Study On Miniature Six-Axis Force/Torque Sensor And Its Auto-Calibration

The miniature six axis force/torque sensor is more and more important, as the robot development towards humanoid, high integrated, intelligentized. The miniature six axis force/torque sensor is one of important component of a robot, which can improve the overall level of the robot's dexterous manipulation and intelligence level. This dissertation is supported by the the High Technology Research and Development Programme of China ("Study on the miniature six axis force/torque sensor based on the MEMS"No.2003AA404220 &"Study on the full surface sensor based on the thin film technology"No. 2002AA423220). The main research contents of this dissertation are the miniature six axis force/torque sensor and its auto-calibration technology.The DLR/HIT dexterous hand is composed of four identical fingers with same structure, which is designed with idea of high integration system based on mechanical structure, sensing, driving, transmission and electronics. The integration system needs every sensor is digital locally, and small size. So, as the finger-tip sensor, the six force/torque sensor should be a small one and digital integration sensor. The ability of auto-calibration is plus. This dissetation focuses on these demands and carries on research works. The main contents are as follows: fabrication of the sensor, study on the key technology of miniaturization of the sensor and fabrication processes based on the MEMS thin film, study on the static & dynamic characteristics of the force sensor, and study on the auto-calibration of the multi-axis force sensor.First, a process of six axis force/torque sensor's sensing element design is presented. For the sensor's micromation, the structure and measurement theory of two types of miniature sensing elements(the thin shell cylinder type of sensing element & the full surface type of sensing element) are presented. A kind of integrated strain gauge is presented, which is fit for the thin shell cylinder type of sensing element to compose the miniature six axis force sensor. A miniature signal conditioning circuit & DSP circuit are also presented. The sensor has a miniature size(16.5mm in diameter, 17.5mm in height) & a miniature embeded DSP circuit. According to the author's references finding, no one six axis force/torque sensor has such small size existed. According to the characteristics of the thin shell cylinder type of sensing element, the process of fabricating the thin film strain gauge on the aluminum sensing element is presented. The structure of the thin film strain gauge is designed and fabricated. By analysing and chosing every materials of function layers, this dissertation carries out experiments on four kinds of material of isolation layer, which fit for the aluminum sensing element. The results reveal that the Al2O3 is the best material, for its good performances as the isolation layer. The wire bonding is adopted to introduce the signals of the measurment Wheatstone bridges into the singnal conditioning circuit board, which improves the reliability of the circuit board and simplifies the wiring works. Every layer of the thin film strain gauge is measured.A method of measuring the thickness of the thin film by the AFM is presented.The results proved that the processes above are feasible and powerful backup for the sensor system realization. Finally, the miniature six axis force/torque sensor based on the thin film technology is fabricated, which realized integration and digital output. This dissertation have conducted investigations in the static & dynamic characteristics of the miniature six axis force/torque sensor. The static outputs and coupling characteristics are outlined by appling static loads. A further study on the static decoupling methods have been done. Using the neural network as a new approch to improve the decoupling performance is presented, which can overcome some nonlinear characteristics of the sensor. By the calibration matrix perturbation analysing, the up limit of the measuring force error caused by the calibration matrix are computed. The static performance of the sensor is investigated. By sensor's dynamic modeling, the sensor's dynamic performance is investigated also. A dynamic compensator is designed, according to the pole assinment. Also the simulation result of the dynamic compensation is presesnted.This dissertation has made a study of the auto-calibration on the six axis force/torque sensor by two approachs. First, the auto-calibration based on a triaxial accelerometer is presented. With the static application of the triaxial accelerometer, the three angles of the gravity vector and the sensor's coordinate can be measured simultaneous. According to these three angles, the loads applied by the weights can be calculated automatically. This process alleviates the burden of appling the weights, that can acquire the calibration matrix automatically.The auto-calibration method based on the"Shape from Motion"is prsented, and is applied on the two-axis, three axis, two axis force one torque, and six axis force/torque sensors. Further more the collaborative calibration is also realized by this method. By random appling loads on the multi-axis force sensor, and recording the outputs of the sensor, the pseudoinverse of of the sensor's calibration matrix and the loads applied on the sensor can be recovered. This method not only improves efficiency of the calibration process, but also avoids the errors introduced by the appling loads, which finally improve the precision of the calibration process.