WEIGHING up the odds

It's been 25 years since Bofor Electronics, now Nobel Systems, launched its Scale-O-Scope electronic weight indicator. With its patented KIS load cell this device could process data and communicate with other Scale-O-Scope terminals, printers and main-frame computers; no mean feat in those days. It did, however, weigh 30kg with a resolution of only 1 in 30 000.

Nobel's latest transmitter has a 1 in 8million resolution, some 260 times greater, and weighs only 3kg.

Expectations of a weighing system have not changed in 25 years. The most common problems still facing systems are calibration, long cable runs, vibration interference, diagnostics, environmental sealing and voltage surges.

Design improvements and sophisticated electronics are addressing these problems and since the 1970s load cell manufacturers have tried to integrate electronics into the load cell itself to produce a digital output.

Analogue load cells require constant manual testing and compensation to achieve acceptable performance. Manual compensation is only a first order correction, and the idea of digital electronics is to perform continual compensations as part of an electronic routine. A digital signal is controlled by a microprocessor within the load cell body and programmed with all the routines needed for filtering, compensation and communication.

Performance parameters such as creep, recovery, linearity and vibration induction of a load cell depend on the element design and the accuracy of the strain gauges that are attached to it. Electronic developments cannot transform a poor load cell design into a good one. However, digital load cells can compensate for these characteristics. Typical analogue weighing systems consist of a number of load cells connected, in parallel, to a weighing indicator. A digital system is the same except each load cell is accessible and can be identified using its own address. Ability to address each device individually has tremendous benefits in terms of calibration, load cell matching and diagnostics.

Analogue cells use a Wheatstone bridge of four strain gauges bonded to an element which deflects under load to produce an analogue signal. The combined signal out of the bridge is an average of the individual cell outputs. Faults are often not apparent until there is a total system imbalance and consequent failure.

Digital cells also use strain gauges, but the ac power supply is in the form of a square wave. The resulting signal is fed into an amplifier and an analogue to digital converter. Because each cell is individually addressable, the system software tracks and corrects the output. Digital load cells have integral diagnostic features which the system software interrogates, so any deterioration in performance is spotted long before a failure occurs. Digital weighing systems can also be monitored over long distances using modems.

Industrial load cells are designed to sense force under a wide variety of adverse conditions. Mechanical overload is one of the greater dangers. There is a limit to how strong a load cell can be; it must be strong enough to carry the load but it must also be weak enough to deflect, or the strain gauge would not register, hence, no signal. Digital load cells have amplifiers mounted very close to the strain gauges so lower strain deflections are registered. This feature allows a wide weight range, less opportunity for overload and also solves the problem of signal loss through long cable runs. Higher signal levels also reduce the effects of temperature and noise.

Because of delicate electronic components, digital load cells may be compromised at levels below 200V and are therefore installed with surge protection devices to prevent damage from high voltage switch gear or lightning.