(a) (b)
Figure 3.8: Placement of gauges on the middle (a) and inner shafts (b).
Figure 3.9: Cable wiring to allow the inner shaft to slide inside the middle shaft in order to accommodate the different tips.
Due to the configuration of the different handles, the inner shaft needs to slide with respect to the middle shaft when the handles are replaced, while still maintaining the ability to open and close the gripper without the cables getting tangled. Figure 3.9 shows how the cables have been wired to allow the inner shaft to slide inside the middle shaft and accommodate the different tips and handles. The inner shaft can slide with respect to the middle shaft without causing the cable to get tangled or pinched.
3.6
Additional Hardware and Software Interface
The equipment used to capture force and position information in real time is shown in Figure 3.10. It includes three major components: a personal computer (PC), the position sensing system, and the force sensing elements.
The microBIRD™Electromagnetic Tracking System (EMTS) is used for position sensing. This system connects directly to the PC through a PCI card, through which the signals from the two electromagnetic sensors are captured.
3.6 Additional Hardware and Software Interface 37
Figure 3.10: Experimental setup.
amplifiers that are powered by a Quanser Universal Power Module (model UPM-1503). The gain from these amplifiers ranges between 40 and 2000 depending on the potentiometer setting. Two Quanser Q8 Hardware-in-the-Loop boards (one for each instrument) are responsible for capturing the signals from the amplifiers.
The personal computer used is an HP xw4100 workstation with an Intel 2.8 GHz Pentium 4 HT Processor, 1 GB RAM, and running Windows XP. Customized software running on this computer serves to capture, process and record the information from the electromagnetic sensors and the strain gauges. The computer also facilitates the acquisition of video streams from an RS-170A compatible endoscope or camera connected to an installed Matrox Meteor II PCI frame grabber.
A customized software interface (see Figure 3.11(a)) was developed in C++ by a research engineer. The graphical user interface (GUI) presents real-time plots that display the force and position data as indicated by the user. On another window, an interface facilitates force calibration (see Figure 3.11(b)). All of the data displayed and computed by the software, as well as the video, can be recorded into a database for offline processing and analysis. See Appendix A for more details on the software.
3.6 Additional Hardware and Software Interface 38
(a)
(b)
Figure 3.11: Customized software interface including real-time plots (a) and the calibration inter- face (b).
3.7 Calibration 39
3.7
Calibration
In order to determine the relationship between the voltage changes measured by the strain gauges and the actual forces and torques being applied at the instrument tip, a calibration procedure was performed using the setup shown in Figure 3.12. The instrument was placed in different positions and orientations in order to apply forces in each of the individual axes during calibration. To calibrate the inner shaft, which measures actuation forces, weights were applied to the moving gripper while the instrument shaft was supported along its entire length. Preloading of the gripper was necessary to remove any play within the actuation system. The x and y moments were calibrated by applying forces at the tip while the instrument was cantilevered, supported at the rotating wheel (Figure 3.12(a)).
A special tip was designed to allow the application of pure torque and pure axial forces to the instrument (see Figures 3.12(b) and 3.12(c)). During the torque calibration, the instrument shaft was fully supported. When calibrating the z axis, the instrument was mounted on a gimbal that ensured that the forces were applied axially (Figure 3.12(d)). In each case, the forces and torques were calibrated by measuring the voltage increase when weights were applied in 100 g increments from 0 to 1000 g. The calibrated values obtained are as follows: Sii = -.0022, corresponding to
the slope of the actuation force; Sxx = -.0295, corresponding to the slope of the x forces; Syy =
-.0297, corresponding to the slope of the y forces; Stt = -.0051, corresponding to the slope of the
torsional forces; and Saa = -.00051, corresponding to the slope of the axial forces.
In an ideal situation, the strain gauges would be completely decoupled from each other and would only sense forces applied in the direction they were designed to measure. This, unfortunately, is not the case in practice and so a method for decoupling the different signals was developed. It was observed that the forces applied in thex and y directions were decoupled from the torsional forces and from the axial forces. The only coupling present was caused by the grip. The coupling factor in this case was determined by trial and error, adjusting it until actuation of the gripper produced no observable force in thex andy directions.
To decouple the actuation force from the other signals, data were recorded for about 1 minute, during which time forces were applied in all directions except for actuating the gripper. The data were then run through an optimization process using the fminimax function in MATLAB. The goal of the optimization was to find the optimal parameters (Pix,Piy,Pit andPia) that minimized
3.7 Calibration 40
(a) (b)
(c) (d)
Figure 3.12: Instrument placement for calibration for: x andy moments (a), axial (b); and torsion (c) . (d) Shows a close up of the gimbal designed for the application of axial forces. Note that in these pictures, the stainless steel outer shaft was replaced by an ABS plastic shaft. Photo credit: Meg Woodhouse.