In multi electron beam systems, "Neighbours Matter"

In the Multi beam source (MBS) of our Multi Beam Scanning Electron Microscope (MBSEM), an aperture lens array (ALA) splits the emission cone of the Schottky field emitter into multiple beamlets. When the apertures in the ALA are close to each other, the ALA can introduce aberrations to these beamlets through the electrostatic interaction of neighbouring apertures with each aperture's lens field. When the apertures are arranged in a square grid pattern, the aberration causes fourfold astigmatism. The effect on the beam spot is analyzed through a combination of 3D simulations and experimental validation. To counterbalance the fourfold astigmatism, a correction scheme is proposed in which a slightly non-round profile is applied to the aperture lenses.

Skateboards, Bicycles, and Three-dimensional Biped Walking Machines: Velocity-dependent Stability by Means of Lean-to-yaw Coupling

One of the great challenges in the development of passive dynamic walking robots (useful for an understanding of human gait and for future applications in entertainment and the like) is the stabilization of three-dimensional motions. This is a difficult problem due to the inherent interaction between fore-aft motions and sideways motions. In this paper we propose a simple solution. Conceptually, one can avert a sideways fall by steering in that direction, similar to skateboards and bicycles. We propose to implement this concept for walking robots by the introduction of an ankle joint that kinematically couples lean to yaw. The ankle joint has an unusual orientation; its axis points forward and downward, without any left-right component. The effect of the ankle joint is investigated in a simple three-dimensional model with three internal degrees of freedom: one at the hip and two at the ankles. It has cylindric feet and an actuator at the hip joint, which quickly moves the swing leg to a preset forward position. The simulations show that it is easy to find a stable configuration, and that the resultant walking motion is highly robust to disturbances. Similar to skateboards and bicycles, there exists a critical velocity (as a function of the parameters) above which stable walking motions occur. The critical velocity can be lower for a more vertical ankle axis orientation. As an additional benefit, the ankle joint allows a straightforward implementation for steering; a simple sideways offset of the mass distribution will cause the model to gently steer in that direction. The results show great potential for the construction of a real-world prototype with the proposed ankle joint.

Spectroscopic reflectometry of mirror surfaces during plasma exposure

An in situ spectroscopic reflectometry system has been built to investigate the evolution of the specular reflectivity spectrum of ITER first mirror samples during plasma exposure. Results are presented for three different types of molybdenum mirror samples that were exposed to deuterium plasma, including single crystalline, nanocrystalline, and polycrystalline molybdenum. The results show good agreement with ex situ measurements of the reflectivity spectrum before and after exposure and extend the results obtained in previous experiments.

The effect of ankle foot orthosis stiffness on the energy cost of walking: a simulation study

BACKGROUND

In stroke and multiple sclerosis patients, gait is frequently hampered by a reduced ability to push-off with the ankle caused by weakness of the plantar-flexor muscles. To enhance ankle push-off and to decrease the high energy cost of walking, spring-like carbon-composite Ankle Foot Orthoses are frequently prescribed. However, it is unknown what Ankle Foot Orthoses stiffness should be used to obtain the most efficient gait. The aim of this simulation study was to gain insights into the effect of variation in Ankle Foot Orthosis stiffness on the amount of energy stored in the Ankle Foot Orthosis and the energy cost of walking.

METHODS

We developed a two-dimensional forward-dynamic walking model with a passive spring at the ankle representing the Ankle Foot Orthosis and two constant torques at the hip for propulsion. We varied Ankle Foot Orthosis stiffness while keeping speed and step length constant.

FINDINGS

We found an optimal stiffness, at which the energy delivered at the hip joint was minimal. Energy cost decreased with increasing energy storage in the ankle foot orthosis, but the most efficient gait did not occur with maximal energy storage. With maximum storage, push-off occurred too late to reduce the impact of the contralateral leg with the floor. Maximum return prior to foot strike was also suboptimal, as push-off occurred too early and its effects were subsequently counteracted by gravity. The optimal Ankle Foot Orthosis stiffness resulted in significant push-off timed just prior to foot strike and led to greater ankle plantar-flexion velocity just before contralateral foot strike.

INTERPRETATION

Our results suggest that patient energy cost might be reduced by the proper choice of Ankle Foot Orthosis stiffness.

Mechanical analysis of the preferred strategy selection in human stumble recovery

We use simple walking models, based on mechanical principles, to study the preferred strategy selection in human stumble recovery. Humans typically apply an elevating strategy in response to a stumble in early swing and midswing, for which the perturbed step is lengthened in a continuation of the original step. A lowering strategy is executed for stumbles occurring at midswing or late swing, for which the perturbed swing foot is immediately placed on the ground and the recovery is executed in the subsequent step. There is no clear understanding of why either strategy is preferred over the other. We hypothesize that the human strategy preference is the result of an attempt to minimize the cost of successful recovery. We evaluate five hypothesized measures for recovery cost, focusing on the energetic cost of active recovery limb placement. We determine all hypothesized cost measures as a function of the chosen recovery strategy and the timing of the stumble during gait. Minimization of the cost measures based on the required torque, impulse, power and torque/time results in a humanlike strategy preference. The cost measure based on swing work does not predict a favorable strategy as a function of the gait phase.

Controlling the Walking Speed in Limit Cycle Walking

“Limit Cycle Walking” is a relatively new paradigm for the design and control of two-legged walking robots. It states that achieving stable periodic gait is possible without locally stabilizing the walking trajectory at every instant in time, as is traditionally done in most walking robots. Well-known examples of Limit Cycle Walkers are the Passive Dynamic Walkers, but recently there are also many actuated Limit Cycle Walkers. Limit Cycle Walkers generally use less energy than other existing bipeds, but thus far they have not been as versatile. This paper focuses on one aspect of versatility: walking speed. We study how walking speed can be varied, which way is energetically beneficial and how walking speed affects a walker's ability to handle disturbances (that is, disturbance rejection). The study is performed using one prototype and one simulation model. The speed of these two walkers is adapted by changing three parameters: the amount of ankle push-off, upper body pitch and step length. The study has resulted in four conclusions. (1) Steady-state speeds between 0.24 and 0.68 m s-1(for a 0.6 m leg length) were obtained, with loss of stability determining the lower limit and actuation limits determining the upper limit. This result shows the applicability of Limit Cycle Walking for versatile walking machines. (2) For any speed, powering the gait by leaning the body forward costs less energy than using ankle push-off. (3) In contrast to the apparent tradeoff between speed and stability in traditional walking robots, in Limit Cycle Walking we find that increasing the walking speed, independent of how this is done, automatically results in an increasing disturbance rejection. (4) A combination of feedforward actuation adjustment and step-to-step feedback from walking speed shows that it is possible to change walking speed in only a few steps and maintain a desired speed when performing tasks such as carrying loads and walking on slopes. In particular, this fourth conclusion underlines the applicability of the concept of Limit Cycle Walking for versatile two-legged walking machines.

Ankle Actuation for Limit Cycle Walkers

Limit Cycle Walkers are bipeds that exhibit a stable cyclic gait without requiring local controllability at all times during gait. Well-known example are McGeer's “Passive Dynamic Walkers”, but the concept expands to actuated bipeds as involved in this study. Current state-of-the-art Limit Cycle Walkers excel in being very energy efficient, but their ability to handle disturbances (i.e. disturbance rejection) is still limited. A way to improve this ability while maintaining low energy consumption is the use of ankle actuation, which has so far seen few applications in this type of walker. In this paper we study the effect of (1) applying (passive) stiffness in the ankle joint, (2) applying control in the stance ankle based only on local sensor information and (3) modulating ankle push-off. For all three strategies the paper shows how they influence energy use and disturbance rejection of a simple point mass walking model, a more realistic model and a physical prototype. We find that applying a passive ankle spring that results in premature heel rise is energetically optimal and gives an actuation pattern that largely resembles that of humans. Local stance ankle control and ankle push-off modulation can improve the disturbance rejection of a Limit Cycle Walker by at least 60%, without increasing its energy use. These findings are substantiated by showing that our prototype is able to handle large disturbances such as a step-down of 5% of its leg length, while walking efficiently at a mechanical cost of transport of 0.09.

[Synopsis of various electrophysiological tests in early glaucoma diagnosis--temporal and spatiotemporal contrast sensitivity, light- and color-contrast pattern-reversal electroretinogram, blue-yellow VEP]

BACKGROUND

Of the three glaucoma-defining criteria intraocular pressure, optic-nerve damage, and visual field damage, the latter is a late symptom. Therefore, in order to improve an early sensory diagnosis, new tests are necessary. It is the aim of the present paper to test new sensory methods, to rank them in an order of sensitivity, and to base them on possible pathophysiological mechanisms.

PATIENTS AND METHODS

The tests were carried out in subjects of the Erlangen Glaucoma registry: Normals, patients with ocular hypertension, and patients with open-angle glaucoma without or with field defects. The tests are designed to preferentially probe the function of different groups of ganglion cells. Psychophysical methods: Temporal contrast sensitivity in a ganzfeld as "Erlangen flicker test" and spatio-temporal contrast sensitivity to test Magno-cell function. Electrophysiological methods: Pattern-reversal electroretinogram with a luminance-contrast pattern to test Magno-cell function, color-contrast pattern electroretinogram for Parvo-cell function, and blue-on-yellow visual evoked potential to test the "blue-sensitive" pathway.

RESULTS

The most sensitive test is the temp.CS, it is significantly reduced in OHT (p < 0.01). The spatio-temp.CS is reduced in perimetric stages (p < 0.01). The BY-VEP is altered in the preperimetric stage (p < 0.01), the PR-ERG in perimetric stages (p < 0.01). The CC-ERG is reduced in even later stages. These results are in agreement with the hypothesis that tests selective for non-redundant neurons are of early diagnostic value. Multivariate analyses increase the early diagnostic value when different functions are tested in combination.

CONCLUSIONS

When a particular test is taylored to the the special needs of certain groups of ganglion cells sensory defects can be observed before the occurrence of optic-nerve damage (OHT). The most sensitive psychophysical test is the "Erlangen flicker test" which is a screening test selective for M cells. The most sensitive electrophysiological test is the BY-VEP testing the blue-sensitive ganglion cells.

Quantitative analysis of visual field and optic disk in glaucoma: retinal nerve fiber bundle-associated analysis

BACKGROUND

A study was performed to evaluate whether visual field analysis using a perimetric nerve fiber bundle map gives information additional to global visual field indices and cumulative defect curves for early glaucoma diagnosis.

METHODS

One hundred and four control subjects, 124 patients with ocular hypertension (OHT), 97 patients with high-tension glaucoma without visual field defects (preHTG) and 91 patients with open-angle glaucoma with visual field defects [30 low-tension glaucoma (LTG), 61 high-tension glaucoma (HTG)] were included in this study. Correlation analyses were performed between (a) global visual field indices and total neuroretinal rim (NRR) area; (b) local mean values of four visual field areas and the NRR area of the corresponding four optic disk sectors; and (c) local mean values of 10 perimetric nerve fiber bundles (PNFB 1-10) according to Weber and Ulrich (1991) and the four optic disk sectors. The correlations were adjusted for global mean defect and total NRR.

RESULTS

There were no significant correlations between NRR area and visual field in control subjects or in patients with OHT or preHTG for all three analyses. Significant correlations were found between the global visual field indices and the total NRR area for LTG and HTG. Significant correlations between local mean defects and NRR area of corresponding optic disk sectors were found only in LTG for the superior and inferior visual field area and the PNFB covering these areas.

CONCLUSION

The method used for visual field analysis and sectorization of the optic disk does not give additional information on visual field defects in patients with normal global visual field indices and a normal cumulative defect curve. The nerve fiber bundle-related visual field analysis allows the topographical determination and quantification of glaucomatous damage.

Multivariate approach for quantification of morphologic and functional damage in glaucoma

PURPOSE

To determine the usefulness of confirmatory factor analysis in examination of morphometric, electrophysiological, and psychophysical quantitative methods that measure the extent of global glaucomatous damage without referring to a preselected gold standard.

METHODS

In a cross-sectional clinical study, 406 eyes of 203 glaucoma patients and 200 eyes of 100 normal control subjects 18 to 70 years old underwent optic disc morphometry, automated perimetry, measurement of temporal contrast sensitivity by a full-field flicker test, blue-on-yellow visually evoked potential (VEP), and black-and-white pattern-reversal electroretinogram (ERG). Diagnosis of glaucoma was based on a qualitative classification of the optic nerve head and retinal nerve fiber layer independent of intraocular pressure and visual field. Confirmatory factor analysis was performed in the patient group as a whole and in a subgroup showing moderate to advanced glaucomatous optic nerve head damage.

RESULTS

The confirmatory factor analysis models explained the data satisfactorily (P > 0.18, all patients; P > 0.34, subgroup). Global glaucomatous damage was quantified best by the mean defect of automated perimetry (r = 0.81; r = 0.87), followed by the area of the neuroretinal rim (r = 0.64; r = 0.73), the full-field flicker test (r = 0.59; r = 0.65), the pattern-reversal ERG amplitude (r = 0.54; r = 0.55), and the VEP peak time (r = 0.55; r = 0.54).

CONCLUSIONS

Confirmatory factor analysis allows quantification of the validity of established and new procedures that measure global glaucomatous damage using cross-sectional data. The results are not dependent on the preselection of a specific gold standard. Psychophysical testing and morphometry quantified glaucomatous damage best, compared with electrophysiological procedures.

Migraine and tension headache in high-pressure and normal-pressure glaucoma

PURPOSE

To analyze the association of normal-pressure glaucoma and migraine.

METHODS

In a prospective study, 154 patients with glaucoma (56 normal-pressure subgroup and 98 high-pressure glaucoma subgroup), 55 patients with ocular hypertension, and 75 control subjects were analyzed by means of a standardized questionnaire based on International Headache Society criteria.

RESULTS

According to the questionnaire, 46 patients (17%) were classified as suffering from migraine and 20 (7%) from tension headache (episodic and chronic). The prevalence of headache, migraine, and tension headache did not vary significantly among control subjects, patients with ocular hypertension, and patients with glaucoma, but migraine was significantly more common in patients with normal-pressure glaucoma (28%) compared with control subjects (12%; P<.05) and patients with high-pressure glaucoma (10%; P<.01).

CONCLUSION

The results suggest an association of normal-pressure glaucoma and migraine and a potential, common vascular etiology of both diseases.

A multivariate sensory model in glaucoma diagnosis

PURPOSE

To evaluate whether the combination of two psychophysical and two electrophysiological procedures improves diagnostic validity compared with single procedures.

METHODS

In a clinical study, 73 patients with glaucoma from the University Eye Hospital in Erlangen and 122 healthy control subjects from the university staff, ranging in age from 19 to 62 years, underwent measurement of temporal contrast sensitivity using a full-field flicker test, spatiotemporal contrast sensitivity, blue-on-yellow visual evoked potential (VEP), and a black-and-white, pattern-reversal electroretinogram. Diagnostic reference criteria included applanation tonometry, optic disc morphometry, and automated perimetry. Sensitivity was determined univariately with a fixed specificity of 80% and in a multivariate approach using logistic regression analysis. The classification rate was estimated using the leaving-one-out method. The correlation with intraocular pressure, visual field defects, and optic nerve defects was determined.

RESULTS

Contrast sensitivity measurements and the blue-on-yellow pattern-onset VEP showed comparable sensitivity (85%, 84%, and 85%) with 80% specificity, and a pattern-reversal electroretinogram showed lower sensitivity (64%). The first three methods contributed independent information to a diagnostic score. This score improved sensitivity to 94%, with a specificity of 89%. All procedures moderately correlated with the neuroretinal rim area of the optic disc (r=0.32-0.46). The psychophysical tests showed a higher correlation with visual field defects (r > 0.5) than the electrophysiological tests (r < 0.3).

CONCLUSIONS

The multivariate approach substantially increased the diagnostic validity compared with single procedures. This was probably because the diagnostic procedures under investigation tested different aspects of visual function.

[Use of a new anomaloscope test in diagnosis of glaucoma]

BACKGROUND

The Color Vision Meter 712 (CVM) is a new automatic computerized anomaloscope relying on both the Rayleigh and the Moreland equation. In the present study the diagnostic value of Color Vision Meter was examined in glaucoma for the first time and was compared with the Farnsworth 100 hue test.

PATIENTS AND METHODS

33 normals, 15 patients with ocular hypertension (OHT) and a heterogenic group of 31 glaucoma patients were tested with the Nagel anomaloscope, the Color Vision Meter 712 and the Farnsworth 100 hue test. The following determinations were made in all subjects: 1. Anomalous quotient of the Rayleigh equation of the Nagel anomaloscope and of the Color Vision Meter, 2. Mean tritan score of 100 hue test, 3. The matching range, mid matching point, and anomalous quotient of the Moreland equation with the Color Vision Meter.

RESULTS

While in the OHT group only the matching range of the Moreland equation was enlarged, all three variables (matching range, mid matching point and anomalous quotient) of the Moreland equation were significantly changed in the glaucoma group. The mean tritan score of the 100 hue test showed in the OHT group only a slight difference compared to normals, and in the glaucoma group a low significance and a low sensitivity. The matching range of the Moreland equation seems to be most useful with a sensitivity of 87.1% and a specificity of 93.6%.

CONCLUSION

Our results show that the new anomaloscope Color Vision Meter 712 should be considered as a quick screening test for the examination of blue-color vision disturbances in glaucoma because of its higher sensitivity, its easier use for examiner and patients, and its shortened examination time (5 min per equation).