Robotics/Projects

A Simple Creeper Robot.

GIBIN N GEORGE

St. Joseph’s College of Engineering and Technology, Choondacherry, Palai.

(PUBLISHED IN THE PROCEEDINGS OF ASPIRE- 2008 INTER-COLLEGIATE TECHNICAL FEST AT SJCET, PALAI, KERALA, INDIA)

ABSTRACT- Robotics is an engineering discipline where mechanical and electronic systems have to co-exist and interact successfully to give the desired output. In this paper we try to design and construct a creeper robot which is simple in design. Creepers are ‘walking’ robots. They mimic the creeping motion of insects in nature. Ours is a BEAM robot. We used the 74HC240 IC for driving the servo motor that powered the legs of the creeper. In order to mimic the creeping motion effectively, we need continuous to and fro motion of the motor. To implement the to and fro motion electronically, the chip was bicored using suitable bicoring circuit. Bicoring implies that only one half of the IC produces a signal at a time. The two sides alternate continuously and produce signals in opposite directions. The 74HC240 is an octal buffer/ line driver; 3 state inverting IC. The IC has 20 pins of which two are enabling pins. The bicoring circuit for the available IC and motor was designed by us. The motor shaft was connected directly to a servo horn on which the legs were mounted. The legs of the creeper are made of guage10 copper wire and are fixed to the horn. The whole system is built around the motor body itself. The motor we used needed 6V dc supply which was given from 4 AAA dry cells. The cells were placed in battery holders which were mounted on motor body.

I. INTRODUCTION

A robot is a mechanical or virtual artificial agent. In practice, it is usually an electro-mechanical system which, by its appearance or movements, conveys a sense that it has intent or agency of its own. BEAM robotics is the brain child of Mark W. Tilden who is currently working at "Los Alamos National Laboratory" in Los Alamos, New Mexico, US. BEAM is a triple acronym which stands for:

Biology Electronics Aesthetics Mechanics Building Evolution Anarchy Modularity Biotechnology Ethnology Analogy Morphology An important area of research in legged robotics concerns walking and running over challenging terrain including robots with the ability to traverse horizontal and vertical surfaces. Generally, the design of robots involves complex mathematics and electronics. Our project is an attempt to keep things as simple as possible, and yet come up with a robot, that can walk over level terrains.

II. MATERIALS AND METHODS

Our creeper uses the minimum number of parts, most of which are cheaply available or can be claimed from old home appliances. Also, the bicore circuit that we use is a simple one that is commonly used for similar purposes.

A. Actuators: The actuators are the parts which convert stored energy into movement. By far the most popular actuators are electric motors, but there are many others, some of which are powered by electricity, while others use chemicals, or compressed air. We used a 6V dc servo motor for actuating our machine. The servo motor we purchased had an inbuilt control circuit with it, so we had to remove it so as to incorporate our own. The power from the motor may be transferred to the legs of the motor; either by coupling the legs directly with the motor shaft or with a gear drive mechanism. The gear drive mechanism offers smooth power transfer, but makes the robot bulky. The construction of idler shaft for the gears was also another problem. Hence we decided to fix the legs directly to the motor shaft with a horn fixed to the shaft.

B. Manipulators: Robots 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 robot are often referred to as end effectors, while the arm is referred to as a manipulator. Most robot arms have replaceable effectors, each allowing them to perform some small range of tasks. Some have a fixed manipulator which cannot be replaced, while a few have one very general purpose manipulator, for example a humanoid hand. Our robot does not have a manipulator in the real sense. As the purpose of this robot is to walk over level surfaces, the only considerations before us regarding legs were good strength and weight of legs. We used guage 10 aluminium wire to make the legs.

C. Control: The mechanical structure of a robot must be controlled to perform tasks. The control of a robot involves three distinct phases - perception, processing and action (robotic paradigms). Sensors give information about the environment or the robot itself (e.g. the position of its joints or its end effector). This information is then processed to calculate the appropriate signals to the actuators (motors) which move the mechanical structure. The processing phase can range in complexity. At a reactive level, it may translate raw sensor information directly into actuator commands. Sensor fusion may first be used to estimate parameters of interest (e.g. the position of therobot's gripper) from noisy sensor data. An immediate task (such as moving the gripper in a certain direction) is inferred from these estimates. Techniques from control theory convert the task into commands that drive the actuators. Our creeper doesn’t use any sensors. However it has a control circuit that alternates the direction of rotation of the actuators so as to create the walking motion of the robot. This circuit uses an IC of the 74**240 series. The control circuit and bicoring of the IC are explained in detail below.

D. The Bicore Circuit and the 74HC240 IC: The first thing we’ll want to do in building our robot is to assemble its brain. The Walker makes use of the ingenious BEAM Bicore circuit. It’s prefixed bi because it has two states, or nodes, and core because; it is the central part of the robot’s nervous net. Our Bicore uses the 74HCT240 integrated circuit. This chip is an inverting octal buffer. This means that it is a chip with eight logic gates that invert the signals going into them. Whatever signal goes in each gate gets inverted, so a low signal becomes a high signal and a high signal becomes a low one. By combining the three gates on one side and three gates on the other (by soldering their pins together), we end up with two “teams” inverting gates that “buffer” the signal and make it more powerful. The signal passing back and forth between the two nodes sends high and low (or “on” and “off”) pulses to our servo motor. The result is back and forth movement of the motor shaft, which is transferred to our gears to create a reciprocating walking motion. The remaining two gates are used as sort of the controller for the two three-gate teams.

E. Bicore Circuits: The bicore is the core of most BEAM devices. Its unique structure makes it very easy to customize for a wide range of uses. These uses vary from blinking LED’s to walkers.

F. Neurons: The heart of core technology is the use of neurons. Neurons are most common in living creatures. It supplies the creature with information processing abilities, caused by the loads of neurons connected together. The difference between a neuron and a transistor is that the transistor has only a “yes” or “no” if used in logic circuits. Its input is directly related to its output, and therefore it is not very suitable for circuitry with a certain “intelligence” of its own. In this case we use an “ideal” inverter; meaning that it’s switching point (threshold-value) is at exactly one half of the voltage difference between “1” and “0”. For example, if ground is 0V and Vcc is 5V, the inverter will switch at 2,5V. Now imagine, a positive (5V) pulse is given to the “in”, causing the area between the capacitor and the inverter to be 5V too. The capacitor starts discharging through the resistor. If the voltage between the cap and the resistor passes the 2,5V mark, the inverter will “switch”. The time between the pulse and the changing state of the inverter is the delay time. It is easy to see that this time is determined by the capacitance of the capacitor and the resistance value of the resistor.

G. Bicore: A bicore is obtained by coupling together two neurons. (Fig.2). If both resistors and both capacitors are equal in value, the bicore will have a duty-cycle of 50% and thus be symmetric.

H. Suspended Bicore: To increase the symmetry of the bicore, it is possible to change the resistors for one resistor, connecting the inverters together as shown in Fig.3. It’s easy to imagine that in the original bicore the voltage at the resistors is the opposite of each other. The suspended bicore can be imagined as a normal bicore in which the one corner functions as a “ground” for the other corner. The beginning situation is when the sides of the resistor have opposite voltages (Vcc x ground). For this example we take Vcc = 5V and ground = 0V.Because of the inverters, which invert the voltage of the opposite side of the resistor, the voltage over the capacitors is 0V. Because the voltage difference over the resistor is 5V, current starts to flow from one end to the other, causing the capacitors to gain a voltage difference. If the voltages reach the threshold-value of the inverter, the inverters start to change state, creating a new voltage difference over the resistor, only with the voltage levels reversed. This is half an oscillation. If we look at the voltage of the outputs of the inverters, they have an almost perfect "block"-oscillation, which graphically would look like the graph given in Fig4. The replacement of the two resistors of a normal bicore with one as used in the suspended bicore brings up another problem. One side of the resistor has the ground voltage, the other Vcc. If the current flows through the resistor, both sides of it will approach 0.5Vcc but will mathematically never reach it, because if the voltage difference over the resistor decreases, the current will decrease as well, causing the change in voltage difference to drop as well. In designing a suspended bicore, one needs to prevent this effect. 74xx240 Chips have a system that protects the inverters for over or under-voltage.

I. Bicoring the 74HC240:

We used the following connections to bicore our IC. ■ Left side of IC: PINS 4-6, 3-4, 5-7, 6-8, 7-9 ■ Right side of IC: PINS 14-16, 13-15, 12-14, 11-13, 15-18 ■ From left side to right side: PINS 1-19 ■ Capacitors: 2-3 and 18-17 ■ Resistor (across IC): 2-17 (Refer figures 1 and 3).

III. CONSTRUCTION: BUILDING THE WALKER BODY

The body for our creeper is basically the motor casing for the servo motor itself. The control circuit goes on top (actually, the servo is flipped upside down, so it’s technically the bottom of the motor), the two AAA battery packs go on the sides, and the on/off switch gets attached to the back. The motor is oriented so that the drive shaft protrudes from the bottom, delivering power to two sets of gear/legs, one set for the front two legs, and one set for the back. The ingenuity of this design is that four legs are controlled, and that a single motor can achieve a reasonable walking gait. Robotic walking technology is usually hard. The simplest walkers usually have at least two motors and two control circuits (one “master” circuit and one “slaved” to that). This walking machine doesn’t have the most elegant gait in the robot kingdom, but it does work, and it shows the kind of insect like movement and persistence that’s the hallmark of biologically inspired robots. A. Parts List ■ (1) Servo motor. ■ (1) 1.5 inch (4cm) plastic gear (around 40 teeth are good) ■ (1) 2 feet of 10–gauge wire. ■ (2) AAA battery holders (each holds two AAA batteries) ■ (2) .22 μF monolithic capacitors ■ (1)Resistor ■ (1) 74HCT240 integrated circuit (IC) ■ (1) 20-pin DIP IC socket ■ (1) on/off toggle switch ■ (2) leg mounting pads. ■ 2-part epoxy ■ Superglue ■ Metal ruler ■ (4) AAA batteries B. Creating the Leg Assemblies We made two sets of legs by bending the wires into a ‘U’ like shape and epoxying them to the gear halves. (Fig.5)

C. Final Assembly Now we have to attach the power and the motor wires to the control circuit and then the control circuit assembly to the top of the walker. Now attach the epoxied leg assembly to the servo motor horn. This completes our construction.(Fig.6)

IV. FIGURES AND GRAPHS

SORRY BUT I COULDN'T UPLOAD THEM. MAIL ME A REQUEST TO idestinyschild1989@gmail.com

V. REFERENCES

[1]Definition of robotics - Merriam-Webster Online Dictionary.

[2]Asimov, Isaac (2003). Gold. Eos.

[3]C.S.G. Lee & R.C. Gonzalez & K.S. Fu,Tutorial on robotics.

[4]Internet