SYMBIOSIS OF FAST DC DRIVE AND PRECISION PIEZO MOTOR FOR HIGH ACCURACY LINEAR POSITIONERS
Miniaturization is much in demand. Whether in the field of electronics, mechanical engineering or biology, high demands have to be met in terms of dimensional accuracy of ultra-miniature structures in particular. To be able to measure structures in the sub-micrometer range reliable, specialist know-how is essential; simply adopting a downsized standard solution from the „big world“ is not an option. Linear positioners, permitting tiny increments beneath a measuring probe by means of piezo actuators, have been a tried and proven method for a long time. They do a reliable job even in the nanometer range. One of the disadvantages of them, though, is the low dynamism of their drives and their limited feed length. A patented system comprising a fast DC drive in combination with a high-precision piezo rotary drive mounted on a common spindle has provided a solution to this dilemma. Rapid delivery to the measurement point, then minimal feed rate for maximum resolution over long positioning distances is no longer a problem.
Ultra-fine motion control in the production of high-purity crystals and in focusing, scanning, adjustment, inspection and measurement tasks in the sub-μm range demands highly accurate, reproducible movements. The conventional approach to this is to run the object being measured past a measuring probe or actuator on a linear positioner. Piezo drives are known for their capacity to deliver ultra-fine stepwidths, but unfortunately their dynamism is insufficient to transport the payload into the work area. The traditional solution too – by which these very low speeds are attained at very low resolutions based on multi-stage, wide-stepped, play-free reduction gearing – means minutes of approach work to reach the measuring position. But long setup times cost money. Precision movement specialist Feinmess Dresden GmbH has now come up with a patented solution for this dilemma. The fast non-precision transport is provided by a geared DC drive, with the fine adjustment then being handled by a highprecision piezo motor. In cooperation with FAULHABER, the drives were optimized to perform this combination of tasks in harmony.
Time is money Ultra-small scale motion involves different principles than those on which normal positioners operate. Because of the short distances covered, the key factor in terms of positioning time is not the maximum speed of the drive but the acceleration and deceleration time and the span of the mechanical excursion of the overall setup. So to improve the dynamism of the positioning process, those three factors have to be paid special attention. The precision measurement specialists delivered the solution by „splitting“ the task, with a tailored drive solution provided for each type of movement. The best solution for housing two drives on a linear positioner is a ball screw drive unit. A drive can be placed at each end of it. The positioning system produced in this way can also be fitted with extra components to enhance efficiency in many applications. And there is another advantage: there is theoretically no limit to the feed length; the spindle can be made as long as required. This also enables larger measurement objects with multiple measuring points to be processed rapidly in one clamping operation. By contrast, conventional (stacked) piezo drives are restricted to just a few millimetres of positioning width.
Utilizing natural properties A good solution means achieving maximum benefit at minimal cost. Even high-tech solutions can be implemented with relatively simple components – you just have to know how. For fast positioning in high-speed mode, for example, a conventional brush motor with rotary encoder connected to the shaft by a bellows coupling is adequate as the drive element. Since its runs for a relatively short time, the heat input by the motor is negligibly low. Depending on the spindle pitch used, a speed range from 0.5 mm/s to 100 mm/s is practicable. This corresponds to the value for „rough positioning“ in many standard solutions. After switching to high-precision mode the special feature of the new solution becomes clear: In positioning mode, the system switches powerlessly – so with no heat input – to the drive featuring a rotary piezo motor at a speed of 0.5 mm/s by way of a permanent magnetic coupling. At rest, the drive then works as a passive spindle brake, damping oscillation and preventing unwanted movement of the positioner system. A high-resolution linear measuring system continuously records the movement and transmits the information to the motor controller. In this way, the drive moves the linear positioner in highprecision mode at a speed of 0.00002 mm/s to 0.15 mm/s – that is to say, a minimal 20 nm per second! The speed constancy at the bottom end of the range is only dependent on the resolution of the linear scale used. The repetition accuracy is <100 nm. The speed range of the positioning system can attain a ratio of over a million to one between maximum and minimum based on the task-splitting technique.
Optimum movement Precision does not have to cost the earth. Conventional precision motors with graphite commutation are suitable for use in dual-speed linear positioners. FAULHABER adapted the standard motors to the application, so saving development lead time and money. Different motors are used depending on the torque required. The 23 mm diameter motors, for example, reach around 7000 rpm and up to 16 mNm. They are specifically designed for use in conjunction with magnetic encoders. These two-channel incremental encoders are available with 64, 128, 256 or 512 pulses per revolution. Plug-in gear units are available for the 22 mm diameter motor to provide the necessary speed reduction and increase. They come in many stepped reduction ratios. FAULHABER GROUP company PiezoMotor AB in Sweden supplies the piezo rotary motor for the high-precision drive. These drives are likewise very compact, being just 32 x 23 mm (L x D) in size and weighing just 70 g. They work with control voltages in the range from 0 to 3000 Hz and reach 13.5 rpm at 2100 Hz. Their turning torque is 80 mNm, and their holding torque 90 mNm. The maximum incremental stepwidth is 0.35 mrad. A 3 mm thick, 6.5 mm long shaft provides the mechanical link to the permanent magnetic coupling or other applications. Efficiency for systems involving ultraprecise positioning in the nanometer range can be dramatically improved by the right idea. Longer strokes, higher precision and faster positioning than conventional models save users valuable production time. Despite these advantages, the new solution employs relatively simple drive components. Tried and proven miniature drive products optimized to the specific application case can usually handle even the most demanding tasks reliably