The optical strain sensor is comprised of five components, a transmitter/receiver plate, a reflector plate, transmitting/receiving fibers, red LED’s and photodiodes. The transmitter/receiver plate is a 6 mm diameter, 2 mm thickness elastomeric plate with eight outer holes for mounting the fiber optic. The jacket of fiber optic cable used has been removed and the cable consists only of the core and cladding, with a diameter of 1 mm. The eight holes are in pairs, one hole for the light transmitting fiber and one hole for the receiving fiber. One central hole is included for the catheter tube to pass
Figure 4.2: A view of the catheter tip, camera and strain sensor in the experimental setup.
through. The reflector plate, of the same material, diameter and thickness, is coated with a layer of 3M engineering grade reflective tape. One central hole is made for the catheter. The red LED light passes through the light-transmitting fiber; the light reflects off the reflector plate and some of it is sent back through the light-receiving fiber. The light-receiving fiber leads back to the photodiode, which detects the change in light intensity captured by the light-receiving fiber and relays a corresponding analog voltage to the data acquisition card. As the catheter bends the plates become off plane with respect to each other, changing the angle of reflection and the amount of light received by the light-receiving fiber. This change in the plane principal, coupled with the distance changes between the fibers and the reflecting plate during bending, creates large (0-5 Volt) changes in the analog voltage with negligible noise and drift. The sensors used show a linear correlation between the analog voltage and the tip angle, and approximately 5 mV changes in signal per degree of tip angle change. The data acquisition card easily detects these voltage changes without any signal filtering, or amplification on the controls or electrical side. The benefit of this optical sensor is that the final model slides over the sheath of the unaltered conventional steerable ablation catheter. The design and construction of a commercially produced catheter will not need to be altered and the final product will not be much larger in diameter,
1.5 2 2.5 3 3.5 4 0 10 20 30 40 50 60 70 80 Tip Angle: Φ (deg) Analog Voltage (V) Tip Angle vs. Voltage from Top Fiber Tip Angle vs. Voltage from Bottom Fiber
Figure 4.3: Relation between analog voltage and tip angle (solid lines are fitted curves)
therefore should not restrict blood flow during catheter insertion. This allows for easy installation and a cost effective and efficient solution for position feedback and force feedback. The use of four light-receiving fibers allows the sensor to provide 3-axis force and 3-axis torque measurements through relative light intensity values between the fibers. The final model of the sensor will be approximately 2.8 mm in diameter for a 7-Fr catheter. Since the diameter of the fibers used are smaller (250 µm), signal amplification and filtering may be necessary to improve the accuracy of the sensor. It is worth noting that although the fiber bragg grating (FBG) technology is a means to monitor and track the shape of a flexible tool,e.g. a catheter, along its length [23,24], the noticeable costs currently associated with this technology hinders its extensive use in experimental setups/clinical applications.
4.3.1
Mapping Analog Voltage to Tip Angle
The purpose of our first experiment with the optical strain sensor was to find an ex- perimental correlation between the analog voltage input and the catheter tip angle in single plane bending. For this experiment, a light transmitting fiber and light receiv-
0 2 4 6 8 10 1.5 2 2.5 3 3.5 4 Handle Displacement (mm) Analog Voltage (V)
Handle displacement vs. Voltage from Top Fiber Handle displacement vs. Voltage from Bottom Fiber
Figure 4.4: Relation between handle displacement and analog voltage (solid lines are fitted curves)
ing fiber were fixed in both the top and bottom portions of the fiber mounting plate. In Figure 4.3, the catheter tip angle is shown versus values taken from the receiving fibers mounted on the top and bottom portions of the plate. The experimental results show that the unloaded catheter tip angle varies linearly with analog input voltage using the criterion of R-squared value above 0.98:
φ(vt) =−36.12vt + 136.7
φ(vb) =−77.61vb + 275.5
(4.1)
where Φ is the catheter tip angle, and vt and vb represent the voltage values read
from the top and bottom fibers respectively.
Moreover, it is shown that the angle information provided by the top receiving fiber of the strain sensor is accurate to 0.36 degrees using an acquisition system, such as PCI 6224 with millivolt accuracy. Lastly, it is shown that the sensitivity of the upper portion of the system is higher, which is because as the tip bends the angle of reflection increases and the distance between the top portion of the mounting and reflection plates increases. Both of these factors diminish the analog voltage values.
4.3.2
Mapping Handle Displacement to Analog Voltage
The optical sensor output voltages were also mapped against the catheter handle displacement in order to create a usable correlation for feedback position control during tele-robotic surgery:
vt(d) =−0.002618d3+ 0.006711d2+ 0.007023d+ 3.685
vb(d) =−0.001666d3+ 0.009509d2−0.01346d+ 3.481
(4.2) where d is the handle displacement, and vt and vb represent the voltage values read from the top and bottom fibers respectively. Figure 4.4 shows that once the handle reaches 4 mm the optical sensor is able to track the handle displacement to within 0.01 mm with a data acquisition system having an accuracy of 1 mV.