Device and method for measuring of the complex transfer function of an object

Abstract

The invention is related to the field of measurement electronics and related to the measurement of complex transfer functions (e.g. of impedance), especially in the presence of large disturbances and for measuring of small changes of the characteristics of the object under test. The goals of the proposed invention are achieved by detuning of the excitation and reference signals, e.g. by adding incremental phase-shift to the excitation signal to generate the reference signal.

Claims

1. A device for measuring the complex transfer function of an object 1, comprising a generator 2 for generating an AC excitation signal 21 and an AC reference signal 22, a multiplier 3 for multiplying a response (from the object) signal 11 and a reference signal 22, a low pass filter 4 connected to the output of the multiplier and a data processing unit 5 connected to its output, wherein the generator 2 is configured to generate the excitation signal 21 and the reference signal 22 in such way, that the reference signal 22 is tuned away (detuned) from the excitation signal 21 so that the delay of the reference signal 22 relatively to the excitation signal 21 increases (in discrete steps or continuously) so that at a beginning and at an end of a measurement period the excitation signal 21 and the reference signal 22 coincide.

2. The device according to claim 1, wherein in order to delay the reference signal 22 with respect to the excitation signal 21, the device comprises means for adding a phase shift to the phase of the reference signal 22 at least once at each predetermined period nT of the integer excitation signal 21 (n=1, 2, 3, 4, . . . ), where Δϕ=i*360/p, where p is the repeat-rate, i.e. the number of steps during which Δϕ becomes equal to 2π and i=0, 1, 2, . . . p−1.

3. The device according to claim 1, wherein frequency of the reference signal 22 is selected to be different from frequency of the excitation signal 21 in order to delay the reference signal 22 with respect to the excitation signal 21.

4. The device according to claim 3, wherein the frequency of the excitation signal 21 is selected to be in the range of 10 kHz to 10 MHz, and the frequency of the reference signal 22 differs from it by 300 Hz to 3 kHz.

5. A method for measuring the complex transfer function of an object 1, comprising the steps of: generating an alternating voltage excitation signal 21 which is fed to the object 1 to be measured; generating an AC reference signal 22; a response signal 11 generated by the excitation signal 21 is received from the object to be measured; the response signal 11 is multiplied by the reference signal 22, wherein the reference signal 22 is tuned with respect to the excitation signal 21 so that during the measurement cycle the reference signal 22 is increasingly delayed (in discrete steps or continuously) relatively to the excitation signal 21 so, that at a beginning and at an end of a measurement cycle the excitation signal 21 and the reference signal 22 coincide.

6. The method according to claim 5, wherein to delay of the reference signal 22 relatively to the excitation signal 21, a phase shift is added to the phase of the reference signal 22 at least once for each predetermined period nT of the integer excitation signal 21 (n=1, 2, 3, 4, . . . ), where Δϕ=i*360/p and p is the repeat rate, i.e. the number of steps during which Δϕ becomes equal to 2π, while i=0 . . . p−1.

7. The method according to claim 5, wherein the frequency of the reference signal 22 is selected to be different from the frequency of the excitation signal 21 in order to delay the reference signal 22 with respect to the excitation signal 21.

8. The method of claim 7, wherein the frequency of the excitation signal 21 is selected to be in the range of 10 kHz to 10 MHz, and the frequency of the reference signal 22 is selected to be 300 Hz to 3 kHz different from this.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Implementation examples of the invention are described below with references to the accompanying drawings, in which:

[0016] FIG. 1 is a block diagram of a device according to an embodiment of the invention.

[0017] FIG. 2 is a block diagram of a generator used in the invention.

[0018] FIGS. 3A, 3B and 3C show the waveforms (and their arrangement in relation to each other) of the excitation, reference and response signals and the same for the multiplication product of response and reference signals used in the device and method according to the invention.

[0019] FIGS. 4A, 4B and 4C show the waveforms (and their arrangement in relation to each other) of the excitation, reference and response signals and of the multiplication product of response and reference signals used in the device and method according to the second embodiment of the invention.

DETAILED DESCRIPTION THE INVENTION

[0020] One embodiment of the invention is shown in the FIG. 1. A device for measuring the complex transfer function of an object 1, e.g. a bio-object, e.g. electrical impedance or conductivity, comprises a generator 2 configured to generate an AC excitation signal 21 and a reference signal 22, a multiplier 3, a low pass filter 4 and a data processing unit 5.

[0021] The device works as follows. The alternating voltage excitation signal 21 generated by the generator 21 is applied to the object to be measured 1 and the object response signal 11 is applied to the input 31 of the multiplier 3. The reference signal 22 generated by the generator is applied to the second input 32 of the multiplier. The output signal 33 of the multiplier 3 is fed to a low pass filter 4, which suppresses the high frequency and noise components of the output signal 33 and separates the low frequency components, characterizing the properties of object 1.

[0022] On the basis of the excitation signal 21 in the generator 2 a reference signal 22 is formed, which is tuned away (“detuned”) from the excitation signal 21 so, that during the measurement cycle the reference signal delay increases (relatively to the excitation signal 21) in discrete steps or continuously so, that the excitation signal 21 and the reference signal 22 coincide at the beginning and the end of the measurement cycle. According to one embodiment, the sinusoidal reference signal is generated as follows: after each predetermined integer excitation signal sampling period nT (n=1, 2, 3, 4, . . . ), a certain phase shift Δϕ is added to the phase of the reference signal. The value of the phase shift Δϕ is determined by the relationship (1):

Δϕ=360/p*i,  (1)

[0023] where

[0024] p is the repeat-rate, (i.e. the number of steps during which Δϕ becomes equal to 2π) and

[0025] i=0 . . . p−1.

[0026] Such periodically varying phase offset of the excitation signal and the reference signal provides a variable frequency (f=1/Tp) signal at the output 42 of the lowpass filter 4, the amplitude of which depends on the excitation signal frequency and phase shift Δϕ values, as well as on the complex transfer function and static and dynamic values of it (reflecting impedance, conductivity).

[0027] After multiplying and filtering the reference signal and the response signal, a periodic signal (preferably 300 Hz to 3 kHz) is formed at the output 42 at a repeat-rate defined by the excitation rate of the excitation signal 21 and the reference signal 22, from which static and dynamic parameters 52 are determined in the data processing unit 5 from the complex transfer function.

[0028] In a preferred embodiment of the data processing unit, the input signal 43 is sampled at such phase shift values Δϕ, at which the baseline measurement value of the alternating component at f=1/Tp is minimal, so allowing additional gain to be applied in the proposed solution without suppression or compensation of the baseline value. So improved determination of the impedance (conductivity) static and dynamic parameters, even if they are significantly (hundreds, thousands or more times) lower than the baseline value of the signal, is achieved.

[0029] The advantage of the invention is that the measurement of a complex transmission, i.e. its real and imaginary part (e.g. active and reactive resistance) takes place in a single measurement channel due to the continuous phase separation of the excitation signal and the reference signal.

[0030] In a preferred embodiment, a reference signal 22 is generated, differing from the excitation signal 21 (typically 10 kHz to 10 MHz for a bio-object; it is clear to one skilled in the art that the reference signal frequency is selected according to the object and may be much higher or lower). For example, in accordance with established practice, it has been considered technically reasonable to select a phase separation frequency range of 300 Hz to 3 kHz for impedance measurements affected by cardiac activity, to monitor higher harmonics up to about 50 harmonics. It will be clear to a person skilled in the art that, depending on the nature of the object under study and the phenomenon under study, a much lower or higher repeat-rate may be chosen.

[0031] In another preferred embodiment, a reference signal 22 is generated with a different than the excitation signal frequency 21. In the case of a bio-object, f=ω/2π is usually selected from 10 kHz to 10 MHz. It is clear to the person skilled in the art that the reference signal frequency can be significantly higher or lower than in the case of bio-objects. A certain frequency shift (usually Δf=Δω/2π=300 Hz to 3 kHz for cardiac monitoring) is selected, to give required repeat-rate. The corresponding relation is described by equation 2.

r(t)=sin((ω+Δω)*t)  (2)

[0032] where r (t) is the excitation signal,

[0033] ω is the frequency of the excitation signal,

[0034] Δω is the frequency offset (detuning value) and

[0035] t is the current time

[0036] One embodiment of the generator 2 is shown in the FIG. 2. The generator 2 comprises two DDSs (direct digital synthesizers) 23 and 24 controlled by pulses of a reference clock 25. One DDS is configured to generate an excitation signal 21 and the other DDS being configured to generate a reference signal 22, according to the above embodiment examples.

[0037] According to another embodiment, the generator 2 may be implemented as a tabulated function of a pre-recorded periodic waveform (e.g.: sine signal, binary signal), from which the instantaneous values of the signal are read out at variable delay rates.

[0038] FIGS. 3 and 4 illustrate the waveforms and arrangement (in relation to each other) of the excitation, reference and response signals and the response and reference signals used in the device and method, according to the two embodiments described above. In the example of FIG. 3, the excitation signal frequency is 100 kHz and the repeat-rate is 6 (preferably p=300 to 3000 in real bioimpedance measurement applications). In the FIG. 3A the waveforms and arrangement (relatively to each other) of the reference signal and the excitation signal are shown. In the FIG. 3B the waveforms and arrangement of the reference signal and the response signal, their waveforms and arrangement relatively to each other are shown. In the FIG. 3C the waveforms and arrangements of the response signal, the reference signal and the product of the response and reference signal relatively to each other are given. As shown in this Figure, the reference signal and the excitation signal are triggered simultaneously, but the reference signal is applied to the excitation signal with increasing phase shift with respect to the excitation signal with each subsequent time interval. It can also be seen that the response signal is delayed with respect to the excitation signal due to the complex transmission (impedance or conductivity) contained in the reactive component of the object. The product of the response and the reference signal oscillates at twice of the frequency with respect to the excitation and reference signals, the amplitude of which in turn oscillates with a five-fold period with respect to the excitation signal. The product of the reference signal and the response signal is fed to a low-pass filter, which suppresses the high-frequency components of the signal. The maxima of the low-frequency signal here indicate the moment when the response signal and the reference signal are in the same phase, i.e. their product is proportional to the real part of the complex transmission of the object (for example, the active resistance). At the zero point of the low frequency signal, the response signal and the reference signal have a phase shift of 90 degrees, i.e. at this point the response signal is proportional to the imaginary part of the complex transmission (e.g. reactive resistance).

[0039] In the example of FIG. 4, the excitation signal frequency is 100 kHz (in real bioimpedance measurement applications between 10 kHz and 10 MHz) and the reference signal frequency is 16.7 kHz (thus 16.7 kHz resolution; in real bioimpedance measurement applications 300 Hz to 3 kHz). The waveforms and their arrangement (and their arrangement in relation to each other) of the excitation, reference and response signals and the response and reference signal product used in the devices and method according to the second embodiment are given there.

[0040] In the FIG. 4A the waveforms and arrangement (relatively to each other) of the reference signal and the excitation signal are shown. In the FIG. 4B the waveforms and arrangement of the reference signal and the response signal, their waveforms and arrangement relatively to each other are shown. In the FIG. 4C the waveforms and arrangements of the response signal, the reference signal and the product of the response and reference signal relatively to each other are given. As shown in the figure, the reference signal and the excitation signal are triggered simultaneously, but due to the difference in their frequencies, an incremental phase shift occurs between them, which increases to 2π over six periods (i.e., the phase shift returns to zero). The product of the response and the reference signal oscillates at twice the frequency with respect to the excitation and reference signals, the amplitude of which in turn oscillates with a five-fold periods with respect to the excitation signal. The product of the reference signal and the response signal is fed to a low-pass filter, which suppresses the high-frequency components of the signal. From the maxima and zeros of the low frequency signal, the complex transfer values are determined as described above.

[0041] In order to determine the complex transmission of the object, e.g. impedance or conductivity, both the response signal and the product of the response signal and the filtered reference signal are preferably stored in digital form. This enables to determine the time instances suitable for sampling from the product of the filtered reference signal and to calculate the real and imaginary parts of the complex transfer function from the appropriately acquired response and excitation signals, as well as to compensate the baseline value of the complex transfer function to determine the small changes in the complex transfer function.

[0042] In the embodiments described above, sinusoidal signals are used. Instead of sine signals, multisine signals can be used, but also square-wave signals, including so-called shortened square-wave signals, the generation of which is simpler and less energy consuming than the generation of sine signals. The solutions described above are suitable for use in portable devices, such as sports or health watches, in which heart rate and respiration parameters are detected by measuring bioimpedance, for example to monitor the shape of a heart wave and determine its parameters.

[0043] It is to be understood that the above description and appended drawings shall be interpreted as illustrative and not in a limiting sense. Those skilled in the art will appreciate that variations and modifications may be made to the illustrative examples without departing from the scope of the invention.