I live for Fridays. That's because I usually spend that day hiking through the San Diego badlands with an eclectic assembly of iconoclasts, including several brilliant technologists and some of my dearest friends. We connect through our love of instrumentation and our shared passion for developing inexpensive solutions to various experimental challenges. This common interest leads to friendly rivalries, the results of which often feed this column.
Take for instance the problem of measuring extremely tiny masses. George Schmermund developed a fantastic approach, which I described in these pages in June 1996. George extracted the coil and armature from a discarded galvanometer and mounted them upright, so that the needle of the meter moved in a vertical plane. He then connected the coil to a variable voltage and adjusted it until the needle was exactly horizontal. A tiny mass of known weight placed at the end of the needle pulled it downward. George then increased the voltage until the arm returned to its starting position. Because a heavier mass required a proportionally larger increase in voltage to balance it, the change in voltage indicated the weight of a sample. George's electrobalance was able to weigh masses as small as 10 micrograms (that is, 10 millionths of a gram).
That achievement was stunning enough for me, but recently the organizer of our weekly outings, Greg Schmidt, realized that even this amazing performance could be improved on. Greg's design eliminates the need to adjust the needle manually: the balance automatically zeros (or "tares") and levels itself, and it can continuously track how an object changes in mass¿the rate at which a single ant loses water through respiration, for instance. The result is an extremely versatile electrobalance with microgram sensitivity that can be built for less than $100.
Here's how it works. Greg took George's basic design and added an inexpensive microcontroller (a small computer with its central processing unit and memory all on a single chip), instructing it to send 2,000 weak current pulses through the coil each second. The inertia of the armature and needle prevents them from responding to each short pulse, so the deflection reflects the average current in the coil. The individual pulses do, however, seem to be large enough to vibrate the bearings of Greg's galvanometer. He believes that this slight jitter reduces "stiction," the tendency of a bearing to lock in place when it is not moving. This effect seems to account for why an inexpensive meter like his can respond to the tug of such tiny masses.
Greg didn't design his circuit to reduce stiction, though. This feature turned out to be an unforeseen benefit of using "pulse width modulation" to control the average current sent through the coil. With this scheme, the time between successive pulses is kept the same, but the microcontroller varies the duty cycle¿the fraction of the cycle during which the current remains on. Pulse trains with short duty cycles energize the coil for only a smidgen of the total time and so can lift only the smallest weights, whereas pulse trains with longer duty cycles can hoist heavier loads. Greg's microprocessor can generate 1,024 different values for the duty cycle. That number sets the dynamic range of the balance. If the maximum current is set so that the apparatus can lift up to one milligram, for example, the smallest detectable mass will be about one microgram.