Foreword
In automotive applications, magnetic sensors can be used for speed detection such as gearbox speed, wheel speed, camshaft speed, and more. In practical applications, external disturbances such as vibration can cause additional disturbance or noise to the signal processing. In order to ensure the accuracy of the output signal to ensure system reliability, these disturbances or noise must be suppressed by certain algorithms.
1.0 Infrared Magnetic Sensor Basic Hysteresis Algorithm
The magnetic speed sensor is different for different applications in the automotive field, and its corresponding application environment is also different, such as the target wheel pitch and vibration environment. In order to better adapt to different applications for better performance, Infineon magnetic sensors offer flexible hysteresis algorithms. There are four main hysteresis algorithms: HF (Hidden Fixed), VF (Visible Fixed), HA (Hidden AdapTIve), and VA (Visible AdapTIve).
1.1 Basic hysteresis algorithm
For the Hidden hysteresis algorithm, the signal switches at the zero crossing. Since the switching point of the Hidden hysteresis algorithm is at the zero crossing point, it can be avoided by the magnitude of the signal amplitude that varies with the air gap. Therefore, for small pitch target wheels, Hidden hysteresis algorithm can still obtain the best phase accuracy under the influence of signal amplitude variation.
For the Visible hysteresis algorithm, the signal is switched at the level of the hysteresis comparator, and the signal in the hysteresis band is suppressed as noise. Therefore, for a target wheel with a long tooth pitch, such as a gearbox target wheel, the Visible hysteresis algorithm is used to obtain a relatively stable output signal.
The speed sensor speed detection generally uses the differential principle, that is, the magnetic speed sensor detects the target wheel speed and the direction of rotation by detecting a differential magnetic field change. The distance between two magnetic probes (such as Hall probes) of Infineon's magnetic speed sensor is generally 2.5mm, so the ideal pitch of the signal output is 2.5mm*2 equal to 5mm. If the pitch is too large, there will be a distance on the gear due to the equal value of the magnetic field detected between the two Hall probes, ie ΔB is zero. If Hidden hysteresis is selected, the output signal at the long tooth (or missing) is more jittery. With the Visible hysteresis algorithm, the output signal is relatively stable because the switching point is not at the zero crossing. As shown in Figure 1, the Hidden hysteresis and Visible hysteresis algorithm are compared. It can be seen from the figure that the Visible hysteresis algorithm output signal is stable compared to the Hidden hysteresis algorithm for the target wheel with long or long (the legend is crankshaft application). .
Figure 1: Comparison of Visible hysteresis and Hidden hysteresis
Infineon magnetic sensors offer two hysteresis comparator levels, Fixed and AdapTIve.
The so-called Fixed hysteresis, that is, the hysteresis band threshold is a certain value, and will not be adjusted according to the magnitude of the signal that varies with the change of the air gap.
The so-called AdaptTve hysteresis, that is, the hysteresis band threshold is dynamically adjusted according to the input signal to adapt to the air gap change, thereby outputting better phase accuracy. The Adaptive Hysteresis algorithm is used to suppress vibration to a certain extent to eliminate the influence of changes in the air gap or temperature on the input signal.
1.2 Hysteresis algorithm applied in gearbox speed detection
Depending on the type of gearbox, the gearbox system is typically equipped with one or more gearbox speed sensors for detecting the input and output shaft speeds. Some direction signals are also required for the output shaft. The body computer can be used for vehicle speed calculation, electronic clutch control, gear position control, ramp assist, etc. according to gearbox speed and direction information. In order to ensure accurate control of the gearbox, the system requires that the signal vibration amplitude be controlled within a certain range, such as ±1°. This requires the sensor to suppress signal interference due to noise or system vibration, such as engine vibration, axial vibration during shifting of the transmission, noise and vibration caused by low-speed operation of the transmission.
In addition, the gearbox gear pitch is relatively large compared to other speed detection applications such as wheel speed. In combination with the foregoing, in order to adapt to the requirements of the transmission speed detection application, to suppress the noise or interference caused by the vibration, and to reduce the signal jitter caused by the large pitch, the Visible Adaptive hysteresis algorithm is generally adopted for the transmission speed sensor.
2.0 new generation gearbox speed sensor
The Infineon TLE495X series of sensors are a new generation of speed sensors developed specifically for gearbox applications, including the TLE4951C, TLE4954C, TLE4954CE1, TLE4954C E2 and more. The TLE4951C only detects speed information, and the TEL4954C can detect the direction of rotation of the target wheel in addition to the detection speed.
The TLE495X series sensor is a two-wire PWM current protocol. It uses Visible Adaptive hysteresis algorithm. The signal switching hysteresis magnetic comparator level is the peak-to-peak value of the signal, and the signal within the signal amplitude is suppressed. And because it is Visible hysteresis algorithm, it can avoid noise and vibration at the zero crossing of the signal.
2.1 Principles of Vibration Suppression Algorithm
The TLE495X series sensors have three Hall probes, the left and right probes are used to detect speed, and the middle Hall probes are used to detect direction and vibration suppression algorithms. For the TLE4951C, although there is no output direction information, there is still an intermediate Hall probe, which is mainly used for vibration suppression.
The TLE495X series of sensors have excellent vibration suppression capability, and mutual monitoring of speed signals and direction signals is used to judge whether the signal vibrates or not. As shown in Fig. 2, the amplitude signal channel amplitude and phase are monitored at the zero crossing point of the direction signal channel, and the amplitude and phase of the direction signal channel are monitored at the zero crossing point of the speed signal channel. The input differential magnetic field signal is valid only when the speed signal amplitude is greater than ΔBspeed_limit and the direction signal amplitude is greater than ΔBdir_limit.
Figure 2: TLE495X Series Vibration Suppression Algorithm
Through the signal amplitude and direction information for judging and detecting signal noise and vibration, additional magnetic field signals due to radial (angle change) or axial (air gap change) vibrations are suppressed. Magnetic field signal noise and interference judgment principles have the following three main points:
1) The speed signal amplitude is less than the internal hysteresis band threshold
- The speed signal amplitude does not exceed the hysteresis comparator level and is suppressed as noise or interference.
2) The amplitude of the direction signal is smaller than the internal hysteresis band threshold
- Determine the validity of the direction signal by calculating the difference between the direction signal and the speed path actual signal and the last sampled signal. That is, ΔBspeed=dBspeed(n)-dBspeed(n-1) and ΔBdir=dBdir(n)-dBdir(n-1);
- If the difference is too small, it is determined that the direction signal is invalid and is regarded as noise suppression.
3) The direction signal contains the left and right direction information
- The direction signal contains the direction information of the left and right.
2.2 Dynamic check
The TLE495X series of gearbox sensors are dynamically inspected during operation to identify and suppress noise and interference from radial and axial vibrations.
2.2.1 Radial vibration suppression
As shown in Figure 3, when the rising edge and the falling edge of the speed channel signal are sampled to the same amplitude of the direction signal, the suppression algorithm determines that it is radial jitter, such as the direction change or angle jitter caused by the gearbox commutation. At this time, the output signal will be suppressed.
Figure 3: Radial vibration suppression
2.2.2 Axial vibration suppression
As shown in Figure 4, when the direction signal channel samples the velocity signal of the same phase and amplitude, the suppression algorithm determines that it is axial vibration, and the output signal is suppressed.
Figure 4: Axial vibration suppression
3.0 Summary
Infineon's magnetic sensors are leading the market, and their magnetic speed sensors offer a variety of hysteresis algorithms to meet speed detection applications in different automotive sectors. The latest generation of speed sensors, the TLE495X series of sensors, are specially developed for gearbox speed detection with excellent vibration suppression algorithms and output performance.
Appendix: References :
1. TLE4954C datasheet
2. TLE492XC FAQ
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