| With the continuous development of precision manufacturing,the requirements for force during machining operations are becoming increasingly high.In order to avoid unnecessary damage and destruction to the object to be operated,it is necessary to keep the force constant during the operation.The introduction of constant force mechanisms in the operating system can be a good solution to the problem of unstable operating forces.The use of constant force mechanisms can reduce the use of auxiliary equipment such as force sensors and controllers in the system and avoid the design and use of complex force feedback control procedures,thus reducing the complexity of the whole system.It is also simple to operate and cheap to manufacture.The material deformation based compliant mechanism has the significant advantages of no frictional wear and drive clearance,no assembly,no lubrication and allows for high precision movement.The connection of the compliant mechanism does not require hinges or bearings,but uses its own deformation to transmit motion and force,and has the characteristic of coupling force and displacement.Therefore,a reasonable structural design of the compliant mechanism can make it have unique mechanical properties.The compliant constant force mechanism,as a kind of compliant mechanism,is an effective means to solve the problem of constant force manipulation,and has been widely studied in recent years.This thesis presents a detailed study of the technical requirements of compliant constant force mechanisms in practical operations,focusing on the design of a compliant constant force mechanism based on a combination of positive and negative stiffnesses,the modelling theory to characterise an accurate model of the force-displacement relationship,and the exploration of active stiffness adjustment methods based on magnetorheological materials.The main elements of this thesis are described as follows:(1)Based on the study of design methods for flexible constant force mechanisms,three common types of positive stiffness mechanisms(flexible straight beams,U-shaped beams and V-shaped beams)are analysed.By comparing the three mechanisms,a positive stiffness mechanism(double V-shaped beam)with linear large deformation is designed,while a negative stiffness mechanism(bi-stable beam)with flexural characteristics is designed.Based on the principle of combining positive and negative stiffnesses,a new parallel compliant constant force mechanism with large stroke is proposed,which meets the requirements of constant force operation required in the field of precision operation.(2)Based on the study of the modelling method of the compliant constant force mechanism,the pseudo-rigid body model and the chain beam constraint model are used to establish the kinematic static theoretical models of the positive and negative stiffness mechanisms respectively.The stiffness characteristics of the positive and negative stiffness mechanisms are analysed,and the overall stiffness model of the mechanism is established.Based on the theoretical model of the mechanism,the influence of different materials on the output constant force characteristics of the compliant constant force mechanism is investigated.(3)The simulation of the designed compliant constant force mechanism is carried out using finite element simulation software,and the stress and strain of the mechanism during deformation are analysed.The constant force characteristics of the mechanism are verified through simulation analysis.Due to the influence of parameters on the mechanism,the influence of structural geometry parameters on the constant force output characteristics is analysed.A prototype of the mechanism has been prepared and an experimental system based on this constant force mechanism has been built.The relationship curves obtained from the experiments are analysed to verify the constant force output performance of the mechanism.(4)The flexible beam forming the compliant constant force mechanism is hollowed out and filled with magnetorheological material.The mechanism of the influence of the magnetorheological material on the stiffness characteristics of the mechanism under the influence of an external magnetic field is analysed.A mechanical model of the magnetorheological fluid is analysed and a model of the overall stiffness of the compliant constant force mechanism filled with magnetorheological material is developed.The mechanism of stiffness change under external magnetic field is revealed and the characteristic of the mechanism with different magnitude of constant force output is realised. |
