Research On Constant Force Control In Pneumatic Gripper Based On Smart Foam

As a common actuator at the end of robotic arms,the gripper is in direct contact with the workpieces.With the advantages of faster response,smaller size,and lower cost,the pneumatic gripper is highly preferred in industrialized fields.It is critical to maintain a constant grasping force when grasping fragile or easily deformed objects.However,the pneumatic gripper suffers from a lack of tactile feedback and difficulty outputting a constant force.To address this problem,this paper aims to prepare regular porous foams using direct ink writing.And a force sensing resistor(FSR)is embedded during the printing to obtain smart foams with both mechanical and electrical properties.In this paper,the effect of different sizes of porous structure on the performance of smart foams is studied from theoretical analysis,finite element simulation,and experiment design.Finally,the smart foam is applied to the pneumatic gripper to realize the function of constant force control of the gripper system by using its mechanical and electrical properties.The major contents of this thesis are as follows:1.Study of preparation and performance of smart foams of different sizes.To avoid damage to the workpieces when using a pneumatic gripper,polydimethylsiloxane(PDMS)with high flexibility and stable chemical properties is used as the material of the smart foam.A regular porous silicone foam with stress plateau is obtained by pneumatic extrusion on a three-dimensional motion platform.And FSR is embedded to fabricate a smart foam with both mechanical and electrical properties.A total of seven groups of smart silicone foams with different sizes are prepared,and the measured filament widths and sample weights reflected the stability of the preparation process.The effects of the length and width of the foam,the number of layers,and the line spacing on its mechanical properties are investigated.The results are compared with the results of finite element simulations.And it is found that the experimental results are similar to the simulation results,with the most significant effect of the line spacing on the stress-strain curve.In addition,the influence of size on the electrical properties of smart foam is analyzed theoretically and verified experimentally using a self-built pressure-resistance test rig.The experimental results show that the theoretical predictions and experimental data agree well at low and medium pressures.However,the repeatability and linearity of the smart foam decrease as the line spacing of the sample increases.2.Constant force control of pneumatic gripper.Applying the prepared smart foam to the pneumatic grippers,this thesis can achieve the constant force control of the pneumatic grippers from both two aspects: 1)Active constant force control with feedback by using the electrical sensing characteristics of the smart foams;2)Passive constant force control without feedback by using the mechanical characteristics of the stress plateau of the smart foams.In the feedback control,a pneumatic constant force control system is designed after spring selection,system dynamics analysis,and control system mathematical model acquisition.The prepared smart foam is mounted on the pneumatic grippers as the feedback input,and the control signal is output using PID control algorithm to adjust the grasping force of the grippers by controlling the proportional pressure reducing valve.The established closed-loop system can output the specified size of force smoothly and can resist a certain degree of disturbance.Within the sensing operating range of the smart foam(5～12N),its maximum error does not exceed 1.5N,which can meet the demand of controlled constant force control.In the case of no-feedback control,unlike using only FSR,the use of smart foam can maintain a constant force of 14.6N in the air pressure range of 450～560k Pa.