With the intelligent robot technology, robots are playing an increasingly important role in production and life [1]. The mobile robot is a member of the robot family with greater flexibility, greater autonomy, and higher intelligence, and can realize the functions of autonomous navigation, environmental recognition, and safety obstacle avoidance [2]. The omnidirectional mobile robot has the ability to move forwards, backwards, leftwards, and rotationally on the plane of motion [3], so it is possible to achieve a zero turning radius or to move in any direction while keeping the robot's attitude unchanged. Due to its unique sports performance, omnidirectional mobile robots have obvious advantages in smart wheelchairs, material handling robots, inspection robots and other products [4-7].
At present, typical motion structures of omnidirectional mobile robots include Mcnam wheel, continuous switching wheel, and concentric steering wheel [8-10]. The Mcnam wheel and the continuous switching wheel have two degrees of freedom: an active driving freedom along the tangential direction of the wheel face, and a follow-up freedom with a fixed angle to the tangential face of the wheel face. The rotation of the wheel body is driven by the motor, and the roller passively rotates under the action of the ground friction force, and the robot moves omnidirectionally by controlling the rotation of the wheel body [11]. However, the rollers on the outer edge of the Mcnam wheel alternately contact with the ground. During the rolling process, the roller is continuously subjected to the impact load of the ground, so that the height of the contact point with the ground during the rotation of the wheel constantly changes, resulting in the vibration or slippage of the vehicle body.[12] . The omnidirectional mobile robot composed of steered wheels is composed of a plurality of independently steered conventional wheels. The deflection of the wheel direction and its own rotation are all driven by independent motors. The omnidirectional movement of the robot is achieved by controlling the deflection angle and rotation speed of the wheels. ]. Since the deflection of the wheel orientation requires the active steering structure to be driven, a complicated steering structure is required, and when the robot runs in a straight line, the steering drive structure becomes an additional load and the energy utilization rate is low.
To sum up the above problems, this paper adopts a passive concentric steering structure instead of active steered wheels to design a multi-tracked omnidirectional mobile robot. The robot does not require complicated active steering structure and has the characteristics of stable running and heavy load capacity of the crawler robot.
1 Robot Structure DesignThe robot is composed of a main body and four differential crawler units. Each crawler unit constitutes a passive concentric steering structure and is mounted under the robot body through an angular contact bearing. The crawler unit can be deflected by ±90° around the yaw axis. The deflection angle is measured by a precision rotary potentiometer. The rotary axis of the potentiometer is fixed to the yaw axis of the crawler unit via the coupling. Since the angular contact bearing can simultaneously bear a large radial load and axial load, the crawler unit can not only perform the role of carrying the weight of the robot body and the load, but also can move the crawler robot horizontally through the crawler operation. The bottom plate of the robot body adopts an aluminum alloy structure, and the equipment and structure of the robot body are supported above, and there are four crawler units below. The side plate and the supporting structure are 3D printed and fixed on the bottom plate, which mainly play the role of support and fixation. Figure 1 shows the three-dimensional model of the robot structure.
Each crawler unit consists of crawler belts, crawler support wheels, drive motors, motor reducers, speed encoders, and semicircular shading panels. Figure 2 shows the crawler unit structure model.
The robot uses a lithium battery with a nominal voltage of 12 V and a capacity of 31 200 mAh. The power management module stabilizes the voltage to 12 V and 5 V. The robot uses the ROS (Robot Operating System) as the control and communication system. The ROS core runs in the Raspberry Pi3. The bottom robot driver uses the NUCLEO-F446 development board based on mbed. The UART interface is used to communicate with the Raspberry Pi. The system passes the router. Remote connection and data transmission with the PC. The robot uses 4 dual H-bridge circuits to drive 8 DC motor movements. The motor drive can independently drive the motor motion in each track unit under the control of the controller. The deflection angle is measured by a potentiometer and passes through the 12-bit ADC in the processor. Take the acquisition. Its circuit system diagram is shown in Figure 3.
Through the modeling and processing of mechanical structures and the construction of circuit systems, a multitrack crawler omnidirectional mobile robot was finally developed, as shown in Figure 4.
The speed of two motors on each crawler unit of the robot is measured by the speed encoder fixed on the rear of the motor. The speed of the robot can be calculated by the reduction ratio, the radius of the driving wheel and the thickness of the crawler belt. The running speed of the two crawlers can be decomposed into two degrees of freedom for each track unit movement, one degree of freedom controls the front and back movements, and the other degree of freedom controls the direction of movement. Through the analysis of the robot structure, the control flow of a single track unit is designed, as shown in FIG. 5 .
The movement of the crawler unit can be decomposed into linear movement and rotation around the yaw axis. The input control amount is the movement speed and deflection angle of the two crawlers. Firstly, the difference between the actual deflection angle and the deflection angle in the control signal is measured, and the PID controller outputs the adjustment values ​​of the two motor rotation speeds through the deflection angle. The actual speed of each motor and the speed difference in the control command are superimposed on the motor speed adjustment value and input to the motor speed PID controller. The speed of each motor is adjusted, and the torque output by the motor is transmitted to the track drive wheel through the speed reducer. The yaw angle and the moving speed of the crawler unit are output through the cooperation of the direction and the speed to complete the differential movement of the single track unit.
Every crawler unit of the robot needs to receive the control signal of the robot controller, and the movement instruction of the robot is the movement speed, direction and main body orientation of the main body. Therefore, the robot controller is required to decompose the robot's instructions into the control commands of each crawler unit according to the kinematics posture settlement, and the robot body is driven to move through each crawler unit. The control flow is shown in FIG. 6 .
The robot has several typical movement modes such as straight line, lateral movement, arc movement and rotation, as shown in Fig.7. Among them, the straight movement and the lateral movement are translational movements. The robot keeps the main body orientation unchanged. The rotation movement belongs to the circular arc movement. At this time, the direction of the main body of the robot constantly changes. Therefore, the omnidirectional motion of the robot is viewed as a combination of translational motion and arc motion.
Through the robot model, the robot's overall movement speed, movement direction and body can be converted into three control amounts for each track movement speed and each track unit deflection angle. According to the overall robot control flow, each track unit movement can be controlled. In order to achieve the omnidirectional movement of the robot.
3 Experiments and AnalysisThe developed robots were tested for translational motion and arc motion. Use the MATLAB program running on the PC to connect to the Raspberry Pi3 and NUCLEO F446 control boards running ROS indigo through the wireless router on the robot. Release the robot's motion instruction topic to the robot via the ROS command, and read the robot's published status at the same time. The data on the topic collects the movement speed of the robot crawler and the deflection angle of the crawler unit.
During the translational movement of the robot, the path of the given robot is a regular pentagon with a side length of 600 mm, the movement speed is set to 40 mm/s, the body orientation is unchanged, and the speed at which the crawler unit changes the deflection angle is set to 20 mm. /s, Fig. 8(a) is the movement speed of the eight tracks of the robot in a 100 s positive pentagonal translational motion, and Fig. 8(b) is the angle value of the deflection angle of the tracked unit with time. When the robot performs arc motion, it needs to calculate the deflection angle of the crawler unit of the robot and the movement speed of each crawler through the attitude calculation according to the center point of the arc and the rotation angle speed. The trajectory of a given robot motion is a circle with an angular velocity of 0.05 rad/s and a circular arc radius of 500 mm. Fig. 9(a) shows the movement speed of the eight tracks of the robot in a 100 s circular motion, and Fig. 9(b) shows the angle values ​​of the deflection angles of the tracked units over time.
Collect the speed and deflection angle data of the robot during the movement process, use the path-tracking algorithm to calculate the trajectory of the center of each crawler unit of the robot, and fit the trajectory of the center of the robot body through the four trajectory points obtained each time. Figure 10 and FIG. 11 is a trajectory diagram of the center point of the four crawler units and the position of the center of the robot when the robot moves in a regular pentagonal shape and an arc shape. In the figure, Tc represents the center point of the crawler unit, and "x" and "o" represent the positions of the center point of the robot and the center point of the crawler unit at the beginning, respectively.
The experimental results show that the robot can move in different directions while keeping the head orientation unchanged. The robot completes a pentagonal translation movement every 65 s. The robot body remains stationary during the robot's change of the deflection angle of the crawler unit. The deflection angle of the crawler unit during the arc movement of the robot remains stable, and a circular motion is completed in 32 s. The movement direction and speed of each track are basically unchanged.
4 ConclusionThis paper introduces the structure and circuit design, control flow and experimental test of a passive concentric steering multi-track omnidirectional mobile robot. Due to the crawler structure of the robot, the contact area with the ground is large, and slipping is not likely to occur during the movement, and the crawler structure has a better carrying capacity than the wheeled structure. Each crawler unit has the ability to turn and move forwards and backwards. All crawlers can provide driving power during the operation of the robot. At the same time, a plurality of smaller-powered drive motors can be used to generate a larger driving power for the robot as a whole. There is no vibration in the structure during the movement, and it can move omnidirectionally in poor ground conditions. The experimental results verify the ability of passive concentric steering multi-track omnidirectional mobile robots to move omnidirectionally, and prove the correctness of structural design and control flow.
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