A piezoelectric force sensor for mill-scale chip refiners

Abstract

Refiners are used in the pulp and paper industry to separate wood chips into individual fibres and to develop the morphology of fibres to be suitable for the type and grade of paper to be produced. Within a refiner are discs, at least one of which rotates at high speed and all of which are lined with radial patterns of bars on their opposed surfaces. A mixture of chips and water is accelerated radially through the space between the discs. Compressive and shear forces are applied to chips and fibres as the bars on the opposed discs cross each other. Experiments have shown that certain process variables can greatly affect the forces applied to chips or fibres when bars cross. These findings suggest that the contact mechanics of bar-crossings are a significant factor in the development of fibre properties. To investigate these contact mechanics in operating refiners, a piezoelectric-based sensor has been developed to measure the forces applied to the chips and fibres by the bars. The sensor replaces a small section of a refiner bar and is sensitive to both axial and tangential forces. The current paper describes the evolution of the design of this sensor from a proof-of-concept prototype for a laboratory-scale refiner to the latest generation sensor for a mill-scale refiner. Performance during the most recent mill trials is presented.