Abstract:
A circuit for measuring body impedance comprising a voltage source having a predetermined frequency and a current source; a first pair of electrodes adapted to receive one portion of the body for applying said current source to said body; a second pair of electrodes adapted to receive another portion of the body for sensing a voltage therebetween; a comparator having first and second inputs and an output for producing an output signal based on signals applied to said first and second inputs; an instrumentation amplifier arrangement coupled to one of said comparator inputs and responsive to the voltage across the second pair of electrodes for providing an input signal to said first comparator input; a variable resistor coupled to the second input of said comparator and responsive to said voltage source for developing a first voltage signal applied to said second comparator input; and a processor responsive to the output signal of the comparator for controlling the resistance of the variable resistor to cause a corresponding change in said first voltage, whereby when said voltage at said first input corresponds to the voltage at said second input of said comparator; said comparator output signal transitions from a first state to a second state to cause the processor to terminate resistor adjustment, whereby the body impedance value corresponds to the resistance value of the variable resistor.

Description:
FIELD OF INVENTION 
     The present invention relates to measurement devices in general and more particularly to a method and apparatus for measuring impedance of a biological organism. 
     DESCRIPTION OF PRIOR ART 
     There exist in the prior art numerous methods and apparatus for measuring or determining body impedance and body composition (i.e. body fat). 
     For instance, U.S. Pat. No. 4,144,763 discloses a method of measuring body fat using Boyle&#39;s law. U.S. Pat. No. 4,831,527 discloses a system where a fat-to-lean ratio is measured by having a subject stand on a platform and raise his or her heels and then allowing the weight to fall near a transducer to produce a force. The subject&#39;s stomach and other flesh continue to move in a downward direction after skeletal motion has stopped, resulting in a downward force which is registered as a data peak and measured by a computer. A technique for measuring body fat by immersing the subject in a liquid is disclosed in U.S. Pat. No. 5,052,405. U.S. Pat. No. 5,105,825 teaches a method of measuring body fat by transferring controlled volumes of gas between two chambers and measuring pressure while U.S. Pat. No. 5,335,667 measures body composition using bioelectric impedance measurements. 
     In U.S. Pat. No. 5,372,141 a body fat calculator is described which basically measures body fat by the impedance of the network and uses various algorithms after measuring the impedance of the body. 
     A physical fitness evaluation system is disclosed in U.S. Pat. No. 5,435,315 which, besides measuring body fat, can calculate the overall fitness of the individual. This means for measuring body fat uses a large caliper in conjunction with a variable resistor. Pat. No. 5,579,782 shows a high frequency signal generator which is used to measure impedance. The body fat ratio is calculated from those measurements. 
     U.S. Pat. No. 5,595,189 relies on measuring body fat by determining a subject&#39;s weight using two gases at different densities. The weight of the person is measured with a precision balance and the person&#39;s body fat is determined by the density of both of the gaseous environments. 
     U.S. Pat. No. 5,415,176 issued on May 16, 1999 entitled APPARATUS FOR MEASURING BODY FAT, to Sato et al. discloses a method of determining body impedance using two pairs of electrodes placed at the toes and heels of a person, applying a constant current to the toe electrodes, measuring the voltage at the heel electrodes, and calculating the impedance as the ratio of the measured voltage over the constant current. The body fat is then calculated from the body impedance. U.S. Pat. No. 5,611,351 issued on Mar. 18, 1997 entitled METHOD AND APPARATUS FOR MEASURING BODY FAT, to Sato et al. is a continuation of U.S. Pat. No. 5,415,176 and calculates body fat from the impedance value and the person&#39;s height and weight. 
     However, numerous problems and limitations are encountered with the application of each of the above systems and methods for determining body impedance (or body fat). Such limitations include accuracy and reliability, portability and ease of use, size and circuit complexity, and response time. Accordingly, a method and apparatus for determining body impedance which obviates these shortcomings is highly desired. 
     SUMMARY OF THE INVENTION 
     A circuit for measuring body impedance comprising a voltage source having a predetermined frequency and a current source; a first pair of electrodes adapted to receive one portion of the body for applying the current source to the body; a second pair of electrodes adapted to receive another portion of the body for sensing a voltage therebetween; a comparator having first and second inputs and an output for producing an output signal based on signals applied to the first and second inputs; a differential amplifier arrangement coupled to one of the comparator inputs and responsive to the voltage across the second pair of electrodes for providing an input signal to the first comparator input; a variable resistor coupled to the second input of the comparator and responsive to the voltage source for developing a first voltage signal applied to the second comparator input; and a processor responsive to the output signal of the comparator for adjusting the resistance of the variable resistor to cause a corresponding change in the first voltage, whereby when the voltage at the first input corresponds to the voltage at said second input of said comparator, the comparator output signal transitions from a first state to a second state to cause the processor to terminate resistor adjustment, whereby the body impedance value corresponds to the adjusted resistance value of the variable resistor. 
     An apparatus for measuring body composition in a patient&#39;s body comprising a measuring station for measuring impedance between the patient&#39;s feet and an operating circuit coupled to the measuring station, the measuring station including a base on which a patient can stand, first and second pairs of electrodes mounted on the base, the first pair of electrodes adapted to receive one portion of the body for transmitting a drive current thereto; the second pair of electrodes adapted to receive another portion of the body for sensing a voltage therebetween; a comparator having inputs for receiving first and second input signals and an output terminal for providing an output signal; an amplifier arrangement responsive to the voltage across the second pair of electrodes for providing one input signal to the comparator; a digital potentiometer coupled between the first pair of electrodes and the second input of the comparator for providing a second input signal thereto; and a processor responsive to the output signal of the comparator for producing a control signal to the potentiometer for adjusting the resistance of the potentiometer to cause a corresponding change in voltage of the second input signal; whereby when the voltage of the second input signal corresponds to the voltage output produced by the amplifier arrangement, the comparator output signal transitions from a first state to a second state indicative of a null condition, whereby the body impedance value corresponds to the resistance of the potentiometer; and an operating circuit coupled to the measuring station for determining body composition as a function of body impedance value. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A illustrates a schematic circuit for measuring body resistance or impedance according to an embodiment of the present invention. 
     FIG. 1B illustrates a schematic circuit for measuring body resistance or impedance according to an alternative embodiment of the present invention. 
     FIG. 1C is a more detailed schematic circuit for measuring body resistance or impedance as shown in FIG.  1 B. 
     FIG. 1D is a circuit schematic representation of a typical voltage divider construct of a potentiometer. 
     FIG. 2 is a schematic view of a body impedance or body fat measuring device incorporating the body resistance circuitry of FIG. 1 according to an embodiment of the present invention. 
     FIG. 3A is a block diagram of the major functional components of the body impedance or body fat measuring device according to an embodiment of the present invention. 
     FIG.  3 B and FIG. 3C represents a more detailed schematic circuit of the body fat scale according to an embodiment of the present invention. 
     FIG. 4 is a flow chart depicting the method for determining body resistance or impedance according to the present invention. 
     FIG. 5 is a flow chart illustrating the operation of the body fat scale according to an aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Before embarking on a detailed discussion, the following should be understood. As is well known, the use of body impedance analysis (BIA) to estimate body composition is based on the volume conductor theory. This theory suggests that the volume of a conductor can be determined by its impedance to current flow. The impedance of a conductor is proportional to its length and is inversely proportional to its cross-sectional area. Thus, the impedance Z of a conductor may be characterized by the equation Z=w*(L/A) where w is specific impedance, L is conductor length, and A is cross-sectional area of the conductor. Similarly, the volume V of a conductor can be calculated by measuring the length and the specific impedance of the conductor (V=w*(L*L/Z). Lean Body Mass (LBM), defined as total body mass less fat body mass, may be estimated since it is known that LBM is a function of total body weight. Once LBM is known, the percentage of body fat (% BF) can be determined according to the equation % BF=100*(Wt−LBM)/Wt where % BF is percent body fat, LBM is lean body mass, and Wt is total body weight. 
     The present invention provides several advantages over the prior art. First the present invention eliminates the need for handgrips which are often used to determine bio-impedance. Second, the body impedance measurement apparatus and method of the present invention does not require measurement of phase, reactance values, nor require the measurement of multiple frequency signals and voltages to arrive at a body impedance. Still further, conventional devices which drive the body with a known reference current and which use a second pair of electrodes to measure a voltage therebetween require circuitry for demodulating, rectifying, and performing A/D conversion of the measured voltage signal in order to arrive at a body impedance estimate (Z=V/I). The present invention eliminates the need for such complex and costly conversion circuitry through the application of a synthesized reference value and matching of an unknown value (Z) to the reference value. The invention thus provides a simple, fast, accurate, and cost-effective way to determine body impedance and hence body fat. 
     Referring now to FIG. 1A, there is depicted a schematic view of an electronic circuit  10  for measuring body resistance or impedance according to the present invention. The circuit comprises a sine wave (steady state) voltage source  11  of about 50 kHz (kilohertz) which is converted to a current drive source of less than 1 mA (milli ampere) at 50 kHz using a conventional voltage to current converter circuit  30 . In the preferred embodiment the current is a constant current of approximately 800 uA (micro amperes). The low constant current source applied at this frequency is intended to reduce skin capacitance. Drive electrodes  21 , 22  interface to a first portion of the body (e.g. are coupled to the toes) and are in the feedback loop of amplifier  30 . This same 50 kHz sine wave voltage source is also fed to terminal  44  of digitally controlled potentiometer  40 . The potentiometer has terminal  45  coupled to ground or reference potential while the center tap  46  or wiper position for this potentiometer is selected digitally via microcontroller  50 . The tap of the potentiometer is n times the voltage of the 50 kHz oscillator  11  (0&lt;n&lt;1) and is applied to the input terminal  62  of comparator  60 . In a preferred embodiment, the digital potentiometer is stepped in increments of 10 ohms so as to range between 0 and 1000 ohms (1K). Thus, step n ranges from 0 to 100 steps in 10 ohm increments. 
     FIG. 1D shows a circuit schematic representation of a typical voltage divider construct of the potentiometer wherein: 
     
       
         Ra+Rb=Rtotal=Fixed=1000 ohms (1kohm);  
       
     
     
       
         [ Rb/ ( Ra+Rb )]* Vi=Vout    
       
     
     
       
           n* 100= Rb.   
       
     
     The second pair of electrodes  25 ,  26  are used as voltage sense electrodes where no current flows. The electrodes interface to a second portion of the body (e.g. are coupled to the heels). The potential across these electrodes is applied as input to a standard 3 op-amp instrumentation amplifier (differential amplifier) arrangement  70 . The amplifier arrangement  70  comprises first and second buffer amplifiers  701  and  703 , each buffer having its non-inverting input terminal coupled to electrodes  25  and  26 , respectively. The outputs of each of buffer amplifiers  701  and  703  pass through respective resistors R 1  and R 3  and are fed into terminals  708  (non-inverting input) and  709  (inverting input) of differential amplifier  710 . Resistor R 2  and capacitor C 2  are serially coupled between node  705  (non-inverting input) and ground potential and operate to filter noise components and to protect the bias point of differential amplifier  710  (V/2). The output signal  72  of the differential amplifier arrangement is applied to terminal  64  of comparator  60  through coupling capacitor C 3 . Comparator  60  accepts as an input at terminal  62  the voltage signal  42  developed at the center tap of digital potentiometer  40 . The comparator compares the microcontroller-selected voltage signal  42  developed at the tap position of digital potentiometer  40  with the voltage signal  72 . The comparator outputs a signal  68  at output terminal  66  based on the magnitude of the two input signals  42  and  72 . 
     The comparator output  68  from output terminal  66  indicates whether the center tap of the potentiometer  40  exceeds the voltage signal  72 . The comparator output is, in the preferred embodiment, a binary output signal corresponding to either a “high” (binary 1) or “low” (binary 0) state. The output signal  68  is applied via line  90  to microcontroller  50 . If signal  42  is greater than signal  72 , the output signal  68  from the comparator is “high”. In a preferred embodiment, this “high” signal indicates to the microcontroller to provide a control signal to decrease the resistance at the center of digital potentiometer  40  so as to decrease the voltage of signal  42 . The digital potentiometer comprises n steps of a predetermined increment (for example 10 ohms). In this manner the resistance of the potentiometer and hence voltage signal  42 , is incrementally adjusted in response to the comparator output based on the step count n and the increment value. That is, the microcontroller, in response to output signal  68 , sets or adjusts the center tap of the digital potentiometer to a different resistance value each iteration in order to find the point where the output of comparator  60  experiences a state transition. At the point where the voltage signal  42  equals (or is less than) voltage signal  72 , the output of the comparator transitions from a “high” to “low” value. When the output of the comparator changes state (from “high” to “low”, for example) the comparator is effectively nulled, and the microcontroller in response to detection of a state change terminates further adjustment of the potentiometer resistor value. The bio impedance Z is then determined directly as the number of steps or adjustments n, times the value of the center tap resistance increment (e.g. 10 ohms) of the digital potentiometer  40 . This corresponds to the voltage value of the sense electrodes divided by the constant current at the drive electrodes  21 ,  22 . 
     Note that while the above description is predicated on a “high” to “low” transition of the comparator and decrementing the voltage signal  42  applied at comparator terminal  62  through potentiometer adjustment, it is understood that a “low” to “high” transition detection and incrementing of the voltage signal  42  through adjustment of the potentiometer resistance is of course, also contemplated. 
     From the above discussion one can see that the microprocessor in combination with the digital potentiometer and comparator operate as a closed-loop servo mechanism to arrive at a bio-impedance based on the drive current and sensed voltage in a very short time. In operation, the aforementioned servo structure advantageously arrives at the body impedance in substantially less time than other techniques (approximately one second), and without the use of additional electronic circuitry, such as analog to digital converters. This is because the impedance is measured directly. The circuit of the present invention thus enables one to measure bio impedance with increased speed and accuracy and with a minimum of circuit components. 
     An alternative embodiment of the aforementioned impedance measurement circuit for determining bio impedance is illustrated in FIG.  1 B. As shown in FIG. 1B, the potentiometer  40  is coupled between differential amplifier arrangement  711  and body electrodes  21 ,  22  within the feedback loop of the circuit such that the drive current I through the body electrodes  21  and  22  also passes through the potentiometer  40 . The circuit components  701 ,  703 , and  710  coupled to voltage sense electrodes  25  and  26  (in electrical contact with the heels of a person) for generating voltage signal  72  for input to the inverting input of comparator  60 , have been discussed above with reference to FIG. 1A, and thus will not be detailed further. The pair of electrodes  21  and  22  are in electrical contact with the toes of the left and right feet, respectively and receive the constant 50 kHz drive current I generated by voltage to current converter arrangement  30  acting on sinusoidal voltage signal  11 . The voltage signals at terminals  45  and  46  indicative of the voltage drop across potentiometer  40  are applied to differential amplifier  711  so as to produce signal  42  at the non-inverting input of comparator  60 . As in FIG. 1A, the output  68  of the comparator represents a binary “high” or “low” signal which is fed back to micro controller  50 . In response to the output signal  68 , the micro controller provides a digital control signal to potentiometer  40  to adjust the potentiometer resistance a predetermined amount. If signal  42  is greater than signal  72 , the output signal  68  from the comparator is “high”. In a preferred embodiment, this “high” signal indicates to the microcontroller to provide a control signal to decrease the resistance of digital potentiometer  40  so as to decrease the voltage of signal  42 . The digital potentiometer comprises n steps of a predetermined increment (for example 10 ohms). In this manner the resistance of the potentiometer and hence voltage signal  42 , is incrementally adjusted in response to the comparator output based on the step count n and the increment value. That is, the microcontroller, in response to output signal  68 , sets or adjusts the digital potentiometer to a different resistance value each iteration in order to find the point where the output of comparator  60  experiences a state transition. At the point where the voltage signal  42  equals (or is less than) voltage signal  72 , the output of the comparator transitions from a “high” to “low” value. When the output of the comparator changes state (from “high” to “low”, for example) the comparator is effectively nulled, and the microcontroller in response to detection of the state change, terminates further adjustment of the potentiometer resistor value. The bio impedance Z is then determined directly as the number of steps or adjustments n, times the value of the tap resistance increment (e.g. 10 ohms) of the digital potentiometer  40 . 
     FIG. 1C is a detailed schematic diagram which includes specific circuit components which perform the actual functions depicted in the high level schematic circuit shown in FIG.  1 B. Dashed lines and corresponding reference numerals have been included within the actual circuit implementation of FIG. 1C to better illustrate those parts corresponding to that described within FIG.  1 B. 
     For example, referring now to FIG. 1C, the constant voltage source  11  is formed by a 4 MHz oscillator connected to the microprocessor  50  to develop a square wave voltage signal at approximately 50 KHz (actually 56 KHz) which is acted on by the low-pass filter arrangement  12 ,  13   14 . The low-pass filter arrangement filters the first, third and fifth harmonics of the square wave voltage signal to provide a sinusoidal voltage signal  11  of approximately 50 kHz. The voltage to current converter arrangement  30  includes circuit component designated U 2 A and having part number JRC2112 as manufactured by Japan Radio Corporation (JRC). Resistor R 20 , which is an arbitrary schematic designation, has a value of 2000 ohms (2K) and corresponds to set resistor Rs depicted in FIG.  1 B. Calibration pads CAL 1 , CAL 2  are illustrated in FIG. 1C to tune and calibrate the circuit. The differential amplifier arrangement  711  comprises amplifiers designated as U 3 A and U 3 B coupled to digital potentiometer  40  connected within the feedback loop of current electrodes  21  and  22 . The second pair of electrodes  25 ,  26 , are used as voltage sense electrodes where no current flows. The potential across these electrodes is applied as input to instrumentation amplifier arrangement  70  consisting of circuit components designated as U 3 C and U 3 D and associated resistor and capacitor network circuitry. The circuit diagram of FIG. 1C also includes rectifier circuitry  712  to provide rectified DC voltage signals to the input terminals of comparator  60 . 
     The micro-controller having exemplary part number designation KS57C2308 as manufactured by Samsung and arbitrary identifier U 7  is coupled to digital potentiometer  40  (U 4 , exemplary part number X9C102) for providing chip select or enable line (CS), step (INC) and direction (U/D) associated with the resistance of the potentiometer. As previously mentioned, in response to the signal at the output of the comparator, the microcontroller provides a control signal to adjust the resistance of the potentiometer by a predetermined amount. This, in turn, causes a corresponding change in the level of voltage signal  42  developed at the input of the comparator. The comparator output is again fed back to the microcontroller which adjusts the resistance value based on the output of the comparator. Thus, the micro controller operates as a servo mechanism by adjusting the resistance of the potentiometer in a stepped amount in response to the comparator output signal to determine body impedance. Note that, while the above servo structure has been described with reference to a potentiometer, it is also contemplated that another electronic device, such as an A/D converter or other device which associates a voltage signal with a resistance value, may also be utilized. 
     The electronic circuit for carrying out the body impedance measurement method may be placed within the housing of an electronic body weighing scale  100  as depicted in FIG.  2 . Referring now to FIG. 2, the electronic body weighing scale comprises a platform  118 , supports  128  positioned beneath the platform, and a display assembly  122 . Each support  128  contains a piezoresistive sensor element or load cell (not shown) that changes an electrical parameter (e.g. resistance) in response to a weight applied to the platform. Conductors (not shown) operate to couple each of the sensors to the display assembly  122 . The display assembly is attached to the platform so as to provide a convenient interface for user input and for display purposes. 
     The display assembly is located at the top front portion of the scale platform and comprises a display portion  124  such as an LCD display, for example, for viewing the results of a measurement. The assembly further includes an interface portion  126  for entering user input data. Interface portion  126  includes push buttons sized sufficient to enable input via a user&#39;s toe. In a preferred embodiment, a protective member  150  is hingedly connected to the platform to cover and protect the display area when in the down position, and to provide access to the display area in the up position. The protective member is preferably made of a durable material such as plastic having a transparent portion  155  for viewing the display portion when in the closed or down position. 
     Referring again to FIG. 2, the platform is formed of an insulating material, and includes a pair of conductive electrodes  25 ,  26  for a patient&#39;s heels and a pair of electrodes  21 ,  22  for a patient&#39;s toes. The electrodes are mounted symmetrically with respect to the width of the platform. This allows the patient to stand upright so that the feet are not touching. In a preferred form of the invention, the electrodes  25 ,  26  and  21 ,  22  can be at least partially, if not wholly, formed of a conductive resin, such as a carbon filled resin. Alternatively, a conductive metal coating on a plastic, chroming or a conductive paint can be utilized. The electrodes are set sufficiently far apart from one another so as to eliminate the influence of a potential distribution. 
     When the patient&#39;s heels and toes are contacted by corresponding electrodes  25 ,  26  and  21 ,  22 , and the scale is activated, the oscillator circuit  11  generates a 50 kHz constant current source at the drive electrodes  21 ,  22  and the impedance measurement process is initiated. The sensed voltage at the heel electrodes  25 ,  26  is applied through the instrumentation amplifier arrangement  70  (FIGS.  1 A,B,C) and output signal  72  is applied to the input terminal  64  of comparator  60 . Note that, while the preferred embodiment has been described wherein the body impedance is determined using electrodes positioned at the heels and toes of a patient, the present invention is not limited to such an application. It is contemplated that the body impedance measurement technique described herein may be utilized by connecting the electrodes to other body portions. For example, body impedance may be measured through a first pair of electrodes disposed within a set of hand grips, for example, for contacting a patient&#39;s hands, and a second pair of electrodes in contact with portions of a patient&#39;s feet. Similarly, a pair of electrodes coupled to a belt fastened around a patient&#39;s waist and a second pair of electrodes coupled to a patient&#39;s arms, chest, legs or feet, for example, may also be implemented to determine the body impedance. Numerous other electrode locations and positions for placing the electrodes onto body portions to measure the body impedance are contemplated. 
     The scale of FIG. 2 further includes a body impedance/body fat activation and user selection switch  200  located at the bottom front portion of the scale platform for activating the body impedance and body fat determination process. Depressing the switch to either the first position (1) or second position (2) activates the body impedance circuitry and causes the microprocessor to retrieve the input characteristics of the particular user associated with the selected switch position. 
     As shown in the block diagram of an exemplary electrical circuit for the weight and body fat scale in FIG. 3A, force sensor module  110  comprising each of the piezoresistive sensors within the platform operates to sense the weight of a user for generating a signal indicative of the relative amount of weight sensed. Analog electronics module  120  include calibrating circuits which enable each of the individual sensor elements to provide a response that reflects an accurate proportional share of the total weight applied to the platform and a combining junction for combining each of the individually calibrated piezoresistive sensor signals. The analog electronics circuit is coupled to sensor module  110  via conductors. Analog to digital converter  130  operates to convert the analog calibrated signal into a digital signal representation for input into processor (microcontroller)  50 . 
     Processor  50  comprises a digital microprocessor controller having a clock oscillator  170  operating at approximately 4 MHz to generate a 50 KHz signal, memory  160  and user input interface  126  for accepting data from a user. The digital microprocessor includes software programs or algorithms which operate to calculate body fat based on the determined body weight, the determined body resistance (module  10 ) and patient data, such as height, age, and gender, for example. 
     FIG.  3 B and FIG. 3C represents a more detailed schematic diagram which includes specific circuit components which perform the actual functions depicted in the high level schematic circuit shown in FIG.  3 A. FIG. 3B also illustrates the detailed circuit schematic of the body impedance measurement circuit shown in FIG.  1 C. Dashed lines and corresponding reference numerals have been included within the actual circuit implementation of FIG. 3D to better illustrate those parts corresponding to that described within FIG.  3 A. 
     A diagram of the impedance measurement routine of the microprocessor is depicted in FIG.  4 . Referring to FIG. 4, the resistance or impedance measurement process is initiated at block  400 . The microprocessor provides a digital control signal to cause the digital potentiometer tap to step to the highest (block  410 ). A wiper_number variable is incremented (wiper_number=wiper_number+1) and stored in memory within the microprocessor (block  420 ). The wiper_number is a RAM address within the processor. As shown in block  430 , the output signal of the comparator is then compared with a threshold value to determine if the comparator output is at a “high” or “low” state If the output signal of the comparator corresponds to a “high” level, the microprocessor causes the potentiometer tap to step down a predetermined level or increment (block  450 ), the wiper_number variable is incremented (block  460 ) and the comparator output is again checked to determine if it is at a “high” or “low” level (block  470 ). In a preferred embodiment, the step increment is 10, so that the tap resistance decreases by the step value of, for example, 10 ohms. However, any number of increments or step values is contemplated depending on the required accuracy. 
     If, however, after the first iteration, the microprocessor detects a “low” signal at the comparator output at block  430  (FIG.  4 ), a measuring error  440  has occurred and the microprocessor returns to block  400  and makes another attempt to measure the body impedance. 
     The process of stepping the digital potentiometer tap resistance, incrementing the wiper_number, and determining the output level of the comparator, is repeated until the microprocessor detects a state transition of the comparator output from a “high” to “low” state ( 500 ). Upon detection of the state transition, the variable wiper_number is retrieved from memory and multiplied by the step increment (in this case 10 ohms) to arrive at the calculated body resistance (block  510 ). The body resistance is stored in memory and the processing sequence is terminated. 
     Note that, as shown in decision block  480 , the microprocessor also includes error detection and processing circuitry to ensure correct processing of the data. In block  480 , the microprocessor compares the value of the variable wiper_number with a predetermined upper limit (for example 100). If the variable value equals (or exceeds) this limit, a measuring error  490  has occurred and the microprocessor resets and restarts the measurement process  400 . If, on the other hand, the upper limit has not been reached and the comparator output has not changed state, processing returns to block  450  to continue the stepping of the digital potentiometer resistance. 
     From the above description, it is apparent that the present technique is a form of substitution principle where the voltage at the sense input is determined using a successive approximation or substitution principle. 
     The equations are as follows: 
     
       
           I  body= V (50 kHz)/ RS    
       
     
     
       
           V  sense= n×V (50 kHz) (based upon comparator detection) 
       
     
     Therefore, the body resistance is given as: 
     
       
           R  Body= V  sense/ I  Body= n×R  set. 
       
     
     The microprocessor also includes algorithms which operate to calculate a patient&#39;s body fat as identified below from the patient&#39;s measured weight using the weight determined from the electronic scale measurement, and the individual&#39;s body impedance as determined above. For example, the microprocessor may utilize the determined values of body weight and body impedance or resistance determined via the aforementioned process, in combination with user input data such as height, age and gender, to calculate lean body mass (LBM) according to the equation: 
     
       
           LBM=K+A (ht) 2   −B ( R )+ C (Wt)− D (age)+ E (Male), 
       
     
     where 
     Ht=user height (centimeters, cm); 
     R=determined body impedance (ohms); 
     Wt=calculated total body weight (kilograms, Kg); 
     Age=user age; 
     Male=0 if male, else 1; 
     and where A, B, C, D, and E are coefficients stored within the microprocessor memory and K is a constant corresponding to the Y-intercept. In a preferred embodiment the coefficients are A=0.00104, B=0.036, C=0.17, D=0, and E=−3.6 
     Fat mass (FM) may then be calculated to be 
     
       
         
           FM=Wt−LBM; 
         
       
     
     and percentage of body fat (% BF) is then obtained as 
     
       
         %  BF =( FM/Wt )*100. 
       
     
     Each of these values may then be displayed to the user via the display unit  122  (see FIG.  2 ). 
     FIG. 5 illustrates an exemplary flow diagram associated with user interaction and display portion of the body composition scale  100  according to a particular embodiment of the invention. Note that character references labeled “A”,“B”,“C” . . . through to “O” associated with blocks  730 ,  735 ,  740 ,  745 ,  750 ,  755 ,  757 ,  759 ,  760 ,  762 ,  764 ,  768 ,  770 ,  780 ,  785 , and  790 , respectively, are used to indicate correspondingly labeled display results to be displayed via LCD display unit  122 . Referring to FIG. 5, once a user&#39;s personal data has been preprogrammed into the scale (module  700 ), the user may then press a user selection switch  200  at the bottom of the platform to identify himself as the appropriate user (e.g. user  1  or user  2  on unit  200 ). Upon initiation (module  710 ), display screen  122  then displays the user number, gender, age and height of the individual (module  720 ). If the data needs to be corrected (module  722 ) or if one&#39;s personal data was not preprogrammed into the scale, the user depresses the set key  350  and user key  310  as appropriate to identify the user to the unit and to enter the relevant physiological data, including height (module  750 ), age (module  760 ), and gender (module  770 ). Depressible button  320  on the keypad operates to toggle between height and age parameter data while button  330  provides for gender selection. Up/Down arrow keys  340  and  350  provide the ability to increase or decrease these parametric values. Set key  350  operates to enter the data into memory for later retrieval and or analysis. When all of the user-related physiological data is entered (module  780 ), display  122  will shut off and the user will provide the appropriate user selection for recalling his or her personal data. The user then steps onto the scale (module  790 ) and the scale will then calculate the current weight. With the user&#39;s feet positioned on the corresponding electrodes as previously described, the body impedance circuit will also be activated to determine the body impedance and software programs will operate on the determined body weight and body impedance to calculate the percent of body fat. The weight reading will then be displayed for a predetermined time period of, for example, a few seconds (module  800 ) on LCD display  122 . Upon expiration of the predetermined time interval, the system will display the calculated % body fat. The user then steps off the scale which causes the system to power down (module  810 ) If an error occurs during the calculation process (module  820 ), the user may be given the option as to whether or not another weight reading (or body fat reading) is to be made. Note that, by not depressing activation button  200 , the scale according to the present invention operates as simply a body weight scale and does not determine body fat. 
     It is contemplated that various algorithms may be utilized to arrive at lean body mass, fat mass and body fat composition values based on well known formulas and equations. It is to be understood that one skilled in the art may make many variations and modifications to that described herein. For example, in a preferred embodiment, the potentiometer may be implemented as a half bridge, temperature compensated device for providing a substantially temperature invariant voltage. Alternatively, a device having associated resistance and voltage values selectable by the microcontroller such that the resistance associated with the signal value that causes the comparator output to be nulled represents the body impedance may also be utilized. Numerous other implementations of a potentiometer or such device are contemplated. All such variations and modifications of the above components and structures are intended to be included within the scope of the invention as defined in the appended claims.