Mass air flow circuit having pulse width modulation feedback control
View Patent ↗A circuit for measuring the mass air flow rate through a duct is disclosed. The circuit includes a bridge circuit, a first amplifier, an oscillator, a comparator and a switch. The bridge circuit has a plurality of resistors arranged to output first and second bridge voltage signals. The first amplifier circuit receives the first and second bridge voltage signals and generates a differential bridge voltage signal. The oscillator provides an oscillating control signal. The comparator is in communication with the first amplifier circuit and the oscillator for comparing the differential bridge voltage signal to the oscillating signal to generate a pulse width modulated differential bridge voltage signal. The switch is in communication with the bridge circuit and the comparator. The switch selectively provides a power voltage signal to the bridge circuit at a frequency corresponding to the pulse width modulated differential bridge voltage signal.
1. A circuit for measuring the mass air flow rate through a duct, the circuit comprising:
a bridge circuit having a plurality of resistors arranged to output first and second bridge voltage signals;
a first amplifier circuit for receiving the first and second bridge voltage signals and generating a differential bridge voltage signal;
a compensation network for receiving the differential bridge voltage signal, changing the phase of the differential bridge voltage signal, and generating a compensated signal;
an oscillator for providing an oscillating control signal;
a comparator in communication with the first amplifier circuit, the compensation network, and the oscillator for comparing the compensated signal to the oscillating signal to generate a pulse width modulated differential bridge voltage signal; and
a switch in communication with the bridge circuit and the comparator, wherein the switch selectively provides a power voltage signal to the bridge circuit at a frequency corresponding to the pulse width modulated differential bridge voltage signal.
2. The circuit of claim 1 wherein the bridge circuit has at least four resistors arranged in a Wheatstone bridge configuration.
3. The circuit of claim 1 wherein the oscillator provides a triangular shaped oscillating control signal.
4. The circuit of claim 1 wherein the first amplifier circuit is an operational amplifier.
5. The circuit of claim 1 further comprising a filter circuit for filtering at least one of the first and second bridge voltage signals.
6. The circuit of claim 5 wherein the filter circuit includes a resistor in series with a capacitor.
7. The circuit of claim 1 further comprising a second amplifier circuit for amplifying one of the first and second bridge voltage signals to generate an amplified output signal.
8. The circuit of claim 7 further comprising a gain circuit for setting the gain provided by the second amplifier circuit.
9. The circuit of claim 7 further comprising a voltage to frequency converter in communication with the second amplifier for converting at least one of the first and second bridge voltage signals from an analog signal to a frequency.
10. The circuit of claim 1 further comprising an auto oscillate circuit in communication with the comparator and the switch, the auto oscillate circuit operable to determine whether the pulse width modulated differential bridge voltage signal is present, the auto oscillate circuit further operable to generate a default pulse width modulated signal if the auto oscillate circuit determines that no pulse width modulated differential bridge voltage signal is present.
11. A circuit for measuring the mass air flow rate through a duct, the circuit comprising:
a bridge circuit having a plurality of resistors arranged to output first and second bridge voltage signals;
a first amplifier circuit for receiving the first and second bridge voltage signals and generating a differential bridge voltage signal;
an oscillator for providing an oscillating control signal;
a comparator in communication with the first amplifier circuit and the oscillator for comparing the differential bridge voltage signal to the oscillating signal to generate a pulse width modulated differential bridge voltage signal;
a switch in communication with the bridge circuit and the comparator, wherein the switch selectively provides a power voltage signal to the bridge circuit at a freguency corresponding to the pulse width modulated differential bridge voltage signal; and
a three-way switch for selectively switching between one of a bridge voltage signals, a frequency signal and a programming signal.
12. A circuit for measuring the mass air flow rate through a duct, the circuit comprising:
a bridge circuit having a plurality of resistors arranged to output first and second bridge voltage signals;
a first amplifier circuit for receiving the first and second bridge voltage signals and generating a differential bridge voltage signal;
a compensation network for receiving the differential bridge voltage signal, changing the phase of the differential bridge voltage signal, and generating a compensated signal;
an oscillator for providing an oscillating control signal;
a comparator in communication with the first amplifier circuit, the compensation network, and the oscillator for comparing the compensated signal to the oscillating signal to generate a pulse width modulated differential bridge voltage signal; and
a switching means in communication with the bridge circuit and the comparator, wherein the switching means selectively provides a power voltage signal to the bridge circuit at a frequency corresponding to the pulse width modulated differential bridge voltage signal.
13. The circuit of claim 12 wherein the oscillator provides a triangular shaped oscillating control signal.
14. The circuit of claim 12 wherein the first amplifier circuit is an operational amplifier.
15. The circuit of claim 12 further comprising a filter circuit for filtering at least one of the first and second bridge voltage signals.
16. The circuit of claim 15 wherein the filter circuit includes a resistor in series with a capacitor.
17. The circuit of claim 12 further comprising a second amplifier circuit for amplifying one of the first and second bridge voltage signals to generate an amplified output signal.
18. The circuit of claim 17 further comprising a gain circuit for setting the gain provided by the second amplifier circuit.
19. The circuit of claim 18 further comprising a voltage to frequency converter in communication with the second amplifier for converting at least one of the first and second bridge voltage signals from an analog signal to a frequency.
20. The circuit of claim 12 further comprising an auto oscillate circuit in communication with the comparator and the switching means, the auto oscillate circuit operable to determine whether the pulse width modulated differential bridge voltage signal is present, the auto oscillate circuit further operable to generate a default pulse width modulated signal if the auto oscillate circuit determines that no pulse width modulated differential bridge voltage signal is present.
21. A circuit for measuring the mass air flow rate through a duct, the circuit comprising:
a bridge circuit having a plurality of resistors arranged to output first and second bridge voltage signals;
a first amplifier circuit for receiving the first and second bridge voltage signals and generating a differential bridge voltage signal;
an oscillator for providing an oscillating control signal;
a comparator in communication with the first amplifier circuit and the oscillator for comparing the differential bridge voltage signal to the oscillating signal to generate a pulse width modulated differential bridge voltage signal;
a switching means in communication with the bridge circuit and the comparator, wherein the switching means selectively provides a power voltage signal to the bridge circuit at a freguency corresponding to the pulse width modulated differential bridge voltage signal; and
a three-way switch for selectively switching between one of a bridge voltage signals, a frequency signal and a programming signal.