Compliance Control Of Pneumatic Actuators And Its Application

As the world's population ages,there is a growing demand for machines that assist humans in their work or care for the elderly.This also puts forward a higher requirement for the human-machine interaction performance of the actuator.Pneumatic actuators use compressed air as power,which is clean and low cost.Due to the compressibility of gases,pneumatic actuators have "soft" output characteristics,making them easier to meet "human-friendly" requirements.However,the current position control method of pneumatic system cannot meet the above requirements.In this paper,an endoscopic assisted robot for minimally invasive thoracic surgery is selected as the target application background to study the compliance control of pneumatic actuators.It is pointed out that it is difficult for doctors to obtain the contact force between endoscope and human body when controlling the movement of field of vision.With 2D image,distance judgment is hard and the vision is narrow.It is possible that endoscope contact and damage tissues.In order to improve the safety of endoscopic assisted robot,it is necessary to control the actuator's compliance.Firstly,the working principle of proportional flow valve controlled pneumatic system is analyzed,and the nonlinear mathematical model of the system is established by using force balance equation,pressure differential equation and valve orifice flow equation.In order to obtain the parameters of the model,the pneumatic system is experimentally studied.The cylinder friction was measured and it fits the Stribeck model.The displacement of the spool and flow characteristics of the valve are measured,and the parameters of the flow equation of the valve port are obtained,and the effective throttle area is modified by series throttle orifice theory.Then the remote center mechanism of the endoscopic assisted robot was designed according to the operation requirements,and the kinematics and dynamics were analyzed.Load characteristic is important for the control strategy research,in order to set reasonable control parameters,this paper analyzes the minimally invasive thoracic surgery operation,selected the chest wall as the research object,points out that the thoracoscope may contact with the chest wall,or generate too much leverage on the ribs,or contact with the chest wall in the blind area.The mechanical properties of biological tissues load were measured with lamb chop as the experimental object.In this paper,the characteristics of several viscoelastic models were compared,and a parallel combination model of variable stiffness spring model and Maxwell model was finally determined to model intercostal muscles,ribs and other biological tissues.In order to realize the motion of the thoracoscope following the input displacement in free space and avoid big contact force when contacting biological tissues,this paper adopts the position-based impedance control as the basic control idea.According to the characteristics of stress relaxation in biological tissue,the impedance controller was improved to control the contact force at a lower level and finally out of contact,and the expected stiffness of actuator translational freedom can reach 0.01N/mm.At the same time,it is pointed out that in order to ensure the field of vision,ordinary impedance controller should still be used for the rotational degrees of freedom,with the expected stiffness down to 0.05N·m/deg.Finally,pneumatic actuators compliance control experiment platform was designed,and the experimental results verified the control performance of the control strategy in this paper.