The Mars-bound 2020 rover, known as Perseverance, will be retrieving soil samples from the planet, and even launching its own helicopter to demonstrate the technologies required for flying in the Martian atmosphere.
To perform these giant feats requires a number of small parts — specifically motors and drives.
NASA’s Jet Propulsion Laboratory asked the Switzerland-based manufacturer maxon motor to produce a set of drives for Perseverance.
Nine brushless DC motors will assist with the rover's objective of obtaining surface samples — and sealing them up tight. The drives found in the rover’s sample caching system and sample handling arm will support the gathering of the Martian soil for investigation.
maxon also has six brushed DC motors built for the Ingenuity helicopter aboard the rover. The 10-millimeter-diameter components will control the tilt of the drone's rotor blades and the direction of flight.
Brushed motors use brushes and commutator contacts to deliver current to the motor windings. The brushed motor is somewhat simple: You apply voltage and it spins.
The more complex brushless motor requires an electronic controller and additional cabling. Brushless motors switch the motor’s field via an amplifier.
In a live Tech Briefs presentation, readers had a number of questions about the role of motors — both brushed and brushless — in the Mars 2020 missions. Read the (edited) responses from Robin Phillips, Aerospace Project Engineer at maxon, below:
Have any of your motors ever failed on Mars, and are you worried about this for the upcoming missions?
Robin Phillips, Aerospace Project Engineer, maxon: Am I worried? Yes, always.
There have been no motors that have failed on Mars, that we're aware of. There were a couple of actuators on Spirit , however, that famously failed. But the feedback we've received from JPL is that most of those were from failed electronics.
There was one case where JPL thought the gearbox, which fortunately wasn't from us, failed, and there was also one motor that failed because a heater blanket got stuck on it. The blanket was used to heat up the motors after the cold Martian night, and the blanket didn't turn off when it was supposed to, and fried the motor basically.
It's always nerve-racking when you've spent all this time building something that you know is critical for somebody else, and I won't be relaxing until the rover is down on the ground, the helicopter is flying, and the sample collection has started next year.
What are the key modifications and differences that are needed for a Martian or lunar operational motor vs. an industrial standard one?
Robin Phillips, Aerospace Project Engineer, maxon: What's different is the shock and vibration of launch, as well as the temperature cycling.
So, if you take any of our standard motors and put a big shock on them, you'll normally find that you'll separate the rotor from the stator. That's the big upgrade that we need to design for. It's not particularly difficult. You add press rings, change some materials, and put in a few extra welds. You have to look at the materials and make sure you have expansion coefficients matched.
Brushed motors normally require an atmosphere to work. They require humidity, and you don't have that on Mars or in space. What you can do, however, [to allow brushed motors to operate on Mars] is you can impregnate them with lubricant. We've done some more development on that. We've tweaked the mixture, and we've found something that works pretty well without an atmosphere.
What are the tradeoffs of using brushed vs. brushless motors for space applications?
Robin Phillips, Aerospace Project Engineer, maxon: Most of the tradeoff is not with us; it's with the customers. It's whether or not you want to deal with the electronics.
So, motor controllers for a brushless motor going to go to Mars have to be radiation-hard. They've got to be able to survive certain amounts of temperature swings.
The complexity of designing space-rated electronics is, from what I can conceive from all of our customers who struggle with it, not to be underestimated.
If you have a brushless motor, then you do get a motor that's not life-limited by brushes. That's the big problem with brush motors: not getting enough lifetime out of them.
It depends on the application really. If you're building something like a solar panel that's rotating in Earth orbit, it has to spin millions of times over its life. That's generally not too good for a brushed motor. But the rovers on Mars, and the number of cycles that you'll need out of them, are well within the capabilities of a brushed motor design. And so you can save yourself the complexity there.
What do you think? Share your questions and comments below.