Design and Development of Robotic Arm for Cutting Tree
Abstract The Robot Institute of America defines a robot as “a programmable multifunctional manipulator designed to move material, parts and tools of specialized device through variable programmed of variety of tasks.” Today, in this modern fast growing industrial age, every company is looking for speed in manufacturing to meet the needs and requirements of its clients. Robots are more quickly, cheaply, and accurately than humans have ever been. One type of robot mostly used in industry is a robotic manipulator or simply robotic arm. It is open or closed kinematic chain of rigid links interconnected by movable joints. In some configurations, links can be considered to correspond to human anatomy as waist, upper arm and forearm with joint at shoulder end elbow. A machine was designed and fabricated which can move around the tree trunk smoothly while carrying the cutting system. A mechanize motor system also was designed and assembled on the carrier machine for moving the cutting machine forward and backward along the tree trunk radius. For a successful and smooth cutting process, two direct current (DC) motors were used for carrier machine. Cutting machine consist of a mechanism for cutting and a cutting blade. A reciprocating mechanism was used in this project because of the added advantages of this method as compared to others. Design of the blade tooth for doing a fast and clean cut was an important parameter in this project. An ATmega2560 microcontroller were used to control the cutting system.
Keywords: Robot Arm, Design, Development, Implementation, DC Motors, Manufacturing, Microcontroller, End-Effector, Robot Gripper, Cutting Tree
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1. Introduction The word ‘robotics’, meaning the study of robot was coined by Isaac Asimov. Robotics involves elements of mechanical, electrical and software engineering, as well as control theory, computing and now artificial intelligence (Selig, 1992). Robotics research today is based on developing systems that exhibit modularity, flexibility, faulttolerance, a general and extensible software environment and seamless connectivity to other machines, by providing sensor based intelligence to the mechanical arm. The goal of agricultural robots does not only apply to robotics technologies in the field of agriculture, it also applies to using agricultural challenges to develop new techniques and systems. An agricultural robot must deal with an unstructured, unknown and varying environment. In recent years, harvester robots have been among the noteworthy topics studied by researchers. First, as modern industry has become more complex, there has been a growing need for getting work done in environments that
are very dangerous for humans. As an example, work in a nuclear reactor plant often requires contact with radioactive materials. Second, as robots became more advanced and less expensive, they are being set up in industry situations where working conditions are not so much dangerous as unpleasant for various reasons. These situations typically involve high degrees of the following: - Heat, Noise, Poisonous gases, risk of injury by machines. Simple robots do many routine jobs in industry. Robots are useful in sample assembly operations such as stuffing printed circuit boards and loading and unloading parts from machines. Due to increase using of industrial robot arms, an evolution to the topic began trying to imitate human movements in a detail mode. For example a group of students in Korea made a design of innovations that robotic arm take account of dancing hand, weight lifting, Chinese calligraphy writing and color classification. Another group of engineers at USA develop eight degrees of freedom robot arm. This robot is able to grasp many objects with a lot of shapes from a pen to a ball and simulating also the hand of human
Figure 1. Components of Mechanical Arm
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being. In space, the Space Shuttle Remote Manipulator System, known as SSRMS, and its successor is example of multi degree of freedom robot arms that have been used to perform a variety of tasks such as inspections of the space shuttle using a specially deployed boom with cameras and sensors attached at the end effector.
2. Components of Mechanical Arm Figure 1. The structure of a mechanical arm is usually mostly mechanical and can be called a kinematic chain. The chain. The chain is formed of links, actuators, and joints which can allow one or more degrees of freedom. Most contemporary mechanical arm use open serial chains in which each link connect the one before to the one after it. These mechanical arm are often resemble the human arm. Mechanical arm used us manipulators have an end effector mounted on the last link. This end effector can be anything from a welding device to a mechanical hand used to manipulate the environment.
A DC motors (Figure 2) is any of a class of electrical machines that converts direct current electrical power into mechanical power. Now a days, DC motors plays a vital role in most of the industrial areas, it can be seen in most of the electronic devices. They are mainly used for the mechanical movements of physical applications such as rolling the bundle of sheets or CD drivers, lifts etc. Many methods evolved to control the revolution of a motor. DC motors can be controlled either by software or directly by hardware. Software controlling needs computers which are bulky and common man cannot afford for it, so hardware controls are in use. Even in hardware if it is programmable device then it is preferred because it can be modeled according to the requirements of the user. Advantages of using PIC over other controlling devices for controlling the DC motor are given below: 1. SPEED: The execution of an instruction in PIC IC is very fast (in micro second). One instruction generally takes 0.2 microseconds. 2. COMPACT: The PIC IC will make the hardware circuitry compact. 3. RI SC PROCESOR: The instruction set consists only 35 instructions. 4. EPROM PROGRAM MEMORY: Program can be modified and rewritten very easily.
Figure 2. DC Motor
5. INBULT HARDAWARE SUPPORT: Since PIC IC has inbuilt programmable timers, ports and interrupts, no extra hardware is needed.
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6. POWERFUL OUTOUT PIN CONTROL: Output pins can be driven to high state, using a single instruction. The output pin can drive a load up to 25mA. Vision – Computer vision is the science and technology of machines that see. As a scientific discipline, computer vision is concerned with the theory behind artificial systems that extract information from images. The image data can take many forms, such as video sequences and view from cameras. In most practical computer vision applications, the computers are preprogrammed to solve a particular task, but methods based on learning are now becoming increasingly common. Computer vision system rely on image sensor which detect electromagnetic radiation which is typically in the form of either visible light or infra-red light. The sensors are designed using solidstate physics. The process by which light propagates and reflects off surface is explained using optics. Sophisticated image sensor even require quantum mechanics to provide a complete understanding of the image formation process.
Figure 5. Human and Robot Grippers
Figure 3. Components of Machine Vision Senors Diagram
Manipulation – Mechanical arm which must work in the real world require some way to manipulate objects; pick up, modify, destroy, or otherwise have an effect. Thus the ‘hands’ of a mechanical arm are often referred to as end effector, while the arm is referred to as a manipulator. Mechanical Grippers – One of the most common effectors is the gripper. In its simplest manifestation it consists of just two fingers which can open and close to pick up and let go of a range of small objects.
Figure 4. Robot Gripper
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3. Cutting Machine Two essential parameters for designing a cutting machine for any material are the cutting mechanism and the cutter blade design. Common types of cutting machines include reciprocating saws, horizontal endless band saws, universal tilt frame band saws, abrasive saws, and cold saw. Rotary saw cutting is a cutting method which is versatile and effective for a variety of industrial applications. The advantages of using the rotary saw method are: faster cutting times, closer tolerances, better finishes, less kerf, easier tool changes, better tool life, and an overall wider range of applicability. The reciprocating saw mimics the back and forth motion of a common hacksaw. The gearing system inside the reciprocating saw will cause the saw blade move back and forth across the material that needs to be cut. This saw is usually used to cut wood, plaster, plastic or some other soft material. The important parameters to be considered when choosing a blade for cutting are the type of metal, width, blade set, thickness, tooth form and length of the blade. Most saws are designed to create a kerf that is wider than the saw blade. This difference is called the side clearance which is the most important parameter for doing a curve cut. The teeth of the blade used in this project were bent alternately in opposite directions to create a kerf wider than the body. The amount of bending is usually proportionate to the thickness of the body. The amount of bending is usually proportionate to the thickness of the blade body and is usually 25% of the blade thickness. The angle at which the
points of the saw tooth make contact with the material is an important factor of the effective cutting performance of a saw blade. Figure 6 shows the design of the alternate tooth style and teeth angles of the blade used in this project for cutting the trees. The radial lengths of the circles that can be cut by a blade depend on the ratio of the width of the blade to the width of the saw kerf. Since the diameter of the trunk tree is not constant, the blade was designed based on the minimum size of the three which is 32 cm. The appropriate size of the blade width is calculated using: W2/2 = (R+K)2 – (R+T)2 , where W is the blade width, R is the radius of the tree trunk, K is the size of the kerf width, and T is thickness of the blade.
Figure 6. The alternate set of style and its angles
4. Mechanical Design The mechanical design of the robot arm is based on a robot manipulator with similar function to a human arm. The links of such a manipulator are connected by joints allowing rotational motion and the links of the manipulator is considered to form a kinematic chain. The business end of the effector or endof-arm-tooling and its analogous to the human hand. Figure 7 shows the Free
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Figure 7. Free Body Diagram of the Robot Arm
Body Diagram for mechanical design of the robotic arm. As shown, the end effector is not included in the design because a commercially available gripper is used. This is because that the end effector is one of the most complex parts of the system and, in turn, it is much easier and economical to use a commercial one than build it. The DC motor was installed in such position so that the teeth of its sprocket were placed inside the chain. The rotation of the DC motor causes the sheet metal to move forward and backward. One limit switch was installed at the head of the cutting machine. When the cutting machine moves forward, the limit switch touches the tree surface and stops the motor. Controlling of the DC motors is done by the operator using the remote controller. Figure 8 shows the work region of the robotic arm. This is the typical workspace of a robot arm with four degree of freedom (4 DOF). The mechanical design was limited to 4 DOF mainly because that such a design allows most of the necessary movements and keeps the costs and the complexity of the robot competitively.
Figure 8. Work Region of the Robotic Arm
In choosing the materials and the shape for the fabrication of the robotic arm, the following were taken into consideration:
The ease of manufacturing the parts The mode of manufacturing Ease of assembly Strength and durability of the parts Weight of robot Cost
The principal requirements for power transmission of robots are:
Low weight and moment of inertia High effective stiffness Accurate and constant transmission ratio Low energy losses and friction for better responsiveness of the control system Elimination of backlash Small size
Hence, the combination of these factors has influence all the choices made in the design selection of the robotic arm.
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Figure 9. Force Diagram of the Robotic Arm
Figure 9 shows the force diagram used for load calculations. The calculations were carried out only for the joints that have the largest loads, since the other joints would have the same motor. The calculations considered the weight of the motors, about 50 grams, except for the weight of motor at joint B, since it is carried out by link BA. Figure 10 shows the force diagram on link CB, which contains the joint (B and C) with the highest load (carry the links DC and ED) and the calculations are carried out as follows. The values used for the torque calculations: Wd = 0.011 kg (weight of link DE) Wc = 0.030 kg (weight of link CD) Wb = 0.030 kg (weight of link CB) L = 1 kg (load) Cm = Dm = 0.050 kg (weight of motor) LBC = 0.14 m (length of link BC) LCD = 0.14 m (length of link CD) LDE = 0.05 m (length of link DE) Performing the sum of forced in te Y axis, using the loads as shown in Figure 10, and solving for CY and CB, see Equations (1)-(4). Similarly, performing the sum of moments around point C, Equation (5), and point B, Equation (6), to obtain the torque in C and B, Equations (7) and (8), resectively.
Figure 10. Force Diagram of Link CB
∑ Fy =(L + Wd + Dm + Wc + Cm)g – CY = 0 CY = (1.141 kg) 9.8 m/s2 = 11.18 N
(1) (2)
∑ Fy =(L + Wd + Dm + Wc + Cm + WB)g – CB = 0 (3) CB = (1.171 kg ) 9.8 m/s2 = 1.4758 N (4) ∑MC = - (Wc LCD / 2) – WD (LCD + LDE/ 2) -L (LCD + LDE) – Dm(LCD) + MC = 0
(5)
∑ MB= - L(LBC + LCD + LDE) - WD(LBC+LCD+LDE/2) - Dm (LBC+LCD) – Wc (LBC+LCD/2) - Cm (LBC) – WB (LBC/2) + MB = 0 (6) Mc = 1.968 Nm = 278.6 oz/in MB = 3.554 Nm = 503.38 oz/in
(7) (8)
5. Remote Controller A remote controller system comprises of hardware and software designed and fabricated for operator to control the cutting system. The remote conroller and the cutting system each have one HM-RF transmitter receiver device and an Atmega2560 microcontroller as the main hardware components. A specific program was written for each microcontroller to receive the data, analyze them and finally generate and send the appropriate commands to other components, such as DC motors and HM-RF devices. A HM-TR Transparent Wireless Data Link Module consists of two parts, both of which can send and receive data wirelessly. The first part is used by the remote controller to send the operator’s commands and the second part is installed
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Figure 11. Block Diagrams of a Microcontroller
on the cutting system to receive the commands from the remote controller. The data transferred between these parts uses a radio frequency that ranges between 310.24 and 929.27 MHz. This paper focuses more on the processes needed to transfer the data from the HMTR Module to the microcontroller, and the analysis of them.
SOFTWARE When the operator pushes a button of the remote controller, the remote controller sends three numerical codes to the receiver of the robot. The first number informs the microcontroller that a new command is being sent by remote controller and it readies the microcontrolles to receive the next two numbers. The second number is the code of the command that is allocated to a button of the remote controller. By analyzing this number, the microcontroller understands what the operator wants the robot to do. The third number is a summation of the first and the second numbers and is called the check sum number. The microcontroller also adds up the first and the second received numbers
and compares the result with the check sum. If these numbers are the same, the microcontroller understands that it has received the data from the remote controller completely and correctly. In the next program, the microcontroller sends the appropriate command to each DC motor based on the value of the second number. To change the motors rotation directions, a miniature intermediate power relay was used for each motor. The operator can start and stop the DC motors for moving the cutting system around the tree trunk and also move it forward and backward.
6. Electronic Design Electronic design involves designing and choosing of the electrical components as per our requirements. Some of those used in our project are given below. MICRO CONTROLLER A micro controller (Figure 11) is similar to a processor in a computer. The primary difference is that the micro controller has the CPU, the flash memory, the RAM and external ports all buit into a single chip.
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All this makes the micro controller very cheap and is true value for money. The various components of the MCU shown in Figure 11 are explained below:
7. Advantages and Disadvantages
Random Access Memory (RAM): RAM is used for temporary storage of data during runtime.
arm have the capacity to dramatically improve product quality. Applications are performed with precision and high repeatability every time. Production - With mechanical arm, throughput speeds increase, which directly impacts production. Because an automated mechanical arm has the ability to work at a constant speed without pausing for breaks or sleep, it has the potential to produce more than a human worker. Safety - Mechanical arm increase workplace safty. Workers are moved to supervisory roles where they no longer have to perform dangerous applications.
ROM: ROM is the memory which stores the program to be executed. SFR Registers: Special Function Registers are special elements of RAM. Program Counter: This is the „engine“ which starts the program and points to the memory address of the instructuion to be executed. Control Logic: As the name implies, it which supervises and controls every aspect of operation within MCU. A/D Converter: A/D stands for analog to digital. They convert analog to digital signals. I/O Ports: To be of any practical use, microcontrollers have ports which are connected to the pins on its case. Oscillator: This is the rhythm section of the MCU. The stable pace provided by this instruments allws harmonius and synchronus functioning of all other parts of MCU. Timers: can be used for measuring time between two occurrences and can also behave like a counter. Power Supply Circuit: this powers the MCU.
- Advantages Quality - Industrial automated mechanical
Savings - Improved worker safety to financial savings. There are fewer healtcare and insurance for employers.
- Disadvantages Expense - The initial investment to integrated automated robotics into your business is significan, espcially when business owners are limiting their purchases to new robotic equipment.
ROI - Incorporating industrial robots does not guarantee results. Without planing, companies can have difficulty achieving their goals. Expertise - Employees will require training program and interact with the new robotic equipment. This normally takes time and financial output. Safety - Robots may protect workers from some hazards, but in the meantime, their very presence can create other safety problems. These new dangers must be taken into consideration.
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8. Conclusions This paper presents the design and development of robot arm for cutting tree, which has the talent to accomplish simple task, such as rotate, lower and raise its arm, by being controlled by the microcontroller is build successfully. The weights of all the motors were put into consideration when calculating the torque by each joint. This is to make sure the gears are arranged in a way that will produce enough torque to carry the succeeding load. Use of DC motor at the base. The output torque of the DC motor is higher than that of the stepper motors. Using DC motor at this joint brought us a step closer to solving the problem of insufficent torque to carry the arm. We used a microcontroller which provides enough pins for selecting motors, hence eliminating the need for decoders. From our work, we deduced that in comparison to humans, robots can be much stronger and are therefore able to lift heavier
weights and exert larger forces. Thay can be very precise in their movements, reduce labor costs, improve working conditions, reduce material wastage and improve product quality. This is why they’re very important in industries because the overall objective of industrial engineering is productivity. The design of the robot arm was limited to four degrees of freedom since this design allows most of the necessary movements and keeps the costs and the complexity of the robot competitively. The end effector is not included in the design because commercial available gripper is used since it is much easier and economical to use a commercial one than build it.
9. References [1] Manipulating Industrial Robots – Vocabulary, International Organization for Standardization Standard 8373, 1994. [2] B. Siciliano, L. Villani and G. Oriolo, „Robotics, Modeling, Planning and Control,“ Springer, London, 2009. [3] R. J. Wang, J. W. Zhang, et al., „The Multiple-Function Intelligent Robotic Arms,“ FUZZ-IEEE Journal, Korea, 20-24 August 2009 [4] microElectronika (2004). Architecture and programming of 8051 MCU (Chapter 1). [5] Valavanis, K. P., & Saridis, G. N. (1992). Intelligent robotics. Academic Publishers. [6] Appin Knowledge Solutions. Robotics. ISBN: 978-1-934015-02-5 [7] A. Ghosal, „Robotics“, Oxford, New Delhi, 2006. [8] Brumbach ME, Clade JA (2003). Industrial Maintenance, Published by Delmar Learning.
