1.  Field of the Invention
The invention has to do with a process for establishing the scanning range of vehicle-activated measuring apparatuses, such as a hot axle detecting apparatus, in which a wheel sensor is fitted with a transmitter and two receivers arranged on either side of the transmitter, whereby the signals of both receivers are compared with one another, and a measurement is made which depends on the result of the signal comparison.  The invention also has to do with an apparatus for the adjustment and correction of measuring apparatus on tracks relative to wheel sensors with a chassis, on which the wheel sensors and measuring apparatus can be set at a geometrically defined distance from one another, where the wheel sensors each have a transmitting coil and two receiving coils. 
2.  The Prior Art
In hot axle detectors until now, wheel sensors have been used whose sending or receiving units are mounted on both sides of the head of the rail.  By these well-known arrangements, the transmitter signal, which is influenced by the wheel and in turn is detected by the receiver, is transformed on reaching a definite threshold value by means of an evaluator switch into a gate opening signal for the hot axle detector.  This gate opening signal is influenced greatly by the speed of the vehicle or the temperature of the surroundings.  This leads to delays in the gate opening and closing signals, as a result of which the exactness of the measurement of the parts to be checked, such as the wheel bearings especially, suffers. 
With the wheel sensor elements set on both sides of a rail head,-placement of the hot axle detector in a location where the wheel sensors also determine exactly the overshooting of a wheel is not possible, for reasons of space. 
From EP-A 340 660, wheel sensors have become known which are distinguished by the fact that they can be set on only one side of the head of a rail.  The wheel sensors disclosed in EP-A 340 660 consist of a coil system which can be mounted, for example, on the inner side of a rail of the track, with a transmitter coil fed by alternating current and two receiver coils to go with it.  The transmitter coils induce a voltage in the receiver coils.  When one receiver coil is aligned before and the other after the transmitter in the direction of the rail, then if the receiver coils are identically formed and set at equal distance from the transmitter coil, and identical voltage is induced in both receiver coils, it can be concluded that a wheel is running over the middle of the sensor.  An electrical evaluator for signals of this kind of sensor has already been proposed in EP-A 340 660; and it is possible with a signal intersection evaluating circuit of this kind to locate exactly the center of the wheel, or the wheel's axis.  Through the exact determination of a geometrical location of the wheel which is made possible in this way, the hot axle detector can also be exactly adjusted; at the same time, and because only one side of the rail head is needed for the setting of the necessary elements for a sensor of this kind, an exact geometrical correlation can also be maintained. 
An exact measurement requires not only a geometrically unambiguous positional location for the ranges to be measured by the hot axle detector.  Just as necessary is the establishment of a gate opening or closing signal, with which the moment of measurement is clocked exactly in relation to the spatial geometry of the measurement.  Therefore, in the equipment known up to now, two wheel sensors are set to generate the gate opening and closing signal.  Wheel sensors of this kind must be adjusted lengthwise along the rail, whereby the corresponding settings can only be varied to a limited extent due to mechanical distortion of the setting.