By Jim Renehan, Contributor
Catchweighers, which in North America are more commonly known as in-motion scales, are an essential aspect of material handling, as well as parcel processing and distribution. In a fast-paced distribution environment, catchweighers must operate both swiftly and with exacting accuracy; in terms of precision, the weigh cell becomes the central, mission-critical element.
Like many crucial machinery items, technology has evolved in recent years. The latest and greatest catchweighers incorporate weigh cells featuring electromagnetic force restoration, or EMFR. Let’s explore what this entails and how it compares to conventional weighing methods.
What Is EMFR?
EMFR systems can be compared to a simple beam balance—although the intricacies are anything but. Incoming weight on the load plate causes the lever arm (1) to leave its nominal resting position. The position detector (2) recognizes this slight position change and forces the coil (4) on the other side of the lever arm to move out of the magnet’s field (3). When this occurs, a photoelectrical beam recognizes any minute deviations in lever arm position and immediately sends its findings through a measurement resistor, transforming this data into a digital signal via an analog-to-digital (A/D) converter. The resulting digital weight value is determined by an advanced digital signal processor at exceptionally fast speeds, with an internal sampling rate of 1mS, or 1,000 weight values per second.
Figure 1. Block Diagram of an EMFR Weigh Cell
With their lightning-quick sampling rates, EMFR systems enable extremely accurate weighing results that provide space- and cost-savings when integrated into existing systems. They possess high throughput rates—some EMFR-equipped triple scales can weigh up to 19,000 parcels per hour—while other smaller and mid-range scales offer fantastic price-to-performance ratio. All EMFR scales deliver increased plant efficiency, thanks to precise and reliable weighing results. It’s no wonder EMFR-based solutions are used by packaging and supply chain industry leaders, such as DHL, FedEx, UPS and the U.S. Postal Service.
The Benefits of EMFR
Compared to conventional strain gauge scales, EMFR scales offer a number of benefits. When compensating for a dead load, traditional strain gauge-based checkweighers require a stiffer load cell to accurately and effectively process the weight. With strain gauge checkweighers, the resolution of the weighing range declines. EMFR scales can balance dead load with software commands, keeping the weighing range available and the resolution intact.
When measuring dynamic performance, EMFR checkweighers have a short settling time and active attenuation by the electronic controller, while preventing sensitivity and resolution from changing—eliminating the need for constant calibration checks. Strain gauge weighers generally possess less effective dampening oscillatory systems and a higher resonance frequency while settling, while the stiff load cell reduces sensitivity and resolution, requiring consistent calibration and maintenance. In short, EMFR scales are faster, more efficient, more precise—and require less maintenance than conventional weight cells.
Figure 2. Settling Behavior of an EMFR Weigh Cell vs. a Conventional Load Cell
Active Vibration Compensation
One benefit of strain gauge checkweighers is that they possess a lower point-of-entry price than their EMFR counterparts. However, the front-end savings can be offset by costly rework, should the weight that appears on the scale not match the advertised weight on consumer packaging.
Strain gauge checkweighers are also more susceptible to environmental factors and external vibration that can negatively impact weight results, leading to inaccurate readings. Certain EMFR weight cells can be equipped with Active Vibration Compensation (AVC) technology, which greatly reduces the amount of operational interference that could affect measuring results. This is accomplished by mounting two weigh cells within one housing, where one cell measures only the load to be weighed (1), while the other measures only the vibrations (2). AVC electronics removes much of the vibration component from the load signal by calculating and comparing the two measurement curves (3). The obtained measurement result is very close to a result that would be achieved without any influence through vibration (4). The net result is that dynamic weighing systems can be used reliably in places they otherwise could not, due to external vibration sources, such as sorters or transport systems like forklifts. AVC technology can reduce external vibrations, particularly those under 10hz, to one-tenth of their value while maintaining a high level of weighing accuracy.
Figure 3. Active Vibration Compensation (AVC) Control Signals
For material handlers, AVC offers the potential to increase profits through gram-precise weighing results. This makes transport billing fees more exacting and ensures parcels are placed into the proper pricing category. Many companies in the courier, express and parcel (CEP) sector use AVC technology in their weighing systems—and increase their profit margins by doing so.
When factoring in pricing categories, AVC ensures that packing and shipping companies will meet a certain weight threshold, allowing them to not only begin billing early, but ensuring their charges are correct, based on precise measurements offered by EMFR and AVC. WMHS
About the Author
Jim Renehan is senior marketing manager for WIPOTEC-OCS, Inc., a leading manufacturer and supplier of precision in-motion product weighing, X-Ray scanning, product inspection, parcel dimensioning, package marking, and serialization and aggregation equipment. The company’s weighing, inspection, and track and trace solutions serve applications in a broad array of industries, including pharmaceutical, food, chemical, cosmetics, healthcare, eCommerce and logistics. www.wipotec-ocs.com/us/