Cad
This document was uploaded by user and they confirmed that they have the permission to share it. If you are author or own the copyright of this book, please report to us by using this DMCA report form. Report DMCA
Overview
Download & View Cad as PDF for free.
More details
- Words: 1,822
- Pages: 7
Dept of Mech. Engg.
IV Sem Mech.
Chapter 9 Computer Aided Design CAD Definition: Computer aided design (CAD) can be defined as the use of computer systems to assist in the creation, modification, analysis or optimization of a design. The computer system consists of hardware and software to perform the specialized design functions required by the particular user firm. The CAD hardware typically includes the computer, one or more graphics display terminals, keyboards and other peripheral equipment. The CAD software consists of the computer programs to implement computer graphics on the system (system software) and application programs to facilitate the engineering functions of the user company. The application programs include stress-strain analysis of components, heat transfer calculations, lubrication, combustion, kinematics and dynamics of machinery, etc. The collection of application programs will vary from one user firm to the next depending upon product lines manufacturing process and customer markets. Scope of CAD System: 1. to increase the productivity of the designer 2. to improve the quality of design 3. to improve communications 4. to create database for manufacturing Benefits of CAD: 1. Improved engineering productivity 2. Shorter lead times 3. Reduced engineering personnel requirements 4. Customer modifications are easier to make 5. Improved accuracy and design 6. Reduced errors in programming 7. Easy inspection of complicated parts 8. Saves materials and machine time and also contributes in reduced wastage 9. Better communication interfaces 10. Leads to standardization of design
Compiled by NSY
Page 1of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
Hardware in CAD Secondary Storage
Design Workstation
CPU
Graphics Terminal
Input Devices
Output Devices and Peripherals
The configuration of a CAD system is as shown above The input devices can be keyboard, mouse, lightpen, digitizer, scanners, etc., while output devices may be printer, plotter, etc. Secondary storage devices such as tapes, discs, floppies, pen drives, portable hard disks, etc. helps in taking the backup of drawing. The CPU is the main controlling part of the entire CAD system. For a CAD system the processor should be fast enough to handle huge amount of data to carry out complex mathematical calculations. Design Workstation: The CAD workstation is the system interface with the outside world. It provides a mode to communicate with the computer. The workstation must accomplish five functions: 1. it must interface with the central processing unit 2. it must generate a steady graphic image for the user 3. it must provide digital descriptions of the graphic image 4. it must translate computer commands in to operating functions it must facilitate communication between the user and the system The Graphics Terminal: The graphic terminals available today generally use the cathode ray tube (CRT) as the display device. The graphic terminals are available in different sizes.
Compiled by NSY
Page 2of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
A CRT is shown in the sketch below
A heated cathode emits a high speed electron beam onto a phosphor coated glass screen. The electron energize the phosphor coating, causing it to glow at points where the beam makes contact. By focussing the electron beam, changing its intensity, and controlling its point of contact against the phosphor coating through the use of a deflector system, the beam can be made to generate a picture on the screen. Operator Input Devices Input devices are needed for graphics workstation to facilitate convenient communication between the user and the system. A number of input devices can be grouped into three categories. They are: 1. Cursor control devices 2. Digitizers 3. Keyboard terminals The first two are used for graphical interaction with system and the third one is used for input of alphanumeric commands and text.
Compiled by NSY
Page 3of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
1. Cursor Control Devices The cursor normally takes the form of a bright spot on the CRT screen. The computer is capable of reading the current position of the cursor. Hence the user’s capability to control the cursor position allows location data to be entered into the CAD system database. A typical example would be for the user to locate the cursor to identify the starting point of a line. Another, more sophisticated case would be for the user to position the cursor to select an item from a menu of functions displayed on the screen. For instance, the screen might be divided into two sections, one of which is an array of blocks which correspond to operator input function. The user simply moves the cursor to the desired block to execute the particular function. There are variety of cursor control devices which have been employed in CAD systems. These include: a. The mouse b. The thumbwheel c. Joystick d. Track ball e. Light pen f. Touch screen Mouse Mouse is the most popular input device. Mouse is a small hand held puck like instrument which is attached to the computer. Mouse can be moved around by the operator on any flat surface to provide graphic input. Its ability to rapidly position the cursor on the screen is its most important advantage. Mouse is available as a mechanical or optical graphic input device. In the case of a mechanical mouse, the rolling ball at the bottom of the mouse causes two encoders to rotate. The movement of the mouse is thus converted into pulses which move the cursor in the X and Y direction in proportion to the movement of the mouse. An optical mouse works on the basis of signal generated by the reflecting light from the plate on which the mouse is moved. The source of light is an LED located underneath the mouse. There are no moving parts in an optical mouse and that is an
Compiled by NSY
Page 4of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
advantage over the mechanical mouse. Slippage at the contact surfaces is a disadvantage of mouse. Thumbwheel Two wheels are used to control the cursor position along the horizontal and vertical direction. The position is indicated by a cross-hair on the screen. Thumbwheels are mounted on the keyboard by the side. Thumbwheels are used in animation packages. Two of them are provided for X and Y movements of the cursor. Joystick Joystick is a potentiometric device that contains sets of variable resistors which feed signals that indicate the device position to the computer. These devices rely on the operator’s sense of touch and hand-eye co-ordination to control the position of the cursor on the screen. Joystick devices are normally set so that side-to-side movement produces change in X coordinate and front to back movements produce change in Y co-ordinates. Thus they are best suited for pointing in tasks such as menu selection or creating simple schematics. Three dimensional capability is possible by moving the handle up and down by twisting it to provide data entry in the Z-axis. Track ball Track ball has a ball and socket construction but the ball must be rolled with fingers or the palm of the hand. The cursor moves in the direction of the roll at a rate corresponding to rotational speed. Its function is similar to joystick and allows closer control over the cursor movement. These are ideal for flight simulation control through CAD software packages. Light Pen The light pen is a pointing device in which the computer seeks to identify the position where the light pen is in contact with the screen. Light pen is nothing but a detector of
Compiled by NSY
Page 5of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
light on the CRT screen and uses a photodiode, phototransistor, or some other form of light sensor. Selection is done by pressing a push button in the pen. Touch Screen Touch screens are direct devices. They are used by simply touching CRT display with one’s finger or a pointing device. Two types of touch screens namely, mechanical and optical are used in CAD applications. Mechanical type is a transparent screen overlay which detects the location of the touch. Optical touch screen systems use rows of light emitters and receptors mounted just in front of the screen with the touched location determined by broken beams. 2. Digitizer Digitizer boards or tablets are electro-mechanical vector graphic input devices that resemble a drafting board. These are used together with a movable stylus called puck. The puck can be moved over the surface to following existing lines. It is a common technique in CAD systems for taking x and y co-ordinates from a paper drawing. The tracking device (puck) contains a switch for the user to record the desired x and y coordinates. The co-ordinates are either directly sent to the computer or stored in secondary storage devices like magnetic tapes. Digitizers with a large board provide high resolution and accuracy. Digitizers are more frequently used to create drawing using an interactive drafting package to enter a sketch or a drawing which can be redrawn or edited as the display appears on the CRT screen. Modern day digitizers can pick 3-dimensional information from a model, moving the tip of a wand along the model. These systems generate signals that are analysed to produce co-ordinates. This type of digitizer is very useful for reverse engineering. 3. Keyboard Terminals It is an alphanumeric terminal used to enter commands, co-ordinates, functions and process data to the CAD system. The information typed is displayed on the CRT screen. The typed information displayed on the CRT screen can be changed. CAD systems make use of special function keyboards. These function keys are provided to eliminate extensive typing of commands. The number of function keys varies from about 8 to 80.
Compiled by NSY
Page 6of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
Soft and Hard Prototyping Prototyping is the process of building a model of a system. A prototype is often used as part of the product design process to allow engineers and designers the ability to explore design alternatives, test theories and confirm performance prior to starting production of a new product. A solid model of a part, or an assembly of solid parts, provides a more complete product definition. Boeing Commercial Airplane Group uses the CATIA solid modelling product. The solid models let Boeing spot interference problems before expensive mock-ups are built. This results in significant savings. One of the biggest paybacks Boeing is experiencing is the visualization benefits of solid modelling. It gives their engineers a realistic view of the part. They can print colour images of the parts and share them with other groups inside and outside the organisation. The process of building a prototype and analysing it using the computer is known as soft prototyping. When a hard copy output is taken of the prototype it is known as hard prototyping.
Compiled by NSY
Page 7of 7
MDD- Chapter 9
IV Sem Mech.
Chapter 9 Computer Aided Design CAD Definition: Computer aided design (CAD) can be defined as the use of computer systems to assist in the creation, modification, analysis or optimization of a design. The computer system consists of hardware and software to perform the specialized design functions required by the particular user firm. The CAD hardware typically includes the computer, one or more graphics display terminals, keyboards and other peripheral equipment. The CAD software consists of the computer programs to implement computer graphics on the system (system software) and application programs to facilitate the engineering functions of the user company. The application programs include stress-strain analysis of components, heat transfer calculations, lubrication, combustion, kinematics and dynamics of machinery, etc. The collection of application programs will vary from one user firm to the next depending upon product lines manufacturing process and customer markets. Scope of CAD System: 1. to increase the productivity of the designer 2. to improve the quality of design 3. to improve communications 4. to create database for manufacturing Benefits of CAD: 1. Improved engineering productivity 2. Shorter lead times 3. Reduced engineering personnel requirements 4. Customer modifications are easier to make 5. Improved accuracy and design 6. Reduced errors in programming 7. Easy inspection of complicated parts 8. Saves materials and machine time and also contributes in reduced wastage 9. Better communication interfaces 10. Leads to standardization of design
Compiled by NSY
Page 1of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
Hardware in CAD Secondary Storage
Design Workstation
CPU
Graphics Terminal
Input Devices
Output Devices and Peripherals
The configuration of a CAD system is as shown above The input devices can be keyboard, mouse, lightpen, digitizer, scanners, etc., while output devices may be printer, plotter, etc. Secondary storage devices such as tapes, discs, floppies, pen drives, portable hard disks, etc. helps in taking the backup of drawing. The CPU is the main controlling part of the entire CAD system. For a CAD system the processor should be fast enough to handle huge amount of data to carry out complex mathematical calculations. Design Workstation: The CAD workstation is the system interface with the outside world. It provides a mode to communicate with the computer. The workstation must accomplish five functions: 1. it must interface with the central processing unit 2. it must generate a steady graphic image for the user 3. it must provide digital descriptions of the graphic image 4. it must translate computer commands in to operating functions it must facilitate communication between the user and the system The Graphics Terminal: The graphic terminals available today generally use the cathode ray tube (CRT) as the display device. The graphic terminals are available in different sizes.
Compiled by NSY
Page 2of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
A CRT is shown in the sketch below
A heated cathode emits a high speed electron beam onto a phosphor coated glass screen. The electron energize the phosphor coating, causing it to glow at points where the beam makes contact. By focussing the electron beam, changing its intensity, and controlling its point of contact against the phosphor coating through the use of a deflector system, the beam can be made to generate a picture on the screen. Operator Input Devices Input devices are needed for graphics workstation to facilitate convenient communication between the user and the system. A number of input devices can be grouped into three categories. They are: 1. Cursor control devices 2. Digitizers 3. Keyboard terminals The first two are used for graphical interaction with system and the third one is used for input of alphanumeric commands and text.
Compiled by NSY
Page 3of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
1. Cursor Control Devices The cursor normally takes the form of a bright spot on the CRT screen. The computer is capable of reading the current position of the cursor. Hence the user’s capability to control the cursor position allows location data to be entered into the CAD system database. A typical example would be for the user to locate the cursor to identify the starting point of a line. Another, more sophisticated case would be for the user to position the cursor to select an item from a menu of functions displayed on the screen. For instance, the screen might be divided into two sections, one of which is an array of blocks which correspond to operator input function. The user simply moves the cursor to the desired block to execute the particular function. There are variety of cursor control devices which have been employed in CAD systems. These include: a. The mouse b. The thumbwheel c. Joystick d. Track ball e. Light pen f. Touch screen Mouse Mouse is the most popular input device. Mouse is a small hand held puck like instrument which is attached to the computer. Mouse can be moved around by the operator on any flat surface to provide graphic input. Its ability to rapidly position the cursor on the screen is its most important advantage. Mouse is available as a mechanical or optical graphic input device. In the case of a mechanical mouse, the rolling ball at the bottom of the mouse causes two encoders to rotate. The movement of the mouse is thus converted into pulses which move the cursor in the X and Y direction in proportion to the movement of the mouse. An optical mouse works on the basis of signal generated by the reflecting light from the plate on which the mouse is moved. The source of light is an LED located underneath the mouse. There are no moving parts in an optical mouse and that is an
Compiled by NSY
Page 4of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
advantage over the mechanical mouse. Slippage at the contact surfaces is a disadvantage of mouse. Thumbwheel Two wheels are used to control the cursor position along the horizontal and vertical direction. The position is indicated by a cross-hair on the screen. Thumbwheels are mounted on the keyboard by the side. Thumbwheels are used in animation packages. Two of them are provided for X and Y movements of the cursor. Joystick Joystick is a potentiometric device that contains sets of variable resistors which feed signals that indicate the device position to the computer. These devices rely on the operator’s sense of touch and hand-eye co-ordination to control the position of the cursor on the screen. Joystick devices are normally set so that side-to-side movement produces change in X coordinate and front to back movements produce change in Y co-ordinates. Thus they are best suited for pointing in tasks such as menu selection or creating simple schematics. Three dimensional capability is possible by moving the handle up and down by twisting it to provide data entry in the Z-axis. Track ball Track ball has a ball and socket construction but the ball must be rolled with fingers or the palm of the hand. The cursor moves in the direction of the roll at a rate corresponding to rotational speed. Its function is similar to joystick and allows closer control over the cursor movement. These are ideal for flight simulation control through CAD software packages. Light Pen The light pen is a pointing device in which the computer seeks to identify the position where the light pen is in contact with the screen. Light pen is nothing but a detector of
Compiled by NSY
Page 5of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
light on the CRT screen and uses a photodiode, phototransistor, or some other form of light sensor. Selection is done by pressing a push button in the pen. Touch Screen Touch screens are direct devices. They are used by simply touching CRT display with one’s finger or a pointing device. Two types of touch screens namely, mechanical and optical are used in CAD applications. Mechanical type is a transparent screen overlay which detects the location of the touch. Optical touch screen systems use rows of light emitters and receptors mounted just in front of the screen with the touched location determined by broken beams. 2. Digitizer Digitizer boards or tablets are electro-mechanical vector graphic input devices that resemble a drafting board. These are used together with a movable stylus called puck. The puck can be moved over the surface to following existing lines. It is a common technique in CAD systems for taking x and y co-ordinates from a paper drawing. The tracking device (puck) contains a switch for the user to record the desired x and y coordinates. The co-ordinates are either directly sent to the computer or stored in secondary storage devices like magnetic tapes. Digitizers with a large board provide high resolution and accuracy. Digitizers are more frequently used to create drawing using an interactive drafting package to enter a sketch or a drawing which can be redrawn or edited as the display appears on the CRT screen. Modern day digitizers can pick 3-dimensional information from a model, moving the tip of a wand along the model. These systems generate signals that are analysed to produce co-ordinates. This type of digitizer is very useful for reverse engineering. 3. Keyboard Terminals It is an alphanumeric terminal used to enter commands, co-ordinates, functions and process data to the CAD system. The information typed is displayed on the CRT screen. The typed information displayed on the CRT screen can be changed. CAD systems make use of special function keyboards. These function keys are provided to eliminate extensive typing of commands. The number of function keys varies from about 8 to 80.
Compiled by NSY
Page 6of 7
MDD- Chapter 9
Dept of Mech. Engg.
IV Sem Mech.
Soft and Hard Prototyping Prototyping is the process of building a model of a system. A prototype is often used as part of the product design process to allow engineers and designers the ability to explore design alternatives, test theories and confirm performance prior to starting production of a new product. A solid model of a part, or an assembly of solid parts, provides a more complete product definition. Boeing Commercial Airplane Group uses the CATIA solid modelling product. The solid models let Boeing spot interference problems before expensive mock-ups are built. This results in significant savings. One of the biggest paybacks Boeing is experiencing is the visualization benefits of solid modelling. It gives their engineers a realistic view of the part. They can print colour images of the parts and share them with other groups inside and outside the organisation. The process of building a prototype and analysing it using the computer is known as soft prototyping. When a hard copy output is taken of the prototype it is known as hard prototyping.
Compiled by NSY
Page 7of 7
MDD- Chapter 9
