Active bandsaw control
Thesis DisciplineMechanical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis investigates the modelling and active control of narrow and wide bandsaw blades, with application to the sawmilling industry. Strings, beams and plates are considered in the modelling work, with advances made in the modelling of exogenous influences and multispan saw blades. Beams and plates are considered in the control work, with classical and optimal controllers considered. Importance is placed on closed-loop robustness with respect to parametric variation, closed-loop performance in vibration suppression and in providing a physically realisable solution to the control problem. In the string and beam work exogenous influences are modelled by pointwise and distributed forces, including; lateral stiffness, lateral damping and a "follower" force that comprises an in-line and a lateral component. Pointwise actuation and arbitrary disturbance forces as well as pointwise sensing are also included. Successful comparison with results of other contributors, as well as comprehensive experimental work, validates the modelling. The experimental validation also concentrates on system damping and the integration of sensing and actuation. The plate work considers the single-span cutting blade presented by other contributors, and extends it to include saw guides and partial-span cutting forces. These cutting forces include damping, stiffness and follower loads, and act over a partial length of the cutting edge. While this three span model is not experimentally verified, it is shown to produce credible results. The control work is in two parts. A comprehensive study of the robustness of various controllers with respect to translation speed and band tension is performed for the beam; theoretically and experimentally. The theory-practice gap was small regarding trends in robustness, but unmodelled effects such as the band weld degraded the agreement of absolute values at higher band speeds. Classical controllers were abandoned due to high frequency noise amplification, and a near optimal H∞ loop shaping controller was found to be superior to others of its type and various H₂ formulations. The plate work is entirely theoretical, but uses the same actuator and sensor dynamics that were successful in the beam work to maintain the physical feasability of the controllers. Both single span and multispan systems are considered, with the central cutting span of the blade being controlled via actuation and sensing of the upstream and downstream noncutting spans. Robustness studies were conducted, with satisfactory robustness achieved with respect to a large number of parameters. Furthermore, substantial increases in maximum allowable cutting loads were achieved, as well as reduced vibration energy. The control actuation used in this work is electromagnetic force, with eddy current sensors used to sense the blade position. A noncontacting collocated actuatorsensor was developed that, with appropriate control of the winding current, performed excellently in both the validation and control work. Further development of this could lead to a versatile tool in experimental vibration analysis and distributed systems control research and applications.