A wide-range variable stiffness mechanism for semi-active vibration systems

A machine with large output stiffness variation can meet the requirement in different working environments. This paper presents an approach to design a linear variable-stiffness mechanism (LVSM) with adjustable zero-to-infinite stiffness. The idea is to use circular slots to adjust the initial rotation of two parallel connected springs. The output force to displacement curve can exhibit zero to very large stiffness depending on the rotation of the springs. Infinite stiffness is achieved by using mechanical stoppers to constrain the displacements of the springs. The merit of the proposed LVSM is that zero and infinite stiffness can be simultaneously achieved in a compact space. To further reduce size and complexity, specifically designed planar springs are proposed to replace commercially available coil springs. Force and stiffness analyses are presented to design a LVSM with the largest stiffness variation. The effects of various parameters on the stiffness variation are discussed. The results are numerically verified with a prototype illustrated.

Designing robotic systems capable of low-power operation, inherent to their compliant actuation, has been elusive in practical application. In this paper, we propose a physical measure to mathematically define mechanical designs that are suitable to realize stiffness modulation with low power cost. Using this measure, we present a mathematical formulation of an ideal variable stiffness mechanism unaffected by the external load during its operation. We then analyze several existing mechanisms from the literature to relate design features with analytical conditions inherent to low power stiffness modulation in practical designs. Through this analysis, we identify an approximate practical realization of an ideal actuator capable of stiffness modulation with inherently low power cost. Similar to a number of existing efficient variable stiffness mechanisms, this mechanism is able to hold a given stiffness setting with zero input force under no external load. However, unlike many other previously designed mechanisms, it enables infinite range stiffness modulation using finite control forces. A practical variable stiffness mechanism that is capable of infinite range stiffness modulation using finite control forces leads to lower power cost and reduced energy consumption.

This paper presents a compact variable stiffness mechanism (VSM) with reconfigurability. The VSM is able to achieve variable stiffness in adjustable ranges, due to its reconfigurable design. The new VSM features with a compact and light weight structure, and can be used as a stand alone module, or integrated in wearable elbow exoskeletons and other transmissions. In the paper, mechanics models are developed for VSM to reveal the stiffness performance and torque-deflection characteristics. A prototype of VSM is constructed and static testing results validate the models. The proposed VSM is finally integrated in a wearable elbow exoskeleton.

Semi-active vibration isolation system with variable stiffness and damping control

Wearable robots, especially power suits to enhance human activity are one of most interesting topics. This study aims to develop a power suit installing a variable stiffness mechanism. In a previous study, we showed that the relation between impedance parameters of power suits and human performance, and verified the effectiveness of self impedance matching method using soft computing. In this paper, we conduct fundamental experiments in order to design a variable stiffness mechanism, which can tune the stiffness of joint mechanically. Experimental results show the effectiveness of variable stiffness for improving human performance and reducting muscle fatigue. Moreover, we try to design the variable stiffness mechanism, and to verify the effectiveness by simulation.

In this paper, we developed two knee assist exoskeletons with a variable stiffness mechanism. These exoskeletons support human knee torque through increasing the stiffness during stance phases and decreasing the stiffness during swing phases. We conducted experiments to evaluate the assist effects. In the experiment, we compared three conditions: the subject did not wear the exoskeleton, the subject wore the exoskeleton and the exoskeleton did not support torque of the subject, and the subject wore the exoskeleton and the exoskeleton supported torque of the subject. We measured metabolic energy of the all conditions. As a result, we verified that the variable stiffness exoskeleton reduce 13.4[%] of metabolic energy. We made an improved exoskeleton that is 1.2[kg] lighter than the old one, and has a variable stiffness mechanism using not a servomotor but an electromagnetic clutch. This mechanism can realize more quick stiffness adjustment and energy saving. We verified that this improved exoskeleton can adjust stiffness.

Configuration synthesis of variable stiffness mechanisms based on guide-bar mechanisms with length-adjustable links

In this paper, a novel variable stiffness mechanism (VSM) based on specially designed S-springs made from shape memory alloy (SMA) is developed. Based on the stiffness model, by changing the state combination of the SMA S-springs with different thicknesses, the actuator's stiffness can be discretely adjusted online. The stiffness adjustment range of the actuator can be conveniently extended almost from 0 N/m to infinity by changing the preset spring angle manually offline without a complex mechanical mechanism or replacing any parts. A linear digital variable stiffness actuator (LDVSA) is also designed with the VSM. Some tests of the SMA S-springs, as well as the actuator's stiffness under different configurations, comparisons with other actuators and its trajectory tracking capacity are conducted to verify its design. Then, a dynamic model of the actuator is established and its bandwidth is analysed. Based on this, explosive experiments are designed and performed to explore the application potential of the actuator. Experimental results are provided to illustrate the explosive capacity and energy efficiency of the proposed design.

Soft robots have advantages such as low weight and compact size compared to rigid robots. Variable stiffness is one of the key methods of improving the performance of a soft robot. The soft gripper grasps objects of various shapes or sizes based on the variable stiffness. In a previous study, we validated the variable stiffness mechanism where flexible and rigid segments were connected alternately in series. This paper presents a soft variable stiffness gripper that can be used to control the stiffness by pulling tendons. The soft gripper has three variable stiffness structures acting as fingers and the stiffness can be controlled using two motors by winding tendons. To understand the tendency of the stiffness variation and determine the design parameters, a compliant mechanism was developed using a pseudo-rigid-body model (PRBM). The experimental results show that the difference between the lowest and highest stiffness values of the fabricated variable stiffness gripper was 5.6 times the original. Similarly, the difference in the gripping weight was 19 times. Using the experimental results, the variable stiffness gripper can be designed and manufactured based on the required stiffness and used to grip various types of objects.

A novel variable stiffness mechanism (i.e., variable spring) based on the concept of tensegrity structures is presented. Variable springs have extensive applications in noise and vibration control. The proposed method builds upon the prestress stiffness in tensegrities, which occurs along infinitesimal mechanisms and is fully controllable through force control in the members. A criterion is given to select a suitable tensegrity structure and an infinitesimal mechanism to develop a variable spring. Also, a mathematical model is developed for the stiffness components in an n-gon tensegrity prism. The variable components of the stiffness are then utilized to create a translational or rotational variable spring. In order to elaborate on the feasibility of the concept, a case study is presented on the engine mount of a vehicle. Parameters of a possible design of a variable stiffness mount are given, and the characteristics are compared with those of a conventional passive mount. This is followed by a detailed discussion on the properties of such a variable spring and the effects of various parameters.

Variable stiffness mechanisms as Jamming Transition draw huge attention recently in Soft Robotics. This paper proposes a comprehensive design methodology for developing variable stiffness devices based on layer jamming. Starting from pre-existing modelling, we highlight the design parameters that should be considered, extracting them from literature and our direct experience with the phenomenon. Then we validated the methodology applying the design process to previous layer jamming cases presented in literature. The comparison between the results obtained from our methodology and those presented in the analyzed previous works highlights a good predictive capability, demonstrating that this methodology can be used as a valid tool to design variable stiffness devices based on layer jamming transition. Finally, in order to provide the scientific community with an easily usable tool to design variable stiffness structures based on layer jamming transition, we have elaborated a Matlab script that guides the user through the main design parameters implementing the proposed methodology in an interactive process.

Human lower limbs have particular flexibility. Both the efficiency of bipedal walking and the ability to protect actuators with low energy loss are worthy references for the design of bipedal robots. This paper proposes a design for a biped robot with joints of variable stiffness. The robot has three degrees of freedom in the sagittal plane in each leg. The hips and knees are driven directly by the motor, while the ankles are passive joints with adjustable stiffness. After a comprehensive investigation, a variable stiffness mechanism was introduced based on lever principles, and driven by a motor that can realize real-time adjustment. Simulations verified the necessity of variable stiffness joints in the robot. The variable stiffness joint can absorb the ground impact on each joint, reduce the energy loss of the motor, and improve the efficiency of movement.

A new variable stiffness suspension system based on a recent variable stiffness mechanism is proposed. The overall system is composed of the traditional passive suspension system augmented with a variable stiffness mechanism. The main idea is to improve suspension performance by varying stiffness in response to road disturbance. The system is analyzed using a quarter car model. The passive case shows much better performance in ride comfort over the tradition counterpart. Analysis of the invariant equation shows that the car body acceleration transfer function magnitude can be reduced at both the tire-hop and rattle space frequencies using the lever displacement transfer function thereby resulting in a better performance over the traditional passive suspension system. An H∞ controller is designed to correct for the performance degradation in the rattle space thereby providing the best trade-off between the ride comfort, suspension deflection and road holding.

This paper proposes a semi-active variable stiffness vibration isolation system based on electromagnetic spring for the low-frequency vibration isolation of mass-varying objects. It is achieved by four straight leaf springs in parallel to an electromagnetic spring system composed of a single electromagnet and a permanent magnet. The equivalent magnetic circuit method is used to compute electromagnetic force of the electromagnetic spring system, and mathematical model of the semi-active vibration isolation system is established according to Maxwell's equations. The nonlinear mathematical model is linearized at the equilibrium point by using the Taylor series expansion theorem to establish linear state-space representation of the system, and then using the traditional PID control method, a double closed-loop feedback control system of the inner current loop and outer location loop is designed. By controlling the current in the coil, the equivalent stiffness and electromagnetic force of the system are variable to achieve semi-active control. Furthermore, the control block diagram of the semi-active vibration isolation system is built based on Simulink software, then make a simulation analysis to the vibration isolation performance of the system and compare the effects of vibration isolation with inner current loop control and without inner current loop control, respectively. Finally, the experiments prove the correctness of the theory. It concludes that this semi-active vibration isolation system is a vibration isolation system with broad application prospects, which has fast current response, high vibration isolation efficiency, and an excellent vibration isolation effect for the low-frequency disturbance of mass-varying objects.

The applications of robotic fish require high propulsive efficiency mechanism to prolong the mission time. Though many methods were applied, robotic fish still suffers from low efficiency. To improve the efficiency of robotic fish, this paper proposes a variable stiffness mechanism which is based on the negative work. The live fish adjusts its body stiffness to save energy when the muscles do negative work. Inspired by the live fish, a control mechanism based on negative work is proposed to change the stiffness of the robotic fish for higher efficiency. Changing the stiffness of the robotic fish is to change the joint-stiffness. A fuzzy controller is introduced to mimic the variable stiffness mechanism of the fish and depicts the relationship between the stiffness and the negative work. To evaluate the performance of this controller, a two-joint robotic fish model is established based on its kinematic model and hydrodynamic model. The evaluation results show that the robotic fish reduces the energy consumption and improves the propulsion efficiency when introducing the variable stiffness mechanism. Different environments with the control mechanism impact differently on propulsive efficiency. This mechanism may provide a high efficient propulsion control method for the robotic fish.