Aerostatic thrust bearings are currently used in applications where high accuracy is required, e.g., in machine tools, measuring instruments, manufacturing and medical equipment. Their appeal is due to the absence of contact between their moving and stationary parts, which ensures relative motions characterised by low friction, low wear and limited heat generation. However, in some operating conditions, these systems may present low specific stiffness and be subject to instability. This thesis presents the design, modelling and testing of an actively compensated aerostatic thrust bearing. A support compensation strategy has been adopted as an active compensation method. The support compensation method consists in integrating conventional passive bearings with piezoelectric actuators which make it possible to compensate for variations in the bearing position. This compensation is obtained by exploiting changes in the stroke of the actuators and makes it possible to enhance both the static and the dynamic bearing performance. This integration has been possible thanks to the presence of a compliant mechanism which gives a directional guide to and opportunely pre-loads the actuator. Preliminary experimental tests have consisted in a "classical" static characterisation of the bearing where the load, air consumption and stiffness of the bearing have been obtained. Moreover, concerning the design of bearings, their dynamic performance is becoming a crucial aspect, due to the continual demand for higher performance from today’s applications. For this reason, a dynamic linear time-invariant model of the system has been proposed. Here, the linear approximation is made by presuming that around each operating condition the system motion may be considered as an equilibrium state, and the bearing is modelled as a SDOF system because of the physical constraints imposed. The model validation has been performed with the aid of step force and tracking tests. Step force tests have been firstly carried out to dynamically characterise the bearing in its passive configuration (without the automatic control) by utilising and comparing the Logarithmic Decrement and the Half Power Bandwidth methods. Secondly, step force tests have also been used to assess the capacity of compensation of the active bearing (with the automatic control). On the other hand, tracking tests have been performed to verify the positional accuracy of the bearing and assess its bandwidth. Results demonstrate both the effectiveness of the prototype and the sound of the adopted mathematical model.

Design, Test and Identification of an Active Aerostatic Thrust Bearing with a Compliant Mechanism and Piezo Actuator / Lentini, Luigi. - (2017).

Design, Test and Identification of an Active Aerostatic Thrust Bearing with a Compliant Mechanism and Piezo Actuator

LENTINI, LUIGI
2017

Abstract

Aerostatic thrust bearings are currently used in applications where high accuracy is required, e.g., in machine tools, measuring instruments, manufacturing and medical equipment. Their appeal is due to the absence of contact between their moving and stationary parts, which ensures relative motions characterised by low friction, low wear and limited heat generation. However, in some operating conditions, these systems may present low specific stiffness and be subject to instability. This thesis presents the design, modelling and testing of an actively compensated aerostatic thrust bearing. A support compensation strategy has been adopted as an active compensation method. The support compensation method consists in integrating conventional passive bearings with piezoelectric actuators which make it possible to compensate for variations in the bearing position. This compensation is obtained by exploiting changes in the stroke of the actuators and makes it possible to enhance both the static and the dynamic bearing performance. This integration has been possible thanks to the presence of a compliant mechanism which gives a directional guide to and opportunely pre-loads the actuator. Preliminary experimental tests have consisted in a "classical" static characterisation of the bearing where the load, air consumption and stiffness of the bearing have been obtained. Moreover, concerning the design of bearings, their dynamic performance is becoming a crucial aspect, due to the continual demand for higher performance from today’s applications. For this reason, a dynamic linear time-invariant model of the system has been proposed. Here, the linear approximation is made by presuming that around each operating condition the system motion may be considered as an equilibrium state, and the bearing is modelled as a SDOF system because of the physical constraints imposed. The model validation has been performed with the aid of step force and tracking tests. Step force tests have been firstly carried out to dynamically characterise the bearing in its passive configuration (without the automatic control) by utilising and comparing the Logarithmic Decrement and the Half Power Bandwidth methods. Secondly, step force tests have also been used to assess the capacity of compensation of the active bearing (with the automatic control). On the other hand, tracking tests have been performed to verify the positional accuracy of the bearing and assess its bandwidth. Results demonstrate both the effectiveness of the prototype and the sound of the adopted mathematical model.
2017
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11583/2670377
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