85
CHAPTER 6
PRELIMINARY EXPERIMENTAL EVALUATION
AND CONCLUSIONS
The chapter will explain:
- a preliminary experimental evaluation
- the conclusion and the possible future works regard to the work presented in this thesis.
6.1 – Experimental evaluation and results
To generate the magnetic field and, as a consequence, to have the magnetic forces and torques to translate and rotate the capsule, it was necessary to apply an electrical current to EES.
As shown in Fig.64, the user, through a joypad regulates the current intensity and an human machine interface (HMI), implemented with Labview (National Instruments Labview 2012, Texas, USA) gets the input from the joystick showing the value of the selected current on the PC screen and sends the right command to the power supply in order that it delivers the requested current to the EES.
86
Fig.64. Functioning of EES
The selected power supply (Fig.65) is PS-8080 2U (Elektro-Automatic GmbH & Co. KG, Germany). It has an output voltage range from 0 to 80 V, an output current range from 0 to 60 A (range of power between 0 to 1500 W) and it allows to be interfaced with the HMI through an USB communication.
Fig.65. PS-8080
The HMI combined simultaneously the setting of the current by the input of the joypad with the setting of the output current from the power supply. The
87
communication with the power supply was implemented by using predefined Labview packages, that are directly furnished by the company of the power supply, reported in Fig.66. Details of “Select device”, “Set remote” and “Joypad management” are shown in Fig.67.
88
Fig.66. Block diagram composed by: “Device scan”, ”Select device”, “Set remote” and “Close communication”
89
As well as the communication with the power supply, a simple routine by means of Mathscript node (Fig.68) was implemented to set the current by the user through the joypad which, in addition to be widely used (such as in video games or automotive applications), is highly intuitive. In particular the analog signal generated by the joypad is acquired and computed by the software shown in Fig.68 in order to calculate the required current and send the appropriate command to the power supply.
Fig.68. MathScript Node
The user can control the amplitude of current by deflecting the left knob. The maximum deflection of the knob corresponds to the maximum values of current imposed in the software interface. The maximum value is chosen within a set of values (from 5 A to 30 A) by the user pushing different times the X button depending on the working configurations and difficulty to control the capsule. The triangular button is instead used by the user when he/she choose a previous maximum value in the set. However the maximum value is always displayed in the HMI and visible to the user. The analog signal generated by the joypad is acquired, computed by the software in order to
90
calculate the required current and send to the power supply the appropriate command.
In order to evaluate the final design of EES, a preliminary experimental test was carried on by means of prototype of the EES, that has a height of 110 mm, an external diameter of 97.5 mm and an internal diameter of 45 mm. To simulate the abdomen of the patient, a sheet of plexiglass was used, and placed on tables in order to have a distance compatible with anatomical constraints (5-7 cm) between the EES and the capsule, as shown in Fig.69. The used capsule, made of plastic, had spherical shape, where inside there was a further sphere, in which the permanent magnet was fixed. The two spheres can move freely one inside the other.
The preliminary experimental evaluation was performed to demonstrate the working principles explained in this thesis and thus validate the final design of EES. It is worthwhile to note that the positive results of the experimental tests made on EES prototype, in principle are valid also for the final EES resulting from the design described in this thesis. These due to the fact that the difference between them, listed in Tab.14, are influencing the capability to generate a greater or a weaker magnetic field gradient - and thus the maximum acceptable distance between EES and the capsule - or the possibility to use a lower current density but not the working configuration neither the navigation strategy. Moreover it is possible to observe that using the same current density and the same distance between EES and capsule, EES prototype is capable to produce a gradient equal 0,57 T/m while the final EES will be able to generate a gradient equal to 0,74 T/m (30% greater), considering C1 configuration, demonstrating that the final EES is more efficient than the prototype.
91 Final EES (mm3) EES prototype (mm3) Difference (%) Copper volume 1046 * 103 646 * 103 62%
Iron core volume 427 * 103 175 * 103 144%
IPM volume 400 936 -57%
Tab.14. Differences about the size between Final EES and EES prototype
The below figures show the capsule movements in the working conditions C1 and C2.
Fig.69 shows the cylindrical IPM inside the spherical capsule and the EES in configuration C1. As expected, the capsule was attracted in the center of the EES and IPM was placed vertically, because its magnetization is direct along the Y-axis.
Fig.69. Attraction of the capsule in configuration C1
Also in this configuration, the position of EES was able to allow the capsule translation along the three axes. In particular Fig.70 shows the locomotion along X axis.
92
Fig.70. Locomotion of the capsule in configuration C1
Fig.71 shows the configuration C2 (the EES is rotated of an angle of 90°). Also in this condition the capsule was lifted and was translated, respectively along three axes. Moreover, Fig.71 shows an example of locomotion in configuration C2 with capsule positioned at the edge of the coil.
Fig.71. Attraction and locomotion of the capsule in configuration C2
Finally Fig.72 and 73 show respectively pitch rotation, around axis X, and yaw rotation, around Y axis, when EES is in configuration C2.
93
Fig.72. Pitch rotation in configuration C2
Fig.73. Yaw rotation in configuration C2
6.2 – Conclusion
Before to draw the final conclusion it is worthwhile to summarize the target of this thesis, which was the design and the development of a reliable, compact, suitable for transporting and cost effective external electromagnetic system. The electromagnetic system , supplied with an electric current, has to generate an appropriate magnetic field, outside the human body, for the navigation of a endoscopic capsule along colon removing the inconvenient of the classic endoscopy procedure that is considered traumatic and frequently poor tolerate by patients (e.g. due to the need to insufflate with air the colon or possible pain caused by the stretch of the colon).
In short the steps that allowed the final design of the external electromagnetic system have been the following:
94
- definition of the capsule navigation strategy in order to manoeuvre the capsule along 5 degrees of freedom so identifying 2 working configurations, C1 and C2, of the EES;
- definition of the features of the permanent magnet, to be inserted inside the capsule, necessary to create a link with the external magnetic field generated by the EES. having a trade-off between the target values of the forces and torque and the encumbrance inside the capsule;
- definition of the weight of the capsule to be considered to calculate magnetic force and torque necessary to move it;
- definition of the smallest wire diameter, to be used to build the EES, capable to sustain the necessary current density to generate the requested force balancing that such current density has to be lower than the imposed limit equal to 3,5 A/mm2. This limit is recommended when the wire is used in compact design and there is not circulation air around it;
- definition of the best iron core features to be inserted inside the EES allowing a minor leakage of the magnetic flux;
- choosing the best modelling tool (between one based on finite element method and another based on Biot-Savart’s law) to be used during the design of EES in order to simulate magnetic torque and force. The selection process needed to perform also some experimental measurements (by using a hall-effect probe) in order to compare the simulation outputs with the reality;
- considering all the above and that height and external diameter of the EES were imposed to change within a range from 100 to 130 mm (balancing the requirements of medical team to have the final EES with a good ergonomic size and the fact that EES has to be higher as much as possible allowing the appropriate magnetic attraction to have the best maneuverability of the capsule), in order to find the suitable dimensions of EES, an iterative procedure was implemented to try different combinations of height, external diameter and internal diameter (that is equal to iron core diameter) for each of them calculating, through the selected tool simulations, the generated magnetic forces and torques.
95
Finally the design of the final EES was achieved and a preliminary experimental evaluation has demonstrated that an electromagnetic system was designed in order to navigate, a magnetic colonoscopy capsule fulfilling all the requested requirements.
As future works, the electromagnetic system will be developed and tested in a real operating scenario and a custom-made manipulator will be developed for supporting and moving the electromagnetic system within the colonoscopy medical procedure.
