Wheel Speed Sensor

A wheel speed sensor or vehicle speed sensor (VSS) is a type of tachometer. It is a sender device used for reading the speed of a vehicle's wheel rotation. It usually consists of a toothed ring and pickup.

Special purpose speed sensors

Road vehicles

Wheel speed sensors are used in anti-lock braking systems.

Rotary speed sensors for rail vehicles

Many of the subsystems in a rail vehicle, such as a locomotive or multiple unit, depend on a reliable and precise rotary speed signal, in some cases as a measure of the speed or changes in the speed. This applies in particular to traction control, but also to wheel slide protection, registration, train control, door control and so on. These tasks are performed by a number of rotary speed sensors that may be found in various parts of the vehicle.

In the past, sensors for this purpose often failed to function satisfactorily or were not reliable enough and gave rise to vehicle faults. This was particularly the case for the early mainly analogue sensors, but digital models were also affected.

This was 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)).

Rotary speed sensors for motors

Bearingless motor speed sensors

Although rail vehicles occasionally do use drives without sensors, most need a rotary speed sensor for their regulator system. The most common type is a two-channel sensor that scans a toothed wheel on the motor shaft or gearbox and therefore does not require a bearing of its own.

The target wheel can be provided especially for this purpose or may be already present in the drive system. Modern sensors of this type make use of the principle of magnetic field modulation and are suitable for ferromagnetic target wheels with a module between m =1 and m = 3.5 (D.P.=25 to D.P.=7). The form of the teeth is of secondary importance; target wheels with involute or rectangular toothing can be scanned. 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.

This type of sensor normally consists of two hall effect sensors, a rare earth magnet and appropriate evaluation electronics. The field of the magnet is modulated by the passing target teeth. This modulation is registered by the Hall sensors, converted by a comparator stage to a square wave signal and amplified in a driver stage.

Unfortunately, the Hall effect varies greatly with temperature. The sensors’ sensitivity and also the signal offset therefore depend not only on the air gap but also on the temperature. This also very much reduces the maximum permissible air gap between the sensor and the target wheel. At room temperature an air gap of 2 to 3 mm can be tolerated without difficulty for a typical target wheel of module m = 2, but in the required temperature range of from 40°C to 120°C the maximum gap for effective signal registration drops to 1.3 mm. Smaller pitch target wheels with module m = 1 are often used to get a higher time resolution or to make the construction more compact. In this case the maximum possible air gap is only 0.5 to 0.8 mm.

For the design engineer, the visible air gap that the sensor ends up with is primarily the result of the specific machine design, but is subject to whatever constraints are needed to register the rotary speed. 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.

The primary signal from a Hall sensor loses amplitude sharply as the air gap increases. For sensor manufacturers this means that they need to provide maximum possible compensation for the Hall signal’s physically induced offset drift. The conventional way of doing this is to measure the temperature at the sensor and use this information to compensate the offset, but this fails for two reasons: firstly because the drift does not vary linearly with the temperature, and secondly because not even the sign of the drift is the same for all sensors.

For a new sensor generation it was therefore necessary to find another way: an integrated signal processor now corrects the offset and amplitude of the Hall sensor signals. This correction is so effective that one can almost double the maximum permissible air gap at the speed sensor. On a module m = 1 target wheel these new sensors can tolerate an air gap of 1.4 mm, which is wider than that for conventional speed sensors on module m = 2 target wheels. On a module m = 2 target wheel the new speed sensors can tolerate gap of as much as 2.2 mm. It has also been possible to markedly increase the signal quality. 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.

In addition, in spite of the complex electronics it has also been possible to increase the MTBF for the new speed sensors by a factor of three to four. So they not only provide more precise signals, their signal availability is also significantly better.

These new sensors, still with the familiar appearance, thus open up whole new possibilities for the designers of drives for rolling stock. The sensors are attractively priced and operate without wear and tear.

Motor encoders with integrated bearings

There is a limit on the number of pulses achievable by sensors without integrated bearings: with a 300 mm diameter target wheel it is normally not possible to get beyond 300 pulses per revolution. But many locomotives and electric multiple units (EMUs) need higher numbers of pulses for proper operation of the traction converter, for instance when there are tight constraints on the traction regulator at low speeds. Such applications really need encoders with built-in bearings, which can tolerate an air gap many orders of magnitude smaller because of the greatly reduced play on the actual sensor as opposed to that of the motor bearing. This makes it possible to choose a much smaller pitch for the measuring scale, right down to module m = 0.22. There are a number of types of encoder with this property. One of them is used in large numbers in EMUs. It can be used to achieve values from less than 100 to more than 130 000 pulses per revolution. In railway applications however, the maximum possible pulses per revolution are not required.

For even greater robustness and signal accuracy a precision encoder can be used.

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. Arctangent interpolation is used to generate up to 512 rectangular pulses from each of the 256 signal periods per revolution. The precision encoder also possesses amplitude and offset correction functions that are housed in the external interpolation unit. This makes it possible to further improve the signal quality, which has a very positive effect on the traction regulator.

Speed sensors on the wheelset

Bearingless wheelset speed sensors

Bearingless speed sensors may be found in almost every wheelset of a rail vehicle. They are principally used for wheel slide protection and usually supplied by the manufacturer of the wheel slide protection system. These sensors require a sufficiently small air gap and need to be particularly reliable. One special feature of rotary speed sensors that are used for wheel slide protection is their integrated monitoring functions. Two-wire sensors with a current output of 7 mA/14 mA are used to detect broken cables. Other designs provide for an output voltage of around 7 V as soon as the signal frequency drops below 1 Hz. Another method used is to detect a 50 MHz output signal from the sensor when the power supply is periodically modulated at 50 MHz. It is also common for two-channel sensors to have electrically isolated channels.

Occasionally it is necessary to take off the wheel slide protection signal at the traction motor, and the output frequency is then often too high for the wheel slide protection electronics. For this application there is a speed sensor with an integrated frequency divider. There are now products available that are compliant with all the usual standards, with considerably improved technical properties and markedly longer useful lives.