Application Of Fractional Order Feedforward-Feedback Control In Atomic Force Microscope System

Atomic force microscopy(AFM)is an important tool in the field of nanotechnology for exploring materials at the nanoscale,and is capable of measuring the surface morphology of material objects with atomic-level resolution.However,Conventional PI control struggles to overcome the effects of system non-linearity when AFM system perform high-speed scanning imaging,which has become a major factor limiting the imaging performance of AFM system.In this paper,to address the problem of inadequate control performance of traditional PI control in AFM systems,analyze the influence of control parameters on AFM performance by establishing a system model of AFM.A fractional order-based Feedforward-Feedback controller was designed and applied to a homemade AFM scanning imaging system to improve the control performance at high AFM imaging speeds and thus improve the imaging quality of AFM.The main work of this paper is as follows:The laboratory home-made AFM system was modeled and the AFM system simulation model was built in the SIMULINK environment.The effects of control parameters,nonlinear characteristics,etc.on the performance of AFM systems are investigated.The AFM motion control method based on the fractional-order PI~λfeedback controller was studied to address the problems of inflexible parameter adjustment and low control accuracy of the traditional PI control for AFM systems.In this design,the OUSTALOUP approximation method was used to solve the problem that the fractional differential operator in the controller are difficult to implement directly numerically.The results of the trajectory tracking simulation and AFM system imaging experiments show that the designed fractional-order PI~λcontrol can effectively improve the control accuracy of the AFM system and have better imaging results.The AFM motion control method based on fractional-order Feedforward-Feedback control was studied to address the problems of poor dynamic characteristics and image distortion of the AFM system during high-speed scanning.In the Feedforward path,a fractional-order PD~αiterative learning algorithm was designed to achieve fast shape tracking in the z-axis of the AFM,and this scheme can make the probe cantilever beam maintain the imaging force well near the set point.Fractional PI~λcontrol algorithm was used in the feedback loop,which plays a very good role in maintaining the imaging accuracy and quality of the system.The results of trajectory tracking experiments show that the designed control scheme can track the desired trajectory in a short time and achieve the desired control accuracy.The results of the AFM scanning imaging experiments show that the control scheme can effectively overcome the non-linearity of the system and the adverse effects of external interference on the imaging,and still achieve high imaging quality at high scanning speeds,thus improving the control performance of the AFM for nanomanipulation and measurement.