ABS new generation domestic Hall wheel speed sensor platform with ISO26262 ASIL-B functional safety mechanism
Introduction to vABS (Anti lock Braking System)
ABS( Antilock Braking System )： Anti lock braking system for automobiles.
The Development and Current Status of vABS
(1) ABS was first used on airplanes and trains.
(2) In the 1950s, Ford moved the ABS of airplanes to Lincoln sedans.
(3) In the 1980s, ABS began to be popularized in Europe and America.
(4) ABS is now a standard feature of vehicles with more mature technology.
The function of vABS
When the front wheels of a car brake and lock up, the car will lose its steering ability, and when the rear wheels lock up, it will cause the car to spin and spin.
The anti lock braking system is designed to prevent the wheels from getting stuck during the braking process, increase braking deceleration, shorten braking distance, and effectively improve the directional stability and steering control ability of the vehicle.
The impact of anti lock braking system on vehicle performance is mainly manifested in reducing braking distance, maintaining steering control ability, improving driving direction stability, and reducing tire wear.
The structure of vABS
The ABS system usually consists of wheel speed sensors, hydraulic control units (hydraulic regulators, brake pressure regulators), and electronic control units (ECU).
The workflow of vABS
In the entire ABS system, the speed sensor is the information input terminal for system execution and plays an important role
Introduction to vABS wheel speed sensor
ABS wheel speed sensor, commonly referred to as ABS sensor, is used to detect the wheel speed and transmit the speed signal to the ECU to determine whether to apply anti lock braking.
A typical car has an ABS wheel speed sensor on each of its four wheels, which consists of a sensor head and a gear ring. According to the shape of the sensor, it can be divided into two categories:
Convex shaft type (used with gears), end cover type (used with ring gears).
Visual ABS common wheel speed sensor appearance
According to its structure and working principle, ABS wheel speed sensors are divided into two categories: electromagnetic and Hall sensors.
VABS Electromagnetic Principle (Taking Micro Transmission AMR Chip VCW1866W PWM Output as an Example)
When the ring gear rotates, the tooth tips and clearances alternate towards the polar axis. During the rotation of the ring gear, the magnetic flux inside the induction coil alternates, generating an induced electromotive force. This signal is input into the ABS electronic control unit through the cable at the end of the induction coil.
When the speed of the ring gear changes, the frequency of the induced electromotive force also changes. The ABS electronic control unit detects the wheel speed by detecting the frequency of the induced electromotive force.
VABS Hall Principle (Taking Saizhuo SC9641 as an Example)
The Hall type ABS wheel speed sensor is based on the Hall effect. The Hall chip inside the sensor is backed by a permanent magnet, and there are two Hall elements inside the Hall chip. When the tooth peak is facing the Hall element, the magnetic field lines passing through the Hall element are concentrated, and the magnetic field is relatively strong. When the tooth valley is facing the Hall element, the magnetic field lines are dispersed, and the magnetic field is relatively weak. Therefore, when the gear rotates, a sinusoidal magnetic field difference (∆ B) will be generated between the two Hall elements. The internal circuit of the Hall chip processes the magnetic field difference signal and finally outputs a high and low current signal. An external sampling resistor is used to convert the current signal into a voltage signal.
When the speed of the ring gear changes, the frequency of the voltage signal also changes, and the ABS electronic control unit detects the wheel speed by detecting the frequency of the voltage signal.
VABS wheel speed calculation
Whether it is electromagnetic or Hall type, the ABS electronic control unit finally detects the frequency of the voltage signal, and calculates the wheel speed based on the frequency, gear teeth, and wheel rolling radius. The calculation formula is as follows:
1) Measure the period T of the voltage signal (T=1/f)
2) Number of teeth Z
3) Wheel rolling radius r
4) Gear rotational angular velocity ω=2 π/ZT
5) If ω=V/r, then V=2 π r/ZT
Comparison of vABS schemes
electromagnetic type
Advantages: Simple structure, low cost
Disadvantages: 1. The output signal amplitude varies with the speed, and if the vehicle speed is too slow, the output signal amplitude will be too large
Low, the electronic control unit cannot detect.
2. The response frequency is not high, and when the speed is too high, the sensor frequency response cannot keep up.
3. Poor ability to resist electromagnetic wave interference.
Hall type
Advantages: 1. The amplitude of the output signal voltage is not affected by the rotational speed.
2. The frequency response is as high as 20KHz, which is equivalent to the signal frequency detected at a speed of 1000km/h.
3. Because it is a current transmission, it has strong resistance to electromagnetic wave interference.
VABS wheel speed sensor market
In recent years, with Bosch's 8.0 version ABS and ESP systems and TRW's EBC 450 version ABS and ESP systems being used in many car models, two-wire (power, signal) Hall type wheel speed sensors have been widely used. Currently, 80% of ABS wheel speed sensors on the market are two-wire Hall type.
VABS two-wire Hall type wheel speed sensor structure
The two-wire Hall type ABS wheel speed sensor has a simple structure, generally including Hall IC, magnet, positioning sleeve, and cable.
Common magnet specifications for vABS
There are generally two types of magnets for backing: square and cylindrical. The recommended specifications are shown in the following figure.
Three commonly used output waveforms for vABS
Common production processes of vABS (taking Infineon TLE4941 as an example)
Among the many processes related to chips, important processes include magnetic poles, chip bending and shearing, hot riveting and chip pin welding, and injection molding.
Magnetic steel and chip bonding
Magnetic steel is generally made of neodymium iron boron magnets with a surface magnetic capacity of 3800GS. The magnet size should be able to completely cover the chip, and the distance from the chip to the left and right should be equidistant. The back of the chip should be completely in contact with the surface of the magnet. It is recommended to add a thin iron sheet between the magnet and the chip to collect the magnetic field. This way, even in the case of a back bias of the magnet, the offset of the two Hall plates inside the chip is close to 0.
Chip bending
The bending of the magnetic steel chip should be placed in the fixture, and the bending part should be located below the center of the hole (specific to the actual size of the chip).
Hot riveting and chip pin soldering
The hot riveting process mainly uses the soldering iron tip of the hot riveting machine to rivet the plastic protrusions of the skeleton stuck between the chip legs, which serves to fix the chip. During the hot riveting process, do not apply too much force, and the direction of riveting should be from the power pin of the chip to the ground pin to prevent excessive force on the power pin and damage to the chip package.
Hot riveting and chip pin soldering
The welding process mainly involves welding the leads with metal sheets onto the chip pins, usually using a pressure welding machine. During this process, attention should be paid to anti-static measures during welding.
Glue injection molding
The injection molding process is the most important process in the production of ABS wheel speed sensors, and attention should be paid to the design of molds and flow channels in this process.
① The space for fixing the mold and chip skeleton should not be too small to prevent slight displacement of the skeleton during the injection molding process, which may cause the chip to come into contact with the mold wall, resulting in excessive pressure on the chip surface and damage to the chip.
② The injection pressure should be moderate. If the injection pressure is too high, it will cause the wafer inside the chip to break and fail. The flat pressure that the chip package can withstand should be below 3400N.
Common Failure Modes of vABS
Taking the example of the Saizhuo SC9641 (SC9642), Saizhuo products have shipped over ten million units in the front-end market, and their failure modes can be mainly divided into two categories:
1) Chip damage leads to failure
a. Chip internal wafer breakage
b. The bonding wire at the power end is broken
c. The internal circuit of the chip is burnt out
2) Incorrect application method leads to failure
a. Detection distance too far
b. The width of the ring gear or gear varies
d. Magnet back bias or tilt
Chip wafer fragmentation
SC9641 and SC9642 are special TS-2 packages with thin packaging. During processes such as chip bending, hot riveting, and injection molding, excessive or uneven mechanical stress on the chip surface or legs can cause wafer breakage inside the chip.
The bonding wire at the power end is broken
Due to the special architecture of the SC9641 and SC9642 packages, the power pins of the chip are only connected to the wafer by a single bonding wire. If the power pins are subjected to high stress, the bonding wire may break, causing the power terminal to break and fail.
The internal circuit of the chip is burnt out
The maximum withstand voltage value of the power ports of SC9641 and SC9642 is 30V, and they do not have reverse withstand voltage capability. In practical applications, high voltage or back pressure may enter the interior of the chip, causing damage to the internal circuit due to overvoltage and resulting in failure.
Detection distance too far
According to the SC9641 detection principle, the condition for the chip to achieve high and low current flipping is that during the gear rotation process, the magnetic field difference ∆ B sensed by the two Hall plates inside the chip is at least 5GS. If the sensor is too far away from the gear, this condition cannot be met, and normal flipping cannot be achieved, resulting in wave loss and poor duty cycle.
The optimal detection distance for SC9641 after demagnetization is 0.5mm~1.5mm (the distance from the chip surface to the gear), and the farthest detection distance is 2.8mm. SC9642 detection distance refers to the output mode of SC9642.
The width of the gear or ring gear varies
If the width of the gear or ring gear is not uniform, and some teeth are too wide, it will cause the magnetic field sensed by the two Hall plates inside the chip to be indistinguishable at the point where the output current should have flipped. When it reaches the next normal tooth, it will return to normal flipping. From the output waveform, the output pulse width is wide, indicating poor duty cycle.
Magnet back bias or tilted wheel
According to the detection principle of the chip, in an ideal state, the difference ∆ Bstart between the magnetic fields sensed by the two Hall plates inside the chip is 0GS when there is no static state after demagnetization. However, in reality, there is always a certain offset, and the chip itself has the ability to correct this deviation to 0GS, but the correction ability is limited. The maximum offset value that can be corrected is 200GS. If it exceeds this value, it will cause flipping abnormalities, manifested as abnormal output waveform duty cycle or wave loss.
VABS new generation domestic Hall wheel speed sensor platform with ISO26262 ASIL-B functional safety mechanism
The previous generation of products has been steadily shipped in over 10 million units in the automotive factory/front-end market, covering 99% of automotive OEMs. The new generation of sensor platforms has made significant improvements in performance, stability, and reliability, meeting automotive standards
AECQ100 GRADE 0 level, meets ISO26262 ASIL-B functional safety requirements
The design and development follow the ISO26262 standard and meet the ASIL-B level;
• Built in active diagnostic capability;
Optimized the calculation method for sensing points to meet the requirements of larger air gap applications;
• Use laser burning to permanently solidify internal configurations Comprehensive internal circuit inspection capability during wafer testing phase;
• Equipped with vibration suppression capability;
• Better EMC protection capabilities;
Welcome to leave a message or contact the sales manager in charge of the region to obtain detailed product information, development tools, technical support, etc. Starrymicro provides you with super added value and serves you wholeheartedly!
