Speed ​​sensors used on rail vehicles

Abstract: On rail vehicles, the stability of the vehicle system depends largely on the reliability and accuracy of the speed signal it collects, and the collected speed signal includes the current speed value and the speed change. The collection of speed signals is involved in the traction control, wheel slip protection, train control, and door control of the locomotive. We can find that in various rail vehicles, this task is completed by many speed sensors.

Overview
On rail vehicles, the stability of the vehicle system depends largely on the reliability and accuracy of the speed signal it collects, and the collected speed signal includes the current speed value and the speed change. The speed signal collection problem is involved in the locomotive traction control, wheel slip protection, train control, and door control process. We can find that in various rail vehicles, this task is completed by many speed sensors. In
the past, the sensors used to measure speed usually had unstable performance and were prone to failure, which often caused vehicle accidents. The main reason is that analog sensors were mainly used in the early days, and the digital sensors used at that time were also very poor. The main reason for the above speed sensor problems is that the environment in which rail vehicles are used is extremely harsh.
After years of research and accumulation of practical experience, Lenord+Bauer, a German company, has developed high-quality, multifunctional speed sensors with very stable performance, which are widely used in the rail train industry with harsh working conditions.

Bearingless Speed ​​Sensor
Although some rail vehicles do not use sensors, most locomotive control systems require speed sensors.
The most commonly used speed sensor type is the dual-channel speed sensor (see Figure 1 and Figure 2). This sensor directly scans the gear on the locomotive motor shaft or the reducer, so the sensor itself does not need a bearing.
The target measurement gear can be specially customized according to the user's requirements or the existing measurement
gear in the equipment can be used.
The speed sensor uses the principle of magnetic field modulation (see Figure 3) and is suitable for ferromagnetic measuring wheels with a module of 1 and a module of 3.5. The shape of the teeth of the gear being measured is also an important factor, because the speed sensor can measure square-toothed gears and gears with involute teeth. Depending on the diameter and number of teeth of the measuring wheel, the resolution of the speed sensor ranges from 60 pulses per revolution to 300 pulses per revolution, which can meet the requirements of general locomotive motor drives.

This type of speed sensor usually consists of 2 Hall sensors, a permanent magnet, and a signal processing circuit. When the speed sensor scans the rotating gear, the magnetic field of the permanent magnet changes. The change in the magnetic field is recorded by the Hall sensor, converted into a square wave in the comparison link of the circuit, and amplified in the drive link.

However, the performance of the Hall sensor is greatly affected by temperature. Therefore, the factors that determine the sensitivity of the speed sensor and the phase difference of the signal are not only the installation air gap of the gear, but also the temperature. The influence of temperature greatly reduces the maximum allowable value of the installation air gap between the sensor and the gear. At room temperature, the installation air gap of a standard measuring gear with a module of 2 can be 2-3mm, but when the required temperature range is from -40 degrees to +120 degrees, the maximum allowable air gap is reduced to 1.3mm.
We usually require our measuring gears to have not only high resolution but also small size, so under this requirement, the maximum air gap of the measuring wheel is smaller. The maximum allowable air gap range of a high-resolution pinion with a module of 1 is 0.5-0.8mm.
For design engineers, the air gap of the sensor, if the speed sensor requires a smaller installation air gap, the higher the overall design requirements of the equipment. The small allowable range of the installation air gap limits the mechanical installation tolerance of the motor housing under test and the allowable error range of the measuring gear for the output signal. Therefore, for locomotive motor manufacturers and operators, they are willing to choose speed sensors with a larger installation air gap range.
In actual operation, the amplitude of the speed sensor output signal decreases rapidly as the installation air gap increases (as shown in Figure 4). For sensor manufacturers, they need to compensate the signal amplitude as much as possible, and at the same time compensate for the phase difference accordingly. The usual practice is to measure the working temperature of the sensor, and then compensate the phase difference based on the temperature information, which is what we usually call temperature compensation. However, there are two disadvantages to doing this: First, the phase difference of the signal is not linearly related to the temperature. Second, not every sensor has the same phase difference. Therefore, the adaptability of traditional sensors to temperature needs to be improved.

The new generation of Lenord+Bauer speed sensors has found a new way to address the shortcomings of traditional sensors. It uses an integrated signal processor to adjust the amplitude and phase difference of the signal, thereby increasing the sensor's mounting air gap to about twice the original. With this sensor, the mounting air gap can be as large as 1.4mm for a gear with a modulus of 1, which is larger than the mounting air gap of a gear with a modulus of 2 measured by traditional sensors. For the new generation of sensors, the mounting air gap can reach 2.2mm for a gear with a modulus of 2. At the same time, the new generation of sensors has greatly improved the signal quality. Faced with the same air gap fluctuations and temperature changes, the stability of the duty cycle and phase offset of the new sensor's two channel signals is 3 times that of the traditional sensor. Moreover
, although the circuit of the new sensor is more complex, its MTBF value is higher than that of the traditional sensor. The new sensor not only provides higher signal accuracy than the original, but also better signal availability than the original.

This new sensor has a similar appearance to traditional sensors (see Figure 5) and can be applied to all vehicles currently in use.