In order to achieve this high path accuracy, PI developed a hybrid concept where a motor drive screw that is suitable for heavy loads and long travel ranges is combined with a piezo actuator (Figure 5). Serial combination of both very different drives results in a powerful and high-precision positioning system (Figure 6).
Piezo Actuators Position with Sub-Nanometer Accuracy
Precision motion that results when an electrical voltage is applied to a piezoelectric material is of particular importance for nanopositioning. The electrical power is converted into mechanical energy directly inside the crystalline solid state, which means that there are no rotating or frictional parts.
The piezo actuators not only work with high precision but are also maintenance-and wear-free. They can move large loads with weights up to several tons. Electrically, they act as capacitive loads and need virtually no power in static operation. The behavior in the power circuit is very much like an electrical capacitor. Similar to capacitors, they do not generate any heat in a static condition.
The lifetime of piezo actuators is also convincing: In the case of PICMA multilayer actuators (Figure 7), the active layers consist of thin ceramic films and are surrounded by an all-ceramic insulating layer that protects the actuators against air humidity and failure from leakage current. The monolithic piezoceramic block of such an actuator is very reliable even under extreme ambient conditions and high temperature differences.
The piezo actuators used in the hybrid drive for the telescope segments are also encapsulated in sealed metal bellows filled with nitrogen (Figure 8) in order to reach the 30-year lifetime necessary in the adverse ambient conditions in the Atacama Desert.
A High-Resolution Sensor for Both Drive Systems
A further feature of the hybrid drives is the common high-resolution sensor that helps to control both drives simultaneously and continuously. This is the only way to implement the high resolution of the piezo actuators over the entire travel range.
The high-resolution sensor is an incremental optical encoder placed near the drive axis (Figure 9). It operates at a resolution of 0.1 nanometer and is also not sensitive to the changing environmental conditions prevailing at the telescope's location in the desert.
The motor drive screw is suitable for heavy loads and long travel ranges starting at a few millimeters and going up to one meter. The piezo actuator provides a nominal displacement of approximately 0.1 to 0.15 percent of the actuator length but nevertheless, achieves a positioning accuracy in the sub-nanometer range with one high-resolution sensor and can therefore compensate for the inaccuracies of the motor drive screw. The drive screw is driven by a brushless, high-torque motor via a high-ratio reduction gearhead.
The gearhead ensures zero-play operation and guarantees a constant transmission ratio. The motor can, therefore, be very small even though large masses have to be moved. The high transmission also supports self-locking of the motor when at rest.
A dedicated controller controls both drives simultaneously and also controls the high-resolution position measuring system. The servo-algorithms consider the motor and the piezo system as a single drive unit and compare the actual motion with a calculated trajectory.
The control principle of the hybrid drive is easy to understand (Figure 10). The motor voltage is derived from the control voltage of the piezo. The greater this voltage, the faster the motor runs. When the piezo expands, the motor drives the drive screw in the same direction. In this way, the rough positioning of the drive screw is supplemented by the fine positioning of the piezo. At the same time, the drive screw always moves the piezo near to its zero position automatically. This gives it the best chance of correcting the position in both directions. In this way, relatively long travel ranges can be combined with an extremely high positioning accuracy.
The characteristics of these types of hybrid drives are not only useful for telescopes, but are always a practical solution when a position needs to be detected with high precision and moved repeatedly over long travel ranges, or when a target position needs to be reached with nanometer precision.
This article was written by Oliver Dietzel, Team Leader for Project Management Corporate Platform Development, Physik Instrumente (PI), Auburn, MA. For more information, visit here .