A micro-robot is a promising device for minimally invasive surgery or drug delivery inside of the body for medical purpose. Due to its small size, the micro robot should float in the blood vessel to destination. Therefore, the propulsion mechanism of micro-robot have become an active area of study these days. Recent studies of the propulsion of these robots have focused on the insertion of permanent magnets with Helmholtz coil which can generate magnetic field for propulsion of the micro-robot. However, this type of propulsion system has limited in that the DC magnetic field delivers propulsion but does not deliver any electric power to the robot. Inevitably, the micro-robot only perform limited mission-like drilling and moving without any electric power sources. If propulsion and power can be delivered to the micro-robot at the same time, the robot can contain active devices and can thus perform much more complex and important medical missions. The WPT system generally consists of the source part and the load part, where power is transferred from the source to load. Therefore, if the propulsion force can be generated at the load coil, the electric power and propulsion force are transferred simultaneously. In this paper, we propose a propulsion method for wireless power transfer (WPT) coil to delivers both the propulsion force and electric power simultaneously. By changing phase difference between the source and load currents, which is determined by the capacitance of load coil, the magnitude of generated current and propulsion force can be controlled.
II. PROPULSION OF A MICRO-ROBOT USING A WPT SYSTEM
As the source coil and load coil are carrying AC current, they behave as electromagnets. However, they are designed to transfer electric power, and no propulsion force is generated in typical WPT coils or the force is ignored because the weight of the system is much larger than the force. If we enhance and control the propulsion force, and if the load coil is very light to be affected by the propulsion force, we can utilize the system for implantable micro-robot application. The WPT coils are modeled as two inductors as shown in Fig. 1. If the AC source current (iS) flows in the source part, the time varying magnetic field is generated from source coil and the load current (iL) is generated according to the Faraday’s law. The induced current is expressed as IL = IS jωM / ((RL+jωLL+1/(jωCL))), where IS and IL denote the phasor forms of load current and source current. The capacitor (CL) is inserted to maximize the induced current by constructing resonance with LL. The phase difference between iS and iL is expressed as α = arctan( RL/ (ωLL-1/(ωCL)) ). The phase difference, α, is 90 ° in general WPT system for maximum power transfer, therefore, the attraction force or repulsion force, which is expressed as F = k |IS| cos(ωt) |IL| cos(ωt+α), where k is the geometrical constant, becomes zero in average over the time. If the phase difference, α, is controlled to have proper value by changing the capacitor CL, the attraction or repulsion force can have non-zero value, and it can be used as propulsion force for small coil.
III. TEST SETUP AND RESULTS
We designed a 100-turn circular winding with a diameter of 127 mm as a source coil, carrying a 1-A current at the frequency of 20 kHz, and the load coil with a 10-turn circular winding with a diameter of 10 mm. Distance between source and load coils is 50 mm. Fig. 2 shows the simulation setup and distribution of magnetic field using 3-D field solver ANSYS Maxwell. The induced current is maximum when α = 90°, however, the propulsion force is nearly zero. By changing the phase difference, α, we can control the magnitude of the induced current and the propulsion force. The result shows that 7.7 A of induced current and 54 μN of propulsion force can be delivered to the load coil when α = 60°.