While at some level all industrial weighing has to be carried out very precisely, there are different levels of precision that must be met by different industries. When a food processing plant is mixing large amounts of different ingredients for a recipe, the measurements of each ingredient must be precise, but small deviations of a few grams above and below the target weight will not drastically affect the quality of the final product. When a patient’s weight is taken in a hospital, it is necessary to use an electronic scale that is accurate to within a tenth of a pound. Any digital scale that is less accurate than this will not prevent the patients from receiving the proper medical care. On the other hand, when a plastics company must combine various materials to make a polymer, it is imperative that each ingredient be weighed out with extreme precision. Even the slightest deviation will cause the polymer to have different physical characteristics than were otherwise intended. In the automotive industry, colors of paint are mixed very accurately so that one blue car looks identical to another blue car.
When ultra-precise weighing is not required, the most cost-effective digital scales on the market are strain gauge scales. These industrial scales use specially machined blocks of metal, called load cells, that have certain spring characteristics. Several strain gauges, a type of flexible resistor, are placed on the load cell. When a force is placed on the load cell, it bends, thereby bending the strain gauges and changing their resistance. This change in resistance is detected by an analog to digital converter which outputs a weight reading. Until recently, only one type of technology, called force motor, or magnetic force restoration was available for situations that required more accuracy than strain gauge technology. While typical strain gauge electronic scales have a readability of 1 part per 5,000, force motor technology allowed for scales with a readability of 1 part per 150,000 or even more. Unlike load cells used in strain gauge scales which consist of one big block of metal with a few strain gauges connected to it, force motor scales require weight transducers made up of dozens of tiny moving parts. When a weight is placed on an industrial scale platform, it pivots a beam lever that is part of the transducer. This lever has an LED at the end of it that shines light at two photodiodes. When the lever pivots, one photodiode receives more light than the other. Also connected to the beam lever is a bobbin that is wound tightly in coil that is called the force coil. This bobbin is hanging in a permanent magnet called the force motor. When one photodiode receives more light than the other, the balance feeds the force coil with more voltage or current so that the magnetic field pushes it up, balancing out the lever once more. At the point that both photodiodes are again receiving the same amount of light, the amount of voltage that is being fed to the force coil is converted by an analog to digital converter and a final weight reading can be given. Since force restoration scales are made of such a large number of small parts, they are extremely expensive and are prone to damage from overload and shock. Recently, Arlyn Scales developed ultra-precision electronic scales that use Surface Acoustic Wave (SAW) technology. Like in strain gauge scales, metal load cells are used. This technology uses two SAW transducers. One is a surface acoustic wave transmitter that sends a signal across the surface of a semiconductor substrate to a second SAW transducer that acts as a receiver. The same signal that is produced by the transmitter is induced in the receiver. It is then amplified and fed back into the transmitter, causing an oscillation between the two SAW transducers. When the load cell bends, the distance between the two semiconductor substrates changes and the oscillations between the substrates change as well. Like force motor scales, these ultra-precision SAW scales have readabilities of 1 part per 150,000 and higher. This means that SAW scales with 10 pound capacities have readabilities up to four decimal places. Since Arlyn’s SAW scales contain fewer parts, they are much cheaper than force motor scales. In addition, they are sturdier than force motor scales and are much less likely to be permanently damaged by shock and overload.
