FORCE/TORQUE SENSORS

In order to know precisely the intentions of a human interacting with a robot, it is possible to use a force/torque sensor to measure the forces and moments applied on a handle. The advantage of this type of sensor, in comparison with a sensitive robotic skin covering the whole manipulator, is that it allows the precise measurement of the applied moments and shearing forces instead of the normal forces alone.

The commercially available force/torque sensors are built from strain gages, which have the disadvantage of providing a noisy signal. This is problematic because control algorithms used in human-robot interaction often require the variation rates of efforts. Yet, differentiating noisy signals with respect to time is often only possible by filtering the signal, which reduces the performances of a collaborative robot. Furthermore, force/torque sensors based on strain gages output signals which drift over time even if the applied efforts remain constant. Thus, a collaborative robot could detect small forces applied on it and start moving even if nobody touches it.

For these reasons, a new technique to measure the applied efforts is used to build sensors from compliant mechanisms and photo-interruptors (PI). A PI is an electronic component which comprises a light-emitting diode (LED) and a photo-resistor. It allows the measurement of the obstruction of the emitted light. By combining it with a compliant mechanism, a uniaxial force sensor is obtained as seen in Figure 1.

Fig. 1: One degree of freedom force sensor based on a compliant mechanism and a photo-interruptor.

The next figure shows a force signal measured over time by a commercial sensor and a sensor based on PI. Both sensors were placed in series during the experimentation such that the applied efforts were the same. The noise level difference for the two sensors is clearly visible in the figure.

Fig. 2: Force signal over time for a sensor based on PI compared with a commercial sensor.

It is possible to assemble several uniaxial force sensors in order to measure efforts applied on a handle. Figures 3 and 4 present an assembly of sensors which allow the measurement of forces applied in all directions in addition to the moment applied with respect to a vertical axis.

Fig. 3: Assembly of four compliant mechanisms with photo-interruptors.

Fig. 4: Complete handle which allows the measurement of forces in x, y and z directions and the moment along a vertical axis.

More compact compliant mechanisms were also built to create a 3 degree of freedom force sensor which has the same size and shape as standard commercially available sensors. Figure 5 presents the developped sensor, placed on the wrist of a PR2 robot from Willow Garage as it is using a pen to write on a white board.

Fig. 5: PR2 robot from Willow Garage equipped with a three degree of freedom force sensor (work executed in collaboration with Robotiq and the Biomimetics & Dexterous Manipulation Laboratory of Stanford University).

The Laboratory has also collaborated with Robotiq to develop a 6-axis force torque sensor.