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Robot-assisted prostate intervention under magnetic resonance imaging (MRI) guidance is a promising method to improve the clinical performance compared with the manual method. An MR conditional 6-DOF prostate intervention serial robot is developed and a binocular vision system (BVS) is established to evaluate the needle placement accuracy and located the penetration point precisely. The robot is designed by the MR conditional criteria. The serial configuration of the robot provides adequate flexibility and large workspace and excellent friendliness to the physicians. The kinematics are deduced and the needle placement control flow is proposed according to the configuration of the robot. The robot-assisted prostate intervention is divided into two phases including needle placement and needle penetration. A custom-made robust BVS is developed to obtain the needle tip position automatically in the needle placement phase where the needle cannot be detected by the MRI for lack of hydrogen atom. A simple and general algorithm used for needle tip camera coordinate estimation is proposed. Experiments on the BVS validation and robot accuracy evaluation are performed. The experiment results show that the errors of the BVS are under 0.3621 mm and the position error of the proposed robot is 2.815 mm which indicate the adequate accuracy for the prostate intervention. Graphical abstract.

The control performance of a butterfly-shaped linear ultrasonic motor is not good when the target speed is less than 1 mm/s by traditional methods. To improve low-speed controlling characteristics of linear ultrasonic motor, a closed-loop control strategy by using both the step control and the fuzzy PID control was proposed. The controller was constructed with the function of providing a closed-loop control of the speed by adjusting the driving voltage amplitude in stepping driving mode. The corresponding step controlling parameters were determined by experiments. Comprehensive experiments on the developed control strategy were conducted under different target speeds. There was a maximum of 24.5% speed error at the target speed of 10 μm/s, meanwhile, the coefficient of variation and the response time were 16.3% and 0.11 s, respectively. With the triangular or sinusoidal waves speed tracking curves at amplitude of 1 mm/s, the maximum speed tracking error were 0.1 mm/s and 0.15 mm/s respectively. Thus, a good speed tracking performance was obtained. These results support the functional ability and potential of the proposed method as a low-speed precision control method for linear ultrasonic motors.