HARDWARE INTERFACE FOR A 3-DOF SURGICAL ROBOT ARM Ahmet Atasoy 1, Mehmed Ozkan 2, Duygun Erol Barkana 3 1 Institute of Biomedical Engineering, Bogazici University, 2 Electrical and Electronics, Yeditepe University Istanbul, Turkey ABSTRACT Robotic surgery aims minimum soft tissue damage and minimum operator intervention by employing automated, precise electro mechanical devices. In this project the hardware components of a 3 DOF prototype robot arm are being integrated and controlled with MSP430 microcontroller to operate a surgical end-effector that is aimed for precise orthopedic surgery. The early research and applications on robotic systems are in serial robot structures, but the poor positioning capabilities, pave the way to parallel robot applications for more precise positioning. In this project our design includes both parallel robot "Stewart platform" with a 6-DOF and a 3-DOF serial robot arm. The robot arm carries the stewart platform as its end effector. Figure 1. shows the overall mechanical structure. ROBOT INTERFACE The robot interface is responsible for driving the AC servo motors and handling the feedback signals. The motors are driven by Toshiba RA motor drivers. Figure 3.1. illustrates robot interface for one motor. The motor driver has an enable /start pin (E) for activating the motor, that requires 10V for activation. A second control pin (SPD) inputs speed proportional control signal. When the SPD input is at “0” volts the motor is locked at its current position. For the motor to be activated a voltage between ±10 Volts is required. For our purposes we need only voltages between ± 2.5 Volts. This analog control signal is supplied by DAC7624; four channel digital to analog converter IC from TI. To get position of links existing quadrature encoders on motors were used for this project. To keep system safety, limit switches were placed just before this physical limits of the system. Figure 1 –Whole System In this study, we present the controller of the 3 DOF Robot Arm that is mainly driven by MSP430s. The Robot Arm is responsible for positioning the surgical tool on the correct position in relation to the end effector orientation. End effector is designed to hold drilling tool at position with 1mm position accuracy. For this purpose an electronic circuit was implemented. The circuit comprises modules for user and robot interfaces, motor drivers, sensory feedback. Sensory data that are gathered are processed to control the actuators by a control algorithm running on the MSP430 microcontroller. ROBOT ARM The Robot Arm has three degrees of freedom;translate, rotate, rotate. All the joints of the robot are driven by 3-phase AC servo motors (M1, M2, M3) with feedback capabilities. The robot arm which we use in our design was originally designed for painting applications and the original control structure is as presented in Figure 2.1. Comprises, a PC, 3 PCI cards, and interface bus. The proposed architecture offers a much simplified solution with the employment of MSP430FG439 microcontroller as are shown in Figure 2.2. Figure 2.1. – Robot Arm Old System Components Figure 2.2. – Robot Arm System Components Electronic Control Card has a user inteface and a robotic interface modules under the control of microcontroller unit. In this study MSP430FG439 is selected for the electronic control hardware for its functional capabilities. For controlling Robot Arm position, inputs and outputs of the system are configured with necessary circuit adaptations. For user inputs a keypad unit is designed with eight push buttons. Six of these buttons are adjusting X, Y, Z coordinates or θ 1, θ 2, θ 3 for desired position. Other buttons are used for selected functions and for enter input data and to pause or stop the system. LCD modul is designed to see the data entered by the user and the actual position of the robot arm. The robot interface collect feedback data and drive motors with a control algorithm for desired position of the robot arm. Figure 2.3.– Block Diagram of Electronic Control Card Figure 3.1.– Robot Interface For One Motor The overall 3 DOF Robot Interface based on 1-DOF modules in Figure 2.5. is illustrated in Figure 3.2. Figure 3.2..– Robot Interface A simple application program is employed as illustrated in Figure. 4. The system calibrates the robot position for all joints by searching for the limit switches. When the user enters the target end-effector position, a P- controller algortim runs the motors to the targetted joint angles computed through inverse kinematics. CONCLUSION It is intended to devise a hardware interface to control "robot arm" and hold the end effector at the desired position. For this purpose an electronic hardware card has been designed and implemented to Robot Arm as a control unit for this study. Robot Arm controlled with a firmware on a microcontroller successfully. System modules on electronic hardware card almost works properly. It is possible to read encoder data four times better with different methods. It is also possible that different control algorithms can be implemented. Further more a serial communication can be added instead of LCD and button modules to system to control robot arm and end effector together via matlab simulink program. Position accuracy for robot arm can be reduced by reducing encoder resolution. Also there is a way to get position of the links with 3-axis accelerometers to make better results for positioning. Figure 4.– A simple Flow Chart to Control Robot Arm Wait for ENTER button Write “Robot Arm” on the LCD Write “Calibration” on the LCD 1 1 Enter desired position Enable Motors Give Speed Referen Motor 2 goes up limits ces Motor 1 goes left limit Motor 2 goes up limit Motor 3 goes up limit Set X, Y, Z to zero Write Position “X:000 Y:000 Z:000” on the LCD Wait for ENTER button if enter Go to desired position Write “Actual Position and desired position” on the LCD Write “Enter New Posisiton” on the LCD End if enter 1 Start