![]() ![]() When the end effector containing the sample comes sufficiently close to the sample storage location, the engaged 6-DOF joint between the end effector and the sample is disengaged and an initially disengaged 6-DOF joint between the sample and the storage location is engaged. This 6-DOF joint has tight position limits to keep the sample nearly constrained to the end effector. When the end effector is sufficiently close to the sample, an initially disengaged 6-DOF joint connecting them becomes engaged. We use a simplified model for the interaction between the end effector and the sample that leverages joint mode switching. To mimic sensors like encoders, the subsystem outputs the joint angles from each of the six revolute joints. Its actuators correspond to six torque-actuated revolute joints. The manipulator is modeled as a 6-DOF arm mounted on the front end of the chassis. We first use the geometric bicycle model and the target direction angle ( α) (provided by the Pure Pursuit Controller) to obtain the bicycle steering angle ( δ) and the turn radius ( R) as shown below. The steering angles for each of the four corner wheels of the rover are derived in two steps. ![]() This is a geometric algorithm that computes a target direction angle ( α) needed to move the robot from its current position to reach some look-ahead point in front of the robot. Pure Pursuit (Robotics System Toolbox) is used for path tracking. The wheels are assumed to roll without slipping.īased on the above considerations, a six wheel rover can be equivalently represented by a geometric bicycle model. The front and the rear wheels are considered to be steered symmetrically. This simplification is done by representing each pair of wheels by a single wheel located in the middle and a single steering angle corresponding to the turn radius of the center of the rover. The Ackerman steering geometry is simplified by assuming a 2D geometric bicycle model with an equivalent turn radius. Based on this capability, the rover is considered to be using Ackerman steering. The four corner wheels of the rover have independent steering which can enable the rover to perform Ackerman steers. To simplify the kinematics formulation, the rover is assumed to be moving on a planar surface. Mars rovers are typically assumed to have low forward velocity (on the order of cm/s), therefore the dynamics of the motion are ignored and the controls problem is approached using kinematic equations only. The goal of this subsystem is to first, compute the necessary steering angles and the wheel speeds needed to follow a desired path and a desired chassis linear velocity and second, to compute the necessary actuator torques needed to achieve these steering angles and wheel speeds.įor developing the path tracking controller, the following considerations are made: These waypoints can be loaded using the roverDesiredPath.mat file. These waypoints are assumed to be provided by a high-level path planner and would represent an obstacle free path for the rover. The path consists of ordered waypoints in the X-Y plane which the rover is desired to pass through. This subsystem models rover's path tracking control system. Refer to the file rover_rigid_terrain_params.m to setup the parameters needed to create the Grid Surface from a STL file. ![]() To model a Martian surface, a rigid terrain is created using the Grid Surface block. The points on each wheel's grousers are created using the Point Cloud block. The contact between the wheels and the rigid terrain is modeled using Point Cloud and Grid Surface contact pairs along with the Spatial Contact Force block. In addition to this, three main components of the suspension mechanism are also modeled, namely the differential arm, rocker and bogie. The rover's actuators correspond to the six torque-actuated revolute joints mounted to each of the six wheels for speed control and the four torque-actuated revolute joints mounted to the top of four corner wheels used for steering. The CAD parts for the geometry are imported into Simscape Multibody™ using the File Solid. This subsystem models various components of the rover like chassis, rocker-bogie suspension and wheels. ![]()
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