Ultra Precision Industrial Scales with New Technology Introduced By Arlyn Scales

Share Article

Electronic Industrial Scales are among the most common instruments used throughout a variety of different industries. Industrial engineers and purchasers of electronic scales have two overriding characteristics that they must trade off in their purchasing decision; accuracy and cost. There have traditionally been two predominant technologies that are used by the vast majority of these scales. The first is strain gage load cells, which offer good accuracy and lower costs. The second is force motor, or force restoration technology which offers excellent accuracy and high costs. Arlyn Scales has developed a second generation of Ultra Precision Scales using an internationally patented third technology, Surface Acoustic Wave (SAW) load cells, which provide superior accuracy to even the force motor scales at a cost which is comparable to high quality strain gage digital scales.

News Image
We have been able to eliminate some of the precision analog to digital circuitry previously required. Instead, we have added significant processing capability to the scale controller that results in a fully featured, easy to use, highly accurate scale

Electronic Industrial Scales are among the most common instruments used throughout a variety of different industries. Industrial engineers and purchasers of electronic scales have two overriding characteristics that they must trade off in their purchasing decision; accuracy and cost. There have traditionally been two predominant technologies that are used by the vast majority of these scales. The first is strain gage load cells, which offer good accuracy and lower costs. The second is force motor, or force restoration technology which offers excellent accuracy and high costs. Arlyn Scales has developed a second generation of Ultra Precision Scales using an internationally patented third technology, Surface Acoustic Wave (SAW) load cells, which provide superior accuracy to even the force motor scales at a cost which is comparable to high quality strain gage Digital Scales.

"Technology remains fairly constant over fairly long periods of time" says Arlyn Scales President, Arnie Gordon. "There may be ongoing refinements and improvements which produce incremental benefits. But it is the brand new innovation, the "out of the box concept" that drives technology to an entirely new level. Arlyn's SAW technology represents just such a breakthrough. By using measurement methods that have never previously been applied to scales, Arlyn Scales has developed an industrial scale that has almost ideal weighing characteristics, but is quite reasonable in cost."

Strain gage scales have been available for more than fifty years. They are based on the concept that the resistance value of a resistor will change if the shape of the resistor is allowed to change. Strain gages are, in fact, resistors. They are usually produced from a flat foil material shaped into a long, serpentine path. The gage is carefully bonded onto some sort of a spring element, called a load cell. As the load cell bends, the strain gage will be stretched, so that the long serpentine path will become slightly longer. As the path becomes longer, the resistance will increase slightly.

There are a number of difficulties with this concept that limit the overall accuracy of the scale. A perfect load cell would be a perfect spring. It would bend in a manner that was perfectly proportional to the load placed on it. When the load was removed, it would return to exactly the same position that it started from. But it is impossible to make a perfect spring. The spring element must not be allowed to bend too much. If it does, it will undergo plastic deformation, and it will lose some of the characteristics that make it a good spring. So the overall stress on the load cell must represent only a very small fraction of its modulus of elasticity. Because of this, the strain gage will only change its resistance by a very small percentage of its starting value.

Bonding the strain gage to the load cell with glue will also introduce problems. Good load cell manufacture requires limiting the glue thickness to a very thin, smooth layer. But even with these efforts, glue makes a very poor spring. This also affects the overall spring characteristics of the load cell. In addition, the glue will never perfectly transmit the exact bending of the load cell to the strain gage.

While there are problems with the spring element, there are additional problems introduced because the strain gage can never be a perfect resistor. Ideally, its resistance would only change because its length changed. And the resistance would change in exact proportion to its change in length. Unfortunately, every type of real world resistor is affected by many other influences. Changes in temperature usually cause the largest errors. Other errors may be caused by microscopic abnormalities in the materials of the strain gage, and even ongoing changes as the resistor ages.

Industrial scale manufacturers go to great lengths to minimize these affects by a number of means. For high quality scales, four strain gages are generally used together in the form of a full bridge. This provides a significant amount of compensation for various mechanical errors in the load cell. There will be some correction for errors caused by the strain gages being placed off of the center axis of the spring element. Some of the temperature affects will also be cancelled out. Additional temperature compensation resistors are also often added to the circuit. They are designed to have temperature characteristics that are generally opposite to those of the strain gages. These compensation efforts do greatly reduce many error terms, but of course they are not completely removed.

Force motor scales work on an entirely different concept. The overall concept is to have an electromagnet support the scale platform. As a load is placed on the platform, the amount of electrical current required to support the scale platform will change. By measuring this current, the scale can determine the amount of the load. This is an intrinsically accurate, but quite costly method for determining weight. Nonetheless, some errors are still introduced by such things as changes in temperature. The accuracy characteristics of the scale do change with time. To reduce this type of error, many force motor scales are constructed with an internal calibration mass, and a method for applying this mass to the sensor. For best accuracy results, this re-calibration should be performed quite often. But the calibration mass device adds even more cost to the scale. Also, force motor scales start to become somewhat impractical at higher capacities. Either great amounts of electrical current would need to be used to support large loads, or increasingly expensive and complicated systems must be employed to use mechanical advantage to reduce the load seen by the force motor.

Arlyn's Ultra Precision Surface Acoustic Wave industrial scales utilize a device fabricated with semiconductor type of technology. "The transmission of a bulk wave from a transmitter to a receiver that is a known distance away will take a very predictable amount of time" noted Dr. Slava Kats, Arlyn's Director of SAW Technology. "When the distance between them changes, the transmission time will also change. Accurately measuring this transmission time will give a very accurate measurement of the distance. In a unique and exciting way, this technology allows weight measurement using a spring element load cell without the disadvantages found in strain gage load cells"

Because SAW load cells are not dependent on the strain or stress on the spring element, it can be reduced by 90% or even more. Most of the typical load cell errors will now be reduced to a level that cannot even be measured. As there are absolutely no strain gages used, all of the errors associated with the gages themselves and the bonding techniques will also be eliminated. Because the measurements being taken are time measurements, they are automatically in a digital format. It is so longer necessary to convert analog resistance measurements into digital readings.

"Overall accuracy of Arlyn SAW scales are approximately twenty times greater than strain gage scales" states Project Engineer Murtaza Karim. "We have been able to eliminate some of the precision analog to digital circuitry previously required. Instead, we have added significant processing capability to the scale controller that results in a fully featured, easy to use, highly accurate scale".

Currently, Arlyn offers the SAW technology in bench scales and counting scales in capacities of 5 lb, 10 lb, 25 lb, 50 lb, 100 lb and a 200 lb model. These are ideal for precise formulation, ink and dye manufacturing, and all other areas where precision is important. The SAW scales are particularly appropriate for parts counting, because the accuracy of counting is dependent on accurately weighing the original samples. Typically, these are just a small percentage of the capacity of the scale, so lower weight resolution and accuracy will have a very major impact on the overall count accuracy. These scales provide much higher accuracy with similar costs to high quality strain gage scales offered by other well known industrial scale companies. In the higher capacities, the cost of the SAW scales are generally less than one third of the cost of force motor scales, but the accuracy is at least as good.

Arlyn Scales expects to expand this technology to other types of industrial scales, including cylinder scales, drum scales and floor scales. Gordon noted that "There is no technical reason why the same technology cannot be used across the entire range of industrial weighing. We are expecting to see our SAW scales take a notable position in the field."

###

Share article on social media or email:

View article via:

Pdf Print

Contact Author

Arnie Gordon
Visit website