Design of signal conditioning circuit in intelligent vehicle magnetic navigation

The magnetic navigation group is introduced for the first time in the upcoming 2010 Fifth National College Students "Freescale" Cup Smart Car Competition. The new competition system stipulates that an enameled wire is laid under the center line of the track, through which an alternating current of f=20 kHz, I=100 mA is passed, the frequency is (20±2)kHz, and the current is 50~150 mA. The electromagnetic group is not allowed to detect the path by obtaining optical information of the road, and can only guide the car to travel along the current line by detecting the magnetic field around the enameled wire. Considering the working frequency, the size of the output signal, the device cost, the magnetic field strength and other aspects, the most suitable sensor for magnetic navigation track detection is the induction coil. After the detection coil is installed on the smart car body, the spatial orientation between the coil and the navigation current line during the smart car's forward movement determines the induced electromotive force output by the coil. Then, it is equipped with an appropriate signal conditioning circuit, and the electrical signal output by the detection coil is amplified, detected, and finally converted into a signal that can be received by the smart car's single-chip computer, providing navigation basis for the smart car. This is an important basic work for the magnetic navigation smart car to correctly find the path and travel at high speed. So far, there is little research on magnetic navigation detection. This article will discuss the signal conditioning circuit connected to the detection coil.

1 Detecting the induced electromotive force in the coil

Since the size of the racing car is much smaller than the length of the track, the current-carrying wire can be approximately regarded as an infinitely long straight wire. The magnetic induction lines around the current-carrying long straight wire are concentric rings with the wire as the axis. The direction of B is the right-hand spiral tangent of the current i. The magnetic induction intensity at point P, a distance from the wire, is

Where μ0 is the magnetic permeability of vacuum, i is the alternating current in the straight conductor, excited by sinusoidal current (if it is a non-sinusoidal wave, it can be regarded as the linear superposition of a series of sinusoidal waves), i=Ipsin2πft, so B is the alternating magnetic field, which is converted into an induced electromotive force by the detection coil placed around the conductor.

Assume that a rectangular detection coil with an area of ​​S and N turns is placed vertically above the current-carrying wire. At this time, the magnetic induction intensity is perpendicular to the plane of the coil. The magnetic flux φ passing through the coil can be estimated based on the magnetic induction intensity at the center point P of the coil.

Formula (4) shows that when the coil is wound, the number of turns IV and area S of the coil are determined. The induced electromotive force output by the detection coil is also proportional to the amplitude of the excitation current, Ip and the frequency f. The excitation current frequency specified in the competition system is (20±2)kHz, and the change does not exceed 10%. However, the current range is 50~150mA, and the change can reach 3 times, which will have a great impact on the induced electromotive force output by the coil.

If N=20, μ0=4π×10-7N/A2, S=0.002 m2, f=20 kHz, Ip=150 mA, a=0.03m, the amplitude of the induced electromotive force can be estimated to be E=5 mV, but this is only an order of magnitude estimate. In fact, if the coil deviates from the current-carrying wire, or the exciting current in the current-carrying wire is reduced, or a smaller detection coil is used, the amplitude of the induced electromotive force will decrease rapidly.

During the process of the car seeking the track, the car and the detection coil fixed on the car will always deviate from the current-carrying wire to the left and right. The task of the detection circuit is to determine the relative position of the car and the current-carrying wire track at any time, so as to control the steering angle of the servo on the car according to the degree of deviation of the car from the track and the speed of the car. In order to achieve the relative positioning of the car and the current-carrying wire track, multiple identical detection coils must be arranged on the car. Correspondingly, each detection coil is equipped with the same signal conditioning circuit, and only the circuit output signal corresponding to the coil located directly above the current-carrying wire track is the largest.

In other words, the relative position of the car and the current-carrying track is determined by the relative maximum value of the output signals in the multiple detection coils, and has no direct relationship with the signal size output by each detection coil. By finding the maximum value of the induced electromotive force in each coil, it can be known that the track is below the coil. Although changes in the frequency and amplitude of the excitation current will significantly affect the induced electromotive force output by the coil, these factors have the same impact on all detection coils, and the above-mentioned "find the maximum" idea of ​​achieving track positioning is not affected, thereby improving the adaptability of the detection circuit to the track.

2 Requirements of intelligent vehicle control circuit for detection signal

The induced electromotive force output by the detection coil must be amplified and processed as necessary, and finally provided to the single-chip microcomputer of the smart car for A/D conversion sampling to obtain the position information of the track. The A/D input terminal of the single-chip microcomputer of the smart car requires a unipolar voltage between 0 and 5 V. For this, two different signal types can be provided to the single-chip microcomputer, and the single-chip microcomputer adopts different sampling methods.

Method 1: Amplify the 20 kHz, millivolt-level signal output by the detection coil by about 1,000 times (60 dB), and then perform amplitude detection and convert it into a DC voltage. The microcontroller can know the signal size by sampling each detection signal only once, and collect multiple voltages for comparison, and track positioning can be achieved by "finding the maximum".

Method 2: directly collect the amplified 20 kHz signal (superimposed on the DC bias voltage), but the A/D acquisition rate of the microcontroller is required to be much greater than 20 kHz. The microcontroller continuously collects voltages of multiple cycles, finds the maximum and minimum values ​​from the collected data according to the periodicity of the signal, and obtains the peak-to-peak value of the AC signal according to the difference between the two. In this method, the microcontroller must quickly sample each signal many times to obtain the size of the signal. Similarly, it is necessary to collect multiple voltages in a circular manner and achieve track positioning by "finding the maximum".

In order to achieve accurate positioning of the car and the current-carrying track, the use of multiple detection coils is an inevitable choice. The competition system stipulates that up to 16 detection sensors can be used. Comparing the above two methods, the signal processing circuit of method 2 does not need the detection function, but it takes up a lot of working time of the microcontroller. Therefore, method 1 is a reasonable choice.

3 Signal conditioning circuit design

From the previous analysis, it can be seen that the tasks and working conditions of the signal conditioning circuit are: 1) Bandwidth and gain, amplifying the 20 kHz, millivolt-level signal by about 1,000 times, and with a large dynamic range; 2) Power supply, powered by the vehicle battery , using a single-power amplifier circuit, the rated battery voltage is 7.2 V; 3) Signal conversion, amplitude detection of the amplified signal.

Although this function can be achieved by using discrete components to build a circuit, the circuit is complex, debugging is inconvenient, and the circuit performance will change with the fluctuation of the battery voltage. Common general-purpose op amps such as OP07, LM324, LM358, etc. cannot meet the bandwidth and gain requirements for 20 kHz signals, and their output swing is small. Some new integrated operational amplifiers that have appeared in recent years can well undertake the above tasks. Such as OPA228 series op amps and MAX445l series op amps. In particular, the MAX4451 dual op amp has a -3 dB bandwidth of 210 MHz, can work under a single power supply of +4.5 to +11 V, has a large output swing, has rail-to-rail output, and an open-loop gain of more than 50 dB. It is fully capable of using two-stage amplification plus negative feedback. The actual circuit is shown in Figure 1.

Smart cars are powered by batteries. As the working time continues, the battery voltage will inevitably drop. Since the common mode rejection ratio of the op amp MAX4451 is extremely high, with a typical value of CMRR = 95 dB, it can work normally under single power supply conditions, and the fluctuation of the battery voltage basically does not affect the working performance of the op amp.

In Figure 1, L1 is the detection coil. The voltage divider R1 and R2 provides the input bias voltage for the op amp. The input bias voltage of the amplifier can be changed by properly adjusting R2. Since the gain of the second-stage amplifier circuit is set to (R5/R4) = 30 times, R3 can be appropriately selected to change the amplification factor of the first stage according to the magnitude of the induced electromotive force output by the detection coil L1, so that the total gain meets the requirements. R7 is introduced to reduce the DC gain of the first-stage amplifier circuit, thereby improving the stability of the static operating point. However, the introduction of R7 reduces the AC amplification capability of the first-stage circuit, so C4 = 0.47μF is connected to achieve AC bypass. VD1, R6 and C3 form an amplitude detection circuit. VD4 is a high-frequency germanium diode with a small voltage drop. The time constant of the detection circuit is τ = R6C3, which is generally selected to be 3 to 5 times the period of the excitation current (f = 20 kHz). The larger the capacity of C3, the smaller the 20 kHz ripple in the DC voltage output to the A/D terminal of the microcontroller. However, if the capacity of C3 is too large, the circuit response time will be long and the reaction to the deviation of the smart car from the track will be slow. The actual value of C3 should be determined through testing based on this estimate.

In addition, according to common sense, the R1=R2 voltage divider should provide half of the power supply voltage VCC, about 3.6 V, for the op amp input bias. However, since VD1, R6 and C3 form a positive half-cycle peak envelope detection circuit, the greater the induced electromotive force of the detection coil L1, the higher the DC potential output by the detection circuit. As mentioned above, the induced electromotive force output by the coil is affected by many factors and has a large range of variation. In order to increase the output swing of this circuit, R1=20 kΩ and R2=5.1 kΩ are selected to reduce the input bias voltage of the op amp's in-phase terminal to about 1.8 V, so as to reduce the initial DC potential at the output of the detection circuit and increase the dynamic range of the circuit.

4 Conclusion

The above circuit can meet the signal detection requirements of the magnetic navigation smart car. Circuit adjustment method: Under static conditions, adjust R2 so that the output voltage of the detection circuit is about 1V; under dynamic conditions, when the vertically placed detection coil is closest to the track current line and the excitation current is 150 mA, adjust R3 so that the output voltage of the detection circuit is close to but not more than 5 V, so as to meet the requirements of the A/D terminal collection voltage of the single-chip microcomputer.

Since the induced electromotive force output by the detection coil will decrease rapidly as the distance between the coil and the track current line increases, in order to improve the sensitivity and accuracy of the track detection, it is not enough to use a single detection coil. In fact, multiple vertical detection coils can be placed in parallel at the front end of the car, and each detection coil is equipped with the same signal conditioning circuit. The single-chip microcomputer on the smart car must quickly patrol and collect the output voltage of each conditioning circuit, find the maximum output voltage, and then determine that the track current line is directly below the detection coil.

When the single-chip microcomputer collects the output voltages of each conditioning circuit in a loop, it only needs to compare which output voltage is the largest to achieve track seeking, and does not care about the specific voltage value. The advantage of this "find the maximum" method is that the output voltage of the signal conditioning circuit is related to the track excitation frequency ((20±2)kHz) and excitation current (50～150 mA), but the excitation frequency and excitation current have the same effect on the detection voltage of each channel. The above "find the maximum" method can always determine the track position based on the relative maximum value of the output voltage, which makes the track seeking detection circuit have good adaptability to the track.

Multiple detection coils can also be placed horizontally with the same connection circuit. However, it is not difficult to see that if the detection coil is placed horizontally directly above the current-carrying wire, the induced electromotive force is zero; when the detection coil is located on one side above the current-carrying wire, the induced electromotive force is large; when the detection coil is located on one side above the current-carrying wire and deviates far away, the induced electromotive force decreases. At this time, the single-chip microcomputer on the smart car should quickly collect the output voltage of each conditioning circuit and find the minimum output voltage, so as to determine that the current-carrying wire of the track is directly below the detection coil.