Study on grouser mechanism to directly detect sinkage of wheel during traversing loose soil for lunar exploration rovers
© Iizuka et al.; licensee Springer. 2014
Received: 17 March 2014
Accepted: 16 August 2014
Published: 21 October 2014
The rovers, which some researchers and agencies are developing, have many functions to sense a lot of information from peculiar environments like the lunar surface for localization, path planning and so on. On rough terrain, without artificial ground maintenance, like the lunar surface, the rovers avoid obstacles by using sensors, which they have.
However, if the rovers traverse loose soil, like the planetary surface, there exists the sinking behavior. This sinkage is caused by the weight of the rover. At present the rover sensors are unable to sense it. Actually, Mars Exploration Rover (MER, NASA/JPL), during its exploration on Mars, was disabled because of this. If MER could have known the subsidence of each wheel in time, MER may have avoided this catastrophic condition.
We therefore propose to have the grouser mechanism, which can detect sinkage of the wheels of rovers, installed on the wheels of rovers. The grouser mechanism is needed to traverse loose soil like lunar surface and mars surface. When the wheel rotates on loose soil, the resistance force from the loose soil is given to the grouser. However, initially, for the wheel contacts on the surface of loose soil, the resistance from the loose soil is small. We will develop the grouser mechanism with a function that can detect the whole range of the effective resistance force from loose soil including this small resistance force.
This study moreover carries out experiments with various loads and verifies the effectiveness of the proposed mechanism, using pictures, of contact with the loose soil surface.
Robots are among the most important mission systems for planetary exploration. These robots are designed to travel along planetary surfaces to gather precise information regarding, among other things, the origin of the solar system . The NASA Mars mission in 1997 utilized the micro robot Sojourner, which moved about and explored the surface of Mars, transmitting important data and detailed images of the Martian surface back to Earth . The Sojourner mission demonstrated the importance of mobile exploration. In 2003, NASA/JPL sent Mars Exploration Rover (MER) to Mars, which also transmitted important data .
Planetary exploration rovers are required to traverse rough terrain such as those found in craters and cliffs, which are scientifically important locations for exploration. Rovers are required to travel over these rough terrains and must avoid tipping over and becoming stuck. The surface of the Moon, which is the target of this study, is covered with regolith that is soft and slippery. This regolith, which is made up of fragments of rock broken from the moon and other celestial bodies, undergoes chemical changes caused by the granular phenomenon. As this regolith is different from weathering soil on Earth, a conventional wheel would not achieve the necessary traction efficiency for movement on this surface. The lunar environment is a vacuum and is exposed to radiation from space. Moreover, the temperature differences between day and night are intense. In such environments, conventional tires, meant for use on Earth, cannot be used. Therefore, wheels made of a rigid material, such as metal, are used for lunar and Mars rovers.
The problem is, when a rover with rigid wheels traverses loose soil, there is a possibility of extremely bad traction conditions. This means that slipping and sinking behaviors can occur. The wheels of the MER was sinking into the soil and could not move; a situation that is not easily rectified. When the rovers are traversing loose soil, we have to consider the condition of the ground under the wheels. The relation between the wheel and loose soil can be described using Terramechanics . Using terramechanics, the entrance angle, which can indicate the depth that the wheel is sinking into the soil, is used to lead the drawbar pull. Many researchers are studied about Terramechanics –. Ding et al. summarized the most recent work on Terramechanics . It is necessary to understand the range of the contacting angle between the wheel and loose soil for the derivation of drawbar pull. The contacting angle is shown using the entrance angle and the leaving angle. If the entrance angle can be detected while the rovers are traversing, the drawbar pull and the sinkage can be derived “real time”. It is effective to detect sinkage to determine the rovers moving path.
The poor traction conditions can be avoided if the entrance angle is known while the rovers are traversing loose soil. Some research has been performed to acquire the range of contacting angles between the wheel and loose soil. A.Krebs et al. developed a flexible wheel with tactile sensors . They used IR distance sensors to detect the contacting angle between the flexible wheel and the ground. Nagatani et al. developed a wheel with a built-in force sensor array (containing many sensors on the surface of the wheel) that can measure the normal stress distribution . This study focused on the grousers mounted on the surface of the wheels. When the grouser touches on the surface of loose soil, it can acquire the resistance force from loose soil. If we can acquire this information, the sinkage can be predicted. However, the resistance from loose soil in the first place is very small . We need to consider the grouser mechanism with a function that can pick up the small resistance force from loose soil immediately at touching. This paper proposes the grouser mechanism to detect immediate information at the first touching of loose soil. The proposed mechanism consists of the principle of leverage and supposes that the small resistance force can be acquired at the moment of touching the surface of loose soil.
In this paper, we will present a developed wheel with grousers, which have tactile sensors to calculate the entrance angle. Firstly, we mention the sinkage of the wheel with grousers. Secondly the proposed grouser mechanism is explained. Then the experimental device and condition are mentioned. Finally, we will give the conclusions of the study after the results and a discussion are presented.
2Sinkage of wheel with grousers on loose soil
The sinkage of the wheel while traversing loose soil can be calculated by using the conventional terramechanics model . When the weight of the wheel is given at first, the normal stress can be calculated as static condition (not dynamic condition). The entrance angle and the exit angle are given when the value between the weight of the wheel and force calculated by using the normal stress is equal . However, there are some problems with that situation. The sinkage including the height of grousers is not contained in this calculation using the conventional terramechnics model. The sinkage is not known by the dynamic condition. The wheels for rovers have installed some grousers to get the effective driving force on loose soil. So, the sinkage including the height of grousers and the dynamic situation are needed to understand this. Some researchers have considered the running situation using wheel with “grousers”. Ding et al. and Sutoh studied the influence of grousers experimentally –. The sinkage is measured by using some extrinsic sensors, for instance motion capture and the rotary encoder and so on. This means that the wheel cannot calculate its own sinkage in time. From this problem, this paper suggests a wheel that can measure the sinkage directly.
3.1 Detection mechanism using grousers
Characteristics of strain gage
1.98 ± 0.1%
120.4 ± 0.4 ohm
Adapt linear expansion coefficient
1.9 *19−6 / °C
3.2 Proposed wheel with grousers mechanism which can detect sinkage
3.3 System flow of proposed wheel
The next section describes how to judge the angle where the grouser contacts the loose soil surface.
3.4 How to judge the contacting angle to loose soil
4Experimental device and condition
4.1 Single wheel test bed
4.2 Experimental condition
5, 10, 15
5Results and discussion
The surface of loose soil is very soft and the shearing strength is nonlinear. Therefore, the wheel needs to detect the tactile information directly and sensitivity. On this paper, the proposed grouser mechanism mounted on the wheel could detect sinkage directly and a difference between the correct values and data of experiments using single wheel test bed that could traverse loose soil was very small.
The wheel with proposed grousers which can detect sinkage directly will be able to utilize as advanced applications, for instance as measurement systems to solve some problems (measurement of accurate entrance angle and so on) which are occurred between the wheel and the loose soil (“problems of Terramechanics”). In the near future, we will be able to suggest the rovers without the poor traversing condition that is caused by sinking behavior.
This work was supported by KAKENHI (23760223).
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