Ultrasound Transducers for Measuring Martian Wind Speeds

We have yet to fully understand the wind on Mars, even though it transfers heat, momentum, and molecules from the surface. Traditionally, heat loss and motion detectors have been used to measure wind speeds. Robert D. White (Tufts University) discusses his team's work on ultrasound transducers that may offer a more precise way to measure turbulent eddies on the  Red Planet.

Associated paper: Robert D. White, Rishabh Chaudhary, Zijia Zhao, Luisa Chiesa, Ian Neeson, and Don Banfield. "Modeling and characterization of gas coupled ultrasonic transducers at low pressures and temperatures and implications for sonic anemometry on Mars." J. Acoust. Soc. Am. 156, 968- 988 (2024) https://doi.org/10.1121/10.0028008.

Read more from The Journal of the Acoustical Society of America (JASA).
Learn more about Acoustical Society of America Publications.

Music Credit: Min 2019 by minwbu from Pixabay. 

Transcript

Kat Setzer  00:06

Welcome to Across Acoustics, the official podcast of the Acoustical Society of America's publications office. On this podcast, we will highlight research from our four publications. I'm your host, Kat Setzer, editorial associate for the ASA.

Kat Setzer  00:25

Over the past few decades, space exploration has been able to bring rovers to the surface of Mars and help us better understand Earth's planetary neighbor. Today, I'm talking to Robert White, whose current research aims to help us better understand Mars' environment. We'll be discussing his recent JASA article, "Modeling and characterization of gas-coupled ultrasonic transducers at low pressures and temperatures and implications for sonic anemometry on Mars," which also gained some extra attention through a press release. Thanks for taking the time to speak with me, Rob, how are you?

Robert White  00:53

I'm doing great, Kat. How are you this morning? 

Kat Setzer  00:56

Good! So first, tell us a bit about your research background.

Robert White  00:59

So I'm a mechanical engineering professor at Tufts University in Boston. I've been here for quite a while and worked on a lot of different things over the years. I got started in MEMS transducers originally, and worked on some MEMS sensor technologies of different kinds, and then got involved in aerodynamic measurement technologies. And over the last few years, with the help of some of my coauthors, I've been inducted into planetary sciences and now working on sensing systems for planetary science, often using acoustic methods.

Kat Setzer  01:31

Cery fun. So who did you work with on this study?

Robert White  01:35

So I was very pleased to work with an excellent team. So I really want to thank my collaborators, Don Banfield at NASA Ames and Ian Neeson at VN Instruments in Ontario. We've worked together for quite a few years now, and I never would have gotten involved with this at all without Don calling me up. So that's been excellent. And then I also want to thank the students who've worked on this, so Rishabh Chaudhary and Zijia Zhao are both students on this paper, and they were spending many hours in the lab, and then also my collaborator at Tufts, Luisa Chiesa, who was helpful in giving us access to her cryogenic test system so we could do the low temperature measurements. So really appreciate these coauthors, and then also the rest of the team members on the sonic anemometry project.

Kat Setzer  02:17

So why do researchers want to know that the speed of surface winds on Mars?

Robert White  02:21

So we're very interested in the Martian atmosphere. There's only a few planets that have atmospheres that we have access to, and we want to understand those. So we're very interested in not only the winds at the surface, but the winds throughout the atmosphere. And when we put landers on the surface of Mars, we can do really high-quality measurements of winds at the surface, and that can tell us all kinds of things about the atmospheric dynamics of Mars, and then that may have implications for our understanding of atmospheres in general, including, of course, the very important question of understanding our own atmosphere.

Kat Setzer  02:51

Okay, okay, that totally makes sense. So what are the current methods used to measure Martian wind speed, and what are their benefits and limitations? 

Robert White  02:58

So a number of different rovers and landers have measured wind speeds at the surface of Mars. They've really used two methods. One is to use heat transfer analogies, to look at the heat loss from a heated patch or a heated wire of some kind, and then try to infer the wind speeds from how quickly heat is lost from that heated element. The second method is to use the high quality cameras that are on the landers to image some kind of a telltale which blows mechanically in the wind, and then try to infer wind speed from the motion of those telltales. So those are the really the two methods that have been used on various different landers, and those methods have yielded great data, very interesting data, high-quality data. 

Robert White  03:38

Probably the two limitations that we see that are making us want to propose a sonic anemometer for a lander are speed and ability to measure slow flows. So in terms of speed, all the systems that have flown to Mars, about the fastest they can measure is one wind speed measurement per second, and we think we can do probably 10 times or more better than that. And then also these methods struggle a little bit to measure slow speeds. So any wind speeds that are under about maybe one meter per second or half a meter per second are difficult to measure using the techniques that have flown, and we think that we can measure all the way down to probably five centimeters per second, maybe even a little slower speeds than that. So... And the reason we care about measuring slow speeds and getting a high update rate on those wind speed measurements is we'd like to know not only kind of mean winds that you might be concerned about for loading on a structure, but we're also interested in turbulence and mixing. So if there are turbulent eddies or other kinds of mixing phenomenon, there's some interesting dust devils that fly around on the surface of Mars that have very quickly fluctuating wind speeds. We'd like to be able to measure at those high rates and slow speeds, to understand those sort of mixing dynamics and the atmospheric dynamics.

Kat Setzer  04:47

Okay, okay, so what is Sonic anemometry, and why do you believe it could be a better method for measuring wind speed on Mars?

Robert White  04:54

So Sonic anemometry works by using acoustic pulses that are sent through the atmosphere over a short distance between a transmitter and a receiver, and we measure the time of flight of those sonic pulses that are sent across that gap. And if there's no wind, those just travel at the speed of sound. And if you measure that time of flight, and you know the distance, then you can measure the speed of sound, which might tell you something about the temperature of the atmosphere. If there was interest in gas composition, you might be able to get some information about that. But if there's a background wind, then if you send a pulse in the forward direction and the backward direction, the one traveling with the wind will appear to travel a little faster, and the one traveling against the wind will appear to travel a little slower. And so by looking at the difference in the two times of flight in the opposite directions, you can back out the background wind speed in that direction. So you put three of those pairs in x, y and z directions, you can measure the three components of the wind vector at that location in space. So that's how sonic anemometry works. 

Kat Setzer  05:52

Okay, it reminds me of those classic word problems from math classes when you're little, where they're like, if you have this one train going at this speed in this direction, and this speed and this other direction... I know it's nothing like that, but...

Robert White  06:05

Yes, it's all about it's all about measuring that difference in speed in the two directions. That's what makes it very accurate. Is because anything else that might change the measurement, maybe the temperature changes and the electronics change a little bit, it tends to affect both directions the same. So whatever differences might be caused by that other parameter that you don't want to measure, they tend to cancel out when you take that difference. So by doing this sort of differential measurement, you get a lot of improvements in measuring kind of small fluctuations.

Kat Setzer  06:34

Right, right. Okay, so what was the goal for this study? 

Robert White  06:38

Right. So this study was focused on the transducers. So we need a transmitter and a receiver that will send and receive the acoustic pulses in the atmosphere. And people do sonic anemometry on Earth all the time. But Earth has this really thick, kind of soupy atmosphere, right? We have, we can fly airplanes around in it. It's really heavy, and so it's pretty easy to transmit sound through Earth's atmosphere, but the Martian atmosphere at the surface of Mars is about 1% the density of the atmosphere on Earth at sea level, and so there's not as nearly as much gas to transmit through. So a big concern for an instrument like this is, is there going to be enough sound to be able to get a good measurement in this thin atmosphere? And so that was the first thing that we were concerned about with these, with these transducers, primary thing. And so we worked on that, and we were able to get enough signal so we were able to do that. 

Robert White  07:27

But then the secondary concern is that on the Martian surface, the temperature changes a lot over the course of the day. Since there's such a thin atmosphere, when the sun comes up and when the sun goes down, you get really large temperature fluctuations at the surface, which depends on where you are, and what time of year is, and so on. But big, big temperature changes. And so if your transducers are affected by temperature changes, maybe that is going to give you a false wind measurement. Maybe you're going to get a shift in the transducer characteristics, and maybe that's going to fool you into thinking that there's some wind that's not really there. So we wanted to characterize some candidate transducers for this project at relevant pressures and over a large range of temperatures. So that's what we were doing in this paper. Is looking at how do we looked at four different transducers we're thinking of using. How do their characteristics change as temperature swings over a range of relevance to Mars and as pressure changes over a smaller range also of relevance to the surface of Mars?

Kat Setzer  08:21

Okay, interesting. So how could the dynamics of transducers impact ultrasound systems, and why is it of importance for considering the systems for Mars? 

Robert White  08:31

Yeah, so. So the method relies entirely on measuring this time of flight as the acoustic pulse travels across a small gap, and that's a pretty fast travel. It's not a lot of time, so you have to measure that accurately. And the transducers, they have their own dynamics, so they introduce kind of effective delays in that path. And so if those effective delays are changing because maybe temperature or pressure changed, then that might spoof you into thinking that there's a difference in the time of flight, and maybe that will appear to you to be a wind velocity change. So that's the concern. The differential nature of the measurement helps a lot, because if the transducers change the same way in both directions, then it tends to cancel out. But we wanted to be able to characterize this, and then in the paper, we also make some approximate estimates from what we see about the transducers, how big of an error that might create a wind measurement and I think we're able to show that the measurement errors are quite small, only maybe a couple of percent at most. So that that's really good news.

Kat Setzer  09:29

Oh yeah, that is. How did you determine the necessary transducer parameters for a Martian anemometry system? 

Robert White  09:36

So we had specific goals for our wind measurement system. So we wanted to be able to measure wind speeds below five centimeters per second, and we wanted to be able to measure 20 times per second, and we wanted to do this over the relevant pressure and temperature ranges. So that's what we were shooting for. So we wanted to see if we have a model that connects changes in the transducer characteristics to what it might create in terms of wind measurement errors. Then we could try to estimate those and see if we're able to meet our specifications with the transducers that we're testing.

Kat Setzer  10:05

Okay, so what did you find with regards to the effect of varying pressure and temperature on transducer dynamics? 

Robert White  10:10

So we did find that all the transducers, unsurprisingly, are affected by temperature and pressure. The different transducers are sort of different in their construction, so they behave somewhat differently, but we were able to identify at least one transducer which could operate over the entire temperature range of relevance to the surface of Mars. And we were able to operate at the six millibar pressure we need to operate at. And we were able to show, I think, in this paper, that at least in terms of the transducers themselves, the errors that they contribute to velocity measurements should be less than a couple of percent, which is quite good and really allays any concerns we might have originally had about how the transducer characteristics changing with temperature and pressure might impact our measurement quality. 

Kat Setzer  10:11

What are the implications of these results on the development of a sonic anemometry system for Mars? 

Robert White  10:20

Well, I think that they're very exciting results. I'm really hopeful we'll get to fly this instrument. So at this point, our instrument is at a pretty high technology readiness level. We've built some instruments with collaborators at NASA, and we've done a lot of testing on these, and we'd love to fly an instrument. So I think the implication is we're ready to go, and we need the go ahead and the mission to get on to. So we have a great instrument. We're going to be trying to convince folks that it should get a spot on a mission, and we'd love to fly it. So we and we have gotten to fly some of these on Earth on high altitude balloons. So that's been exciting. So there's some data that we're already gathering here on earth with this system. And of course, we'd love to fly it to Mars. Yeah,

Kat Setzer  11:39

That sounds very fun. What do you think are the key takeaways from this study?

Robert White  11:42

I think probably the biggest takeaway is that if folks are concerned about whether we have the transmitters and receivers we would need for a system like this under these low pressure and low temperature conditions, the answer is yes, we do, and there are transducers available which will work for this scenario. I think there are still other things that we need to keep working on on this system. We need to work on more flow characterization studies. We need to work on looking at the electronics and how they may be impacted by environmental changes as well. So we're continuing to work on those things, and then just demonstrating the whole system in as tough of an environment as we can probably on these high altitude balloons. So there's more, more work to be done, but in terms of the transducers, I think we're pretty happy with the results.

Kat Setzer  12:24

That's great. So actually, good segue. What are the next steps for this research?

Robert White  12:28

So right now, the team is building kind of what we're calling our engineering test unit, which will be our highest-quality instrument to date. And that instrument is going to go through shock and vibe testing, temperature and vacuum testing out at NASA Ames, and then it's going to be put into the Martian simulation wind tunnel in Aarhus, Denmark, and we're going to run it through a series of wind tests out there so that engineering test unit will be fully qualified. Hopefully by the end of the fall, we'll have some great data on that that we'll put out there. We also have a couple of units that were built at Tufts University in my group, which have a few differences to the engineering test unit, but are a similar concept that are flying on two high-altitude balloons. So one of them is flying out of New Mexico in a few weeks time, in the end of August, and then the other one's supposed to fly out of Antarctica in December. So I'm hopeful that those two missions will go well, and we will have some interesting stratospheric wind data from those two instruments, also over the course of the next year. So those are the things that we're focused on in the near term. And then in the longer term, we're going to be continuing to present and talk to mission planners and administrators and see what opportunities there might be for operating on a future mission to Mars. And we're also looking at other ways that we can take this technology and apply it to different problems. So we're interested in gas composition sensing using ultrasonic techniques for the giant planets, for Saturn and Uranus, and then we're also interested in potential missions to Venus. So we're continuing to talk to other collaborators and trying to develop the instruments directions as well. 

Kat Setzer  13:58

Very, very cool. This is all so exciting. 

Robert White  14:01

Yeah, it's really a very fun project. I'm so happy to have gotten the opportunity to work with my co authors on this. It's been great. 

Kat Setzer  14:07

Yeah, well, I really hope that your sonic anemometry system ends up on a future Mars rover. 

Robert White  14:13

Yeah, me too. 

Kat Setzer  14:15

Wish you the best of luck in your research, and thanks for taking the time to speak with me. 

Robert White  14:18

Yeah, thank you very much.

Kat Setzer  14:22

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