Development of a piezoelectric force sensor for a chip refiner

Abstract

Abstract

Chip refiners are used to separate individual fibres from the wood matrix through the application of cyclic compressive and shear forces. The work presented here deals with the development of a two-axis piezoelectric force sensor to measure these cyclic forces in directions normal and tangential to the motion of refiner bars. The sensor consists of a small probe tip that replaces a portion of a refiner bar and is supported on four piezoelectric elements inside a housing. Stresses applied to pulp and wood material, at the surface of the probe, are thus transmitted to the piezoelectric elements, which respond by producing voltage signals. Signals from two of the four piezoelectric elements are used to determine forces in the normal and tangential directions during refining. A prototype sensor was tested in an atmospheric-discharge laboratory refiner. Impacts from individual bar crossings could clearly be discerned even at the maximum operating speed of the refiner. At low refiner speed, detailed measurements of the magnitude of the normal and tangential forces throughout a bar crossing were obtained. However, resonant vibrations of the sensor made it difficult to obtain such information when running the refiner at maximum speed. A number of design modifications are discussed, with the aim of improving the sensor performance for applications in larger-scale commercial refiners.