【正文】 owamplitude signal can even be susceptible to corrosion at the connector terminal. Another problem with most wheelspeed sensors is that the impulse ring and sensor are subjected to detrimental environmental conditions. Some factors that must be considered are temperature influences, dynamic stresses, and corrosion. Temperature buildup near the wheelspeed measuring equipment es form three sources。 wheel bearings, drive shafts, and brakes. Low temperatures, to 40176。 pose no problem for ABS ponents. High temperatures, however, are another case. Heavy braking, such as in steep mountain descents, produces tremendous heat, and rotor or drum temperatures may exceed 1,000℉ When the brakes get hot, they also heat the bearings. Fortunately, most sensors operate properly at any temperatures withstood by bearing lubricants. Therefore, bearing manufacturers installing sensors with bearings have already dealt with lubrication heat problems and have learned to dissipate heat. Mechanical properties and temperature effects on the magnetic field influence material selection, especially for impulse rings fashioned from multipolar permanent magnets. In addition to heat, axles and their wheel bearings are subjected to dynamic stresses Driving surface conditions only slightly influence these accelerations, and under normal driving conditions, they range from 2 to 4 g. However, braking increases the acceleration amplitude up to 50g. These acceleration forces act in the driving direction and perpendicular to it. Speedsensing ponents must be firmly attached to withstand these forces. Corrosion also haunts impulse rings mounted on the outer ring of a bearing. Galvanic reactions even corrode stainlesssteel rings. But, surface coatings for corrosion protection must be heat coatings are preferred. Corrosion protection normally does not influence sensor function, but it increases their costs. Antilock braking systems also require low cost and weight, precise impulsering geometry, and sufficiently sensitive sensors for electromagnetic requirements. What’s more, the signal generator and cable must tolerate reactions to external electromagnetic fields or impulse disturbances. Impulsering geometry, both sensortoring gap and feature intervals on the ring, affects ABS accuracy. Air gaps between rings and sensors influence signal amplitude of passive sensors and the working range of active ones. The diameter, width, and running accuracy. For example, radial and axial run out of the impulse ring simulates nonexisting speed changes,. Impulsering pitch(toothtotooth or featuretofeature spacing) errors also simulate speed changes. A typical allowable deviation in distance between adjacent teeth in relation to the mean deviation of all teeth is %, with a maximum nonrepeatable deviation of %.Sensor alternative In contrast to passive sensors, activeelectronic ponents powered by the vehicle electrical system avoid many passivesensor shortings. One advantage is that they supply a constantamplitude signal over a wide range of wheel speeds. This increases signal usefulness by allowing accurate readings even from zero wheel speed. Additional application for active sensors include better traction control, more accurate speedometers and odometers, navigation systems and better engine and transmission management. Active sensors also tolerate wider air gap variations and higher signal voltages which reduce susceptibility to terminal corrosion and electromagnetic interference. Active sensors also tend to be smaller than passive sensors. Two technologies are answering the call for a better ABS: magnetoresistant sensors and Halleffect sensors. Each has advantages. In magnetoresistive elements, for instance, the electrical resistance changes with magnetic field. These elements are small and easily incorporate into a bearing in a halfbridge electrical circuit that pensates for temperature changes. A metallic target and a small magnet behind the magnetoresistance(this arrangement is called back biasing) gives the variable magnetic field. Alternatively, the target can be formed as a magnetized impulse ring. Sinusoidal signals are amplified and shaped into pulses by a small electronic circuit placed close to the measuring elements. Magnetoresistive elements operate up to 200176。C, well above temperatures in wheel bearings. The electronic circuit can be kept away from heat sources, by mounting it, for example, in the cable connector. Halleffect sensors incorporate the activesensing element as well as amplification and signalshaping electronics within a small integrated circuit. Signals from Hall sensors also vary with magnetic field, sensing down to zero speed. Back biasing with a magnet or using a magnetized impulse ring produces a variable magnetic field sensed by Hall sensors. Hall sensors were limited by high temperature and peaks to 190176。C。 Although designers can choose from an assortment of sensing devices, several are dominant in auto applications. Variablereluctance (VR) sensors predominate in ABS wheelspeed sensing and for output speed sensing of automatic transmissions. Hallbased active sensors are most widely used in ignition systems, while optical sensors and accelerometers are used in suspensioncontrol systems. Function, reliability, and cost are major deciding factors in sensor selection. A change in any of these three opens the door to reevaluation. For example, if an ABS sensing system must also be useful for traction control, closetozero speed capability bees more important. VR sensors give weak signals at low speeds, thus the designer has two options. He can continue to use a VR sensor system that promises tractioncontrol performance at low speed, or change to sensors that perform better in that range, a Hall sensor for example.A mount solution Space to acmodate the sensor system is not usually available unless the sensor is