How do you design sensors that can work equally well on two planets?
Humidity and pressure sensors from an electronics expert are currently operating in every continent on Earth, as well as on space missions such as NASA’s Curiosity and Perseverance rovers. These motorised vehicles have been working on Mars since August 2012 and February 2021, respectively, and the technologies in the Vaisala sensors is the same as those deployed in multiple applications on Earth. So, how do you design a sensor that is able to operate in such different environments? Anni Torri, a senior scientist in Vaisala’s R&D division offers some valuable insight.
With over 20 years of experience at the company, Torri is well placed to comment. However, if you ask her how you design sensors for two planets, she simply says, “You don’t! Instead, you design sensors that out-perform customers’ needs here on Earth, which means designing for accuracy, stability and longevity. Happily, by developing such high-quality sensors, we also met the requirements of the teams working on the Mars rovers.”
Vaisala’s humidity sensor technology Humicap was initially launched in 1973, and the Barocap pressure sensor followed in 1985. Since that time, Torri says: “The technologies have benefited from continuous improvement, driven by innovation within our team, as well as new and challenging requirements from customers. In addition to the core sensor technology (chip), we have also developed monitoring instruments, transmitters, probes and modules to meet a wide variety of customers’ needs.
“Applications include meteorological measurements in the extremes of the polar regions and the hottest deserts, as well as industrial operations with a range of conditions. Consequently, the sensors evolved and became able to deliver the highest levels of performance in the most demanding of applications.”
Environmental Monitoring On Mars
NASA’s Curiosity and Perseverance rovers feature pressure and humidity monitoring devices designed and built by the Finnish Meteorological Institute (FMI). Thanks to the long-term stability and accuracy of these sensors, as well as their ability to tolerate dust and harsh environmental conditions, these technologies are especially suitable for such demanding applications. The FMI devices onboard the rovers employ standard Humicap humidity sensors and specially customised Barocap pressure sensors. The modification of the pressure sensor is mainly a slightly thinner membrane to accommodate the lower pressure conditions.
In comparison with Earth, Mars is a dry planet; relative humidity is practically zero (0% RH) during the day and rises during the night as the temperature falls. There have been observations of small amounts of water in the atmosphere in the past, though never in liquid form. Martian surface temperatures vary during the night from -135°C to -70°C and the Martian surface layer along its equator achieves positive temperatures during the day time. Considering the temperature and low pressure of the atmosphere on Mars, it was previously estimated that liquid water could not be present on Mars. However, in May 2015, supported by the measurements made by FMI and Vaisala technology, combined with numerical models, the first evidence of ancient liquid water on Mars was confirmed.
Curiosity’s mission was to determine whether Mars has ever had the right environmental conditions to support microbial life forms. The discovery of traces of ancient liquid water was therefore a major success, and led to the mission objective for Perseverance: to search for signs of ancient microbial life.
Environmental Monitoring on Earth
Meteorological and aviation measurements rely on the same pressure and humidity technologies that are deployed on Mars. However, terrestrial applications also include building management systems and industrial monitoring where products are dried or moistened, or where process measurements, such as gas levels, require compensation.
On Mars, sensor service and calibration is not possible for obvious reasons, but on Earth it is relatively simple to perform. However, terrestrial customers are also looking for sensors with high levels of accuracy and stability, long life expectancy and extended periods between service or calibration. This is important because unreliable measurements could have catastrophic consequences and because frequent service and calibration would interrupt production and increase operational costs.
Despite the long-term stability of the Vaisala sensors, it is still necessary to conduct regular calibration to check that the instrument is within specification and capable of producing valid data. Unless required by regulation, it may be possible to extend the period between calibrations as an instrument is proven to be stable.
Humidity Monitoring Technology
The Humicap sensors employ a capacitive thin-film polymer sensor consisting of a substrate (typically glass or ceramic) on which a film of polymer is deposited between two electrodes. The polymer either absorbs or releases water vapour as the relative humidity of the ambient air rises or falls, which changes its dielectric properties, and thereby the capacitance of the sensor, which is measured and converted to a humidity reading.
Pressure Monitoring Technology
Using a single-crystal silicon material, Barocap is a micromechanical pressure sensor that measures dimensional changes in its silicon membrane. As the surrounding pressure increases or decreases, the membrane bends, thereby increasing or decreasing the height of the vacuum gap inside the sensor. Opposite sides of the gap act as electrodes, and as the distance between them alters, the sensor capacitance changes, which is measured and converted to pressure readings.
Although participation in projects such as the Mars missions is an exciting and challenging diversion from the daily business, Vaisala’s engineers are keen to point out that invitations to be involved in such work only come when product development aims for ‘best in class’. Torri says: “Our sensors have become popular for three main reasons. Firstly, we focus intensely on the voice of the customer; listening to their needs and driving our R&D to meet and exceed expectations. Secondly, as part of our constant drive for improvement, we seek and encourage innovation in everything we do. Thirdly, our brand stands for quality and reliability, so before any product can be awarded our logo, it has to earn that right by passing a rigorous and demanding performance test programme.”