Weighing systems tend to be installed in difficult to reach locations, such as underneath truck unloading systems, on beams supporting vessels, below conveyor belts, etc. And these locations are also often widely dispersed. For these reasons, a weighing system needs to be simple, small and reliable. Strain gauges are simple, relatively low-cost, reliable and robust, making them one of the most commonly used technologies in weighing systems. Strain gauges change resistance, with the typical range being between 30 ohms and 3,000 ohms. However, resistance is not a very useful form of measurement, at least not directly, especially as it can change with temperature. So, it's typically converted to voltage or current.
The Wheatstone bridge, with which we're all familiar, is well suited to measure the small changes in resistance required when measuring strain gauges at the required level of accuracy. The Wheatstone bridge can respond if the strain gauge is either in tension or compression, with a corresponding increase or decrease in resistance unbalancing the bridge. This produces a voltage indication that corresponds to the displacement at the gauge. If the strain increases, the voltage difference across the meter terminals increases. If the strain is zero, then the bridge balances and the meter will show a zero reading.
We now have a voltage or current reading that, like any other measurement, can be scaled and transmitted over longer distances to be recorded or used for control.
As stated above, with strain gauge systems being in difficult to reach locations and widely distributed, they appear to be a good fit for wireless connectivity. However, it appears that the major manufacturers of these devices only offer systems as wired outputs, which have the advantage of providing the required power supply, or transmit the signal using proprietary, license-free, typically 900 MHz, wireless systems. The proprietary systems are effectively a "bolt-on" adapter, meaning the same approach could be used if you wanted to install a wireless sensor network (WSN) weighing solution in your facility.
Considering the apparent good fit for wireless in weighing applications, why are there not more wireless weighing systems?
- Power—Despite the fact that batteries used for industrial applications are now capable of a 10-year lifespan on long update cycles, weighing systems need to be regularly calibrated, and access to the weighting cell itself can be challenging;
- Signal path—The cells tend be installed within large steel structures that effectively shield the signal, thus affecting reliability;
- Electrical interference—Some installations, particularly for conveyors, are near large motors and other sources of electrical interference;
- Price—Development of a wireless device comes with a price, as does the associated ongoing support;
- Market demand—Weighing system users haven't identified a niche application requiring wireless sensor level connectivity, or if they have, they've chosen to use adapter technology; and
- Certification—Once certification is included, WSN standard chip sets are more expensive than a proprietary solution, and with no clear WSN leader for their target market(s), suppliers will not see a return on the required development effort.
Though for the reasons above, WSN applications for weighing systems are not very common, Wi-Fi is being used for communication between systems, such as at truck stops to interact with the drivers and reporting systems. These applications, however, still use wired signals to the controller, then an access point to share the information with humans.
As we have seen, what may appear to be a “perfect fit” for a technology is not necessarily always so. Time and again, engineers, including myself on more occasions than I should, continue to promulgate our stereotype of a better way to build a mousetrap, and instead are caught in the trap of developing a solution for which we do not have a problem.
About the author: Ian Verhappen
