New Weighing Technology: Surface Acoustic Wave Load Cells

Most industrial scales use strain-gage load cells. This technology is dependable, economical, and appropriate for a broad range of general weighing applications. However, a conventional strain-gage scale may not provide enough readability when a process must detect extremely small weight changes while supporting a relatively heavy load.

This frequency-based digital measurement allows an Arlyn Ultra Precision Scale to provide substantially finer readability than a typical industrial scale while retaining useful platform sizes, practical industrial capacities, and strong overload protection.

How Does Surface Acoustic Wave Weighing Technology Work?

An Arlyn SAW load cell uses crystalline transducers attached to a carefully designed mechanical element. One transducer generates an acoustic signal, while another receives it. The signal repeatedly travels through a feedback path, creating a stable oscillating frequency.

When an object is placed on the scale:

1. The applied force produces a very small displacement in the load-cell structure.
2. The displacement changes the physical relationship between the crystalline transducers.
3. The frequency of the acoustic signal changes.
4. The scale electronics measure the frequency digitally.
5. The calibrated system converts the frequency change into a weight value.

Frequency can be measured with very high precision. The load cell therefore requires less mechanical deformation than a conventional strain-gage design needs to produce a usable electrical signal.

This allows the SAW load cell to use a more rigid mechanical structure while still detecting extremely small changes in applied load.

Why Is SAW Considered a Digital Weighing Technology?

A conventional strain-gage load cell produces a very small analog voltage. That signal must be amplified, filtered, and converted into digital information before the indicator can display a weight.

An Arlyn SAW load cell represents the measurement as a change in frequency. The electronics count and process that frequency directly. This reduces the system’s dependence on accurately measuring very small analog voltage changes.

How Does a Strain-Gage Load Cell Work?

A strain-gage load cell contains a metal spring element with thin electrical-resistance sensors bonded to carefully selected locations. These sensors are connected in an electrical circuit, typically a Wheatstone bridge.

When weight is applied:

1. The metal spring element bends or deforms slightly.
2. The bonded strain gages stretch or compress with the metal.
3. The electrical resistance of the strain gages changes.
4. The bridge circuit produces a small analog voltage.
5. An amplifier and analog-to-digital converter process the voltage.
6. The indicator converts the processed signal into a displayed weight.

Strain-gage load cells are available in an extremely broad range of capacities and configurations. They remain the most practical and economical choice when an application does not require unusually fine readability.

Their limitation becomes important when the smallest meaningful weight change is below the scale’s displayed readability. A representative 100 lb industrial strain-gage scale may display in increments of approximately 0.02 lb. That is sufficient for many production applications, but it cannot directly display an ingredient addition or material loss of only 0.005 lb.

How Does Magnetic Force Restoration Technology Work?

Magnetic Force Restoration, also known as Electromagnetic Force Restoration or EMFR, is commonly used in precision laboratory balances and specialized high-speed weighing systems.

An MFR system operates like an electronically controlled balance:

1. An applied load attempts to move the balance mechanism away from its neutral position.
2. A position sensor detects that movement.
3. An electronic controller increases current through a coil located within a magnetic field.
4. The electromagnetic force returns the mechanism to its equilibrium position.
5. The balance calculates weight from the current required to restore equilibrium.

MFR systems can provide extremely fine resolution and rapid measurement. Their tradeoffs may include greater mechanical complexity, smaller weighing platforms, lower maximum capacities, greater sensitivity to installation conditions, and higher cost.

Strain-Gage vs. SAW vs. MFR Weighing Technology

The main difference between these technologies is how each system converts an applied load into a measurable signal. The following table shows where each technology generally fits.

How Does SAW Compare with Other Load-Cell Technologies?

Strain-gage and MFR systems are the most relevant comparisons for SAW, but several other technologies are used for specialized force-measurement requirements.

These technologies serve important specialized purposes. However, the central benefit of SAW is its ability to combine fine readability with the capacities and platform sizes expected from an industrial scale.

Why Does SAW Fill an Important Gap in Weighing Technology?

A standard strain-gage scale is usually the correct choice when moderate readability is sufficient. An MFR balance is often appropriate when the user requires analytical-level performance at a relatively low capacity.

The difficulty appears when an application needs both:

• A larger platform or higher maximum capacity
• The ability to detect very small changes in weight

SAW technology addresses this combination. Arlyn Ultra Precision Scales provide up to 10 to 20 times finer readability than representative industrial scales. Premium Ultra Precision models provide even finer increments, while Large Ultra Precision models extend SAW measurement to loads as high as 1,000 lb.

Current Arlyn Ultra Precision models also provide up to 250% safe overload protection. This does not permit routine operation above rated capacity, but it helps protect the weighing mechanism from certain accidental overload conditions.

Arlyn Surface Acoustic Wave Scale Offerings

Arlyn offers several SAW product families for different combinations of capacity, platform size, and readability. The examples below demonstrate the available range without listing every individual configuration.

Note: Displayed divisions are calculated by dividing maximum capacity by the displayed readability. Available models and specifications should be confirmed before ordering.

Which Weighing Technology Should You Choose?

The correct weighing technology depends on what the scale must measure—not simply on which system displays the most decimal places.

• Choose a strain-gage scale when general industrial readability is sufficient and cost, broad capacity selection, or very high capacity is the primary concern.
• Choose an Arlyn SAW scale when the process must detect substantially smaller weight changes while retaining practical industrial capacity, platform size, and overload protection.
• Choose MFR or EMFR technology when analytical-level resolution is required and the available platform size, capacity, environment, and budget fit the application.
• Choose piezoelectric technology when the primary requirement is measuring impacts, vibration, or rapidly changing forces.
• Choose hydraulic or pneumatic technology when the operating environment makes electronic sensing at the load point undesirable.

Readability is only one part of scale performance. Accuracy also depends on calibration, linearity, repeatability, temperature, installation, vibration, load placement, and mechanical interference.

Frequently Asked Questions