Saturday, February 11, 2006

BLOCKDIAGRAM AND ITS BRIEF DESCRIPTION - Part 2

The main block in the Block diagram is Obstacle Sensing circuit Designed with IR Sensors. The obstacle sensing block is designed with LM567 IC, this is a tone decoder IC, also it generates tone frequency. For identifying the obstacles 3 sets of sensors are used with three different 567 IC’s, similarly for detecting the pit or valleys another set of sensors are used. All these four sets of sensors are arranged at front side of the vehicle. Each sensing block is designed with two IR LEDs , Namely transmitting LED and receiving LED. Both the sensors are arranged side by side with in half-inch distance. The tone generator part of the IC is configured as astable mode of operation, which produces a perfect square wave of 10 KHz approximately and it is amplified using a transistor. The amplified signal is radiated through the transmitting IR LED. The signal delivered by the IR LED transmits in a line like LASER beam, when ever this signal is interrupted by an object, the radiating signal will be spread in the air because of the object, this signal is tracked by the another IR LED which is called as optical signal sensor. On receipt of optical signal, the tone decoder part of the IC detects the signal through the optical sensor and generates a high signal for the micro controller. Like wise the controller is getting signals from the four sensing blocks, according to the received information from the sensors, the controller controls the stepper motors in all directions.

Especially the new era of Robotic field mainly deals with fuzzy logics and artificial intelligence. That means, these gadgets should be in a position to identify the obstacle before it and do some appropriate calculation to overturn. This project also uses this theory. The actual theory is “Collision avoidance theory”. This theory was developed by using range finding sensors. As this is a prototype we are least bother about the range . When object obstacles in that range the IR beam will be reflected back and the Receiver will detects it. Here the receiver is connected to microcontroller. Infrared reflex sensors are most typically used for distance measurements by transmitting a modulated infrared light pulse and measuring the intensity of the reflection from obstacles nearby. In practice, infrared sensors can only be used for detection of objects, not for range measurements.

MICRO-CONTROLLER

The received information from the optical sensors fed to micro-controller, for storing as well as controlling stepper motors. Micro-controller unit is constructed with ATMEL 89C51 Micro-controller chip. The ATMEL AT89C51 is a low power, higher performance CMOS 8-bit microcomputer with 4K bytes of flash programmable and erasable read only memory (PEROM). Its high-density non-volatile memory compatible with standard MCS-51 instruction set makes it a powerful controller that provides highly flexible and cost effective solution to control applications.

Micro-controller works according to the program written in it. Most micro-controllers today are based on the Harward architecture, which clearly defined the four basic components required for an embedded system. These include a CPU core, memory for the program (ROM or Flash memory), memory for data (RAM), one or more timers (customisable ones and watchdog timers), as well as I/O lines to communicate with external peripherals and complementary resources — all this in a single integrated circuit. A microcontroller differs from a general-purpose CPU chip in that the former generally is quite easy to make into a working computer, with a minimum of external support chips. The idea is that the microcontroller will be placed in the device to control, hooked up to power and any information it needs, and that's that.

A traditional microprocessor won't allow you to do this. It requires all of these tasks to be handled by other chips. For example, some number of RAM memory chips must be added. The amount of memory provided is more flexible in the traditional approach, but at least a few external memory chips must be provided, and additionally requires that many connections must be made to pass the data back and forth to them.


For instance, a typical microcontroller will have a built in clock generator and a small amount of RAM and ROM (or EPROM or EEPROM), meaning that to make it work, all that is needed is some control software and a timing crystal (though some even have internal RC clocks). Micro-controllers will also usually have a variety of input/output devices, such as analog-to-digital converters, timers, UARTs or specialized serial communications interfaces like I²C, Serial Peripheral Interface and Controller Area Network. Often these integrated devices can be controlled by specialized processor instructions.


Originally, micro-controllers were only programmed in assembly language, or later in C code. Recent micro-controllers integrated with on-chip debug circuit accessed by In-circuit emulator via JTAG (Joint Text Action Group) enables a programmer to debug the software of an embedded system with a debugger.


More recently, however, some micro-controllers have begun to include a built-in high-level programming language interpreter for greater ease of use. BASIC is a common choice, and is used in the popular BASIC Stamp MCUs (Master Control Unit). Micro-controllers trade away speed and flexibility to gain ease of equipment design and low cost. There's only so much room on the chip to include functionality, so for every I/O device or memory increase the microcontroller includes, some other circuitry has to be removed. Finally, it must be mentioned that some microcontroller architectures are available from many different vendors in so many varieties that they could rightly belong to a category of their own. Chief among these are the 8051 family.


Stepper Motor Drive Circuit


The output of the microcontroller is used to drive the stepper motor through drive circuit, and the motor used in this project work is having four windings, therefore the controller drives the motor through four outputs. The stepper motor windings are energized one after another in a sequence according to the code produced by the controller through motor drive circuit. This motor rotates in step wise and the step angle is 1.80. The speed of the motor can be varied by varying the pulse rate. The pulses are produced by the controller can be controlled through the program by which motor speed can be varied. The stepper motor used in this project work is capable to drive up to 5kg load.




ABOUT STEPPER MOTOR


The stepper Motor used in this project work is indigenous one,which is an easy and reliable device to convert electrical energy into mechanical motion. It does not have the accuracy or the response speed of a DC motor. It is, however, utilized in many applications such as disk drives, printers, recorders, plotters, copiers, scanners, fax machines, robots, machine tools, automobiles, and medical equipment for its ease of use. Since each input change causes exactly one step rotation, a stepper motor may be operated in an open loop system. Typical step angles are 0.9o, 1.8o, 3.6o, 7.5o, 15o, and 30o.


Stepper motors are frequently applied to problems that require precision positioning without rotor position feedback. The most common stepper motors have multiple field windings and a permanent magnet rotor. The rotor is made to rotate by means of electronically commutating (switching) the current in the field windings. These motors are design to operate indefinitely with DC voltage applied to one or more fields in order to hold the rotor in a fixed position.


The rotor will rotate in discrete steps when the fields are energized in a specific sequence. Depending upon the sequence, the rotor may rotate clockwise (CW) or counter clockwise (CCW). Stepper motors are designed to rotate a fixed number of degrees with each step. A 1.8-degree stepper motor requires 200 steps for the rotor to make a full revolution.


Stepper motors have multiple stepping modes, full stepping, half-stepping and micro stepping. During full stepping, the rotor rotates the designed angular distance (1.8 degrees for example) each step. To rotate the motor, only four distinct input combinations or states are required to rotate the rotor. Repeating the sequence of states in the proper order results in what appears to be a continuously rotating rotor.


Half stepping is achieved on the same stepper motor by using an 8-state sequence. The rotor now rotates only half the designed angular rotation per half step. For a 1.8-degree stepper motor, the rotor will rotate 0.9 degrees for each half-step thus requiring 400 half steps for the rotor to make a full revolution. The chief advantage of half stepping is higher position control precision. Micro stepping requires extremely complex field current switching and allows an infinitely small rotation. Micro stepping is beyond the scope of this experiment. (For more information on stepper motor operations, read chapter – 11).


There are inherent problems with stepper motors that designers must be aware of to properly apply them. Due to rotor mass and finite energy in the field, stepper motors can only step so fast before the rotor will begin to skip steps and eventually stop completely. The primary cause of this high-speed stall is reduced field current at high speeds. This reduced field current is caused by back EMF generated by the turning rotor. One way to increase the top speed is to increase the field current at higher speeds. Elaborate field current control schemes have been devised to increase the current at high speeds without melting down the motor at slow speeds.


TRIGGER CIRCUIT BLOCK


This block is designed with 555 timer IC and an LDR is used as a light sensing device, the same is wired with timer IC. The idea of building this block is to energize the vehicle head-lamps automatically, whenever the natural light disappears. Two lamps are provided at the front side of the vehicle and these lamps energized through the relay contact.

The timer IC configured as Schmitt trigger mode of operation, triggers at 1/3Vcc. When the LDR is exposed to the light intensity, the resistance of the LDR will become less than 1K and makes the voltage at comparator input less than 1/3Vcc which in turn triggers the timer IC and generates a high signal at its output (Pin No.3). When the IC is triggered relay will be energized automatically, this relay contact is used to provide supply to the lamps. For this purpose normally closed contact is used, when the relay energized closed contact becomes open and breaks the supply to the lamps. When the natural light disappears, the resistance of the LDR will become more than 500K, which in turn comparator input voltage increases more 2/3Vcc, there by the relay remains in de-energized condition. When the relay remains in de-energized condition, normally closed contact remains in closed condition and provides supply to the lamps. Hence these lamps energized automatically when the natural light disappears.

1 Comments:

At 4:11 PM, Blogger geetha said...

can u give me transmitter section ckt diagram....what r the inductor values used in it...

 

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