There always seems to be some medical widget at my local Walgreen's that gets the wheels of curiosity turning. This time it was Omron's $50 (or less) HBF-306 personal body fat meter. It's not ordinarily the kind of device people pick up for fun; but along with technical curiosity about how it worked, I thought it might be good to check my progress in keeping off the pounds. As it turns out, the device functions on a surprisingly simple premise with correspondingly Spartan—and mostly analog—electronics used for implementation.
The HBF-306 resembles a handheld game controller, with two handles gripped by the subject during the measurement process and a center "pod" containing measurement electronics. The handles contain two electrodes each, formed by pre-sprung metal encircling the plastic enclosures. Test subjects wrap their hands around the grips, making contact with the electrodes. AAA batteries power the device, further evidence of the elegantly simple electronics within.
With a retail price as low as $30, there's little budget for fancy medical-grade monitoring. Traditionally, accurate body composition measurement relies on dual X-ray absorptiometry or hydrostatic weighing, which records the apparent weight of a subject underwater with the lungs expelled. Measurements of body folds and other physical characteristics are also common. The Omron unit clearly lacks X-rays, a test pool or a set of calipers, so something else is going on.
With a bit of poking around on Omron's Web site and through the patent literature, it becomes clear that the HBF-306 relies on bioelectrical impedance analysis. BIA measures the subject's body impedance, recorded under carefully controlled measurement conditions, and combines the results with user input and correlated lab-gathered data to estimate body fat percentages.
The impedance to be measured is a complex matter in and of itself (pun intended). The human body's conduction has both a resistive and reactive component, with the resistance being driven by water-bearing tissue and the capacitive reactance stemming from the insulating membranes around each cell. Lean tissues are characterized by a relatively low resistance, with fat and bone being poor conductors, given their low retention of fluids and electrolytes. Determining a precise RC model to mimic the human body has been a subject of debate, but to a first order, the impedance can seemingly be modeled as a parallel RC circuit.
Measurements begin by having the subject enter sex, height, weight and age. The device adjusts for variables that affect impedance and its ultimate correlation to body fat percentage. From what I could determine, Omron accumulated a database of fat percentages with a large and presumably varied group of test subjects using the traditionally accurate means mentioned previously. By knowing lab-grade numbers for each person and measuring the impedance corresponding to each case, Omron developers created the database. The assumption is that any given result with the HBF-306 can be assigned a percentage of body fat corresponding to more rigorously acquired lab data (once corrected according to the user-specific inputs).
The method isn't perfect and is subject to other variables. The hand contact during measurement must be steady, and Omron's patents indicate a narrow range of time-lapsed readings within which impedance must be seen, or the test will abort. The instructions also indicate that the user must have both arms extended to 90° angles in a standing position and with feet spread slightly apart. These requirements all speak to the need for a predictable body geometry that doesn't cause electrical shunts, which might poison the measurement and correlation. Finally, because the body's impedance varies over the course of the day with fluid intake and activity, Omron outlines specific times and circumstances under which reliable readings may be achieved. Adjustments for musculature and nominal subject conditioning ("Athlete Mode") are also possible.
The Omron device essentially comprises a simple impedance meter and a little database. Because complex impedance is to be measured, an ac signal must be used; here, Omron implements a constant-current 500-µA, 50-kHz signal applied across two of the four electrodes—one for the left hand, the other for the right. The two remaining electrodes serve as the pickups to sense the impedance by measurement of the voltage rise and phase shift caused by the equivalent RC circuit of the body.
Electronics for the analog-meter front end are limited to an STMicroelectronics TS924 quad op amp, likely used partly for the oscillator, partly for the detected voltage amplifier to determine resistive values and partly for implementation of a demodulator, which serves for phase detection. Two Texas Instruments SN74LV4052 four-channel analog mux/demux chips and a single TI SN74LV4066 quad bidirectional analog switch create what might be an analog signal fabric, suggestive of a reversing circuit to make measurements in two directions. That bit is a little unclear.
(Click on image to enlarge)
The biggest IC is a Renesas M38223M4H 8-bit microcontroller with 16-kbyte ROM and 512-byte RAM--nothing new, given a die-level 1986 copyright date. The microcontroller has A/D circuit peripherals for digitizing demodulator and amplifier outputs, along with the digital smarts to do measurement lookup and correlation of body fat percentage.
The user interface of front-panel switches and the direct-drive monochrome LCD are also in the line of duty for the Renesas part. A 1-kbit Seiko Instruments serial E2PROM supplements the controller's on-chip memory, perhaps storing code, correlation data or user measurement results.
Along with a minimalist component set, a CEM circuit board provides a low-cost substrate on which to mount the ICs and 80-odd discrete and passive components. The CEM board uses cost-saving silver-paste filled via holes with a sealing overprint for oxidation prevention.
All in all, a few bucks for chips and support components and less than $10 for display and casing add up to a roughly $15 parts cost that-—combined with some clever software—gives folks a simple and inexpensive way to estimate body composition.
I did OK on my own test, but didn't exactly knock it out of the park; continued exercise and intake moderation would probably still be a good idea. n
David Carey is president of Portelligent, the Austin, Texas, company produces teardown reports and related industry research on wireless, mobile and personal electronics (www.teardown.com).
