Development of Eddy Current Sensor systems in artificial heart for noncontact gap sensing. භ`. C. B. Ahn1,3, K. H. Kim1,3, K. C. Moon3, K. S. Jeong1,3, H. C. ...
Proceedings of the 2005 IEEE Engineering in Medicine and Biology 27th Annual Conference Shanghai, China, September 1-4, 2005
Development of Eddy Current Sensor systems in artificial heart for noncontact gap sensing භ`
C. B. Ahn1,3, K. H. Kim1,3, K. C. Moon3, K. S. Jeong1,3, H. C. KIM1,3, J. J. Lee3, C. M. Hwang2,3, K. Sun2,3,4 1. Biomedical Engineering of Brain Korea 21 Program, Korea University, 2. Department of Biomedical Engineering College of Medicine, Korea University, 3. Korea Artificial Organ Center, Korea University, 4. Department of Thoracic and Cardiovascular Surgery, College of Medicine, Korea University j
Abstract— The axial flow pump has been developed in Korea Artificial Organ Center. It consists of an impeller, a motor and a magnetic bearing. The magnetic bearing fully levitates the impeller not to contact with other parts of pump. However, in order to control the gap between the impeller and other parts, continuous gap sensing is necessary. The conventional gap sensors are relatively large to implant in artificial heart. Thus, the compact eddy current sensor system proper for artificial heart was developed and the performances were evaluated. It showed good results and has small size. However, the dependency of the sensor upon temperature and target material was shown also. Moreover, the output of sensor had nonlinear responses. These must be calibrated in further study.
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I. INTRODUCTION
he artificial heart is categorized as pulsatile and nonpulsatile device. The pulsatile blood pump provides pulsatile blood flow as the natural heart. However, the nonpulsatile blood pump provides a continuous blood flow. There are two major types of nonpulsatile pump: the axial flow pump and the centrifugal pump. An axial flow pump has been developing in Korea Artificial Organ Center. The axial flow pump consists of an impeller, a motor, a magnetic bearing. The magnetic bearing is adopted in order to levitate impeller magnetically and to remove contacts between the impeller and other parts of pump completely. This reduces the destruction of blood cells, namely, hemolysis and improves biocompatibility. However, to levitate the impeller and control the gap, continuous gap sensing is essential [4]. There are several contactless gap sensors as ultrasonic, capacitive, optic and eddy current sensor [1]. However, in this study, the eddy current sensor system proper for artificial heart was developed and the performances were evaluated.
0-7803-8740-6/05/$20.00 ©2005 IEEE.
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Fig. 1. Block diagram of the artificial heart control system
II. METHOD The general eddy current sensor circuit is as figure 2. There are a resonant part and amplification part in this circuit. The sensor is placed in a resonant part as inductor and the resonant part changes its resonant voltage as the inductance changes by eddy current according to the gap between sensor and sensing target. The resonant voltage is amplified to appropriate scale. The transistors are used in each part.
G Fig 2 The conventional circuit
In this study, the OP-AMP is used in a resonance and amplification part to reduce the circuit size and meet the demands as implantable device. The circuit is as figure 3, and the OPA2604(Texas Instrument, USA) is used. Moreover, the input voltage is reduced from 24 V to 12V.
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The sensor coils are made also with various coil thicknesses and turns. These several sensors are tested with various gaps, temperatures and target materials. III.
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RESULT
Each sensors showed different results. The sensor of high inductor showed higher output with same gap. The results according to various sensors and gaps are shown in figure 4-1 ~ 2.
Fig. 4-2. Three individual sensors with same inductance were tested for four different inductance values.
Figure 5 shows the dependency of sensors upon temperature. Higher temperature yields higher output voltage. Moreover, the output voltages were different with different target materials as figure 6. With magnet material, the sensor shows higher slope. 6
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Fig. 5. The sensors show different output voltage with different temperature.
Fig. 4-1. Each sensor with different inductance shows different results according to various gaps
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IV. DISCUSSION The compact sensor driver circuit was developed using OP-AMP rather than transistor. This circuit showed satisfactory results and could be implantable in human body because it occupies smaller room than existing circuit with transistors. The sensor coils were developed as small dimension. Since the sensor is located in axial blood pump, its size must be as small as possible. The sensor coils showed various measurement ranges according to the inductance values. However, each sensor show nonlinear output characteristics. Moreover, its performance has dependency on temperature and target material. These problems would be fixed by calibration. Further study is planned to improve the performance of the sensor by considering the material of sensor coil.
ACKNOWLEDGMENT • This study was supported by a grant of the Korea Health 21 R&D Project, Ministry of Health & Welfare, Republic of Korea. (02-PJ3-PG6-EV09-0001) • This work is supported by the Brain Korea 21 Project of the Ministry of Education and Human Resources Development, Republic of Korea.
REFERENCES [1]
Yamagishi, H. et al., "Development of built-in type and noninvasive sensor systems for smart artificial heart", Asaio J. vol. 49, no. 3, pp.265-70, May-Jun. 2003.
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Chin E. Lin et al., "AN Active Suspension of Linear Oscillatory Actuator Using Eddy Current Sensor", IEEE. vol. 2, no. 2, pp.1072-1077, May. 1997. DeBakey, M. E. et al., "A miniature implantable axial flow ventricular assist device", Ann Thorac Surg. vol. 68, no. 2, pp.637-40, Aug. 1999. John G. Webster, Editor, "medical instrumentation : Application And Design", Third Edition, pp.577-622, 1998. S.S Udpa, et al, “Electromagnetic Nondestructive Evaluation(ญ)", August 1-3, 1999.
