JP3849955B2 - Body fat meter and body weight scale with body fat meter - Google Patents

Description

[0001]
[Industrial application fields]
The present invention simultaneously measures bioimpedance and body weight between both feet in the standing position, and calculates and estimates the amount of fat in the body based on physical characteristics such as height, gender, and age that are input separately from the measured values. More particularly, the present invention relates to a weight scale with a body fat meter that corresponds to variations in contact impedance between the impedance measurement electrode and the sole of the foot, which is a measurement main body.
[0002]
[Prior art]
The composition of the human body can be estimated using the impedance between living body terminals obtained by applying a small constant current from the end of the subject and measuring the voltage drop between the electrodes (The Ameri-can Journal of Clinical Nutrition , 41 (4) 810-817 1985 "Body fat meter using" Assessment of fat-free mass using bioelectrical impedance measurement of the human body "" has been proposed, (USP. 4,008,721, Japanese Patent Publication 5- (49050, JP-A-7-51242, etc.) These proposed products have been commercialized.
[0003]
In particular, in the product related to Japanese Patent Publication No. 5-49050, a flat metal electrode for measuring bioimpedance is placed at a position where the soles of both feet come into contact with each other when the subject puts on the weight scale. By simply placing it on the surface of the table and placing it barefoot on the specified position of the measuring instrument, among the factors for estimating the amount of fat in the body, the inter-body impedance and body weight that change with each measurement are measured simultaneously and calculated. It has been commercialized as a device that can be incorporated into the equation and can estimate the amount of fat in the body most simply, a weight scale with a body fat scale.
[0004]
A conventional internal fat meter first inputs physical data such as height, gender, weight, etc. of a measured person via an input device such as a key switch, and stores it in a storage device. The impedance measurement device is driven, the impedance value of the living body is output as an analog signal, converted into a digital value by an AD converter, taken into the arithmetic device, and a digital value indicating the obtained biological impedance value is stored in the storage device. The body fat mass is calculated from the stored physical data such as height and output to the display device. Unlike other physical data, the body weight is easily fluctuated, so it is necessary to input it every time it is measured, so a gravity measuring device is provided, and the weight value of the subject is also measured each time impedance is measured. What is to be measured is a body scale with a body fat meter.
[0005]
This body weight scale with body fat meter is provided with a changeover switch between the analog output of the impedance measurement device and the AD converter, and the other end of the changeover switch is connected to the analog output of the gravity measurement device. The control terminal is connected to the control device. First, the output of the weight measuring device is input to the AD converter with the changeover switch, the weight value of the person to be measured is converted into a digital value, and the value is stored in the storage device. After storing, switch the changeover switch, input the output of the impedance measuring device to the AD converter, convert the measured impedance value into a digital value, and share the AD converter between the weight measuring device and the impedance measuring device Yes. (Figure 1)
[0006]
Further, the conventional bioimpedance measuring device for a weight scale with a body fat meter employs a four-terminal method in order to eliminate the influence of variations in contact resistance between the electrode and the living body on the measured value. (Fig. 2), (Fig. 3)
Electrodes A1, A2, B1, and B2 are arranged so that the toe part and the heel part of both feet are in contact with the toe part and the heel part when the subject is placed on the platform of an electronic weight scale that measures the weight of the subject. The current terminal of the AC constant current source having a known current value iR at 50 kHz is connected to the electrodes A1 and A2, and the measurement terminal of the AC voltmeter is connected to the electrodes B1 and B2. The AC voltmeter is designed so that almost no current flows through the measurement terminal.
Here, the body impedance is ri, the contact impedance of the right toe is rA1, the contact impedance of the left toe is rA2, the contact impedance of the starboard is rB1, and the contact impedance of the port is rB2.
The AC constant current iR flows through rA1, does not leak into rB1 and rB2, flows directly into ri and rA2, and returns to the current terminal.
At this time, the voltage drops generated in rA1, ri, and rA2 are respectively
vA1 = iR × rA1 (1)
vi = iR * ri (2)
vA2 = iR × rA2 (3)
Here, since almost no current flows in each measurement terminal of the AC voltmeter, the voltage drop generated in rB1 and rB2 is regarded as 0, respectively, and the influence of rB1 and rB2 is eliminated. In the AC voltmeter, vi is Observed as is.
From the equation (2), the internal impedance ri is ri = vi / iR (4)
Since iR is a known value, it can be easily obtained from the observed value vi.
[0008]
The AC constant current source is composed of an AC constant voltage source, a resistor R1, and an OP amplifier. (Fig. 4)
The output of the AC constant voltage source is connected to one end of R1, and the other end of R1 is connected to the negative terminal of the OP amplifier. The electrode A1 is connected to the negative terminal, the electrode A2 is connected to the output terminal of the OP amplifier, and the positive terminal is connected to GND (0 V).
As long as the output terminal is not saturated, the − terminal of the OP amplifier is at the same potential as the + terminal, and no current flows from the − terminal into the OP amplifier.
Therefore, the current flowing through R1 flows directly to the electrode A1, flows through the living body, reaches A2, and is absorbed by the output terminal of the OP amplifier.
If the output voltage of the AC constant voltage source is v, the voltage across R1 is v.
ri = v / R1 (5)
Since v and R1 are known, a known constant current can be obtained.
[0009]
The AC voltmeter includes a differential amplifier, a rectifier, a low-pass filter, and an AD converter. First, an amplified output obtained by multiplying the difference voltage between the electrodes B1 and B2 by N times is obtained by a differential amplifier.
At this time, the output voltage v of the differential amplifier is
v = N * Vi = N * iR * ri (6)
When this output is input to a half-wave rectifier, the rectifier outputs only the positive portion of the AC voltage. This is converted into direct current by a low-pass filter and input to the AD converter, thereby obtaining a digital value proportional to the internal impedance ri.
With this configuration, it is possible to measure bioimpedance that is not affected by foot contact impedance.
[0010]
However, in order to measure as accurately as possible, it is usually designed on the assumption that the contact impedance is somewhat small and that it is used with bare feet. The contact impedance of the normal sole of the foot is within 1 kΩ, and the maximum impedance measurement current is designed to be 1 mA or less.
When the peak voltage of the AC constant voltage source is 0.8 V and the resistance R1 is 1 kΩ, the current value at this time is 800 μA from the equation (5).
Assuming that the bioimpedance is 500Ω and the contact impedance is 1 kΩ, the voltage Vo of the electrode A2 is
Vo = 800 μA × (rA1 + ri + rA2) = 2V
The contact impedance of the usual sole of the foot is within 1 kΩ, but becomes extremely large when wearing socks.
Now, assuming that the contact impedance is 10 kΩ, the calculated voltage Vo of the electrode A2 is
Vo = 800 μA × (10 kΩ + 500Ω + 10 kΩ) = 16.4 V (8)
It becomes.
[0011]
On the other hand, body fat scales are often made on the premise of carrying, and the circuit is mainly driven by a battery. Therefore, the circuit voltage is limited to about ± 5V. The voltage at the output terminal of the OP amplifier used for the constant current source is also within that range. When wearing socks (contact impedance is 10 KΩ) as described above, the OP amplifier is saturated and constant. Current cannot flow. Therefore, ri is not a known constant current, and the waveform is distorted and not a sine wave. However, the AC voltmeter detects the distorted and inaccurate voltage output vi and uses the value to calculate the body fat mass and outputs it to the display device.
[0012]
As described above, the conventional bioimpedance measuring device for a body weight with a body fat meter is designed on the premise that it is placed on a measuring device with bare feet, and when the sock is worn, the electrodes of the measuring device If the contact impedance with the measured object becomes excessive, the voltage drop between the electrode of the measuring device and the measured object will be large due to the system configuration, deviating from the normal operating range of the constant current power supply, Current could not flow in, and accurate measurement was impossible.
[0013]
[Problems to be solved by the invention]
The problem to be solved by the present invention is that the amount of fat in the body can be estimated easily and accurately even when the contact impedance between the living body and the electrode for measuring the biological impedance becomes large, such as wearing socks. It is to provide a weight scale with a built-in body fat meter.
[0014]
[Means for Solving the Problems]
Supply current for bioimpedance measurement corresponding to the contact impedance between the sole of the subject's foot and the bioimpedance measurement electrode between both feet by monitoring the generated voltage by installing a saturation detector of a constant current source OP amplifier The bioimpedance is measured only when the value is changed and the voltage generated by the constant current source is equal to or lower than a predetermined voltage.
[0015]
[Action and effect]
Since the constant current power supply is always used in the normal operating range, a predetermined stable and accurate current is supplied, and accurate bioimpedance can be measured even when the contact impedance is large.
[0016]
Embodiment
A pair of electrodes A (A1, A2) for injecting a minute current into the body that contacts the toes of both feet when the subject is placed on the platform of an electronic weighing scale that measures the weight of the subject when the subject is placed. ) And a pair of electrodes B (B1, B2) that contact the heel of both feet and measure the potential difference between the heels, and two pairs of bioimpedance measuring devices between the feet, height, age, sex, etc. An input device for inputting physical features is provided. The electronic scale is provided with an oscillator for obtaining a constant voltage of a sine wave, a voltage-current converter for obtaining a plurality of constant currents, and a determination device for a voltage generated by the voltage-current converter. An output of a voltage-current converter that is equal to or lower than a predetermined voltage is connected to the electrode A. A potential difference corresponding to the voltage drop from the electrode B is taken out by a differential amplifier, subjected to waveform shaping, rectification, etc., converted into a direct current and then converted into an AD, and an output current of a connected voltage-current converter is used to Imported into the calculation unit as digital impedance data. The calculation unit is configured to calculate the amount of fat in the body based on the input physical characteristics such as height, age, and sex, and the measured body weight and bioimpedance, and display the calculated amount on the display.
[0017]
【Example】
Details of embodiments of the present invention will be described in detail with reference to the drawings. As a specific example 1 of the saturation detector of the constant current source OP amplifier, SW1 for selectively connecting the B1 and B2 terminals and the A1 and A2 terminals to the differential amplifier is provided, and the control terminal of SW1 is provided to the control device. When the constant current source is connected and is surely operating normally, the maximum digital value obtained by AD-converting the voltage between the A1 and A2 terminals is stored as NMAX.
A resistor R4 is connected to R3 so that it can be connected in parallel by SW2. The control terminal of SW2 is connected to the control device. (Fig. 6)
[0018]
The control device first connects R2 in parallel with R1 through SW3.
When R1 = 5 kΩ R2 = 1.25 kΩ, the parallel resistance value R ′ is
R ′ = 1 / (1 / R1 + 1 / R2) = 1 kΩ
If the peak voltage of the AC constant voltage source is 0.8 V, the inflow current value in the body is
iR = 0.8 V / 1 kΩ = 800 μA
The control device connects the A1 and A2 terminals to the differential amplifier by SW1, converts the output into a DC voltage by a half-wave rectification and low-pass filter, and inputs it to the AD converter.
[0019]
If the digital value obtained from the voltage between the A1 and A2 terminals is smaller than NMAX when iR is applied, the B1 and B2 terminals are connected to the differential amplifier by SW1, and the AD is passed through the half-wave rectification and low-pass filter. Based on the digital value proportional to the bioelectrical impedance ri obtained by the converter and the body data of the person to be measured, the arithmetic unit calculates the amount of fat in the body and outputs it to the display device. (Fig. 8)
[0020]
If the digital value obtained from the voltage of the constant current source, that is, the voltage between the A1 and A2 terminals is larger than NMAX, it can be determined that the constant current source is saturated.
When it is larger than NMAX, the control device turns off SW3 and disconnects R2.
Since the resistance that generates a constant current is only R1, the inflow current iR 'in the body is
iR ′ = 0.8 V / 5 kΩ = 160 μA = iR / 5
Calculating an example when wearing socks and each contact impedance is 10 kΩ,
Vo = 160 μA × (10 kΩ + 500Ω + 10 kΩ) = 3.28V
Thus, it can be seen that saturation is prevented when the constant current source power supply voltage is ± 5V. If the digital value obtained from the voltage between the A1 and A2 terminals is smaller than NMAX when iR 'is applied, the B1 and B2 terminals are connected to the differential amplifier by SW1. Since the obtained digital value is proportional to the inflow current in the body, a value of 1/5 is obtained. Accordingly, the arithmetic unit calculates the obtained digital value × 5 and the arithmetic unit calculates the body fat mass based on the value and the physical data of the measurement subject, and outputs it to the display device.
[0021]
When the sock is thick and the contact impedance is 25 kΩ, Vo = 160 μA × (25 kΩ + 500Ω + 25 kΩ) = 8.8 V
Then Vo becomes larger than NMAX. In this case, it is determined that the OP amplifier is in a saturated state and the constant current source is not normally operating properly. That is, when Vo is greater than or equal to NMAX, “Error” is displayed and measurement is not normally performed. That the contact impedance between the measurement object and the bioimpedance measurement electrode is too high.
[0022]
The OP amplifier saturation detector may directly monitor the output voltage of the OP amplifier. (Fig. 7)
The terminal A2 is connected to the + terminal of the comparator, and the-terminal is set to the upper limit voltage VMAX at which the power supply OP amplifier operates reliably. The output of the comparator is input to the control device.
The comparator outputs Hi when the voltage at the terminal A2 exceeds VMAX, and outputs Lo when the voltage is lower than VMAX.
The control device monitors whether the output of the comparator becomes Hi, and when it becomes Hi, SW3 is turned off and R2 is disconnected, and the resistor for generating a constant current is only R1. In this state, if it is Hi, it is determined that the OP amplifier is saturated, “Error” is output, and if it remains Lo or changes to Lo, it is considered that normal measurement is possible and calculation is performed. Displays the amount of fat in the body.
[0023]
An amplifier 2 is further provided after the differential amplifier. Usually, when the voltage of the differential amplifier is doubled, the contact impedance is high, and the impedance measurement current is 1/5 of the normal, the voltage of the differential amplifier is reduced. If the output is multiplied by 5, the obtained digital value does not need to be multiplied by 5 in the calculation, and the digital value actually measured by AD conversion becomes 1/5, and the resolution is lowered. Can be avoided. Specifically, one end of R3 is connected to the output of the differential amplifier, and the other end is connected to the-terminal of the amplifier 2 . Further, the negative terminal and the output terminal of the amplifier 2 are connected by R5, and further R4 is connected to R3 so that it can be connected in parallel by SW2. The control terminal of SW2 is connected to the control device. R3 = 5 kΩ R4 = 1.25 kΩ R5 = 5 kΩ First, SW2 is turned off and R4 is disconnected. The amplification factor of the amplifier 2 is represented by R5 / R3 and is 1 time. When the contact impedance is high and the measurement current is reduced, SW2 is turned on. The parallel resistance value R ″ of R3 and R4 is
R ″ = 1 / (1 / R3 + 1 / R4) = 1 kΩ
The amplification factor is R5 / R '' = 5 times. In the above example, the constant current stage is described as two stages for the sake of simplicity. However, it is obvious that three or more stages can be similarly realized.
[0024]
In the high contact impedance mode, the inflow current in the body becomes small, so the error rate increases and a slight decrease in accuracy may be observed.
In the above example, the constant current is automatically switched. However, there may be a case where it is troublesome for the accuracy to change every time the person to be measured changes.
Therefore, by adding the “High Contact Impedance Mode” key to the input device and pressing the “High Contact Impedance Mode” key, the storage device memorizes the “High Contact Impedance Mode” and turns off SW3 and maintains the state. In addition, by maintaining a low constant current, it is possible to prevent the accuracy from changing even if the measurement subject changes.
It can be used with confidence even when measuring when there are many socks.
[0025]
In addition, the “high accuracy” and “low accuracy” marks are provided on the display device, and “high accuracy” is lit in “normal mode” and “low accuracy” is lit in “high contact impedance mode”. Even if it changes, it can be displayed in an easy-to-understand manner and mistakes can be avoided.
It is also an effective method to distinguish between the display by displaying in 0.5% units in "normal mode", displaying in 1% units in "high contact impedance mode", and not displaying decimals. is there.
[0026]
In the above example, the description of the weight scale is omitted for the sake of simplicity. However, a changeover switch is provided between the analog output of the impedance measurement device and the AD converter, and the other end of the changeover switch is connected to the gravity measurement device. By connecting to the analog output and connecting the control terminal of the changeover switch to the control device, first, the output of the weight measuring device is input to the AD converter with the changeover switch, and the weight value of the person being measured is a digital value After storing the value in the storage device and inputting the output of the impedance measuring device to the AD converter, the device can be shared with the impedance measurement, and it is easy to use and inexpensive. can do.
[0027]
As mentioned above, the technique for measuring the body fat percentage with higher accuracy and preventing mistakes has been described, but with this technique , measurement can be performed with high accuracy and without making mistakes even while wearing socks. .
[Brief description of the drawings]
1 is a block diagram of a conventional weight scale with a body fat meter. FIG. 2 is a schematic diagram of body fat measurement. FIG. 3 is an explanatory diagram of bioimpedance measurement. FIG. 4 is a constant current block diagram. FIG. 6 is a block diagram of an embodiment of the present invention. FIG. 7 is a block diagram of another embodiment of the present invention. FIG. 8 is an operational flow diagram of an embodiment of the present invention.
A1 Power supply terminal of impedance measuring means A2 Power supply terminal of impedance measuring means B1 Voltage drop measuring terminal of impedance measuring means B2 Voltage drop measuring terminal of impedance measuring means rA1 Contact impedance between living body and each terminal rA2 Between living body and each terminal Contact impedance rB1 Contact impedance between living body and each terminal rB2 Contact impedance between living body and each terminal ri Internal impedance

Claims (5)

Input means for inputting physical data such as the height of the person to be measured, an AC constant current source for supplying a current to the body of the person to be measured and measuring bioimpedance, and electrodes for current application and voltage measurement An impedance measuring means comprising: an arithmetic processing means for calculating a body fat mass by arithmetically processing the physical data input by the input means and the impedance value measured by the impedance measuring means; and the arithmetic processing means. A body fat meter having a display means for displaying the obtained body fat mass,
The AC constant current source includes an AC constant voltage source, an OP amplifier, and a plurality of resistors connected in parallel between the AC constant voltage source and the OP amplifier to generate a constant current.
Wherein the impedance measuring means, detecting means for detecting a voltage of the current application electrodes as the information related to the contact impedance, the voltage detected by the detection means and the between the current supply electrode and the body of the subject Current value changing means for reducing the current value supplied to the body by disconnecting some of the plurality of resistors constituting the AC constant current source when the constant current source is larger than the upper limit voltage at which the constant current source operates reliably. A body fat meter characterized by further comprising: An input means for inputting physical data such as the height of the measurement subject, a weight measurement means for measuring the weight of the measurement subject, and an AC constant for measuring the bioimpedance by supplying current to the measurement subject's body. Impedance measuring means having a current source and electrodes for current application and voltage measurement , physical data input by the input means, body weight value measured by the weight measuring means, and impedance measurement means A weight scale with a body fat meter, having a processing means for calculating the body fat mass by computing the impedance value, and a display means for displaying the body fat mass obtained by the processing means,
The AC constant current source includes an AC constant voltage source, an OP amplifier, and a plurality of resistors connected in parallel between the AC constant voltage source and the OP amplifier to generate a constant current.
Wherein the impedance measuring means, detecting means for detecting a voltage of the current application electrodes as the information related to the contact impedance, the voltage detected by the detection means and the between the current supply electrode and the body of the subject Current value changing means for reducing the current value supplied to the body by disconnecting some of the plurality of resistors constituting the AC constant current source when the constant current source is larger than the upper limit voltage at which the constant current source operates reliably. And a body weight scale with a body fat meter. The arithmetic processing means reduces the current value supplied to the body by the current value changing means, and the voltage of the current application electrode detected by the detection means is higher than the upper limit voltage at which the constant current source operates reliably. The body fat meter or body weight meter with a body fat meter according to claim 1 or 2 , wherein when it is large , the body fat mass is not calculated. In the display device, after the current value supplied to the body by the current value changing unit is decreased , the voltage of the current applying electrode detected by the detecting unit is larger than the upper limit voltage at which the constant current source operates reliably. In such a case, the body fat mass is not displayed, and the body fat meter or body weight scale with body fat meter according to claim 1 or 2 . Body fat meter or body fat meter with body weight meter according to claim 1 to 4, wherein the configured by 4-terminal circuit the impedance measuring means.

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