How Does Surface Acoustic Wave Technology Work?

cylinder-scale-2In the age of technology we are always expecting everything to continue on the path of better, smaller, and cheaper. This has been the case with many different kinds of products and services. Not very long ago, telephones needed to be cranked to contact an operator who would connect your line to another operator and then another until your call was finally connected. The cost of the phone call could be extremely high. Great progress was made when the standard black dial telephone was perfected, allowing the user to directly connect with their desired party. Phone calls were also less expensive, as they did not require operator assistance. With the introduction of Princess phones, there was a major increase in style and color and a decrease in size of the phone instrument. The introduction of the Flip phones provided a significant decrease in size.

A common theme throughout of all of these phones is that they had to be connected to a phone jack. The arrival of the first Brief case cell phones provided a new world of convenience in telephony. In rapid succession, the brief case was replaced by the Brick phone, which was then replaced by slimmer, lighter, better working cell phones. The past few years have witnessed a very rapid migration to pocket sized, lightweight smart phones. These include beautiful color screens that can be used for web browsing, TV viewing, and shooting video. It is an absolute certainty that the evolution of the phone is going to continue in the coming years.

There are many other types of devices that have followed very similar development curves. This is particularly true in the medical field where items as diverse as pace makers and hearing aids have progressed from large, bulky machines to tiny, full-featured marvels. It is also an extremely obvious trend in computers and related equipment.

Unfortunately, not every technology is able to continually improve. There are many examples of items that have come close to, or have already reached their theoretical limits. One example of this is the efficiency levels of gas heaters for buildings. Obviously, the maximum efficiency of converting natural gas to heat is 100%. There are currently heaters available at 95% and even 97% efficiency levels. While it is desirable to increase the efficiency even more, there is not much that can be gained by doing so.

Electric heaters share a similar scenario. Almost by definition, electric heaters will convert 100% of the electricity that they use into heat. If the heater uses 1,000 watts of electricity, it will provide 1,000 watts of heat. There are no technological improvements that will increase this efficiency. On the other hand, a new technology has been introduced that changes the method of producing heat. Heat pumps offer a method of taking heat energy from one source, concentrating it, and providing it to the user. The source of the heat can be from the outside air, in the example of a standard heat pump, or from underground, in the case of a geothermal heat pump. The electric energy is used to pump water from the ground, which in turn heats or cools the Freon in the heat pump system. More energy is used to compress the Freon, which then brings heat into the building. The advantage is that for every watt of electricity used by the system, up to 5 watts of heat are developed. This is a case where a new technology has been able to break through previous limitations.

Industrial electronic scales have seen a very similar type of trajectory. The initial breakthrough that allowed electronic scales to be developed was the invention of the strain gauge. This was originally a coil of very thin wire that was bonded to a spring element, called a load cell. When a load is placed on the scale platform, the spring bends a small amount. Because the thin wire is bonded to the spring, it will closely follow its movement. When the spring material stretches, the wire will also stretch. When the spring material is compressed, the wire will also be compressed. In a non-stressed state, the wire has a particular resistance. When the wire is stretched, its resistance will increase. When it is compressed, it will decrease. By measuring the resistance of the wire, it is possible to determine how much it has stretched, and therefore how much the spring has stretched. Assuming that the load cell is a perfect spring, it follows that it is possible to then calculate how much weight has been placed on the scale platform causing the spring to stretch.

As with other technologies, there have been many incremental improvements to strain gauges and load cells. The coil of thin wire evolved into a metal foil, which was cut into a pattern that provided a long, thin path. This gave considerably better control over the thickness of the wire, and therefore its resistance. The resistance of metal changes with changes in temperature, which could cause errors in weight readings. The alloy of metal used was revised to produce a strain gauge with smaller and very predictable responses to temperature. This allowed a variety of techniques to compensate for temperature changes. With the advancement of photolithography, the strain gauge could be produced with etching methods, allowing further refinement. The metal foil was placed on a thin, flexible backing for ease of installing on the load cell. The materials of the backing also advanced in order to minimize their negative affect on the spring constant of the load cell. The type of bonding adhesive also underwent improvement, allowing very good connections of the strain gauge to the load cell.

Electronics also played a large role in improving the performance of electronic scales. The nature of strain gauges and load cells determine that there are a number of different types of error terms. These include a nonlinear output, changes in both sensitivity and absolute value due to changes in temperature, long-term drift, and slow changes in the output when the load cell is fully loaded. As long as these errors are predictable and measurable, electronics can provide a good level of compensation. Close control of the manufacturing process improved the predictability of many characteristics.

Even with the many advances during the past fifty years, the electronic strain gauge scale has been stuck at a particular level of accuracy. This can be noted by the various ratings provided by national and international bureaus of standards. Although they all use somewhat different parameters, there have not been any strain gauge scales that significantly raise the bar. This can be attributed to the natural limitations of the metals used for the load cells, and the material properties of the strain gauges, backings and adhesives. As a direct parallel to the electric heaters discussed earlier, standard industrial electronic scales have run against their limitations, and are simply not going to get any more accurate.

Fortunately, as with the comparison of heat pumps to electric heaters, there are other weighing technologies that provide a path to greater weighing accuracy. One type of scale, known as Electromagnetic force restoration, or Force motor provides much higher accuracy. These rely on an electromagnet system to counter the weight of the load on the scale platform. Unfortunately, especially for higher capacity units, the cost of this type of scale is so much higher than strain gauge scales that they cannot be considered.

A third technology has recently been invented by Arlyn Scales that provides very high accuracy with cost levels that are comparable to strain gauge scales. We incorporate an opposed pair of unique Surface Acoustic Wave sensors, and are also known as SAW scales. The sensors on these ultra precision units are fabricated using similar methods to digital integrated circuits. We develop a high frequency wave that propagates along the surface of a special crystalline structure. The amount of time that it takes for the wave to travel from the end of one sensor to the opposite end of the opposed unit is precisely related to the weight on the scale platform. Digital time measurement is then processed to provide the weight readout on the scale.

The SAW scales change the rules of the game, and therefore bypass the accuracy limitations of industrial scales. They are available in the format of bench scales with capacities ranging from 10 lb up to 200 lb. They are also available as platform scales going from 200 lb up to 1000 lb. And they are recently introduced in much higher capacities up to 5000 lb. They are at least ten times more accurate than the equivalent strain gauge scale. Typically, a highly accurate piece of equipment implies one that is also highly sensitive and prone to accidental damage. But SAW scales are exactly the opposite. Their overload and shock load capacity are more than twice as good as any other technology, providing a rare situation where you can have your cake and eat it, too.

It is encouraging to learn that the world of high technology has joined the world of high accuracy industrial weighing. Arlyn Ultra Precision Scales certainly provide the Better and cheaper attributes desired in a rugged industrial package.