Basic Theoretical And Experimental Research On Large Range Parallel Six-Axis Force Sensor

The large range and high accuracy six-axis force/torque sensor is urgently neededand has extensive application prospects in aviation, automobile, construction,shipbuilding and some other fields. Parallel mechanism is characterized by excellentrigidity, good structure stability, large load-carrying capacity, not-accumulative error andhigh accuracy. It is one of the best structure patterns in design and manufacture of largerange and dimension six-axis force sensors, especially fully isotropic parallel six-axisforce sensor has great application potentials. This paper focuses on isotropy analysis oflarge range parallel six-axis force sensor, key theory and calibration experiment ofstatically indeterminate pre-stressed six-axis force sensor, aiming to enrich and perfectthe design theory of parallel six-axis force sensor, as well as building the theoretical andexperimental basis for the design and manufacture of six-axis force sensor withindependent intellectual property rights.Statics analysis of statically determinate Stewart platform-based force sensor isconducted using screw theory. The structural features of six-axis force sensor based ontraditional 6/6 Stewart platform and two-circle 3-3/3-3 Stewart platform are analyzed andcompared, and a 6/3-3 Stewart platform-based force sensor with equal leg lengths isproposd. The mathematical expressions of force mapping matrices and other featurematrices for these three structures are obtained.Isotropy indices of six axis force sensor are studied systematically. In combinationwith physical significance, isotropy indices of the past are analyzed and compared one byone. By introducing decoupling performance constraint condition, the force and torqueisotropy indices satisfying decoupling performance are defined, and the concept of fullisotropy is proposed. The sensitivity indices in all directions and comprehensivesensitivity are analyzed concretely when full isotropy are satisfied. The physicalsignificance of isotropy is summarized from the viewpoint of global stiffness andsensitivity. The proposed isotropy indices and the concept of full isotropy are extended toevaluate Stewart platform-based force sensor with arbitrary limbs. By using analytic approach, the isotropy performance of the traditional 6-6 Stewartplatform-based force sensor is studied in detail based on the modified isotropy indicessatisfying decoupling performance, the result indicates that it is impossible to realize fullisotropy in theory, and the best comprehensive isotropy performance is obtained bycompromising the isotropy indices. The isotropy performance of the 3-3/3-3 and 6/3-3Stewart platform-based force sensors are studied systematically. The analytic relations ofkey structural parameters leading to fully isotropic configuration are obtained, classes ofisotropic configurations can be easily obtained from the analytic results. The validsolutions and the valid range of the solutions are determined by using a numericalalgorithm. The numerical examples and fully isotropic structural examples are given.A series of statically indeterminate pre-stressed six-axis force sensor are proposed.The force analysis of the statically indeterminate pre-stressed structure is carried out byconsidering the stiffness properties and compatibility in deformations, the analyticalrelationship between the axial force and the six-dimensional external force is obtained ina neat form. Furthermore, the statically indeterminate force-distribution difficult problemof Stewart platform with arbitrary limbs is solved thoroughly. On the basis of above, thedifference and relation between the deduced theoretical solution and the solution basedon force mapping matrix's Moore-Penrose generalized inverse are analyzed andcompared detailedly. The solving formula based on Moore-Penrose generalized inverseincluding the effect of stiffness of all the limbs and force mapping matrix is given.Systematic analysis of statically indeterminate pre-stressed Stewart platform-basedforce sensor is presented. The fully isotropic design of statically indeterminate Stewartplatform-based force sensor is completed, and the analytic relations of key structuralparameters leading to fully isotropic configuration are obtained. To ensure the structuralstability and measurement validity, the mathematical relation between the pre-tighteningforce and expected range of external loads is obtained by solving the conditionalextremum of the Lagrange function. The key structure parameters of sensor prototype aredetermined based on the deduced analytical expression leading to full isotropy. The detaildesign of the pre-stressing limb and the whole design of sensor prototype are completed,and the prototype of large range six-axis force sensor is manufactured. The calibration methods of statically indeterminate six-axis force sensor arediscussed using linear and nonlinear scheme. A statically determinate calibration methodof statically indeterminate six-axis force sensor is proposed. The method based on neuralnetwork is introduced to calibrate the statically indeterminate six-axis force sensor. Thestatic calibration system of six axis force sensor is designed. The static calibration deviceis manufactured and the data acquisition hardware and software system is developed. Thestatic calibration experiments of sensor prototype are carried out, the comparison anddiscussion of the calibration results using the various calibration methods are presented.By means of mechanism modeling method, finite element method and step responseexperiment method, the dynamic characteristic of sensor prototype is studied. Thedynamics theory model of sensor prototype is built. The non damping free vibrationequation of the sensor system is derived out, and the theoretical natural frequency andvibration mode of the prototype are given. The finite element model of sensor prototypeis built and the modal analysis is presented. The preliminary study of dynamic calibrationexperiment is carried out and the dynamic performance indices of the sensor prototypeare obtained.