Automatic zeroing system and electronic level adjustment of pressure transducer applied to vital signs monitors

Abstract

The present invention provides automatic zeroing and electronic level adjustment of pressure transducer in relation to patient, applied to vital signs monitors, where the automatic zeroing of circuit of pressure consists of circuit and software able to remove the value of the virtual ground voltage from the pressure calculation, and the electronic level adjustment of transducer in relation to patient consists of compensating, through software, the value in mmHg related to level difference in cmH.sub.2O informed by the user by means of monitor interface.

Claims

1. Automatic zeroing system and electronic level adjustment of pressure transducer applied to vital signs monitors, comprising circuit and software working together to remove the value of virtual ground voltage from a pressure calculation, and electronically adjusting the level of the pressure transducer in relation to a patient, through software for value compensation in mmHg related to level difference in cmH.sub.2O informed by a user by means of monitor interface through the software; wherein said automatic zeroing system uses two differential amplifiers with inputs inverted to each other adding to said inputs the virtual ground; and wherein said automatic zeroing and the electronic adjustment at the level at the pressure transducer with respect to the patient are simultaneously performed according to the equation:
Pressure=(ValueCH1−ValueCH2)×scaleAdjustment+levelAdjustment where: Pressure—final pressure value calculated in mmHg; ValueCH1—number value achieved by an analog-to-digital converter corresponding to an output signal of a first differential amplifier; ValueCH2—number value achieved by the analog-to-digital converter corresponding to an output signal of a second differential amplifier; scaleAdjustment—conversion factor achieved by the analog-to-digital converter in mmHg; levelAdjustment—value in mmHg, corresponding to level difference in cmH.sub.2O between the patient and the pressure transducer, informed by the user through the monitor interface.

2. The system according to claim 1, wherein said first differential amplifier provides voltages higher than the virtual ground voltage for positive pressures and said second amplifier provides voltages higher than the virtual ground voltage for negative pressures, and, in condition of zero pressure, both differential amplifiers provide a voltage equal to the virtual ground voltage.

3. The system according to claim 2, wherein said voltages are applied to inputs of two different channels of the analog-to-digital converter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Automatic zeroing system and electronic level adjustment of pressure transducer in relation to patient, according to the present invention, will be best understood from the attached illustrative figures, which represents in a schematic and non-limitative way of its scope:

(2) FIG. 1—Typical invasive pressure monitoring system of the prior art;

(3) FIG. 2—Scheme of a pressure transducer circuit of “Wheatstone” bridge type of the prior art;

(4) FIG. 3—Block diagram featuring a traditional pressure measurement circuit;

(5) FIG. 4—Block diagram featuring pressure measurement circuit according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(6) FIGS. 1 and 2 regarding the monitor and “wheatstone” bridge type circuit of the prior art were already described above for reference.

(7) Thus, the present invention provides the automatic zeroing and electronic level adjustment of pressure transducer in relation to patient, in order to provide the assemble with no requirement of running the pressure sensor zeroing, either in initiation as well as every 8 hours, and to connect the pressure transducer to the monitor and initiating the pressure monitoring is the only requirement.

(8) In block diagram of FIG. 3, it is shown a traditional pressure measurement circuit, in which an floating reference is part of calculation of pressure value; i.e., assuming the use of analogic to digital converter of 10 bits, this will indicate values between 0 and 1023 in its output, representing voltages between 0 and Vref, and assuming Vref is equal to +Vcc, which in turn is equal to +5 V, having the virtual ground at 2.5 V, and, for such voltage value, the analogic to digital converter will indicate the value 512 in its output. When the zeroing is executed, the microprocessor is informed that the value 512 is corresponding to zero pressure, and, therefore, values above 512 are considered positive, and values below 512 are considered negative; i.e., the voltage at the output of differential amplifier is equal to:
+Out=(((+Sig)−(−Sig))×gain)+Vcc/2

(9) where: +Out—output voltage of differential amplifier; +Sig—positive output voltage of pressure sensor; −Sig—negative output voltage of pressure sensor; Gain—voltage gain of differential amplifier; +Vcc/2—half of power supply voltage of pressure sensor and of amplifier.

(10) If value of +Vcc/2 fluctuate, the output voltage will also fluctuate, even if the pressure is the same, and, therefore, the zeroing every 8 hours is required, and constitutes the trouble of pressure monitors of the prior art.

(11) In automatic zeroing provided by the present invention, the solution for avoiding the reference fluctuating trouble (+Vcc/2) consists of removing the reference from the pressure calculation, what should be done with circuit of FIG. 4, in conjunction with specific software, using two differential amplifiers, with inverted inputs to each other to amplify the signal, else adding the virtual ground (+Vcc/2), and where the first differential amplifier provides voltage higher than +Vcc/2 for positive pressures, while the second differential amplifier provides voltage higher than +Vcc/2 for negative pressures, and, at condition of zero pressure, both differential amplifiers provide voltage equal to +Vcc/2, having the following equations for output voltages from two amplifiers:
Out1=((+Sig)−(−Sig))×gain+Vcc/2
Out2=((−Sig)−(+Sig))×gain+Vcc/2

(12) where: Out1—output voltage of differential amplifier 1; Out2—output voltage of differential amplifier 2; +Sig—positive output voltage of pressure sensor; −Sig—negative output voltage of pressure sensor; Gain—voltage gain of differential amplifier; +Vcc/2—half of power voltage of pressure sensor and of amplifier.

(13) Applying these voltages to analogic to digital converter inputs, the following values are achieved:
ValueCH1=value(((+Sig)−(−Sig))×gain)+value(Vcc/2)
ValueCH2=value(((−Sig)−(+Sig))×gain)+value(Vcc/2)

(14) where: ValueCH1—number value achieved by analogic to digital converter corresponding to output signal of differential amplifier 1(Out1); ValueCH2—number value achieved by analogic to digital converter corresponding to output signal of differential amplifier 2(Out2); value(((+Sig)−(−Sig))×gain)—number value achieved by analogic to digital converter corresponding to output voltage of differential amplifier 1 without virtual ground; value(((−Sig)−(+Sig))×gain)—number value achieved by analogic to digital converter corresponding to output voltage of amplifier 2 without virtual ground. value (Vcc/2)—number value achieved by analogic to digital converter corresponding to virtual ground, in which the optimal conditions would be 512.

(15) As can be seen from the block diagram of FIG. 4 and from the above equations, the value related to voltage of virtual ground is present in values of ValueCH1 and ValueCH2, therefore, when it is subtracted one from another, the value of Vcc/2 is eliminated, not impacting in final pressure value, as can be noted from the following equation:
Pressure=(ValueCH1−ValueCH2)×Scaleadjustment

(16) where: Pressure—final pressure value calculated in mmHg; ValueCH1—number value achieved by analogic to digital converter corresponding to output signal of differential amplifier 1(Out1); ValueCH2—number value achieved by analogic to digital converter; corresponding to output signal of differential amplifier 2(Out2); Scaleadjustment—conversion factor of value achieved by analogic to digital converter for value in mmHg.

(17) Therefore, using software suitable for calculating the equations above, a circuit able to measure pressures both positive and negative can be achieved, without requiring to run the zeroing, which results in great advantages, such as, e.g., the prompt start of monitoring, just by connecting the pressure transducer to monitor for initiating it, dispensing the requirement of waiting the “warm-up time”, for measurements effect, disregarding the voltage fluctuations of virtual ground; the no requirement for opening the system to expose the transducer to atmospheric pressure which eliminates the contamination risk and damage to electrical connections due to serum leakage during the zeroing; circuit immune from noises due to the fact that both amplification as well as pressure calculation are performed in a differential way.

(18) The electronic level adjustment of transducer in relation to patient may be performed by deploying the following equation in software:
Pressure=(ValueCH1−ValueCH2)×Scaleadjustment+Leveladjustment

(19) where: Pressure—final pressure value calculated in mmHg; ValueCH1—number value achieved by analogic to digital converter corresponding to output signal of differential amplifier 1 (Out1); ValueCH2—number value achieved by analogic to digital converter corresponding to output signal of differential amplifier 2(Out2); Scaleadjustment—conversion factor of value achieved by analogic to digital converter for the value in mmHg; Leveladjustment—value in mmHg, corresponding to level difference in cmH.sub.2O between patient and pressure transducer, informed by the user through a monitor interface.

(20) This value is positive when the transducer is higher than patient, and negative when the transducer is lower than patient, and the level difference value should be measured by the user and informed to the monitor through its interface, and, therefore, the transducer may be positioned anywhere, with no requirement that it stays in support close to patient; further, if patient change from position, or in case of transportation, it is enough to inform the height difference between transducer and patient at the monitor, and the level difference will be automatically corrected. Other observed advantage is the possibility for measuring the patient pressure against other points of body, besides the heart.

(21) The skilled in the art will understand that the features shown herein are not limited to monitoring of parameters disclosed herein as example, but also to other ones no mentioned herein.