Electromagnetic compatibility in myoelectrode amplifiers: Isolation, impedance and CMRR

SUMMARY: Electromagnetic compatibility of myoelectrode amplifiers for prosthetic control is important for safe operation in electrically noisy conditions. Factors affecting susceptibility to interference (impedance, common mode rejection ratio (CMRR) and isolation) were studied using a commercial amplifier. INTRODUCTION: Electrical interference can enter the system either directly between the active electrodes as a differential signal, or via the common electrode as a common mode signal. The direct path is susceptible to interference due to the high gain of the amplifier. Shielding and electrode geometry design can help reduce the effects. A notch filter is generally used to reduce greatly the amplifier gain at mains frequency, though harmonics are still passed. Common mode interference is mitigated by the normally very high CMRR of the amplifier; however, common mode signals can also be converted to differential signals through imbalances in electrode impedances (Winter and Webster, 1983; Scott and Lovely, 1986). Common mode interference mainly enters via the power and output leads of a myoelectrode amplifier, so effective isolation should reduce the effect. METHODS: Two Otto Bock 13E125 active myoelectrodes were used for the investigations. Sinusoidal signals of different frequencies were applied in differential and common mode configurations and the output measured. The myoelectrode and measuring equipment were all battery powered to avoid conductive mains interference. Connection to the electrode contacts was through an assembly of spring-loaded platinum contacts, and the myoelectrode was shielded from the external environment in a die-cast box. Measurements were made with different balanced and unbalanced input impedances to represent the skin interface. The measurements were made with the myoelectrode alone and also in combination with an isolation amplifier (Burr-Brown ISO124), and a d.c.-d.c. converter (Murata MEA1D0505SC) to provide isolated power. RESULTS: The d.c. input impedance and intrinsic CMRR of the myoelectrode amplifier were extremely high by design (>40 M? and >90 dB). The response to differential signals was greatest at 200 Hz to 1 kHz, with a sharp notch at 50 Hz. The CMRR was significantly reduced for a.c. signals if the electrode impedances were unbalanced. As the amplifier gain was non-linear and frequency dependent, a protocol was followed where the imbalance in electrode impedances was adjusted to give the same output as a known differential input. Thus the effect of impedance imbalance could be separated from the intrinsic CMRR of the amplifier. The results obtained fitted the Winter-Webster model with input capacitances of 400 pF. The centre earth electrode was found to be strongly coupled to the 0V lead at a.c. via a capacitance of 1 ?F. Isolating the amplifier improved the rejection of common mode signals introduced via the 0V lead. There was a 20 dB increase in CMRR with the isolation components used. CONCLUSION: Common mode interference can enter via the leads of a myoelectrode amplifier. Even if the intrinsic CMRR of the amplifier is very high, common mode is converted to an interfering differential signal if the electrode impedances are unbalanced. This is very likely to be the case, due to differences in skin contact. It has been demonstrated that the common mode route can be blocked by an isolating amplifier and dc-dc converter, though at the expense of extra complexity and the need to power these components. Practical implementations may come from developments in low power circuitry for applications such as wireless sensor networks. REFERENCES: Winter B. and Webster J. 'Reduction of interference due to common mode voltage in biopotential amplifiers', IEEE Transactions on Biomedical Engineering, BME-30 (1), 58-62, 1983. Scott R.N. and Lovely D.F., 'Amplifier input impedances for myoelectric control', Medical & Biological Engineering & Computing, 24, 527-530, 1986

Use of accelerometers in the control of practical prosthetic arms

Accelerometers can be used to augment the control of powered prosthetic arms. They can detect the orientation of the joint and limb and the controller can correct for the amount of torque required to move the limb. They can also be used to create a platform, with a fixed orientation relative to gravity for the object held in the hand. This paper describes three applications for this technology, in a powered wrist and powered arm. By adding sensors to the arm making these data available to the controller, the input from the user can be made simpler. The operator will not need to correct for changes in orientation of their body as they move. Two examples of the correction for orientation against gravity are described and an example of the system designed for use by a patient. The controller for all examples is a distributed set of microcontrollers, one node for each joint, linked with the Control Area Network (CAN) bus. The clinical arm uses a version of the Southampton Adaptive Manipulation Scheme to control the arm and hand. In this control form the user gives simpler input commands and leaves the detailed control of the arm to the controller

Plan for the uniform mapping of earth resources and environmental complexes from Skylab imagery

There are no author-identified significant results in this report

Acoustic confinement and Stimulated Brillouin Scattering in integrated optical waveguides

We examine the effect of acoustic mode confinement on Stimulated Brillouin Scattering in optical waveguides that consist of a guiding core embedded in a solid substrate. We find that SBS can arise due to coupling to acoustic modes in three different regimes. First, the acoustic modes may be guided by total internal reflection; in this case the SBS gain depends directly on the degree of confinement of the acoustic mode in the core, which is in turn determined by the acoustic V-parameter. Second, the acoustic modes may be leaky, but may nevertheless have a sufficiently long lifetime to have a large effect on the SBS gain; the lifetime of acoustic modes in this regime depends not only on the contrast in acoustic properties between the core and the cladding, but is also highly dependent on the waveguide dimensions. Finally SBS may occur due to coupling to free modes, which exist even in the absence of acoustic confinement; we find that the cumulative effect of coupling to these non-confined modes results in significant SBS gain. We show how the different acoustic properties of core and cladding lead to these different regimes, and discuss the feasibility of SBS experiments using different material systems

Plan for the uniform mapping of earth resources and environmental complexes from Skylab imagery. Assessment of natural vegetation, environmental, and crop analogs

The author has identified the following significant results. For interpreting a wide range of natural vegetation analogs, S-190A color infrared and the ERTS-1 color composite were consistently more useful than were conventional color or black and white photos. Color infrared was superior for five vegetation analogs while color was superior for only three. The errors in identification appeared to associate more with black and white single band images than with multiband color. For rice crop analogs, spectral and spatial discriminations both contribute to the usefulness of images for data collection. Tests and subjective analyses conducted in this study indicated that the spectral bands exploited in color infrared film were the most useful for agricultural crop analysis. Accuracy of crop identification on any single date of Skylab images was less than that of multidate analysis due to differences in crop calendar, cultural practices used, rice variety, planting date, planting method, water use, fertilization, disease, or mechanical problems

Market and Coordination Failures in Poor Rural Economies: Policy Implications for Agricultural and Rural Development

This paper argues that the disappointing outcomes of adjustment policies in poor rural economies, principally in sub-Saharan Africa, can be partly attributed to weaknesses in the neo-classical theory which underlies these polices and from associated failures to recognise structural changes (or transitions) in growing agricultural economies. After a brief description of agricultural policy changes in sub Saharan Africa, the mixed achievements of market liberalisation policies are explained using new institutional economic arguments regarding inherent difficulties in economic coordination in poor economies, difficulties which markets themselves cannot overcome. A novel framework is put forward for understanding coordination failure and integrating it with other causes of under-development notably low levels of technical and institutional development and poor governance. The paper concludes by considering the implications of these arguments for development policies in different sub-Saharan economies.development, coordination, markets, institutions, Marketing, O12, O17, Q12,

First Year Projects and Activities of the Environmental Remote Sensing Applications Laboratory (ERSAL)

Activities, pilot projects, and research that will effectively close the gap between state-of-the-art remote sensing technology and the potential users and beneficiaries of this technological and scientific progress are discussed in light of the first year of activity. A broad spectrum of resource and man-environment problems are described in terms of the central thrust of the first-year program to support land use planning decisions with information derived from the interpretation of NASA highlight and satellite imagery

The low-temperature geochemical cycle of iron: From continental fluxes to marine sediment deposition

Suspended sediments from 34 major rivers (geographically widespread)and 36 glacial meltwater streams have been examined for their variations in different operationally-defined iron fractions; FeHR (iron oxides soluble in dithionite), FePR (iron soluble in boiling HCl but not in dithionite) and FeU (total iron less that soluble in boiling HCl). River particulates show a close association between FeHR and total iron (FeT), reflecting the effects of chemical weathering which derive oxide iron from, and retain it in close association with, total iron. Consistent with this, continentalscale average FeHR/FeT ratios vary with runoff ratios (average river runoff per unit area/average precipitation per unit area). By contrast, the diminished effects of chemical weathering produce no recognizable association of FeHR with FeT in glacial particulates, and instead both FePR and FeU are closely correlated with FeT, reflecting essentially pristine mineralogy. A comparison of the globally-averaged compositions of riverine particulates and marine sediments reveals that the latter are depleted in FeHR, FePR and FeT but enriched in FeU. The river and glacial particulate data are combined with estimates of authigenic, hydrothermal, atmospheric and coastal erosive iron fluxes from the literature to produce a global budget for FeHR, FePR, FeU and FeT. This budget suggests that the differences between riverine particulates and marine sediments can be explained by; (i) preferentially removing FeHR from the riverine particulate flux by deposition into inner shore reservoirs such as floodplains, salt marshes and estuaries; and (ii) mixing the resulting riverine particulates with FeHRdepleted glacial particulates. Preliminary measurements of inner shore sediments are consistent with (i) above. Phanerozoic and modern normal marine sediments have similar iron speciation characteristics, which implies the existence of a long-term steady state for the iron cycle. This steady state could be maintained by a glacioeustatic feedback, where FeHR-enriched riverine particulates are either more effectively trapped when sealevel is high (small ice masses, diminished glacial erosion), or are mixed with greater masses of FeHR-depleted glacial particulates when sealevel is low (large ice masses, enhanced glacial erosion). Further important controls on the steady state for FeHR operate through the formation of euxinic sediments and ironstones, which also provide sealevel-dependent sinks for FeHR-enriched sediment