Load Cell Primer

The following is a nice explanation of how the load cell functions and interfaces with your data acquisition system, courtesy of Massload.  Additionally you can download this Load Cell Primer from Revere, lots of great information, a 160k .pdf
 
  System Components: In contemporary control applications, weighing systems are used in both static and dynamic applications. Some systems are technologically advanced, interfacing with computers for database integration and using micro-processor based techniques to proportion material inputs and feed rates. To send the weight information to computers, signal conditioners are utilized to permit direct communication from the load cell via conversion of the load cellķs analog signal to a digital signal.

An entire system can be constructed, one piece at a time, from   basic modules.

Parts of a system can include: 

• Load cells 
• Cable 
• Junction Box (summing up the load cell signals up to one output),
• Instrumentation (indicators, signal conditioners, etc), 
• Peripheral Equipment (printers, scoreboards, etc.)

Fundamentals: A load cell is classified as a force transducer. This device converts force or weight into an electrical signal.

The strain gage is the heart of a load cell. A strain gage is a device that changes resistance when it is stressed. The gages are developed from an ultra-thin heat-treated metallic foil and are chemically bonded to a thin dielectric layer. "Gage patches" are then mounted to the strain element with specially formulated adhesives. The precise positioning of the gage, the mounting procedure, and the materials used all have a measurable effect on overall performance of the load cell.
 
 

Each gage patch consists of one or more fine wires cemented to the surface of a beam, ring, or column (the strain element) within a load cell. As the surface to which the gage is attached becomes strained, the wires stretch or compress changing their resistance proportional to the applied load. One or more strain gages are used in the making of a load cell.

Multiple strain gages are connected to create the four legs of a Wheatstone-bridge configuration. When an input voltage is applied to the bridge, the output becomes a voltage proportional to the force on the cell. This output can be amplified and processed by conventional electrical instrumentation.