The magnetic navigation group is the first to be introduced in the upcoming 5th National College Student "Freescale" Cup Smart Car Competition in 2010. The new competition system stipulates that the enameled wire should be laid under the center line of the track, which has f=20 kHz, I= 100 mA alternating current, frequency is (20 ± 2) kHz, current is 50 ~ 150 mA, the electromagnetic group is not allowed to pass the optical information of the road for path detection, only by detecting the magnetic field around the enameled wire to guide the carriage along the path Carrying a streamline. Considering the working frequency, the size of the output signal, the cost of the device, and the strength of the magnetic field, the most suitable sensor for magnetic navigation track detection is the induction coil. After the detecting coil is mounted on the smart car body, the spatial orientation between the coil and the navigation current-carrying line during the advancement of the smart car determines the induced electromotive force of the coil output, and then is coupled with an appropriate signal conditioning circuit to detect the electrical signal output by the coil. After amplification, detection and other processing, it is finally converted into a signal that the smart car MCU can receive, providing navigation basis for the smart car. This is an important basic work for the magnetic navigation smart car to correctly seek and travel at high speed. So far, the research on magnetic navigation has been studied very little. This paper will discuss the signal conditioning circuit of the detection coil.
1 Detecting the induced electromotive force in the coilSince the scale of the race car is much smaller than the length of the track, the current-carrying wire can be approximated as an infinitely long straight wire. The magnetic induction line around the long straight current carrying wire is a concentric ring with the wire as the axis. The direction of B is the right helical tangential direction of current i, and the magnetic induction intensity of point P from the wire a
Where μ0 is the vacuum permeability, i is the alternating current in the straight wire, excited by a sinusoidal current (if it is a non-sinusoidal wave, it can be regarded as a linear superposition of a series of sine waves), i=Ipsin2πft, so B In order to alternate the magnetic field, the alternating magnetic field is converted into an induced electromotive force by a detecting coil placed around the wire.
Suppose that a rectangular detection coil with an area of ​​S and a number of turns N is placed vertically above the current-carrying wire. At this time, the magnetic induction intensity is perpendicular to the plane of the coil, and the magnetic flux Φ passing through the coil can be estimated by the magnetic induction at the point P of the coil center.
Equation (4) shows that the coil turns IV and area S have been determined when the coil is wound. The induced electromotive force of the detection coil output is also proportional to the magnitude of the excitation current, Ip and frequency f. The excitation current frequency specified by the competition system is (20 ± 2) kHz, and the variation does not exceed 10%. However, the current range is 50 to 150 mA, and the variation can be up to 3 times, which will have a great influence on the induced electromotive force of the coil output.
If N=20, μ0=4π&TImes; 10-7N/A2, S=O. 002 m2, f=20 kHz, Ip=150 mA, a=0.03m, then the magnitude of the induced electromotive force E=5 mV can be estimated, but this is only an order of magnitude estimate. In fact, if the coil deviates from the current-carrying conductor, The magnitude of the induced electromotive force will decrease rapidly when the excitation current in the current-carrying conductor is reduced or when a smaller-sized detection coil is used.
During the car seeking process, the trolley and the detection coil fixed on the trolley will always deviate from the current-carrying wire. The task of the detection circuit is to judge the relative position of the car and the current-carrying track at any time, so as to deviate from the car according to the car. The degree of the road and the speed of the trolley control the steering angle of the steering gear on the trolley. To achieve the relative positioning of the trolley and the current-carrying track, it is necessary to arrange a plurality of identical detection coils on the trolley. Correspondingly, each detection coil is matched with the same signal conditioning circuit, only in the current-carrying line. The circuit corresponding to the coil above the track has the largest output signal.
That is to say, the relative position of the trolley and the current-carrying track is determined by the relative maximum value of the output signals in the multi-detection coil, and is not directly related to the magnitude of the signal output by each detection coil, and the induced electromotive force in each coil is found. The maximum value shows that the track is below the coil. Although the variation of the excitation current frequency and amplitude will significantly affect the induced electromotive force of the coil output, these factors have the same effect on all detection coils. The above-mentioned “finding the maximum†to achieve the track positioning is not affected, thus improving the detection circuit pair. The adaptability of the track.
2 Smart car control circuit requirements for detection signalsThe induced electromotive force of the detection coil output must be amplified and necessaryly processed, and finally provided to the smart car's MCU for A/D conversion sampling to obtain the position information of the track. The A/D input of the MCU of the smart car needs a unipolar voltage between 0 and 5 V. For this, two different signal types can be provided for the MCU, and the MCU samples in different ways.
Method 1: Amplify the output of the detection coil at a frequency of 20 kHz and a signal of about millivolts, and the amplification factor is about 1 000 times (60 dB), and then amplitude-converted into a DC voltage. The MCU can only know the signal size by sampling only one detection signal per channel, and collecting multiple voltages for comparison, and the track positioning can be realized by “finding the maximumâ€.
Mode 2: directly collect the amplified 20 kHz signal (added on the DC bias voltage), but require the A/D acquisition rate of the microcontroller to be much larger than 20 kHz. The microcontroller continuously collects multiple cycles of voltage, according to the periodicity of the signal. Find the maximum and minimum values ​​from the collected data, and obtain the peak-to-peak value of the AC signal based on the difference between the two. In this mode, the MCU needs to quickly sample each signal many times to obtain the size of the signal. Similarly, it is necessary to patrol the multi-channel voltage and achieve the track positioning by “finding the maximumâ€.
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