Open-loop Mechanisms Research Articles

Tree-type hybrid mechanism configuration synthesis based on modular decomposition and power-concentrated joint closed-loop design

Mechanisms with multiple end-effectors are widely used in engineering due to their ability to synchronize and collaborate on multiple processes. However, this kind of mechanism is difficult to design due to its high topological complexity and complex applied loads. In order to solve this problem, this article proposes a configuration synthesis method for tree-type hybrid mechanisms from two aspects: modular decomposition of open-loop mechanisms and closed-loop design of power-concentrated joints. According to the correlation of the space of degrees of freedom between the end-effector, the mechanism is decomposed into different modules for hierarchical design to simplify the mechanism structure. The unit power concentration index is utilized to find the joints where the power output is relatively concentrated and replace them with low-complexity single-degree-of-freedom closed-loop structures to improve the structural stiffness and dynamic performance of the mechanism. Finally, the feasibility of the proposed method is verified by taking a steel arch looping mechanism with four end-effectors as an example. The aim of this article is to design a simple and reliable multi-branch mechanism with high load carrying capacity to make it more adaptable to the complex and harsh operating environment, which can provide a reference for the design of similar engineering equipment.

Active Haptic Exploration of Softness: Indentation Force Is Systematically Related to Prediction, Sensation and Motivation.

Active finger movements play a crucial role in natural haptic perception. For the perception of different haptic properties people use different well-chosen movement schemes (Lederman and Klatzky, 1987). The haptic property of softness is stereotypically judged by repeatedly pressing one’s finger against an objects’ surface, actively indenting the object. It has been shown that people adjust the peak indentation forces of their pressing movements to the expected stimulus’ softness in order to improve perception (Kaim and Drewing, 2011). Here, we aim to clarify the mechanisms underlying such adjustments. We disentangle how people modulate executed peak indentation forces depending on predictive vs. sensory signals to softness, and investigate the influence of the participants’ motivational state on movement adjustments. In Experiment 1, participants performed a two alternative forced-choice (2AFC) softness discrimination task for stimulus pairs from one of four softness categories. We manipulated the predictability of the softness category. Either all stimuli of the same category were presented in a blocked fashion, which allowed predicting the softness category of the upcoming pair (predictive signals high), or stimuli from different categories were randomly intermixed, which made prediction impossible (predictive signals low). Sensory signals to softness category of the two stimuli in a pair are gathered during exploration. We contrasted the first indentation (sensory signals low) and last indentation (sensory signals high) in order to examine the effect of sensory signals. The results demonstrate that participants systematically apply lower forces when softer objects (as compared to harder objects) are indicated by predictive signals. Notably, sensory signals seemed to be not as relevant as predictive signals. However, in Experiment 2, we manipulated participant motivation by introducing rewards for good performance, and showed that the use of sensory information for movement adjustments can be fostered by high motivation. Overall, the present study demonstrates that exploratory movements are adjusted to the actual perceptual situation and that in the process of fine-tuning, closed- and open-loop mechanisms interact, with varying contributions depending on the observer’s motivation.

Dynamic analysis of open-loop mechanisms with multiple spatial revolute clearance joints

Dynamic model of a typical open-loop mechanism with multiple spatial revolute clearance joints were established based on the Newton–Euler equations and the Hertzian contact deformation theory. An augmented constraint violation correction method was presented to solve the nonlinear dynamic equations of motion, which improved the global convergence and stability effectively. The nonlinear dynamic behaviors of a serial robot manipulator with two spatial revolute clearance joints were studied to demonstrate the effects of the location and coupling relationship of the clearance joints. Numerical results show that the influence of spatial revolute clearance joint on the dynamic behaviors of the open-loop mechanism is relatively stronger than that of the planar ones. The location of the spatial revolute clearance joints is an important factor to dynamic behavior of the system. The closer the spatial revolute clearance joint is to the end-effector, the stronger influence it has on the system. The spatial revolute clearance joints interact significantly with each another, which exhibits vigorous vibration with a higher frequency, larger amplitude, and deeper penetration. This work provides new insights into investigating the nonlinear dynamic behaviors of the systems with spatial revolute clearance joints.

An uncertainty analysis method for error reduction in end-effector of spatial robots with joint clearances and link dimension deviations

The position accuracy of the robot end-effector is inherently affected by uncertainties. In order to design and manufacture robots with high accuracy, it is essential to know the effects of these uncertainties on the motion of robots. Uncertainty analysis is a useful method which can estimate deviations from desired path in robots caused by uncertainties. This paper presents an applied formulation for 3D statistical error analysis of open-loop mechanisms and robotic manipulators. In order to have an accurate analysis, uncertainty effects of both the link dimension deviation and the joint clearance in performance of the spatial open-loop mechanisms and the robots are considered. The maximum normal and parallel components of the position error on the end-effector path are introduced as error bands for all over range of motion. Furthermore, the percent contributions of manufacturing variables are estimated and the corresponding tolerance that has the most significant effects on the uncertainty zone of the end-effector position is determined. The proposed method is illustrated using a spatial manipulator with three revolute joints and verified with a Monte Carlo simulation method. The results of applying this method demonstrate that estimating the position error and its reduction in mechanisms and robots can be done efficiently and precisely.

A new approach for obtaining the dynamic balancing conditions in serial mechanisms

Adaptive balancing means that the mechanical structure of the manipulator is modified in order to achieve the decoupling of dynamic equations. This work deals with a systematic formulation for the adaptive balancing. Basically, two traditional balancing techniques are employed here: the addition of counterweight and counter-rotating disks coupled to the moving links. In addition, the feasibility of the dynamic decoupling for three distinct types of serial manipulators is discussed regarding the achievement of such balancing and the complexity level of the modified mechanical structure. The balancing conditions are developed here for 3-dof spatial and planar open-loop kinematic chain mechanisms, whose topologies are composed of revolute and prismatic joints.

Error Reduction in Spatial Robots Based on the Statistical Uncertainty Analysis

ABSTRACT Kinematic accuracy of the robot end-effector is decreased by many uncertainties. In order to design and manufacture robots with high accuracy, it is essential to know the effects of these uncertainties on the motion of robots. Uncertainty analysis is a useful method which can estimate deviations from desired path in robots caused by uncertainties. This paper presents an applied formulation based on Direct Linearization Method (DLM), for 3D statistical uncertainty analysis of open- loop mechanisms and robots. The maximum normal and parallel components of the position error on the end-effector path are introduced. In this paper, uncertainty effects of both linear and angular variations in performance of spatial open-loop mechanisms and robots are considered. Based on the relations for the percent contributions of manufacturing variables, for the position error reduction, the tolerances that have the most significant effects on the commutated uncertainty zone of the end-effector position can be modified. The proposed method is illustrated using a spatial manipulator with three-revolute joints and verified with a Monte Carlo simulation method. Finally, normal and parallel distances to end-effector path are determined as error bands for all over range of motion. The results of applying this method demonstrate that estimating the position error in mechanisms and robots can be done efficiently and precisely.

Closed and open loop mechanisms controlling arterial blood pressure in waking state in human and experimental animal

Time-honored indices of classical, closed loop arterial blood pressure (BPa) control have recently been supplemented by analyses thought to be sensitive to both closed and open loop mechanisms. These newer tools include, among others, power spectral analyses of heart rate (HR) and BPa, and the cross-correlation between changes in BPa and changes in HR or R-R interval. The „baroreceptor effectiveness index‟ (BEI) is significantly larger in supine, healthy individuals as compared to diabetic patients with peripheral neuropathy. Such findings give promise that BEI may provide a means to assess the development of autonomic neuropathy in diabetic or in other at-risk patients. The cross-correlation between BPa and heart rate in man includes a peak where the two are positively correlated with HR changes leading pressure changes, suggestive of open loop control, and a sharp nadir where the two are negatively correlated and BPa changes precede HR changes, suggestive of closed loop control. In rat the positive cross-correlation depends upon intact spinal sympathetic pathways. The magnitudes of both the positive and negative cross correlations are weaker in tetraplegic patients as compared to neutrally intact subjects. Both peaks remain relatively strong in healthy subjects during the challenge of head up tilt, but decline precipitously in tetraplegic patients. The recognition that both closed and open loop mechanisms are operative and functionally important suggests multiple additional possible etiologies for cardiovascular dysfunctions, including hypertension. The advent of non-invasive means to quantify autonomic function promises new insights into control of the cardiovascular system in health and disease

On Equations of Motion of Elastic Linkages by FEM

Discussion on equations of motion of planar flexible mechanisms is presented in this paper. The finite element method (FEM) is used for obtaining vibrational analysis of links. In derivation of dynamic equations it is commonly assumed that the shape function of elastic motion can represent rigid-body motion. In this paper, in contrast to this assumption, a model of the shape function specifically dedicated to the rigid-body motion is presented, and its influence on elastic motion is included in equations of motion; the inertia matrix related to the rigid-body acceleration vector depends on both shape functions of the elastic and rigid elements. The numerical calculations are conducted in order to determine the influence of the assumed shape function for rigid-body motion on the vibration of links in the case of closed-loop and open-loop mechanisms. The results of numerical simulation show that for transient analysis and for some specific conditions (e.g., starting range, open-loop mechanisms) the influence of assumed shape functions on vibration response can be quite significant.

Can vibratory feedback be used to improve postural stability in persons with transtibial limb loss?

The use of vibration as a feedback modality to convey motion of the body has been shown to improve measures of postural stability in some groups of patients. Because individuals using transtibial prostheses lack sensation distal to the amputation, vibratory feedback could possibly be used to improve their postural stability. The current investigation provided transtibial prosthesis users (n = 24, mean age 48 yr) with vibratory feedback proportional to the signal received from force transducers located under the prosthetic foot. Postural stability was evaluated by measuring center of pressure (CoP) movement, limits of stability, and rhythmic weight shift while participants stood on a force platform capable of rotations in the pitch plane (toes up/toes down). The results showed that the vibratory feedback increased the mediolateral displacement amplitude of CoP in standing balance and reduced the response time to rapid voluntary movements of the center of gravity. The results suggest that the use of vibratory feedback in an experimental setting leads to improvements in fast open-loop mechanisms of postural control in transtibial prosthesis users.

Brain-Machine Interfaces: Electrophysiological Challenges and Limitations

Brain-machine interfaces (BMI) seek to directly communicate with the human nervous system in order to diagnose and treat intrinsic neurological disorders. While the first generation of these devices has realized significant clinical successes, they often rely on gross electrical stimulation using empirically derived parameters through open-loop mechanisms of action that are not yet fully understood. Their limitations reflect the inherent challenge in developing the next generation of these devices. This review identifies lessons learned from the first generation of BMI devices (chiefly deep brain stimulation), identifying key problems for which the solutions will aid the development of the next generation of technologies. Our analysis examines four hypotheses for the mechanism by which brain stimulation alters surrounding neurophysiologic activity. We then focus on motor prosthetics, describing various approaches to overcoming the problems of decoding neural signals. We next turn to visual prosthetics, an area for which the challenges of signal coding to match neural architecture has been partially overcome. Finally, we close with a review of cortical stimulation, examining basic principles that will be incorporated into the design of future devices. Throughout the review, we relate the issues of each specific topic to the common thread of BMI research: translating new knowledge of network neuroscience into improved devices for neuromodulation.

Stochastic approach to kinematic reliability of open-loop mechanism with dimensional tolerance

Open-loop mechanisms are usually employed in industrial applications to position or orient an object, where high accuracy, repeatability and stability of operations are required. Deviations in link dimensions and joint clearances in manufacturing and assembling operations decrease the degree of accuracy and repeatability of the systems. Due to the random nature of dimensional tolerance and clearance, a stochastic approach to figure out reliability of the open-loop mechanism should be required. In this work, firstly, with the assumption that all kinematical parameters are normally distributed random variables, stochastic model of links with dimensional tolerances and of revolute joints with clearances are presented. The kinematical reliability for the positioning and orientation repeatability is then calculated analytically based on an advanced first-order second moment (AFOSM) method and verified with a Monte Carlo simulation method. Finally, the proposed methods are numerically illustrated with an RRR spatial manipulator.

Multiple timescales in postural dynamics associated with vision and a secondary task are revealed by wavelet analysis

Discrete wavelet analysis is used to resolve the center of pressure time series data into several timescale components, providing new insights into postural control. Healthy young and elderly participants stood quietly with their eyes open or closed and either performed a secondary task or stood quietly. Without vision, both younger and older participants had reduced energy in the long timescales, supporting the concept that vision is used to control low frequency postural sway. Furthermore, energy was increased at timescales corresponding to closed-loop (somatosensory and vestibular) and open-loop mechanisms, consistent with the idea of a shift from visual control to other control mechanisms. However, a relatively greater increase was observed for older adults. With a secondary task a similar pattern was observed-increased energy at the short and moderate timescales, decreased energy at long timescales. The possibility of a common strategy-at the timescale level-in response to postural perturbations is considered.

Limitations of feedforward control in multiple-phase steering movements

When attempting to perform bi-phasic steering movements (such as a lane change) in the absence of visual and inertial feedback, drivers produce a systematic heading error in the direction of the lane change (Wallis et al., Curr Biol 12(4):295-299, 2002; J Exp Psychol Hum Percept Perform 33(55):1127-1144, 2007). Theories of steering control which employ exclusively open-loop control mechanisms cannot accommodate this finding. In this article we show that a similar steering error occurs with obstacle avoidance, and offer compelling evidence that it stems from a seemingly general failure of human operators to correctly internalise the dynamics of the steering wheel. With respect to lateral position, the steering wheel is an acceleration control device, but we present data indicating that drivers treat it as a rate control device. Previous findings from Wallis et al. can be explained the same way. Since an open-loop control mechanism will never succeed when the dynamics of the controller are internalised improperly, we go on to conclude that regular, appropriately timed sensory feedback-predominantly from vision-is necessary for regulating heading, even during well-practiced, everyday manoeuvres such as lane changing and obstacle avoidance.

Oscillations in a Simple Neuromechanical System: Underlying Mechanisms

A half-center neural oscillator was coupled to a simple mechanical system to study the closed-loop interactions between a central pattern generator and its effector muscles. After a review of the open-loop mechanisms that were previously introduced by Skinner et al. (1994), we extend their geometric approach and introduce four additional closed-loop mechanisms by the inclusion of an antagonistic muscle pair acting on a mass and connected to the half-center neural oscillator ipsilaterally. Two of the closed-loop mechanisms, mechanical release mechanisms, have close resemblance to open-loop release mechanisms whereas the latter two, afferent mechanisms, have a strong dependence on the mechanical properties of the system. The results also show that stable oscillations can emerge in the presence of sensory feedback even if the neural system is not oscillatory. Finally, the feasibility of the closed-loop mechanisms was shown by weakening the idealized assumptions of the synaptic and the feedback connections as well as the rapidity of the oscillations.

Acute Effects of a Single Bout of Resistance Exercise on Postural Control in Elderly Persons

Many elderly persons are engaging in resistance exercise to counter muscle atrophy due to aging. Here, the acute effects of resistance exercise on postural control mechanisms were examined. Postural control was quantified by mean square center-of-pressure displacements were calculated utilizing force vectors in accordance with previously developed equations. Stabilogram-diffusion plots utilized the displacements as data points after curve-fitting techniques were applied. Two regions, representing the open-loop and closed-loop postural control mechanisms, are shown by the plots and separated at the critical point, which represents the shift in control mechanisms. 21 older adults (age M=71.2, SD=3.84, range 66-81 years) performed three sets of 10-12 repetitions for six resistance exercises for the lower extremity until fatigue. Immediately after exercise, postural stability was reduced. This was represented by a shift of the critical point to the right, indicating an increase in open-loop control. Since resistance training has an acute negative effect on postural control, it is advised to assist elderly clients carefully and immediately after resistance training.

Decoupling of dynamic equations by means of adaptive balancing of 2-dof open-loop mechanisms

Adaptive balancing is a technique that brings some modifications to unbalanced mechanisms in such a way to obtain static balancing and complete decoupling of dynamic equations, even when payload changes. These goals are accomplished by adding movable compensation inertias to the mechanism kinematic chain. The article develops this technique for two-degree-of-freedom planar articulated open-loop mechanisms. Through a performed simulation, a comparison between a mechanism using the proposed method, an unbalanced one and one only statically balanced is also provided.

Role of eye, head, and shoulder geometry in the planning of accurate arm movements.

Eye-hand coordination requires the brain to integrate visual information with the continuous changes in eye, head, and arm positions. This is a geometrically complex process because the eyes, head, and shoulder have different centers of rotation. As a result, head rotation causes the eye to translate with respect to the shoulder. The present study examines the consequences of this geometry for planning accurate arm movements in a pointing task with the head at different orientations. When asked to point at an object, subjects oriented their arm to position the fingertip on the line running from the target to the viewing eye. But this eye-target line shifts when the eyes translate with each new head orientation, thereby requiring a new arm pointing direction. We confirmed that subjects do realign their fingertip with the eye-target line during closed-loop pointing across various horizontal head orientations when gaze is on target. More importantly, subjects also showed this head-position-dependent pattern of pointing responses for the same paradigm performed in complete darkness. However, when gaze was not on target, compensation for these translations in the rotational centers partially broke down. As a result, subjects tended to overshoot the target direction relative to current gaze; perhaps explaining previously reported errors in aiming the arm to retinally peripheral targets. These results suggest that knowledge of head position signals and the resulting relative displacements in the centers of rotation of the eye and shoulder are incorporated using open-loop mechanisms for eye-hand coordination, but these translations are best calibrated for foveated, gaze-on-target movements.

Simulation of The Epidermal Growth Factor Signal Transduction Pathway

In ischaemic exercise ATP is produced by glycogenolysis and net phosphocreatine (PCr) splitting, and lactatem' generation is opposed by net H+ 'consumption' as a consequence of PCr splitting [I-31. Here we examine proton buffering [41, control of glycogenolysis [ I , 21, and the interplay between these and the constraints imposed by the creatine kinase equilibrium [2] in magnetic resonance spectroscopy studies of human muscle in vivo. Spectra (8-scan at 1.5 s i.p. delay) were acquired in a 4.7 T magnet using a 5 cm surface coil placed over finger flexor muscles in 9 males (25-45 y) while cuff ischaemia was maintained during and for 3 min preceding dynamic finger flexion exercise (7% maximum voluntary contraction force at 0.7 Hz). During exercise pH and [PCr] fell and [ADPI rose, as expected (end-exercise values: 6.50k0.05, 1322 mM and 48r6 pM respectively: meankSEM). PCr depletion rate ( D ) at the start of exercise was used to calculate total ATP turnover rate (F = 26+4 mM/min). During exercise, glycogenolytic ATP synthesis rate (L , estimated as F-D) [ I , 31 increased while D declined, reaching 2323 mM/min by 3 min. The total H' load to the cytosol, calculated as A[lactate] less net Hf consumption by PCr splitting, was linear with pH, suggesting overall non-Pi buffer capacity = 27r5 slykes [ I ] . The quantity LMAx (estimated L at saturating [Pi], which is proposed as an approximate measure of Ca''-dependent generation of the 'active' a form of glycogen phosphorylase [2]) started off low, increasing to an approximately constant plateau of 45k2 mM/min. This is consistent with the hypothesis that glycogenolysis increases partly as a result of the 'open-loop' Ca*'-dependent conversion of phosphorylase b to a , and the 'closed-loop' increase in Pi consequent on PCr splitting. The proportionality between L and F in studies at different powers [ I ] implies that similar open-loop mechanisms play a major part in the activation of other glycolytic enzymes, notably phosphofructokinase [ 11. I . Conley KE, Blei ML etal. [I9971 Am. J. Physiol. 273: C306-C315. 2. Kemp GJ (19971 Magma 5 : 231-241 3. Wackerhage H e t a l . [1996] Magma4: 151-5. 4. Kemp GJ et al. [ 19931 NMR in Biomed. 6: 73-83.

Evaluation of Robotic Open Loop Mechanisms using Dynamic Characteristic Charts.

Previously, dynamic characteristic charts were presented as the evaluation method of dynamic characteristics of the robotic parallel crank mechanisms. The dynamic characteristic charts reveal the change in dynamic torque which applied to each driving shaft of robots as they move the whole working space. The dynamic equations of the parallel crank mechanisms do not involve effects due to Coriolis' force so that the dynamic characteristic charts can be made easily. On the contrary, it is difficult to make the same charts for open loop mechanisms since their dynamic equations involve effects due to Coriolis' force. However, open loop mechanisms are the most commonly used so that an appropriate dynamic characteristic evaluation method is particularly needed for them. Therefore, we propose use of the dynamic characteristic charts for open loop mechanisms by transforming the dynamic equation. Furthermore, this paper describes a simple measurement method for dynamic torque applied to driving shafts of robotic mechanisms. Using the method, changes in the dynamic driving torque of robotic open loop mechanisms are measured practically, and the utility of the dynamic characteristic charts is confirmed based on those results.

Characteristic Evaluation and Comparison of Robotic Mechanisms : Evaluation and Comparison Based on Static Characteristics

This paper compared and evaluated several kinds of representative robotic mechanisms based on static characteristics. Especially, open-loop mechanisms and two kinds of closed-loop mechanisms which are parallel crank mechanisms were adopted as the representative robotic mechanisms. On the characteristic comparison of the robotic mechanisms, the previous research had been carried out for the condition under which the configurations of the mechanisms were limited to a certain form. However, static characteristics of the robots are greatly changed by the mechanism configurations. Therefore, this paper paid special attention to the changes of each mechanism characteristic by these configurations. Particularly, geometrical characteristics, strength characteristics and characteristics based on input/output displacement of those robotic mechanisms were compared and evaluated. As a result, the superiority or inferiority in the characteristics of these robotic mechanisms was successfully revealed.