After hours of inspections, Mimas was finally ready to attempt a driving test! It has been carried out using the DC actuators only, powering them at 10V 1.5A. The rover drove over the obstacle without a single issue! The wheels' spokes assembly were a bit too bendy, but this did not represent a problem. However, it might become an aspect of major concern when the rover's weight will increase.
After partially disassembling the rover for the journey from Bainfield to Merchiston, it was decided to test all the components to ensure no parts have been damaged during the transfer. This was also a great occasion to test the computer vision algorithm for the first time, in an open-space environment: the lab is large enough to allow drive tests, which will be conducted in the following weeks. At the current state of things, the motors are still not wired and the only fully functional component is the Mast Tower. The "Green Lines algorithm" has been tested using a cardboard box as a rock. For the test, Mimas was placed on custom-made metal perches that keep the rover at the right height even without wheels.
On the 8th of February 2023, Mimas was moved from Bainfield Student Accommodation to Merchiston Campus, room D66. Thanks to Brian Davison, Alexandros Gkanatsios and Stuart Thomas, we got access to a brand-new laboratory to work on the rover! The lab has a workstation for soldering, whiteboard tables for sketching (and, most importantly, a lot of convenient electric plugs!). It took several hours to pack Mimas, ensuring she is safe during the journey from Bainfield to Merchiston, and when the rover arrived at its destination, the help from the ENU Formula Student team was invaluable. Mimas will stay in D66 for 8 weeks and during this relatively short period of time, it will undergo several changes, wiring, and drive testing. As the Honours Project report submission day gets closer and closer, the construction of the rover becomes more challenging.
The "rocker" joints were always one of the major issues with the Rocker Bogie designed for Mimas: since the entire weight of the rover relies on them, these particular pieces have to be extremely solid. The first design suffers from excessive stress, especially in the area of connection between the joint and the rover body. A deformation of <1mm was measured: this was not a big threat, however, the design has been revisited. The new rocker joint has a thicker structure in all directions, and the connection between the joint and the rover body is reinforced by a custom M8 metal flange. This helps better distribute the load on the joint. In addition, four threaded inserts have been added to make the rocker joint tube attachment stiffer.
Stereovision is quite an important part of Mimas: it is not really compulsory, but it may massively facilitate autonomous navigation if running a good algorithm. In addition, stereo cameras can be used to take pictures that can be "sent to Earth" (or in other words, to my PC). Using a pair of red-and-blue glasses, it is possible to obtain an old-fashioned 3D photo effect. As mentioned in previous posts about Computer Vision, a PS4 camera was selected to be assembled on the Mast Tower. This required designing a custom enclosure for the cameras' motherboard and conducting some tests before assembling it to the bottom of the Imager. These tests were run in the open air, placing the stereo cameras on top of a tripod.
The stereo vision cameras attached to the Remote Sensing Mast came with a proprietary enclosure designed by Sony, but unfortunately, it has a shape that is not ideal for our purpose. For this reason, the original black casing has been removed and the camera lens protectors reattached after that. The cable that has been modified in October also needed some improvements, mainly in terms of shielding the wires.
After several upgrades to the suspension system, after installing the plastic plates that were realised by Brian Black and Paul Murray (technician at Merchiston Campus's workshop), Mimas was finally ready for another standing test! The Rocker-Bogie (RB) successfully held the rover in position, guaranteeing sufficiently strong support for the heavy rover body. The elasticity of the in-wheels suspensions ensures a springy reaction to bumps and terrain irregularities. The RB, however, still needs upgrades since the hollow PVC tubes are severely stressed by the rover's weight. This topic will be covered in a future post.
Since July 2022, the Mast Tower v.1 has had a cheap 720p webcam installed at the top, operating as the "Imager". When the time has come to choose the camera for the Mast Tower v.2, originally a 1080p camera or 2K camera was considered. However, the purchase has been delayed for several months due to a lack of budget. In the end, it was decided to adopt a different approach: instead of buying a new camera, the hardware used for Mast Tower v.1 was moved to v.2. This choice, apart from being partially obliged due to financial needs, carries an emotional aspect: the camera inside Mast Tower v.1 passed the last 6 months on the workstation, partially abandoned, "looking" at the rover slowly but progressively being built. Now, however, it got a new life, moving from Mast Tower v.1 to Mast Tower v.2.
Horizontal Ultrasonic Mapping And Navigation (HUMAN) is a simple system that Mimas inherited from the RC-VSTB, on which it was the only navigation system available. The RC-VSTB was essentially driving blind, detecting obstacles using a single ultrasonic sensor. This system was definitely not accurate, but it was precise enough to avoid major hazards. On Mimas, a proper device has been designed to hold two ultrasonic sensors just in front of the rover's belly. This system would not work on Mars, since its extremely thin atmosphere does not ensure correct transmission of sound, hence correct measurements using ultrasounds. However, Mimas does not drive on Mars! So, HUMAN represents a great and cheap way to help the rover's autonomous navigation. The cameras have been tested, showing relatively low performances in dark environments, whereas ultrasonic sensors are not affected by light conditions. HUMAN cannot be used on its own, because the accuracy is too low, but it will be used to enrich the data collected by the 7 onboard cameras.
Samples catching is a crucial aspect of this project: Mimas itself is made to carry a set of sensors and devices to collect information from the surrounding area. Collecting small samples of the terrain is a great way to study a planet's geological history and look for signs of life. After all, these are the objectives of the rovers sent to Mars! For this project, hermetically sealed sample tubes are not an option, since they would require a level of designing and machining far more complex than what we can do in such a short period of time. For this reason, the sample tubes made for Mimas are PVC tubes with a custom 3D-printed base that will be held in position by the sample tubes carousel. This mechanism is inspired by the drill bit carousel used for Perseverance, but instead of delivering the correct drill bit for the robotic arm, it rotates, exposing the first empty sample tube ready for filling.
