Mobile Agricultural Robot for Remote Sensing Applications -Electronics Projects
Mechanical Electronics Projects: Mobile Agricultural Robot for Remote Sensing Applications
The number of electronic devices connected to agricultural machinery is increasing to support new agricultural tasks related to the Precision Agriculture such as remote sensing and spatial variability mapping. Based on the necessity of projecting more automated agricultural machines and implements, a current trend in the agricultural area is the development of mobile robots and autonomous vehicles. These robots and vehicles developed with the same technologies existing in agricultural machinery can be more efficient doing specific tasks than traditional large tractors, giving the same, or even greater, overall output as conventional systems. One of the major challenges in the design of these robots is the development of the electronic architecture
for the integration and control of the several devices related to the motion, navigation, data acquisition and communication (or tele operation) systems. A technology that has strong potential to be applied on the devices interconnection in agricultural machinery is the CAN protocol. This technology provides significant benefits and has been used as an embedded control network in agricultural robots and vehicles. The implementation of the ISO11783 (ISOBUS) standard represents the standardization of the CAN protocol for application in agricultural machinery.This work describes the design and implementation of a mobile agricultural robot for remote sensing applications. The discussions are focused on the developed electronic architecture, the wireless communication system for tele operation and the distributed control based on CAN protocol and ISO11783 for the mobile agricultural robot. The evaluation of the developed system was based on the analysis of the performance parameters obtained with the robot operation. The results show that the developed systems meet the design requirements for an accurate robot movement and an acceptable response time for control commands and supervision.
It is expected that this paper can also support the development of mobile agricultural robots and CAN andISO11783 based distributed control technologies.
Mechanical Structure of the Robot
The agricultural mobile robot was designed to be used as an experimental platform for development of control, navigation and data acquisition technologies to the agricultural area. The major application of the robot is to do the remote sensing of agronomic parameters of most important Brazilian culture in large areas. It doesn’t require actions that demand high power, as in agricultural operations, but only moving efficiently in this environment. The mechanical structure, showed in above Figure1, was designed by the studying of work conditions required in field and desired characteristics of the project. It was established that the structure should be in portico with 2m of height and 2,5m of length, capable of operating in cultures up to 1.5 m of height, with adjustable gauge (width of 1,5 to 2,5m) to operate in various row spacing cultivation. To accomplish this, the system was designed in independent modules (side frame–number 1 and top frame–number 8 in Figure 1), together by telescopic bars (number 10), to meet the maximum possible situations.
The steering module (number 6), the propulsion module (number 5) and central box (number 9)complete the system. The structure also should be light and flexible compared with commercial agricultural vehicles, with the possibility to insert new sensors and actuators. The side boxes (number 7) contain the electronic systems to communicate with the CAN field bus and the motor controllers and also protect these devices from weather, dusty and vibrations. It is important to observe that heavier items in the robot such as batteries (number 4), propulsion and steering systems and side boxes are at least one meter of the soil, contributing to lower structure center of gravity, increasing its stability on sloping land.
The robot architecture with distributed CAN field bus was designed symmetrically between right and left sides of the structure, which allows the homogeneous distribution of weight, simplifies the development, reduces design time and costs and the amount of cables, and accomplishes the maintenance of equipment installed in the system.
Electronic Systems and CAN Based Electronic Control Units
The propulsion system of the robot needs to have accuracy in direction, low power consumption and low cost. Propulsion systems with wheels are cheap and, in function of the low need for traction and load to be distributed, meet the needs of this project. In this project, we adopted a four wheels system (number 3 in Figure 1) and to increase the ability of vehicle pull in adverse conditions, we adopted independent traction in each wheel. Each propulsion system is composed by a Roboteq AX2850 controller, a Bosch GPA 750W 24V DC motor, a 75:1 reduction system (25:1 of a planetary gear head plus a 3:1 crown, pinion, chain transmission) and a Hohner Serie 75 incremental encoder with 100 pulses per revolution.
Among the steering systems found, there are differential steering, articulated steering and wheel steering. Differential steering works by the difference between the speed of rotation of right and left wheels. In function of structure configuration is in portico format and with adjustable gauge,it was chosen a system that could be independent for each wheel, with easy construction and
accuracy of steering, so we opted by the system Ackerman in front wheels. Each steering system is composed by a Maxon Motor kit (EPOS 70\10 positioning controller with CAN interface, a RE40 150W 48V DC motor with a 2 30:1 reduction planetary gear head GP22C and MR incremental encoder with 500 pulses per revolution). However until the conclusion of this paper,the integration of the EPOS steering motor controllers in the CAN field bus is not finished.Because of this, the first operation tests are done with differential steering system for the mobile robot.
For integration (communication by the network, information exchange and control) between electronic devices, it was deployed a CAN field bus network based on ISO11783 protocol in the agricultural mobile robot. An electronic control unit (ECU), or CAN interface, develop in our laboratory (Sousa, 2002) was used for this devices integration. The Figure 2 presents the schematic diagram of a standard ECU with CAN communications capabilities.
Figure 2 show that the ECU has three main components: the micro controller, the CAN and the RS232 transceiver. The micro controller used was the PIC18F258. This chip provides the logic operations for the CAN protocol communication and implements the programs for data acquisition of the devices connected to the I/O ports, such as sensors and actuators. A MCP2551 transceiver was incorporated into the ECU to provide switching between the digital TTL logic of the micro controller and the differential output required on the CAN bus. And a MAX232 transceiver provides switching between the TTL logic and the output required by the serial RS232 port. To communicate in agreement to the ISO11783 standard, a micro controller library was develop and inserted in the ECU. This micro controller library is according to the specifications of parts 1 to 11 of ISOBUS documents. The implementation of the high-level ISOBUS functions (initialization, management and communication) was in C language and used as a basis a J1939 library for the Microchip micro controller.
The CAN network developed not only enables the integration of sensors, actuators and computer systems relative with tasks of navigation (motor controllers,D GPS Trimble AG-114 and digital compass Vector 2X), but also enables the devices integration related to data acquisition of agronomical variables, which will eventually compose the architecture of the robot. Until the moment, we used ultrasonic sensors (Polaroid 6500) and an active reflectance sensor (Crop Circle ACS-210) for information correlation with agronomical variables. The agronomical information acquired with the mobile robot georeference (latitude and longitude coordinates of the DGPS) allows building spatial variability maps. In the architecture developed, the mobile robot is teleoperated. A teleoperation station showed in Figure1, has the function of managing the operations performed by the robot, permitting planning, controlling and monitoring tasks in real-time via a Wireless Ethernet network based on IEEE 802.11 performed through a VNC connection. A directional antenna in both systems (teleoperation station and robot) allows the data communication and teleoperation up to 5 km of distance.