Soft, Stiffness-Controllable Sensing Tip for On-Demand Force Range Adjustment With Angled Force Direction Identification

Abstract

Force sensors are essential for measuring and controlling robot-object interactions. However, current force sensors have limited usability in applications such as grasping and palpation, where the range of angled forces changes between tasks. To address this limitation this paper proposes a novel optical-based soft-tipped force sensor capable of adjusting its range and sensitivity through pneumatic modulation. This research describes the sensor's design and examines the relationship between the internal pressure of the sensor and its sensing range, sensitivity, single-axis force-sensing accuracy, and capability of measuring the angle and magnitude of non-normal forces. Results indicate that by increasing the pressure in the sensor, the sensing range can be increased and the sensitivity decreased. These results demonstrate that the sensor can measure normal forces reliably at each pressure using 4th order fits with root-mean-square error (RMSE)∈ [0.032 N 0.110 N]. Finally, it is also demonstrated that by using a neural network, the sensor can measure the angle and magnitude of non-normal forces with RMSEs on trained variables of 0.0120 Rad for Y-angle (θ Y ) measurements, 0.0109 Rad for X-angle (θ X ) measurements, and 0.102 N for force measurements.

Soft-Tipped Sensor With Compliance Control for Elasticity Sensing and Palpation

“…An approximation of spring stiffness could be calculated from (3) using the relationship between membrane tip displacement, force, and tracking point displacements at each internal pressure. These relationships could be obtained using a linear rail and force sensor in line with the experiments in [27]. However, this may not fully capture the interaction between the tissue and the dome-shaped membrane as forces would not be applied in the same manner as a flat surface on the membrane tip.…”

“…The membrane was cast from Ecoflex 00-50, mixed with a ratio of 5 g part A to 5 g part B, to 0.2 ml of Silc Pig black. This is stiffer than the membrane used in [27] to reduce the effects of shear forces during palpation. Hemispherical tracking points, 1.4 mm in diameter, were cast on the membrane's inner surface and painted with a mixture of Smooth-On Psycho Paint and Smooth-On Silc Pig Cyan.…”

“…Once in contact, the internal pressure was increased in steps of 0.061 kPa. After a 1 s pause at each step for membrane adjustment, tracking point locations were measured five times, with a 0.2 s gap between measurements, in line with our previous force-sensing procedure [27]. This was repeated up to a pressure of 9.77 kPa, then the experiment was reset.…”

“…However, this sensor interacts using stiff-tipped indenters, which may damage tissue [25], [26]. On the other hand, it has been shown in previous work [27] that the pressure modulated optical tracking (PMOT) sensor (Fig. 1) can measure force with a variable range through compliance control of a soft membrane.…”

ESPRESS.0: Eustachian Tube-Inspired Tactile Sensor Exploiting Pneumatics for Range Extension and SenSitivity Tuning

“…This is because a ‘stiffening’ actuator may control the sensitive range available to a sensor it is linked to, which maximises its sensitivity as is appropriate to the stimulus. Pneumatic pressure modulation of a soft sensor membrane has also been used as a mechanism to increase the sensor’s stiffness to help improve the tactile sensing performance across a variety of tactile applications [ 20 , 21 , 22 ], with specific benefits towards enhancing edge detection and reducing `cross-talk’ between sensory units [ 17 , 23 ].…”