Is there life on Mars? Load cells help find the answers

Published On: February 25 2021


If like us you’ve watched the Mars Perseverance Rover touch down gently on the surface of the red planet, you’ll know just what a nail-biting landing this was. Since it launched in July 2020, the rover has travelled 293 million miles across space, finally touching down in the Jerezo Crater.

The Perseverance is about the size of a car, being 3 meters long, 2.7 meters wide, and 2.2 meters tall (excluding arms), and weighs just over 1000kg. It was successfully lowered down to the surface on three mechanical cables. While we don’t know if the cables had load cells attached, load cells would have certainly been used in the testing and process of getting the $2.2billion rover sitting safely on six wheels in Martian dust.

Armed with load cells

The one location where we know load cells are used on Perseverance is in the robotic arm that enables the rover to extract samples and drill into the planet surface. According to strainblog, a 6 axis load cell allows the rover arm to sense forces and torque in different directions. Referred to as a six-degree-of-freedom (6DOF) sensor, this was:

“Incorporated into the shoulder joint of a robotic arm to provide sensing and structural support. As the rover traverses the Martian landscape performing tests, taking samples and gathering data, the sensor provides complex force data to ensure the entire system operates within pre-determined limits as well as providing data regarding hardness of samples.”

As the blog goes on to explain:

“There is a structural member connecting a turret instrument package to the robotic arm. The sensing section is an ultra-thin walled titanium material. Six fully redundant Wheatstone bridge circuits are used to measure 3 forces in the x,y,z axis and 3 moments in the x,y,z axis.”

Not sure where the turret is? Neither were we until we found the superb 3D interactive model of the rover on NASA’s website:.You can check out all the parts at NASA’s interactive 3D visualization.

The journey of 293 million miles starts with – a push

The rover was constructed at NASA’s jet propulsion laboratory in Pasadena, California, before being carefully wrapped, crated and shipped via cargo plane to the Kennedy Space Centre in Florida. With such an expensive cargo, it’s no wonder that whilst most the lifting was done with winches, some of the moving into position was done the old fashioned way, with a team gently pushing the rover’s crate. See how they did the move, and count how many different lifts were required, in this video from NASA.

The next generation of Mars rovers

A team from MarsWorks , part of the Sheffield Space Initiative (SSI) are addressing the issues of “Autonomy Challenges of the next generation of Mars Rovers”. Load cells are very much part of their work, and gives clues as to where else on the current Perseverance rover they may be deployed.

This starts with the interpretation of basic data, as the team explain:

“Basic properties of the collected (Martian) soil sample have to be measured (temperature, density, humidity), which inevitably involves the fusion of signals from very different transducers, but also filtering, interpolation, or downsampling to enable the variables to be manipulated together in calculations. One basic example is the density determination, which involves combining the readings from a load cell (mass) with the time of flight data (volume occupied); therefore, the sampling rates of the two had to be matched and different low pass filters had to be implemented according to the known transducer dynamics.”

In their Rover, soil samples were stored in three containers.

“Each container is equipped with an Instrumentation Control Board, which performs sampling, signal amplification, analog-to-digital conversion (especially for the low voltages coming from the load cells), as well as preliminary filtering before transmitting the data through CAN bus.”

And finally, it was important that only the precise right weight of soil was collected and stored.

“To achieve this, the weight measurement is used as a feedback signal for the robotic arm upon releasing the sample. It was therefore critical to ensure that no spikes are present in the load cell signal (which often proved to be the case during the first seconds of auto-calibration) as those would lead directly to violent control actions in the robotic arm.”

The team’s solution was to apply a low pass filter with a cut-off frequency of 0.5 Hz, which helped smooth any spikes in the auto-calibration procedure.


Load cell solutions for “sky’s the limit” projects

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