Series elastic actuators (SEAs) achieve output torque and stiffness control by managing the deformation of a spring arranged between the motor and the output. SEAs are ideal for tasks involving human-robot interaction and unstructured environments. The stiffness, size, and torque capacity of the spring are crucial for the performance of an SEA. To increase the torque capacity of an SEA while maintaining a compact size, this paper proposes a novel helical flexure as the spring in an SEA. The helical flexure is formed on the thin wall of a tube to generate high torque output with minimal reaction forces and moments. The resulting compliant tube can be combined with other transmission components to reduce the number of components and allow the inner passage of cables and shafts. Simulation comparisons and experimental testing verify the merits of the proposed helical flexure. An SEA prototype is fabricated to demonstrate the performance of the new helical flexure during zero-torque and high-torque motion.

This paper presents the design and experimental validation of a torque-controlled robotic wrist for peg-in-hole tasks. The proposed wrist features a serial pitch-yaw joint configuration that enhances dexterity while maintaining compactness. The design integrates stepper motors, harmonic geartrains, and compliant mechanisms to optimize torque output and control accuracy. A compliant pulley and a compliant cap are introduced, enabling embedded torque sensing without the need for external sensors, thereby reducing system complexity and improving response time. Experimental results demonstrate the effectiveness of the wrist in torque accuracy and misalignment correction during peg-in-hole assembly, highlighting the benefits of the compliant-driven torque sensing approach. Compared to existing robotic wrists, the proposed design achieves a higher torque density. The findings contribute to advancing robotic wrist technology, particularly in applications requiring precise force modulation, high dexterity, and adaptable compliance.

Various sensorized grippers have been developed to handle delicate objects safely. These grippers have sensors mounted on their fingers’ surface that provide direct force measurements. However, multiple sensors are often required on one finger, leading to significant sensor placement and wire routing complexity. Finger-based sensors are limited to sensing external gripping force, and fingers cannot be easily replaced to meet the requirements of objects with specific geometries. To overcome the complexity and limitations of finger surface sensors, this paper proposes a force-controlled two-fingered gripper that relies on the deformation sensing of elastic elements in the drivetrain to obtain finger force. By using a minimum number of optical encoders placed in the drivetrain, accurate position and force sensing can be achieved at any location of each finger. When gripping an object, the size and stiffness of the object can thus be accurately measured. Simulation and experimental results demonstrate the proposed gripper’s merits. We expect this new gripper to provide a more competitive solution for robots that need to manipulate objects and check their mechanical qualities at the same time.

Industrial arm-type robots have multiple degrees-of-freedom (DoFs) and high dexterity but the use of the roll-pitch-roll wrist configuration yields singularities inside the reachable workspace. Excessive joint velocities will occur when encountering these singularities. Arm-type robots currently don't have enough dexterity to move the end-effector path away from the wrist singularities. Robots with redundant DoFs can be used to provide additional dexterity to avoid the singularities and reduce the excessive joint velocity. An end-effector wrist module is proposed to provide two redundant DoFs when interfaced with an existing 6-DoF robot. The new 8-DoF robot has a compact roll-pitch-yaw wrist that has no singularities inside the reachable workspace. The highly redundant robot can also be used to avoid collisions in various directions. Path tracking simulation examples are provided to show the advantages of the proposed design when compared with existing redundant or nonredundant robots. We expect that this module can serve as a cost-effective solution in applications where singularity-free motion or collision-free motion is required.

Controlling the contact force on workpieces has been a challenging task for industrial grinding/deburring operations. Its realization often requires a grinding spindle with a multi-axis force sensor and controller feedback. The spindle needs to frequently vary its position in order to maintain a constant contact force. The use of sensors and control is costly and introduces extra complexity for grinding tools. To improve the polishing quality of handling workpieces of irregular contours, this paper presents a novel force regulation mechanism (FRM) to be installed on grinding tools. Without using additional sensors and control, the FRM can passively produce an adjusTable NORMAL Contact force between the tooltip and workpiece of various geometry. the spindle does not have to move to regulate the contact force. together with a simple grinder which is much less expensive, this approach offers a more attractive solution in terms of cost and complexity. in this paper, the design concept and simulation results are presented and discussed. a prototype of a grinder with the proposed FRM is illustrated to demonstrate the effectiveness and accuracy of force regulation. this novel mechanism is expected to serve as a reliable alternative for industrial grinding/deburring operation.

Controlling the gripping force on fragile objects has been a challenging task for industrial grippers. The solution often requires an electric gripper with embedded force sensors and control feedback. This approach is costly and leads to extra gripper complexity. To avoid damage while handling fragile objects, this paper presents a novel force regulation mechanism (FRM) to be installed on pneumatic grippers. Without using additional sensors and control, the FRM can passively produce an adjustable contact force between the gripper jaws and objects of various sizes. Together with pneumatic grippers that have a higher gripping speed, lower cost, and simpler structure, this approach offers a more attractive solution than the use of electric grippers. In this paper, the design and analysis of the FRM are presented. A prototype of the FRM is illustrated to demonstrate the effectiveness and accuracy of force regulation. This novel mechanism is expected to serve as a reliable alternative for fragile object manipulation. Note to Practitioners-Pneumatic grippers are popular due to their low cost and simple structure. However, it is difficult to control the gripping force to handle fragile objects without damage. The FRM has a small size and can be easily installed on pneumatic grippers to provide fast force regulation. The gripping force depends on axial preload of the FRM rather than air pressure of the gripper. Axial preload is passively regulated using a screw or spacer. No additional source of power or sensor is required. The FRM has a large force adjustment range and can grip objects of various sizes using the same force. Applications include the automation in electronics, food, agricultural, and medical industries.

Powered exoskeletons can provide motion enhancement for both healthy and physically challenged people. Upper limb exoskeletons are required to have multiple degrees-of-freedom and can still produce sufficient force to augment the upper limb motion. The design using serial mechanisms usually results in a complicated and bulky exoskeleton that prevents itself from being wearable. This paper presents a new exoskeleton design aimed to achieve compactness and wearability. We consider a shoulder exoskeleton that consists of a parallel spherical mechanism with two slider crank mechanisms. The actuators can be placed on a stationary platform and attached closely to human body. Thus a better inertia property can be obtained while maintaining lightweight. Through the use of a gravity-balancing mechanism, the required actuator power becomes smaller and with better efficiency. A static model is developed to analyze and optimize the exoskeleton. Through illustrations of a prototype, the exoskeleton is shown to be wearable and can provide adequate motion enhancement of a human's upper limb.

It has been a challenge to design robots that possess intrinsic compliance, especially for robots that are required to achieve multi-DOF manipulation. Inspired by human limbs, robotic manipulators with internal compliance can perform high-quality force/torque control and better human-robot interaction. This paper presents a robotic wrist whose size, range, and torque output are comparable to those of a human wrist. To achieve two collocated and perpendicular axes of compliant actuation, two linear compliant couplers are proposed. Through slider crank and spherical mechanisms, the linear elasticity is converted to rotary elasticity to control the pitch and yaw torques at the same time. This new compact design realizes series elastic actuation in both axes without increasing size and complexity. Static and dynamic models of the compliant wrist are developed to analyze the motion. Through experiments of a prototype, the wrist is shown to achieve accurate and fast force/torque control. We expect this novel compliant wrist to serve as an alternative for applications involving human-robot interaction.

Force regulation is a challenging task of forceps used for robot-assisted surgical manipulation. To avoid excessive force applied on soft tissues, sophisticated sensors with computerized precise control are often required. Without using additional electronic elements, this paper presents a passive mechanism to maintain a constant contact force between forceps jaw tips and tissues given a pre-specified force magnitude. The mechanism consists of symmetric flexible structures specifically designed to generate a constant torque regardless of input rotation. The constant torque is converted to a constant force through an adjustable lever arm. When the force is further transmitted to jaw tips, it keeps a nearly constant contact force regardless of tissue stiffness and size. After a formulation to find the optimal mechanism configuration, the design is verified by comparing experiment and simulation results. A prototype of the adjustable constant-force forceps is finally illustrated and discussed. The novel forceps is expected to serve as a reliable alternative for robot-assisted surgeries.

Force regulation is a challenging issue of robot end-effectors when interacting with unknown environments. It often requires sophisticated sensors with computerized control. This paper presents a constant-force mechanism (CFM) to regulate the contact force of a robot end-effector. The proposed CFM is a monolithic compliant mechanism that has no frictional wear and is capable of miniaturization. Due to the passive mechanism, additional sensors and control effort are minimized. We propose a design formulation to find the optimal CFM shape that produces the most constant force. The reaction force to input displacement curve is invariant of size and flexural rigidity. The curve can be manipulated depending on the desirable situations. The CFM is validated through an experiment. When equipped with the CFM, an illustrative end-effector can adapt to a surface of variable height, without additional motion programming. With the merits shown, we expect this type of elastic mechanism can be utilized in robot end-effectors to provide friendly contact with environment.

This paper investigates and realizes the self-sensing capability of shape memory alloy (SMA) actuators. SMA exhibits large stroke, high energy density, and requires low driving voltage. To make SMA more applicable to small scale robotic manipulations, its motion control using accurate self-sensing is necessary. The presented technique builds a self-sensing model by measuring the SMA electrical resistance. Effects of pretension force on strain and force self-sensing are investigated. The model is polyfitted to replace sensor electronics for strain or force feedback. A pretensioner is specifically designed to provide sufficient pretension force without affecting the subject to be actuated. The advantages gained from using polyfitted self-sensing models are demonstrated through several step response control experiments. With the merits shown, we expect this technique can be utilized for SMA actuators in meso to micro scale applications.

This paper presents the design, fabrication, and control of a rotational manipulator using shape memory alloy (SMA) wire actuated flexures. Monolithic flexure mechanisms have no friction/backlash and are capable of miniaturization. They are well-suited for tasks that required high precision and packed space. To explore flexure shapes beyond traditional notch hinges and leaf springs, we present a general two-step design method to find the optimal flexure shapes for maximal rotation without yield. The advantages gained from shape variations are shown through a simulation example. We further use a SMA wire to drive the flexure. SMA exhibits large stroke with high power density and requires low driving voltage. By using versatile SMA wire, the rotational range of the manipulator can be significantly increased while the overall size can be kept compact. A feedback PID control algorithm with fuzzy-tuned gains is implemented to precisely control the response of the manipulator. We illustrate its performance by tracking and step response experiments. With the merits shown, we expect this type of manipulator can be utilized in meso to micro scale applications.

The analysis of compliant mechanisms has traditionally based on known initial shapes and external forces. For applications, it is often required to find an initial shape for the specified deformed shape of the mechanism. We present here the global coordinate model (GCM) with a numerical solver that is capable of forward and inverse analysis. The model uses the arc length as the independent variable so that the shape of straight and curved links can be easily expressed. The resulting governing equations are a generalization of Timoshenko's beam theory that accounts for the effects of bending and shear deformations on large-deflected links. The effect of shear deformation on link deflection will also be investigated. Systematic procedures are developed to analyze generic compliant mechanisms. Both forward and inverse illustrations are presented. Their applications for robotic handling of bio-material are also shown. It is expected that the proposed model can give more insight on the analysis and design of compliant mechanisms

We consider here a class of compliant mechanisms that consist of one or more flexible beams, the manipulation of which relies on the deflection of the flexible beams. As compared with traditional rigid-body mechanisms, compliant mechanisms have the advantages of no relative moving parts and thus involve no wear, backlash, noises and lubrication. In this paper, we present a formulation based on shooting method (SM) along with two numerical solvers to facilitate the analysis and the design process of a compliant mechanism. Unlike finite difference method (FDM) or finite element method (FEM) that offers accurate solutions at discrete nodes, the computed solution of SM, which treats the boundary value problem (BVP) as an initial value problem, is continuous. Three example compliant mechanisms are formulated to illustrate the generalized shooting method and the computed results are validated by comparing those obtained using FEM.

Motivated by the applications of flexible fingers (capable of offering large deflections to accommodate object variations) in grasping, we present several computational models that characterize the large deflection of a flexible finger (beam). Specifically, we develop analytical methods for analyzing the design of cantilever-like fingers or elements of a machine that is designed primarily to support forces acting perpendicular to the axis of the member. Both uniform and non-uniform beams are considered. The methods were numerically validated by comparing the computed results against those obtained using the closed-form solutions, where exact solutions are available for fingers with a uniform cross-section. To extend the closed-form solution for predicting the shape of a non-uniform finger, we compute numerically an effective EI that approximates the non-uniform finger as a uniform finger at the point of contact. The approximate model has been examined experimentally. The results show excellent agreement. We expect that the methods presented here will have other engineering applications.