Special Steel Transducers Improve Industrial Scales

The most critical part of any industrial scale is the transducer which senses force as an input and provides an electrical signal as an output. These transducers are used in most standard types of digital scales. For smaller capacities, a single transducer in the middle of the scale platform is utilized for bench scales. Parts counting scales up to about 150 pound capacity use a similar device. Larger electronic scales may use four identical transducers, one in each corner of the platform. This is the case with platform scales within the range of about 1000 pound capacity. Drum scales use almost identical transducers. Large floor scales use a larger variety of this type of transducer to accept larger loads, up to 20,000 pounds, and even larger. All of these transducers share many similar characteristics. The most common type is known as a strain gage transducer, or a strain gage load cell. The load cell device is designed so that it will bend in a very controlled pattern when it is acted upon by a force. This is the case when an item is placed on the industrial scale platform. Gravity acts upon this item, and the load cell will bend. The amount of bending is directly proportional to its weight. The load cell is limited in the total amount that it will be allowed to bend. If it bends too much, it will not return to exactly its initial position when the weight is removed from the platform. Instead, the material of the load cell will undergo a change which may be referred to as plastic deformation. The bending of the load cell in a manner which is proportional to the force that it experiences, and the return of load cell to its original position is actually a very good description of a spring. And, in fact, the design of a load cell is meant to produce a transducer that is as close to an ideal spring as possible. A special type of electrical resistor, known as a strain gage, is carefully attached to the load cell. Most commercial digital scales will use four strain gages in order to minimize the effect of temperature on the resistors. As the load cell material bends, the strain gages that adhere to it will stretch with it. When the load cell again returns to its non-loaded position, the strain gage will also return to a non-stretched condition. When the strain gage stretches, its resistance will increase. As it returns to the original position, the resistance will also return to its original value. By measuring the amount of resistance, the amount that the strain gage has stretched can be calculated. By association, this will be proportional to the amount of bending of the load cell, and therefore the amount of weight that has been placed on the industrial scale platform. This change of resistance produces an analog signal which may then be converted to a digital representation. A microprocessor device within the digital scale will receive the digital value, will process it appropriately, and then sends the weight value to the digital display. The accuracy of the electronic scale is completely dependent on the transducer. For this reason, the material and design of the load cell should be selected to achieve the best spring performance possible. A very popular transducer material is aluminum. It is quite inexpensive, and it is very easy to machine into the desired shapes. But aluminum also has some non-desirable characteristics. Most importantly, the modulus of elasticity is quite low when compared with some other materials. Under conditions of high load, the aluminum can deform past the point where it will return to the non-loaded position. In this case, the transducer will have been damaged beyond repair. Steel is a better choice for the transducer. It will accept significantly higher load conditions without damage. But because steel will corrode, it is generally plated with nickel. Unfortunately, even with the plating, corrosion will eventually occur. This is especially damaging when this happens underneath the areas where the strain gages have been bonded to it. A special stainless steel alloy, known as 17-4 material, is a much better choice. Although it is considerably more expensive, and much harder to machine into its shape, it will reject shock loads that would damage other types of transducers. The best quality load cells, such as those found in Arlyn Scales line of industrial scales, will use this alloy that has been precipitation hardened. After machining, it is further heat treated to an even better condition. Finally, strain gages are bonded to the surface, and then protected with a number of layers of coatings.