Haptic interaction plays an important role in the virtual reality technology, which let a person not only view the 3D virtual environment but also realistically touch the virtual environment. As a key part of haptic interaction, force feedback has become an essential function for the haptic interaction. Therefore, multi-dimensional force sensors are widely used in the fields of virtual reality and augmented reality. In this paper, some conventional multi-dimensional force sensors based on different measurement principles, such as resistive, capacitive, piezoelectric, are briefly introduced. Then the mechanical structures of the elastic body of multi-dimensional force sensors are reviewed. It is obvious that the performance of the multi-dimensional force sensor is mainly dependent upon the mechanical structure of elastic body. Furthermore, the calibration process of the force sensor is analyzed, and problems in calibration are discussed. Interdimensional coupling error is one of the main factors affecting the measurement precision of the multi-dimensional force sensors. Therefore, reducing or even eliminating dimensional coupling error becomes a fundamental requirement in the design of multi-dimensional force sensors, and the decoupling state-of-art of the multi-dimensional force sensors are introduced in this paper. At last, the trends and current challenges of multi-dimensional force sensing technology are proposed.

This paper presents a spherical actuator-based hand-held device that provides lateral force feedback for user interaction with the touch screen. The spherical actuator is built around a ball containing the magnetorheological elastomer (MRE). This design of the actuator not only allows it to move in multiple degrees of freedom but also realizes its direct interaction with the touch screen and enhances its lateral force feedback capability by utilizing the conductive and soft characteristics of the MRE. Meanwhile, using the magnetically conductive properties of the MRE, the lateral force can be controlled by current. In this paper, we introduced the overall structure of the device, described the fabrication of the MRE, and tested the relative permeability and surface friction properties of the MRE. Then, based on the structural parameters obtained by lateral force modeling and finite element analysis, we fabricated a prototype of the actuator and determined the lateral force control method through calibration tests. Finally, through physical measurements and psychophysical experiment, we comprehensively evaluated the lateral force performance of the actuator and its ability in displaying virtual surface friction. The experimental results confirm the effectiveness of the actuator in interacting with the touch screen and displaying the virtual surface friction characteristics. INDEX TERMS Hand-held haptic device, spherical actuator, magnetorheological elastomer, touch screen interaction, friction display, lateral force feedback.

The haptic interface plays an increasingly important role in enhancing the realism and immersion of the user's interaction with the touch screen. Inspired by the wearable haptic system, this paper proposes a finger wearable device called FW-Touch for touch screen interaction. The device provides normal force, lateral force, and vibrotactile feedback for the interaction of the finger and the touch screen through three internally integrated actuators. By displaying the hardness, friction, and roughness of a virtual surface, the device is capable of simulating the active exploration and sensing process of the finger on a real surface. This paper describes the design and specifications of the FW-Touch, and details the design process of a magnetorheological (MR) foam actuator that uses a Hall sensor to correct the output force. Through physical measurements and psychophysical experiments, we comprehensively evaluated the force feedback performance of the FW-Touch and its ability in displaying the stiffness and friction of the virtual surface. The results show that improving the accuracy of force feedback is necessary for virtual stiffness display, and the accuracy and effectiveness of the FW-Touch in displaying virtual surface features can be confirmed from the measured stiffness and friction Weber fractions.

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