Systems and methods for calibrating a distorted signal with another signal of known calibration

• US 6,662,130 B1
• Filed: 06/13/2002
• Issued: 12/09/2003
• Est. Priority Date: 06/13/2002
• Status: Expired due to Fees

• Current Assignee: Omron Healthcare Company Limited (Omron Corporation)
• Original Assignee: Southwest Research Institute
• Inventors: Peel, Harry Herbert III, Bartels, Keith Alan
• Primary Examiner(s): SHAH, KAMINI S
• Application Number:
  US10/167,448
• Publication Number:
  US 20030233204A1
• Time in Patent Office: 544 Days
• Field of Search: 702/104, 702/66, 600/485, 600/494, 600/500, 735/79, 736/02, 736/22
• US Class Current: 702/104
• CPC Class Codes: A61B 5/021 Measuring pressure in heart or blood vessels A61B5/0205 takes precedence
• CPC Class Codes: A61B 5/7257 using Fourier transforms

Abstract

An uncalibrated sensor located at a first location relative to a physical phenomenon is calibrated using a calibrated sensor spaced away from the uncalibrated sensor at a second location relative to the physical phenomenon and a frequency-domain transfer function that relates the physical phenomenon at the second location to the output of the uncalibrated sensor.

Claims

1. A method for calibrating an uncalibrated sensor, comprising:

sensing at a first location a time-varying physical phenomenon using a calibrated sensor to determine an input waveform of the time-varying physical phenomenon at the first location;

sensing at a second location the time-varying physical phenomenon using the uncalibrated sensor, the uncalibrated sensor outputting an output waveform corresponding to the time-varying physical phenomenon at the second location;

determining a frequency-domain transfer function that relates the output waveform to the input waveform based on the input and output waveforms;

determining a calibration coefficient for the uncalibrated sensor based on the determined frequency-domain transfer function; and

determining a calibration constant for the uncalibrated sensor based on the determined calibration coefficient, the input waveform and the output waveform, wherein the determined calibration coefficient and the determined calibration constant calibrate the uncalibrated sensor to the time-varying physical phenomenon at the second location.

2. The method of claim 1, wherein the time-varying physical phenomenon is blood pressure.

3. The method of claim 2, wherein the calibrated sensor is a calibrated oscillometric blood pressure monitor.

4. The method of claim 2, wherein the uncalibrated sensor is a tonometer.

5. The method of claim 4, wherein the tonometer is an electronic applanation tonometer.

6. The method of claim 1, wherein the calibrated sensor has a known frequency and amplitude response to the time-varying physical phenomenon at the first location.

7. The method of claim 1, wherein determining the frequency-domain transfer function based on the input and output waveforms comprises determining an estimated frequency-domain transfer function based on the input and output waveforms.

8. The method of claim 7, wherein determining the calibration coefficient for the uncalibrated sensor based on the determined frequency-domain transfer function comprises:

determining a value of the estimated frequency-domain transfer function at a frequency where a second frequency-domain transfer function that relates the input waveform to a second input waveform of the time-varying physical phenomenon at the second location has a determinable value;

determining the calibration coefficient based on the value of the estimated frequency-domain transfer function at the frequency and the determinable value of the second frequency-domain transfer function.

9. The method of claim 7, wherein the frequency is zero.

10. The method of claim 9, wherein the second frequency-domain transfer function has a value of 1 for the frequency of zero.

11. The method of claim 1, further comprising:

determining a mean value of the input waveform; and

determining a mean value of the output waveform;

wherein determining the calibration constant for the uncalibrated sensor comprises determining the calibration constant based on the calibration coefficient, the mean value of the input waveform and the mean value of the output waveform.

12. The method of claim 1, wherein determining the frequency-domain transfer function that relates the output waveform to the input waveform based on the input and output waveforms comprises:

obtaining at least one full waveform for each of the input and output waveforms;

defining an estimated transfer function in the complex z-domain that corresponds to the frequency-domain transfer function, the estimated z-domain transfer function having a plurality of parameters; and

determining values for the parameters such that the estimated z-domain transfer function is fitted to the obtained at least one full waveforms for the input and output waveforms.

converting the fitted z-domain transfer function to the frequency-domain transfer function.

13. The method of claim 12, further comprising:

determining a frequency where the estimated frequency-domain transfer function has a determinable value; and

converting the determined frequency to an equivalent z-domain value.

14. The method of claim 13, further comprising:

determining the calibration coefficient based equivalent z-domain value and the determined values for the parameters of the estimated z-domain transfer function.

15. The method of claim 12, wherein defining the estimated z-domain transfer function comprises defining a polynomial numerator and a polynomial denominator for the estimated z-domain transfer function having the form:

$H_{\mathrm{IO}}<br><br>\text{\hspace{1em}<br><br>}<br><br>\left(z\right)=\frac{\underset{n=1}{\overset{N_b}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>b_n<br><br>\text{\hspace{1em}<br><br>}<br><br>z^{-\left(n-1\right)}}{1+\underset{n=1}{\overset{N_a}{\sum <br><br>}}<br><br>\text{\hspace{1em}<br><br>}<br><br>a_n<br><br>\text{\hspace{1em}<br><br>}<br><br>z^{-n}}$

where the polynomial numerator has order N_b−

1, the denominator has order N_a, and the plurality of parameters are the coefficients a_n and b_n.

16. The method of claim 1, further comprising calibrating a second uncalibrated sensor to the time-varying physical phenomenon at the first location to obtain the calibrated sensor, such that the second uncalibrated sensor, once calibrated, can be used as the calibrated sensor.

17. The method of claim 16, wherein calibrating the second uncalibrated sensor to the time-varying physical phenomenon at the first location comprises:

obtaining values of the input waveform of the time-varying physical phenomenon at the first location at at least a first time and a second time;

obtaining values of a second output waveform output by the second uncalibrated sensor corresponding to the time-varying physical phenomenon at the first location at at least the first time and the second time;

determining a second calibration coefficient for the second uncalibrated sensor for the time-varying physical phenomenon at the first location based on the obtained values for at least the first time and the second time of the input and second output waveforms; and

determining a second calibration constant for the second uncalibrated sensor based on the determined second calibration constant, the input waveform and the second output waveform, wherein the determined second calibration coefficient and the determined second calibration constant calibrate the second uncalibrated sensor to the time-varying physical phenomenon at the first location.

18. A method for calibrating an electronic applanation tonometer, comprising:

measuring a brachial artery blood pressure with a calibrated oscillometric blood pressure monitor to generate a calibrated oscillometric blood pressure signal;

measuring a radial artery blood pressure with an uncalibrated electronic applanation tonometer to generate a voltage signal;

determining a frequency-domain pressure-to-voltage transfer function based on the calibrated oscillometric blood pressure signal and the voltage signal of the uncalibrated electronic applanation tonometer;

determining a calibration coefficient for the uncalibrated electronic applanation tonometer based on frequency-domain pressure-to-voltage transfer function;

determining a calibration constant for the uncalibrated electronic applanation tonometer based on the determined calibration constant, the calibrated oscillometric blood pressure signal and the voltage signal, wherein the determined calibration coefficient and the determined calibration constant calibrate the uncalibrated electronic applanation tonometer to the radial artery blood pressure.