SYLLABUS

AERONAUTICAL ENGINEERING

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AUTOMOBILE ENGINEERING

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BIOMEDICAL ENGINEERING

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CIVIL ENGINEERING

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CSE

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ECE

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EEE

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EIE

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IT

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MECHATRONICS ENGINEERING

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MECHANICAL ENGINEERING

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Servo Motor

 What are Servo Motors?

Servo refers to an error sensing feedback control which is used to correct the performance of a system. Servo or RC Servo Motors are DC motors equipped with a servo mechanism for precise control of angular position. The RC servo motors usually have a rotation limit from 90° to 180°. Some servos also have rotation limit of 360° or more. But servos do not rotate continually. Their rotation is restricted in between the fixed angles.

Where are Servos used?

The Servo motors are used for precision positioning. They are used in robotic arms and legs, sensor scanners and in RC toys like RC helicopter, airplanes and cars
ervo Motor manufacturers

There are four major manufacturers of servo motors: Futaba, Hitec, Airtronics and JR radios. Futaba and Hitec servos have nowadays dominated the market. Their servos are same except some interfacing differences like the wire colors, connector type, spline etc.

Servo Motor Manufacturers

Servo Motor wiring and plugs
The Servo Motors come with three wires or leads. Two of these wires are to provide ground and positive supply to the servo DC motor. The third wire is for the control signal. These wires of a servo motor are color coded. The red wire is the DC supply lead and must be connected to a DC voltage supply in the range of 4.8 V to 6V. The black wire is to provide ground. The color for the third wire (to provide control signal) varies for different manufacturers. It can be yellow (in case of Hitec), white (in case of Futaba), brown etc.

Futaba provides a J-type plug with an extra flange for proper connection of the servo. Hitec has an S-type connector. A Futaba connector can be used with a Hitec servo by clipping of the extra flange. Also a Hitec connector can be used with a Futaba servo just by filing off the extra width so that it fits in well.

Hitec splines have 24 teeth while Futaba splines are of 25 teeth. Therefore splines made for one servo type cannot be used with another. Spline is the place where a servo arm is connected. It is analogous to the shaft of a common DC motor.

Structure of Servo Motor


Unlike DC motors, reversing the ground and positive supply connections does not change the direction (of rotation) of a servo. This may, in fact, damage the servo motor. That is why it is important to properly account for the order of wires in a servo motor.

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What is the difference between a DC motor and servo motor?

A DC motor has a two wire connection. All drive power is supplied over these two wires—think of a light bulb. When you turn on a DC motor, it just starts spinning round and round. Most DC motors are pretty fast, about 5000 RPM (revolutions per minute).

With the DC motor, its speed (or more accurately, its power level) is controlled using a technique named pulse width modulation, or simply PWM. This is idea of controlling the motor’s power level by strobing the power on and off. The key concept here is duty cycle—the percentage of “on time” versus“off time.” If the power is on only 1/2 of the time, the motor runs with 1/2 the power of its full-on operation.

If you switch the power on and off fast enough, then it just seems like the motor is running weaker—there’s no stuttering. This is what PWM means when referring to DC motors. The Handy Board’s DC motor power drive circuits simply switch on and off, and the motor runs more slowly because it’s only receiving power for 25%, 50%, or some other fractional percentage of the time.

A servo motor is an entirely different story. The servo motor is actually an assembly of four things: a normal DC motor, a gear reduction unit, a position-sensing device (usually a potentiometer—a volume control knob), and a control circuit.

The function of the servo is to receive a control signal that represents a desired output position of the servo shaft, and apply power to its DC motor until its shaft turns to that position. It uses the position-sensing device to determine the rotational position of the shaft, so it knows which way the motor must turn to move the shaft to the commanded position. The shaft typically does not rotate freely round and round like a DC motor, but rather can only turn 200 degrees or so back and forth.

The servo has a 3 wire connection: power, ground, and control. The power source must be constantly applied; the servo has its own drive electronics that draw current from the power lead to drive the motor.

The control signal is pulse width modulated (PWM), but here the duration of the positive-going pulse determines the position of the servo shaft. For instance, a 1.520 millisecond pulse is the center position for a Futaba S148 servo. A longer pulse makes the servo turn to a clockwise-from-center position, and a shorter pulse makes the servo turn to a counter-clockwise-from-center position.

The servo control pulse is repeated every 20 milliseconds. In essence, every 20 milliseconds you are telling the servo, “go here.”

To recap, there are two important differences between the control pulse of the servo motor versus the DC motor. First, on the servo motor, duty cycle (on-time vs. off-time) has no meaning whatsoever—all that matters is the absolute duration of the positive-going pulse, which corresponds to a commanded output position of the servo shaft. Second, the servo has its own power electronics, so very little power flows over the control signal. All power is draw from its power lead, which must be simply hooked up to a high-current source of 5 volts.

Contrast this to the DC motor. On the Handy Board, there are specific motor driver circuits for four DC motors. Remember, a DC motor is like a light bulb; it has no electronics of its own and it requires a large amount of drive current to be supplied to it. This is the function of the L293D chips on the Handy Board, to act as large current switches for operating DC motors.

Plans and software drivers are given to operate two servo motors from the HB. This is done simply by taking spare digital outputs, which are used to generate the precise timing waveform that the servo uses as a control input. Very little current flows over these servo control signals, because the servo has its own internal drive electronics for running its built-in motors.

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Applications of Microcontroller

The reason I find microcontrollers fascinating is that they have been and continue to be such an important part of the electronics industry.  Over the past decade and more microcontrollers have been creeping into our daily lives.  Figure 1 shows how microcontrollers increased in popularity from 1991 through 1996.

In today’s world, microcontrollers are used in just about every electronic object in the household and place of business.  Just about the only common object in the house that does not have a microcontroller in it is the light bulb.  In fifteen years or so even that may not be the case.

The reason microcontrollers have become so common is that they are more than merely reliable.  By adding a small computer to many devices it is possible to increase efficiency or safety, or any number of other features.

Timing devices are now composed almost entirely of microcontrollers.  This has made them unbelievably accurate.  They are also cheap, and much more reliable.  A digital watch today, which has no moving parts, is almost impossible to break through normal use.  It is easy to adapt digital watches to extreme environments such as the deep sea or vacuum.

Telephones and other personal communication devices also use microcontrollers. With such devices it is possible to have wireless telephones and cellular phones, each capable of maintaining a connection between the phone unit and some sort of base station.  These phones can also encrypt data as it leaves and decrypt it as it comes in.

Televisions and stereos use microcontrollers.  Neither is the mess of tubes that was synonymous with televisions and radios.  As a result, both produce better quality picture and sound, have more features, and weigh less per unit volume.

All kinds of transportation systems use microcontrollers.  Cars use them in fuel injection systems, brakes, airbags, and just about any other piece of equipment.

Airplanes are going to a “fly-by-wire” control system.  This is a complex computer interface between the controls that the pilot uses and the control surfaces of the plane. Such interfaces are controlled by microcontrollers.

Modern traffic lights are controlled by microcontrollers.  Emergency phones by the highways of America have solar panels, batteries, and a microcontroller circuit to operate in remote areas. 

            Microcontroller's use increased rapidly. Now these are used in almost every electronic equipment like Washing Machines, Mobile Phones and Microwave Oven. Following are the most important facts about Microcontrollers, which causes rapid growth of their use:

Microcontrollers are cheap and very small in size, therefore they can be embedded on any device.

Programming of Microcontrollers is simple to learn. Its not much complicated.

We can use simulators on Computers to see the practical results of our program. Thus we can work on a Embedded project without even buying the required Components and Chips. Thus we can virtually see the working of our project or program.

Microcontrollers are mostly used in following electronic equipments :

• Mobile Phones
• Auto Mobiles
• CD/DVD Players
• Washing Machines
• Cameras
• In Computers-> Modems and Keyboard Controllers
• Security Alarms
• Electronic Measurement Instruments.
• Microwave Oven

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ARCHITECTURE OF 8051

Each block will be discussed step by step:

 ALU — Arithmetic Logical Unit
This unit is used for the arithmetic calculations.

 A-Accumulator
This register is used for arithmetic operations. This is also bit addressable and 8 bit register.

 B-Register
This register is used in only two instructions MUL AB and DIV AB. This is also bit addressable and 8 bit register.

PC-Program Counter
• Points to the address of next instruction to be executed from ROM
• It is 16 bit register means the 8051 can access program address from 0000H to FFFFH. A total of 64KB of code. 16 bit register means. Initial value 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 (0000H)
Final value 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 (F F F F H)
• Initially PC has 0000H
• ORG instruction is used to initialize the PC ORG 0000H means PC initialize by 0000H

• PC is incremented after each instruction.

Example:

Mnemonics Machine codes

MOV R5, #25H 7D 25

MOV A, #00H 74 00

ADD A, R5 2D

HERE: SJMP HERE; 80 FE

• When 7D is accessed then PC locate the 0001H (next instruction to be executed)

• When 00 is accessed then PC locate the 0004H (next instruction to be executed)

 ROM Memory Map in 8051

→ 4KB, 8KB, 16KB, 32KB, 64KB on chip ROM is available.

→ Max ROM space is 64 KB because 16 bit address line is available in 8051.

→ Starting address for ROM is 0000H (because PC which points the ROM is 16 bit wide).

 8051 Flag Bits and PSW Register

→ Used to indicate the Arithmetic condition of ACC.

→ Flag register in 8051 is called as program status word (PSW). This special function register PSW is also bit addressable and 8 bit wide means each bit can be set or reset independently.

There are four flags in 8051

• P → Parity flag → PSW 0.0

1 – odd number of 1 in ACC

0 – even number of 1 in ACC

• OV(PSW 0.2) → overflow flag → this is used to detect error in signed

arithmetic operation. This is similar to carry flag but difference is only that

carry flag is used for unsigned operation.

• RS1(PSW0.4) RS0(PSW0.3) Register Bank Select

0                              0                            Bank 0

0                              1                            Bank 1

1                              0                            Bank 2

1                              1                            Bank 3

for selecting Bank 1, we use following commands

SETB PSW0.3 (means RS0=1)

CLR PSW0.4 (means RS1=0)

Initially by default always Bank 0 is selected.

• F0 → user definable bit

• AC → Auxiliary carry flag → when carry is generated from D3 to D4, it

is set to 1, it is used in BCD arithmetic.

Since carry is generated from D3 to D4, so AC is set.

•CY → carry flag → Affected after 8 bit addition and subtraction. It is used to detect error in unsigned arithmetic opr. We can also use it as single bit  storage.

SETB C → for cy = 1

CLR C → for cy = 0

 Structure of RAM or 8051 Register Bank and Stack

→ 128 byte RAM is available in 8051

→ 128 byte = 2^7B

Address range of RAM is 00H to 7FH.

→ In MC8051, 128 byte visible or user accessible RAM is available which is shown in figure. Extra 128B RAM which is not user accessible. 80H to FFH used for storage of SFR (special function register)

→ Four Register Banks

→ There are four register banks, in each register bank there are eight 8 bit register available from R0 to R7

→ By default Bank 0 is selected. For Bank 0, R0 has address 00H

R1 has address 01H

. . . . . . . . . . . . . . . .

 . . . . . . . . . . . . . . . .

R7 has address 07H

For Bank 1, R0 has address 08H

R1 has address 09H

. . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . .

R7 has address 0FH

For selecting banks we use RS0 and RS1 bit of PSW.

→ R0 to R7 registers are byte addressable means.

 If we want to set the bit 3 of R0 then we can’t use SETB R0.3  We use MOV R0, #08H;

 For changing single bit we can modify all the other bits of R0.

→ Locations 20H to 2FH is bit addressable RAM means each bit from 00H to FFH in this we can set or reset CF rather than changing whole byte.

→ Locations 30H to 7FH is used as scratch pad means we can use this space for data reading and writing or for data storage.

 Stack in 8051

→ RAM locations from 08H to 1FH can be used as stack. Stack is used to store the data temporarily. Stack is last in first out (LIFO)

→ Stack pointer (SP) →

• 8bit register

• It indicate current RAM address available for stack or it points the top of stack.

• Initially by default at 07H because first location of stack is 08H.

• After each PUSH instruction the SP is incremented by one while in MC after PUSH instruction SP is decremented.

• After each POP instruction the SP is decremented.

Example:

MOV R6,#25H;

MOV R1,#12H;

MOV R4,#OF3H;

PUSH 06H;

PUSH 01H;

POP 04H;

→ if we want to use more than 24byte (08H to 1FH) of stack. We can change SP to point RAM address 30H to 7FH by MOV SP, #XX Any value from 30 to 7FH

Conflicting of Register Banks and Stack

→ We know locations from 08H to 1FH is used as stack and it is also used as register bank.

→ If in the program, we use the Register Bank 1 to 3 and also use the stack then conflicts exist and error can be possible. For removing this situation we use the stack from location 30H to 7FH by shifting SP to 2FH. MOV SP,#2FH;

DPTR

→ Data Pointer in 8051

→ 16 bit register, it is divided into two parts DPH and DPL.

→ DPH for Higher order 8 bits, DPL for lower order 8 bits.

→ DPTR, DPH, DPL these all are SFRs in 8051.

Special Function Register

→ (See Fig.) RAM scratch pad, there is extra 128 byte RAM which is used to store the SFRs

→ Following figure shows special function bit address, all access to the four I/O ports CPU register, interrupt control register, timer/counter, UART, power control are performed through registers between 80H and FFH.

Byte Addressable SFR with byte address

SP – Stack printer – 81H

DPTR – Data pointer 2 bytes

DPL – Low byte – 82H

DPH – High byte – 83H

TMOD – Timer mode control – 89H

TH0 – Timer 0 Higher order bytes – 8CH

TL0 – Timer 0 Low order bytes – 8AH

TH1 – Timer 1 High bytes = 80H

TL1 – Timer 1 Low order byte = 86H

SBUF – Serial data buffer = 99H

PCON – Power control – 87H.

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