Motion control technology from PI powers Mars science
Ferried along by the rugged Curiosity rover, the Mars Science Laboratory is the biggest, most advanced suite of instruments ever sent to the surface of another planet. The record of the planet's climate and geology is essentially "written” in the rocks and soil—in their formation, structure and chemical composition. The rover's onboard laboratory will study rocks, soils and the local geologic setting in order to detect chemical building blocks of life (e.g., forms of carbon) on Mars and will assess what the martian environment was like in the past.
In designing the MSL, developers had the difficult task of building a maintenance-free, autonomous science laboratory able to conduct experiments sensitive enough to determine whether the chemical building blocks of life exist in the soil. Compounding this challenge was the need for instrumentation rugged enough to survive a harsh environment and deliver scientifically useful results.
The instrument suite includes more than a dozen distinct systems, and two of the most important include the ChemCam and CheMin. Both of these instruments required the use of piezoelectric resonators to provide reliable, high-frequency motion control.
NASA’s Jet Propulsion Laboratory sought out two companies who had deep expertise in this type of product: Physik Instrumente (PI) and miCos. During the development phase, their piezo and motion control technologies would be stretched further than ever before.
The CheMin, short for “chemistry and mineralogy”, has been used to identify and quantify the minerals present in Mars’ rocks and soil, and can accurately do this above detection limits in a variety of complex natural mineral samples, including basalts. The instrument has been key in recent discoveries that have helped determine the presence (or absence) of water.
A powerful x-ray diffraction instrument that also has x-ray fluorescence capabilities, CheMin will analyze as many as 70 or more samples during MSL mission, each of them taking up to 10 hours of analysis.
In addition to the challenge posed by engineering a sampling arm long and strong enough to hold the weight of CheMin, which is situated near the sample collector, developers had to devise an automated and maintenance-free sample handling solution.
After being collected with the arm rock and soil tools, the sample is reduced to a powder of micrometer-sized grains that are then dumped into a funnel. A small amount of the sample is delivered to the sample chamber for analysis. The powder in the tunable sample cells must be shaken at variable amplitudes and frequencies in the range of 0.9 to 2.2 kHz to homogenize particle size or density segregations.
PI Physik Instrumente’s contribution to the Mars mission, the PI Ceramic monolithic piezo actuators (PICMA) were put to use in the Chemistry and Mineralogy instrument (CheMin).
The precisely controlled vibrations are generated by PI Ceramic monolithic piezo actuators (PICMA) from PI Ceramic. These multilayer ceramic-encapsulated actuators emerged after rigorous tests by NASA’s Jet Propulsion Laboratory as the highest reliability piezo stacks commercially available, and were put to use in CheMin.
According to Stefan Vondran, marketing director at PI, multiple PICMA samples passed 100 billion cycles of life testing with no failures, maintaining greater than 96% of their performance at the end of the test.
The actuators had already been run through demanding tests of the course of several years, but at lower frequency and in higher humidity.
“For earthly applications, humidity is a major lifetime limiting factor of piezo actuators; when PI started development of the PICMA actuators a decade ago the focus was on the ceramic insulation because it provides a barrier to moisture,” says Vondran.
Conformal coatings of conventional actuators, he continues, get penetrated by water molecules which can lead to dielectric breakdown.
The ChemCam, short for “chemistry and camera”, is a laser-induced breakdown spectrometer joined with a high-resolution visible image camera. The determines through laser-based remote pre-testing which rocks in the vicinity of the rover (distances up to 9 m) warrant a closer examination with in situ instruments. The high-power laser beam evaporates the rock and the light emitted from the plasma is analyzed with the spectrometer.
A specially modified and space-qualified MT-40 linear positioning stage from PI miCos focuses the analyzing system at the desired distance by translating the secondary mirror of the telescope. Due to violent shocks and vibration during launch and landing, every component in the stage from the stepper motor to the crossed roller bearings and limit switches had to be closely examined and optimized to eliminate the possibility of failure or performance loss. For fast and precise focusing, high resolution and linearity are required and backlash has to be controlled tightly. This is not an easy task given the very wide operating temperature range of the instrument. Modeling and thermal compensation were used as well as special vacuum compatible lubricants for all moving parts.
In the end, says Vondran, the modified stage passed all the tests and the decision to go with a COTS part for cost control reasons proved right.
Importance of COTS to cost
Shortly before Curiosity left Earth in November 2011, miCos joined the PI family of companies and PI is now prepared for more space-related testing. Failure was not an option, but cost definitely was an issue and budgets are tighter now than in the early days of space exploration. Notably, the technology supplied by PI and miCos used largely commercial off-the-shelf equipment. The PICMA actuators do not differ from the products available to customers, and the MT-40 was slightly modified mainly for vacuum and low temperature operation.
Manufacturers with the best commercial off the shelf products (COTS), says Vondran, proven quality systems and the ability to make the necessary modifications for the tough space requirements will be in demand for 21st century space missions.
“This is our first involvement with NASA’s rover development. We would love to be able to contribute to other missions,” says Vondran.
PICMA actuator test study: Piezoelectric multilayer actuator life test