|From Tommy Thompson to Ronald McDonald™, everyone seems to be step counting these days. But what separates the tools from the trinkets? Let's look inside popular pedometers and find out.|
Pedometers can be broken down into a few basic pieces. Those pieces determine ACCURACY, RELIABILITY, and COST
|Pedometer mechanics measure the physics of body motion. As a person walks, the legs fight the pull of gravity and cause the body core to accelerate and decelerate vertically. The foot strike causes a sharp change in motion.
In a suspended lever arm pedometer, a lever is suspended by a spring. When your foot strikes, the arm continues downward due to inertia, closes a circuit, and counts a step. The quality of these moving parts in the pedometer affects accuracy, reliability, and the overall life (durability).
Most pedometers use a thin hairspring, which is less expensive and smaller than a coil spring. Hairsprings lose strength quickly and the lever arm droops, causing inaccuracy after just a few months of walking in most cases. I have found a mere two pedometers in the US and Japanese markets that feature a coiled spring: the Yamax® Digiwalker™ and the Robic® M307 Step & Stride Counter, which is a lower-cost imitation of the Digiwalker™.MouseOver the pedometer to zoom in on the spring.
A long, well-balanced lever arm and spring determines the sensitivity of the pedometer. The force required to move the lever and close the circuit is measured by the manufacturer in g's, a unit of acceleration where
It appears that most require approximately .5g in addition to gravity to count one step. All pedometers have a way to calibrate the sensitivity either at the factory, by the user, or both. Coiled springs can be tightened or loosened, and hairsprings can be relocated to change the effective length of the spring. Beware of pedometers with external calibration switches or buttons that allow the owner to modify the sensitivity. This generally signifies that a manufacturer focused on saving time and money and did not calibrate the unit at the factory. If the pedometer is not calibrated at the factory, it will almost never achieve an accurate sensitivity setting. A good pedometer will be precisely calibrated at the factory and will retain its calibration. Most hairspring pedometers will retain calibration for less than 1 million steps before the spring weakens and the pedometer begins to overcount. Coiled spring pedometers generally retain calibration for tens of millions of steps. Research is currently being performed on the overall durability of spring mechanisms.
Quality pedometers will show quality engineering at the point of contact between the lever arm and the circuit. Some pedometers use a magnetic switch to close the circuit, avoiding any contact at all, but creating a 'clicking' noise due to the technology (cyclists will be familiar with this click on their cyclo-computer's sensor). Low-quality pedometers have loud metal to metal contact whereas better ones have contacts covered in non-corrosive conductive rubber, which dampens the noise and prevents corrosive buildup that shortens the working life of the metal-to-metal contact type.
In an accelerometer, a strain gauge deforms due to inertia, and the extent of that deformation is measured as time vs. acceleration. This allows the accelerometer to know how hard or soft each step is. According to the latest research, accelerometers, sometimes referred to as piezoelectric pedometers, are the most accurate and reliable step counting mechanism that exist. The drawbacks of accelerometers are shorter battery life, due to the constant sampling of the strain gauge, and a higher price. Of the companies producing accelerometer-type pedometers, only a few are marketed to the general public in the United States, the NL-2000 by New-Lifestyles®, the HJ-112 by Omron®, and the Fitware pedometer by Highgear®. The Tritrac™ and the Actigraph™ appear to be marketed more toward researchers and are quite expensive at $500 and ~$800 respectively. The NL-2000, Omron HJ-112, and the Fitware run $55, $30, and $30 respectively. The Biotrainer™ by IMsystems® and the Caltrac™ count calories and body movements, but not steps. Several models of consumer-targeted accelerometers exist in Japan, which is several years ahead of the US in mainstream pedometer design and usage.(MouseOver the pedometer to zoom in on the strain gauge)
photo ©2003 NEW-LIFESTYLES pedometers ; used with permission.
JSC Engineering LLC is an Electrical Engineering Consultant Group specializing in product design & performance testing.
© 2008 JSC Engineering, LLC. Direct reprint requests to firstname.lastname@example.org.