Six-axis Load-sharing Heavy Force Sensor Based On The Stewart Platform Structure

Heavy equipments like huge manufacturing equipments, huge operation equipments and huge excavation equipment are characterized by heavy load, large inertia, multi-degree-of freedom, multi-dimensional control of forces and positions. In order to increase productivity and avoid accidents caused by sudden change and of heavy forces, terminus executive arms of heavy equipments should be in compliance with force and displacement. And compliance control and multi-degree harmonious control of terminus executive arms is premised on six-axis heavy force measurement in real time. Therefore, development of six-axis heavy force sensors is of vital importance for real-time measurement of six-axis heavy force. The contradiction of multi-axis and heavy force leads to the difficulty of six-axis heavy force research. In this paper, A six-axis load-sharing heavy force sensor based on Stewart structure is researched, which presents a novel method for multi-axis heavy force measurement.To solve the problem of multi-axis heavy force measurement, a six-axis heavy force measurement method and sensor is proposed. Furthermore, the decoupling algorithm of the six-axis heavy force sensor is deduced from the force mapping matrix of the Stewart structure, the forward kinematics of the Stewart structure and the element stiffness matrix of deep beam. A finite element modal of the six-axis heavy force sensor is established, and analysis of statics and dynamics of the modal is conducted, which verifies the decoupling algorithm. The defination of six-axis heavy force load sharing ratio is presented. The influence of the load bearing beam size on the load sharing ratio is analyzed. All the results above present a theoretical base for performance indice analysis and designment of the six-axis heavy force sensor.To satisfy the designing requirements, the isotropy and sensitivity performance indices of the six-axis heavy force sensor are analyzed according to the force mapping matrix. The performance atlases of the change law of performance indices with the structure parameters are drawn. The entire analysis presents an important theoretical base for the selection of structure parameters.Multi-axis dynamic measurement is necessary for huge operation equipments, so piezoelectric quartz is selected as the sensitive element of the sensor after comprehensive analysis of all sensitive elements. To apply the preload on each measurement rod, a preload structure with adjustable preload and six-site local preload method is presented. Preload of each rod can be applied by ajusting the preload screw bolt and slider, which ensures close preloads on each measurement rod. According to the requirement of measurement and the assembly condition, the wedge expansion technique is used to fasten the sensor on the load bearing beam, which solve the problem of the sensor fasten and force transmission.A six-axis heavy force calibration equipment with single force source is established. Six-axis heavy force sensors with 30kN and 200kN measurement ranges are developed. A six-axis heavy force sensor with measurement range of 200kN is calibrated on the calibration equipment, and the calibration matrix of the sensor is obtained. Results show that the repeatability errors are within 1%, the non-linearity errors are within 1.2% and the interference errors of each direction are within 5%. The load sharing ratio of force is 1.8%,1.6% and 0.28% in X, Y and Z axis respectively. The load sharing ratio of moment is 17.2%,19.1%, and 11.5% in X,Y and Z axis. The calibration results verify the validity and feasibility of the six-axis load-sharing heavy force sensor based on the Stewart structure. The change law of isotropy and sensitivity performance indices with the load bearing beam parameters is obtained by changing the length and radius of the load bearing beam. The change law in experiment is in accordance with the theory curve. The dynamic characteristis of the sensor is researched with force hammer impacting method. The natural frequencies of the sensor in each channel are obtained.Combining the force condition of the manipulator with the calibration principle of "SFM", the on-machine "SFM" calibration method for six-axis heavy force sensor is presented. Calibration of two-axis force, three-axis and six-axis force with "SFM" method is conducted to the finite element modal of the six-axis heavy force sensor, And the sensor is calibrated as two-axis force sensor, which verifies the feasibility of the "SFM" method for the calibration of multi-axis heavy force sensor.In general, the decoupling modal of the six-axis heavy force sensor, the static and dynamic force analysis, the analysis of performance indices, the structure design of the sensor, the calibration equipment, the dynamic character analysis and calibration on machine presented in this paper provide a significant theory and experiment basis for design, development and application of the six-axis heavy force sensor.