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The 2,800 rpm rotation was successful, but flight 14 was delayed after the conjunction
Several Martian days, or soils, have been hectic since our last blog post, so we wanted to provide everyone with a status update. March. In our last article, we explained that we were preparing to fly with a higher rotor speed to compensate for the decrease in atmospheric density caused by seasonal changes on Mars. The increase in rotor speed is a big change from the way we’ve been flying so far, so we wanted to go carefully. The first step was to perform a high speed spin test at 2,800 rpm on the ground and, if all went well, the second step was to perform a short flight, briefly overflying our current location, with a rotor speed of 2,700 rpm.
The high-speed spin test was successfully completed on September 15, 2021 at 11:29 p.m. PDT, 11:11 a.m. LMST local time in March (Sol 204 of the Perseverance mission). Ingenuity’s motors spun the rotors up to 2,800 rpm, briefly held that speed, then spun the rotors to a stop, all exactly as sequenced for the test. All other subsystems were working fine. Of particular interest was to determine whether the higher rotor speeds cause resonances (vibrations) in the structure of Ingenuity. Resonances are a common challenge in aerial rotorcraft and can cause detection and control problems, and can also lead to mechanical damage. Fortunately, data from this last high-speed spin showed no resonance at the highest rotor speeds. The successful high speed spin was an exciting achievement for Ingenuity and gave us the green light to conduct a test flight with a rotor speed of 2,700 rpm.
Ingenuity upper swashplate assembly: The upper swashplate of NASA’s Ingenuity Mars helicopter controls the pitch of the upper rotor blades as they rotate and is essential for stable and controlled flight. The swashplate is driven by three small servomotors. Credit: NASA / JPL-Caltech.
The test flight was scheduled to take place on September 18, 2021 (Sol 206) and was to be a brief hover at 16 feet (5 meters) above sea level with a rotor speed of 2,700 rpm. The flight turned out to be uneventful, as Ingenuity decided not to take off. Here’s what happened: Ingenuity detected an anomaly in two of the small flight control servos (or simply “servos”) during its pre-flight self-check and did exactly what it was supposed to do: it canceled the flight.
Ingenuity controls its position and orientation during flight by adjusting the pitch of each of the four rotor blades as they rotate around the mast. The blade pitch is adjusted by a swashplate mechanism, which is operated by servos. Each rotor has its own independently controlled swashplate, and each swashplate is powered by three servos, so Ingenuity has six servos in total. Servo motors are much smaller than the motors that turn the rotors, but they do a tremendous amount of work and are essential for stable and controlled flight. Due to their criticality, Ingenuity performs an automated servo check before each flight. This self-test drives the six servos through a sequence of steps on their range of motion and verifies that they reach the commanded positions after each step. We affectionately refer to the Ingenuity Servo Self-Test as the “Servo Shaker”.
The pre-flight abnormal servo movement data shows that two of the upper rotor swashplate servos – servos 1 and 2 – began to oscillate with an amplitude of about 1 degree around their commanded positions just after the second stage. of the sequence. Ingenuity’s software detected this wobble and quickly canceled the self-test and theft.
Our team is still looking for the anomaly. To collect more data, we asked Ingenuity to run additional wiggle tests over the past week, with a wiggle test on September 21, 2021 (Sol 209) and one on September 23, 2021 (Sol 211). Both oscillation tests were successful, so the problem is not fully reproducible.
One theory explaining what is happening is that the moving parts of servo reducers and swashplate linkages are starting to show some wear now that Ingenuity has done well over twice as many flights as originally expected (13 completed against five planned). The wear of these moving parts would cause increased backlash and increased backlash, and could explain the servo oscillation. Another theory is that the high speed spin test left the top rotor in a position that loads servos 1 and 2 in a unique, oscillating-inducing way that we’ve never encountered before. We have a number of tools available to work on the anomaly and we are optimistic that we will overcome it and resume flight soon.
Our team will have a few weeks to complete our analysis as Mars will be in solar conjunction until mid-October, and we will not be transmitting any command sequences to Ingenuity during this time. The conjunction is a special time when Mars moves behind the Sun (as seen from Earth), making communications with spacecraft on Mars unreliable. However, ingenuity will not be completely inactive during this time; Ingenuity and Perseverance will be set up to keep each other company by communicating approximately once a week, with Ingenuity sending basic system status information to its base station on Perseverance. We will receive this data on Earth once we get out of the conjunction and learn how Ingenuity behaves over a long period of relative inactivity on Mars. See you on the other side of the conjunction!
Written by Jaakko Karras, Deputy Chief Operating Officer of Ingenuity Mars Helicopter at NasaJet propulsion laboratory
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