FI109942B - Method for testing a tetrapolar impedance measurement system - Google Patents

Description

1 109942

The present invention relates to a method for testing the operation of a tetrapolar impedance measurement system according to the preamble of claim 1.

The problem with impedance measurement methods is the connection between a measuring device having at least 5 power supply and voltage measuring circuits and the object to be measured. Interface refers to the connection from the measuring device to the object being measured, for example, an interface cable from the power supply circuit terminal to the electrode and from the electrode to human skin. In particular, there may be problems at the interface between the electrode and the object being measured, i.e. the electrical resistance of the interface may be too high. In the known methods of electrode-to-object 10 electrical resistance testing, one pair of conductors and electrodes is measured separately. In a multi-channel device, it is difficult to implement interface testing automatically. Even if each pair of interfaces is measured individually, this information does not yet indicate whether the total electrical resistance of the interfaces will affect the reliability of the measurement. The effect of the resistors on the interfaces depends on the ability of the 15 devices to produce a constant current regardless of the electrical resistance and the input current of the voltage measuring circuit.

It is an object of the invention to eliminate this drawback and to provide. . an impedance measuring arrangement check method for checking the performance of the entire measuring system so that the electrical resistance between the power supply and voltage measuring terminals of the measuring device and the object to be measured does not affect the measurement result.

This objective can be achieved by applying the principle of reciprocity, ie: * reciprocity in impedance measurement. In accordance with the principle of reciprocity '·; · 'The power supply and the voltage measurement connections can be interchanged without any measurement result.:: 25 changes. According to the invention, if any connection is disturbingly large,:. ·: Reciprocal measurement does not give the same result. In this case, the instrumentation-object interface problem affecting the operation of the measuring system can be detected.

More specifically, the method of the invention is characterized in what is stated in the characterizing part of claim 1.

The invention will now be described in more detail with reference to the accompanying diagram which illustrates a tetrapolar impedance measurement system and the use of the method according to the invention.

5 Tetrapolar impedance measurement uses at least four switching points, C1, C2, C3, C4. Each coupling point may consist of one or more contacts (Cia, Cib, ...; C2a, C21 ,, .. ·; C3a, C31 ,, ...; C4a, C4b, ...), for example an electrode, into the object to be measured. For example, switching layout 1: The constant current source terminals Bi and B2 are connected to two supply points, Ci and C4. The voltage caused by the current is measured at the two remaining switching points 10, in this case A1 is connected to switching point C2 and A2 is connected to switching point C3. The voltage-to-current ratio gives an impedance proportional to the conductivity distribution of the body. When the measuring apparatus (1) is ideal, the interfaces (Z1, Z2, Z3, Z4) between the current supply and voltage measuring circuits and the object to be measured, consisting of each contact (Cia, Cib, ...; C2a, C2b, ...; C3a, C3b). , ...; C4a, 15 C4b, ...) and the total impedance (Zi -Z4) of the connecting conductors, do not affect the measurement, only the conductivity distribution of the measured part / a- · In practice, one or; "* 'As the number of interface impedances (Zi - Z4) increases, the measurement no longer describes the conductivity of the object being measured. This magnitude of impedance or impedance •' 'depends on the implementation and characteristics of the instrumentation used.

*. ·. * 20 In accordance with the idea of the invention, it is determined whether one or more of the interfaces distort the measurement result by performing a reciprocal or reciprocal impedance measurement.

The reciprocal measurement of the measuring set-up described above is carried out as follows: '·': the power supply terminals B1 and B2 are connected to points C2 and C3. Voltage measurement terminals Ai •; · * And A2 are connected to points C1 and C4. In accordance with the principle of reciprocity: · · · • '.: With 25 instrumentation working perfectly, the reciprocal measurement gives exactly the same' ... 'result. If one or more interfaces (Z1-Z4) are distorting the measurement, the reciprocal:: ': measurement will not give the same result.

Knowing the behavior of instrumentation current and voltage circuits as access impedances increase, interfering access impedances (Zi-Z4) can also be located by making additional measurements. For example, with the arrangement shown in Figure (1), in addition to the above-mentioned 3 109942 reciprocal measurements, two other switching arrangements and their reciprocal measurements can be measured. These other switching configurations are: switching setup 2: Ai switching Cj, A2 switching C3, B1 switching C2 and B2 switching C4 and switching setup 3: Ai switching C1, A2 switching C2, B1 switching C3 and B2 switching C4.

The following is an example of how the invention is used to detect and locate a measurement distorting impedance impedance (Z 1 - Z 4). Assume that the impedance impedance Z2 is so large that it interferes with the operation of the power supply circuit by reducing the current supplied by the circuit to the object to be measured. In practice, the power supply circuit of the measuring apparatus is generally more sensitive to the increase in the impedance impedance because the voltage measuring circuit is 10 more technically easier to implement than the power supply circuit.

In the exemplary case, none of the switching arrangements shown here gives the same measurement result as the reciprocal measurement of each switching configuration, and thus the presence of an impedance impedance (Z1 to Z4, in Example Z2) is known. The high coupling impedance (Zi-Z4) due to an incorrect measurement result can be localized 15 by using the three switching arrangements shown separately and their mutual measurement. When the constant current source terminal (B | or B2) is connected to the measurement impedance interface impedance, the impedance obtained from the measurement is proportional to the conductivity / cy of the measured object when: '· · the constant current source terminals (Bi and B2) are not connected liityntäimpedanssiin. Using a switching arrangement of 1, the reciprocal measurement gives a »» · · · · · · · · in impedance value in the example case. Thus, it is known that one or both of the interfaces (C2 and C3) connected to the power supply circuit (Bi and B2) in the mutual measurement * * · '·' 'distort the measurement result. Next, normal and reciprocal '; '' Measurement with switching setup 2. In the example case (when the input impedance Z2 distorts '; · 25 of the measurement results), a switching setup with 2 normal measurement gives a lower result because:'.: The constant current source is connected to C2 and C4. From the available measurement results, it is possible to deduce »t '· ·' that the measurement impedance Z2 causes an incorrect measurement result and to correct the situation.

::: An attempt can be made to automatically eliminate the effect of a bad connection impedance:. j by adjusting the amount of current supplied by the constant current source, or by correcting the connection to a sufficiently low impedance.

Claims (5)

Method for testing a tetrapolar impedance measurement system, which method is based on the principle of reciprocity, characterized in that impedance is measured using a first measurement arrangement between a measuring equipment (1) and a measuring object (2), after which the impedance is measured using a second measurement arrangement. which has been determined according to the reciprocity principle of said first measurement arrangement such that the power supply coupling and the voltage measurement coupling are interchanged, the measurement result from the first measurement and the second measurement being compared with each other, the basis of the coupled result being determined by the between the measuring equipment and the measuring object causes distortion in the measurement result of the impedance. Method according to claim 1, characterized in that said interfering coupling can be located by using all possible different measuring arrangements and their reciprocal arrangements. Method according to claim 1, characterized in that when the coupling consists of several contacts to the measuring object, a reciprocal imbalance in the impedances between the object and the contacts is observed. 4. Measuring means for measuring the impedance, characterized in that the measuring means comprise couplings with which the coupling for the current supply and the coupling for the voltage measurement can be reciprocally shifted to measure the impedance with '': 'measuring arrangements determined according to the reciprocity principle, the connection between the measuring device and the measuring object which causes the distortion in the measurement result of the impedance. »* Method according to claim 4, characterized in that the said, the analyzing means calculates said abusive coupling using all possible different coupling arrangements and their reciprocal arrangements. 't *