Output Stiffness Research Articles

Design, Analysis and Testing of a Mirco/Nano-Transmission Platform

On the basis of geometrical characteristic of the mirco/nano-transmission platform(MNTP), its kinematics model and input/output motion are established by the theory of coordinate transformation. The stiffness model was derived using elasto-dynamics and Lagrange equation. In terms of ANSYS, the output displacement and stiffness of different degrees of freedom(DOF) was simulated. Actual output displacement about x axis and stiffness along z axis were obtained by experimental testing, the results of them obtained by theoretical computation, ANSYS simulation and testing were compared. A good agreement was achieved, which shows that the kinematics model, stiffness model, the design principle and experiments of the MNTP are correct.

Multiscale modeling of skeletal muscle properties and experimental validations in isometric conditions

In this article, we describe an approach to model the electromechanical behavior of the skeletal muscle based on the Huxley formulation. We propose a model that complies with a well established macroscopic behavior of striated muscles where force-length, force-velocity, and Mirsky-Parmley properties are taken into account. These properties are introduced at the microscopic scale and related to a tentative explanation of the phenomena. The method used integrates behavior ranging from the microscopic to the macroscopic scale, and allows the computation of the dynamics of the output force and stiffness controlled by EMG or stimulation parameters. The model can thus be used to simulate and carry out research to develop control strategies using electrical stimulation in the context of rehabilitation. Finally, through animal experiments, we estimated model parameters using a Sigma Point Kalman Filtering technique and dedicated experimental protocols in isometric conditions and demonstrated that the model can accurately simulate individual variations and thus take into account subject dependent behavior.

A novel SU-8 electrothermal microgripper based on the type synthesis of the kinematic chain method and the stiffness matrix method

This paper presents a new systematic design and optimization procedure used for the microgrippers driven by a chevron electrothermal actuator. The procedure includes three steps: first, a suitable rigid-body gripper mechanism is selected using the type synthesis of the kinematic chain method; then, the rigid-body mechanism is transferred into a compliant microgripper; finally, by the stiffness matrix model and the genetic algorithm, a geometry parametric optimization with the high output stiffness objective is carried out. Using this procedure, a novel SU-8 electrothermal microgripper is obtained. According to the FEM simulation, the microgripper meets the design requirements and satisfies the constraints. To eliminate the out-of-plane actuation, a novel processing technology is implemented to fabricate the microgripper with a sandwich structure actuator. The experimental results demonstrate that a jaw gap change of 107.5 µm requires only 73.6 mV, 25.61 mW and only 44.92 °C temperature increase at the actuator and the out-of-plane actuation is almost eliminated. A micromanipulation of a micro blood vessel specimen and a micro-assembly for micro-tensile testing studies of fine hair are demonstrated. Hence, the design procedure is valid to generate novel compliant micro mechanisms. The fabrication process can be used in the fabrication of other SU-8 MEMS devices actuated by the electrothermal actuator.

Embodying Desired Behavior in Variable Stiffness Actuators

AbstractVariable stiffness actuators are a class of actuators with the capability of changing their apparent output stiffness independently from the actuator output position. This is achieved by introducing internally a number of compliant elements, and internal actuated degrees of freedom that determine how these compliant elements are perceived at the actuator output. The introduction of a mechanical compliance introduces intrinsic, passive oscillatory behavior to the system, but rather than trying to minimize this effect, the question arises if it can be exploited for the actuation of periodic motions. In this work, we propose a strategy to control the variable stiffness actuator optimally, with respect to a cost criterion, to a desired periodic motion of the output. In particular, the cost criterion provides a measure of embodiment of the desired behavior in the passive behavior of the variable stiffness actuator, i.e., the variable stiffness actuator is controlled such that its passive behavior is as close as possible to the desired behavior and thus that the control effort is minimized.

Energy Efficient Control of Robots with Variable Stiffness Actuators

Variable stiffness actuators realize a particular class of actuators characterized by the property that the apparent output stiffness can be changed independently of the output position. This is feasible due to the presence of internal springs and internal actuated degrees of freedom. In this work, we establish a port-based model of variable stiffness actuators and we derive an energy efficient control strategy. In particular, when the variable stiffness actuator acts on a mechanical system, the internal degrees of freedom are used to achieve the desired nominal behavior and the internal springs are used as a potential energy buffer. The release of energy from the springs, as well as the apparent output stiffness, are regulated by control of the internal degrees of freedom. Simulation results on a robotic joint illustrate the effectiveness of the control strategy during the tracking of a periodic motion in presence of disturbances.

Stable haptic interaction using a damping model to implement a realistic tooth-cutting simulation for dental training

It is difficult to implement a stable and realistic haptic simulation for cutting rigid objects that is based on a damping model because of an inevitable conflict between stability and high output force. This paper presents passivity techniques to show that an excessive damping coefficient causes the output stiffness to exceed the maximum output stiffness of the haptic device, leading to instability. By analysing the damping model of a haptic dental-training simulator, we construct a relationship among the damping coefficient, position resolution, sampling frequency, human operation, and the maximum achievable device stiffness that will still maintain device stability. A method is also provided to restrict the output stiffness of the haptic device to ensure stability while enabling the realistic haptic simulation of cutting rigid objects (teeth) that is based in a damping model. Our analysis and conclusions are verified by a damping model that is constructed for a dental-training haptic display. Three types of haptic devices are used in our analysis and experiments.

Measurement of output force distribution and stiffness in human limbs

Measurement of output force distribution and stiffness in human limbs

Amplification Ratio and Stiffness of Bridge-Type Flexure Hinge

Bridge-type flexure hinge is a classical flexure amplification mechanism. The amplification ratio and stiffness are the key parameters in an amplification mechanism. Elastic mechanics was used to analyze the input and output stiffness of bridge-type amplification mechanism in relation to the translational and rotational stiffness of the flexure pivots. The result showed that the stiffness model can explain well the outer characteristic of bridge-type flexure hinge.

Simultaneous Force and Stiffness Control of a Pneumatic Actuator

This paper proposes a new approach to the design of a robot actuator with physically variable stiffness. The proposed approach leverages the dynamic characteristics inherent in a pneumatic actuator, which behaves in essence as a series elastic actuator. By replacing the four-way servovalve used to control a typical pneumatic actuator with a pair of three-way valves, the stiffness of the series elastic component can be modulated independently of the actuator output force. Based on this notion, the authors propose a control approach for the simultaneous control of actuator output force and stiffness. Since the achievable output force and stiffness are coupled and configuration-dependent, the authors also present a control law that provides either maximum or minimum actuator output stiffness for a given displacement and desired force output. The general control and maximum/minimum stiffness approaches are experimentally demonstrated and shown to provide high fidelity control of force and stiffness, and additionally shown to provide a factor of 6 dynamic range in stiffness.

Design, Modeling, and Initial Experiments on Microscale Amplification Device

This article proposes the design of several microelectromechanical amplification devices formed of many identical microunits that are connected in a serial–parallel configuration, each being individually actuated and amplifying its own input motion. The microdevices realize the border-crossing from the micro-to the meso-scale displacement domain as they combine the micron-level individual inputs into millimeter-range output levels. The base structural unit is a flexure-based compliant device that is capable of transforming the input from a thermal actuator into an amplified displacement, about a direction perpendicular to the input one. The base unit is designed based on performance criteria, such as displacement amplification, input stiffness and output stiffness by utilizing finite element simulation, and an algorithm based on closed-form compliance equations of the incorporated flexure hinges. The microelectromechanical amplification device is monitored by means of embedded capacitive displacement sensors for the input port. The feasibility of the device design was verified through a numerical simulation and some initial experimental results are presented.

High cardiac output and arterial stiffness evaluated by research cardiovascular profiling system in a hypertensive patient with pseudoxanthoma elasticum

Pseudoxanthoma elasticum (PXE) is an inherited systemic disorder characterized by generalized fragmentation of elastic fibers and progressive calcification of elastic tissue, affecting the skin, the eyes and the vascular system. Typically these patients present arteriosclerosis, hypertension, ischemic attacks and occlusive vascular changes at young ages. The aim of our study was to specify the cardiovascular changes in a case of pseudoxanthoma elasticum by a non-invasive haemodynamic evaluation. We present a case of a 50 years-old woman with a clinical diagnostic of pseudoxanthoma elasticum. She had no toxic habits and refered an active lifestyle but without regular exercise. Except for hypertension history, treated during the last four years, there was no other modificable cardiovascular risk factor. The patient had familiar history of early cardiovascular death. In the physical examination were present the typical skin lesions and the angioid streak of the retina. Age, sex and blood pressure are determinant of elastic properties, so we compared our patient with age, sex and BP matched controls. In both cases we determinated the following parameters by CR-2000 Research Cardiovascular Profiling System: Large and Small artery elasticity index (ml/mmHgx100), systemic vascular resistance (dyne.sec.cm5), total vascular impedance (dyne.sec.cm5), average pulse rate (beats / minute), Stroke volume (ml/beat), Cardiac output (ml/minute). Statistical analysis: Signed-Rank test (G-Stat 2.0) There is lower elasticity in large arteries (p = 0,006) but not in small arteries. There is no difference if we compare this value with an elder control group. There is a high cardiac output with respect control group (p = 0,006). There is higher total vascular impedance (p = 0,006) respect the control group but there is no difference compared with the elder group. There is a high cardiac output, total vascular impedance and large arterial stiffness in our patient with respect control group. We suggest that patients with PXE present vascular changes comparable with elder patients. These differences can not be explained by the hypertension.

Analytical model of displacement amplification and stiffness optimization for a class of flexure-based compliant mechanisms

The paper formulates an analytical method for displacement and stiffness calculations of planar compliant mechanisms with single-axis flexure hinges. The procedure is based on the strain energy and Castigliano’s displacement theorem and produces closed-form equations that incorporate the compliances characterizing any analytically-defined hinge, together with the other geometric and material properties of the compliant mechanism. Displacement amplification, input stiffness and output stiffness calculations can simply be performed for any serial compliant mechanism. The class of amplifying compliant mechanisms that contain symmetric corner-filleted or circular flexure hinges is specifically addressed here. A parametric study of the mechanism performance is performed, based on the mathematical model, and an optimization procedure is proposed, based on Lagrange’s multipliers and Kuhn–Tucker conditions, which identifies the design vector that maximizes the performance of these flexure-based compliant mechanisms. Independent finite element simulation confirms the analytical model predictions.

Amplitude of the human soleus H reflex during walking and running.

1. The objective of the study was to investigate the amplitude and modulation of the human soleus Hoffmann (H) reflex during walking and during running at different speeds. 2. EMGs were recorded with surface electrodes from the soleus, the medial and lateral head of the gastrocnemius, the vastus lateralis and the anterior tibial muscles. The EMGs and the soleus H reflex were recorded while walking on a treadmill at 4.5 km h-1 and during running at 8, 12 and 15 km h-1. 3. The amplitudes of the M wave and the H reflex were normalized to the amplitude of a maximal M wave elicited by a supramaximal stimulus just after the H reflex to compensate for movements of the recording and stimulus electrodes relative to the nerve and muscle fibres. The stimulus intensity was set to produce M waves that had an amplitude near to 25 % of the maximal M wave measured during the movements. As an alternative, the method of averaging of sweeps in sixteen intervals of the gait cycle was applied to the data. In this case the amplitude of the H reflex was expressed relative to the maximal M wave measured whilst in the standing position. 4. The amplitude of the H reflex was modulated during the gait cycle at all speeds. During the stance phase the reflex was facilitated and during the swing and flight phases it was suppressed. The size of the maximal M wave varied during the gait cycle and this variation was consistent for each subject although different among subjects. 5. The peak amplitude of the H reflex increased significantly (P = 0.04) from walking at 4.5 km h-1 to running at 12 and 15 km h-1 when using the method of correcting for variations of the maximal M wave during the gait cycle. The sweep averaging method showed a small but non-significant decrease (P = 0. 3) from walking to running at 8 km h-1 and a small decrease with running speed (P = 0.3). The amplitude of the EMG increased from walking to running and with running speed. 6. The relatively large H reflex recorded during the stance phase in running indicates that the stretch reflex may influence the muscle mechanics during the stance phase by contributing to the motor output and enhancing muscle stiffness.

Shaping static elbow torque-angle relationships by spinal cord circuits: A theoretical study

Static torque-angle relationships (invariant characteristics) as measured by Feldman [Feldman A. G. (1980) Neuroscience 5, 81–90] at the human elbow joint for constant descencling excitatory drive have a monotonic convex shape determining joint angle-dependent stiffness. In contrast, for constant activation of elbow flexors, the torque increases, peaks and decreases again with increasing angle because of related moment arm alterations [Hasan Z. and Enoka R. M. (1985) Expl Brain Res. 59, 441–450]. Conversion of such constant-excitation torque-angle shapes into an invariant characteristic might result from action of the stretch reflex which adds excitation with increasing joint angle. To test whether a simple linear model of the stretch reflex could convert constant excitation torque-angle relationships into invariant characteristics, the following assumptions were made. (1) Muscle fibre length increases linearly with joint angle. (2) Reflex muscle excitation (electromyogram) is linearly related to muscle (fibre) length. With these assumptions, invariant characteristic shape cannot be derived from constant excitation torque-angle relationships because it would be sigmoid at low and nearly straight at large joint angles, whilst real flexor invariant characteristics are more convex at large than small angles. It is suggested that recurrent inhibition via Renshaw cells contributes to bend the invariant characteristics into their right shape. Renshaw cells show a nonlinear saturating dependence on motor axon input rate and amount of excitation, i.e. number of active axon collateral synapses. These relationships can contribute to shape motoneuron output so as to yield convex invariant characteristics. Whilst it is not quite clear whether the gain of recurrent inhibition from and to skeleto-motoneurons is high enough to co-determine the invariant characteristic shape significantly, recurrent inhibition of Ia inhibitory interneurons mediating reciprocal inhibition between antagonists is supposed to be quite strong and may influence joint stiffness by interacting with reciprocal inhibition. The arguments presented here extend those of Feldman and co-workers concerning the role of recurrent inhibition and in addition provide a possible explanation for the functional role of mutual inhibition between Renshaw cells. Together with reflex feedback, recurrent inhibition thus contributes to fine-regulate force output and joint stiffness. To account for this cooperation and to make another step towards a general theory of spinal cord circuits, major traits of a new concept are briefly outlined.

Grasp synthesis for upper-extremity FNS

Functional neuromuscular stimulation (FNS) has been used in upper extremity neuroprostheses to provide grasp and release for quadriplegic individuals. The goal of this project was to determine the electrode/muscle input/output characteristics with the greatest influence on the grasp output. This was evaluated by simulating the grasp synthesis procedure, using a model of the electrode/muscle output as a function of stimulus level and joint angle. The parameters of this model were determined from experimental data from 112 electrodes. The grasp output for 500 different modelled pairs of electrodes were analysed, with each pair consisting of a thumb flexor and a thumb extensor. The simulation results indicate that the most influential electrode input/output characteristics are the output stiffness, defined as the change in force output for a unit change in joint angle at a constant level of stimulation, and the length dependency mean-squared error. Recruitment gain was found to be of secondary influence on the grasp output, and the threshold, force direction and non-linearity of the stimulus level to force relationship were found to have little influence on the grasp output. These results establish criteria for electrode selection and implantation for use in upper extremity FNS.

A Total Hydraulically Powered Prosthetic Arm System

The multi-functional externally powered arm prostheses currently available in the UK, suitable for fitting to children suffering from congenital deficiencies of both upper limbs, are all powered by compressed carbon dioxide gas's2. Arm articulation is accomplished by using three-way control valves in association with differential area piston actuators, or two three-way valves mounted back-to-back with double acting actuators, in closed loop position controlled servo-mechanisms. Several advantages are associated with the use of hydraulic fluid as the powering medium for such devices3 These mainly accrue from the relative compressibility of the two media. The virtually incompressible nature of hydraulic fluid results in safe operation of systems at higher working pressures and consequently results in better power to weight ratios. Thus hydraulically powered position control systems, designed to conform to the prosthetic requirements of safety, compactness and lightness, are able to meet load, response and stability specifications whch exceed the capabilities of their pneumatic counterparts. The output stiffness of hydraulic servos is also much greater, which decreases their disturbance by external loads. The construction of a hydraulically powered arm prosthesis has not been possible to date due to lack of a suitable power source. However, recent successful development of a prototype miniature hydraulic power unit indicates that portable hydraulically powered systems are viable'? 7 1 Energy storage for this hydraulic supply is accomplished by electric batteries which offer great advantages in ease of recharging or replacement compared with compressed carbon dioxide storage cylinders. This should make such a system popular in the United States where, due to the logistic difficulties of supplying gas cylinders, most externally powered prostheses are actuated by electric motors, resulting in heavier arm systems with poor dynamic performance. Other areas of hfficulty in implementing a hydraulic arm prosthesis include the development of suitable control valve and actuator system^^^^^ and the adaptation of suitable terminal devices to make them compatible with the arm structure and hydraulic power. This article describes the current state of the hydraulic prosthesis under construction at University College London, under the four main headings of arm structure, portable power supply, valve and actuator system and terminal device. It should be stressed that this project is aimed at producing a prototype hydraulic arm and that the arm now nearing completion will not be used for clinical trials, but could form the basis of a system which, suitably modified to permit small batch production, could be used by bilateral dysmelics.