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

Energy Efficient Relay-Assisted Cellular Network Model using Base Station Switching

Cellular network planning strategies have tended to focus on peak traffic scenarios rather than energy efficiency. By exploiting the dynamic nature of traffic load profiles, the prospect for greener communications in cellular access networks is evolving. For example, powering down base stations (BS) and applying cell zooming can significantly reduce energy consumption, with the overriding design priority still being to uphold a minimum quality of service (QoS). Switching off cells completely can lead to both coverage holes and performance degradation in terms of increased outage probability, greater transmit power dissipation in the up and downlinks, and complex interference management, even at low traffic loads. In this paper, a cellular network model is presented where certain BS rather than being turned off, are switched to low-powered relay stations (RS) during zero-to-medium traffic periods. Neighbouring BS still retain all the baseband signal processing and transmit signals to corresponding RS via backhaul connections, under the assumption that the RS covers the whole cell. Experimental results demonstrate the efficacy of this new BS-RS Switching technique from both an energy saving and QoS perspective, in the up and downlinks

Energy savings using an adaptive base station-to-relay station switching paradigm

Applying a Base Station (BS) sleep approach during low traffic periods has recently been advocated as a strategy for reducing energy consumption in cellular networks. The complete switching off of certain BS however, can lead to coverage holes and severe performance degradation in terms of off-cell user throughput, greater transmit power dissipation in both the up and downlinks, and more complex interference management. This paper presents a novel cellular network energy saving model in which certain BS rather being turned off are switched to Relay Station (RS) mode during low traffic periods. The switched RS and other shared RS deployed at the cross border of each cell are responsible for upholding the same quality of service (QoS) provision as when all BS are active. A centralised adaptive switching threshold algorithm is also introduced to undertake the switching decision, instead of using a fixed threshold. Simulation results confirm the new BS-RS Switching model using an adaptive threshold can reduce network energy consumption by more than half, as well as improving off-cell users’ throughput

Dynamic spectrum access based on cognitive radio within cellular networks

Overlay transmissions in cognitive radio (CR) permit a secondary system to use spectrum concomitantly with a primary system, though adopting this spectrum sharing strategy presents a number of challenges, such as the requirement for a secondary user to have a priori knowledge as side information about the primary user. In this paper, a cognitive cellular network is proposed which uses an overlay approach to dynamically share its radio resource by incorporating cognition, leading to enhanced cell capacity. To compensate for the interference caused by the overlay, cognitive base stations use robust dirty-paper coding in combination with variable transmission powers, which are set depending upon the location of the mobile stations. A detailed performance analysis is presented to corroborate the improved spectrum utilization achieved using this technique

Traffic-and-interference aware base station switching for green cellular networks

Base station (BS) sleeping in cellular networks has emerged as a promising solution for more energy efficient communications, concomitant with lowering the network carbon footprint. Switching off specific BS entirely however, can lead to coverage holes and severe performance degradation. To avoid coverage holes, the transmit power of neighbouring BS must be commensurately increased, which can cause higher interference to other cell users. Recently a BS-RS (relay station) switching model has been proposed where the BS changes operating mode to a RS during off-peak periods rather than being completely turned off. This paper presents a traffic-aware and traffic-and-interference aware switching strategy for both the BS sleeping and BS-RS switching paradigms, which dynamically establishes the conditions for a BS to alter its working mode. The switching is based upon a dynamic traffic threshold allied with the received BS interference level. Analysis corroborates both new algorithms significantly improve network energy efficiency, while upholding the requisite quality of service provision

An interference-aware virtual clustering paradigm for resource management in cognitive femtocell networks

Femtocells represent a promising alternative solution for high quality wireless access in indoor scenarios where conventional cellular system coverage can be poor. They are randomly deployed by the end user, so only post deployment network planning is possible. Furthermore, this uncoordinated deployment creates severe interference to co-located femtocells, especially in dense deployments. This paper presents a new architecture using a generalised virtual cluster femtocell (GVCF) paradigm, which groups together FAP into logical clusters. It guarantees severely interfering and overlapping femtocells are assigned to different clusters. Since each cluster operates on different band of frequencies, the corresponding virtual cluster controller only has to manage its own FAPs, so the overall system complexity is low. The performance of the GVCF algorithm is analysed from both a resource availability and cluster number perspective. Simulation results conclusively corroborate the superior performance of the GVCF model in interference mitigation, particularly in high density FAP scenarios

A Universal Method for Analysing Copolymer Growth

Polymers consisting of more than one type of monomer, known as copolymers, are vital to both living and synthetic systems. Copolymerisation has been studied theoretically in a number of contexts, often by considering a Markov process in which monomers are added or removed from the growing tip of a long copolymer. To date, the analysis of the most general models of this class has necessitated simulation. We present a general method for analysing such processes without resorting to simulation. Our method can be applied to models with an arbitrary network of sub-steps prior to addition or removal of a monomer, including non-equilibrium kinetic proofreading cycles. Moreover, the approach allows for a dependency of addition and removal reactions on the neighbouring site in the copolymer, and thermodynamically self-consistent models in which all steps are assumed to be microscopically reversible. Using our approach, thermodynamic quantities such as chemical work; kinetic quantities such as time taken to grow; and statistical quantities such as the distribution of monomer types in the growing copolymer can be derived either analytically or numerically directly from the model definition.Comment: 24 pages, 11 figure

Novel instrumented frame for standing exercising of users with complete spinal cord injuries

This paper describes a Functional Electrical Stimulation (FES) standing system for rehabilitation of bone mineral density (BMD) in people with Spinal Cord Injury (SCI). BMD recovery offers an increased quality of life for people with SCI by reducing their risk of fractures. The standing system developed comprises an instrumented frame equipped with force plates and load cells, a motion capture system, and a purpose built 16-channel FES unit. This system can simultaneously record and process a wide range of biomechanical data to produce muscle stimulation which enables users with SCI to safely stand and exercise. An exergame provides visual feedback to the user to assist with upper-body posture control during exercising. To validate the system an alternate weight-shift exercise was used; 3 participants with complete SCI exercised in the system for 1 hour twice-weekly for 6 months. We observed ground reaction forces over 70% of the full body-weight distributed to the supporting leg at each exercising cycle. Exercise performance improved for each participant by an increase of 13.88 percentage points of body-weight in the loading of the supporting leg during the six-month period. Importantly, the observed ground reaction forces are of higher magnitude than other studies which reported positive effects on BMD. This novel instrumentation aims to investigate weight bearing standing therapies aimed at determining the biomechanics of lower limb joint force actions and postural kinematics