JPH0143049Y2 - - Google Patents

Description

[Detailed description of the invention] This invention uses an excitation coil to excite the blood flow with an alternating magnetic field of a square wave, and demodulates the induced voltage detected by the detection electrode in accordance with the velocity of the blood flow to generate a blood flow signal. The present invention relates to an electromagnetic blood flow meter.

This type of blood flow meter uses Faraday's law to detect the induced voltage at the detection electrode by blood flowing through a magnetic field, but normally the detection signal is generated by an excitation coil connected to the lead wire of the detection electrode. In addition to the superposition of transformer components due to cross magnetic fields,
As shown in Figure 1, an electrocardiogram signal (hereinafter referred to as an ECG signal) induced directly in the detection electrode regardless of excitation.
Also mixed in. Since this ECG signal has a lower frequency component than the carrier of the blood flow signal, it can be removed by first passing it through a high-pass filter, but if an attempt is made to significantly reduce it, it will inevitably affect the blood flow signal. Therefore, from the perspective of removing ECG signals, it is better to make the excitation frequency as high as possible;
Conversely, the transformer component increases correspondingly, resulting in a loss of zero stability.

Therefore, this transformer component is, for example,
39750 may be used to remove it.
That is, as shown in Figure 2, if a non-excitation period is inserted into each excitation period, and the transformer component of the detected induced voltage waveform during the excitation period is V _TM and the transformer component during the non-excitation period is V _T , then V There is a relationship: _T = KV _TM + Vo (K: constant of proportionality, Vo: offset voltage), and therefore V _TM = V _T - Vo/K, so it is possible to detect V _T and find V _TM . By sampling the terminal part of each induced voltage,
Subtracting V _TM allows a nearly pure blood flow signal to be generated. However, even if this method is used, if the carrier frequency is increased to about 500Hz,
A phenomenon occurs in which the proportional relationship that existed between the transformer component V _TM during the energized period and the transformer component V _T during the non-energized period is lost, making it difficult to remove the transformer component.

Therefore, an object of the present invention is to provide a rectangular wave alternating magnetic field type electromagnetic blood flow meter that can avoid a decrease in zero stability even when the excitation frequency is increased.

Next, an embodiment of the present invention will be explained based on FIGS. 3 and 4.

In FIG. 3, reference numeral 1 denotes an excitation circuit that excites the probe 2 with a rectangular wave, and its excitation frequency is temporarily switched from a normal high frequency, such as 500 Hz, to a low frequency, such as 50 Hz, during input of the gate pulse a.
3 is a feedback amplifier which inputs the induced signal detected by the probe 2 and has at least a high-pass filter or high-pass characteristic for removing the ECG signal;
This is a normal blood flow signal detection circuit including a demodulation circuit. 4 is an averaging circuit that averages the blood flow signal b which is the output of the blood flow signal detection circuit 3, and 5 is a gate pulse a.
6 is a holding circuit that holds the output of the average circuit 4 immediately after the disappearance of gate pulse a; 7 is a holding circuit that holds the output of the average circuit 4 immediately after the gate pulse a disappears; 7 is the output of the holding circuit 5 from the output of the holding circuit 6; 8 is a subtraction circuit that subtracts the output d−c of the subtraction circuit 7 from the blood flow signal b. When the gate pulse a is generated periodically, the circuits 1, 4 to 8 described above function as an automatic zero balance circuit, and when the gate pulse a can be generated at any time by manual switch operation, it becomes a manual zero balance circuit. Furthermore, the generation time width of the gate pulse a, that is, the 50 Hz carrier, is set to be at least longer than the time constant of the averaging circuit 4 in order to allow the averaging circuit 4 to perform complete averaging.

The operation is as follows.

During normal measurement, a blood flow signal with an excitation frequency of 500 Hz induced by the probe 2 is supplied to the blood flow signal detection circuit 3 and passed through its high-pass filter.
The components of the ECG signal are significantly reduced without causing distortion, further amplified and demodulated, and output as a blood flow signal b. That is, although the ECG signal has been removed, the transformer component, which has become a DC component due to demodulation, is superimposed.

When gate pulse a is generated, the excitation frequency is 50
Hz, the influence of the transformer component on the blood flow signal b becomes almost zero, and is averaged by the averaging circuit 4, and only the pure blood flow signal is held in the holding circuit 5 as the averaged holding signal c. . On the other hand, the blood flow signal b with an excitation frequency of 500 Hz immediately after the disappearance of the gate pulse a is averaged including the superimposed transformer component in the averaging circuit 4, and is held as the holding signal d in the holding circuit 6. The subtraction circuit 7 continuously outputs a subtraction signal of the holding signal c from the holding signal d, that is, a transformer component signal. This results in
The subtraction circuit 8 removes the influence of the transformer component from the blood flow signal b, and finally the ECG signal is greatly reduced corresponding to the normal excitation frequency of 500Hz, and the excitation frequency is 50Hz.
A substantially pure blood flow signal e from which the transformer component is largely removed is output in accordance with the frequency of Hz.

Note that if the excitation frequency for generating the holding signal c is selected to be 50Hz or lower, the influence of ECG signals and other noises will likely appear on the output signal of the averaging circuit 4.
If the blood flow signal from which the transformer component is more completely removed is detected in the blood flow signal detection circuit 3 based on the method according to the above-mentioned Japanese Patent Publication No. 54-39750,
A purer blood flow signal e that is not affected by ECG signals or other noises can be obtained. In addition, the holding circuits 5 and 6 are equipped with an A/D converter, a latch circuit, and a D/D converter in order to reduce fluctuations in the holding circuit c over time.
It can be constructed from an A converter and held as a digital value. The timing of sampling and holding to the holding circuit 6 can also be set immediately before the gate pulse a by supplying another sampling pulse before the gate pulse a is generated.

Holding of the transformer component signal is performed by a holding circuit 11 following the subtraction circuit 7 as shown in FIG.
Either one of the holding circuits 5 and 6 may be abolished. That is, the sampling pulse f causes the holding circuit 10 to hold one of the average signals at the time of excitation frequency switching, and causes the holding circuit 11 to hold the output of the subtraction circuit 7 at that time. Figure 6 shows A/
A microcomputer 14 equipped with the subtraction circuit and holding function of the circuits shown in Figs. 3 and 5 is inserted between the D converter 12 and the D/A converter 13. When gate pulse a is generated, the average Value data is taken in, the subtracted data is held in memory, and is continuously supplied to the A/D converter 13 as output data.

As described above, according to the present invention, by increasing the excitation frequency in a rectangular wave excitation electromagnetic blood flow meter,
ECG signal contamination can be greatly reduced by using a high-pass filter, and the correspondingly increased transformer component can be removed by calculating the level of the blood flow signal with a temporarily lowered excitation frequency. ,
It has excellent zero stability and can obtain blood flow signals with less ECG signal contamination.

[Brief explanation of the drawing]

Figure 1 is a normal electrocardiogram signal waveform, Figure 2 is an explanatory diagram of the transformer component removal method according to Japanese Patent Publication No. 54-39750,
Figure 3 shows an example of the circuit configuration of the electromagnetic blood flow meter according to the present invention.
FIG. 4 shows waveforms of various parts thereof, and FIGS. 5 and 6 show modified examples of the circuit shown in FIG. 3.

Claims (1)

[Scope of utility model registration request] An excitation circuit that excites a probe with a rectangular wave and whose excitation frequency temporarily switches from a normal frequency to a lower frequency in response to a switching command signal, which receives the detection signal of the probe as input, and removes the ECG signal. a blood flow signal detection circuit including a high-pass filter and a demodulator; and an averaging circuit for averaging the blood flow signal output from this circuit; a subtraction and holding circuit that subtracts and holds the output of the averaging circuit for the lower frequency from the output of the averaging circuit for the frequency of An electromagnetic blood flow meter characterized by having a subtraction circuit that outputs a flow signal.