Surface Acoustic Wave (SAW) Scales and Strain Gage Scales

Industrial scale technology has been fairly stagnant for a number of years. The predominant technology has been strain gage load cells. When increased accuracy is required, magnetic force restoration, also known as magnetic force motor transducers are used. The disadvantage of force restoration is significantly increase cost. Recently, though, Surface Acoustic Wave (SAW) scales have been introduced by Arlyn Scales. These Ultra Precision scales provide the accuracy of force restoration at a much lower cost . Strain gage scales have been available for more than sixty years. As is the case with all load cells, the strain gage cell is actually based on a spring element. It is usually made from aluminum or steel. It is machined so that it will deform in a very controlled manner as load is applied to it. When the load is removed, this spring element is designed to return to its original shape. A strain gage is actually a type of resistor. It is usually produced as a metal pattern with a thin film backing. An adhesive system is used to bond the strain gage to the spring element. Great care must be taken during this process to make sure that the adhesive layer is very thin and very uniform. Clearly, the bending characteristics of the spring will be compromised by the very poor bending characteristics of the adhesive. Furthermore, the strain gages must be very accurately placed. The load cell is designed to only detect vertical forces. But there are also horizontal forces that must be rejected. The placement of the strain gages with reference too the central axis of the load cell will affect this. As is the case with all resistors, strain gages are subject to change in resistance with changes in temperature. Although temperature compensation is attempted, the load cell output will still vary with temperature. In general, overall accuracy is limited to about 1/10 of 1%, and resolution is typically from 1 part in 5,000 up to 1 part in 10,000. Force restoration scales can provide much better accuracy and resolution. The weighing system consists of an electromagnet that supports the scale platform. The amount of current in the electromagnet required to counteract the weight on the platform will be proportional to that weight. The weight will be accurately known by carefully determining the amount of current. The platform must be directed by a very precise flexure system. Unfortunately, these flexures are prone to damage from side forces and shock loading. Also, the amount of current required rises quickly as the load on the platform increases. For scales with larger capacities, the flexure system needs to incorporate some mechanical advantage, or else the current requirements would become prohibitive. The final result is a somewhat sensitive but very accurate scale, at a cost that is often three to five times higher than strain gage scales. The newest technology SAW scales, also use load cells. But instead of measuring the amount of strain in the spring element, it measures the amount of displacement. This results in strain levels that are less than 10% of those found in strain gage cells. All of the errors associated with strain of the spring element are not measurable in SAW load cells. The displacement is measured with extreme accuracy by Surface Acoustic Wave transducers. The transducers are fabricated using the same technology as that used to produce integrated circuits. The substrate material is a crystalline structure made from lithium niobate. A metallized pattern, acting as a transmitter, has a signal applied that produces a bulk wave in the substrate. This wave travels down the length of the transducer and excites an equivalent signal on a parallel substrate. Another metallized pattern on the second substrate acts as a receiver. The signal receive is amplified, and then fed back to the transmitter. The frequency of oscillation of this feedback loop will be dependent on the distance between the transmitter and receiver, which is actually the displacement of the spring element. SAW scales provide accuracies of about 1/100 of 1%. This is equivalent to force restoration scales. Because the strain levels of the load cell are very low, they are about five times more rugged than strain gage scales. They are ten to twenty times more rugged than force restoration scales, making them suitable for use in harsh industrial environments. The cost of this technology is just slightly more than high quality strain gage scales, and only about one third the cost of force restoration scales.