" for measurement applications. The reasons are that it exhibits a vanishing sensitivity at low fields;that the direction of the external field cannot be determined; and except for the region around 45°,there is nearly no signal linearity. To make a usable device, a trick shown in Figure 7 is applied.Adding small structures made of a highly conductive material such as aluminum to the stripcreates a barber pole configuration. In the simplest case, barber poles are just tiny blocks ofmaterial sitting on top of the Permalloy. Their typical linear dimension in all directions is a coupleof microns, and they are arranged at an angle of <45° to the long axis of the strip. Serving as localshorts for the Permalloy, they force the current to flow through it at that angle.Thus, an angle of 45° between current flow and magnetization, M, results when no external fieldis applied. This gives the barber pole device the desired maximum sensitivity at zero field and acorresponding linearity around the working point. Working at roughly an average value betweenmaximum and minimum resistance of a simple strip, the barber pole device reacts in anunequivocal way to changes in the external field direction.In the majority of practical applications AMR sensor chips are composed of barber pole deviceswired as Wheatstone bridges. AMR chips have significant advantages over other technologiessuch as Hall devices:They can be used at high frequencies (some MHz are no problem) and high temperatures such asthose in automotive environments (continuous service at 150°C, peak values at 190°C), andexhibit low and stable offset values. Due to their high sensitivity (~10 3 that of Hall devices) theycan be used to detect very weak fields (the present limit is ~10 nT).25Current Sensors in Tomorrow’s CarsFuel efficiency first—this is the present motto of automotive development worldwide. Not onlythe combustion engine, but all other automobile components as well are being optimized toachieve this goal. Many components that are at present powered by the car‘s engine will in thenear future operate on electricity—air conditioning compressors, water pumps, oil pumps, and thelike. Due to continuously changing engine speed, these components have rather poor efficiencyratings. Electric motors have the capability of optimizing their operation, independent of enginespeed.This change, though, would overload the conventional 12 V car generators and batteries. Ratherthan increasing the current and corresponding cable cross-sections, it makes more sense to make42 V the future standard. One immediate consequence will be a combined starter-generator placeddirectly on the crankshaft.Prototypes of this unit were presented by Mannesmann-Sachs AG at the 1999 IAA car exhibitionin Frankfurt, Germany. The unit is a three-phase synchronous engine using a permanentexcitation. It is integrated into the flywheel of the clutch and can be used as either a starter or agenerator. Efficiency is increased significantly in both modes of operation. On one hand, theengine is started at a higher starter speed, and on the other, the new generator will produce muchmore electric power without increasing power demand on the combustion engine. Very effectivestart/stop systems can thus be realized: The engine is switched off whenever the vehicle stops andstarted again automatically when the driver wants to go on. These measures should lead to fuel26savings of around 30% compared to today‘s vehicles.A prototype of the starter-generator control unit made by Mannesmann-Sachs (see Photo 5),requires the use of three-phase MOSFET alternators. For control and safety reasons, thesealternators need current sensors for at least two of these phases. Mannesmann uses a customer- specific version of Sensitec‘s NTS-xx system, in this case an NTS-xx hybrid without aconductor for the working current.The conductor in Photo 6 is configured as a 0.8-mm-thickcopper bus bar. At the location of the NTS-xx, the bar takes aU-shaped configuration to generate the necessary magneticfield gradient. For this application, the NTS-xx is mountedPhoto 5. A three-phase alternatordirectly above the bar and electronically calibrated in situ.designed for an automotiveThis arrangement has many advantages. Production is muchstarter-generation was introducedeasier because there is no need to break or disconnect thein prototype in 1999.conductor carrying the working current or even feeding itthrough the NTS-xx itself. Furthermore, because this versionof the NTS-xx can be used for practically all measurement ranges (e.g., 10 A, 100 A, or 1000 A),provided the busbar is adapted correspondingly, a great reduction in manufacturing cost can berealized.Fuel efficiency first—this is the present motto of automotive development worldwide. Not onlythe combustion engine, but all other automobile components as well are being optimized toachieve this goal. Many components that are at present powered by the car‘s engine will in the27 "