Motion simulation is crucial in the evaluation of sensors and detectors, including vibrating testing of vehicles or airplanes, response testing of accelerometers or gyroscopes, and image sensors such as those used in high end cameras and even smartphones. Repeatable multi-axis motion simulation can help tweak advanced algorithms in image stabilization systems and verify the performance of opto-mechanical or electronic stabilizers.
Recent advances in precision 6-axis parallel mechanisms (hexapods) and controllers allow for the design of very compact test set-ups. Parallel-kinematics positioners provide very flexible motion in all degrees of freedom, not unlike the human hand – which is why they have also found use in surgical robots. For meaningful test results, simulators need to provide highly repeatable motion mimicking precisely the same trajectory, acceleration and deceleration profiles as the original motion. This involves low friction, low-noise mechanics with sub-micron resolution and very powerful controllers capable of crunching huge amounts of data because in a parallel-kinematics system, even motion in one degree of freedom involves coordinated motion of all 6 actuators.
Reduce Vibration and Enhance Optical Resolution
With the new simulation tools that optical engineers have at their disposal, the performance of image stabilization mechanisms and algorithms can be easily improved. One way to actively enhance resolution and reduce vibration in optics or imaging chips is based on piezo-ceramic drive technology.
Piezo actuators, a special form of electro-ceramics, are widely used for applications where precision, speed, and stability are required in a small package. To make them more accessible for OEM designers, manufacturers package the actuators inside an arrangement of flexures (typically made of aluminum, steel or titanium) providing precision guidance and amplified motion along with a simple mounting interface.
Amplifying the motion of the piezo ceramics comes at a small price. With increasing amplification ratios, the responsiveness is reduced. Nevertheless, well-designed piezo flexure actuators can still provide sub-millisecond step-and-settle times, significantly faster than any other conventional actuator.
Designed into flexure arrangements, piezo mechanisms can provide multi-axis motion, for example, in the form of compact planar scanners or tip/tilt devices. These can be easily integrated in high-end custom or scientific cameras for image stabilization or resolution enhancement purposes.
Piezo actuators are free of wear, no lubrication is required, they are non-magnetic and vacuum compatible. With the absence of friction and wear, they can provide billions of cycles of maintenance-free service. The ceramic encapsulated PICMA® piezo actuators, developed by PI, were life tested by NASA/JPL and survived 100 billion cycles without failures – they have been running on the Mars Rover’s Science Lab for almost 4 years.
Improving the Performance of High-End Detectors
Low-light conditions create difficulty for imaging applications, from astronomy to microscopy. Low-light intensity means neither the resolution of the chip nor the exposure time for changing or moving objects can be freely chosen. Typical applications are fluorescence microscopy, white-light interferometry (OCT in medical technology or general surface structural analysis), surveillance cameras and cameras for aerial photography. Further fields of application are scanners used to digitize analog data, for example, technical drawings, art and paintings. Pixel sub-stepping makes it possible to significantly improve the resolution with relatively little effort.
The resolution of digital recording methods is determined by the number of imaging pixels of a CCD or CMOS chip, for example. If one wishes to increase the resolution, the number of imaging pixels must be increased. There are basically two solutions – either increase the size of the recording chip or decrease the size of the pixel. The first case requires a larger recording device and different imaging optics. In the second case, the light sensitivity decreases with the pixel size. This reduces the separation between image signal and noise signal which, in the end, may even decrease the image quality despite the higher resolution.
Pixel Sub-Stepping for Super-Resolution Imaging
With pixel sub-stepping, the recording area is moved on a defined path with a defined frequency. This dithering, where the travel is a fraction of the size of a pixel, causes the pixel to be exposed several times on the recording area, producing a virtual pixel multiplier which can significantly improve the resolution. The various images produced in this way are subsequently super-imposed to form the final, high-resolution image, a process also known as super resolution.
Since these stabilization and pixel sub-stepping methods are based on motion, a drive is required which meets all the performance criteria for mechanical precision and lifetime. The drives differ, with the application, but they all have crucial features in common: the motion must be reproducible in multiple dimensions, and the travel is of the order of the pixel size, such as a few tens of micrometers or less.
The dynamics required range from a few hertz for still images up to the kilohertz range for video recordings. The basic requirement for high-resolution biometric CCD/CMOS scanners used to identify persons by their fingerprints is a scanning frequency of between 1 Hz and 5 Hz, at a response time of less than 1msec, for example. The travel for the drives is between 5 µm and 15 µm, with a precision of better than 0.5 µm. The drive solution must occupy the smallest possible mounting space.
Microscopy and Astronomy Applications
Fast piezo-driven scanning devices also find application in astronomy and microscopy. In astronomy, piezo-driven active tip/tilt mirrors (usually secondary or tertiary mirrors) correct for atmospheric distortions in real time, basically un-twinkling the stars. This requires bandwidth of several 100 Hz and large optics up to 300-mm diameter.
To break the diffraction limit in light microscopy, several scanning techniques are available, requiring nanometer resolution and again, relying on piezoelectric drives. Fast multi-axis motion also allows the real time recording of 3D images with nanometric resolution.
Super resolution microscopy is based on scanning techniques with nanometer and sub-nanometer resolution. Faster scanning means faster acquisition of images and lower risk of photo bleaching effects.
About the author(s)
Stefan Vorndran is VP of marketing and tactical engineering at PI (Physik Instrumente) L.P. He holds an MS in EE and brings with him over 25 years of experience with nano-positioning and piezo-motion applications.
Scott Jordan is Director of NanoAutomation Technologies for PI (Physik Instrumente) L.P. A physicist by training with an MBA in New Venture Management and Finance, he publishes frequently on nanotech developments, including novel instrumentation, software and microscopy techniques.
About Physik Instrumente L.P.
Physik Instrumente L.P. (PI) is a manufacturer of nano-positioning, linear actuators and precision motion-control equipment for photonics, nanotechnology, semiconductor and life science applications. PI has been developing and manufacturing standard and custom precision products with piezoelectric and electromagnetic drives for over 40 years. The company has been ISO 9001 certified since 1994 and provides innovative, high-quality solutions for OEM and research. PI has a worldwide presence with 10 subsidiaries and over 750 staff.
For more information please contact Stefan Vorndran, VP Marketing for Physik Instrumente L.P.; 16 Albert St., Auburn, MA 01501; Phone 508-832-3456, Fax 508-832-0506; email [email protected]; www.pi-usa.us.