Design and Control of a 1-DOF MRI-Compatible Pneumatically Actuated Robot With Long Transmission Lines

Abstract

This paper presents the design and control of an MRI-compatible 1-DOF needle driver robot and its precise position control using pneumatic actuation with long transmission lines. MRI provides superior image quality compared to other imaging modalities such as CT or ultrasound, but imposes severe limitations on the material and actuator choice (to prevent image distortion) due to its strong magnetic field. We are primarily interested in developing a pneumatically actuated breast biopsy robot with a large force bandwidth and precise targeting capability during radio-frequency ablation (RFA) of breast tumor, and exploring the possibility of using long pneumatic transmission lines from outside the MRI room to the device in the magnet to prevent any image distortion whatsoever. This paper presents a model of the entire pneumatic system. The pneumatic lines are approximated by a first order system with time delay, because its dynamics are governed by the telegraph equation with varying coefficients and boundary conditions, which cannot be solved precisely. The slow response of long pneumatic lines and valve subsystems make position control challenging. This is further compounded by the presence of non-uniform friction in the device. Sliding mode control (SMC) was adopted, where friction was treated as an uncertainty term to drive the system onto the sliding surface. Three different controllers were designed, developed, and evaluated to achieve precise position control of the RFA probe. Experimental results revealed that all SMCs gave satisfactory performance with long transmission lines. We also performed several experiments with a 3-DOF fiber-optic force sensor attached to the needle driver to evaluate the performance of the device in the MRI under continuous imaging.

Needle-Guiding Robot for Laser Ablation of Liver Tumors Under MRI Guidance

“…The reaction time quoted by the valve manufacturer is less than 10 ms and the corresponding bandwidth is above 100 Hz. Consequently, the valve dynamics is much faster than the rest of the system [24]. The pressure dynamics in supply pipes and cylinder chambers can be derived from the state equations of ideal gas assuming isothermal conditions [39]:…”

“…As a result, accurate models of the pressure dynamics typically rely on continuous pressure measurements in the cylinder chambers [42]. Consequently, the pressure dynamics was generally disregarded in the design of previous controllers for similar pneumatic systems [20], [21], [24]. In this work, the pressure dynamics due to valve, pipe, and cylinder chamber was experimentally evaluated with a step command issued to the proportional valve and a simultaneous pressure measurement.…”

“…Since in this application the piston is only required to perform slow movements, the pressure dynamics was approximated with a first order transfer function. This choice is based on the following considerations: firstly, the dynamics of the pipe can be described with a first order transfer function as shown in [24]; secondly, the proportional valve has a much higher bandwidth than the rest of the system. This approach allows simplifying the design and the tuning of the controller, which requires only three parameters, at the cost of lower tracking accuracy.…”

“…Differently from SMC schemes [24], [25], [42], [45], TDC does not require the assumption of bounded disturbances. This is beneficial here since friction forces caused by the robot structure can be difficult to estimate.…”

“…The comparative study specifically refers to pneumatic actuations with long supply lines, defined as in previous works [20], [24]. The performance of the controllers was evaluated for a single cylinder mounted horizontally, similarly to the actuators in the robot (Fig.…”