In industrial production, the vast majority of sensors work with wires. This approach, however, prevents many applications directly from having moving parts.
Wireless sensors offer an alternative here, but the problem of power supply and therefore the maintenance involved must be carefully considered. A new class of sensors combines wireless functionality with energy-efficient operation, enabling new applications without downstream maintenance.
This paper examines the requirements of sensors in the industrial internet of things and discusses the possibilities and limitations of radio-based sensors.
Energetic wireless sensors in quality control
Quality monitoring is a central aspect of the production process. It ensures that the final product of the production process is characterised by previously defined parameters.
In order to achieve this goal, a variety of parameters must be monitored, for example environmental factors such as temperature, humidity and air quality; process factors such as speed, force, pressure and temperature or material factors such as considering the starting materials used.
Many of these parameters are suitable for automated monitoring by sensors. In practice, however, this is not yet fully implemented. The reasons are complex, the most common aspects are the necessary investment in new tools, machines or sensors, the necessary training of the employees, the costs of maintenance and a difficult estimate of profitability.
Ideally, tools with integrated sensors, which fit directly into existing production processes, would not require special training and generate no follow-up costs.
These requirements can be met by energy-efficient sensors, as the following example shows: In many cases, a relevant data acquisition must be close to the boundary between the tool and the workpiece.
Parameters such as feed, contact pressure, surface temperature, vibration and noise can be recorded, which can be subjected to a downstream analysis. A challenge here is that the monitoring often has to be carried out at moving and / or difficult to access locations.
The use of conventional wired sensors is thus difficult to achieve. Compact, wireless sensors offer decisive advantages for this application. Since they do not require any wiring, they can be mounted directly on moving parts.
Furthermore, these sensors can also be used in a hermetically encapsulated environment (for example, in a vacuum or under a protective atmosphere).
Maintenance-free process monitoring
The goal of process monitoring is to ensure a defined productivity, ie to achieve a production quantity based on a certain amount of time, material and personnel. Deviations in the production process must be detected and avoided as far as possible.
At the same time, the integration of wireless sensors into the production facilities offers decisive advantages: hermetically encapsulated, wireless sensors can be used, for example, in pipelines to measure the flow, pressure and temperature of liquids or gases.
Condition-based maintenance with battery-free sensors
A central aspect of quality assurance is the monitoring of machines used for production. These are subject to wear and tear and early recognition of any problems and therefore timely countermeasures for affected items are therefore important prerequisites for continuous quality assurance and to protect against production losses.
A fundamental problem of maintenance scheduling is calculating the intervals between each maintenance visit. On the one hand, the time between maintenance visits must be sufficiently short to detect possible deviations before the occurrence of a major problem. On the other hand, maintenance is costly in terms of time labour and inactive machines.
Ideal would be the so-called condition-based maintenance, defining acceptable ranges. A typical example of this is the regulation of the minimum depth of tread on car tyres. When the depth of the tyres is below the defined limit, it is time to change them.
Such an approach would undoubtedly be desirable for machines from the point of view of cost efficiency, since maintenance is only necessary when this is required.
The main difficulty in implementing a condition-based maintenance is the continuous monitoring of the object in order to be able to reliably determine the time of the necessary maintenance.
Even with a superficial view of the number of critical parameters of a machine tool, it is quickly clear that it would not be possible or indeed very difficult to monitor all relevant data. The associated costs would only be useful in very rare cases.
However, in many cases it is possible to gain valuable information by monitoring fewer, simpler parameters.
Examples of such parameters are temperature, since wear often leads to higher friction which, in turn, leads to a temperature rise on the machine. Another parameter is sound because experienced employees can often recognise wear and tear on machines based on deviating noises.
Vibration also plays a role, since the combination of wear and friction frequently leads to asymmetries in the machine geometry, which are expressed through vibrations, particularly in the case of rotating machines. Wireless sensors, which can be directly integrated into the machine, also offer decisive advantages.
Self-powered radio sensors open up new possibilities
From the foregoing considerations, it becomes clear that wireless sensors offer decisive advantages for various applications in production. However, these advantages have to be weighed against the challenge that cable sensors must be continuously supplied with energy.
Particularly when used in places which are difficult to access (for example, integrated in machines, tools, leads, etc.), the disadvantage of a possible maintenance due to the necessary battery change must be carefully considered. Battery solenoids (energy-efficient) wireless radio sensors allow completely new approaches.
Battery-powered radio sensors use light, movement or temperature differences in their immediate environment as a source of energy. In this process, energy converters the smallest amounts of energy for the operation and the radio communication of sensors, switches or actuators.
In the industrial sector, in particular kinetic and thermal energy generators are of interest. Kinetic energy generators gain energy from movement, for example, by lateral movement (as when a switch is pressed), vibration or rotation about an axis.
Thermal energy generators use temperature differences to generate energy. Here, the combination of a thermoelectric converter and a voltage amplifier can already convert temperature differences of two degrees Celsius into usable current.
The energy gained by such energy generators is often low. Their use therefore requires the optimisation of the entire sensor architecture to include efficient measurement methods and energy-saving radio solutions. Such sensors have been developed by EnOcean since 2001 and are now used in several million sensor nodes.
The way to the battery-free internet of things
Today, wireless sensors (for example, as limit switches) and wireless incremental encoders (for example for voltage monitoring in rotary chucks or for monitoring the tightening torque in torque wrenches) are already widely used today. In both cases, the tool remains unchanged in form and mode of operation, thus enabling simple use in existing systems.
For these applications, there are already energy-efficient implementations which enable a permanently maintenance-free operation.
Environmental parameters such as temperature and humidity can already be measured well today. There is great interest in sensors that detect corrosion. Here too, energy-efficient operation is also conceivable in many cases.
The project “Optimised Resource Efficiency in Production through Energy- Energetic Sensor Technology and Interaction with Mobile Users” or ESIMA, has developed an energy-efficient compressed-air sensor that simultaneously measures pressure and flow and transmits the data by radio to a base station.
The project partners, including Festo, Varta, Daimler and EnOcean, have integrated this sensor directly into a turbine-generator unit. This can be operated directly with the compressed air used in the production process and provides the necessary electrical energy for supplying the electrical components.
Versatile projects in the area of ??condition-based maintenance based on monitoring of temperature, vibration or acoustics are in the development stage. These applications are only the beginning of a series of new applications.
They show that the use of energy-efficient wireless sensors offers new possibilities for better monitoring of important production parameters within the industry 4.0.