Basic Theoretical And Application Research On Double-layer And Pre-stressed Six-Axis Force Sensor

With the rapid development of science and technology, sensor technology has beenwidely used in the fields of measurement, control and information, and has become oneof the cores of the high-tech. In all kinds of sensors, the six-axis force/torque sensor isone kind of the most important sensors as it can measure the six-dimensional force andmoment information in the space, and has broad application prospects in the field ofaerospace, robotics, automotive manufacturing and biomedical. This paper presents thein-depth research on the design theory, calibration experiments, as well as applications ofa double-layer pre-stressed six-axis force sensor, and builds the theoretical andexperimental basis for the design and application of high-precision six-axis force sensorwith independent intellectual property rights. The main contents of this paper are asfollows:A double-layer and pre-stressed six-axis force sensor with multiple limbs isproposed, and the viable number of the measuring limbs of the sensor structure isanalyzed by using the theory of convex analysis. The mathematical model of thedouble-layer and pre-stressed six-axis force sensor is built, and the static mapping matrixbetween the six-axis force/torque applied on the sensor and the axial force on themeasuring limbs is derived out using screw theory.The problem of force distribution of the double-layer and pre-stressed six-axis forcesensor is derived systematically. Because the structure of the sensor is staticallyindeterminate, the solution of the reacting force on the measuring limbs is solved byintroducing the concept of weighted generalized inverse of limb’s stiffness, and thephysical meanings of the calculation results are discussed. The forces on the measuringlimbs are decomposed into two parts, one is produced by the external force and the otheris related to the pre-tightening force. On the above basis, the pre-tightening force, whichcan ensure the sensor structure stably measure, is determined by a linear transformationmethod. Finally, the numerical example and experiment verify the correctness of theabove derivation. A method to optimize the structure parameters of the six-axis force sensor isproposed based on the design measurement range. The relationship between themaximum axial force of measuring limbs and the structural parameters of the sensorwithin the full scales of force/torque applied is established. The curves of the maximumaxial force and the structural parameters are drawn, and the optimized structureparameters are obtained when the maximum axial force achieves the minimum. Thestructural parameter optimizations of double-layer pre-stressed seven-limb andeight-limb six-axis force sensors are carried out respectively. Based on the optimizedstructure parameters, the structural design of the seven-limb six-axis force sensor iscompleted, and the prototype of the sensor system is manufactured.The calibration device for six-axis force sensor is manufactured and the static anddynamic calibration software is developed based on LabVIEW. The linearity,repeatability and hysteresis error of six-axis force sensor are derived. A multi-pointloading method to apply the loading force on each dimension step by step in the fullscales of force and torque is adopted. The static calibration experiments of sensorprototype are carried out, and the static performance indices of the sensor are obtained byprocessing the experimental data. The experimental result shows that the sensorprototype possesses good static performance.Sources of measuring error of the parallel six-axis force sensor and the method toimprove the accuracy are studied. The influence of the axial deformation of themeasuring limbs to the measurement accuracy of parallel six-axis force sensor isanalyzed. The influence and compensation algorithm of the gravity of the robot’s endeffector when sensor applications are deduced. The influence of the pre-tightening forceto the measurement accuracy of pre-stressed six-axis force sensor is analyzed and amethod to reduce the measuring error by increasing the pre-tightening force is proposed.By using screw theory and multi-degree-of-freedom system vibration mechanics, thevibration system simplified model of the double-layer and pre-stressed six-axis forcesensor is established, and the motion differential equation of the vibration system isdeduced. The response of the system in the initial excitation and the natural frequenciesof the sensor are obtained by solving the differential equations of motion. The dynamic calibration experiments are also carried out using the step signal response method, andthe natural frequency of the sensor is obtained. The experimental results are consistentwith the theoretical calculations and simulation.The application research of six-axis force sensor is carried out based on two typicalmotion model of surface tracking and peg-in-hole assembly. The inadequacy of thesensor when measures the six-axis force online is pointed out by considering the threeforms of the generalized six-dimensional force/torque. The trajectories of the two motionmodels are planned, and the feedback control algorithm based on the measuringinformation of six-axis force sensor is deduced. The robot force control experimentalsystem is set up, and the control flow and motion control software based on the feedbackalgorithm are completed. Finally, the force feedback control experiments of surfacetracking and peg-in-hole assembly are carried out.