Sliding Mode Control of Piezoelectric Valve Regulated Pneumatic Actuator for Mri-compatible Robotic Intervention

This paper presents the design of a magnetic resonance imaging (MRI) compatible pneumatic actuator regulated by piezoelectric valve for image guided robotic intervention. After comparing pneumatic, hydraulic and piezoelectric MRI compatible actuation technologies, we present a piezoelectric valve regulated pneumatic actuation system consisted of PC, custom servo board driver, piezoelectric valves, sensors and pneumatic cylinder. The system was proposed to investigate the control schemes of a modular actuator, which offers fully MRI compatible actuation; the initial goal is to control our MRI compatible prostate biopsy robot, but the controller and system architecture are suited to a wide range of image guided surgical application. We present the mathematical modeling of the pressure regulating valve with time delay and the pneumatic cylinder. Three sliding mode control schemes are proposed to compare the system performance. Preliminary simulation results are presented to validate the control algorithm. INTRODUCTION Recent years have witnessed the flourishing of MRI compatible robotics and its application in image-guided interventions and surgery. MRI provides high-fidelity soft tissue contrast and high spatial resolution. However, the highfield magnetic field limits the available technologies. There are a number of alternative MRI compatible actuation paradigms ranging from the traditional methods including hydraulic and ultrasonic piezoelectric to non-conventional actuation including electro-rheological fluids and electro-strictive polymer[1]. Although piezoelectric motor is the most prevalent actuator choice by far for its good dynamic performance, ease of control, compactness and low noise induced to the imaging process with proper shielding [2], the underlying friction driven working principle, presents challenge in design and application for force feedback due to nonlinearities, non-backdrivability and high wear. The high braking torque can be either an advantage or a detriment, with respect to a specific application [3]. Pneumatics is a preferred actuation method over hydraulics with regard to its cleanliness, ease of connectivity and ability to be operated at higher speeds. Recently, Stoianovici et al. [4] presented a new type of pneumatic motor encoded by optical sensors. Fischer, et al proposed a MRI compatible pneumatic robot for prostate intervention [5] and presented a comparative study which clearly indicated the advantage of pneumatic actuation under MRI environment [6]. Pneumatic actuation is promising to address the intrinsic compatibility issue by taking advantage of entire nonmagnetic parts and dielectric materials. The desired position tracking accuracy of our actuator is 0.1 mm for image guided intervention [7-9]. The nonlinearity of pneumatics system presents significant challenges to accurate servo control. Also the difficulties in precise modeling and the varieties of surgical applications call for control robustness. Paul, et al. proposed a reduced order sliding mode controller (SMCr) for pneumatic actuators [10]. Acarman, et al. presented a SMCr with an observer for estimating position and 1 Copyright © 2010 by ASME Proceedings of the ASME 2010 Dynamic Systems and Control Conference DSCC2010 September 12-15, 2010, Cambridge, Massachusetts, USA