Inductive displacement transducers sense how cherries grow
Long-term measurement of cherry growth on the tree requires slim displacement transducers with high resolution, low forces, high protection class and low power consumption
This measuring task requires a displacement transducer that works wear-free in field tests at high humidity levels – including droplet formation.
The inductive sensor of the SM34 series fulfills these requirements and was selected for this task. The displacement transducers detect measuring distances of up to ± 10 mm with resolutions in the µm range.
The plunger is pressed into the rest position by a spring. The measuring tips and the spring can be exchanged for different measuring tasks. The robust, encapsulated design of the probes allows them to be used under difficult environmental conditions. In the application described (carried out at the University of Hanover, Institute for Horticultural Production Systems, Department of Fruit Growing), the probes are attached to cherry trees to measure the change in diameter of the cherries. The diameter is measured with a resolution of 3 µm and is used to calculate the volume of the fruit. Thanks to its compact design in a housing just 10 mm thick and its low weight, the displacement transducer is easy to attach to the branches of the cherry trees. The integrated microcontroller evaluates the axial displacement of the Mu metal core. The complete electronics are integrated in the housing and provide an output signal proportional to the displacement. Low power consumption enables the sensor to be supplied with an inexpensive power bank. The output signals from 26 displacement transducers are read out simultaneously and stored on a data logger.
Installation of the displacement sensor on cherries in the tree
(Martin Brüggenwirth, Andreas Winkler, Moritz Knoche. 2016. xylem, phloem and transpiration flows in developing sweet cherry fruit. Trees, DOI 10.1007/s00468-016-1415-4)
The typical design of an inductive displacement transducer consists of a hollow coil body with strictly symmetrically wound coils, a magnetic shield with high permeability and a round, stainless steel housing. The spaces between the coil and housing are completely filled with hardening plastic. The plunger made of a nickel-iron alloy moves through the coil body and changes the inductance of the two coil halves in opposite directions. The bore of the coil body and the diameter of the plunger are selected so that contactless, wear-free movement is possible. Contactless measurement is a major advantage of inductive displacement transducers and enables them to be used in material testing machines and on vibration test tables, for example. Due to the infinite resolution, the smallest changes in the displacement of the plunger can be measured, limited only by the noise of the integrated electronics.
The virtually unlimited service life and high reliability also enable use in sensitive applications such as satellite technology, aircraft or industrial automation. The integrated electronics evaluate the change in inductance caused by the axial displacement of a NiFe metal core. Operating voltages of 5, 12 or 24 VDC and the low operating current also enable use in mobile systems. The proportional, analog output signal of 0 – 10, 0 – 5 or 0 – 4 VDC can be processed directly by many evaluation units. The version with current output 4 – 20 mA is available in 2-wire technology.
The miniature sensors with IP67 protection can be used in extreme environmental conditions such as oil mist, mud, rain and dust and can withstand shock loads of up to 250 g SRS and vibration loads of up to 20 g rms.
