Research On Bilateral Control Method For Surgical Robots

Using robot system to assist surgery has significant advantages in improving the quality of surgery, reducing the trauma of patients, reducing the labour intensity of doctors, etc. Master-slave position control is used in most of the commercial surgical robot systems to accomplish the surgery by making the slave hand follow the move of the master hand. Adding additional force feedback, i.e. bilateral control, will make the doctor performing the surgery feel telepresence. As a result, the doctor can make full use of his perception to make the surgery process more efficient and safe.Bilateral operation system is a complex system with multivariate. Firstly in this work, input-output relationship (i.e. causality analysis) of the system is discussed. The input signal of most surgery is analyzed to be velocity or displacement, which is the first feature of surgery. Causality analysis figure of the whole system is drawn by analyzing the control process inside the body. Next, based on the admittance/impedance classification method of the bilateral operation system, the block diagram under ideal condition is obtained. Except from velocity sas input signal, another two features of surgery are high requirement of transparency and low frequency of the input signal.Based on the features of the surgical robot system, control structures, which are beneficial for the transparency and stability, are built. A force-position two channel structure is chosen from them as the control structure in this work. Then, control strategy which can realize the ideal transparency is derived by using the design idea based on transparency. Low pass filter is introduced and the controlling block diagram is transformed in order to solve the problem of using acceleration signal in the control strategy. Disturbance observer is introduced in order to solve the problem of differences between the dynamic model and the real model. Finally, a force-position control method based on disturbance observer and low pass filter is obtained.Based on theoretical analysis, it is proved that an ideal low-frequency transparency can be achieved by the control method above. It can also be proved that stability of the system can be achieved when time delay and time delay condition are ignored. By setting parameters suitable for surgery, simulation of the control method is carried out using the Simulink toolbox of Matlab. The efficiency of the method is proved by the simulation. The control method in this work is compared with the method proposed by Lawrence which can achieve low-frequency transparency by simulation. Based on that, the effect of parameters estimation for the slave side and the selection of low pass filter’s cutoff frequency is studied. Stability margin of the system is analyzed using bode diagram. Then the control method is perfected by adding low pass filter in the slave side.Experiments of bilateral control are carried out to verify the final control method obtained in this paper. The control software is written in VC++. The computer and PMAC motion control card are used as host and slave computer to build the hardware platform for bilateral operation system. With Omega.7master hand controlling needle-driven robot, experiments of pig liver puncturing are conducted using different control parameters. Velocity tracking curves, force feedback curves and control errors are obtained with the experimental data. The results show that the force-position control method based on disturbance observer and low pass filter can achieve good transparency with low-frequency input signals.