Abstract: This paper addresses the shortcomings of current road marking machines, which require manual marking before mechanical spraying. A guiding robot is proposed to provide guidance for the road marking machine . This robot uses a PLC control unit to process signals from ultrasonic sensors and control the speed of the DC motors of the left and right drive wheels, enabling the robot to move and spray along the center line of the road. The robot has a simple structure and a highly stable control system, making it particularly suitable for harsh field operations.
Keywords: Highway marking machine; Marking and guiding robot; Design; PLC; Ultrasonic sensor
Design of the Line-oriented Robot for Marking-lines MachineAbstract: A new design of the line-oriented robot has been accomplished aimed to solve the problem of the way when the marking-lines machine works, which is used to mark lines after the manual marking. In order to make the robot walking along the middle line, the robot controls the speed of DC motor which equipped in the driving wheel by signals from ultrasonic sensor dealt with by PLC control unit. The robot has simple structure and it's control system has stronger stability, which particularly suitable for the wild environment.
Keywords： Marking –lines machine;Line-oriented robot;Design;PLC;Ultrasonic sensor
1 Introduction
Highway marking machines are specialized machines used to mark various road markings on high-grade highways and urban roads. They are also used to mark new markings on old markings during road maintenance [1]. Currently, there are many types of marking machines, which are used in different situations, but they all share the common feature of first marking manually and then mechanically spraying. Manual marking is labor-intensive, slow, and has poor accuracy. In response to this phenomenon, this paper introduces a highway marking machine marking guide robot, which is specifically designed to provide a more accurate route for the marking machine, reduce the workload of manual marking, and improve the accuracy of marking.
2. Overview of Line Marking and Guiding Robots
The basic components of a line-marking and guiding robot system include a guidance control system, a motor-driven tracked walking system, and a paint spraying system. This line-marking and guiding robot is mainly used for marking and guiding the center line of highways. Ultrasonic ranging modules are installed at both ends of the robot to measure the distance from the road edges on both sides to the robot. The signals obtained after processing by a Siemens S7-200 PLC software program drive the DC motor, enabling the robot to quickly approach the center line and then move along it. Once the movement is stable, the spray guns in the paint spraying system begin spraying paint onto the ground, providing a more accurate line marking. A schematic diagram of the robot control system is shown in Figure 1.
Figure 1. Schematic diagram of robot control
3. Guiding Control System
The basic components of the guidance control system are: an ultrasonic ranging module and a PLC control unit.
An ultrasonic ranging module generally consists of hardware and software components. The hardware includes a control unit, ultrasonic sensor, transmitting and receiving circuits, etc.; the software is the program fixed in the control unit. Currently available ultrasonic rangefinders, with slight modifications, can also meet the requirements of this paper. Current ultrasonic rangefinders can achieve a detection distance of approximately 10 meters, far exceeding the requirements of this paper.
This robot has an ultrasonic ranging module symmetrically installed at each of its left and right ends. The two sensors simultaneously emit ultrasonic waves to opposite directions. By comparing the time difference between the emission and reception of the two sensors, the robot determines whether it is veering to the right or left, and then controls the movement of the DC motors of the drive wheels, thus ensuring the robot moves along the center line of the road. After receiving data from the ultrasonic ranging modules, the PLC control unit compares the two sets of data. If the data on the right is smaller, the emitted ultrasonic wave takes less time to return to the right, so the robot veering to the right of the center line. In this case, the right drive wheel rotates faster than the left drive wheel. This adjustment is repeated until the robot is positioned on the center line of the road. Once stable, the control unit stores the ultrasonic data at this point in its storage area. The robot's drive wheel speed will then maintain this constant speed, adjusted based on real-time data from the sensors. The adjustment range is limited to a certain value range. If the value exceeds this range, the motors will start rotating to adjust the distance between the robot and the center line, ensuring it moves as quickly as possible.
The PLC control unit software includes: a main program, a subroutine for reading data from the ultrasonic ranging module, and a motor adjustment subroutine. Its flowchart is shown in Figure 2.
Figure 2 Program Flowchart
4. Motor-driven tracked walking system
Robots can move in three ways: wheeled, walking, and tracked. Walking robots have complex structures and move slowly, but they can adapt to poor road conditions. Wheeled structures are highly mobile, but they are only suitable for flat terrain. Tracked structures, on the other hand, have the advantages of smooth movement, strong climbing ability, and less tendency to tip over; they overcome the disadvantages of wheeled mobile mechanisms, such as poor obstacle crossing ability, poor ditch crossing ability, and slippage [2-3].
The line-marking guide robot proposed in this paper is applied to highways with good road conditions. However, considering that this robot is to find the center line of the highway, although the wheel structure has good mobility, its adaptability to the road surface is far inferior to that of the tracked movement mode. In combination with various situations, this robot adopts a two-section parallel track connection mode, and its movement mode is similar to the walking mechanism of a tank. The tracks on both sides are installed on the front and rear wheels. The rear wheel is driven by a DC motor, which drives the front wheel to rotate together through the track. Its structural schematic diagram is shown in Figure 3. The tracked structure robot can not only move straight, but also turn in place by using the differential of the track. The structure is simple and does not require a differential, steering mechanism, etc. It is easy to control, has a small turning radius, and the motor can provide both forward power and steering power [3].
The drive wheel DC motor is a 24V DC motor with a gearbox. The battery voltage is 24V, including a working battery and a backup battery. The working battery powers not only the DC motor but also the control unit. The backup battery is used in case the working battery fails or suddenly loses power.
Figure 3. Schematic diagram of tracked mobility structure
5. Paint spraying system
According to the materials used for marking lines, line marking machines are divided into cold paint line marking machines and thermoplastic line marking machines. Cold paint line marking machines spray paint at room temperature. There are two types: air spraying and airless spraying. Air spraying is to atomize the paint with compressed air and then spray it onto the ground through the nozzle. Airless spraying line marking machines use a special high-pressure pump to apply a pressure of 10-15 MPa to the paint and then spray it onto the ground through the nozzle. Thermoplastic line marking machines heat hot melt paint or hot melt plastic (there are two types: granular and block) in a hot melt tank to the operating temperature and then apply it to the ground through the line marking device. The line marking methods of the line marking device are divided into scraping, extrusion and spraying [4]. In conjunction with the application of the line marking guide robot in this paper, the guide robot provides the marking for the line marking machine. Therefore, the paint spraying system of the guide robot is not as complicated as the spraying system on the line marking machine. It only needs to meet the requirement of providing the line marking machine with recognizable markings.
This paper describes a paint spraying system that uses a cold-plastic airless spray method, which involves painting at room temperature using a dedicated high-pressure pump and spray gun. Its basic components include: a high-pressure pump, a paint tank, a spray gun, and a motor controlling the spray gun. The paint spraying process is as follows: the high-pressure pump pressurizes the paint tank; when the motor controlling the spray gun receives a command from the control unit, the motor starts working and sprays paint from the spray gun. This system is installed at the rear of a tracked chassis, with the spray gun positioned on the centerline. As the robot moves, the spray gun is aimed at the ground and sprays paint.
6. Summary
Compared with the traditional road marking machine process, this robot has the following characteristics:
First, this robot autonomously finds its centerline direction without human intervention.
Secondly, this robot provides more accurate centerline markings for road marking machines, eliminating the need for manual marking and greatly reducing the workload of workers.
Third, this robot has a simple structure and a highly stable PLC control system, making it adaptable to harsh outdoor environments.
The innovations of this paper are: the use of a PLC in the control system, which is suitable for harsh field operations and offers high stability; and the application of a tracked mobile structure, which can be used in complex road conditions, resulting in stable robot mobility.
References:
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