Zhu Rong: How to patrol to detect Mars
■Jia Yang
In the 2020 launch window, China’s Zhu Rong and the US’s Perseverance rover set off at the same time, and are currently conducting exploration activities on the surface of Mars.
As a result, my country became the second country to carry out patrol and exploration on Mars and the only country to carry out patrol and exploration on the moon and Mars at the same time.
After the Zhurong rover reached the surface of Mars, relevant departments released photos taken by the rover, which aroused widespread attention.
Especially when they saw the photo of the Mars rover and the landing platform taken from a third-party perspective using the WiFi separation probe, everyone praised the Mars rover Zhurong and called this picture "Rongrong took a photo of Lulu."
As of July 30 this year, the Zhurong rover has been working on the surface of Mars for 75 Martian days, and completed the scientific exploration of the second dune landform that day.
In the future, it will use scientific payloads such as surface composition detectors and multispectral cameras to carry out detailed detection of more targets.
It can be said that people have understood the cuteness and dedication of "Rongrong".
So, what are its technical characteristics?
Let us find out.
Our design is positioned as the "second and a half"
At the beginning of the design of the rover, some foreign experts asked about the design and positioning of China's first rover.
To this question, our answer is: The United States has developed three generations of Mars rover in 25 years. From the weight point of view, they are the first generation of 10 kilograms, the second generation of 100 kilograms, and the third generation. At the 1000 kg level, the capabilities in all aspects have also been continuously improved.
Our Zhurong Mars rover weighs 240 kilograms. From the weight point of view, it belongs to the second generation.
With the development of technology, the design positioning of our Mars Rover Zhurong is "the second and a half generation."
This extra "half-generation" is mainly reflected in the advancement of fire surface movement, survival, and independent technology.
Active suspension is not afraid of any terrain
The surface of Mars has a complex topography, including steep slopes, large rocks, and soft sand.
In the course of its work, the American Mars rover encountered steep sandy slopes that were difficult to climb, and was stuck in the sand and unable to move.
The Zhurong rover uses an active suspension movement system, the purpose of which is to enable the rover to have a strong passing ability under complex terrain conditions.
When moving on a flat hard road, the Mars rover keeps the active joints of the active suspension mechanism locked, and the suspension degenerates into a passive suspension at this time.
In the case of relatively high stone obstacles, the active suspension can be used to raise the car body.
In soft soils that are difficult to pass, especially when the wheels cannot pass smoothly due to a large subsidence, the inchworm movement can be used to get out of trouble.
First, the two front wheels move forward, the middle wheel and the rear wheel do not move, and the height of the car body gradually decreases.
Then, the front wheel does not move, the middle wheel and the rear wheel move forward. During this process, the height of the car body gradually increases.
Then, continue to repeat the above process.
Although the exercise efficiency of this type of inchworm exercise is relatively low, the effect of getting out of the sand is very good.
Relying on heat-collecting windows to achieve "warmth"
The surface temperature of Mars is low. On the top of the rover, a device like binoculars is installed called a heat collector.
The window has a thin film through which visible light can pass through smoothly, but the far-infrared rays emitted by the car body cannot pass through, which has a heat preservation effect.
After the sunlight passes through the heat collecting window, the energy is stored by a substance called n-undecane through phase change.
When the temperature of Mars rises during the day, this material absorbs heat and melts; when the temperature drops at night, this material releases heat energy during the process of solidification.
The energy conversion method has become "light energy-thermal energy-phase change energy-thermal energy", and the efficiency can reach more than 80%.
Currently, it is midsummer on Mars, and the Zhurong rover will "feel" a little hotter.
However, when Mars arrives in the fall, this ability to collect heat will show its effectiveness.
Solar cells remove dust like lotus leaves
Working on the surface of Mars will inevitably be affected by Martian dust.
The most direct effect is to cause the output power of solar cells to decrease.
Because the electric energy required for the work of the rover comes from solar energy, if the electric energy is insufficient, the rover can only "sleep" on the surface of Mars.
In summer, we observe the water droplets on the lotus leaf and we can find that there is no infiltration between the lotus leaf and the water. The water droplets are easy to roll off when the lotus leaf is swaying with the wind.
Drawing lessons from the hydrophobic principle of the lotus leaf in nature, researchers have added a super-phobic microstructure to the battery cover sheet.
The size of these structures is smaller than the characteristic size of Martian dust particles. When Martian dust comes into contact with it, it is equivalent to contacting a nano-scale "bed of needles" instead of a flat surface.
This greatly reduces the contact area between the Martian dust particles and the battery, thereby weakening the adhesion between them, making the Martian dust difficult to deposit, even after it is deposited, it is easier to remove.
The Mars rover uses a super sparse base battery cover sheet, and two of the solar wings can be adjusted to the vertical state, which is convenient for the Martian dust to slide down.
There are many ways to prepare the ultra-phobic microstructure, and the wet chemical etching method is used on the rover.
The test found that the improved solar cell has a dust removal effect of more than 80%, especially for dust particles in the range of 75 to 125 microns in size, the dust removal effect can reach 95%.
It’s up to you to decide when to "sleep" and when to "get up"
There will also be localized sand and dust weather on the surface of Mars. In severe cases, it may even spread to most areas of Mars and become a global dust storm.
The American Mars rover Opportunity has been working on the surface of Mars for 15 years, but it was interrupted by a severe dusty weather.
How does the Zhurong rover cope with such dusty weather?
The developers have designed an autonomous sleep wake-up function for it.
In other words, the rover will decide when to "sleep" and when to "get up" according to changes in the environment.
On the surface of Mars, when the wind speed gradually rises and there is a dusty weather, the first thing the rover feels is that the electric energy output by the solar panels is somewhat insufficient.
At dusk, the battery should be full, "Why is there so little today?" The rover quickly calculated how much electricity it would need to work tomorrow.
If the conclusion is "a little bit too close", then the rover will reduce its working equipment and wait until the next day through the "excessive life" method; if the conclusion is "a lot worse, not enough for tonight", then the rover will be Sleep immediately and power off the entire system.
At this time, it is necessary for the Zhurong Mars rover to "live a bit of hardship."
The temperature of the equipment is getting lower and lower, and the lowest can reach below -100℃.
Even so, there is no way, the rover can only "sleep" in the cold.
There are two necessary conditions for awakening: one is to wait until the dusty weather passes, the sun becomes stronger and the atmosphere becomes clear and transparent, and the power generation of the solar wing of the rover can maintain normal operation; the other is the key equipment of the rover. The temperature meets the working requirements, for example, the battery can be charged normally.
When the conditions are met, no ground control is required, and the rover will "wake up" by itself and continue to work.
Image compression algorithm unfolds its powerful functions
For exploring unfamiliar environments, image information is undoubtedly the most intuitive and core information.
Image information contains considerable time and space redundancy, so the data volume of image information is very large.
In the first three months of the mission, the distance between Mars and the Earth was between 320 and 380 million kilometers. The bandwidth of the communication link from Mars to the Earth was greatly restricted. The image was compressed at the deep space data source. It is the only way to improve the efficiency of information return.
The resources of deep space probes are precious and limited, and the data processing capabilities of the Mars rover will not be as powerful as ground computers.
Therefore, according to the application requirements of the Mars exploration mission, the two links of image data compression and transmission need to be considered uniformly, and the optimal image data compression and transmission scheme needs to be designed.
In response to the image compression processing requirements in the Mars exploration mission, the researchers specially designed image compression algorithms for the Zhurong rover, which realized a variety of camera data storage management, flexible control of image compression ratios, progressive quality transmission, preferential coding of regions of interest, The functions of anti-error spreading and image thumbnail generation and downloading meet the requirements of reliable, efficient and flexible image application of Mars rover.
(The author is a researcher at China Academy of Space Technology)