| 摘要 |
An improved magnetic inductance vehicle detection system is provided which responds to changes in the inductance of a wire loop embedded in the roadway to sense the presence of a vehicle. The system includes a first oscillator connected to the loop whose frequency changes as the loop inductance changes, and it also includes a second oscillator whose frequency is independent of the loop inductance, and which is used as a reference. Appropriate logic circuitry establishes an output signal whenever the frequency of the loop oscillator increases with respect to the frequency of the reference osillator beyond a predetermined frequency differential, so as to indicate the fact that a vehicle has crossed in the area circumscribed by the loop. This output signal is formed into a direct current signal which actuates a relay in a vehicle indicating circuit. The system of the invention also includes a control circuit for compensating for drifts in the system. This control circuit includes a network which responds to an error signal to develop a fast-acting anti-drift control whenever the frequency of the loop oscillator falls below the frequency of the reference oscillator; and which also develops a slow-acting anti-drift control to compensate for frequency drifts in the system by which the frequency of the loop oscillator rises above the frequency of the reference oscillator.
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