IO-Link sensors: where it all went right

IO-Link has enabled more comprehensive sensor communication, enabling sensor condition monitoring, while making set-up and maintenance easier.

In various and evolving forms, sensors have long been an integral component of industrial automation. They perform what initially appear to be very simple tasks, and for the most part they just let the control system know when something is present, absent, correct, incorrect or at a specific position or measuring point.

Of course, it isn’t really that simple. To address the huge variety of automation applications there also needs to be a huge variety of sensors, and choosing the correct sensor for an application is not always straightforward. There are different devices available to detect or measure different materials and there are sensors for speed, temperature, pressure, colour, vibration, humidity, angle, flow, vibration and much more.

There is also a dauntingly wide range of sensing principles in use including, but not limited to, inductive, photoelectric, capacitive, magnetic, ultrasonic, radar, guided microwave, magnetostrictive, mechanical, MEMS, gyroscope, infrared, vortex and calorimetric.

After the introduction of non-contact sensors (usually three-wire devices) the basic operation of sensors stayed the same for a very long time. The sensor was powered up and, when specific set parameters were met (or not met), a signal was sent to the control system. Prior to Industry 4.0, and with a few rare exceptions such as a remote teach wire, the flow of information from sensors was all one way from the sensor to the control system and the information passed on was limited to basic digital or simple analog signals.

Industry 4.0 — and more specifically IO-Link — has now given us the ability to get much more from our sensors and also made using the sensors much easier. There are several ways in which this applies.

Connectivity

Using IO-Link sensors can save on connectivity costs. Most of us by now are familiar with the major benefits of remote I/O when compared to ‘traditional’ hard wiring. These include reduced physical wiring, the reliability of pre-made connectivity and the easy identification of faults. IO-Link technology adds a further level of cost savings when sensors with analog outputs are used. Shielded cables are no longer required and, when a full IO-Link system is used, there is no longer the need for any costly individual analog input cards. Connectivity from the IO-Link master to the control system is via fieldbus but the IO-Link sensor connections are point-to-point from each device to the IO-Link master using standard and reasonably priced industrial connectors, which are usually pre-moulded, double-ended M12 cordsets.

Condition monitoring

Previously, the data received from sensors has been pretty much limited to one, or occasionally two, simple digital signals and the odd basic analog measurement signal. This level of data provides you with the information that you need, but in the ideal world it is probably not everything that you really want. But for a long time that was all that was available.

For the most part, sensors are robust devices that reliably work right up until they don’t — and when they don’t, they should just be replaced. Some sensors, however, such as optical sensors, will require regular maintenance like cleaning to keep them operating correctly. The interesting question here is how frequently? If you don’t do the maintenance often enough, you get failures during production and if you do it too often, you are wasting resources and people will eventually devalue the importance of it.

Picture a retro-reflective optical sensor installed in an occasionally dusty environment where the sensor is not cleaned. When the device is installed the receiver element sees a lot more of the transmitted light than it needs in order to operate, and this ‘extra’ light is known as excess gain. As dust slowly accumulates on the lens of the device or on the reflector, it reduces the amount of light that reaches the receiver. When the light received finally drops too far there is no longer any excess gain, and the device then sees the beam as blocked and the output switches on continuously. This failure usually shuts down the whole line and production stops until the fault is diagnosed and the sensor is cleaned. The cost in faulty product, lost production, idle workers and the ‘knock down effect’ of having other machines that are idle during this period can be substantial.

With some IO-Link retro-reflective optical sensors, whenever the sensor is not blocked you can monitor the level of excess gain. Simply setting an alarm trigger point on the excess gain in IO-Link means you can take the guesswork out of maintenance and clean the device only when it is needed. This will give enough notice for the cleaning to be done during scheduled downtime. This solution remains effective as the device ages and the intensity of the emitter deteriorates, reducing the level of excess gain available, or when there are mechanical issues that create gradual misalignment of the reflector.

Figure 1: The excess gain graph shows the expected excess gain for a specific device with a specific reflector across its rated sensing range. Anything over 1 will operate correctly but higher levels (10 minimum) are recommended to compensate for soiling and misalignment.

Other condition monitoring benefits of IO-Link, depending on the device, include being able to monitor internal device temperature, operating hours, voltage and a host of other parameters. All of this can combine to provide a warning if there is a change in external conditions that may indicate other issues that need to be addressed, and it will also give us a much better indication of the expected lifetime for each unit, enabling us to plan our maintenance in an informed manner.

Configuration

In the Australian market our manufacturing lines need to be versatile. In most cases we simply do not have the volume requirements to run a dedicated line for each individual product that we produce. This means product changes, and at every change we potentially need to make mechanical adjustments and reconfigure at least some of the sensors.

Adjusting the sensors can be a bit of a task. In some rare cases we need to physically move the sensors, but this is fortunately now the exception rather than the rule. Usually, they are adjusted using a potentiometer, teach buttons or an onboard configuration sequence.

When sensor adjustment during a product change is required, just getting access to them can be a challenge. This can be due to other activity on the line or due to the sensors being in awkward locations where access can involve discomfort, the risk of injury, potential contamination or equipment damage. When you finally get to where the sensor is mounted, there is the added challenge of remembering the different teach processes and sequences for each device.

Picture an ultrasonic level sensor mounted in the top of a large enclosed tank. You are doing a product change and you need to adjust the switch point settings in the device. To access the location, you must climb a long ladder and then crawl across the top of the dirty tank in a narrow gap between the top of the tank and the roof of the factory. Often it is a stinking hot or freezing cold day — of course. When you reach the device you need to remember how to change a level setting, do the task and then carefully find your way back down again.

One of the major features of IO-Link is it provides you with the ability to transmit information to the sensors as well as receive data from them. This means that, in applications such as the one above, once the sensor has been installed all configuration and adjustment can be done remotely from anywhere that you have access to your control system and it can be done live while the system is running. In the event of a sensor failure IO-Link also facilitates automatic parameter setting. When this is utilised, any failed or damaged device that is replaced with an identical unit is automatically configured to be the same as the previous unit and production can resume immediately. This can save a lot of time and stress.

Some IO-Link solutions, for example radar level sensors, also have an integrated web server that can be used to visualise the signals being returned and this enables the user to ensure the device is configured so that it ignores interference signals and just looks for the required target.

Summary

From a sensor point of view IO-Link has enabled us to communicate much more comprehensively. It’s sort of like our stereotypical teenage sensors that ignored the world around them, wouldn’t listen and only communicated in grunts with the minimum possible information have now suddenly turned on their senses and joined the world. They have grown up and they are now mature adult sensors that communicate meaningfully and contribute positively to the conversation.

We now have a substantially greater amount of sensing, feedback and condition monitoring data available from our sensors and we can also configure them remotely, all without the need for any additional wiring. In this age of Industry 4.0, where improved technology has provided us with the capacity to collect and analyse much more data, this added functionality enables us to get a much better handle on what is going on in our facilities and what actions we need to take to maintain peak efficiency. Maintenance will always be partially reactive, but with the comprehensive information that we can now collect and process we can increase the amount of proactive work we do and in the long run this will make our sites more reliable and efficient.

Top image: ©iStockPhoto.com/gorodenkoff