Members of the Kameleon Team and Team mentor, assoc. prof. Mirosław Szmajda, Poland

Clever Kameleon is a project carried out at the Opole University of Technology by the Kameleon Team. Since 2015 the goal is to build and develop a multi-purpose Martian rover. The main targets of the project are the development of its members, students of the Opole University of Technology and participation in the annual European Rover Challenge competition. The Team already took part in the competition four times, taking the highest 5th place in its history in 2019. Thanks to the multitasking of the robot, which is a space rover prototype, students from many fields can find areas of their interest and modernize them, while gaining valuable knowledge and skills, and thus developing their qualifications. The Kameleon is a prototype of four-wheeled space rover with each wheel twisted individually and it has manipulator with six degrees of freedom.

The competitions in which we participate are based on the situation of a rover on another planet, which means wireless contact and remote control with the use of on-board cameras. Three out of five competitions require efficient manipulator operation in such activities as: tool handling, soil sampling, control panel operation. One of the competition is solved with navigating the route without view from cameras, but only by planning it or basing on the autonomous rover’s actions.

Until 2018, we had to deal with the problem of frequently failing servo drives responsible for steering and the lack of information about the position of the manipulator joints (no feedback). During the reconstruction, it was decided to change the modeling servos responsible for wheel steering to DC motors with worm gear. In order to know the position of the steering system, it was necessary to read the position of the drive shaft. Just like in a space vehicle, there is little space in the rover and everything must be lightweight and reliable at the same time. For this purpose, the samples from RLS - RMK3 evaluation board were used as part of experiments. Due to the successful results, we asked RLS about the possibility of cooperation. Thanks to the support of the company, in a short time we were in possession of 10 RMB30SI13BC1 encoders and appropriate magnets.

The first application that we implemented was the installation of feedback couplings for the drives responsible for wheel steering. We used 3D print sleeves in which we placed the magnet and PMMA plates on which we placed the encoders. The whole set was enclosed in a printed PET- G housing and the cables were routed through a cable gland to IP68 rating, which was required near the wheels of the vehicle. The STM32 processor with our own PCB board was responsible for encoders interpretarion and implementing drives in the form of servomotors. 

After the implementation of the modified drives, no abnormalities were observed in their operation. The resolution of 13 bits was sufficient and non-contacting structure of magnetic field ensured that the encoder body was insensitive to the strong vibration to which the shaft was exposed regardless of the gearbox body, where we attached the sensor. 

The second application of RLS encoders is the feedback of the manipulator joints. This task was difficult, because the manipulator already existed and its design do not consider mounting any kind of sensors. Thanks to the small size of the RMB30 encoders, they could be installed in such a way that they aren’t interfere with any other components. Due to the gap between the integrated circuit and the encoder, a PMMA plate was used to protect the encoder against grease and other particles from moving parts.

Accuracy at the level of 0.5 degree allowed for sufficient accuracy for our needs, because the longest arm is about 1,5 m long, which gives in theory about 1 cm movement accuracy. In fact, the cheap transmissions we use nowadays do not allow us to work with better precision, so we were not able to challenge it. Until now, the position measurement of 4 out of 6 axes of the manipulator has been implemented, and the measurement of the other two joints is in the conceptual phase. The very low weight of the encoders was especially important near the gripper, where the use of classic absolute encoders, e. g. optical ones, would significantly increase the weight and at the same time reduce the dynamics and load capacity. 

It was our first cooperation with RLS and its encoders. Very effective contact during implementation is crucial in this type of prototype projects, where there is always too little time and too much work to do. We are very satisfied with both the products and the contact with the company and we hope for equally successful joint actions in the future.