This paper describes a flexible and soft tactile sensor that measures the tri-axis force based on inductance measurement. The proposed sensor has four spiral inductors printed on a flexible circuit board and a mounted cylindrical elastomer (silicon rubber). A disk-shaped magnetorheological elastomer (ferromagnetic marker) is embedded in the cylindrical elastomer and its 3-D displacement is estimated by monitoring the inductance changes of the four inductors. In this paper, we investigated the relationship between the applied tri-axis force and inductance changes. Our results can be summarized as follows: 1) the inductance changes of the four inductors were monotonic and linear against the applied normal and shear force; 2) the applied tri-axis force could be estimated well with linear functions of the sum and difference of the measured inductances; and 3) the estimation error of the tri-axis force increased when a larger force was applied and/or faster contact speeds were used.

A significant challenge in robotics is providing a sense of touch to robots. Even though several types of flexible tactile sensors have been proposed, they still have various technical issues such as a large amount of deformation that fractures the sensing elements, a poor maintainability and a deterioration in the sensitivity caused by the presence of a thick and soft covering. As one solution for these issues, we proposed a flexible tactile sensor composed of a magnet, magnetic transducer and dual-layer elastomer, which consists of a magnetorheological and nonmagnetic elastomer sheet. In this study, we first investigated the sensitivity of the sensor, which was found to be high (approximately 161 mV/N with a signal-to-noise ratio of 42.2 dB); however, the sensor has a speed-dependent hysteresis in its sensor response curve. Then, we investigated the spatial response and observed the following results: (1) the sensor response was a distorted Mexican-hat-like bipolar shape, namely a negative response area was observed around the positive response area; (2) the negative response area disappeared when we used a compressible sponge sheet instead of the incompressible nonmagnetic elastomer. We concluded that the characteristic negative response in the Mexican-hat-like response is derived from the incompressibility of the nonmagnetic elastomer.

Soft matter computers (SMCs) are promising control and computation mechanisms that do not use rigid materials. In such mechanisms, conductive droplets are used to switch the state of the voltage applied to connected loads, such as in an electric actuator. Without a complex structure, an SMC can only apply voltage in sequence starting from the first load. Therefore, soft robots with simple SMCs can only perform simple operations. In this study, to apply voltage to connected loads in any order and combination, the SMC design was extended by adjusting the spatial intervals between pairs of facing electrodes. The intervals between the pairs of facing electrodes for each load were designed uniquely. These pairs of facing electrodes are electrically connected by two conductive droplets at the same interval and a voltage is applied to the specified connected load. Based on the arrangement of conductive droplets, voltage can be applied to loads arranged in a pattern on a soft tube in any combination and order. An experimental evaluation was conducted to determine whether the proposed method could apply voltage to loads in a predefined pattern. Additionally, the applicability of the proposed method to soft robots was demonstrated by controlling soft actuators in any combination.

Tactile information is usually important for object manipulation and grasping. Typically, soft hands and tactile sensors can deform passively and contribute to stable grasping. In particular, soft tactile sensors that use conductive materials as sensor elements can acquire contact information, including the contact position and pressure; however, this tactile information cannot be classified. This study attempts to classify tactile information based on conductive material arrangements. To this end, we develop a soft tactile sensor composed of a silicone rubber body with two channels filled with a conductive material. The two channels are arranged so that they are parallel from the top view and angled with different slopes from the side view. Our experimental data reveal that the contact position parallel to the channels can be determined based on the resistance changes in the two channels, whereas the pressure can be obtained through a model based on the estimated value of the contact position.

Musculoskeletal humanoids driven by pneumatic artificial muscles (PAMs) offer high adaptability to environmental interactions, but replicating anatomical joint structures poses stability challenges, particularly at the shoulder. To prevent dislocation in anthropomorphic ball-and-socket joints, contact between the ball and socket must be maintained by antagonistic muscle tension. The resulting friction impedes motion and degrades dynamic performance; however, it also provides holding torque to resist external forces during position holding. This paper proposes an air-lubricated, friction-utilizing shoulder joint to balance holding torque and dynamic performance. In experiments, we compared the proposed joint against a low friction, rolling contact joint. The results demonstrated that the proposed joint achieved a holding torque 1.3 times greater than that of the rolling contact joint. Furthermore, air lubrication mitigated the friction-induced decline in dynamic performance, increasing the effective range of motion by up to 1.6 times and reducing the settling time to one-fifth of the value without air lubrication.