Tactile sensor is one of the most important sensations for human to perceive the external world. This article proposed a novel triaxis soft electromagnetic (EM) tactile sensor based on the principle of a transformer, where the sensitivity or measurement range of the designed tactile sensor can be adjusted by changing the excitation voltage (frequency or amplitude) of the primary coil. It mainly contains a primary coil, four secondary coils, and a PDMS elastomer containing a magnet, where the coils are planar coils and the four secondary coils are a four-quadrant structure. Therefore, the displacement of the magnet in the 3-D space can be obtained by decoupling the output voltages of the four secondary coils, which is equivalent to being capable of detecting the triaxis force. Theoretical analysis and model simulation were first used to study the influence of the excitation voltage of the primary coil on the output voltage of the secondary coil, which provides a theoretical basis for the adjustable sensitivity of the sensor. Then, the designed tactile sensor was prepared, and the typical performances of the adjustable sensitivity were evaluated by a commercial six-axis force/torque sensor. The results have shown the relationship between the sensitivity of the sensor and the excitation voltage of the primary coil and verified its ability to measure the triaxis force. Finally, the adjustable ability of the designed tactile sensor to measure normal force and shear force was verified by a commercial robotic arm with an electric gripper.

A novel Wheatstone bridge adjustment circuit with both rough-tuning and fine-tuning functions is designed for balancing Wheatstone bridge in this paper. Firstly, the parametric circuit structure is constructed to facilitate the design of rough-tuning and fine-tuning sub-circuit parameters; Secondly, the quantitative index of Wheatstone bridge imbalance is defined, and the parameter design flow of the rough-tuning sub-circuit, which consists of two sets of series-parallel-resistor (SPR) structures that are respectively connected in parallel with the two branches of one half-bridge, is given to directly weaken the initial bridge imbalance in the manufacturing stage. Theoretical analysis results show that the SPR structure can effectively approach any expected value and reduce the number of fixed resistor types. Thirdly, parameters of the fine-tuning sub-circuit, which consists of a resistor and a digital-to-analog converter (DAC), are optimized based on any expected adjustable range. By adjusting the DAC input to seek the middle value of an analog-to-digital converter output, indirectly eliminate the additional bridge imbalance caused by mechanical assembly stress, gravity, ambient temperature, and etc. in the application stage. Then the influence of the fine-tuning parameters on linear fitting coefficients of the output vs. input of the Wheatstone bridge is analyzed theoretically. The results show that the DAC output voltage only has a significant linear impact on the intercept coefficient, while its impact on the scale coefficient is negligible. Finally, the effectiveness of the adjustment circuit is verified on a six-axis force/torque sensor that is developed for an Experimental Module Manipulator in the Chinese Space Station.

Purpose: This paper aims to present a smooth transition adaptive hybrid impedance control for compliant connector assembly. Design/methodology/approach: The dynamics of the manipulator is firstly presented with linear property. The controller used in connector assembly is inspired by human operation habits in similar tasks. The hybrid impedance control is adopted to apply force in the assembly direction and provide compliance in rest directions. The reference trajectory is implemented with an adaptive controller. Event-based switching strategy is conducted for a smooth transition from unconstrained to constrained space. Findings: The method can ensure both ideal compliance behaviour with dynamic uncertainty and a smooth transition from unconstrained to constrained space. Also, the method can ensure compliant connector assembly with a good tolerance to the target estimation error. Practical implications: The method can be applied in the connector assembly by “pushing” operation. The controller devotes efforts on force tracking and smooth transition, having potential applications in contact tasks in delicate environment. Originality/value: As far as the authors know, the paper is original in providing a uniform controller for improving force and position control performance in both unconstrained and constrained space with dynamic uncertainty. The proposed controller can ensure a smooth transition by only adjusting parameters.

Force measurements become important once contact has occurred and the object is held or explored by the hand. The uncertainty associated with the fingertip contact position and the soft finger contact torque can lead to inaccurate force measurement when using a model-based method. To address this issue, we utilized a learning-based method that leverages neural networks to concatenate data from tactile, position, and torque sensors to improve force measurement accuracy in soft finger contacts. Due to the requirement for a large volume of data to train the network, we propose a data collection method that is independent of the fingers. This method utilizes the physical model of the finger to rapidly generate a large dataset. The effectiveness of this data collection method is validated on real-world datasets. Comparing the model-based method and the data-driven method using solely tactile input, our proposed force measurement method achieves the lowest force errors in all three axes and magnitude. Particularly, in the x - axis and force magnitude, the proposed method reduces the error by more than 50% compared to the model-based method. Furthermore, our method demonstrates effectiveness and robustness on unseen objects. This method is not only suitable for dexterous hands but also applicable to underactuated hands equipped with torque sensors and high-dimensional tactile.

SUMMARY: For acquiring a broad view in an unknown environment, we proposed a control strategy based on the Bézier curve for the snake robot raising its head. Then, an improved discretization method was developed to accommodate the backbone curves with more complex shapes. Besides, in order to determine the condition of using the improved discretization method, energy of framed space curve is introduced originally to estimate the shape complexity of the backbone curve. At last, based on degree elevation of the Bézier curve, an obstacle avoidance strategy of the head-raising motion was proposed and validated through simulation.

Head control is important for snake robots to work in an unknown environment. However, the existing methods of head control have certain application limitations for snake robots with different configurations. Thus, a strategy for head control based on segmented kinematics is proposed. Compared with the existing head control strategies, our strategy can adapt to different structures of snake robots, whether wheeled or non-wheeled. In addition, our strategy can realize the accurate manipulation of the snake robot head. The robot body is divided into the base part, neck part and head part. First, parameters of backbone curve are optimized for enlarging the area of the support polygon. Then the desired pose for the head link and the dexterous workspace of the head part can in turn derive the desired position and direction of the end frame for the neck part. An optimization algorithm is proposed to help the end frame of the neck part approach a desired one and obtains the joint angles of the neck part. When the actual frames of the neck part are determined, the dexterous workspace of the head part will cover the desired pose of the head link. Then the TRAC-IK inverse kinematics algorithm is adopted to solve the joint angles of the head part. To avoid the collision between the body and the ground, a trajectory planning method of the overall body in Cartesian space is proposed. Finally, simulations validate the effectiveness of the control strategy.