In pressing operations, measuring the force applied by the ram throughout its stroke can provide critical information about the quality of the assembly. For example, in insertion applications, force and position sensors can detect misaligned parts, missing parts, and oversized or undersized parts and bores. They can also determine if the part has been inserted to the correct depth. In wire crimping, force and position sensors detect broken conductors, poorly removed insulation or worn-out crimping tools.
Force is measured with a load cell. The two main technologies for load cells are strain gauges and piezoelectric sensors.
Strain gauges measure force indirectly. When a force is applied to a structure, the length of the structure changes. Strain is the ratio of this change in dimension to the structure's original dimension. This is what a strain gauge measures.
A strain gauge consists of a thin wire or foil arranged in a grid and bonded to a thin backing. The gauge is mounted to the structure so that the conductors are aligned in the direction of the stress. Any distortion in the structure causes a distortion in the conductors and a corresponding change in electrical resistance. By measuring the change in resistance, a controller can determine the strain and thus, how much force was applied to the structure.
For greater accuracy, a load cell often contains two or four gauges in a "bridge" arrangement. "With no load, all four legs of the bridge are balanced," says Eric Lewis, general manager of Veri-Tek Inc. (Wixom, MI). "When you apply force, resistance between the legs changes."
At the heart of a piezoelectric load cell is a crystalline or ceramic material that produces a voltage when compressed. The amount of voltage is directly proportional to the amount of force. Piezoelectric sensors have the widest measuring range of any force-sensing technology. However, because the electrical signal generated by the piezoelectric material decays rapidly after the application of force, these sensors are more suitable for measuring dynamic loads than static ones.
One advantage of piezoelectric load cells is their ability to measure light loads with no loss of resolution. This characteristic gives engineers some leeway to design flexible and durable equipment. "You could use a 10,000-pound piezoelectric load cell in a 200-pound application, and it would work just great," says Larry Stockline, president of Promess Inc. (Brighton, MI).
A strain gauge doesn't have that ability, so there's less margin for error. "And if you overstress a strain gauge, you will destroy it," says Stockline.
Load cells can be mounted between the ram and the tooling; inside the part fixture; or even to the frame of the press. "The most precise location is where the load is," says Stockline. "So in most assembly applications, the load cell should go between the ram and the workpiece."
Load cells can be mounted on any kind of press, including manual ones. The width of the load cell should match the width of the ram. In addition, the maximum force specified for the load cell should be greater than the maximum force the press can produce. The accuracy of load cells varies, but it's typically less than 1 percent of their maximum load, says Lewis.
On electromechanical presses, the position of the ram is measured with an encoder. On pneumatic and hydraulic presses, ram position is usually measured with a linear variable displacement transducer (LVDT). An LVDT consists of a rod-shaped magnetic core that is free to move axially within a series of coil windings. Any movement of the core creates an electromagnetic imbalance between the coils. This imbalance produces an output voltage that is directly proportional to the direction and magnitude of the displacement.