Linear actuators get a servo look

New actuators borrow ideas from linear motors to provide controlled displacement on a budget. Vice President Copley Controls Corp. Canton, Mass. Historically, industry has depended on ball screws, belt drives, and pneumatic mechanisms for automation’s positioning tasks. But the rising demand for millions of operating cycles, speedier throughput, and programming agility has exposed shortcomings in these mechanisms. There are often trade-offs when it comes to obtaining precise, controllable load positioning. Recently, pneumatics makers have upgraded “bang-bang” pneumatic cylinders with external control valves, position encoders, and servoelectronics in an effort to address some of these difficulties. But it is daunting to close a servoloop around a high-friction piston and squashy compressed air. Control is problematic owing to the elasticity of air, finite air-travel time, air friction, and piston friction. Direct-drive linear-actuator technology can provide a better approach. Direct drive refers to motion created by the direct application of electromagnetic drive force, rather than through a belt, ball screw, or some other intermediate drive. Of course, servomotors have been used in direct-drive configurations for many years. A more-recent development in this area is that of direct-drive linear servoactuators. They produce linear motion directly rather than convert rotary motion to a linear displacement. As is the case with rotary servomotors, the shedding of ball-screw mass, friction, wear, cogging, resonance, and so forth gives direct-drive linear actuators agility and lets them be easily controlled. The most recent advance in direct- drive linear actuators is called a tubular linear actuator. It evolved from linear-motor technology. A conventional way to describe a linear motor is to envision a rotary servomotor sliced lengthwise and then flattened. Motion between stator and what was formerly the rotor will now be linear, akin to a maglev train. Tubular linear actuators have a form factor resembling that of a solenoid. The position of the “plunger” (called a thrust rod