“AD22157 is a mixed-signal magnetic field converter. It has a large speed measurement range (0~2500Hz) and a wide operating temperature range (-40~150℃). It also has two-wire current operation, air gap diagnosis and reverse A series of features such as voltage protection (-30V). It can measure the speed and rotation direction of the ferromagnetic target wheel of the car in a larger speed range. In addition, it can also be used for transmission speed measurement and proximity measurement in the transmission system. Displacement measurement, etc. The structure diagram of AD22157 is shown in Figure 1.
AD22157 is a mixed-signal magnetic field converter. It has a large speed measurement range (0~2500Hz) and a wide operating temperature range (-40~150℃). It also has two-wire current operation, air gap diagnosis and reverse A series of features such as voltage protection (-30V). It can measure the speed and rotation direction of the ferromagnetic target wheel of the car in a larger speed range. In addition, it can also be used for transmission speed measurement and proximity measurement in the transmission system. Displacement measurement, etc. The structure diagram of AD22157 is shown in Figure 1.
2､The main characteristics of AD22157
AD22157 adopts a two-wire current loop operation mode, which is suitable for continuous operation in the temperature range of -40~150℃ and +20V DC power supply, and can maintain normal operation when the instantaneous voltage is as high as +27V.
The output current pulse of AD22157 wheel speed sensor is 7mA or 14mA (rest bias value is 7mA). The rising edge of the output current pulse of the sensor can be accurately positioned on the hub of the target wheel. The output pulse width can be determined by the direction of movement of the target wheel and The magnetic field strength is determined, and it can be coded as a set of pre-defined time intervals based on the direction of movement of the target wheel and the magnetic field strength according to the current industry standards recommended by mainstream system manufacturers.
Its pulse width can be different according to the intensity of the measured differential mode magnetic field: ΔB>4mT (normal magnetic field)､2mT
Under different magnetic field strength ranges, the output pulse width is shown in Figure 2.
When the dynamic signal is not detected due to the initial power-on, the target wheel stops or other reasons, a safety stop failure signal will be repeatedly generated at a frequency of about 1.5 Hz.
The AD22157 sensor integrates a Hall unit and has a corresponding circuit to reduce the temperature drift of the Hall device parameters. When used with SmCo magnets, the device has the best compensation effect. This structure gives full play to the linearity of the CMOS circuit. The advantages of high voltage and high voltage of DMOS circuit enable the sensor to work accurately in the required environment.
AD22157 also includes an adaptive differential mode zero-crossing detector, which can accurately detect the position of the target wheel hub. This structure reduces the increase in the output pulse caused by the packaging and temperature of the Hall sensor array. There is a 2% deviation in the time interval between the edge and the rising edge.
In order to ensure the accuracy of the measurement, AD22157 discards the 4 pulse edges each time the power is turned on or when it is stopped. It uses digital signal processing technology to enhance the function, while reducing the false pulse or pulse loss that may be generated under EMC limit conditions Phenomenon.
AD22157 adopts a single-row 5-pin (SIP) package, which is very suitable for use as a vehicle speed sensor. It can be easily assembled with a bias magnet placed behind it. The package of AD22157 is shown in Figure 3.
The main limit parameters of AD22157 are as follows:
Maximum power supply voltage: +27V;
Maximum output current (pin 2): 18mA;
Operating temperature range: -40~150℃;
The highest temperature of the core: 190℃.
3､Principle and application
3.1 Working principle
AD22157 wheel speed sensor is actually a two-wire current modulation transmitter, which can generate corresponding current pulses according to the differential mode change of the magnetic field in space. When it is applied to the wheel speed sensor, the magnetic field detected by it is a The permanent magnet behind it interacts with the iron groove mark on the front of the sensor and the target wheel. Under this condition, the sensor must offset the constant magnetic field bias and amplify the differential mode modulation magnetic field to accurately determine the target The rotation of the wheel.
3.2 Principle of signal detection
The AD22157 wheel speed sensor adopts a Hall plate structure integrated on a silicon substrate to perform spatial differential mode measurement of the magnetic field, thereby canceling the influence of the bias magnetic field. The Hall structure is composed of three sets of Hall cells arranged in a straight line. Output the corresponding quadrature signal to some sawtooth or groove not narrower than 5mm.
Each group of Hall cells is composed of 4 independent Hall plates with a diameter of 200μm and arranged in a spatial cross shape in parallel (as shown in Figure 4). This arrangement is helpful to reduce the increasing inclination during use. The influence caused by the signal voltage.
The Hall array is powered by three sets of matched current sources, and the sensitivity at this current is 5μV/Gauss. The three sets of Hall effect sensors can be divided into two groups, and they are connected to the instrument amplifier respectively, and the middle Hall plate is connected to two at the same time. Two amplifiers are connected. This structure can convert the two sets of spatial differential mode magnetic field signals into electrical signals, and the peak-to-peak value is proportional to the differential mode magnetic field signal and the Hall plate bias current.
Therefore, if the Hall array matches the inclination of the wheel, then the Hall signal measured by the spatial differential mode array in AD22157 will change according to the sine law.
3.3 Error sources before signal modulation
In addition to the required spatial differential mode signal generated by the Hall sensor, the following errors are usually generated:
(1) The compensated magnetic field bias. The error source mainly comes from the mismatch of the Hall plate sensitivity, the mismatch of the Hall plate bias current and the change of the magnetic flux density passing through the surface of the bias magnet.
(2) The inherent misalignment of the Hall plate. This situation stems from the misalignment caused by the uneven contact surface of the Hall plate, the inconsistent tolerance of the Hall plate diffusion required by the manufacturer, and the local flatness change caused by the mechanical pressure of the package. Wait.
(3) The Hall cell is affected by temperature sensitivity. This effect is about 450~±150ppm/℃.
(4) Offset caused by the circuit structure affected by temperature. Generally, the overall effect of the device on the output of the pre-amplifier is on the order of hundreds of mV, and it will vary by tens of mV in the positive or negative direction with the temperature.
From the circuit point of view, the amplifier will further cause the input offset of the signal, but this offset component is generally less than 1mV, usually on the order of several hundred μV.
3.4 Signal adjustment
The function of signal adjustment is to compensate the offset error and accurately determine the zero-crossing point of the differential mode signal (the differential mode signal is a mutually orthogonal sinusoidal signal generated by the Hall unit, and the frequency of the generated sinusoidal signal is determined by the speed of the target wheel). The relationship between them is shown in Figure 5. The phase relationship between the orthogonal signals can be used to determine the direction of wheel rotation.
The signal adjustment of the device uses two separate measurement channels. The first channel is used to detect the zero-crossing information and provide the main signal source of the edge information. The second channel only compares the signal phase to extract the information of the rotation direction. Each channel contains 2 extreme value sample/hold circuits and a 10-bit A/D converter.
Each channel uses a sample/hold circuit composed of two A/D converters to detect the extreme value of each signal. One sample/hold circuit detects the peak value, and the other detects the valley value. The voltage output of the DAC reflects The peak-peak value of the signal at any time. The intermediate value of this voltage can be used as the reference value of the zero-crossing detector in the PWM. This structure can ensure that the 1kHz signal (rising edge to rising edge) can be detected under any operating conditions. 2% phase jitter.
Channel 1 also gives the peak-to-peak value of the measured signal. This result can be used to measure the magnetic field strength directly related to the air gap or for air gap diagnosis. At the same time, the direction information of channel 2 can be combined to calculate the output pulse width in PWM.
Due to different air gap settings, or dynamic changes in the air gap caused by wheel deviation, the output peak-to-peak value of the Hall signal will also change accordingly. Therefore, using a fixed resolution 11-level converter may not be guaranteed The sampling accuracy of the signal peak value. In view of this situation, when the signal cannot be tracked with an 11-level converter, the accuracy of the converter should be adjusted accordingly.
3.5 Hall plate bias
The bias value of the Hall cell should be set so that the sensitivity temperature coefficient of AD22157 is equal to that of rare magnetic materials but opposite in polarity. For example, SmCo=-450ppm/℃? or Alnico5-7=-300ppm/℃ , So that the PWM output value will maintain good stability.
3.6 Operation mode
When receiving the power-on reset signal, stop signal or no magnetic field, the sample/hold circuit of each channel will be reset to their maximum and minimum voltage values, and then track inward until the Hall signal is detected. That is the channel 1 (S/H max) increases to the maximum value of the Hall signal, and channel 1 (S/H min) decreases to the minimum value. Figure 6 shows the signal tracking curve when there is no magnetic field in the power-on and stop domain.
In order to ensure that the peak value of the Hall signal is obtained, the first four zero-crossing events generally do not cause signal output. The acquisition operation mode is not executed before the third zero-crossing signal after reset. With the DAC signal, the peak value of the Hall signal starts to be tracked. , The system will make the converter enter the change mode after four zero-crossing events. This operation mode will make the DAC voltage track and maintain the peak value of the Hall signal, thereby maintaining a valid zero-crossing point for wheel deviation and misalignment.
3.7 PWM process and output process
The last step of pulse width modulator (PWM) to complete signal modulation is to convert the zero-crossing point information of the Hall signal, signal amplitude, wheel rotation direction and other information into a pulse width modulation signal. The first edge of the pulse width is determined by the channel The zero-crossing event of 1 is determined. The pulse width is determined by the direction and signal amplitude, as shown in Figure 6.
All signal modulation events are synchronized with the internal clock. Asynchronous zero-crossing events will be arranged to the next clock edge, and this will result in a maximum delay time of 1.4μs. The output pulse width is modulated by a 19-bit counter, which can both As a pulse width modulator, it can also be used as a watchdog timer.
The counter sequence is as follows:
(1) The counter is reset after receiving a zero-crossing event;
(2) The rising edge of the output pulse after a delay of 45μs;
(3) The amplitude threshold and direction are decoded, and a pulse signal of appropriate width is output;
(4) Counter reset;
(5) If the zero-crossing signal is not received 745μs before the counter overflows, a stop pulse will be output.
The tracker reset is to ensure that when no zero-crossing event occurs, the offset correction circuit can still work, but when there is no zero-crossing event for too long, the offset correction will not work on the drift caused by temperature.
AD22157 sensor can modulate the output current into two current values of 7mA or 14mA according to the input pulse. The current value of 7mA represents the static state or the logic zero state.