The tone wheel is typically made of steel and may be an open-air design, or sealed (as in the case of unitized bearing assemblies).
A passive sensor typically consists of a ferromagnetic rod which is oriented to project radially from the tone wheel with a permanent magnet at the opposite end.
The rod is wound with fine wire which experiences an induced alternating voltage as the tone wheel rotates, as the teeth interfere with the magnetic field.
For example, in the Ford AOD transmission, the VSS is mounted to the tailshaft extension housing and is a self-contained tone ring and sensor.
Speed sensor failures are frequent, and are mainly due to the extremely harsh operating conditions encountered in rail vehicles.
The relevant standards specify detailed test criteria, but in practical operation the conditions encountered are often even more extreme (such as shock/vibration and especially electromagnetic compatibility (EMC)).
The most common type is a two-channel sensor that scans a toothed wheel on the motor shaft or gearbox which may be dedicated to this purpose or may be already present in the drive system.
Depending on the diameter and teeth of the wheel it is possible to get between 60 and 300 pulses per revolution, which is sufficient for drives of lower and medium traction performance.
If this means that the possible air gap has to lie within a very small range, then this will also restrict the mechanical tolerances of the motor housing and target wheels to prevent signal dropouts during operation.
This means that in practice there may be problems, particularly with smaller pitched target wheels of module m = 1 and disadvantageous combinations of tolerances and extreme temperatures.
From the point of view of the motor manufacturer, and even more so the operator, it is therefore better to look for speed sensors with a wider range of air gap.
Both the duty cycle and the phase displacement between the two channels is at least three times as stable in the face of fluctuating air gap and temperature drift.
The functional principles of the two encoders are similar: a multichannel magneto-resistive sensor scans a target wheel with 256 teeth, generating sine and cosine signals.
A rail vehicle, particularly a locomotive, possesses numerous subsystems that require separate, electrically isolated speed signals.
Using a number of bearingless speed sensors would also involve additional cables, which should preferably be avoided for outdoor equipment because they are so susceptible to damage, for instance from flying track ballast.
They have two fundamental problems in functioning reliably for years, the optical components are extremely susceptible to dirt, and the light source ages too quickly.
Further problems are encountered when the pulse generators are used in environments in which the dew point is passed: the lenses fog and the signal is frequently interrupted.
Attempts are made to compensate for this by using special regulators that gradually increase the current through the LED, but unfortunately this further accelerates the aging process.
Magnetic Hall and magnetoresistive sensor systems can be imbedded in plastic or potting material, which increases mechanical reliability and eliminates damage from water and grease.
Pulse generators constructed in accordance with this principle have been successfully field tested by several rail operators since the beginning of 2005.
Inside-journal bogies make particular demands on the pulse generator designer because they have no bearing cover on the end to serve as the basis from which the rotation of the wheelset shaft could be registered.
In this case the pulse generator has to be mounted on a shaft stub attached to the wheelset and fitted with a torque converter connected to the bogie frame to prevent it from rotating.
Some transport companies are faced with a special problem: the circulating air that keeps the motors cool carries swarf abraded from the wheels and rails.