Lightweight Hollow Shaft Servo Actuator

An innovative, compact and lightweight hollow shaft servo actuator, designed and manufactured by Harmonic Drive, is playing an important role in helping to ensure precise alignment of communications antennae mounted on offshore oil and gas platforms. Developed by the Norwegian company, Nera Networks, the antennae systems are used for transmitting voice and data signals between platforms and supply and support vessels, and have to function in some of the world's most inhospitable environments. Nera Networks is part of the Nera Group, one of the world's leading developers of wireless telecommunications using microwave and satellite technology, and has specialised in offshore communications systems for over thirty years. The company's latest systems utilise high performance broadband technology, providing an extended bandwidth of up to 155 Mb/sec via specially designed antennae that are capable of operating reliably even in heavy seas, sub-zero temperatures and aggressive environmental conditions. One of the most challenging tasks in these conditions is to ensure that each antenna is correctly aligned at all times, regardless of the pitching or rolling of the platform to which it is mounted or the prevailing wind speeds. To resolve this problem Nera Networks worked closely with Harmonic Drive to develop a stabilising system based around the FHA-C hollow shaft servo actuator, to control movement of each antenna about the azimuth and elevation axes. The FHA-C combines an AC motor, gearbox and encoder in a single, fully integrated unit, and is capable of producing high levels of torque and positional accuracy from an extremely compact and lightweight unit, which is easy for Nera Network to install and interface to other system components. At the heart of each FHA-C servo actuator is the latest Harmonic Drive gear technology. This is based on a simple three part assembly, with an integrated Wave Generator, Flexspline and Circular Spline forming an extremely robust mechanism. This offers exceptional positioning accuracy, to within one second of arc, with repeatability of just a few seconds of arc, plus a high torque capacity and torsional stiffness, zero backlash and a low weight and small size. Positional accuracy and speed of movement are further enhanced by means of the integrated AC servo motors, which provide exceptionally smooth torque curves, with output speeds up to 200 rpm, while a TTL-encoder mounted directly onto the motor shaft provides precise and immediate feedback data. This level of positional accuracy is crucial for Nera Networks, to ensure that each antenna remains locked onto satellite signals. Perhaps as importantly, however, is the ability to mount each antenna directly to the actuator, by means of a specially developed cross-roller output bearing that resists axial and radial loads and high tilting moments, plus the use of a hollow central shaft that allows cables to be routed directly through the actuator. Combined, these features have enabled Nera Networks to reduce considerably the overall weight and size of each installation, making the systems simpler and less expensive to transport by helicopter and minimising the use of valuable space on each offshore platform.