The broader the decision-making basis in automated machines or plants for their control or higher-level systems, the more precisely the production result can be controlled. Sensors provide the basis for this, which is why more and more of them are being used. Hardware requirements can be limited by using safety-related sensors for even non-safety-related purposes. It is best to factor this in right from the automation concept stage.
Sensors are the eyes, ears and feelers of machines and systems. The signals and information they provide serve as a navigation aid for control and higher-level systems as the basis for optimising operations.
Sensory organs of machines and systems
Data is considered to be the oil of the 21st century. As with the drilling points, the sensors at the field level therefore also form the basis for evaluations and analyses, as a source of information. Ever-increasing quantities of data are collected and evaluated in order to digitalise all of the business and production processes. Developing digital transformation is a prerequisite for solving tasks such as economic predictive maintenance or the transition to Industry 4.0 through the use of the Internet of Things (IoT).
The better these data evaluations are, the more informed companies can be when making important business decisions. In order to broaden the data basis for decisions at all levels, machines and systems are therefore being equipped with an increasing number of sensors. They are often fitted during the course of modernisation of the machines, or retrofitting.
A real need for information and a small budget
The growing hunger for ever more detailed information from ongoing operations poses a huge challenge for the developers of control and automation solutions. On the one hand, they have to provide their designs with ever more numerous data sources and evaluation options. On the other hand, they must meet tight cost targets in order not to jeopardise the marketability of the devices, machines or systems themselves.
Therefore, they cannot increase the number of installed sensors indefinitely. They have to strive to obtain the valuable information from other sources as well. To this end, it is helpful, for example, to evaluate the power consumption of motors and to compare their movements. In this way, developers can gain new insights into the stiffness of mechanical movement axes. If the machines need to be readjusted or relubricated, this will be shown at an early stage.
Activating safety sensors
Like the functional “grey” control, safety circuits or safety-related “yellow” controls also make use of signals from sensors. For this purpose, companies use their own products that meet the requirements for functional safety (FuSa).
Often, these more expensive safe sensors are only connected to the safety circuit or a safety-related controller. Use is then exclusively for these purposes. However, their signals can often be used meaningfully for general control purposes. With a manageable total number of sensors, this makes it possible to build machines with a high level of functionality without having to compromise on functional safety.
If you want to exploit this synergetic potential, it is advisable to take appropriate sensors into account during the planning phase. The desired safety concept should be part of the automation plan from the outset. The synergetic use of different safety components can only be successful if the sensors are able to meet the relevant safety standards.
The development of functional safety
To start, allow us a brief digression into the history of safety technology in mechanical and systems engineering. In the early days it was limited to emergency stop buttons and door contacts. These acted directly, or via relay circuits, on the power supply to the machine and brought it to an immediate standstill in the case of danger. In most cases, however, there are not just one or two points on a machine whose monitoring requires functional safety. This later resulted in complex safety circuits, which ensure an orderly shutdown even in systems with complex geometries.
However, these circuits are inflexible. The effort required to adapt to changing circumstances increases exponentially with increasing system size and complexity. In addition, these circuits are completely self-contained — it is not possible to use the information available for other purposes.
From wiring to programming
In order to increase the functionality and flexibility of safety technology, the industry developed programmable logic controllers (PLCs) with redundant properties for command processing and communication. In these safety-related controllers, the actual “hard” wiring is replaced by programming or parameterising channels and reactions. They can also evaluate more complex signals.
Some of these safety-related controllers can take on normal “grey” control tasks in addition to the safety-relevant “yellow” controls, and this alone helps to limit the amount of hardware required for smaller applications. They can also exchange information between the two internal parts of the system. In this way, not only is the machine standstill brought about via safe channels and actuators, but the operating speed of other system parts is also adjusted via non-safe channels.
Flexible through network communication
Many sensors still only have the connections required for use in safety circuits and controls. Hard-wired circuits cannot, however, pass on information from these sensors, while programmable safety controllers are usually already capable of doing so.
Modern Ethernet-standard safety components communicate with the safety-related control via the so-called black channel. In doing so, they tunnel through the general data stream on the network cables and therefore make use of the existing industrial network infrastructure. In most cases, it does not matter which version of Industrial Ethernet is concerned. It is also irrelevant whether the communication is wired or via Wi-Fi. Most manufacturers also already have devices with safety communication via the manufacturer-independent OPC UA protocol available or at least in development.
Of course, there are safe digital input and output modules for this technology, which can be used to connect simple switches with wire connections. Analogue modules—for example for connecting temperature or level sensors—are also a matter of course nowadays.
In addition, the use of high-speed networks for safety technology has led to the development of components that enable differentiated responses to security breaches. These include, for example, 360° laser scanners or time-of-flight (TOF) cameras or drives which, in addition to an immediate safety stop, can also handle a safe speed or direction, for example.
Communication enables multiple uses
What Ethernet-compatible safety sensors or I/O modules have in common is that they also enable direct access from other systems — without additional wiring. This enables the seamless integration of safety-related hardware into the functional concepts of control and automation solutions.
For example, the individual beams in a light grid are evaluated to determine the shape and speed of an incoming object. As a result, the system is not only able to distinguish between a safety breach by a person or their hand and a material delivery. It is also able to initiate the appropriate further steps for each piece of material delivered. Manufacturers of autonomous mobile robots use safe 360° sensors not only to protect people but also for contour-based navigation.
The list can be continued — imagination knows no bounds. If you consider in advance what information can be obtained from the safety-related sensors, I/O modules and controllers that are already required, you can save on a lot of additional hardware and develop cost-efficient products. By using safety-related components in multiple ways, developers of production systems can increase their accuracy and efficiency, realise better predictive maintenance concepts and gain more meaningful information for operational decision-making.
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