Electricity: measuring and testing – Magnetic – Displacement
Reexamination Certificate
1999-09-30
2001-03-27
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Displacement
C324S236000, C324S207120
Reexamination Certificate
active
06208134
ABSTRACT:
BACKGROUND
The invention pertains to proximity sensor systems. Particularly, it pertains to sensor systems which analyze the impedance of their resonators, and more particularly to such systems in which analyzers eliminate the effects of high levels of electromagnetic interference (EMI) in the sensors' environments.
A class of non-radiating proximity sensors and the associated detection systems rely on inductive properties and rely on the principle that their effective inductance changes in proportion to a conducting object's position.
A common and well-known detection method for this class of sensors is the eddy-current killed oscillator. In this system a circuit, including an inductor in the sensor, is in resonance when there is no detectable object near the sensor. When a detectable object is sufficiently close to the sensor, the change in the effective loss of the inductor in the sensor defeats resonance and thus causes the oscillations in the circuit to cease. If the inductance changes, the resonator may change frequency without quenching the oscillations. It is the change in the loss that spoils the resonance and stops the oscillation. The related-art approach is inadequate for environments with high electromagnetic interference.
The present invention circumvents this problem by utilizing in a particular way the principles of AM radio, without radiating an AM signal, as well as of resonance tracking, namely synchronous demodulation, to analyze the impedances of proximity sensors. The resonance tracking circuit in this invention never stops because the sensor is being driven with a generated waveform, but the change in loss causes the amplitude of the signal to change and ultimately causes the output to change due to the change in amplitude.
SUMMARY OF THE INVENTION
This invention is an electronic system associated with a proximity sensor (e.g., a railcar wheel sensor) that measures the impedance of the sensor in the presence of high levels of electromagnetic interference (e.g., 75 times that of the signal being sensed). Such interference causes related-art proximity sensor systems to malfunction.
The sensor, having an inductor and capacitor in parallel (i.e., a resonant circuit), is excited by a voltage-to-current converter. The converter is fed with a signal from a voltage-controlled triangle-wave oscillator (VCO) in the present circuit and the sensor changes the output signal into a sine wave. These electronics are receptive to interfering electromagnetic signal source or noise. A voltage amplifier senses the voltage at the resonant sensor. The sensed voltage is demodulated with a shifted version of the signal going into the converter. The result is filtered and fed back through an integrator, which integrates the error signal. This integrated voltage signal goes to the VCO and ensures that the VCO is at the resonant frequency of the sensor.
When the VCO is oscillating at the resonant frequency of the sensor, the input to the integrator has no DC component so that the output of the integrator is not changing. This output therefore remains at the level needed to keep the VCO running at the resonant frequency of the sensor. When it is not at the resonant frequency, there is a DC component at the input of the integrator, so the output of the integrator moves to a new level to adjust the frequency of the VCO. The output of the integrator stabilizes when the resonant condition is established. Thus, the VCO is maintained at the resonant frequency of the sensor.
The output of the proximity sensor system is a single, DC signal. This is produced by demodulating the signal from the resonant sensor, using the signal from the oscillator, and filtering out the interfering signals from it. The demodulating signal, the one from the oscillator, is synchronized with the signal to be demodulated, the one from the sensor, since they both have the same source, the oscillator. The filtering is commonly done before the demodulation, but in this design it is done afterward. At the same time, the resonant sensor itself acts as a band pass filter to also attenuate some of the interference before demodulation.
The oscillator with in-phase and quadrature outputs, the feedback paths via the voltage-to-current converter or amplifier, the demodulator and the low-pass filter implement a lock-in amplifier (LIA). This is an amplifier that seeks a signal at a specific frequency to amplify. In this case, it is the frequency of the VCO, which tracks the resonance frequency of the proximity sensor. The impedance, and therefore the resonant frequency, changes in the presence of a metal object proximate to the sensor, thus sensing the object.
This approach is effective for two reasons. First, the design implements a lock-in-amplifier that translates the frequency of interest to DC before filtering out extraneous signals using a multiple-pole (i.e., third order) low pass filter. In the present invention, the sensor's resonant frequency is around 400 kHz and the dominant source of interference is around 165 kHz, so the interference to be rejected is more than 200 kHz away from the sensor's resonant frequency and the third order 300 Hz low-pass filter provides over 100 dB of rejection of the interference.
The second reason for the system's effectiveness is that the resonator or sensor itself implements a pre-filter, and the controlling electronics assures that the filter is centered on the signal. A common technique in lock-in amplification is to pre-filter the signal by putting a band-pass filter in front of the demodulator, but this is not necessary since here the sensor itself acts as a band-pass filter. This combination of the pre-filter and synchronous demodulator is used to recover signals that are buried in broadband noise that is hundreds of times larger than the signals. The use of lock-in-amplification in conjunction with the present resonance-tracking control electronics is an unusual and effective solution to the problem of electromagnetic interference. The present sensor is very robust in very noisy EMI environments (i.e., noise-to-signal ratios greater than 75 times).
REFERENCES:
patent: 4001718 (1977-01-01), Wilson et al.
patent: 4187462 (1980-02-01), Haker et al.
patent: 5420507 (1995-05-01), Laskowski
patent: 5767672 (1998-06-01), Guichard et al.
Honeywell International Inc
Patidar Jay
Shuay Jr. John G.
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