Transcript
A REPORT ON
Design and Development of Wall Climbing Robot
Project Guide: Dr. Y V D Rao B. Madhavi
By JITHIN DONNY GEORGE SARTHAK GHOSH JAIDEEP SINGH
2011A4PS291H 2010A3PS211H 2010B5A4412H AT
BIRLA INSTITUTE OF TECHNOLOGY AND SCIENCE, PILANI HYDERABAD CAMPUS
ACKNOWLEDGEMENT We would like to thank Dr Y V D Rao, Head of the Department, Mechanical Engineering, BITS Hyderabad, for giving us the opportunity to pursue the project in a sophisticated milieu. We would like to thank B. Madhavi, Ph.D. scholar, BITS Hyderabad for acting as our instructor.
Abstract This is a project aimed to design and develop a unique wall-climbing robot. We begin by seeing a lot of existing wall-climbers and the vast variety of mechanisms and methods used. Then, we start the design process and came up with our own design. Our design consists of unique mechanisms for different kinds of motion on the wall and efficient techniques for controlling the robot too. This design can be may modified based on its drawbacks and to incorporate other mechanisms. Then, we plan to manufacture an affordable prototype and see how well it works.
TABLE OF CONTENTS 1. Introduction………………………………………………………...5 2. Objective………………………………………….....................5 3. Ideas from existing research……………………………………………………………...5 4. Proposed Design………............................................7 Mechanism for suction Mechanism for locomotion.. 5. Control of the robot…………………………………….......11 6. Proximity Sensors.................................................. 7. Conclusion............................................................15 8. References…………………………………………………………15
Introduction Climbing robots have been researched and developed all over the world for the main purpose of cleaning outer walls of multi-storied buildings, painting large vessels and inspecting problems in storage tanks. Most climbing robots developed up to now, climb the wall using legged mechanisms or translation mechanisms. The problem of holding on the wall is solved by many methods. There are many factors, which effect in holding, all forces, robot movement and mechanical design. We attempt to make a new and unique robot that is affordable yet efficient. For this, we first define our problem statement elegantly, see the developments in the world and come up with our own design
Objective For the wall-climbing robot that we were planning to build, we outlined these as the primary objectives:
To be able to traverse a vertical perpendicular wall To remain stationary on a vertical perpendicular wall
Ideas from existing research A prototype using SMA actuators to mimic suction in nature In nature, lots of small animals exhibit wall climbing ability and they attach or detach to wall freely just consuming little of power. Strategies adopted in nature to perform wall climbing rely on a wide variety of ingenious mechanisms. For example, octopus and leech use suckers to generate negative pressure to anchor their bodies to the substratum (William, M.K., 2002, Grey, J., 1938). Geckos employ large numbers of very fine hairs that achieve adhesion via van der Waals force (autumn, K., 2002). Tree frogs and aphids employ wet adhesion. Imitating the piston structure of the stalked suckers, prototype 1 is designed. To decrease the volume of the prototype 1, a TWSME SMA spring is used because of no need of bias spring for the TWSME SMA spring. Fig.1 and Fig.2 are elementary and actual picture of the prototype 1. The prototype 1 has an outer and an inner cylinder fabricated by aluminous alloy. When the suction cup contacts with a substratum, there is a sealed air cavity among the two cylinders and the substratum. To keep the sealing of the air cavity, an elastic sealing O-ring is set between two cylinders, and an elastic edge is bonded with the outer cylinder.
Figure 1
Figure 2
If heated by an electrical current, austenitic phase transformation occurs in the TWSME SMA spring, and the spring begins to elongate and pushes the inner cylinder. Up relative to the outer cylinder, so the volume of the sealed air cavity is enlarged and negative pressure is generated. When cooled, the inner cylinder restores to its primitive position under the recovery force of the TWSME SMA spring, so the negative pressure is cancelled. Because of an excellent cooling effect of aluminous alloy cylinders, the enlargement of the sealed air cavity can be seen as an isothermal process. According to the ideal gas law, the pressure difference P generated by volume change V can be derived as,
v
(1)
V =lS
(2)
P=
P V + V00
And,
In equations (1) and (2), P0 denotes the atmospheric pressure. V0 represents the original volume of the sealed air cavity. l is the elongation of the TWSME SMA spring. S is the effective suction area.
An interesting mechanism for locomotion
Figure 3
Robot movement is carefully designed because the robot moves on the wall. The robot must move and hold on the wall in the same time. From this constraint, a robot movement is designed as shown in Fig. 3. It shows six movement steps only in forward direction, but other directions; left, backward and right directions are similar as forward movement steps. As the robot is moving, in each step, robot must remain minimum two vacuum cups to hold on the wall. In designing, robot can move in line, which is controlled by pneumatic cylinders. Figure 3 shows a pattern of robot movement to forward direction for a cycle step by starting from alphabet ordering (A) to (F). Frame (A) shows the initial robot condition and will start movement in forward direction. In frame (B) robot stops air-suction line and changes to air-injection line in vacuum cup 1 and robot lifts the vertical cylinder with vacuum cup 1 then contract parallel cylinder 1 then releases the vertical cylinder 1 with vacuum cup 1 and then expand vertical cylinder 1 and final step vertical cylinder 1 moves in original position and changes to air-suction line again. In frame (C) robot stops air-suction line and changes to air injection line in vacuum cup 2 at left side and in vacuum cup 3 at right side. Then the robot lifts both the vertical cylinder 2 and vertical cylinder 3. In frame (D), robot contracts parallel cylinder 1 and expands parallel cylinder 4 in the same time (robot moves its body to new position from back to forward). In frame (E), robot stops air-injection line and changes to air- suction line.
The proposed Design Primary problem: Creating Suction Suction can be created primarily in two different ways, either by using rotary vacuum device or a piston cylinder arrangement. Both methods have their pros and cons, the suction mechanism for the robot is based on a piston-cylinder arrangement. The piston is connected to a solid shaft which is shaped like a rack; the rack is driven by a high torque DC motor. The system has two suction devices connected via a movable arm. To hold the bot stationary on a surface suction can be created in any of the two cylinders. The control for the same is done using a microprocessor. Suction pressure can be adjusted by changing the length of the rack. The main advantages of this design are: simple implementation possibility of creating high suction pressures smooth and silent operation
Calculations for force required for suction while holding on to a vertical wall Here, we assume the force required against friction to be 4 kg and the coefficient of friction to be 0.6. So, we get pressure required to be 32250.4 Pa. Using 𝑃1 𝑉1 = 𝑃2 𝑉2,
And taking area = 2.0258 *10−3 𝑚2(with a radius of 2 inches), We get the 𝑃1 𝐴1 𝐿1 = 𝑃2 𝐴2 𝐿2 , With 𝐿1 = 5mm, We get 𝐿2 = 15.72𝑚𝑚 and stroke = 10.72mm
The problem of locomotion The use of piston cylinder arrangement limits the possible mechanisms for moving the robot from one place to another. The robot can move by rotating the free arm about the fixed suction head or by crawling the arm with respect to the stationary suction head. The second mechanism does not fit for application on rough and uneven surfaces as crawling of the arm is hindered. Hence, rotating arm mechanism has been chosen for the robot configuration. A DC stepper motor controlled via the micro controller determines the angle of rotation of the arm with respect to its cylinder mounting head. Another identical DC Motor fitted on the other cylinder head also rotates simultaneously to align the suction cylinder with the surface when the rotation process completes. The anti-symmetric arrangement of the two motors and their simultaneous operation ease the control mechanism which is another design advantage.
Final Design
Improvisations: A mechanism for turning (Da Vinci’s turn table) So far, our robot is capable of holding onto the wall and rotating itself in a plane perpendicular to the wall. The following design illustrates a unique mechanism for performing rotation in the plane of the wall. Here, instead of mounting all the motors on the head, we have three planes. The main features are:
The top plane is connected to the bottom plane There are ball-bearings in the slots between the top plane and the middle plane. There are ball-bearings in the slots between the bottom plane and the middle plane. So, the top and bottom plane are free to rotate about the middle plane. The middle plane consists of the motor used for suction. The top plane is partly open. In the closed region of the top plane, the motor for rotation of the arm is kept. The top plane has a crown gear running along its inner periphery. The middle plane has a motor placed on it with its shaft radially outwards with a pinion to rotate the crown gear. This rotation will a maximum of 60 degrees to the left and 60 degrees to the right too.
Control of the robot The preliminary aim of the robot is to be able to climb up a vertical wall, which involves basic synchronization of the four motors being used, two stepper motors and two high torque DC motors. To achieve this simplified functionality of the robot, the control mechanism can be realized with the help of a state diagram. STATE DIAGRAM A state diagram is a type of diagram used in computer science and related fields to describe the behaviour of systems. State diagrams require that the system described is composed of a finite number of states; sometimes, this is indeed the case, while at other times this is a reasonable abstraction. The figure below shows an example of a state diagram of a mealy machine. S0, S1, and S2 are states. Each edge is labelled with "j / k" where j is the input and k is the output
To make a state diagram for the functioning of the robot, we can represent the control signals for the different motors accordingly as shown:
X and Y are the two signals which indicate the up and down motion of the pistons inside the cups. R and S are the signals which control the motors responsible for the rotational motion of the arm of the robot and also the 180 degree rotation of each cup alternatively. Under these assumptions, the state diagram can be represented as:
At the end of state S2, the output becomes 0011 again, we exchange the signals of X and Y, and we again find ourselves at state S1.
The implementation of the state diagram can be achieved by using a simple microcontroller board such as the Arduino Uno.
The Arduino Uno is a microcontroller board based on the ATmega328. It has 14 digital input/output pins (of which 6 can be used as PWM outputs), 6 analog inputs, a 16 MHz ceramic resonator, a USB connection, a power jack, an ICSP header, and a reset button. Using the built in clock of the Uno, the state diagram can be easily implemented. For the control of the motors, a simple H-bridge motor driver circuitry can be used like the one shown below, which gives the circuit for controlling a stepper motor
Proximity Sensors A proximity sensor is a sensor able to detect the presence of nearby objects without any physical contact. A proximity sensor often emits an electromagnetic field or a beam of electromagnetic radiation (infrared, for instance), and looks for changes in the field or return signal. The object being sensed is often referred to as the proximity sensor's target. Different proximity sensor targets demand different sensors. For example, a capacitive photoelectric sensor might be suitable for a plastic target; an inductive proximity sensor always requires a metal target. The maximum distance that this sensor can detect is defined "nominal range". Some sensors have adjustments of the nominal range or means to report a graduated detection distance. Proximity sensors can have a high reliability and long functional life because of the absence of mechanical parts and lack of physical contact between sensor and the sensed object. Proximity sensors are also used in machine vibration monitoring to measure the variation in distance between a shaft and its support bearing. This is common in large steam turbines, compressors, and motors that use sleeve-type bearings.
An infrared proximity sensor
Detection of Right Angled Walls
Just climbing vertical walls may not be enough for a wall crawler or climber, if we think in terms of real world uses of such a robot. Thus in order to improve the functionality of the robot and its overall worth, we have thought of a simple way by which the robot can identify a wall which is at right angle to the current wall, and can align itself accordingly. The angle to which the suction cup has to rotate to align itself to the wall cannot be predetermined because it depends on the distance at which the robot is from the perpendicular wall. The basic approach taken is using proximity sensors or IR feedback sensors. The idea is that whenever the sensor detects an obstacle which is closer than the distance “L”, it tries to align itself so that both the proximity sensors placed on opposite sides of the cup can get equal or almost equal values. The polling of the proximity sensors start when the arm of the robot has rotated half way through. The following diagrams can give a step by step viewing of the situation.
D
