Metrology and Test Equipment for Qualifying Nanopositioning and Motion Control Systems

Part 1: What Tools are Required

When it comes to high performance motion control and nanopositioning equipment, test and metrology data matter, as much as the methods used to acquire the data. PI engineers have been working for years on better ways to measure the figures such as dynamic performance, precision, resolution, repeatability but also geometric accuracy of all kinds of micropositioning and nanopositioning devices, from single axis motorized actuators to complex multi-axis parallel kinematic motion system.

When each individual measurement and qualification step is seen as an instrument for quality assurance, only motion systems and nanopositioning components within the guaranteed specifications will reach the customer.

Depending on the type of motion control component of positioning system, different criteria apply and for highly dynamic applications, rise time, settling time, resonant frequency and body plots may be important, while for static applications stiffness, long term stability, and maximum load and position hold capability without power can be more meaningful.

In either case, the accuracy of the measuring equipment should be significantly better than the component that needs to be tested, in terms of stability, resolution, linearity, etc.

1.1 Nano-Metrology Labs

Nanometer accuracy cannot be tested on an office desk. As the basis for high precision measurements, PI designed special nanometrology labs, with multiple vibration and temperature isolation functions. Anyone who has spent time at the top of a tall TV/radio tower can understand that the place with the lowest amount of vibration and motion is usually found in the basement of a building, which is where the highest performing PI nanometrology labs are located. By separating the foundation of the lab from the rest of the building, seismic influences from the outside are further suppressed. Equipment is mounted on vibration isolated honeycomb tables. A room-in-room design with two stage air conditioning ensures temperature stability better than 0.25 Kelvin over a period of 24 hours. 15 special measuring laboratories are available at PI's headquarters in Karlsruhe alone, with additional labs in its engineering and production facilities in USA and Asia. Five measuring laboratories with special seismic, electro-magnetic, acoustic and thermal isolation allow measurements down to the picometer range and long-term positional stability tests with nanometer precision which is only possible under such conditions.

View into a nanometrology laboratory with six-fold isolation.

Temperature stability of a test lab over several days. The variation is in the .1 degree-range.

Available Metrology Labs at PI HQ. *The VC classification is different for each room. Simple seismic-decoupled rooms achieve classes VC-D to VC-G, corresponding to maximum vibration of <6.25 μm/s in a frequency range of 1 to 100 Hz. Multiple seismic-decoupled rooms achieve classes VC-F to VC-I, corresponding to a maximum vibration of <0.78 μm/s in a frequency range of 1 to 100 Hz.

In (large) series production, a significant part of the tests already take place directly in the production environment allowing deviations in the process to be detected immediately, enabling high yield, and reducing rejects and costs for reworking.

For this purpose, PI develops fully and semiautomatic measuring and test stations that are validated for their suitability before they are used in production.

A great variety of different measurements need to be conducted under specific ambient conditions. This is where the great variety of test tools in our different application laboratories is key. Vacuum chambers with a base pressure to 10-11 hPa allow functional tests and residual gas analysis, which is very important for applications such as crystallography, semiconductor technology or optical coating. Cryogenic chambers are available for low temperature examinations down to 77 °K (condensation point of nitrogen) allowing startup, function tests, and lifetime tests under extreme conditions.

View of semiautomatic test stations for large series production

A high load lifting and tilting rotary table is available to test motion and positioning performance of systems at different orientations. In this picture, a hexapod 6-axis stage with load is measured interferometrically at different angles.

A high load lift and swivel table allows with a load capacity of 5000kg allows qualifying of positioning systems at random angles with application-specific alignment and load. In a special heavy-load hall, three gantry cranes span the working area, and lift and transport the components of the high load precision positioning systems.

All measuring equipment is subject to recalibration (ISO9001) guaranteeing meaningful results with minimal uncertainty and high reproducibility with respect to national and international standards. Interferometric measuring systems are frequently used due to their high resolution, bandwidth, and noncontact, wear-free nature.

Detailed Measuring Equipment Overview (PDF)

In addition to standard measuring tools, PI has access to high end metrology tools. Here are a few examples.

We use laser scanning vibrometers, for high dynamic measurements. They provide insights into unwanted vibration modes and help to improve the structural rigidity of our high precision motion products. We also use them to analyze the motion characteristics of piezoelectric transducers and ultrasonic piezoceramic motors.
Special capacitive gauges and software are used to analyze eccentricity and wobble of rotary tables.
3D CMMs (coordinate measuring machines) are used for incoming inspection and for multi-degree of freedom motion systems.
White light interferometers are available for surface topology analysis and for material research.
A 6-Axis laser tracker is available for the qualification of multi-axis motion systems and 6-DOF hexapod parallel robots.

Laser scanning vibrometry provides real-time data of high-speed motion even at micrometric scales. The graphics shows the deformation of an electrically excited ring-type piezoelectric transducer. Data recorded with Polytec 3D laser scanning vibrometer.

The 6D laser tracker offers position measurements in three linear and three angular degrees of freedom — The 6-axis laser tracker provides high resolution position data in all degrees of freedom.

The ISO 230-2 and VDI/DGQ 3441 standards are the basis for qualification of PI products and evaluation of the measurands. The qualification data is always verifiable for each individual product. The measured data is compiled in a database and used for process control. Traceability at product level is particularly important for large production runs. The metrology report is provided with all high-performance motion and piezo nanopositioning systems many systems.

Typical measurands in a measuring log showing the linearity and impulse response of a piezo system.

Standardized process control with full documentation of the individual measuring logs

Measuring data is transmitted in real time, saved to a database, and visualized on a web-based user interface. Production and test engineers have immediate access to all process data and can intervene immediately if necessary.

Automatic saving of the measured data after measuring
Real-time display of the measured values in the control charts
Multi-control charts for maximum process control
Dynamic calculation of warning and intervention limits
Download of raw data and customer log
Calculation of process capability Cpk level
Pareto chart for process analysis and optimization
Correlation chart to check the dependence of two measurands
Various export functions

The software for controlling the fully and semiautomatic test and measurement stations is developed in-house by PI. All measuring equipment can be configured directly and polled from the software, eliminating possible error sources between for test location and database. Software development makes use of current development standards such as version control systems, bug tracking, automatic build systems, static code analysis, unit tests, and code reviews. This ensures that the software is always traceable and the quality standards are adhered to.

Additional details on metrology for nanopositioning equipment are found here: Measurement Standards for Precision Motion Control Systems