IO-Link transceivers goes even further. The latest IO-Link transceivers from Analog Devices include protection features on each of the IO-Link line interface pins (V24, C/Q, DI, GND). All pins include 1.2kV/500Ω surge protection as well as reverse voltage protection, short-circuit protection, and hot plug protection.

These devices include all of these protection features and integrate an internal DC-DC buck regulator, yet are housed in an extremely small WLP package (4.1 mm x 2.1 mm), enabling miniaturized IO-Link sensor designs.

■ Conclusion

Figure 6 shows how Analog Devices' IO-Link transceiver technology is advancing.
The first generation IO-Link transceiver technology met the requirements of compact sensor designs by integrating LDOs into an easy-to-use TQFN package. As power and size issues become increasingly important, second-generation transceiver technology optimizes power consumption by providing lower RON, further reducing power consumption and enabling smaller WLP packages.

The latest generation of transceivers incorporate protection features and high-efficiency DC-DC buck regulators, further reducing the size of the sensor subsystem and improving thermal performance.

As IO-Link technology becomes more and more prevalent in industrial sensors, these device specifications become increasingly important in achieving small, robust, and power-efficient sensors.

※ Contributor

Suhel Dhanani, Director of Business Development, Industrial and Healthcare Business Unit, Analog Devices