Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
2000-09-29
2001-11-20
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S165000
Reexamination Certificate
active
06320430
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a technique for processing a measurement signal.
Measurement devices, such as sensors, bridge circuits, or the like, have a disadvantage that they often superpose a first offset signal on the measurement signal of interest to the user. This offset signal is due to the mechanical sensitivity of the measurement device, inaccuracies in the manufacturing technique of the measurement devices, and/or the electronics. The magnitude of this offset signal is often unknown to the user and varies in time.
Some measurement devices, such as Hall sensors or Wheatstone bridges can be driven in such a way that the offset signal is alternately added to and subtracted from the measurement signal. Therefore, the offset signal is superposed on the measurement signal as an AC signal, preferably a square-wave signal. Driving the sensors or bridge circuits in this way is often referred to as “chopping”. The frequency that the offset signal changes sign is called the “chopper frequency”.
The chopper frequency is typically selected to have a much higher value than the maximum frequency of the measurement signal, so the high frequency offset signal can be separated from the measurement signal by a low-pass filter to obtain an offset-free measurement signal.
A disadvantage of this technique is the complication associated with the circuitry involved in filtering the signal, and the fact that filtering usually creates a new offset. Therefore, there is a need for a system and method for obtaining the measurement signal without the need for filtering.
SUMMARY OF THE INVENTION
Briefly, according to the present invention, a summation signal delivered by a measurement device, for example a Hall sensor or a Wheatstone bridge, and composed of a measurement signal and an offset signal with a first frequency (chopper frequency), is sampled at the rate of a second frequency so as to provide sample values, the second frequency being a multiple of the first frequency. To create measurement values from the measurement signal, the average of at least two sample values is then formed. These sample values are formed by sampling the summation signal at the interval of a half period of the first frequency or at the interval of an odd multiple of half the period of the first frequency. In forming the average, one always uses at least one sample value for which the value of the offset is added to the measurement value and always at least one sample value for which the value of the offset is subtracted from the measurement value. The offset is thus eliminated by forming an average.
Assuming that the measurement signal changes slowly compared to the first frequency of the offset signal, forming the average of two sample values at an interval of half the period of the offset signal would have no effect on the measurement value. The time delay of the measurement values caused by forming the average, or the time delay of a measurement signal processed and formed from the sample values, relative to the original measurement signal, can often be tolerated in actual practice.
Sampling the summation signal at the rate of the second frequency, which is a multiple of the first frequency, and creating the measurement values by forming averages also at the rate of the second frequency, improves the time resolution of the measurement signal. This is especially necessary for applications in which the zero passage of the measurement signal is to be evaluated.
In one embodiment, the summation signal that includes the measurement signal and the offset signal is amplified before being sampled. This amplification is often necessary if the amplitude of the first summation signal (i.e., the output signal of the measurement device) is too small for sampling and forming an average. Amplification of the first summation signal results in a second summation signal, which is then sampled, and the average of at least two sample values is then formed.
Problems may arise when using amplifiers that add a second offset to their input signal, in the present case the summation signal includes the measurement signal and the first offset signal.
This can be dealt with by changing the sign of the input signal of the amplifier at periodic intervals, or changing the phase of the input signal by 180 degrees, at the rate of a third frequency. This causes the second offset of the amplifier to be added to the negative input signal during intervals in which the sign of the input signal is changed, and to be added to the positive input signal during intervals in which the sign of the input signal is retained.
The sign change of the input signal at periodic intervals at the rate of the third frequency is reversed by changing the sign of the output signal at periodic intervals at the rate of this same frequency, such that the sign of the output signal is changed in phase with the sign change of the input signal. An output signal is thus obtained that corresponds to the input signal amplified by the amplification factor of the amplifier, and to which the second offset of the amplifier has been superposed as a square-wave signal of the third frequency. The method technique is referred to as chopping the amplifier with the third frequency.
The third frequency of the amplifier offset signal preferably corresponds to the first frequency of the offset signal of the measurement circuit or is an even multiple of this first frequency. This ensures that during the subsequent sampling and formation of an average, the value of the second offset of the amplifier is also eliminated.
If the chopper frequency of the amplifier (the third frequency) is identical to the chopper frequency of the measurement device (the first frequency), the signal present at the output of the amplifier corresponds to the amplified measurement signal, on which a total offset signal of the first frequency is superposed. The amplitude of the total offset signal includes the amplified first offset signal of the measurement circuit and the second offset signal of the amplifier. This total offset signal is eliminated by forming an average following the sampling.
The amplification of the amplifier is preferably regulated according to the amplitude of the input signal, so that the amplitude of the amplifier output signal remains constant, and thus essentially independent of the amplitude of the input signal.
In one embodiment, a system according to the present invention includes a sampling device for sampling an input signal, which depends on the summation signal, at the rate of a second frequency to produce a sequence of sample values. The sequence of sample values are input to a device for forming an average of at least two sample values.
The device for forming an average preferably has a memory device for storing sample values, and an adder for adding at least two memory values (i.e., past values) or for adding a memory value (i.e., a past value) and a sample value present at the input of the device.
The number of memory cells of the memory device preferably corresponds to the number of sample values obtained within half a period of the first frequency of the first offset signal.
In another design of the invention, the memory cells store the sample values of an entire period of the offset signal. To form the average, the values of two memory cells are here always added, these values having been obtained within a half period of the offset signal.
These and other objects, features and advantages of the present invention will become apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.
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PROFOS, P., “Handbuch der industriellen Messtechnik,” Essen: Vulkan Verlag Dr. W. Classen Nachf. GmbH
Micronas GmbH
Samuels , Gauthier & Stevens, LLP
Tran Toan
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