Dimensión: Implementación de la gestión logística

battery

inductive

coupling

implant coil

energiser coil

Figure 4.1 Two alternative implant powering schemes: a remotely switched implanted battery (top), and inductive coupling (bottom).

two groups, but problems have been experienced with failure o f the reed switch in the 'on' state, causing the battery to be exhausted rapidly (Davy 1990). Possible solutions to this problem might include a timer to limit the maximum ‘on’ time or the use of two switches in series. However, this all adds additional hardware. The fourth alternative, inductive coupling, is the most attractive option in terms o f reliability, safety and required space (for a small implant coil). Furthermore, implant data may be telemetered over the same link in the opposite direction [#4.4.2.5]. Inductive powering requires two coils, one connected to the implant and the other externally applied, to be sufficiently coupled electromagnetically so as to enable enough power to be induced for the implant to operate, figure 4.1 (bottom). The method has been used in numerous medical as well as other applications (Donaldson 1987a, 1987b, 1987c, Hochmair 1984). This method was chosen for powering the implant.

The four power supply options are assessed in Table 4.1 below.

Direct wire Battery permanently ‘on’ Switched battery Inductive coupling Subject safety

poor average average good

Reliability poor good poor good

Lifetime unlimited* limited to a few months

very good unlimited*

Ease of use average good maybe good average

Use of same link for data

yes no no yes

* except by wire breakage or encapsulant breakdown

4.1.2. Options for the type of strain gauge

A strain gauge is a device which is used to measure the mechanical strain on the surface of a material, in one main direction (the direction o f the principal axis of the gauge). Available in several forms, it essentially consists o f an electrical resistive element, and is bonded to the material on which strain is to be measured. The element geometry is designed to maximise its strain-dependent properties. Three strain gauges would be required to describe the complete state of strain at any one point on a surface. The strain gauge should ideally not affect the stiffness o f the surface (causing reinforcement errors) or be sensitive to parameters other than strain, such as temperature. Several types o f strain gauge are available. The earliest types were made from fine wire, but were improved upon with the invention of the foil strain gauge (Stein 1992). More recent types include semiconductor, thick-film and thin-film gauges. Several relevant parameters for each o f these types of gauge are tabulated in Table 4.2 [#4.1.2.3]. All these gauges depend to some extent on both the dimensional and piezoresistive effects o f the strain-sensitive element (Appendix A). An alternative strain measuring device, the mechanically resonant beam, operates on an entirely different principle and is also included for comparison. The two types of gauge used in the instrumented prostheses will now be described and their merits and drawbacks discussed. The three other types which were not used are discussed in Appendix A. Data from an experiment to assess the likely rates of drift in foil gauges under different conditions are reported. First the common terms applying to all the resistive gauge types will be defined.

Both the dimensional and piezoresistive effects (described in Appendix A) cause the electrical resistance of the conductor to vary with the applied strain. For the small strains usually applied the effects are linear. The term ’gauge factor’ (GF) is used to indicate the per unit change in electrical resistance produced by a strain gauge in response to an applied strain in the direction o f the gauge element. The gauge factor is a function o f the properties of the gauge metal, and has a value of 2.1 for most metals including high resistance alloys commonly used for strain gauges. The gauge alloy is usually specified for use with particular materials for

temperature matching, and temperature coefficients may be quoted for use with other materials. Best results are usually achieved by using four gauges in a full bridge arrangement, figure 4.2, in which all four gauges contribute to the bridge output signal, thereby maximising sensitivity to the desired strain direction as well as providing immunity to strains acting in other directions. Due to the very small changes in resistance which occur for typical strains, a stable high-gain low-noise differential amplifier is usually required [#4.4.3.2, Appendix D]. Half-bridge configurations can also be used, achieving half the full bridge sensitivity.

Positive supply Positive supply

Output 1

Negative supply

Output 2 _Output

Negative supply

Full bridge Half bridge

Figure 4.2 Full and half bridge arrangements for strain gauge connections.

4.1.2.1 The metal foil strain gauge

The metal foil strain gauge is the earliest type o f practical strain gauge. A vast literature exists for this device, which has found extensive application in strain, load and pressure measurement in a variety o f applications (Stein 1992). The notion of bonding a thin metal foil to a surface for use as a strain measuring device was conceived as an extension o f the printed circuit board principle, where thin conductive tracks are bonded to a glass fibre backing material. Necessity was the mother o f its invention, as at the time a means was urgently needed for measuring

strains developed in helicopter rotor blades. It was the pioneer strain measurement device, and still has a number o f advantages over its competitors, including low cost and ease o f application, requiring no expensive capital equipment. Two disadvantages which are of relevance in this application are i) gauge resistance is usually limited to a maximum o f 5kD (with more typical values of 1200 and 3500), the gauges thus drawing several milliamps o f current when used in a bridge configuration, ii) ’zero drift’ may occur under less than ideal environmental conditions, causing apparent strains to be measured [#4.1.2.4].

The modem device consists of a foil of resistance alloy, typically constantan or karma, bonded to a thin insulating backing material. A thin layer of acrylic adhesive is used to bond the backing to the test piece and the strain is transmitted to the foil via the adhesive and the backing. The foil is laid out in the form of a grid, figure 4.3, and the gauge is usually aligned so that the grid lies longitudinally along the axis o f the strain to be measured. At one end o f the grid solid conductor regions are provided for electrical connections, made either by directly soldering small leads to the pads or by ultrasonically wirebonding to an adjacent pad for subsequent solder connection. One uncertainty surrounding the use of foil gauges is the role played by the adhesive layer, which is sometimes suspected of causing artificial strain drift through moisture induced adhesive creep. For this and other reasons [#4.1.4], the foil gauge is best used under dry conditions [#4.1.2.4].

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