A force transducer converts measured forces representing force, weight, and pressure into transmitted output signals. These signals may be sent to indicators, controllers, or data acquisition systems connected to computers. Most force transducers have either analog or digital electronic, pneumatic, or hydraulic outputs, depending on the needs of the application. A force transducer can be as small as the head of a pin, measuring a mere gram of force (1/28th of one ounce) or as large as a bridge column, measuring hundreds of tons (1 ton equals 2000 pounds or 909 kilograms).
Early force transducers were hydraulic. Cylinders, bellows, or diaphragms converted forces to pressure signals that were read on indicator gauges. An advantage of hydraulics is that indicator gauges in different locations on the same line enabled several people to obtain and use the information simultaneously. Another is that hydraulics are inherently sealed systems, making them generally rugged, reliable, and impervious to water and other contaminants.
Later, force transducer systems with pneumatic outputs came into use, which allowed easy integration into pneumatic process control systems using the universal 3-15 psi output convention. Many users of hydraulic systems merely installed a pneumatic pressure transmitter in the hydraulic tubing from the hydraulic force transducer already in place. Pneumatic force transducers were also widely used to directly control winding tension brakes on rolling machines in the paper industry.
Advances in the art of strain gauge sensors since the 1950s changed the way forces would be measured. The sensing element was based on the relationship between electrical resistance and elongation, twisting, or other physical distortion of a conductive filament wire, foil, or thin film from its normal rest position. When these conductors were bonded to beams, pins, shafts, or plates, they allowed the implementation of highly accurate, repeatable, and rugged force transducer systems called load cells. They would be applied to every area of human enterprise from industry and commerce to transportation, health care, scientific research, and personal electronics. They are used in weigh bins, scales, and for medical measurements such as blood pressure.
Force transducer systems based on strain gauge sensors or load cells are generally inexpensive to produce. They include voltage excitation for the sensor and balancing bridge circuit, an amplifier section, scaling, and conditioning electronics for the output. Analog outputs can range from direct current (DC) voltages that predominate scientific, medical, and defense applications, to standard DC current outputs of 4-20 milliamps for industrial control systems. Force transducers directly connected to computers and multiplexers can incorporate RS-232 serial interfaces, Universal Serial Bus (USB) connections, and industrial data highways such as Modbus®. Food scales in dieters’ kitchens or digital tire pressure gauges on key rings both incorporate miniature force transducers with simple liquid crystal displays (LCDs).