Design of magnetic actuator driver system for laser scanning capsule endoscopy applications

Abstract

In capsule endoscopy, laser scanning microscopy can be applied as an alternative to the commonly used camera-based tissue imaging. The most significant advantage of laser scanning microscopy is its ability to image the inner regions of tissues in addition to their outer surfaces. This process is achieved by changing the focal point of a lens that focuses laser beams, allowing the beams to penetrate the tissues to different extents. The lens inside the capsule is mounted on a magnetic actuator. The movement of the lens's focal point is realized through the back-and-forth motion of the magnetic actuator. In this thesis, an energy-efficient magnetic actuator driver system has been designed for laser scanning capsule endoscopy. The designed system is a part of an endoscopic capsule that integrates multiple subsystems. Within the capsule, there are electro-optic and electro-mechanical systems for laser scanning imaging, integrated circuit systems for detecting, processing, and accurately transmitting data from tissues to outside the body. Considering the volume of the capsule, the power capacity of the battery that energizes these systems is quite low. Therefore, the system designed in this study aims to achieve high energy efficiency. To ensure the driver system's energy efficiency and high linearity, a new technique implementing harmonic and amplitude control methods has been developed in this study. This technique aims to suppress the dominant third and multiples of third harmonics in the current supplied to the coil. The inductance of the coil has been measured at 7.7 mH, and the parasitic resistance at 95 Ω. To move the lens focus by 3.22 mm, it is planned to apply a current waveform with a peak value of 17.1 mA and suppressed third and multiples of third harmonics to the coil. The system providing amplitude and harmonic control includes an oscillator circuit, a frequency divider circuit, a combinational logic circuit, and a driver circuit. These circuits were designed using the UMC 180nm fabrication technology in the Cadence Virtuoso simulation program. Pre-layout simulations of the circuits were performed considering random variations in process-voltage-temperature. The layouts of the designed circuits were successfully completed, with the entire driver system occupying an area of 440 μm × 85 μm. The actuator driver was placed on a test chip, with the chip packaged in a QFN32 package, for which a connection diagram has been drawn. Post layout simulations, considering production variations, were also performed. The simulations show that the third harmonic of the output current has been reduced to 75.2 μA. This current level is equal to 1.46% of the third harmonic current that would result from control with a square wave. The initial phase of the measurements was carried out at the ITU VLSI Laboratory. Before the measurement process, coil was optimized and resistance was decreased to 81.2 Ω. During these initial tests, the actuator was operated both with a resistive load and with a coil, resistor, and actuator load. The supply voltage was set to 1.8 V for operation with LDO and 1.4 V for stand alone operation with battery. The electrical measurements indicated that the system operated successfully. The frequency of the output voltage and current waveform was 32 Hz and system utilized the amplitude and harmonic control. Coil current's RMS value was measured as 14.4 mA and efficiency was calculated as 93%. In the final stage of the experiments, the entire system (magnetic actuator, actuator driver system, and laser) was tested. The designed PCB, the wound coil, and the actuator were placed inside a capsule manufactured in the Electro-Optical Devices Laboratory. The coil connected to the PCB was wound around a laser and positioned 5 mm away from the actuator. When the system was activated, the coil driven by the actuator driver system interacted magnetically with the Nd magnets on the actuator. This interaction caused the actuator to move at its resonant frequency of 32 Hz. The collected data indicated that the focal point of the lens on the actuator moved by 3.22 mm. Thus, it was demonstrated that the designed actuator driver system performed its function efficiently and successfully.