Increasing current and voltage sensor accuracy and resolution in electric and hybrid electric vehicles

A method for estimating an electrical measurement, such as a current or a voltage, comprises generating an analog electrical measurement signal using a sensor. An analog filter filters the analog electrical measurement signal. An A/D converter samples the analog signal and generates a digital electrical measurement signal. A matrix is formed based on the characteristics of the analog filter. An estimated actual current or voltage is calculated based on a relationship between the digital signal and the matrix.

FIELD OF THE INVENTION

The present invention relates to current and voltage sensors, and more particularly methods and apparatus for improving current and voltage sensor accuracy and resolution.

BACKGROUND OF THE INVENTION

Electric and hybrid vehicles include a propulsion system that typically includes an electric motor and/or an engine. Current for powering the electric motor is supplied by a battery subsystem. Key performance issues of the electric and hybrid vehicles include fuel efficiency, emissions, and drivability, which depend largely on the operation of the propulsion system.

The battery subsystem is a significant element of the propulsion system of these vehicles. An accurate state of charge (SOC) algorithm improves performance of the battery subsystem, and therefore the vehicle. The SOC algorithm requires an accurate current sensor for sensing current in the battery subsystem. The battery subsystem also requires an accurate analog-to-digital (A/D) converter that communicates with the current sensor. Other applications requiring accurate current sensing include fuel cell and supercapacitor systems.

Current sensor measurements are limited by current sensor accuracy and resolution, as well as A/D converter resolution. One conventional method for improving the accuracy of current sensor measurements uses multiple A/D converters and/or multiple current sensors. Using multiple converters and sensors increases the complexity and cost of the battery subsystem. Another conventional method uses an A/D converter having a higher resolution. However, there are limits to A/D converter resolution.

SUMMARY OF THE INVENTION

A method for estimating current or voltage in an electrical system comprises generating an analog current or voltage signal using a sensor. The signal is filtered using an analog filter. An A/D converter samples the analog signal and generates a digital signal. A matrix based on characteristics of the analog filter is formed. An estimated actual current or voltage is calculated based upon a relationship between the digital signal and the matrix.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements.

Referring now toFIG. 1, one or more inverters10draw current from a power source12. The power source12selectively receives power from the inverters10. The inverters10draw current through a current sensor14. The inverters10change power settings to vary the magnitude of current drawn from the power source12through the current sensor14. A trigger16communicates with the inverters10to synchronize power setting changes. The trigger16may be a physical wire, a serial data message or other suitable trigger. The trigger16may be executed at periodic time intervals. Additional loads18may also receive power from the power source12.

The current sensor14is implemented with current sense hardware such as hall effect sensors, sense resistors, or other appropriate hardware and/or software. An analog filter20receives an analog current signal22from the current sensor14. In the exemplary embodiment, the analog filter20is a low-pass filter. However, other suitable filters may be used. An analog to digital (A/D) converter24converts the analog current signal22from the current sensor14into a digital current signal. The A/D converter24may communicate with the trigger16to sample the analog signal22in sync with inverter10power setting changes. Alternatively, a step inverter26may change power settings in a manner that causes the analog signal22to transition through discrete levels of the A/D converter24.

A microprocessor28receives the digital current signal30from the A/D converter24. The microprocessor28executes a current estimating algorithm to reconstruct the digital current signal30. The current estimating algorithm results in a current estimate with a resolution that is greater than the resolution of the A/D converter24. Additionally, the microprocessor28generates the trigger16to synchronize the sampling of the A/D converter24with power setting changes by the inverters10.

Referring now toFIG. 2, an alternative embodiment uses a voltage sensor32. The inverters10draw power from a battery34having a known source impedance. The voltage sensor32measures the voltage of the battery34. The A/D converter24converts an analog voltage signal36from the voltage sensor32into a digital voltage signal38. The microprocessor28executes a voltage estimating algorithm to reconstruct the digital voltage signal38.

Referring now toFIG. 3, u(t) is an input signal40to a signal flow model of the present invention. The input signal40is the sum of the currents (or voltages) from the inverters10and the other loads18inFIGS. 1 and 2. H(s) is a transfer function42of the analog filter20inFIGS. 1 and 2. The analog filter20filters the input signal40to produce w(t), which is a filtered analog input signal44. The A/D converter24converts the filtered input signal44to a quantized input46y(t).

Referring toFIGS. 1 through 3, the trigger16may synchronize the inverters10to change power settings at first rate such as 20 Hz and to sample the quantized input46at a second rate such a 200 Hz. The second rate is preferably higher than the first rate. As a result, the quantized input46is sampled multiple times for each power setting of the inverters10.

In steady-state (constant current) conditions with no noise, the accuracy of the quantized input46is not impaired. However, variable current and noise levels affect the accuracy of the A/D converter24(as shown inFIGS. 1,2, and3). Although the input signal40may be constant, noise may cause the quantized input46to fluctuate between discrete A/D converter levels. Therefore, the quantized input46may not represent actual current measurements. However, actual current measurements may be estimated if characteristics of the analog filter20are known. Any change in the input signal40can be determined by comparing the quantized input46to a known response of the analog filter20. By observing the quantized input46, the microprocessor28, may estimate values of the input signal40that would result in the quantized input46. The microprocessor28may estimate values that are not constrained by quantization levels of the A/D converter24.

Referring now toFIG. 4, a current estimating algorithm50is executed to correspond with changes in inverter power settings. The current estimating algorithm50improves accuracy and resolution of the quantized input46inFIG. 3. The current estimating algorithm50may be executed at any rate up to the sampling rate of the A/D converter24(as shown inFIGS. 1,2and3). The microprocessor28(as shown inFIGS. 1 and 2) collects current samples from the A/D converter24in step100. The microprocessor28arranges the samples into a column vector Y of measured currents in step102. The column vector Y consists of n samples from Y1to Ym. Y1and Ymare the oldest sample and most recent sample, respectively, taken by the microprocessor28.

The microprocessor28forms a matrix A with m rows and n columns in step104. The matrix A is formed based upon characteristics of the analog filter20ofFIGS. 1 and 2. Relevant characteristics of the analog filter20include linearity, saturation and damping characteristics, and a time constant. The analog filter20is preferably linear. The analog filter20may be nonlinear if adjustments are made to the current estimating algorithm50.

An impulse response of the analog filter20is h(t). For t<0, h(t) is 0. The matrix A is formed as follows:

Aij=∫τ=(j-1)⁢Tcj·Tc⁢h⁡(Ts·i-τ)·ⅆτ
wherein TSis the sampling rate of the quantized input46of the A/D converter24and TCis the current (most recent) A/D converter sample. In step106, the microprocessor28uses the column vector Y and the matrix A to derive a vector U of actual currents. A relationship between measured currents Y, matrix A, and actual currents U is expressed as:
Y=A·U+V
wherein vector V represents quantization noise and noise due to other loads. V is not know in advance and must be estimated through statistics and probability. Noise can be described by a probability density function (a histogram of noise values over a very long period of time) and an autocorrelation function (the extent that noise values are related to previous noise values).

The microprocessor28calculates an estimate of an actual current estimate u using the above relationship in step108. In the preferred embodiment, the microprocessor28uses a least squares method to calculate the current estimate u. The microprocessor estimates a current u based on a history of current measurements Y. The least squares method calculates the estimated current vector Û as follows:
Û=(ATA)−1AT·Y.
Matrix A is known and constant. Therefore, (ATA)−1ATcan be pre-calculated by the microprocessor28, resulting in:
Û=Ā·Y.
Additionally, this calculation may be further reduced to calculate a single current estimate u:
û=Ām·Y
wherein Āmis the last row in Ā=(ATA)−1AT. The result of this simplification is that the current estimate may be reduced to a finite impulse response (FIR) filter. The FIR filter is executed in sync with changes in inverter power settings. Although the least squares method is described, other methods are possible. Alternatively, a convex optimization function may be used to estimate the current u.

Accuracy of the current estimate may be affected by the number of power setting changes and the delay between a power setting change and the execution of the current estimate. Additionally, accuracy of the current estimating algorithm50may be affected by knowledge of the characteristics of the analog filter20. However, knowledge of filter characteristics are limited by manufacturing and temperature variations. The characteristics may be derived by injecting a known signal into the analog filter20and estimating the necessary characteristics.