1 Introduction
With the development of science and technology, human research activities have expanded from land to the seabed and space. Utilizing mobile robots for space exploration and development has become one of the main means for major technologically advanced countries in the 21st century to develop space resources. Researching and developing lunar exploration mobile robot technology will greatly promote research into related cutting-edge technologies, including mobile robots themselves.
A mobile robot is a robotic system capable of sensing its external environment and its own state through sensors, enabling autonomous movement towards a target in obstacle-filled environments to complete specific tasks. In recent years, the increasingly broad application prospects of mobile robots in industry, agriculture, medicine, aerospace, and various aspects of human life have made them a research hotspot in international robotics. Since the 1990s, research on mobile robots has reached a higher level, marked by the development of advanced environmental information sensors and information processing technologies, highly adaptable mobile robot control technologies, and planning technologies for real-world environments. Currently, mobile robots, especially autonomous robots, have become a highly active research area in robotics technology.
Wheeled mobile mechanisms have advantages such as high speed, high energy efficiency, simple structure, convenient control, and the ability to draw on mature automotive technologies. However, their off-road performance is not very strong. But with the emergence of various wheeled chassis, such as NASDA's six-wheeled flexible chassis lunar rover LRTV, TRANSMASH's six-wheeled three-body flexible frame mobile robot Marsokohod, CMU's six-wheeled three-body flexible robot Robby series, and JPL's six-wheeled rocker-suspended planetary rover Rocky series, the off-road capabilities of wheeled robots have been greatly increased, making them comparable to legged robots. As a result, the focus of robot mechanism research has shifted to wheeled mechanisms. In particular, Japan recently developed a unique five-point support suspension structure called Micros, whose superior off-road capabilities surpass those of legged robots [6-8].
Wheeled structures can be categorized by the number of wheels into two-wheeled, three-wheeled, four-wheeled, six-wheeled, and multi-wheeled mechanisms. Two-wheeled mechanisms are very simple in structure, but are highly unstable when stationary or at low speeds. Three-wheeled mechanisms are characterized by their easy assembly; the center of rotation is on the straight line connecting the two drive wheels, achieving zero turning radius. Four-wheeled mechanisms have essentially the same motion characteristics as three-wheeled mechanisms, but the addition of a support wheel makes movement more stable. A common feature of these wheeled mobile mechanisms is that all their wheels can only be fixed on a single plane during movement and cannot be adjusted vertically, thus limiting their adaptability to different terrains. Generally, six-wheeled mechanisms are designed to improve the ground adaptability of mobile robots by adding a rocker arm structure. This allows the robot's wheels to adjust vertically according to the terrain, thereby improving the robot's off-road capabilities.
2 Robot Main Structure Design
The main structure is the core of the robot, containing the control system, four drive motors, and various sensors, including infrared sensors, pressure sensors, and sound sensors, which act as the robot's eyes, touch, and hearing. The structure is primarily a box-like structure, housing the motors and system hardware as needed. The main structure is shown in Figure 1.
Figure 1. Main structure diagram of the robot
Figure 1 shows the main body without any sensors or devices attached to the outer shell. The four smaller boxes house the motors controlling the upper limbs, and the holes on the sides are for wiring. The large box contains the control system's circuit board. After all the components are in place, a plate can be added on top to protect the internal components. My design envisions adding other functions to the main body of this robot. The main body is 1.5 meters long , 0.5 meters wide, and 0.3 meters high.
For the lower limbs, since the Mecanum wheels allow for omnidirectional movement, joints are not required. However, a braking system is necessary to ensure timely stopping and prevent rolling during leg movement. A platform is designed at the connection point between the lower limbs and the tires to house the motor, which drives the tires to rotate. One motor corresponds to one tire, allowing for control of complex movements such as direction by changing the rotational speed of each tire.
In addition to a structure similar to the upper limbs, the lower limbs have two additional box-like structures on the sides, with the wider lower section used to connect to the tires. The upper box houses a small motor, which connects to a shaft via two coaxial holes at the top of the lower limb to control the rotation of the lower limb around the upper limb. The holes at the upper sides of the box are for passing electrical wires. The lower box houses the motor that controls the tires, and on the right side, there is space for mounting gears and through holes for fixing them. The two vertical coaxial holes on the lower limbs are also for passing electrical wires. The total length of the lower limbs is approximately 1 meter, and the width of the main body is approximately 20 centimeters.
3-wheeled movement principle
The main principle of wheel-based movement is the movement principle of the Mecanum wheel.
The Mecanum wheel is a patented design by the Swedish company Mecanum. This omnidirectional movement method is based on a central wheel with numerous axles surrounding it. These angled peripheral axles convert a portion of the steering force of the wheels into a normal force. Depending on the direction and speed of each wheel, the final resultant force vector of these forces produces a resultant force vector in any desired direction, ensuring the platform can move freely in the direction of the final resultant force vector without changing the direction of the wheels themselves. Numerous small rollers are diagonally distributed along its rim, allowing the wheel to slide laterally. The generatrix of the small rollers is unique; when the wheel rotates around a fixed central axle, the envelope of each small roller is cylindrical, allowing the wheel to roll continuously forward. The Mecanum wheel is compact, flexible, and a very successful omnidirectional wheel. Combining four of these novel wheels allows for even more flexible and convenient omnidirectional movement.
The Mecanum wheel resembles a helical gear, with its teeth being rotating drum-shaped rollers whose axes form an angle α with the wheel's axis. This unique structure grants the wheel three degrees of freedom: rotation about its axle, translation along the perpendicular direction of the roller axis, and rotation about the point of contact between the roller and the ground. Thus, the drive wheel possesses active driving capability in one direction while also exhibiting free movement (passive movement) in another. Instead of a standard tire, the wheel's circumference is composed of numerous small rollers, whose axes are tangent to the wheel's circumference and can rotate freely. When the motor drives the wheel, it moves forward in a direction perpendicular to the drive shaft in a normal manner, while the rollers around the wheel rotate freely along their respective axes. Figure 2 illustrates the structure and motion parameters of the Mecanum wheel.
Figure 2. Definition of Mecanum wheel motion parameters
4 Servo Control System Design
The motion control system of a mobile robot is the actuator of the robot system, playing a crucial role in the accurate completion of various tasks. Sometimes it can also function as a simple controller. The components of a robot motion control system include : computer hardware and control software, input/output devices, drivers, and sensor systems. The relationship between them is shown in Figure 3.
Figure 3 Components of a Robot Control System
4.1 Research on Mobile Robot Control System
(1) Mobile robot architecture. Utilizing a distributed intelligent architecture can improve the real-time performance and robustness of mobile robots, and reduce their size and weight, making them lighter and more flexible.
(2) Sensor Technology in Control Systems. Mobile robot sensor technology primarily involves the detection and processing of the robot's internal position and orientation information, as well as information about the external environment. Obtaining accurate and effective environmental information is crucial for the control system's decision-making. Sensors are typically categorized into internal and external sensors. Internal sensors mainly include encoders, linear accelerometers, gyroscopes, and magnetic compasses. External sensors mainly include vision sensors, ultrasonic sensors, infrared sensors, and contact and proximity sensors.
(3) Multi-sensor information fusion technology for control systems. Multi-sensor information fusion integrates the incomplete information about the local environment provided by sensors distributed in different locations, eliminates redundancy and contradictions that may exist between multiple sensors, reduces uncertainty, and forms a relatively complete and consistent perception description of the system environment, thereby improving the speed and accuracy of intelligent system decision-making and planning, while reducing decision-making risks.
(4) Control system development technology. The focus is on open and modular control systems. Standardization of mobile robot controller structures and networked controllers are research hotspots. Further improvements in programming technology to enhance the operability of online programming, more user-friendly human-machine interfaces for offline programming, and the further promotion of natural language programming and graphical programming are also key areas for future research.
(5) Motion control technology. Robot movement must be fast enough, controlled, and safe, avoiding both static and dynamic obstacles. Trajectory tracking, path tracking, and point stabilization are the three fundamental problems in the motion control of mobile robots;
(6) Intelligent technologies for control systems. The intelligent characteristics of control systems include knowledge understanding, induction, inference, reaction, and problem-solving. Areas involved include image understanding, processing and understanding of speech and written symbols, and knowledge expression and acquisition. Intelligent control methods often use neural networks and fuzzy control methods, but the former often requires higher storage capacity and processing speed, which is somewhat different from the high-speed, high-precision motion control requirements of mobile robots. Therefore, fuzzy control methods have a significant advantage in robot control.
Based on the design requirements of the mobile robot control system, and considering the system functions and characteristics of this robot, a general design scheme for the robot control system is proposed according to the modular design concept. See the figure below :
Figure 4 Overall Scheme of Control System
Overall design scheme of control system
This solution is based on the ATmega128 chip and features a modular design. The functions of each sub-module are as follows :
(1) Microprocessor module : It is the core of the control system, including microcontroller and its related peripheral circuits. It mainly processes various information and data and coordinates the functional modules in the system to complete the predetermined tasks.
(2) Drive module : controls the predetermined tasks of the servo motors and sensor modules in the robot system; realizes the control of servo motor speed and position, and completes actions such as forward, backward, straight, turning, obstacle avoidance, and grasping;
(3) Sensor module : It has sensors for speed, position, distance, sound, etc., and is mainly responsible for detecting obstacles and sounds during the movement of the mobile robot;
(4) Power module : responsible for the power supply of the entire mobile robot, enabling the system to move offline. It mainly consists of a 12V battery and related voltage regulation and stabilization circuits.
(5) Serial communication module : communicates with the host computer via serial port according to the RS232 communication standard;
(6) JTAG debugging : Enables online programming, debugging and simulation.
3.2 Robot Drive System
Currently, the most common motion control methods used in robotics are DC motors, stepper motors, and servo motors. For my project, I need a speed-controllable motor for use as a Mecanum wheel, and another motor with precisely controllable and maintainable angles for use as a leg joint. My initial estimate is that the motor speed isn't very high. If I use a DC motor, due to the speed and torque limitations, a speed reducer would be necessary, and angle control wouldn't be possible. If I use a stepper motor, a driver would be required. To meet the system's control requirements and considering cost-effectiveness, I plan to use the Dynamixel AX-12 servo motor, a dedicated servo motor for robots. It not only provides precise angle control for joint angle control but can also be set to infinite rotation mode via software for use as a wheel.
3. Overview and characteristics of the 3AX-12 digital servo motor
A servo motor is a position servo actuator suitable for control systems that require continuous angle changes and the ability to maintain that angle. Its working principle is as follows : the control signal enters the signal modulation chip through the receiver channel to obtain a DC bias voltage. Internally, it has a reference circuit that generates a reference signal with a period of 20ms and a width of 1.5ms . The obtained DC bias voltage is compared with the voltage of a potentiometer to obtain the voltage difference output. Finally, the positive or negative value of the voltage difference is output to the motor drive chip to determine the forward or reverse rotation of the motor. The AX-12 servo motor is an intelligent, modular power unit, mainly composed of a microprocessor, a precision DC motor, a gear reducer, a position sensor, a temperature sensor, and a control chip with communication capabilities. Its internal mechanical structure and circuit control are shown in Figure 4 .
Figure 4.1 Internal structure and control diagram of the servo motor
The AX-12 digital servo motor has a maximum turning angle of 300 degrees when used as a servo motor, and can rotate freely when used as a motor, making it widely applicable. It uses digital signal control, making it more convenient to control. Each servo motor has a unique ID number and uses a network drive mode and Daisy bus connection method, allowing multiple servos to be connected in a mesh series for easy connection. Its specific parameters are shown in Table 1.1 .
Table 1.1 Specific parameters of the servo motor
Project parameters Project parameters
Weight 55g, displacement angle 0-300°, infinite rotation
Reduction ratio 1/254, minimum angle 0.35 °
Operating voltage: 7VDC-12VDC; Communication: half-duplex asynchronous serial communication
Operating temperature: -5 to 85 degrees Celsius; Baud rate: 7343 bps to 1 Mbps
Maximum current 900mA instruction packet digital signal
Input voltage 7V/10V, physical connection to TTL multi-channel (Daisy bus).
Maximum torque: 12 kgf / cm, 16.5 kgf / cm; Material: Engineering plastic
Rotational speed : 0.269 (seconds/60°) 0.196 (seconds/60°) Feedback on position, temperature, load, voltage, etc.
Because the AX-12 is equipped with an ATmega8 microprocessor, which receives data packets sent by the controller and processes them accordingly, it sends PWM signals to the servo motor to control its start and stop. Therefore, controlling the servo is actually controlling the ATmega8 servo. The servo's state and parameters are stored in the corresponding addresses in the ATmega8's RAM and EEPROM. Controlling the servo is essentially the process of reading and writing data to the corresponding addresses of the servo.
5AX-12 Servo Communication Protocol
Unlike typical R/C servo motors (servos) that use PWM control, the AX-12 digital servo uses digital signals for control. The main controller and servo are connected via a TTL-Daisy bus using a half-duplex asynchronous serial communication protocol (8 data bits, 1 stop bit, no parity bit). The main controller controls the servo by sending and receiving data packets. There are two types of data packets : a command packet, which is the instruction sent from the main controller to the servo; and a status packet, which is the feedback from the servo to the main controller. If the main controller sends a command packet to the servo with ID N, only the servo with that ID will respond with the corresponding status and execute the required action. The control principle diagram is shown in Figure 5 .
Figure 5. Schematic diagram of servo motor control principle
6. Conclusion
This paper designs a motion control system for a wheeled mobile robot: a servo-controlled mobile robot motion control system. This system uses numerical control interpolation technology to track the trajectory, featuring high tracking accuracy. A dedicated DSP chip for motor control was selected, simplifying the design, improving module reliability, and leaving sufficient room for future upgrades to the control algorithm. The trajectory estimation mathematical model used in this paper for mobile robot path planning differs from traditional discretization methods that only discretize straight lines and arcs; it can discretize both first-order and second-order curve tracking paths. The motion control system adopts a master-slave control structure, where the master performs complex calculations and transmits the processed data to the slave, which then controls the robot body, conveniently implementing stepper motor control. The slave-executed motion controller is low-cost, powerful, and easy to use, and has a very broad application prospect.
