# A State-of-the-Art Review on Robots and Medical Devices Using Smart Fluids and Shape Memory Alloys
## 2 Robots Using ERF and MRF
### MRF-Based Robots
After analyzing the DDRA in terms of output friction, torque level, backlash, and interaction force, a one-link manipulator was integrated with the DDRA, as shown in Figure 4, in order to control the joint torque, and hence the position of the manipulator. It has shown from the experiment that more accurate position control, higher torque bandwidth, and smaller inertia of the DDRA compared to conventional EM actuators can be achieved utilizing the DDRA. [PERSON] and [PERSON] [27; 28] undertook a research similar to [PERSON] et al. [26]. In these works, MR clutch actuators were proposed as promising candidates for human-friendly manipulators because of the clear benefits of MRFs. After surveying the actuator requirements for human-friendly manipulators, such as torque levels and control bandwidth, a distributed active semi-active (DASA) actuation mechanism, comprising a driving motor and semi-active MR clutch, was designed and manufactured. In the design process, the actuator inertia, mass of MR clutch, and output impedance were crucially considered in order to achieve maximum efficiency as a robot actuator. It was demonstrated, via experimental work, that accurate torque control with high control bandwidth could be achieved, thus validating an excellent actuator for human-friendly manipulators, which require intrinsic back drivability, low output inertia, and accurate control performance with high bandwidth. A novel parallel-planar robot using an MR damper was developed, and its effectiveness was validated by demonstrating tracking control performance in the \(x\)-\(y\) plane by [PERSON] et al. [29]. In this work, a linear motor and two MR dampers were used as actuators. The linear motor is responsible for moving the platform, while the MR dampers guide it through predefined paths by creating adjustable and controllable resistive forces. The platform then follows the path, based on the concept that moving objects intuitively tend to follow the direction that imposes minimum resistance against motion. Figure 5 shows the planar robot manufactured in this work. In order to demonstrate control accuracy of the robot, three different trajectories, straight, inclined, and sinusoidal, were adopted as those to be  
Figure 4: Robot arm featuring dual-differential MR actuator; (**a**) configuration (**b**) section view of DDRA [26]. followed by the robot, and excellent tracking control performance was achieved by implementing a PD controller. This work noted one salient property of the planar robot using an MR damper. If an excessive load is applied to the joints by an incorrect controller command, the joints and links will not be easily damaged because the MR dampers can easily absorb the majority of such loads. MR fluid technology has been extended to a controllable climbing robot, using the adhesive effects of MR fluids. [PERSON] et al. [30] developed a novel type of climbing robot using the field-dependent adhesive effect of an MR fluid. Before building the robot, they investigated the adhesion property of MR fluids with, and without, a magnetic field. From this test, the fluid thickness between two parallel plates was found to have little effect on the adhesive failure strength, and a positive effect on time to failure, while the target surface roughness and orientation were found to have a significant effect on pull-off adhesion, and a positive effect on shearing loads. In addition, in order to investigate the holding time of the MR fluid adhesive, a pseudo-creep test was undertaken, and the maximum allowed holding time until failure was identified as 5 min. They manufactured four feet using MRF, by considering the holding time, failure time, and surface conditions of climbing materials, and tested them on a vertical board covered with 150-grit sanding cloth. It was found in the test that the proposed robot could climb the vertical board with a shear of approximately 7.3 kPa, which was generated by activating the magnets.