wheel speed sensor or vehicle speed sensor (VSS) is a type of tachometer. This is the sender used to read the wheel speed of the vehicle. It usually consists of toothed rings and pickups.
Video Wheel speed sensor
Custom destination speed sensor
Vehicle
The wheel speed sensor is in an anti-lock braking system along with the Electronic Stability Program (Control) system.
Swivel speed sensor for rail vehicles
Many subsystems in rail vehicles, such as locomotives or multiple units, rely on reliable and precise speed signals, in some cases as a measure of speed or speed change. This applies especially to traction control, but also for wheel slide protection, registration, train control, door control and so on. These tasks are performed by a number of rotational speed sensors that can be found in different parts of the vehicle.
Speed ​​sensor failure is common, and is mainly due to the extremely harsh operating conditions encountered in rail vehicles. Relevant standards specify detailed testing criteria, but in practical operation, the conditions encountered are often even more extreme (such as shock/vibration and especially electromagnetic compatibility (EMC)).
Swivel speed sensor for motors
Although rail vehicles sometimes use uncensored drives, most require swivel speed sensors for their regulatory systems. The most common type is a two-channel sensor that scans the toothed wheel on the motor shaft or gearbox that may be dedicated to this purpose or may already exist in the drive system.
This type of Hall effect sensor utilizes the principle of magnetic field modulation and is suitable for the ferromagnetic target wheel with modules between m = 1 and m = 3.5 (D.P. = 25 to D.P. = 7). Tooth shape is of secondary importance; the target wheel with spiral or rectangular motion can be scanned. Depending on the diameter and gear of the wheel, it is possible to obtain between 60 and 300 pulses per cycle, which is sufficient to encourage low and medium traction performance.
This type of sensor usually consists of two hall effect sensors, a rare earth magnet and an exact electronic evaluation. The magnetic field is modulated by the passing target teeth. This modulation is registered by Hall sensor, converted by phase comparator to square wave signal and amplified in driver stage.
Unfortunately, Hall's effects vary greatly with temperature. The sensitivity of the sensor and also the signal balance therefore depends not only on the air gap but also on the temperature. It also greatly reduces the maximum allowable air gap between the sensor and the target wheel. At room temperature the air gap of 2 to 3 mm can be tolerated without difficulty for the typical target wheel of module m = 2, but within the required temperature range from -40 Â ° C to 120 Â ° C the maximum gap for effective signaling drops to 1.3 mm. The smaller pitch target wheels with the m = 1 module are often used to obtain higher time resolution or to make the construction more concise. In this case the maximum air gap possibilities are only 0.5 to 0.8 mm.
For design engineers, the air gap is visible that the sensor ends up with primarily the result of a particular machine design, but is subject to whatever constraints it takes to register the speed of play. If this means that the air gap that may have to be located in a very small range, then this will also limit the mechanical tolerance of the motor home and the target wheel to prevent dropping of the signal during operation. This means that in practice there may be a problem, especially with a smaller target wheel than the m = 1 module and a combination of unfavorable extreme tolerances and temperatures. From the point of view of the manufacturer of the motor, and moreover the operator, therefore it is better to look for a speed sensor with a wider air gap.
The main signal of the Hall sensor loses a sharp amplitude as the air gap increases. For Hall sensor manufacturers this means that they need to provide the maximum possible compensation for offset drift that is physically balanced by Hall. The conventional way to do this is to measure the temperature on the sensor and use this information to compensate offset, but this fails for two reasons: first because the drift does not vary linearly with temperature, and secondly because there is not even the same deviation sign for all sensors.
Some sensors now offer integrated signal processors that try to improve the offset and amplitude of Hall sensor signals. This correction allows maximum permitted maximum air gap at the speed sensor. In the m = 1 target wheel module, this new sensor can tolerate a 1.4 mm air gap, which is wider than that for the conventional speed sensors in the target m = 2 wheels. In the m = 2 module of the target wheel, the new speed sensor can tolerate a gap of 2.2 mm. It is also possible to improve signal quality significantly. Both the duty cycle and phase displacement between the two channels are at least three times more stable in the face of airflow fluctuations and floating temperatures. In addition, apart from complex electronic devices, it is also possible to increase the average time between failures for 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.
An alternative to Hall effect sensors with gears is sensors or encoders that use [magnetoresistance]. Because the target wheel is a magnet, multipole is active, the air gap can be larger, up to 4.0 mm. Because magnetoresistive sensors have angle-sensitive and no-amplitude, signal quality is enhanced through Hall sensors in fluctuating gap applications. Also the signal quality is much higher, allowing [interpolation] in encoder/encoder or with external circuit.
Motorcycle encoder with integrated pads
There is a limit to the number of pulses that can be achieved by Hall sensors without an integrated bearing: with a target wheel diameter of 300 mm is usually impossible to exceed 300 pulses per revolution. But many locomotives and multi-power units (EMUs) require a higher number of pulses for the operation of an appropriate traction converter, for example when there are strict limits on traction regulators at low speeds.
Such Hall effect sensor applications can benefit from a built-in bearing, which can tolerate air gaps of many smaller magnitude orders due to greatly reduced play on the actual sensors compared to motor bearings. This makes it possible to select a much smaller pitch for the measurement scale, down to the module m = 0.22. Likewise, magnetoresistive sensors offer higher resolution and accuracy than Hall sensors when implemented in motor encoders with integrated pads.
For greater signal accuracy, a precision encoder can be used.
Functional principles of two similar code makers: multichannel magneto-resistive sensors scan the target wheel with 256 teeth, producing sine and cosine signals. Arctangent interpolation is used to generate rectangular pulses from periods of sine/cosine signals. Precision encoder also has amplitude and offset correction function. This makes it possible to further improve signal quality, which greatly improves traction regulation.
Speed ​​sensor on wheelset
Wheel speed sensor without pads
Speed ​​sensor without bearings can be found in virtually every wheelset of rail vehicles. They are mainly used for wheel slide protection and are usually supplied by manufacturers of wheel shear protection systems. These sensors require a small air gap and should be reliable. One of the special features of the rotational speed sensor used for wheel slide protection is their integrated monitoring function. Two-wire sensor with 7 mA/14 mA current output is used to detect the damaged cable. Another design provides an output voltage of about 7 V as soon as the signal frequency falls below 1 Hz. Another method used is detecting 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 insulated channels.
Sometimes it is necessary to remove the wheel slide protective signals on traction motors, and their output frequencies are often too high for wheel slide protectors. For this application, speed sensors with frequency dividers or integrated encoders can be used.
Wheelset pulse generator with integrated bearing
Rail vehicles, especially locomotives, have many subsystems that require isolated electrically separated speed signals. There is usually no adequate installation place or there is not enough space where a separate pulse generator can be installed. A multi-channel pulse generator mounted on a cushion or wheel cover offers a solution. Using a number of speed sensors without pads will also involve additional cables, which should be avoided for outdoor equipment as they are very susceptible to damage, for example from the fly track ballast.
Optical sensor
From one to four channels can be implemented, each channel has a photosensor that scans one of at most two signal paths on a perforated disk. Experience shows that the possible number of channels that this technique can achieve is not enough. Therefore a number of subsystems have to perform with a loop-through signal from electronic wheel shear protectors and are therefore forced to accept, for example, the number of pulses available, although a separate speed signal may have several advantages.
The use of optical sensors is widespread in the industry. Unfortunately they have two fundamental flaws that always make it very difficult to keep them functioning reliably for several years, ie - optical components are very susceptible to dirt, and - the source of light ages too quickly.
Even dirt traces greatly reduce the amount of light passing through the lens and can cause the signal to break. Therefore, the encoder must be sealed very well. A further problem occurs when the pulse generator is used in an environment where the dew point is passed: the lens and signal mist are often disrupted.
Light source used is a light-emitting diode (LED). But LEDs are always aging, which for several years leads to a diminished visible light. Attempts are made to compensate this by using a special regulator that gradually increases the current through the LEDs, but unfortunately this speeds up the aging process.
Magnetic sensor
The principle used in scanning magnetic ferromagnetic scale scales does not show this deficiency. Over the years experience of using magnetic encoders existed several times when the seal has failed and the pulse generator has been found completely covered in a thick layer of brake dust and other impurities, but the pulse generator is still functioning perfectly.
Historically, magnetic sensor systems are more expensive than optical systems, but these differences narrow down quickly. Magnetic Hall and magnetoresive sensor systems can be embedded in plastic or pot materials, which increases mechanical reliability and eliminates damage from water and fat.
Wheel speed sensors can also include hysteresis. It suppresses the foreign pulsation when the vehicle is jammed.
The pulse generator built in accordance with this principle has been successfully tested in the field by some rail operators since early 2005. The type tests specified in EN 50155 have also been successfully completed, so that these pulse generators can now be delivered.
Wheelset pulse generator with integrated bearing for in bogies journals
In the journal bogies make special demands on the designer of the pulse generator because they do not have a cover on the end to serve as the basis of the rotation of the wheelset shaft can be registered. In this case the pulse generator shall be mounted on the shaft rod attached to the wheelset and fitted with a torque converter connected to the bogie frame to prevent it from spinning.
The extreme vibration at this location leads to a considerable load on the pulse generator pads, which, by this method of installation must carry not only a relatively small mass of the pulsed generator shaft but of the entire pulse generator. When we consider that the lifetime of the bearing is reduced by at least the third force of the load we can see that reliable and durable pulse generators for such situations can not be simply adapted from a more common standard pulse generator to boge out-journals only by installation and intermediate flanges or similar construction. It is absolutely necessary to have a pulse generator with a modified design tailored to the needs of that location.
Speed ​​sensor for non-magnetic target wheels or apps that generate swarf
Some transport companies are faced with a special problem: the air circulation that keeps the cold motors carrying the lumps scratched from the wheels and rails. It collects on the magnetic sensor head. There are also more and more motors in which the sensor has to scan the aluminum target wheel, for example because the impeller is made of aluminum alloy and the manufacturer does not want to have to shrink on a separate ferromagnetic tooth rim.
For this application there is a speed sensor available that does not require a target magnet. A number of transmission and receiver coils are used to generate an alternating electric field with a 1 MHz order frequency and coupling modulation between the sender and receiver is then evaluated. These sensors are installations and signals compatible with magnetic sensors; for the most common target wheel module, the unit can be easily replaced without any other necessary action.
Speed ​​sensor with interpolation
Customers often want a higher number of pulses per revolution than can be achieved in the space available and with the smallest module m = 1. To achieve this goal, sensors are available that offer interpolation. It offers an output of 2-64X the original amount of gear teeth or magnetic pole on the target wheel. Accuracy depends on the quality of the sensor input: Hall sensor is a lower cost, but lower accuracy, magnetoresistive sensor is a higher cost, but higher accuracy.
Maps Wheel speed sensor
References
External links
- Continental Automotive System
- SafelyThere - Continental Automotive System
- Vehicle Safety Equipment "Encouraging Americans is Safer"
- Installation procedure/release of wheel speed sensor
- Encoder for railroad vehicles
- Sensors for railroad vehicles
- Speed ​​Probe for Rails Application
Source of the article : Wikipedia
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