Differential drive mobile platforms are widely applied in mobile robotics. Omni wheels provide a practical alternative to caster wheels, enhancing mobility and stability by enabling lateral motion while preserving the ground contact point. This research examines a planar mobile platform featuring front differential drive wheels and rear omni wheels, intended for an electric wheelchair. The chassis was constructed from carbon fiber tubes joined by 3D-printed components. Two 4-inch geared BLDC hub motors with integrated hall sensors, controlled by the ODrive motor controller, drive the front wheels, while two$\mathbf{1 2 5}-\mathbf{m m}$omni wheels serve as passive supports on the rear axle. Experiments were conducted by varying the speeds of the front left and right wheels to observe lateral and longitudinal acceleration at three distinct positions: centrally located between the left and right wheels, slightly posterior to the front axle, and adjacent to the left and right omni wheels at the rear axle. During the rotation of the wheelchair around an instantaneous center of rotation, the magnitudes of centrifugal acceleration in the longitudinal and lateral components may vary depending on the passenger's location. The findings from this study can serve as a framework for regulating motor speeds during wheelchair turns to reduce discomfort.

Planetary gears offering high reduction ratios with concentric input and output shafts are widely applied in various mechatronic systems. For optimizing the efficiency of machines across their range of operating conditions, this paper presents a design concept of the planetary geared continuously variable transmission (PG-CVT) using an electromagnetic (EM) brake to adjust the slipping torque of the rotatable ring gear. The input torque is applied to the sun gear while the output shaft is connected to the planet carrier. The secondary velocity source for driving the ring gear is replaced by the brake resisting the rotation, which requires less energy to operate. When the brake is fully locked, the planetary gear operates with stationary ring gear. The brake torque required to support the ring gear is the linear combination of the input torque at the sun gear and the output torque at the carrier. When the brake is freely slipped, the output torque is limited to zero although the sun gear is rotating. The variable transmission ratio can be achieved by adjusting the torque applied to the ring gear, which results in changing the torque conversion ratio. In this paper, the experimental prototype of the PG-CVT was designed and built. At different output torque and brake torque conditions, the constant speed tests were conducted to observe the input torque required for maintaining the constant speeds. The static friction and the damping of the PG-CVT were characterized. It is worth noting that the zero or negative damping phenomenon was observed from a constant brake torque. The findings of this study are fundamental for implementing velocity feedback torque control for achieving desired dynamic responses.

Legged robots have high mobility for application with uneven terrain. Bipedal robots require the lower number of actuators with less complication of control system, but their walking stability is the main challenge. This paper presents a design of the bipedal robot having two actuated joints for the hip and the knee and one passive joint for the ankle of each leg. To minimize the inertia of the moving legs, the four brushless motors with integrated controller and planetary gearbox are concentrically located at the hip axis while the knee joints are driven through the parallel linkages. The zero-moment point (ZMP) along the foot support contacting the floor was derived based on the table-cart model. During the stance phase of stable walking, the desired ZMP must be located within the foot boundary. The ZMP generator for shifting the center of mass (COM) forward according to the stride was simulated along with kinematics of the legs. The hip and the knee joint trajectories were implemented on the bipedal robot prototype. The desired leg kinematics was validated by the experiment. As the load supported by the feet, the effect of gravity on the leg joint positions and torques was also studied. Additional sensing and the control of joint stiffnesses will be applied for achieving the dynamic walking of the bipedal robot.

Image processing was applied for the real-time evaluation of ultrasound imaging quality. Simultaneous observation of the ultrasound probe's contacting force, tilting orientation, and operational trajectory in spatial coordinates can provide useful feedback for supporting ultrasonography training. This paper presents a 6-DOF passive robotic arm enabling the complete tracking of the probe position and orientation. The custom designed handle located at the end effector is assembled from the inner and outer shells, which allows installation of a tiny three-axis force sensor for measuring the probe contact force. The forward kinematics of the robot arm is derived for mapping the joint variables to the position and orientation of the tip. The real-time measurement of the joint angles is achieved from the non-contact magnetic encoders for the first to fourth revolute joints and the rotary potentiometers for the 2-DOF ball joint. The three-dimensional testbed consisting of the radial arrays of hexagonal slots is used for evaluating the position and orientation accuracy of the passive arm. The results of this study can be used as a guideline for further development of passive robotic arms to achieve a higher level of accuracy on tracking the probe trajectory.

Bridging exercise is a widely applied training for stroke rehabilitation to improve balancing ability on weight-bearing activities. Aiming to reduce the workload of physical therapists and enable the systematic recording of motion data, this paper presents an affordable rehabilitation monitoring system using an RGB camera. For predicting the correctness of the bridge posture, the MediaPipe framework is applied for detecting the human body segments which are used as the input data of the decision tree classifier instead of using a complex neural network. The model was trained using the data collected from five healthy participants performing the correct and Wide Knee postures when the knees are separated laterally. The experimental results show that nearly 100 percent accuracy can be achieved in confirming the correct posture and identifying the Wide Knee posture. The time performance of the decision tree classifier trained by the different number of frames is also evaluated. This system is very promising to help therapists monitor patients and provide feedback for improving the effectiveness of the rehabilitation.

Soft pneumatic actuators providing infinite passive degrees of freedom are widely applied for safe physical human–robot interaction and fulfilling the limitation of conventional rigid structures. This paper presents a design of the soft actuator with pneumatic chamber enabling the control of bending and contact force. The actuator is solely fabricated from the thermoplastic polyurethane (TPU 95A) by using the fused deposition modeling (FDM) 3D printing. Several printing setups were performed for trial and error to prevent the air leak. The experimental setup was prepared for measuring the contact force in both vertical and horizontal directions. The grasping force varying against the input pressure ranged from 0 to 58 psi was studied at three contacting points along the longitudinal axis of the actuator with three different bending conditions. The stiffness of the actuator is related to the input pressure by considering the increase of total contacting force at different bending deformations. The results of this study can be applied to the stiffness control of soft pneumatic actuators.

Our goal is to investigate different approaches to modulate stiffness and apply them to human-robot interaction. Here we report on our effort employing the concept of adjustable unsupported-length cantilever leaf spring, which has been previously applied to different designs of variable stiffness actuators. By transmitting the interaction force through the elastic component directly to the supporting structure instead of the actuation unit, this type of actuator requires low power to adjust and to maintain a desired stiffness. In the design of a 1-translational degree of freedom body weight support system of a rehabilitation robot, we used a leaf spring mechanism for stiffness modulation relying only on the spring deflection in combination with a non-backdrivable actuator for adjusting the vertical equilibrium position. This paper describes our approach in determining the spring parameters to attain a desired range of stiffness with a short traveling distance of the adjuster. To model the spring stiffness under deflection, the ideal cantilever support model cannot be assumed for a conventional design of dual roller-pairs slider, especially with a soft spring. A beam deflection model considering the non-zero slopes at the contact points between the rollers and the spring is presented, along with the validation experiments using different spring thicknesses on our prototype.

Manipulating mechanical energy intelligently, passive exoskeletons can improve the energy efficiency of cyclic human motions. Aiming to reduce the energy cost of knee moment during stair ascent, this paper presents a concept of brake-torque support activated when the knee moment is required in the opposite direction to the angular velocity of the knee joint. Integrating an electromagnetic brake to a crossing four-bar knee joint with a compact design, the passive knee exoskeleton enables the polycentric knee center of rotation covering the wide range of knee angle during stair ascent. For preliminary validation, the surface electromyography (EMG) of rectus femoris (RF) and biceps femoris (BF) were studied on a healthy male volunteer wearing the exoskeleton on his right leg. The reduction of peak muscle activity is observed as the brake torque is applied during knee extension while the knee flexion moment is required.

In this paper, we introduce a concept of passive knee exoskeleton for cycling assistance. Considering a knee moment and a knee angle varying with a pedal crank angle, the knee extension moment can be supported by a torsion spring storing energy from knee flexion in order to release it as the knee is extended. The reduction of knee extension effort is corresponding to the torsion spring stiffness and activation range. Exoskeleton prototypes were developed for the concept validation. A crossing four-bar mechanism was chosen for the knee joint to provide kinematic compatibility covering extreme knee flexion. Constant power cycling experiment was performed on a cycling trainer by a healthy subject wearing the exoskeletons on both legs. With the torsion spring support, the surface electromyography recorded from some major leg muscles shows the decrease of knee extensor muscle activity as the leg is moving around the pedal crank top dead center. Verifying the reduction of leg muscle fatigue over repetitive contractions in multiple subjects is our future plan of study.