Introduction to the output protocol of wheel speed sensor chip

1 Introduction

Iris Semiconductor's wheel speed sensor chip provides several different output protocols. Each output protocol has some differences in the output signal, corresponding to different application scenarios and program requirements. It is very important to select a suitable wheel speed sensor output protocol according to the specific situation. This article will introduce the differences and advantages and disadvantages of different output protocols. As shown in Table 1, the output protocols of the most commonly used wheel speed sensors are summarized. (*The version with the suffix "C" at the end of the product model represents a built-in capacitor.)

Table 1 Summary of wheel speed sensor chip output protocol

*The version with suffix “C” in the part number indicates built-in capacitor.

2. Standard Protocol

When the wheel speed sensor measures the gear speed, the moving part being measured will pass through the front end of the sensor, causing a change in the magnetic field density. The standard protocol is the simplest protocol in the wheel speed sensor. CH503 (C) has two built-in Hall plates, which adjust and amplify the differential signal between the Hall plates. When the input signal crosses the arithmetic mean of its maximum and minimum values, it switches (such as the zero crossing point of a sinusoidal signal), that is, the output changes from ON to OFF, and vice versa. A valid sensor output can only be in two states: OFF or ON, represented by low current and high current respectively, and finally outputs a square wave signal, as shown in Figure 1.

Figure 1 Standard protocol output

For applications that only require speed information but not direction information, the standard speed protocol is very suitable. Within the allowable air gap range, the output of the square wave signal is very stable. The duty cycle fluctuation of CH503 (C) is small, the delay is small, and the output parameters are still stable and effective at high wheel speeds. However, the disadvantage of this protocol is that it cannot output other information, such as direction and air gap warnings.

3. PWM protocol

The current of the PWM protocol is similar to the standard protocol, and the on and off states in the circuit are represented by high and low currents. Each zero crossing of the magnetic input signal triggers an output pulse. Usually, a fixed number of pulses are generated per gear tooth rotation cycle. The speed information can be provided based on the output pulse rate. The interval between consecutive rising edges of the output signal is the pulse rate. In addition to the speed signal, the width of the output pulse is changed by PWM modulation to provide more information.

Regarding the definition of direction, as shown in Figure 2, the screen-printed side of the CH504 (C) chip faces the target gear. When the gear rotates from the GND pin side of the chip to the VDD side, the output pulse transmits the DR-R direction information; and when the target gear rotates from the VDD side to the GND side, the output pulse transmits the DR-L information. The two direction information are reflected in different pulse widths, which can help the control unit understand the direction of the gear rotation at this time. Similarly, if the air gap exceeds the critical value, the pulse width change will also give a warning message. The specific pulse width modulation width depends on the programming of the sensor chip. The output waveform of the PWM protocol is shown in Figure 3.

Figure 2 Definition of direction

Figure 3 PWM protocol output

Figure 4 Information transmitted by different pulse widths of PWM protocol

Compared with the standard protocol, the PWM protocol not only provides speed information, but also direction and air gap warning information. The forward and reverse rotation judgment information of the wheel greatly improves the intelligent application of the wheel speed sensor.

4. AK Protocol

The AK protocol contains an IHIGH speed pulse and the subsequent nine data information bits. The speed pulse provides the speed information of the gear, and the subsequent data information bits provide more other information, such as direction and air gap information. These data information bits are Manchester coded. In a frame of data, the waveform of the data segment indicates data 1 if the rising edge of the current is detected, and data 0 if the falling edge of the current is detected. The encoding 1 and 0 are composed of IMID and ILOW. It can be seen that the speed signal of the AK protocol is composed of three currents. The 0 and 1 of the data information bits represent different information, and each data information bit also has its own information, such as direction or air gap warning. The output waveform of CH505 (C) is shown in Figure 3, and the AK protocol data information is shown in Table 2.

Figure 5 AK protocol output waveform

Table 2 Meaning of data information bits of CH505 (C)

It can be seen that the data contains information about air gap validity, direction validity, and forward and reverse rotation. These individual data can be decoded on the electronic control unit to obtain specific status information. Even in the standstill mode, the speed pulse disappears, but the following data bits can still be output, which is very important for applications that require intelligent wheel speed sensors. However, in the AK protocol, since the message transmission time is very long, only a few data bits are output at high speed, so the maximum speed of the target wheel rotation may be subject to certain restrictions.

5 Conclusion

This article introduces some common wheel speed sensor protocols and analyzes their characteristics, advantages and disadvantages. It is not difficult to see that in different application scenarios, it is very important to choose a more suitable output form, and the choice of protocol is highly dependent on the needs of the application. You can choose wheel speed sensors with three output forms of standard protocol, PWM protocol, and AK protocol from the CH50X series according to your needs. More detailed data information can be obtained through the specification sheet. If you need to know more product information, you can send an email to marketing@cosemitech.com for support and help.