Haptics for microrobots

Like a microscope magnifies viewed objects, micro teleoperation systems can be employed to magnify interaction between objects at a scale at which humans are naturally effective. These systems based on the connection between haptic interfaces and microrobots allow human operators to properly interact with microscales. In other words, haptic interfaces give us the option to bring the experience of this physics with the direct reach of the human sensorimotor capabilities. This means that human actions are extended to conduct a manual task in a highly unusual and unpredictable environment. In these systems operator has a key role. As depicted in Fig. 1, the operator perceive information from microscale through the haptic interface and acts accordingly by sending information (commands) to the microrobot interacting with the environment . Thus, the system has to provide operator with a sense of touch to execute his/her task in the microscale in one way. In the other way, it has to transmit operator commands to control the microrobots.

Figure 1 :  Haptic feedback micro teleoperation system architecture. The operator controls the tool trough an haptic interface. The position of the handle is scaled down to control the tool position. The force measured is scaled up (amplified) and transmitted to the operator through the haptic interface.

However, most haptic devices described in the past fail to match the human sensorimotor capacity and thus act as an obstacle between the hand and the phenomenon that could be accessed. Conventional interfaces are subjected to inherent friction and/or high inertia that a ffects the interface transparency, dynamic range, and bandwidth : the higher the torque, the higher the inertia. Consequently the transparency is degraded with inertia. To cope with the limitations of conventional force feedback devices, ISIR developed a dual-stage designs suitable to address the needs of the interaction with the micro/nano scales.

As described in [1,2,3], this haptic device with one degree of freedom is comprised of a large motor coupled to a small one, also carrying the handle, through a passive viscous clutch based on eddy currents (see Fig. 2). This clutch mechanism accurately transforms slip velocity of the large motor into torque and exhibits a linear behavior on a large bandwidth. This first order relationship between the velocity control and the handle eliminates the dynamics of the large motor from the user experience. A feed-forward path is provided through the small motor to fill in the transients or fast variations of the input signal that the large motor could not provide. As this second motor has negligible friction and inertia, unwanted forces could be kept below human detection threshold achieving quasi-perfect transparency. Readers can find more detail about this device in [1,2,3].

Figure 2 : Dual-stage Haptic device designed by ISIR. 

This haptic device was successfully connected to a micro-force sensor in order to interact with a micro/nanoscales. The system was experimentally demonstrated in the complex case which consists in measuring the time-course interaction of a thin glass probe with a water droplet under direct human control (see Fig.3). The task is split in four main phases, namely ‘approach’, ‘contact’, ‘retract’, and ‘contact break’ (see Fig.4). The result, Fig.3, shows that the system remains stable over the experiment and achieves a quasi perfect transparency. In fact, the force felt by the user through the haptic interface is exactly the force measured by the sensor amplified by a constant coefficient. Hence, this result shows the performance and the importance of haptic micro teleoperation systems. 

Figure 3 :  Interaction of a tiny glass probe with a water droplet. (A) Evolution of the measured force as a function of time and as a function of gap probe-droplet. (B) Force felt by the operator hand during the interaction as a function of time and as a function of handle displacement, that accurately replicates the microscopic interaction. (C) The error between the estimation of felt force and the amplification of the measured force. This error, less then 1%, shows the high degree of transparency of the system, where transmitted force replicates accurately measured microscopic force.

Figure 4 : The man phases of interaction between a probe and a water droplet ’approach’, ’contact’ (pull-in), ’retraction’ and ’pull-off’.

The performance reached by this new haptic device make it suitable for micro/nanomanipulation where the operator experience is required to achieve complex tasks. By connecting this device with REMIQUA station, user gets acces to micro/nanoscale throuogh the sens of touch. In other words, with such design end–used (operator) can be kept in the loop in order to interact with micro/nanoscale.

 

[1] G. Millet, S. Haliyo, S. Régnier, and V. Hayward, “The ultimate haptic device: First step,” in Proceedings of the Third Joint Eurohaptics Conference and Symposium on Haptic Interfaces for Virtual Environment and Teleoperator Systems. World Haptics, 2009, pp. 273–278.

[2]  A. Mohand-Ousaid, G. Millet, S. Régnier, S. Haliyo, and V. Hayward“Haptic interface transparency achieved through viscous coupling,” International Journal of Robotics research, vol. 31, no. 3, pp. 319–329, 2011.

[3]  A. Mohand Ousaid, T. Lu, C. Pacoret, S. Régnier, and V. Hayward, Dual Stage Options for Interface Designs Suitable for Haptic Interaction at the Micro-Nano Scales, International Symposium on Experimental Robotics (ISER2014), In press.