Speed sensing devices, termed variously "wheel impulse generators" (WIG), speed probes, or tachometers are used extensively in rail vehicles. Common types include opto-isolator slotted disk sensors[3] and Hall effect sensors.
Hall effect sensors usually use a rotating target attached to a wheel, gearbox or motor. This target may contain magnets, or it could 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 exact distance from the target wheel and detects the teeth or magnets passing its face. issue with this technique 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 encased to prevent ingress from the outside surroundings. The only exposed parts are a sealed plug connector as well as a drive fork, which is attached to a slotted disk internally through a bearing and seal. The slotted disk is usually sandwiched between circuit boards containing a photo-diode, photo-transistor, amplifier, and filtering circuits which produce a square wave pulse train output custom-made to the customers voltage and pulses per revolution requirements. These types of sensors usually provide two 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 opto devices, mounted around the circumference of the disk, provide signals that are phase-shifted relative to another and thus permit the vehicle computer to choose the direction of rotation of the wheel. This is a legal requirement in Switzerland to prevent rollback when beginning 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 show conclusively that the vehicle is stationary, other than by waiting a definite time to make definite that no further pulses occur. This is 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 necessary for train protection systems รข�� notably the European Train Control Technique.
Hall effect sensors usually use a rotating target attached to a wheel, gearbox or motor. This target may contain magnets, or it could 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 exact distance from the target wheel and detects the teeth or magnets passing its face. issue with this technique 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 encased to prevent ingress from the outside surroundings. The only exposed parts are a sealed plug connector as well as a drive fork, which is attached to a slotted disk internally through a bearing and seal. The slotted disk is usually sandwiched between circuit boards containing a photo-diode, photo-transistor, amplifier, and filtering circuits which produce a square wave pulse train output custom-made to the customers voltage and pulses per revolution requirements. These types of sensors usually provide two 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 opto devices, mounted around the circumference of the disk, provide signals that are phase-shifted relative to another and thus permit the vehicle computer to choose the direction of rotation of the wheel. This is a legal requirement in Switzerland to prevent rollback when beginning 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 show conclusively that the vehicle is stationary, other than by waiting a definite time to make definite that no further pulses occur. This is 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 necessary for train protection systems รข�� notably the European Train Control Technique.
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