Artist Concept - Mars Curiosity Rover. Image credit: NASA/JPL-Caltech.If NASA's Mars Exploration Program has its way there ultimately will be – us!  Its Curiosity Rover is currently on a 254 day journey between earth and the red planet (scheduled to land in early August). During the 23 months after landing, equivalent to one Martian year, the rover will spend its time analyzing soil samples to help determine the viability of life. We contributed to the mission’s success by checking the accuracy of key components long before this mobile laboratory was ever launched.

Cameras mounted on the rover’s robotic arm will take extreme close up pictures of rocks, soil and (if present) ice to reveal details smaller than the width of a human hair.  The company which built the robotic arm and stereographic cameras used the ultra high accuracy Leitz PMM-C coordinate measuring machine fitted with an LSPS probe head and PC-DMIS 2010 software to inspect the lenses due to the CMM’s high precision and low probing force.

One measurement required the holding of repeatability of 2 tenth true position tolerance of a 76mm diameter lens over a 508mm travel zone. Although it was theoretically achievable on paper due to the precision level of the instrument, in reality it proved more difficult because of uncertainties in the process. Another measurement challenge the company had was keeping support rails straight and parallel as the lenses were lowered into the housing.

Hexagon Metrology’s applications team carried out a detailed repeatability experiment in an attempt to pinpoint the source of uncertainties. We were able to assess that uncertainty in temperature variation played an important role. An auto probe changer and the correct temperature compensation in the process, along with implementation of basic metrological concepts, helped achieve the desired result. Concerning the fit of lenses into the housing, PC-DMIS measured their position as scientists slid them down the rails. This ensured that the lenses remained centered as the camera zoomed.

Controlling for temperature helped to eliminate errors caused by uneven expansion of machine structures and machine components such as probe extensions, stylus, fixtures etc. Using an auto probe changer and selecting a proper soaking time for the measurement parts also helped the process. Repeatability was brought down from eight tenth to approximately two tenth. This helped to stabilize the manufacturing process and reduced assembly times. It also instilled a high degree of confidence in the manufactured product meeting specification with no error margin. After all, it’s impossible to replace faulty equipment on Mars.

Read more about the Mars Curiosity Rover mission in this Desktop Engineering article.