Electronic industrial scales are among the most common tools used in industry. Industrial engineers and purchasing managers have two principal characteristics that must be considered prior to a purchase: accuracy and cost.
Two technologies currently dominate the weighing industry:
• Strain gage load cells: good accuracy at lower costs
• Force motor, or force restoration technology: excellent accuracy at higher costs
Our second generation ultra precision scales better the accuracy of force motor scales at a cost more in line with high quality strain gage scales with our internationally patented Surface Acoustic Wave (SAW) load cells.
By using a method of measurement never previously applied to scales, we’ve developed an industrial scale with almost ideal weighing characteristics at a quite reasonable cost.
How Strain Gage Scales Work
Available for over half a century, these scales have become the most common weighing device worldwide. The concept behind this technology is that a resistor’s resistance value will change if the shape of the resistor is allowed to change.
Strain gages are a type of resistor usually produced from a flat foil material and shaped into a long, serpentine path. The gage will be carefully bonded to a spring element called a load cell.
The strain gage stretches as the load cell bends, which causes the long, serpentine path to become slightly longer and increases the resistance of the gage.
There Are Several Difficulties With This Concept
1. A load cell is not a perfect spring.
A perfect load cell would be a perfect spring. It would bend in a manner that was perfectly proportional to the weight applied to it. When the weight was removed, it would return to the exact same position it started from.
But it’s impossible to make a perfect spring. When a spring has unlimited potential to bend it will undergo plastic deformation and lose some of the characteristics that make it a good spring. Kind of like the Slinky you stretched too far as a kid, it will never return to its perfect coil.
The stress placed on the load cell must therefore represent only a small fraction of its elastic modulus, which is why the strain gage will only change its resistance by a miniscule percentage of its starting value.
2. Adhesives restrict spring characteristics of the load cell.
Bonding the strain gage to the load cell also introduces problems. Quality load cell manufacture requires a very thin, smooth layer of glue. Yet even with the most careful application, glue simply makes a very poor spring. It restricts the overall spring characteristics of the load cell and will never perfectly transmit the exact bending of the load cell to the strain gage.
3. Strain gages cannot be perfect resistors.
In an ideal world, the resistance of the strain gage would only change because its length changed. It would also change in exact proportion to its change in length. But resistors exist in the real world where many other influences are at play. Changes in temperature generally cause the largest errors, but errors may also be caused by microscopic abnormalities in the strain gage materials and the ongoing changes that occur as the resistor ages.
Scale Manufacturers Go to Great Lengths to Minimize These Effects
High quality scales generally use four strain gages to form a full bridge, which significantly compensates for various mechanical errors within the load cell.
Strain gages are often placed off the center axis of the spring element, which also reduces error.
Additional temperature compensation resistors are often added to the circuit. They’re generally designed to have temperature characteristics opposite to those of the strain gages, which cancels out some of the effects of temperature.
These efforts all work to greatly reduce many error terms, but they cannot remove them completely.
Force Motor Scales Work On An Entirely Different Concept
In this design, an electromagnet is used to support the scale platform. The amount of electrical current required to support it will change as weight is applied. The scale determines the weight of the load by measuring this current.
While this is an intrinsically accurate method for determining weight, it’s also quite costly and still is prone to error. Changes in temperature also cause a large amount of errors for force motor scales as they do strain gage models.
The accuracy characteristics of the scale will also change over time, which can introduce error. To reduce the likelihood of these errors, many force motor scales are constructed with an internal calibration mass and a method for applying this mass to the sensor. Regular recalibration must be performed quite often to maintain accurate results. But the larger issue with the internal calibration mass device is that it adds significant cost.
Another issue for some purchasing managers is that force motor scales become somewhat impractical at higher capacities. Either greater amounts of electrical current are required to support the platform, or increasingly expensive and complicated systems that use mechanical advantage must be implemented to reduce the load seen by the force motor.
Arlyn’s Ultra Precision Surface Acoustic Wave Scales
The concept these scales rely on is that the transmission of a bulk wave from a transmitter to a receiver that are a known distance apart will take a very predictable amount of time.
When the distance between them changes, the transmission time will also change. Accurately measuring this transmission time gives a highly accurate measurement of the distance.
This technology is revolutionary because it allows weight measurement using a spring element load cell without the disadvantages found in the strain gage design.
SAW load cells do not depend on the strain or stress of the spring element, which means these forces can be reduced by 90% or higher. This means that the typical load cell errors are reduced to a level that cannot be measured.
Because no strain gage is used, the errors associated with the actual gages and the bonding technique are also eliminated. The measurements taken are time measurements, which means they are automatically in a digital format. Converting analog resistance into a digital reading is no longer necessary, which makes these scales inherently more accurate.
The overall accuracy of an Arlyn SAW scale is approximately 20 times greater than strain gage scales. With significantly improved processing power in the scale controller, the result is a full-featured, completely digital, easy to use and highly accurate scale.
SAW technology is currently available on bench and counting scales in capacities of 5, 10, 25, 50, 100, 200 and 500 pounds. They are ideal for precise chemical formulation, ink and dye manufacturing and any other industry where precision is important.
They’re particularly appropriate for parts counting because the accuracy of the count depends on the accurate measurement of the original sample weight, which is typically just a small percentage of the scale’s capacity. The lower weight resolution and accuracy have a major impact on the overall accuracy of the count.
Ultimately, our Ultra Precision SAW Scales provide much higher accuracy than strain gage scales at a similar cost offered by other well-known industrial scale companies. In the higher capacities, the cost of our SAW scales is generally less than a third of the cost of force motor scales and the accuracy is at least as good.
We are in the process of expanding this technology to other industrial scale models including cylinder, drum and floor scales.
Order a SAW Scale Today
See the difference SAW technology makes first hand. Order an Arlyn Ultra Precision SAW scale today.