Wireless Sensor in a Screw Head Harvests its Own Energy

A wireless sensor system in a screw head, developed by the Technische Universität Chemnitz and IK Elektronik, is powered by kinetic energy harvesting from vibration in commercial vehicles, writes Andreas Mangler, Head of Strategic Marketing at Rutronik.

This research and development project centres on a wireless sensor system integrated into a screw. The system measures and processes sensor signals and sends them to a measurement data processing system. The electronic system in the screw functions without a battery. It converts kinetic energy from vibrations at the fixing element into electrical energy and uses this energy for sensor signal conditioning and processing. The sensor cannot be distinguished externally from a standard screw.

Figure 1. The sensor screw system developed by Technische Universität Chemnitz and IK Elektronik.

Areas of deployment with robust design

The most important application scenario is using the screw in vehicles at different installation points. Examples for system application are belt buckles, trailer hitches, and particularly sensors in the area of the vehicle’s gearbox and engine. Dust and splash water must not have any effects on the screw itself; deployment in a robust environment must be possible. The housing must be hermetically sealed to the greatest possible extent.

Moreover, in view of the different potential ambient temperatures resulting from winter/summer operation and deployment in different geographical vehicle widths, the system must be stable and independent of temperature influences, particularly regarding the material stretching. The system needs to be as small as possible due to the limited space.

Development challenges

Vibrations in vehicles arise with different amplitudes and frequencies. Energy converters thus generate alternating voltages with different amplitude and frequency. Particularly when it comes to only very small vibration amplitudes, very small alternating voltages arise. Standard rectifiers are able to rectify very small voltages, but not without large losses. The required boost converters must exhibit a high degree of effectiveness and stellar starting behavior. Energy converters and energy management need to be optimized for certain vibration frequency ranges. Additionally, the mechanical specifications and the use in a metallic environment posed great challenges in the development of the energy converter.

The challenge for IK Elektronik in developing the electronics was to obtain a usable direct voltage with highest possible effectiveness from the lowest alternating voltages, and to store the energy in an appropriate manner. Depending on the amount of stored energy, connected electronics may be operated for longer or shorter periods.

The most applied methods of energy generation using vibrations are based on electrodynamic, piezoelectric, electrostatic, or electromagnetic principles. An electrodynamic energy converter, comprising a cylindrical magnet and a surrounding solenoid, is a good choice to be used in a screw. This is because such a converter achieves a large increase in resonance when the system is set up appropriately. The conversion of ambient vibrations into electrical energy is based on the relative movement between magnet and solenoid according to Faraday’s law. Literature on this subject contains various types of electrodynamic energy production, which can be sub-divided according to the mechanical structure of the spring-mass system: use of bending beams, magnetic springs, or spiral springs.

Mechanical and electrical structure

IK Elektronik developed the prototype of an assembly housed in the head of a screw. This contains the rectifiers and voltage multipliers, storage capacitors and energy management wiring, microcontrollers with sensor application (contact sensor, temperature sensor, and pressure sensor), radio transceiver, and antenna.

Figure 2. Arrangement of the electronics in the sensor screw in various configurations (source: IK Elektronik GmbH)

Various applications can be mooted thanks to the combination with the TU Chemnitz electrodynamic energy converter in a screw. Functionality can be optimised based on the vibration energy available in the respective target environment.

Various investigation concepts

Various solutions were investigated regarding the structure of the energy converter. Two concepts were proposed regarding applying the magnetic spring principle. The solution developed by the Professorship of Electrical Measurements and Sensor Technology (MST) at TU Chemnitz uses a moving magnet between two permanently fixed magnets to generate a variable magnetic field depending on the ambient vibrations. The solenoid and its connections are fixed in this structure, meaning that greater reliability is attained when the energy is converted.

Figure 3. Example of a mechanical structure for an energy converter with the fixed and moving elements

The converter’s energy yield is chiefly dependent on the magnet size and the magnetic field strength, number, and quantity of solenoid winds, as well as the excitation frequency and amplitude, irrespective of the configuration in use, i.e. a moving magnet or moving solenoid. For the energy converter design, the FEM analysis (finite element method) was used and can be adjusted to different kinetic profiles and sizes.

 

Figure 4. Finite element analysis regarding the energy converter’s magnetic field distribution in the sensor system

Test series with the energy converter

TU Chemnitz created two prototypes for using a moving magnet; the open circuit voltage for the systems was measured. The applied test structure consists of a vibration shaker as a synthetic external vibration source, monitored by a laser sensor, measuring the deflection in use and frequency stimulation. The vibration shaker is operated in a closed control circuit together with a laser sensor, a controller, and an amplifier; the vibration amplitude can be controlled with this control circuit. The trials have been carried out for a vibration amplitude of 0.5 mm, 1 mm, and 2 mm in a frequency range of 5 to 30 Hz (see figure 5).

The resonance frequency of the intended structure is in the range 25-30 Hz, depending on the solenoid and excitation amplitude in use. By applying the magnetic spring principle, the resonance frequency can also be set to other values relatively simply. As the measurement values show, open circuit peak voltages of over 500 mV were achieved.

   

Figure 5. Open circuit output voltage for the developed magnetic spring energy converter
(a) for excitation amplitudes of 1 mm and 2 mm with a wire diameter of 0.2 mm.
(b) for excitation amplitudes of 0.5 mm, 1 mm, and 2 mm with a wire diameter of 0.09 mm.

 

Research cooperation

This piece of work is a cooperation project with the title ‘Wireless Sensor System with Kinetic Energy Converter in Screw Form for Commercial Vehicles’ between the Technische Universität Chemnitz, Professorship of Electrical Measurements and Sensor Technology (MST) and IK Elektronik, supported by the Central Innovation Programme for SMEs (ZIM) of the Federal Ministry for Economic Affairs and Energy (BMWi).

Rutronik supports both research partners in terms of application consultancy regarding the electronic components, particularly in the field of sensors and wireless products (Rutronik SMART) and collaborates closely as part of Bachelor and Master theses with the Professorship of Electrical Measurements and Sensor Technology (MST) at TU Chemnitz.

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