Coded data generation or conversion – Analog to or from digital conversion – Digital to analog conversion
Reexamination Certificate
2000-12-26
2001-10-23
Jeanpierre, Peguy (Department: 2819)
Coded data generation or conversion
Analog to or from digital conversion
Digital to analog conversion
C341S143000
Reexamination Certificate
active
06307494
ABSTRACT:
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The invention relates to a device and method for the rapid digital/analog (“D/A”) conversion of pulse width modulated (“PWM”) signals, particularly of digitized analog output signals from a lambda probe in a motor vehicle that have been converted into PWM signals.
For cost reasons, modern microcontrollers are increasingly being used for digital signal processor applications in control systems. For such use, the analog measured variable is digitized using an analog/digital (A/D) converter, is modified by a program in the microcontroller (filtering, comparison with nominal variables, PID controller, etc.), and is converted subsequently into an analog signal again using a D/A converter. Although microcontrollers usually contain A/D converters for digitizing analog measured variables, a D/A converter is generally not integrated.
One D/A conversion option is to use an external D/A converter; this is the technically optimum solution, but also the most cost intensive. In a serial D/A converter having a clock frequency of, for example, 500 kHz with an accuracy of 10 bits, the conversion time arising is 2 ms in theory, but approximately 3 ms in practice. The conversion time is incorporated in the phase shift of the control loop as a delay time.
A further option is to convert the digital signal in the microprocessor into a signal having a fixed frequency and having a pulse duty factor that is proportional to the digital value, with subsequent external low pass filtering to remove the AC voltage components. This solution is much less expensive than a D/A converter and is, on many occasions, used for less demanding applications. For use in a control system, however, this solution has the disadvantage that the low pass filter required for filtering greatly delays the phase of the signal (group delay time). If the delay is within the control loop, then the delay contributes to the total phase delay for the control loop.
For example, in a third order low pass filter, the attenuation at a frequency of 100 Hz is −60 dB. In other words, the amplitude of a PWM signal at 5V is attenuated to 5 mV. Then, the group delay time is approximately 50 ms. If the PWM frequency is raised to 250 Hz and the low pass filter is matched accordingly, the group delay time would be reduced to 20 ms, which is roughly equivalent to the technical limit of the system.
Patent Abstracts of Japan Vol. 016, No. 055 (E-1165), Feb. 12, 1992 (Feb. 12, 1992)-&JP 03 254214 A (Fuji Electric Co Ltd), Nov. 13, 1991 (Nov. 13, 1991) disclose a device for the D/A conversion of PWM signals that has a flip-flop, two integrators, and a decoupling amplifier. One of the integrators integrates the PWM signal during a High phase and supplies the D/A converted voltage at the output at least during the subsequent Low phase. In the Low phase, the other capacitor is discharged in the meantime.
Patent Abstracts of Japan Vol. 006, No. 119 (E-116), Jul. 3, 1982 (Jul. 3, 1982)-&JP 57 048826 A (Japan Electronic Control Syst. Co. Ltd.), Mar. 20, 1982 (Mar. 20, 1982) disclose a device for the D/A conversion of PWM signals having a monostable multivibrator, an integrator, a storage capacitor, and a decoupling amplifier, the PWM signal being integrated during its Low phase.
German Patent No. DE 32 37 386 C2 discloses a digital/analog converter of the counter type for processing a pulse code modulated stereo sound signal, in which the alternately appearing digital data words for the left and right audio channels are serially converted into corresponding analog currents. The corresponding analog currents are converted in parallel in a respective integrator into voltage signals that subsequently pass through a respective low pass filter.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a device and method for the rapid digital/analog conversion of pulse width modulated signals that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices and methods of this general type and that provides an inexpensive device and method for the rapid, accurate D/A conversion of PWM signals without using a cost-intensive D/A converter, but with a phase delay that is comparable to that of a D/A converter.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a device for rapid digital/analog conversion of pulse width modulated signals, particularly, for rapid digital/analog conversion of digitized analog output signals from a lambda probe in a motor vehicle that have been converted into PWM signals, including a device input for carrying an input signal, a device output, a first changeover switch having first, second, and third connections, an inverting integrator having a resistor, a first capacitor, a second changeover switch, a first operational amplifier, and an integrator output, the first operational amplifier having an inverting input, a non-inverting input connected to a reference voltage, and an amplifier output, the inverting input selectively connected to the device input through the first and second connections and the resistor, and the inverting input selectively connected to the amplifier output through the second and third connections, the first capacitor disposed between the inverting input and the amplifier output, a second capacitor, a third capacitor, the second changeover switch having fourth, fifth, and sixth connections, the second changeover switch selectively connecting the integrator output to the second capacitor through the fourth and sixth connections and selectively connecting the integrator output to the third capacitor through the fourth and fifth connections, a buffer amplifier having a non-inverting input, an inverting input, and a buffer amplifier output being the device output, a third changeover switch having seventh, eighth, and ninth connections, the third changeover switch selectively connecting the non-inverting input of the buffer amplifier to the second capacitor through the eighth and ninth connections and selectively connecting the non-inverting input of the buffer amplifier to the third capacitor through the seventh and ninth connections, a monostable multivibrator for producing a multivibrator output signal to selectively switch the first changeover switch, the multivibrator connected to the device input for being triggered by the input signal, the multivibrator having a prescribed operating period, and a frequency divider for producing a divider output signal to trigger the second and third changeover switches, the frequency divider connected to the device input for being triggered by the input signal.
The pulse width modulated signal, integrated per period, is alternately supplied to two capacitors, which provide it during the next particular period as an analog signal for the further processing. After analog/digital re-conversion, the signal is subjected to a nominal/actual value comparison with a digital signal proportional to the pulse width modulated signal, and the difference is subsequently used to correct the pulse width modulated pulse duty factor or the pulse width modulated frequency.
In accordance with another feature of the invention, there is provided a microcontroller for converting analog signals into digital pulse width modulated signals at a constant clock frequency.
In accordance with a further feature of the invention, the microcontroller has an output connected to the device input and an input connected to the device output and to the an inverting input of the buffer amplifier.
In accordance with an added feature of the invention, the frequency divider is a flip-flop.
In accordance with an additional feature of the invention, each of the second and third capacitors has a connection to ground.
With the objects of the invention in view, there is also provided a method for rapid digital/analog conversion of pulse width modulated signals, including the steps of providing a conversion device having, a device input for carrying an input signal, a first chang
Greenberg Laurence A.
Jean-Pierre Peguy
Jeanglaude Jean Bruner
Lerner Herbert L.
Siemens Aktiengesellschaft
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