Poppy seeds, lunar rovers and load cells

Published On: June 11 2020


Getting a autonomous robot or vehicle stuck in soft soil and having to rescue it is embarrassing enough. Getting a lunar rover stuck halfway up a slope of “loosely consolidated granular terrain” is much more problematic.

That’s what happened to NASA’s Mars Exploration Rover Spirit, which got stuck in the sands of Troy near the Gusev crater – and it hasn’t moved anywhere since. More modern rovers us a six-wheeled rocker-bogie suspension, but this still won’t help them get out of the soft loose soil known as regolith that covers large areas of both Mars and the lunar surface.


I’m a NASA rover, get me out of here

A team at NASA Johnson Space Center developed a scaled-down prototype robotic lunar rover that can move in three different ways: by wheels, on legs or crawling along. A particular crawling action could be used to enable the rover to get out of soft soils.

To test it the team created a semiautomated test bed filled with their granulated material of choice – dry poppy seeds. Their method was to place the rover model onto the bed of seeds inclined at various angles, and:

“The rover wheels spun for a set time interval of 30 s. After this time interval, the wheels were sufficiently buried to be at the full slip condition, and the rover was entrapped within the granular substrate. The rover then executed the gait, enabling positive locomotion from the embedded state. After the rover stopped executing gait cycles, the trial concluded; the kinematic data … were imported and analyzed.”

Load cells were used as follows:

“To capture the drawbar experiment data, we fastened a Uxcell bar-type load cell to the bed frame. One end of an elastic tube was attached to the load cell and the other end to the rover. An HX711 load cell signal amplifier was used to process and feed the signal into a microcontroller. The microcontroller printed drawbar values at 11 Hz to a connected computer.”

The team discovered that:

“Addition of a cyclic-legged gait to the robot’s wheel spinning action changes the robot dynamics from that of a wheeled vehicle to a locomotor paddling through frictional fluid. … A peculiar gait strategy that agitates and cyclically reflows grains under the robot allows it to “swim” up loosely consolidated hills.”


Excavating granular lunar regolith (aka digging on the moon)

NASA doesn’t just want to get their rovers out of regolith, they want to excavate it too! A team at the NASA Glenn Research Center developed a special arm to dig into regolith, but needed to test the power and forces required to do so. Thier solution was:

“The Advanced Planetary Excavator (APEX), an electrically actuated backhoe arm with a bucket, which is used to dig GRC-3B, a simulant of granular lunar regolith comprised of silica sand and silt. A load cell is mounted between the arm and the bucket to measure digging forces. Dig trajectories files are created and used to control the APEX for repeatability. Comparison of preliminary data recorded for the same trajectory in air and in GRC-3B shows measurable difference in power and load cell measurements. Load cell forces for a linear dig in GRC-3B are presented.”


We have lift-off

The recent launch of SpaceX has rekindled interest in rocket launches, made possible by the NASA Mobile Launcher (ML) located at Kennedy Space Center (KSC). Don’t be fooled by the term “mobile”; this is

“A massive structure consisting of a 345-foot tall tower attached to a two-story base, weighing approximately 10.5 million pounds—that will secure the new NASA Space Launch System (SLS) vehicle as it rolls to the launch pad on a Crawler Transporter, as well as provide a launch platform at the pad.”

The team wanted to undertake modal shaker tests on the structure, but had to devise a new methodology due to the size of the launcher.

”The lateral shaker test fixture consisted of a hydraulic shaker and 2100-lbs of inertial mass plates attached to the top side of a slip plate. On the bottom side of the slip plate were linear bearing assemblies that rode on horizontal rails attached to a base

plate that was bolted to the ML structure. The shaker armature was attached to a vertical arm on the base plate. Excitation forces were generated by oscillating the shaker and slip plate assembly relative to the vertical arm, for a total moving mass of 2867-lbs. Forces were measured with a PCB model 1381-01A rod-end load cell that was installed between the shaker armature and the vertical arm… The rod-end and ring-type load cells installed in both shaker test fixtures provided the force measurements required for calculating driving point frequency response functions (FRFs).”

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Two of the operational shaker test fixtures are shown in Figure 9. In the lateral shaker test fixture, the rod-end load cell can be seen installed between the shaker armature and the vertical arm.


Life on Mars with load cells

If we are to live on Mars, we need to be able to “Have a method of purifying martian ice and turning it into usable water. The extracted water could be used for drinking and growing plants.” A team from the Stevens Institute of Technology developed the Drill-based Extraction of Ice and Martian Overburden System (DEIMOS) system to both extract the ice and turn it into usable water.

In a nod to using ordinary objects for extraordinary tasks that Matt Damon’s “The Martian” would have been proud of, the team used a standard DeWALT SDS MAX Hammer drill mounted on a frame for this so-called digital prospecting. A beam load cell is used to measure the weight on the drill bit.

“In order to excavate into the overburden the drill is positioned horizontally over the test bed. Then the drill is turned on and lowered into the overburden. When the drill bit contacts the regolith, the drill is pushed upward relative to the subframe and apply force to the load cell. Layers within the digital core are assessed by use of a load cell and predetermined weights of the drill, the drill bit and the supporting frame. “

What’s more, a standard load cell should do the job nicely! The only issue is calibration, often a challenge in extreme conditions. The team solved this issue too:

“The new housing for DEIMOS will have an internal frame in which the load cell will be fastened above the drill. This will serve as the basis for calibration while constructing the digital core.”


Stellar quality load cells

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