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Aerospace companies are moving towards more exotic materials in pursuit of weight savings. While these materials are lighter and stronger, they are also taxing to machines. For instance, they may fracture, chip or burn if they are not handled and machined correctly. Since many are composite structures, internal damage might occur where it can only be identified with expensive inspection methods.
A good example would be carbon fiber. This is usually combined into a composite with an extremely high strength-to-weight ratio and is remarkably strong and rigid but at the same time it is also brittle. When carbon fiber is held in place for secondary machining operations, clamping forces should be carefully controlled to prevent damaging or crushing the parts.
Problem
A builder of precision aerospace manufacturing equipment was developing special-purpose CNC machines for drilling holes in thin, long carbon- fiber-composite components. The nature of the part was such that it had to be clamped at each end, to be accomplished by servomotors. However, as some variation was inevitable in part length, moving the clamps to fixed positions risked dropping or damaging the work piece. To steer clear of this, the machine designers wished to maneuver them in to obtain a predetermined load.
In order to reduce automation cycle time, these servomotors would move the clamps in quickly before slowing to 1/10th of an inch per second. Servomotors offer 200-300 pounds of force, so the machine should be able to sense the onset of clamping and halt the servomotors before forces became very high.
Solution
OMEGA faced three challenges due to this problem. First was the need to integrate a compression load cell into the servo-clamping system of the automated machine. Second, as the machine controller would be remote from the load cell, the millivolt output signals required amplification and conditioning to prevent any degradation affecting accuracy. Third, all this had to happen quickly to fulfill the cycle time and speed requirements of the automation controller.
OMEGA recommended the use of LC201 series subminiature load cells for measuring the clamping force. Measuring just 6.4 mm (1/4") thick and 19 mm (0.75") in diameter, these cells determine both tensile and compressive forces up to 500 N (metric models) or 300 pounds. Both millivolt output signals and excitation voltage are handled by a four-conductor shielded cable deals. For convenient installation, the load cells are available with stainless steel construction and dual mounting studs to protect against damage.
A signal conditioner was required for getting the load cell output signals to the machine controller. However, traditional DIN-mounted signal conditioners lacked the speed required for precise motion control. That forced OMEGA to propose the latest IN-UVI high speed signal conditioner. This conditioner has a response time of 200 µS, and would make sure that the controller stopped the clamping motion before affecting the work piece.
The IN-UVI in-line Wheatstone bridge signal conditioner is packaged in a compact IP65 stainless steel enclosure. Ideal for space-constrained applications, it enhances the signal-to-noise ratio, enabling millivolt signals from the load cell to be sent across longer distances. A supply voltage of 22 to 32 Vdc offers the load cell with one of two user-selectable excitation voltages and amplifies the millivolt output signal to suit the controller requirements.
Results
OMEGA’s signal conditioning and load cell solution fully met the customer’s requirements. Over 20 machines have now been outfitted with this clamping force control system. Each system uses two IN-UVI signal conditioners and two LC201- 300-pound load cells (for redundancy). They operate at high speed, and deliver the signals required for precise clamping force control. This has resulted in higher reliability, faster response and most importantly — reduced damage to work piece.
This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.
For more information on this source, please visit OMEGA Engineering Ltd.