HK1092343B - System for measuring and indicating changes in the resistance of a living body - Google Patents

Description

System for measuring and indicating changes in living body resistance

This patent application claims priority to U.S. provisional patent application No.60/455,948, filed on 19/3/2003.

Technical Field

The present invention relates to an improved device for indicating and measuring changes in the resistance of a living body.

Background

Previously, devices for measuring and displaying small changes in the resistance of a living body have been disclosed in U.S. Pat. No.3,290,589 and U.S. Pat. No.4,459,995, and U.S. Pat. No.6,011,992. These devices typically include a resistance measurement circuit, an amplifier circuit, and an indicator circuit. In operation, the device measures small resistance changes using a resistance measurement circuit to produce a measurement signal indicative of the magnitude of the resistance. The measurement signal is then amplified by an amplifier circuit, and the amplified signal is used to drive a display, such as an electromagnetically responsive needle of a meter. The reliability of the readings depends not only on proper calibration of the device, but also on precise adjustment of the gain and sensitivity of the circuitry during in vivo monitoring.

This type of device was first conceived and developed by inventor Hubbard for monitoring or examining (audio) individuals as part of a consultation protocol. The device is used to detect small changes in its resistance while the person under examination considers some aspects of his own presence, in order to improve his ability to verify who and where he is. Detecting and visually perceiving minute, and sometimes very brief, changes in resistance and the particular pattern of change is very important for accurate and most effective inspection of individuals.

Although the prior art devices described above have been suitable for detecting changes in the resistance of a living body, they are difficult to calibrate correctly and are difficult to produce accurately perceptible display readings at all times. These difficulties may arise from the characteristics of signal non-linearity and aging-related and temperature-related component variations that can mask or falsely report small but meaningful measurement changes.

Inventor Hubbard recognized that signal nonlinearity is an important factor that greatly complicates the reliable display of the required information, and that the interaction between the measurement range and sensitivity adjustment of such devices further complicates the ability to obtain clear readings. Mr. Hubbard further identified a small delay of as little as 0.1 seconds in displaying the resistance change as another factor complicating the examination and determined that the most effective display required the least possible resistance change to be perceived with as little delay as possible.

Disclosure of Invention

The present invention is an improved apparatus for measuring and indicating the resistance and changes in resistance of a living body. The device utilizes digital processing to improve display response and accuracy. An automatic calibration sequence substantially compensates for the effects of component aging, temperature variations and measurement tolerances on these very sensitive measurements. The preferred circuit for the sensitivity adjustment means isolates the effect of the adjustment means on the resistance measurement circuit.

These and other features of the present invention will be apparent from the following description of the preferred embodiments, of which the accompanying drawings form a part.

Drawings

In the drawings:

FIG. 1 is a front view of a perspective view of a preferred embodiment constructed in accordance with the present invention for measuring and indicating changes in the resistance of a living body;

FIG. 2 is a top view of a preferred meter 16 used in the apparatus of FIG. 1;

FIG. 3 is a block diagram of a preferred embodiment of circuitry used by the device 10;

FIG. 4 is a schematic diagram of a preferred front-end circuit for constructing the apparatus of FIG. 1 in accordance with the present invention;

FIG. 5 illustrates a correct arrangement of FIGS. 5A-5E, which together illustrate, in turn, a block diagram of a preferred central processing unit for use with the apparatus of FIG. 1;

FIG. 6 is a schematic diagram of a preferred sensitivity adjustment circuit for use with the apparatus of FIG. 1;

FIG. 7 is a schematic diagram of a preferred meter circuit for use with the apparatus of FIG. 1.

Detailed Description

FIG. 1 is a front view of a perspective view of a preferred embodiment constructed in accordance with the present invention for measuring and indicating changes in the resistance of a living body. The device 10 includes a housing 12 having a window 14 through which a meter 16 can be viewed. As will be explained below, the meter is used to display the value of the body resistance (hereinafter simply referred to as "body resistance") of the individual being examined by the device and its changes. Those skilled in the art will appreciate that displays other than the types of meters described herein are also within the scope of the present invention.

The housing 12 includes a second window 18 for viewing a display 20. The display 20 is preferably a Liquid Crystal Display (LCD) that selectively displays information such as the date, time, tone arm (tone arm) position, tone arm movement, elapsed time during the examination, selected display language and other pertinent information.

Three buttons 22A-C are connected to the display 20 for selecting and/or changing the display language, time, date, etc. One of which is used to select a desired menu from a series of sequentially displayed menu titles. The remaining two buttons are used to move the selection bar relative to a menu selected from the plurality of listed options. The first button then functions as a selection button for selecting the identified option.

A second set of three buttons 28a-c are used to select meter sensitivity, respectively "low", "medium" and "high". As will be discussed in detail below, the selected sensitivity is a function of the change in the resistance of the body being examined, and a sensitivity knob 26 works in conjunction with the buttons 28a-c to provide additional sensitivity adjustment. For this purpose, a fixed 32-segment scale is marked on the housing around the knob 26. The arrangement is such that when the button 28a selects the "low" range, the knob setting "32" produces a meter sensitivity equal to the sensitivity of the knob setting "1" when the button 28b selects the medium range, and in the medium range, the knob setting "32" produces a meter sensitivity equal to the sensitivity of the knob setting "1" when the button 28c selects the high range. As further shown below, the low range imparts a gain of about 1 to the measurement signal derived from the bulk resistance, the mid-range imparts a gain of about 9.5 to the signal, and the high range imparts a gain of about (9.5) to the signal²Or a gain of 90.25. Of course, all of these numerical gains and interrelationships can be varied without departing from the spirit and scope of the present invention.

The device 10 further includes a "battery check" button 29, an on/off button 31 and a "meter trim" button 33.

The rotary knob 24 is used to select the appropriate range for the meter 16 as described below, and is typically referred to as the "TA" knob by users who have used these devices. The term "TA" is therefore used herein to denote the gauge range setting from time to time. The TA knob 24 is preferably coupled to an optical encoder within the housing that generates a numerical value indicative of the rotational position of the knob. The rotational position of the knob may conveniently be viewed as the degree to which it is rotated counterclockwise away from the end point, but is conveniently discussed in terms of the TA value represented by its position. Thus, the knob is moved to 24A to indicate a TA value of the fixed, circumferentially imprinted numerical scale 36 on the housing 12. The scale preferably has a scale of "0" to "6" marked in sequence over an arc of approximately 240 deg., and the knob is typically rotated clockwise from a TA value of 0.5 to a TA value of 6.5 during inspection. It should be noted that the numbers and spacing are chosen here to be consistent with previous devices, such as those illustrated and described in U.S. Pat. No.4,459,995, but any series of numbers, letters, or other indicia arranged around any conventional arcuate shape can be employed without departing from the spirit of the present invention.

A pair of electrodes 30, 32 are detachably coupled to a socket 34 on the back of the housing 12 and are adapted to be held by an examinee. However, any other method of attachment to a living body is also within the scope of the present invention. The electrodes can have any suitable shape. Preferably, the examinee holds one electrode per hand, however, it has been found that a generally cylindrical electrode can be comfortably held by the examinee and is therefore preferred.

FIG. 2 is a top view of the preferred gauge 16. The meter is preferably a moving coil meter capable of reading a full range of 0-100 microamperes and has an arcuate scale 38 divided into sections. Approximately one third of the distance from the left end, the scale displays a small portion of the arch, labeled "SET". In operation and during pre-operational calibration of the meter, when the needle points to the portion of the scale marked "SET", its needle 17 is said to be "on SET". The TA knob 24 is used to repeatedly bring the needle back to the area near SET during the examination process and to adjust the sensitivity of the meter using the buttons 22a-22c and knob 26 before or during the examination procedure to obtain the proper meaningful needle range. Preferably, the electrode current flowing through the examined living body does not exceed 50 microamperes. This level has been found to ensure the comfort of the individual, while providing a pointer "reading" of appropriate response during the examination of the individual.

Fig. 3 is a block diagram of a preferred embodiment of circuitry used by the device 10. As shown in FIG. 3, the meter 16 is driven by an analog output signal 480 generated by a digital-to-analog converter 479 in response to a series of digital values 478 produced by the central processing unit 400. Central processing unit 400, in turn, is responsive to respective response input signals 190, 290 and 390 from front-end circuit 100, TA circuit 200 and sensitivity circuit 300.

The front-end circuit 100 is schematically shown in fig. 4. During the monitoring operation, it generates a digital value indicative of the resistance of the examined living body. When the device is powered on, it generates calibration data for use during the monitoring operation.

Front-end circuit 100 includes a resistance sensing circuit 108 for generating a measurement signal indicative of body resistance, a voltage follower 110 for filtering and isolating the measurement signal from the effects of other system components, and an analog-to-digital converter 120a for converting the measurement signal into a digital signal indicative of body resistance as measured by the resistance sensing circuit.

The resistive sensing circuit is preferably of the potentiometer type, electrically coupled to a positive d.c. source voltage V_DDAnd ground GND. During the examination, the resistance sensing circuit includes a 4.99K resistor R1, the bulk resistor R_pc(which is sensed between electrodes 30, 32 coupled to pins 1 and 3 of socket 104), and a 45.3K resistor R3, all coupled in series at d.c. source V_DDAnd the ground. When the monitor electrode is disconnected from the socket 104, the socket is configured to electrically couple pins 2 and 3 together, placing a 5K resistor R2 across the socket.

Resistor R1 is not directly electrically coupled to electrode receptacle 104, but rather is coupled to it (and to the body) through an analog multiplexer/demultiplexer 102Resistance R_pc) Coupling is preferably by Burr brown mc14051 BD. More specifically, R1 is coupled to pin X of analog multiplexer/demultiplexer (hereinafter "multiplexer") 102. The multiplexer 102 is configured to connect its pin X to a selected pin X in response to each associated select signal applied to its pin A, B, C₀，X₁，X₂，X3。

The multiplexer 102 is arranged in connection with a voltage dividing resistor circuit used during the calibration process described below. In a normal biopsy, pin X and pin X are connected₀Electrically coupled to the resistor R1 and the bulk resistor R_pcAre connected in series. The result is an analog measured voltage e₀Which varies with the variation of the bulk resistance according to the following voltage division formula,

(formula 1)

Measurement signal e₀The operational amplifier 110 is fed, which is configured as a voltage follower. The preferred operational amplifier is a Burr Brown LT1677CS 8. The output of the operational amplifier 110 is applied to a 24-bit analog-to-digital converter 120, preferably a BurrBrown ADS 1210U, and generates digital values representing the measured body resistance at its output pins SDO and SDIO, which will be clocked in response to a clock pulse applied to pin SCLK, to pin 66 of a central processing unit ("CPU") 400.

Alignment feature

Those of ordinary skill in the art will appreciate that the bulk resistance R_pcExpressed as the measured voltage e₀There are a number of sources of error. For example, the resistance of the circuit may change over time and may change with temperature, affecting the accuracy of the voltage divider network. In addition, the internal voltage levels, leakage circuits and offset voltages (offset voltages) of the solid state components of the present device can vary with age and/or temperature variations, and can vary from device to device within normal specified and unspecified component tolerances. Although such errors can be minimized using components with extremely tight tolerances, such components are very expensive and the errors can still not be completely eliminated. Since such errors can mask or falsely report small but meaningful measurement changes, this type of device must include a calibration process to minimize such errors. This prior art calibration procedure is very complex, typically requiring the device to be returned to the factory once a year.

According to one aspect of the invention, a calibration circuit is included that is capable of self-calibrating the device each time the device is powered on. First, an actual measurement signal e is obtained at a plurality of reference points₀. These reference points are determined by replacing the bulk resistance R with a known resistance_pcTo be selected. Some or all of these known resistances are normalized values, which correlate to the past TA values. For example, in the past, a bulk resistance equal to 5k ohms was selected as the resistance to bring the meter needle to SET at a TA setting of "2", a bulk resistance of 12.5k ohms would perform the same function for a TA value of "3", and so on.

As described below, the calibration procedure herein replaces R with a 5k ohm resistor, a 12.5k ohm resistor, and a short circuit_pcSo that the measurement signal e for each calibration point can be acquired, digitized and temporarily stored₀The actual value of (c). Then, a model is calculated from these saved signal values, and the meter is set to S for each of the other TA value calculationsET value of the desired measurement signal. During the monitoring process, the checked body resistance is compared to the saved value, the meter will be SET to SET for the TA dialed in, and the difference in signal values will be used to drive the meter needle, thereby compensating for the component variations described above. Of course, the number of calibration points and the TA values used can be varied without departing from the invention.

Thus, CPU400, upon power-up, sends the appropriate select signal to pins 9, 10, 11 of multiplexer 102, thereby coupling its terminal X in turn to terminal X₁、X₂And X₃R is replaced by a 4.99K resistor R2, a 12.4K resistor R4 and a series short with the resistors R1 and R3, respectively_pc. E under each condition₀The values are fed to operational amplifier 110, digitized by converter 120 and output to central processing unit 400. The above values were chosen for R2 and R4 because the TA ranges of "2" and "3" were historically the most frequently used settings when monitoring body resistance. The short circuit condition is used to easily provide an additional data point. Open circuit conditions may also be used.

The value of the measured voltage is related to the resistor value and the d.c. source voltage according to the following formula:

when selecting terminal X₁Then, (formula 2)

When selecting terminal X₂Then, (formula 3)

When selecting terminal X₃Then, (formula 4)

If an open circuit condition is desired, terminal X is selected₄Generating a measuring signal e_errorWherein e is_errorIs any non-zero voltage detected, not the ideal sensed zero voltage. In open circuit conditions, the measured voltage is theoretically zero, but errors due to component offset voltages, leakage currents, etc., produce a voltage across R3 that can be detected and ultimately compensated for.

Once the measured voltages e of the selected resistance values R2(4.99K), R4(12.4K) and zero (short circuit) are obtained₀Can utilize the known V by the CPU400_DDThe values are simultaneously solved by the above formulas 2-4, and effective values of R1 and R3 are calculated.

Once the effective values of R1 and R3 are calculated, the device then calculates the effective resistance associated with the receptacle 104 and the lead associated therewith. The multiplexer 102 outputs itInput terminal X and terminal X₀Are connected. The electrodes 30, 32 are disconnected from the socket 104, and the socket 104 is configured to couple its pins 2 and 3 together. In this step, therefore, the electrodes are open, so that the multiplexer terminal X₀Pin 2 of the socket 104 is connected, thereby connecting R2, R3 and ground. The measurement signal e obtained₀By passing through multiplexer pin X₁The value obtained when R2 is selected is compensated for, which corresponds to the effective socket resistance. Since the socket resistance is added to the sensed body resistance during the inspection, the socket resistance value is calculated, and then the CPU400 subtracts it from the total resistance sensed during the inspection.

The calculated values of R1, R3 and the socket resistance are used in setting other measured signal values used to SET the meter needle on SET, as previously described. Historically, for example, a TA value of 2 would ideally SET the meter needle on SET when a 5K resistor is placed across the electrodes. For TA values of 3, 4, 5, the resistor values are conventionally 12.5K, 30K and 100K. In keeping with prior art devices of this type, it is desirable to maintain the same nominal relationship, although those skilled in the art will recognize that this is not necessary.

Inspection (audio)

After calibration, and during monitoring of a living body, the body resistance R is sensed across the electrodes 30, 32_pcVia the converter 20, the resulting measurement signal e₀Digitized and passed to CPU400 where it is compared to a value corresponding to the TA then dialed in, and the difference between the two signal values is used to drive the meter needle from SET.

Thus, the CPU400 receives two inputs in total. First, it calculates the monitored body resistance R from the digital value of the measurement signal according to equation 1 above and taking into account the socket resistance_pc. This is typically done in real time.

Then, the CPU subtracts a resistance value (R)_TA) Which the CPU calculates from the position of the optical encoder knob 24.In this regard, the TA knob is typically rotated during the auditing procedure to place the meter's needle at or near SET. As the TA knob rotates, it rotates the shaft of the digital decoder 200, producing a digital value to the CPU that represents the rotational position of the knob. This numerical value correlates the TA value to the knob position, which is shown on the scale near the knob. The digital position representative value applied to CPU400 is processed by the CPU to calculate a resistance value for the dialed-in TA position according to a preferred formula:

where TA is the TA value.

The difference between the two values is the value that forms the basis for the needle drive signal applied to the meter 16. The amount by which the pointer is driven off SET is a function of the magnitude of the difference. As will now be discussed, an adjustable "sensitivity" gain can be applied to control the extent to which the difference magnitude moves the pointer. However, it should be appreciated that the difference signal itself is not affected by the sensitivity adjustment.

Sensitivity characteristics

As previously described, CPU400 passes from R_TAThe sampled bulk resistance value is subtracted from each sampled value of the measured voltage to calculate the bulk resistance across the electrodes. If the difference is "0", the processor generates a digital output value that is coupled to the meter via a digital to analog converter and places the meter needle on SET. If the difference is positive, the pointer is driven to the right of SET. If the difference is negative, the pointer is driven to the left of SET. If the operator changes the position of the TA knob, the body resistance will be monitored as the knob is rotated andthe new values are compared until the operator obtains the desired meter reading.

The distance that the needle is driven from SET depends on the setting of the sensitivity knob 20 (fig. 1). The sensitivity adjustment determines the number of increments of pointer movement on the meter scale for signal changes. The increments are conveniently referred to as "T's" because they are a series of inverted T's on the preferred meter (FIG. 2). Thus, the sensitivity setting is such that a given change in bulk resistance determines the amount by which the finger is off SET, and is conveniently referred to as "ohm/T"; that is, the bulk resistance required to move the needle one "T" increment on the meter scale.

Using a suitable algorithm, discussed below, one can make the meter needle reading more accurately indicative of the change in the detected body resistance. For example, a non-linear relationship can be modeled to produce a reading that was previously lost due to the pointer deviating from a relationship with the change in body resistance that is not linear over the entire TA range but is assumed to be linear. Moreover, it has been found that setting the needle on SET at higher TA values, and maintaining the needle within the meter's display range at higher TA values, is much more difficult than at lower TA values. It is therefore highly desirable to separate sensitivity adjustment from range adjustment, which has been achieved as described above. Moreover, it is also highly desirable to automatically reduce sensitivity at high TA values, and to automatically increase sensitivity at low TA values, in order to improve the overall usability of the device.

Thus, the central processor 400 provides an automatically correcting gain factor for the meter drive signal for the purpose of substantially eliminating the possibility of masking and erroneous readings within the available range of TA values. Preferred gain factors are:

for TA value The factor is

2.0≤TA≥5.5： 1

TA＞5.5：

For TA < 2.0:

thus, the pointer drive value sent by the CPU400 to the digital-to-analog converter is first multiplied by the appropriate one of the above three factors, depending on the TA value applied by the optical encoder to pin 36 of the processor 400, before being applied to the converter. The optical encoder is used because it is not temperature sensitive, does not have the life-limiting movable contacts of a potentiometer due to wear, and is capable of producing highly accurate digital values that can be utilized by the CPU without the need for an analog-to-digital converter.

Fig. 6 is a block diagram of a sensitivity adjustment circuit used by the apparatus according to the invention. The CPU400, preferably a Nitsubishi Electric M30624FGAQFP and as shown in FIG. 5), receives a first sensitivity signal at pins 52-54 and 74 indicating a selected button from the high, medium and low sensitivity range buttons 28a-c (FIG. 1) and a second sensitivity signal at pin 93 indicating the setting of the sensitivity knob 26 (FIG. 1).

The CPU400 senses which of the three sensitivity range buttons 28a-c is pressed on pins 52, 53, 54 and 73. The sensitivity buttons 28a-c, along with the other buttons shown in fig. 1, are part of an electronic circuit matrix in which each button is served by a unique pair of conductors conceptually forming rows and columns of the matrix. The pressing of the button may change the logic level of the conductor pair associated with the button, the activation of which is sensed by the processor. For example, three sensitivity range buttons are associated with row 1 of the matrix, and pressing any of these buttons will correspondingly change the logic level of the conductor associated with row 1, which is monitored via pin 74 of the CPU 400. The high sensitivity range button 26a is assigned to the matrix address of column 1 so that when the button is pressed, the conductor associated with column 1 of the matrix undergoes a logic level change, which is sensed by pin 54 of the CPU.

Thus, the CPU digitally amplifies the meter drive signal by a factor of 100, since it senses the appropriate logic level signal at pins 54 and 74 (as explained earlier, the high sensitivity setting provides (9.5)²And the gain provided by the medium and low sensitivity buttons is 9.5 and 1, respectively)

Likewise, the mid-sensitivity range button 26b and the low-sensitivity range button 26c are assigned column addresses of 2 and 3, respectively, and the conductors associated with these columns are monitored by pins 52 and 53, respectively, of the CPU. Those skilled in the art will appreciate that the use of digital amplification eliminates the undesirable amplification of noise associated with analog signal value amplification, making small changes in bulk resistance more readily visually perceptible with the subject device.

Sensitivity knob position signal 532 is applied to pin 93 of CPU400 where it is internally coupled to an analog-to-digital converter that generates a digital value indicative of the knob setting. As shown in FIG. 6, the sensitivity knob is mechanically coupled to a wiper 526A of a potentiometer 526, which is in turn coupled between a 10K resistor 530 and a 10K resistor 532 at a DC source voltage V_DDAnd ground GND. Resistor 530, potentiometer 526, and resistor 532 form a voltage divider network. Thus, the sensitivity knob position signal 532 is a DC level signal that increases as the knob is rotated clockwise and the brushes move away from ground.

The slight unadjusted value of the sensitivity setting is input into the processor during factory calibration when the device is manufactured.

Meter driving circuit

FIG. 7 is a block diagram of a preferred meter drive circuit for use with the apparatus according to the invention.

The output signals from the processor 400 taken at pins 40 and 42-45 are coupled to a digital-to-analog converter 602. The analog output signal 604 of the converter 602 is preferably coupled to a control circuit 606 that compensates for differences in the trajectory of the meter movement from device to device. The control circuit 606 includes an operational amplifier 608 that receives the output from the digital-to-analog converter 602 in the form of a "chip select" input, a "clock" input, a "data" input, a "load" input, and a "clear" input at pins 2, 3, 4, 5, 6, respectively. When the converter 602 receives the appropriate "select" signal at pin 2, it allows the digital data at pin 4 to be clocked at a speed determined by the clock pulse at pin 3. The resulting analog output signal 604 is generated at pin 8 and applied to a current drive circuit 606, which electromagnetically drives the meter's needle through the meter coil 614.

The current drive circuit 606 includes an operational amplifier 608 that receives the analog output signal 604 at its non-inverting input. The output of operational amplifier 608 is partially fed back to its inverting input to the extent controlled by a digital potentiometer 610 in a feedback loop whose resistance is set by data received on pins 1, 2 from processor 400. The digital potentiometer 610 is adjusted during the assembly process to provide the desired amount of meter attenuation (dampening), and the values applied by the CPU preserve this attenuation characteristic.

The meter coil 614 is shunted by an optical FET 612 which provides a short circuit across the meter coil when the device 10 is powered down. The optical FET thereby prevents electromagnetically induced currents in the meter coil due to physical oscillations of the meter when the device is turned off, which would cause the needle to move abruptly beyond the scale range, possibly destroying the needle.

During the factory calibration process, the processor operates under program control to display a query on the LCD display 20 (FIG. 1) so that the technician first moves the meter needle to the far left using the buttons 22B, 22C. When the meter needle overlaps the leftmost end "T" on the meter, the technician is instructed to press the select button 22A. The technician is then instructed to move the meter needle to the right with the buttons 22B, 22C until the needle is on SET, and then to press the select button 22A. In both cases, the signal value at each operative end is then used by the processor to calculate the volts/"T" required to move the pointer to the desired position.

Recording and playback features

According to another feature of the device, digital values representing the TA value, sensitivity and body resistance at all, or selected, instances of time during the examination procedure can be output to a personal computer or other storage device via an RS232 port or other convenient interface. In practice, these values are satisfactorily clocked out and stored at a rate of 120Hz as a 32-bit floating point resistance value, a 16-bit sensitivity value and a 16-bit microphone arm value. These obtained records can then be input back to the central processor unit of the device for demonstration, teaching or record review of the device. In fact, the device responds the same whether the values are generated in real time by examining the living body, or by receiving the values from a personal computer or other storage device.

Remote TA adjustment

Another feature of the apparatus 10 is its preferred ability to utilize the input of a remote TA optical encoder so that the operator can comfortably operate the apparatus 10 and make appropriate adjustments without disturbing the subject under examination. The remote TA optical encoder is electrically coupled to pins 29 and 30 of CPU400 to communicate with the CPU in the manner in which the encoder is coupled to knob 24 on the housing (fig. 1). When the CPU senses a signal on pin 29, it deactivates (deactivates) the optical encoder controlled by TA knob 24 by sending an appropriate signal from pin 35 to the encoder. The remote TA encoder then provides the range value to the processor until deactivated by the operator (deactivating).

While the foregoing description includes details which will enable those skilled in the art to practice the invention, it is to be understood that the description is illustrative in nature and that various modifications and changes in light thereof will be apparent to those skilled in the art. Accordingly, the invention herein is to be defined solely by the appended claims, and therefore, as the claims are interpreted in the breadth to which they are entitled.

Claims (9)

1. An apparatus for indicating a change in the resistance of a living body, comprising:

a resistance measurement circuit having an outer lead for sensing a resistance of a living body disposed across the outer lead;

amplifier means for generating an analog measurement signal indicative of the sensed resistance of the living body;

an indicator circuit for visually displaying a perceptible indicia representation of the sensed living body resistance;

a digital processing unit for digitizing and digitally processing the measurement signal so as to compensate for the effects of component aging, tolerances and temperature on the accuracy of the measurement signal; and

an indicator device, responsive to the processed measurement signal, for visually displaying a small perceptible indicia representative of the sensed change in bulk resistance.

2. The apparatus of claim 1, wherein the digital processing unit comprises:

means for replacing the bulk resistance with a plurality of resistance values for the amplifier means for sensing, the plurality of resistance values simulating various bulk resistance values,

means for digitizing and storing in memory a plurality of measurement signal values corresponding to the plurality of analog bulk resistance values,

means for interpolating between measurement signal values obtained for modeling the bulk resistance values, thereby quantifying expected measurement signal values for a plurality of additional bulk resistance values, an

Means for forming and maintaining a table relating expected measured signal values for each bulk resistance value based on the interpolation.

3. The apparatus of claim 2, wherein the replacing means comprises a multiplexer that places each of the plurality of resistors in the resistance measurement circuit in response to the plurality of select signal values.

4. The apparatus of claim 2, wherein the means for replacing comprises a multiplexer for arranging in the resistance measurement circuit components selected from the group consisting of: (1) an outer lead, and (2) each of the plurality of resistors.

5. Apparatus according to claim 2, including means for automatically activating the means for replacing upon power-up of the apparatus, thereby to form and maintain a table relating expected measured signal values for individual bulk resistance values based on said interpolation.

6. An apparatus according to claim 1, wherein the digital processing unit comprises means for subtracting the monitored bulk resistance value from a user adjustable base value, thereby generating an adjusted measurement signal as the measurement signal to the indicator means,

a manually positionable device operable by a user to adjust the base value, an

An optical encoder coupled to the manually positionable device for generating the base value as a function of the position of the manually positionable device.

7. The device of claim 6, wherein the manually positionable means comprises a manually rotatable knob, and

the optical encoder includes a rotatable spindle coupled to the knob and means for generating a digital output signal indicative of the position of the spindle.

8. The apparatus of claim 7, including means for adjusting the magnitude of the digital output signal from the optical output encoder prior to subtracting the monitored bulk resistance, based on the formula:

wherein

TA is the scale position of the manually positionable device, an

R_TAIs the value of the output signal.

9. The apparatus of claim 6, comprising:

means for repeatedly sampling the in vivo resistance value;

means for subtracting each sample value from the adjusted base value to obtain a measurement signal; and

sensitivity adjustment means for coupling the measurement signal to the indicator means,

the sensitivity adjustment means comprises means for multiplying the measurement signal by a gain factor which is dependent on the position of the manually positionable means.