Design Of The General Purpose Haptic Device And Research On Its Gravity Compensation

A Haptic device is a man-machine interaction device that supplies force feedback and enhances greatly immersion to user interacting with remote and virtual environment.Therefore,a desired haptic device must be required to have high back-drivability,low inertia,low friction,high stiffness,suitable workspace,absence of backlash and high control bandwidth and so on.Unfortunately,the gravity in the haptic device has a serious effect on the performance for the haptic device such as high back-drivability,low inertia,low friction,high stiffness,and control bandwidth.Thus,it is especially desirable to design the haptic device with the favorable gravity compensation for it.A brief on the main work is that a general-purpose haptic device with six degrees of freedom is designed and implemented.Further,three types of the gravity compensation methods are proposed,one is the gravity compensation method based on the ARC(Auto-Regulation Counterweight)mechanism,and the other two are all the gravity compensation based on the linear spring.To solve the complex constrained optimization problems in the gravity compensation methods,two novel swarm intelligence algorithms are presented,and only the first algorithm is employed as the optimization tool for the above compensation gravity methods.The details of the main work are described as follow:The general-purpose haptic device is designed and implemented,and its novelty is that a gravity compensation method based on the ARC(Auto-Regulation Counterweight)mechanism is proposed,which can regulate the position of the counterweight block on the counterweight link in real time to compensate the gravity for the haptic device.The haptic device using the serial link mechanical with six degrees of freedom has six rotary joints,each of its the former three joints can provide the maximum feedback forces of up to 20N,the continuous output forces of up to 8.5N,and the minimum angular resolution of up to 0.018 degree,and each of its the latter three passive joints can provide the minimum angular resolution of up to 0.01 degree.The haptic update rate for the haptic device is from 500Hz to 1000Hz.The cable driver devices with absence of backlash are employed as the speed reducer for the actuators in haptic device.At the same time,actuators are located on the ground base of the haptic device,three-dimension force sensor is located in end effector,and the impedance control method is design and implemented.To solve the complex constrained optimization problems in the gravity compensation methods,two novel swarm intelligence algorithms are proposed as the optimization tool.The first method called MSPSO algorithm removes inertia weight factor,velocity item and the local optimal parameter,and introduces the leap factor.The experiment results show that compared with the SPSO algorithm,the MSPSO algorithm is a simpler,more stable,faster convergence velocity,more high quality solution within shorter calculation time.A novel gravity compensation method based on the ARC(Auto-Regulation Counterweight)mechanism is put forward,the principle and mathematical model deduction of which are given.Further,the ARC mechanism and control system is design and implemented.When designing the ARC mechanism,some influences on the interference between the ARC mechanism and the haptic device,and the gravity of the ARC mechanism are fully considered.Subsequently,a mathematical model for the ARC mechanism with the multivariable and nonlinear constraints is built for the gravity compensation,where the gravity of the counterweight block,the gravity of the counterweight link,and the centre of gravity of the counterweight link,are selected as the optimized variables.By introducing the maximum length of the counterweight link,and curved surface area with multi-angles as fitness functions,the MSPSO is employed to achieve the optimal values of the above-mentioned three variables.When designing the control system for the ARC mechanism,the mathematical model in the control system is established,and the novel feed forward plus MSPSO-PID controller is employed to find the optimal parameter of the PID controller that is applied to regulate the ARC mechanism to compensate the gravity for the haptic device.In addition,the first-order low-pass filter is introduced to the differential item of the PID controller to remove the influence of the high-frequency disturbance in the control system of ARC mechanism.The experimental results indicate that the efficiency of the gravity compensation based on ARC mechanism is up to 96.6%.In addition,compared with the mass counterweight,the gravity compensation method based on ARC mechanism not only compensates the gravity but also brings less inertia to the haptic device.By comparison with the active gravity compensation,it does not weaken the output feedback force of the feedback device.Further,if the ARC mechanism is improved in some aspects,it can also provide the feedback force for the haptic device.A mathematical model Mwith the multivariable and nonlinear constraints is built for the gravity compensation based on the simple spring,where some influences on the position of the spring attachment point,the free length of the spring and the stiffness coefficient for the gravity compensation based on the linear spring,are fully considered.The stretch elongation and free length ratio that is introduced to decrease the number of the optimized variables,is employed as the optimized variable,together with the position of the spring attachment point,in the mathematical model based on the simple spring for the gravity compensation.The average torque error is introduced as the fitness function in the MSPSO algorithm that is applied to obtain the optimal values of the position of spring attachment point,the free length of the spring and stiffness coefficient.Simulations confirm that the efficiency of the gravity compensation based on simple spring with the MSPSO algorithm for the haptic device is about 98%.The novelty of the second method is that the spring and the crown block are not located in the manipulator of the haptic device but rattier in the base ground of the haptic device with the steel cable connected with in the reducing mechanism to mitigate their gravity effects on the haptic device.A mathematical model of the zero-free-length spring is established with length of the radius and the bearing of the crown block for the gravity compensation in haptic device.The sufficient proof is given in theory to confirm that the zero-free-length spring can compensate the whole gravity of the haptic device.