## Special Weighing Problems solved by Custom Load Cells

11.26.2008 · Posted in Technology Articles

Every industrial scale needs a transducer device which will detect the amount of weight being applied and will convert it into an electronic signal. The most common type of electronic scale transducer is called a load cell. It produces a millivolt level analog signal that is exactly proportional to the weight. This low level signal is first amplified, and then converted into a digital number by the circuitry in the digital indicator. The microprocessor within the indicator will process this number in a variety of ways, including linearity compensation and temperature correction. The final value of the digital is then shown on a display screen for use by the scale operator.rnrnMost weighing requirements can be solved by purchasing a scale that was designed for the specific user application. For example, many shipping departments need to determine the weight of the packages that are being sent out. If the weight listed is more than the actual weight of the package, then the shipping costs will be unnecessarily higher. If the weight listed is less, then the transportation company may re-weight and correct the shipment weight, and charge a penalty. Depending on the packages being shipped, a pallet scale, floor scale or platform scale may be the solution of choice.rnrnAnother common industrial requirement is calculating the number of identical pieces on a package or container. The industrial scale that would be appropriate in this case is a parts counting scale. A small sample of parts are counted by hand and placed on the scale platform. The correct sample size is entered, and the digital scale calculates the weight of an individual piece. When an entire container of parts are placed on the scale platform, the scale may divide the net weight by the piece weight, thereby calculating the total number of pieces in the container.rnrnWhile these scale solutions are appropriate for these specific requirements, there are any number of problems where a standard industrial scale will not be useable. One example is the problem of determining the center of gravity of an item. In this case, three custom load cells may be required to support the item. By measuring the weight detected on each of the load cells, calculations may be performed to indicate the center of gravity of that item.rnrnAnother special weighing requirement involves load cells to be placed on a hospital bed. It is necessary to have continuous access to the current weight of the patient. It may be necessary to monitor possible fluid retention. Or it may be used as an alarm to detect that the patient has not left the bed. In this case, a load cell may be placed under each wheel of the bed. But that solution is only effective if the bed is stationary. If it is also a requirement that the bed be rolled around, a custom load cell must be designed for this custom weighing application. In this case, the load cell can be placed between the legs of the bed and the bed wheels, or casters. A receiving slot is formed into the load cell to receive the stem of the caster. Special mounting brackets are attached to the other end of the load cell so that it can be rigidly attached to the bed.rnrnYet another custom weighing application can involve a patient dialysis machine. In this case, the weights of bags of fluids must be carefully monitored. Again, a custom load cell may be built that has a hanger attachment for the bag.rnrnWhile standard load cells technology can solve many problems, in some cases there may be the need for much more accuracy. This can be the case in the formulation of pharmaceuticals. Specially designed custom ultra precision transducers can offer up to twenty times greater accuracy than standard transducers. One choice of technology in this case is known as a force motor, which counteracts the weight of the object with an electromagnetic force. While this does offer the required accuracy, it is also a very expensive solution. An alternative is Arlyn Scales