“It’s been an eventful several Martian days, or sols, since our last blog post, so we wanted to provide everyone with an update on where things stand on Mars. In our last post, we explained that we were getting ready to begin flying with a higher rotor speed to compensate for decreasing atmospheric density caused by seasonal changes on Mars. Increasing the rotor speed is a significant change to how we’ve been flying thus far, so we wanted to proceed forward carefully. Step one was to perform a high-speed spin test at 2,800 rpm on the ground and, if everything went well, step two was to perform a short-duration flight, briefly hovering over our current location, with a 2,700 rpm rotor speed.
The high-speed spin test was completed successfully on Sept. 15, 2021 at 23:29 PDT, 11:11 LMST local Mars time (Sol 204 of the Perseverance mission). Ingenuity’s motors spun the rotors up to 2,800 rpm, briefly held that speed, and then spun the rotors back down to a stop, all exactly as sequenced for the test. All other subsystems performed flawlessly. Of particular interest was determining whether the higher rotor speeds cause resonances (vibrations) in Ingenuity’s structure. Resonances are a common challenge in aerial rotorcraft and can cause problems with sensing and control, and can also lead to mechanical damage. Fortunately, the data from this latest high-speed spin showed no resonances at the higher rotor rpm’s. The successful high-speed spin was an exciting achievement for Ingenuity and gave us the green light to proceed to a test flight with a 2,700 rpm rotor speed.
The test flight was scheduled to take place on Sept. 18, 2021 (Sol 206) and was supposed to be a brief hover flight at 16 feet (5 meters) altitude with a 2,700 rpm rotor speed. It turned out to be an uneventful flight, because Ingenuity decided to not take off. Here’s what happened: Ingenuity detected an anomaly in two of the small flight-control servo motors (or simply “servos”) during its automatic pre-flight checkout 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 spin around the mast. Blade pitch is adjusted through a swashplate mechanism, which is actuated by servos. Each rotor has its own independently controlled swashplate, and each swashplate is actuated by three servos, so Ingenuity has six servos in total. The servo motors are much smaller than the motors that spin the rotors, but they do a tremendous amount of work and are critical to stable, controlled flight. Because of their criticality, Ingenuity performs an automated check on the servos before every flight. This self-test drives the six servos through a sequence of steps over their range of motion and verifies that they reach their commanded positions after each step. We affectionately refer to the Ingenuity servo self-test as the “servo wiggle.”
The data from the anomalous pre-flight servo wiggle shows that two of the upper rotor swashplate servos – servos 1 and 2 – began to oscillate with an amplitude of approximately 1 degree about their commanded positions just after the second step of the sequence. Ingenuity’s software detected this oscillation and promptly canceled the self-test and flight.
Our team is still looking into the anomaly. To gather more data, we had Ingenuity execute additional servo wiggle tests during the past week, with one wiggle test on Sept. 21, 2021 (Sol 209) and one on Sept. 23, 2021 (Sol 211). Both of the wiggle tests ran successfully, so the issue isn’t entirely repeatable.
One theory for what’s happening is that moving parts in the servo gearboxes and swashplate linkages are beginning to show some wear now that Ingenuity has flown well over twice as many flights as originally planned (13 completed versus five planned). Wear in these moving parts would cause increased clearances and increased looseness, and could explain servo oscillation. Another theory is that the high-speed spin test left the upper rotor at a position that loads servos 1 and 2 in a unique, oscillation-inducing way that we haven’t encountered before. We have a number of tools available for working through the anomaly and we’re optimistic that we’ll get past it and back to flying again soon.
Our team will have a few weeks of time to complete our analysis because Mars will be in solar conjunction until mid-October, and we won’t be uplinking any command sequences to Ingenuity during that time. Conjunction is a special period in which Mars moves behind the Sun (as seen from Earth), making communications with spacecraft on Mars unreliable. Ingenuity will not be completely idle during this time, however; Ingenuity and Perseverance will be configured to keep each other company by communicating roughly once a week, with Ingenuity sending basic system health information to its base station on Perseverance. We will receive this data on Earth once we come out of conjunction, and will learn how Ingenuity performs over an extended period of relative inactivity on Mars. See you on the other side of conjunction!