Magnetorheological fluid, whose rheological properties can be continuously and reversely changed, is a new-type smart material. Based on magnetorheological fluid, magnetorheological brake is a potential actuation method in haptics due to its desirable features, such as inherent passiveness and high compactness. Most of the existing magnetorheological brakes are powerful enough, but they are too bulky and heavy. If they were smaller and lighter, they would be more favourable for haptics. In this article, a small-scale yet powerful magnetorheological brake was designed, modelled and evaluated. We adopted a novel multi-drum architecture to activate more shearing areas within a limited volume. To obtain an ideal sealing effect and reduce the off-state friction, the ferro-fluidic sealing technique was used. The ferro-fluidic sealing assemblies can be assembled from both sides of the shaft. The current–torque model of this brake was derived to predict the torque output based on the current input. The optimal design has the diameter of 28 mm and the width of 23.5 mm. It can provide the maximum and minimum torque of 403 and 4 N mm, respectively, giving the torque–volume ratio of 27.864 kN/m2 and the dynamic range of about 40 dB with the 54 ms time constant.

Haptic devices with multi-finger input are highly desirable in providing realistic and natural feelings when interacting with the remote or virtual environment. Compared with the conventional actuators, MR (Magneto-rheological) actuators are preferable options in haptics because of larger passive torque and torque-volume ratios. Among the existing haptic MR actuators, most of them are still bulky and heavy. If they were smaller and lighter, they would become more suitable for haptics. In this paper, a small-scale yet powerful MR actuator was designed to build a multi-finger interface for the 6 DOF haptic device. The compact structure was achieved by adopting the multi-disc configuration. Based on this configuration, the MR actuator can generate the maximum torque of 480 N.mm with dimensions of only 36 mm diameter and 18 mm height. Performance evaluation showed that it can exhibit a relatively high dynamic range and good response characteristics when compared with some other haptic MR actuators. The multi-finger interface is equipped with three MR actuators and can provide up to 8 N passive force to the thumb, index and middle fingers, respectively. An application example was used to demonstrate the effectiveness and potential of this new MR actuator based interface.

For many applications such as tele-operational robots and interactions with virtual environments, it is better to have performance with force feedback than without. Haptic devices are force reflecting interfaces. They can also track human hand positions simultaneously. A new 6 DOF (degree-of-freedom) haptic device was designed and calibrated in this study. It mainly contains a double parallel linkage, a rhombus linkage, a rotating mechanical structure and a grasping interface. Benefited from the unique design, it is a hybrid structure device with a large workspace and high output capability. Therefore, it is capable of multi-finger interactions. Moreover, with an adjustable base, operators can change different postures without interrupting haptic tasks. To investigate the performance regarding position tracking accuracy and static output forces, we conducted experiments on a three-dimensional electric sliding platform and a digital force gauge, respectively. Displacement errors and force errors are calculated and analyzed. To identify the capability and potential of the device, four application examples were programmed.

Based on magnetorheological fluid, magnetorheological brake can exhibit promising characteristics in haptics such as intrinsic passiveness and high torque density. The main difficulty in applying magnetorheological brake lies in the magnetic hysteresis. To deal with the magnetic hysteresis, a magnetorheological brake was combined with a micromotor to construct a hybrid actuator in this article. A novel hollowed multi-drum architecture was adopted for the brake so that the micromotor could be placed inside the brake to obtain a compact structure. The brake produced the maximum torque of 1263.39 mN m with 40 mm diameter and 28 mm length. Through the closed-loop control, no obvious hysteresis loop was observed in the hybrid actuator current–torque figure. The maximum difference between the forward and backward torque was reduced from 7.2% to 1.94% of the total torque range. The dynamic range was increased from 41.17 to 45.42 dB. Furthermore, the experimental results proved that the hybrid actuator could track the reference signals more accurately than the brake.

Wearable fingertip device is a promising form to display haptic stimuli since it provides a lightweight and natural way for operators to grasp or manipulate the objects in the virtual environment. This paper focuses on the analysis and performance evaluation of a wearable fingertip device for haptic applications. The device is equipped with three small servo motors and can provide 3-DOF (degree of freedom) force feedback at fingertip with contact/non-contact capability. It combines a five-bar linkage and a slider-crank linkage, and these two linkages are decoupled, leading to simpler kinematics than some devices with coupled structures. In order to present the device, its mechanical analysis, kinematics analysis, and static force analysis were carried out at first. Then, four experiments were designed and conducted to evaluate the device performance quantitatively. The first experiment aimed to verify the effectiveness in rendering variable stiffness. The second experiment investigated its capability in providing different skin stretch directions for operators. The third experiment evaluated its performance improvement during virtual manipulation. The last experiment aimed to verify the effectiveness in displaying mass information during remote manipulation. The experimental results indicated that this device was capable of rendering various stiffness. It could generate eight clear skin stretch directions. The subjects had better performance during virtual manipulation with cutaneous feedback provided by the device than without cutaneous feedback. The device was also capable of displaying mass information during remote manipulation.INDEX TERMS Haptics, wearable fingertip device, cutaneous feedback, performance evaluation.