Analog to digital converters

A sinusoidal-amplitude-to-digital-output circuit includes a comparator with an input terminal, a reference terminal and an output terminal. A digital bus is connected to the output terminal. A reference voltage source is connected to the reference terminal. A feedback resistor is connected in parallel with the comparator between the output terminal and the input terminal to provide hysteresis for noise rejection such that circuit converts voltage received at the input terminal into a digital pulse-width modulated waveform that varies non-linearly with amplitude of the voltage received at the input terminal.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to signal conversion, and more particularly to analog to digital signal conversion for analog signal health monitoring.

2. Description of Related Art

Sensors, such as those employed on aircraft, commonly employ excitation voltages to sensors for purposes of monitoring a sensed parameter. For example, in rotating machinery like generators and motors resolvers are commonly employed to sense the rotational position and/or speed of a shaft. Such resolvers generally receive an excitation voltage that varies according to a desired sinusoidal waveform and typically provide an output signal indicative of the rotational position and/or speed of the shaft.

When providing a sinusoidal voltage excitation waveform it is often desirable to monitor the amplitude of the sinusoidal excitation voltage applied to the sensor. This is because the output of the sensor can change according to variation in the amplitude of the input excitation waveform, potentially changing output of the sensor for a given sensed parameter value. Monitoring the sinusoidal voltage excitation waveform generally involves feeding the sinusoidal signal through an analog-to-digital converter (ADC) device. ADC devices typically have a limited number of input channels, which can limit the number of parameters sensed in certain applications.

Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there is still a need in the art for improved systems and methods for sensor excitation signal monitoring. The present disclosure provides a solution for this need.

SUMMARY OF THE INVENTION

A circuit to allow an amplitude and frequency of a sinusoidal voltage waveform to be digitally monitored includes an input and an output. The input is configured to receive a sinusoidal voltage waveform. The output is in operable communication with the input and is configured to provide a duty cycle having a nonlinear relationship to the amplitude of the sinusoidal voltage waveform such that an amplitude and frequency of the sinusoidal voltage waveform can be calculated from just the duty cycle.

In certain embodiments, the circuit can be a sinusoidal-amplitude-to-digital-output (SADO). The circuit can include a comparator with an input terminal, a reference terminal and an output terminal. A digital bus can be connected to the output terminal. A reference voltage source can be connected to the reference terminal. A feedback resistor can be connected in parallel with the comparator between the output and input terminals to provide hysteresis for noise rejection such that circuit convert voltage received at the input terminal into a digital pulse-width modulated waveform that varies non-linearly with amplitude of the voltage received at the to the input terminal.

In accordance with certain embodiments, the SADO circuit can include a plurality of interconnected discrete electronic components. A voltage divider circuit connected to the comparator input terminal to attenuate an input voltage received at the comparator input. A source resistor connected in series between a sine wave generator and the comparator input terminal. A ground resistor can be connected in series between ground and the comparator input terminal.

In accordance with further embodiments, a half-wave rectifier circuit can be connected to the comparator input terminal to half-wave rectify and clamp the input voltage received at the comparator input terminal to ground. The half-wave rectifier can include a diode connected between ground and the comparator input terminal. The diode can be arranged to oppose current flow from the comparator input terminal and ground.

It is also contemplated that, in accordance with certain embodiments, a hysteresis circuit. The hysteresis circuit can be connected between the comparator output terminal and the reference voltage terminal. The hysteresis circuit can include a reference resistor connected in series between the reference voltage source and the comparator reference terminal. The feedback resistor can be connected in series between the comparator output terminal and the comparator reference terminal. The feedback resistor can be arranged to set a width of a hysteresis band, i.e., a voltage separation between a low-to-high switching voltage and high-to-low switching voltage, of the comparator. A pull-up resistor can be connected in series between a pull-up voltage source and the comparator output.

A sensor interface includes a SADO circuit as described above, a sinewave generator connected to the comparator input terminal, and a digital bus connected to the comparator output terminal and the comparator reference terminal. In certain embodiments a field programmable gate array (FPGA) device or a microprocessor can be connected to the comparator output by the digital bus. The FPGA or microprocessor can be disposed in communication with a lookup having input voltage waveform frequency associated with input waveform amplitude.

A method of monitoring amplitude and frequency of a sinusoidal voltage waveform is shown. The method includes generating a digital pulse-width modulated (PWM) signal whose duty cycle varies non-linearly with amplitude of the sinusoidal voltage waveform. The digital PWM is analyzed, and amplitude and frequency representative of the sinusoidal voltage waveform are calculated using just the digital PWM signal.

In certain embodiments the method can be a method on monitoring excitation voltage for a sensor. An input voltage with a sinusoidal waveform can be received at a comparator input terminal and a reference voltage at a comparator reference terminal. An output voltage can be received at a comparator output terminal and switched from a first to a second voltage when amplitude of the input voltage rises above a low-to-high threshold. The output voltage can be switched from the second to the first voltage when amplitude of the input voltage drops below a low-to-high threshold.

In accordance with certain embodiments the low-to-high threshold can be less than the high-to-low threshold. The low-to-high threshold can be generated by offsetting the reference voltage by the first voltage after switching from the second to the first voltage. The high-to-low threshold can be generated by offsetting the reference voltage by the output voltage after switching from the first to the second voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made to the drawings wherein like reference numerals identify similar structural features or aspects of the subject disclosure. For purposes of explanation and illustration, and not limitation, a partial view of an exemplary embodiment of a circuit to allow an amplitude and frequency of a sinusoidal voltage waveform to be digitally monitored in accordance with the disclosure is shown inFIG. 1and is designated generally by reference character100. Other embodiments of such circuits, sensors interfaces employing such circuits, and methods of monitoring amplitude and frequency of sinusoidal voltage waveforms in accordance with the disclosure, or aspects thereof, are provided inFIGS. 2-5, as will be described. The systems and methods described herein can be used for monitoring excitation voltages applied to sensors, such as resolvers operably coupled to rotating machinery in aircraft, though the present disclosure is not limited to resolvers or to rotating machinery in general.

Referring toFIG. 1, circuit100is shown. Circuit100generally includes an input101configured to receive a sinusoidal voltage waveform and an output103in operable communication with input101. Output103is configured to provide a duty cycle having a nonlinear relationship to the amplitude of the sinusoidal voltage waveform such that an amplitude and frequency of the sinusoidal voltage waveform can be calculated from just the duty cycle.

In the illustrated exemplary embodiment circuit100is sinusoidal amplitude-to-digital-output (SADO) circuit included in sensor interface10. Sensor interface10is connected to a sensor2and includes a SADO circuit100, a sine wave generator102, a memory104with a lookup table106, and field-programmable-gate-array (FPGA) device or processor108. Sine wave generator102is connected to SADO circuit100and sensor2. FPGA device or processor108is connected to SADO circuit100via a digital bus110and is disposed in communication with memory104. Lookup table106includes an association of sinusoid voltage to amplitude digital output due cycle percentage for an input voltage provided by sine wave generator102to SADO circuit100and sensor2. Sensor2in coupled to a sensed device4for determining a parameter of sensed device4. Sensed device4is resolver arranged to generate signal indicative of rotational positon of rotating machine6using an excitation voltage received at sensor2by sine wave generator102. It is to be understood and appreciated that this is for explanation purposes only, and that sensor2can be any device employing an excitation voltage with a sinusoidal waveform.

With reference toFIG. 2, a circuit diagram of SADO circuit100is shown. SADO circuit100includes a comparator112with an input terminal114, a reference terminal116, and an output terminal118. Digital bus110is connected to output terminal118. A reference voltage source120is connected to reference terminal116. A feedback resistor122is connected in parallel between output terminal118and input terminal114to convert voltage, e.g., sinusoidal voltage shown inFIG. 1, received at input terminal114into a digital pulse-width modulated waveform (shown inFIG. 1) that varies non-linearly with amplitude of the voltage received at input terminal114(shown inFIG. 4). It is contemplated that SADO circuit100include a plurality of discrete electronic components interconnected with one, e.g., resistors and diodes less than fully implemented in silicon.

SADO circuit100also includes a voltage divider circuit124. Voltage divider circuit124is connected to input terminal114and has a source resistor126and a ground resistor128. Source resistor126is connected in series between sine wave generator102and input terminal114. Ground resistor128is connected in series between a ground terminal129and input terminal114. A ground lead131connects ground resistor128to an input lead130at a location between source resistor126and input terminal114. Voltage divider circuit124is arranged, e.g., via respective resistances of source resistor126and ground resistor128, to attenuate voltage of the excitation waveform (shown inFIG. 1) applied to input terminal114.

SADO circuit100additionally includes a half-wave rectifier circuit132. Half-wave rectifier circuit132is connected to input terminal114and includes a diode134. Diode134is connected in series between a ground terminal129and input terminal114, and further connected to input lead130at a location between the connection of ground lead131and input terminal114via a diode lead138. Diode134is arranged to oppose current flow from input lead130, i.e. from input terminal114and sine wave generator102, to ground terminal136to half-wave rectify and clamp input voltage received at input terminal114to ground. In the illustrated exemplary embodiment diode134is a Schottky diode, which provides good efficiency at high switching speeds.

SADO circuit100further includes a hysteresis circuit140. Hysteresis circuit140is connected between output terminal118and reference terminal116and includes a reference resistor142and a feedback resistor144. Reference resistor142is connected in series between a reference voltage source146and reference terminal116. Reference voltage source146sets a trip threshold for comparator112. Feedback resistor144is connected in parallel with comparator112, interconnects output terminal118with reference terminal116, and is connected to reference voltage lead148at a location between reference terminal116and reference resistor142.

It is contemplated that feedback resistor144be arranged to set a width of a hysteresis band (shown inFIG. 3) of SADO circuit100, i.e. a voltage separation174(shown inFIG. 3) between a low-to-high switching threshold172(shown inFIG. 3) and a high-to-low switching threshold170(shown inFIG. 3). The width of the hysteresis band is established at least in part based upon resistance of a pull-up resistor156connecting a digital bus pull-up voltage source158in series to digital bus110.

With reference toFIG. 3, a graph160of an input waveform162and a digital output waveform164are shown. Time is indicated on x-axis166. Voltage is indicted at y-axis168. Input waveform162is provided to SADO circuit100(shown inFIG. 1) in a sampling arrangement, shown in an exemplary way inFIG. 1with SADO circuit100being connected in parallel with an excitation lead extending between sine wave generator102and sensor2, from sine wave generator102(shown inFIG. 1).

As input waveform162arrives SADO circuit100converts input waveform162into digital output waveform164, which SADO circuit100provides to digital bus110. The above-described circuitry pulse width modulates digital output waveform164according to time interval, e.g., the low interval shown inFIG. 3, between a high-to-low threshold170(VH) and low-to-high threshold172(VL). High-to-low threshold170(VH) is greater than low-to-high threshold172(VL), a wide hysteresis band174of SADO circuit100separating high-to-low threshold170(VH) and low-to-high threshold172(VL).

SADO circuit100compares voltage amplitude of input waveform162against high-to-low voltage threshold170during intervals when digital output waveform164is ‘high’, e.g., a ‘1’, and further compares the voltage amplitude of input waveform162against low-to-high threshold172when digital output waveform164is ‘low’, e.g., is a zero. High-to-low voltage threshold170(VH) and low-to-high threshold172(VL) are separated in voltage magnitude by a wide hysteresis band174, which is established by hysteresis circuit140(shown inFIG. 2) and operates to switch the reference voltage applied to reference terminal116(shown inFIG. 2) according to whether digital output waveform164is high or low. The proportion of time during which the digital output waveform164is high corresponds to the duty cycle modulated into digital output waveform164, which is provided in real-time to FPGA device or processor108for determining health of input waveform162.

With reference toFIG. 4, a graph176of duty cycle D born by digital output wavefrom164(shown inFIG. 3) in association with amplitude A of input waveform162(shown inFIG. 3) is shown. Amplitude A is expressed in volts on x-axis178. Duty cycle D is expressed as a percentage on y-axis180. A duty cycle function182extends from 0 volts on the left-hand side of graph176to about 20 volts on the right-hand side of graph176, duty cycle function182varying in a non-linear with amplitude A. As will be appreciated by those of skill in art in view of the present disclosure, the non-linearity of duty cycle function182allows FPGA device or processor108(shown inFIG. 1) to calculate both sinusoidal frequency and amplitude using only a single digital input, i.e., digital output waveform164(shown inFIG. 3).

Frequency is calculated by measuring time intervals between rising and falling edges of digital output wavefrom164. Amplitude of input waveform162is determined by referencing an observed duty cycle (via pulse-width modulation imparted to digital output waveform164) with an associated amplitude according to duty cycle function182. In an exemplary implementation shown inFIG. 4, a duty cycle of 78% is cross-referenced in lookup table106(shown inFIG. 1) to recognize that amplitude of input wavefrom162is about 6 volts. This amplitude can then be compared against a selected voltage value, or minimum to maximum voltage range (i.e., AMINto AMAX), to assess health of input waveform162.

Duty cycle function182is arrived at via SADO circuitry shown inFIG. 2as follows. Input waveform162a sinusoidal waveform with amplitude A and an angular frequency ω, and generally conforms to Equation 1:
Vi(t)=Asin(ωt)
Assuming comparator112(shown inFIG. 2) has a high-to-low threshold VHand a low-to-high threshold VL, then the time associated with input waveform160crossing high-to-low threshold VHis according to Equation 2:

tH=1ω⁢sin-1⁡(VHA)
The time associated with input waveform162crossing low-to-high threshold VLis according to Equation 3:

tL=πω-1ω⁢sin-1⁡(VLA)
The duty cycle D of the input waveform is according to Equation 4:

D=1+ω⁢⁢tH2⁢⁢π-ω⁢⁢tL2⁢⁢π
Substituting Equation 2 and Equation 3 into Equation 4, and accounting for the constraint imposed by the dome of the arcsine yields an expression that is independent of angular frequency according to Equation 5:

D={1A<VH0.5+12⁢⁢π⁡[sin-1⁡(VHA)+sin-1⁡(VLA)]A≥VH
which is illustrated graphically inFIG. 4with duty cycle function182.

In certain embodiments, output of SADO circuit100(shown inFIG. 1) is not as accurate as output from a dedicated analog-to-digital converter. However, using discrete components, embodiments of SADO circuit100can realize amplitude measurement accuracy of about +/−10% of actual amplitude, which is generally sufficient for sensor interfaces, e.g., sensor interface10(shown inFIG. 1).

With reference toFIG. 5, a method200of monitoring amplitude and frequency of a sinusoidal voltage waveform is shown. Method200includes generating a digital PWM signal whose duty cycle varies non-linearly with amplitude of the sinusoidal voltage waveform. The digital PWM is analyzed, and amplitude and frequency representative of the sinusoidal voltage waveform are calculated using just the digital PWM signal. In the illustrated exemplary embodiment a method of monitoring excitation voltage for a sensor, e.g., sensor2(shown inFIG. 1) is shown. It is to be understood and appreciated that this is for illustration purposes only and is non-limiting.

Method200includes receiving an input voltage with a sinusoidal waveform (shown inFIG. 2) at a comparator input terminal, e.g., input terminal114(shown inFIG. 2), as shown with box210. Method200also includes receiving a reference voltage (shown inFIG. 3) at a comparator reference terminal, e.g., reference terminal116(shown inFIG. 1), as shown with box220. Amplitude of the input voltage is compared to the reference voltage, as shown with box230. If the input voltage is not greater than the reference voltage then output of the SADO circuit remains the same and monitoring continues, as shown with arrow250.

When the comparison indicates that the input voltage is greater than the reference voltage then the output of the SADO circuit is toggled between high and low, as shown with arrow260and box270. The reference voltage is then offset with the toggled output, as shown with box280. The offsetting can change the reference from a high-to-low threshold to a low-to-high threshold, as shown with box282. The offsetting can change the reference from a low-to-high threshold to a high-to-low threshold, as shown with box284. The magnitude of the reference voltage changes according when the waveform is switched high or low. It is contemplated that monitoring can continue with output being toggled between high and low according a wide hysteresis defined between the low-to-high and high-to-low thresholds to synthesize a PWM digital output, as shown with arrow290.

When providing a sinusoidal voltage excitation waveform it can be desirable to monitor amplitude of the sinusoid. Usually this involves feeding the sinusoid back through an analog-to-digital converter (ADC). While generally satisfactory for its intended purpose, the number of ADC channels available in a given application may be such that no monitoring channel is available to receive the sinusoidal waveform.

In embodiments described herein a SADO circuit is provided. The SADO circuit is configured to convert an input voltage varying according to a sinusoidal waveform to a digital pulse-width modulated (PWM) output, the PWM output duty cycle varying nonlinearly with amplitude of the sinusoidal waveform. Varying the PWM output duty cycle nonlinearly with the sinusoidal voltage amplitude allows a microprocessor or a field-programmable gate array (FPGA) to calculate both sinusoidal frequency and amplitude using only a single digital input, i.e., the SADO circuit output. This allows for sinusoidal amplitude and frequency monitoring using digital inputs instead of analog inputs, potentially reducing cost as digital inputs can be more readily implemented than analog inputs.

The methods and systems of the present disclosure, as described above and shown in the drawings, provide for sensor interfaces with superior properties including the sinusoidal excitation voltage waveform amplitude and frequency monitoring with the use of an ADC. While the apparatus and methods of the subject disclosure have been shown and described with reference to preferred embodiments, those skilled in the art will readily appreciate that changes and/or modifications may be made thereto without departing from the scope of the subject disclosure.