Editing Tachometer (section)

==In trains and light rail vehicles==
{{Main|Wheel speed sensor}}
Speed sensing devices, termed variously "wheel impulse generators" (WIG), pulse generators, speed probes, or tachometers are used extensively in rail vehicles. Common types include [[opto-isolator]] slotted disk sensors<ref>{{cite web|url=http://www.haslerrail.com/index.php?id=50 |title=HaslerRail Speed Sensors |publisher=Haslerrail.com |access-date=2011-06-02}}</ref> and [[Hall effect sensor]]s.

Hall effect sensors typically use a rotating target attached to a wheel, gearbox or motor. This target may contain magnets, or it may be a toothed wheel.  The teeth on the wheel vary the flux density of a magnet inside the sensor head. The probe is mounted with its head a precise distance from the target wheel and detects the teeth or magnets passing its face. One problem with this system is that the necessary air gap between the target wheel and the sensor allows ferrous dust from the vehicle's underframe to build up on the probe or target, inhibiting its function.

Opto-isolator sensors are completely encased to prevent ingress from the outside environment.  The only exposed parts are a sealed plug connector and a drive fork, which is attached to a slotted disk internally through a [[Bearing (mechanical)|bearing]] and seal.  The slotted disk is typically sandwiched between two [[circuit board]]s containing a photo-[[diode]], photo-[[transistor]], amplifier, and filtering circuits which produce a square wave pulse train output customized to the customer's voltage and pulses per revolution requirements.  These types of sensors typically provide 2 to 8 independent channels of output that can be sampled by other systems in the vehicle such as automatic train control systems and propulsion/braking controllers.

The sensors mounted around the circumference of the disk provide [[quadrature encoder|quadrature encoded]] outputs and thus allow the vehicle's computer to determine the direction of rotation of the wheel. This is a legal requirement in Switzerland to prevent ''rollback'' when starting from standstill. Strictly, such devices are not tachometers since they do not provide a direct reading of the rotational speed of the disk. The speed has to be derived externally by counting the number of pulses in a time period. It is difficult to prove conclusively that the vehicle is stationary, other than by waiting a certain time to ensure that no further pulses occur. This is one reason why there is often a time delay between the train stopping, as perceived by a passenger, and the doors being released. Slotted-disk devices are typical sensors used in [[odometer]] systems for rail vehicles, such as are required for [[train protection system]]s — notably the [[European Train Control System]].

As well as speed sensing, these probes are often used to calculate distance travelled by multiplying wheel rotations by wheel circumference.

They can be used to automatically calibrate wheel diameter by comparing the number of rotations of each axle against a master wheel that has been measured manually. Since all wheels travel the same distance, the diameter of each wheel is proportional to its number of rotations compared to the master wheel. This calibration must be done while coasting at a fixed speed to eliminate the possibility of wheel slip/slide introducing errors into the calculation. Automatic calibration of this type is used to generate more accurate traction and braking signals, and to improve wheel slip detection.

A weakness of systems that rely on wheel rotation for tachometry and odometry is that the train wheels and the rails are very smooth and the friction between them is low, leading to high error rates if the wheels slip or slide. To compensate for this, secondary odometry inputs employ [[Doppler radar]] units beneath the train to measure speed independently.