Precise Control of Force
Sandia National Laboratories Posted 07/09/2008
Systems developed by Sandia's Intelligent Systems and Robotics Center (ISRC) often require precise control of the forces applied. Developers add load cells to robots to sense force and moment. These load cells are called six-axis force-torque sensors because they must accurately and separately measure all three components of an applied force and all three components of an applied moment. Typically these load cells cost around $5000 each and are relatively fragile. Because the high cost and fragility are often an impediment to more widespread application of robots in hazardous environments, we have developed a load cell which will sell for a fraction of the cost and provide sensitivity and accuracy comparable to the existing higher priced units.
A new approach was taken to develop a unique simplified six-axis load cell. It consists of a prismatic section of material with semiconductor strain gauges attached around the circumference. By placing 24 strain gauges in suitable locations around one or more circumferences of the section, six four-arm bridges are constructed so that each bridge produces an analog signal that is proportional to one of the six loads and is unaffected by the other five loads. The combination of very simple geometry and regular positioning of the strain gauges makes this six-axis load cell much less expensive to produce than other designs.
Besides a much lower production cost (less than $400), the new load cells are easier to manufacture, have minimum signal processing electronics, produce direct analog outputs of all six axis loads, and can be manufactured as an integral part of a robot.
A 1-inch-diameter version of the load cell will produce about 1 mV of output for a load of 1 pound or a moment of 1 inch-pound. In addition to being quite sensitive, the load cell is also very rugged. A 1-inch-diameter version can safely handle loads of 1000 pounds and moments of 1000 inch-pounds. The design is readily scaleable over a range of sizes and loads.
The simplified six-axis load cell is well-suited for any application where forces and moments acting on a robot must be monitored. Because of the simple design and rugged construction, it can be used in areas where existing six-axis load cells may not be practical. In addition, the design can be easily modified and simplified even further if less than a full six-axis sensor is required. Since the load cell can be constructed by placing strain gauges on the inner or outer surface of an annulus, an integral version of the load cell can be designed into a robot by properly gauging existing robotic elements. The design is simple enough to make backfitting existing robots with the load cell practical. The simplified six-axis load cell design is protected under United States patent 5,850,044 and is available for licensing from Sandia.