Microscope stages are traditionally positioned using a system of lead screws. These mechanisms have their limits, however. They are usually designed with either high gear ratios for slow and high-resolution movements, or with low gear ratios for velocity, which reduces their accuracy.
These compromises can be avoided by using microscopy stages that operate with ultrasonic piezo drives. As well as eliminating the issues associated with gear ratio choice, piezo-driven stages are also self-locking when at rest, providing an extremely stable image.
Benefits of Piezo Drives for Microscopy Stages
XY stages used for positioning samples in microscopes should be as flat as possible to make them easy to integrate and allow easy access to samples or, in the case of inverted microscopes, permit the revolving nosepiece and other elements below the stage to be reached.
Linear direct drives, powered by compact piezomotors, make many of the attachments required for conventional stages, such as lead screw ducts and flanged-on stepper motors, unnecessary. They also provide unparalleled flexibility of velocity and high resolution, which are essential elements for a microscope which will be used for a variety of different tasks. Their superb image stability and low-noise operation also makes them less likely to tire or interfere with the user over long periods of time using the equipment.
 |
| Figure 1. Versatile PI M-687 microscopy stage, which has a very flat design with a thickness of only 30 mm and does not have any interfering lead screw ducts or motor protrusions. It can be combined with the piezo Z stages which are also very flat |
PI (Physik Instrumente) has used its many years of experience with piezo actuators and nanopositioning systems to develop an extremely versatile microscopy stage that has a very low thickness of only 30 mm and no disturbing lead screw ducts or motor projections (figure 1). It is suitable for displacements of up to 85 × 135 mm. Its large aperture accommodates holders for Petri dishes, standard object slides or microtiter plates.
Velocity Stability
The ultrasonic piezo drives used allow good velocity stability across a very wide range, from 10 µm/s to 100 mm/s. The stages can thus be used for direct visual observation at great magnification (objective 100x at approx. 10 µm/s) without visual artifacts such as "jerking", which can interfere with and tire the user. The same microscopy stage is equally well suited for automated scanning applications that require high velocities of up to 100 mm/s and short settling times.
Dynamic Range
This high dynamic range of the XY stage is due to the piezo ultrasonic linear drives used (figure 2), in which the high-frequency oscillation of a piezoelectric actuator generates the propulsion force. The patented ultrasonic motors work directly without intermediate elements, such as lead screws or gears, are backlash-free due to their great stiffness, and position very precisely. When using the stage along with a linear encoder, positioning resolution is 0.1 µm, and the bidirectional repeatability is 0.4 µm. As a result, so-called "points of interest" can be reliably found again and precisely approached. This is also advantageous in "high content screening" analysis technology, when many samples (e.g. tissue samples) are to be scanned. In addition to short settling times, the good repeatability is important in this kind of application for "tiling" the images.
Low Thermal Drift
Unlike directly driven electromagnetic linear, stepper and DC motors, the piezo-based drive does not require any energy to maintain the position of the stage, and thus no heat is generated whilst the stage is at rest.
This means that there is practically no thermal drift compared to conventional electromagnetic drives, and the position is maintained with a high degree of stability. This is especially advantageous for super-resolution microscopy, where one recording can take several minutes.
Figure 2. The functional principle of PILine® linear drives. They offer maximum performance with respect to accuracy, velocity and acceleration in a very small space.
Components of the Piezo Drive
The drive consists of a stator containing the piezoceramic oscillator and a runner referred to as a friction bar, which is directly fastened to the movable part of a carriage. The oscillation profile of the piezoceramic then produces the forward and backward motion of the drive, making the travel range theoretically unlimited. The ceramic is preloaded against the friction bar and thus keeps it in its ‘at rest’ position when powered down.
Accessories for the PI M-687 Microscopy Stage
XY Controller
Due to its "trace memory", the controller which is adapted to the microscopy stage and has an extensive software package, allows the trajectory to be displayed as a position/time diagram on a connected host PC. Its control parameters can thus be perfectly matched to the respective application.
Piezo Z Stages
The piezo Z stages from the same manufacturer, which also have a very flat design, are ideal for positioning tasks in the direction of the optical axis, e.g. for stack recordings. They are available with a clear aperture of 160 × 110 mm and a stroke of 200 µm and can be mounted on the XY stage without an adapter and controlled with the same controller.
This information has been sourced, reviewed and adapted from materials provided by PI (Physik Instrumente) LP, Piezo Nano Positioning.
For more information on this source, please visit PI (Physik Instrumente) LP, Piezo Nano Positioning.