**2.3.2 Design of the force feedback joystick**

Fig.11 shows a CAD plot of the original prototype design. Principal design objectives were that the device should be low cost therefore plastics were extensively used, only the actual control stick (#5) is made of (metal) aluminium. The other parts shown in Fig.11 (support brackets #2,3,6 , base plate #3) are made of POM. Parts 1 & 6 are digital servo motors (GRAUPNER DS8311 ) and were specifically chosen for their durability (metal bearings), precision (13 bit), compactness and high torque output (112 Ncm @ 6V supply) values.

To enable force sensing and closed loop force feedback 4 strain gauges are placed at the base of the control stick (Part #8) which will allow the force felt by the user in the X & Y axes to be measured. Screws and bearings as well as the electronic control board (integrated into the base plate) are not shown. The base plate has dimensions 140mm x 140mm.

The idea of the haptic force feedback joystick was now to add another pair of strain gauges and add a second servo actuator for force feedback on the second axis.

Fig. 11. CAD design of prototype joystick

Fig. 12a shows the fabricated prototype being tested by a user. All plastic parts were milled from stock POM material while the control stick was turned and then the surfaces for the strain gauges were milled flat. The strain gauges are protected from damage by a rigid plastic cover (#2). Note the servo control cables leading from the servos to the control board under the base plate (#1). Fig. 12b shows a larger view of the sensorized control stick (plastic cover removed) with the 4 strain gauges (#1-4) and the electrical connector (#5).

spring constant to move the joystick back to central position when released. When at a certain angle the catheter touches for instance a wall, a force is sensed in the bowden wires. This force signal is then used to generate a force feedback signal. Figure 10 shows the torques applied by the servo motor in these 3 scenarios. In the simple spring model the torque is directly dependent on the angle (0.5 N cm/°). In the zero actuation force model practically no force is required to move the handle. If the handle is released, it stays at the last position. In the mixed model, a low spring constant was programmed for 0-25°, above that a spring constant of 10Ncm/° was implemented to simulate touching. If the handle is

Fig.11 shows a CAD plot of the original prototype design. Principal design objectives were that the device should be low cost therefore plastics were extensively used, only the actual control stick (#5) is made of (metal) aluminium. The other parts shown in Fig.11 (support brackets #2,3,6 , base plate #3) are made of POM. Parts 1 & 6 are digital servo motors (GRAUPNER DS8311 ) and were specifically chosen for their durability (metal bearings), precision (13 bit), compactness and high torque output (112 Ncm @ 6V supply) values.

To enable force sensing and closed loop force feedback 4 strain gauges are placed at the base of the control stick (Part #8) which will allow the force felt by the user in the X & Y axes to be measured. Screws and bearings as well as the electronic control board (integrated into the

The idea of the haptic force feedback joystick was now to add another pair of strain gauges

Fig. 12a shows the fabricated prototype being tested by a user. All plastic parts were milled from stock POM material while the control stick was turned and then the surfaces for the strain gauges were milled flat. The strain gauges are protected from damage by a rigid plastic cover (#2). Note the servo control cables leading from the servos to the control board under the base plate (#1). Fig. 12b shows a larger view of the sensorized control stick (plastic

cover removed) with the 4 strain gauges (#1-4) and the electrical connector (#5).

base plate) are not shown. The base plate has dimensions 140mm x 140mm.

and add a second servo actuator for force feedback on the second axis.

released, it returns to the central position.

Fig. 11. CAD design of prototype joystick

**2.3.2 Design of the force feedback joystick** 

Fig. 12. a. Fabricated joystick prototype; b. Strain gauges and connector on control stick

Fig.13a shows the electronic control board which is fixed in a cavity under the base plate. This cavity was milled out of the bottom of the base plate. The control system purpose is to control the servo motors to give a resistive force to the user which is determined by the hand movements of the user and the control algorithms used by the microcontroller to vary the user experience. The strain gauges are in a Wheatstone bridge configuration and the differential voltages are fed to the voltage amplifiers (2 x AD623, #3). The amplified voltages (which represent the resistive force felt by the user) are processed by the ATMEL ATmega32 microcontroller (#2) which through a specific algorithm then send digital commands to the servo motors (#4) to continually adjust this force. The force sensed by the user and the position of the control stick in both X & Y axes can be communicated to a PC by the microcontroller. The microcontroller transmits the data to the RS232 chip (#1) which communicates to the external device connected to #6 with the RS232 protocol. The board is powered with a regulated 6V supply connected at #5.

Fig. 13. a. Electronic control board; b. Graphical User Interface

Fig.13b shows a PC user interface developed using Labview which was used to test the performance of the input device in registering both control stick position (#1) and force (#2) and also using the PC interface to change the stiffness (resistance) of the control stick sensed

Sensorized Tools for Haptic Force Feedback in Computer Assisted Surgery 149

electronics was also presented. Based on the same force sensing/control approach a further development of an haptic input device with 3 or more DOF is possible, which can be a low

Force sensorized tool and haptic force feedback input devices for surgical interventions are still in early development stages and only few devices are already used in surgical interventions, but latest developments are promising. These novel devices have the potential to revolutionize robotic aided surgery, as the can be considered as key elements to regain

The work described in this paper was supported by the ASSEMIC project, Marie Curie Research & Training Network (MRTN-CT- 2003-504826). Currently Arne Sieber is funded through the Marie Curie Project Lifeloop (EU FP7-People-IEF-2008, project nr. 237128). A

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**4. Acknowledgment** 

**5. References** 

by the user. Only simple trials were performed with linear spring constants to vary the stiffness of the control stick but for future trials more advanced algorithms will be programmed to also feedback non-linear and damped forces to the user.

A second prototype was designed with the aim of further reducing the size of the input device. Fig.14 shows the CAD design of the second prototype which has a base plate dimension reduced from 140 x 140 mm² to 84 x 84 mm² (40% reduction). The same servos will be used in a different configuration and the supporting brackets have been modified to make more efficient use of space. The material for all load bearing parts (base plate, brackets, control stick) will be aluminium to make the design more robust but will also slightly increase the manufacturing costs.

Fig. 14. CAD model of second prototype with 40% size reduction
