A telerobotic system consists of a pair of manipulators in which an operator handling one manipulator (the master) directly controls the second manipulator (the slave), which acts on an external object. The performance of telerobotic systems can be greatly improved if some form of kinesthetic (or force) feedback is employed. In this study the authors develop haptic systems for telerobotic surgery.
In this study a microstructural, kinetic theory-based model of MR fluids has been developed. For modeling these composite systems, “dumbbell” models in which two beads joined by an elastic connector were investigated. The iron particles are modeled as elastic dumbbells suspended in a carrier fluid. Microscale constitutive equations relating flow, stress, and particle orientation are produced. These new models for MR fluids are three dimensional and applicable to any flow geometry. The model developed in this study is fully vectorial and relationships between the stress tensor and the applied magnetic field vector are fully exploited. The higher accuracy of the model in this regard gives better force representations of highly compliant objects.
A number of different MR fluids are employed: Carbonyl iron powder with two different particle size ranges mixed with silicone oils with five different viscosities as well as mineral oil are used to make different samples of MR fluids. The rheological properties of these MR fluid samples are characterized using a rheometer and their magnetic properties are obtained using a vibrating sample magnetometer. The resulting shear stress-shear rate and magnetization-magnetic field graphs of the MR fluid samples are used to determine the optimal MR fluid sample to be used in the design of two haptic systems.
Next, MR fluids are used in the design of a novel five degree of freedom (DOF) MR sponge-based haptic system (master) for controlling a telerobotic arm used in telerobotic surgery (figure 1a). The master 5-DOF joystick controls the movement of a 5-DOF slave (Lynx 5 by Lynxmotion). A novel multi-axis force sensor is designed by the authors and used at the end effector (EE) of the slave for force feedback control. Force and displacement sensors in the slave sense the environment conditions along which the end effector moves. When the EE contacts a solid object or barrier, the exerted force is sensed by the force sensor and the signals are sent to the 5-DOF MR based master to activate the MR dampers accordingly to replicate the force. The user feels the force from master joystick proportional to the force encountered at the slave. For example if the slave encounters soft tissue (low force, small to no deceleration) a relatively small current signal will be sent to the MR damper which will give the user the slight resistance associated with soft tissue. Likewise if the slave encounters bone (high force, large deceleration) a large signal will be sent to the MR damper, which will give the user a larger resistance.
Figure: 5-DOF MR fluid-based telerobotic Master
Figure: 5-DOF MR fluid-based telerobotic haptic system: Master (right) and Slave (left)
The main objective of any force feedback system is to reproduce the forces encountered by the actual or virtual system at the user's end. This can be seen as a tracking problem where the goal is to track the force at the ‘virtual end' and regenerate it at the user end. The development of the controller must be comprehensive enough that it can handle models that will grow in complexity from both an analytical and experimental point of view. There are two control methodologies in this research: motion control of the slave and force feedback control.
Figure: Simulink block diagram for motion control of haptic system in Real-Time Workshop (RTW)
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