Mars Rover Cruise Stage Vehicle
Client:
NASA-JPL.
Competencies:
Power Circuit Design
PCB Layout
DDR Design
Fly to a Nearby Planet … and Don’t Crash Into It
The launch of an interplanetary probe makes the news when the rocket blasts off. And when the rover heads to the surface, it’s in the news again, since the “Seven Minutes of Terror” that comprise landing make good public viewing.
In between the two, however, is the cruise phase, where the rocket leaves Earth orbit and has to proceed on a course that will put it just ahead of the planet at just the right speed that the planet’s gravity captures it and the rover can detach and head for the surface. That means that small jets of gas need to be ejected from the cruise vehicle to break away from Earth’s gravity and make the minor nudges to the ship that keep it on course as the solar winds blow on it.
Our job was to make sure that the rocket nozzles for those little jets were pointed in the correct direction to keep the probe from being forever lost. And doing that meant moving them to precise positions and holding them firmly as the engines were lit.
On February 18, 2021, NASA got signals back from Mars saying the Perseverance Rover was safely on the planet’s surface.
Mission Accomplished.
What We Did: Diagnosing Motor Noise
The motors pointing the steering rocket nozzles had a tremendous amount of noise on them. Under most circumstances, that wouldn’t have mattered, since motors are inherently noisy, anyway. But these motors were inside a servo loop with position feedback telling the computer directing them which way they were pointing. And when the noise got on the position monitor and the wrong position got reported, there was no controlling the motors at all.
That would have sent the spacecraft hopelessly careening off into space.
Noise all over one of the pulse-width modulated (PWM) windings on the rocket nozzle control motors.
What We Did: Fixing the PCB Layout for the Motor Current Circuits
At lower right of the adjacent PCB layout are circuits MA, MB, and MC — the three current carrying circuits for the motor windings. And these were the ones whose routing we had to address carefully to keep their noise off the other feedback circuits on the board.
Revised artwork for the high-current motor control signals.
What We Did: Fixing the DDR2 Memory
Some of the people who work on interplanetary probes are the best, brightest young scientists out there. But what flies into space is often old technology. There are a number of reasons for this. The older stuff has a track record. And in the case of computer chips, which are always being made smaller, they’re also becoming more fragile and vulnerable to the harsh environment of space. DDR memory is a good example. It’s the older generation stuff that flies outside the protection of the Earth’s magnetosphere since the silicon dies can be more readily made to withstand the bombardment of radiation of the solar wind.
But not all of the best, brightest minds have even seen the older — and more difficult to design — DDR2 memory arrays. We have, and all those serpentine bends in PCB traces of the adjacent image are where we modified a board so signals met both the impedance and time-of-flight constraints of an older generation of computer memory that is radiation-hardened but, in spite of its lower speed, is still far less forgiving of poor PCB layout.
DDR2 memory connected to digital signal processing chip handling much of the motor control system.
Our Passengers
Delivered safely across millions of kilometers.
Perseverance rover and Ingenuity helicopter on Mars (artist conception).
John Carter of Virginia (and Mars)
Because it’s a lot easier to get to Mars by lying down in an Arizona cave and entering a trance that opens a portal through space than it is actually to fly there
