Computer-aided design

Under CAD (of English. Computer-aided design [ ː tə kəmpju eɪdɪd dɪzaɪn ] to German computer aided designing ) refers to the construction of a product by a computer program. Originally referred to as an aid in technical drawing with CAD using a computer. Today professional CAD applications are complex expert systems for the design and construction of technical solutions.

Meanwhile, the third dimension ( 3D) is added in almost all CAD applications. So that CAD refers to the formation of a virtual model of three-dimensional objects by means of a computer. From this the usual technical drawings can be derived and output. A particular advantage is to generate an arbitrary spatial mapping of the already existing virtual three-dimensional object. Due to the material properties determined can use with CAD computer models of different nature for finding an appropriate solution, beyond, manufacturing information (eg rapid prototyping) to derive and create product documentation.

CAD is used in almost all branches of engineering: architecture, civil engineering, mechanical engineering, electrical and all their disciplines and mutual combinations through to dental technology.

  • 2.1 Mechanical Design
  • 2.2 Circuits
  • 2.3 Data Formats

Basics and Technology

Drawings created with CAD: 2D CAD

At the beginning of CAD was merely a tool for technical drawings. The addition 2D was only necessary as technical objects not only drawn with computer assistance, but as a virtual three-dimensional body ( 3D) could be treated.

With the help of a so-called 2D CAD system spatially extended bodies are created exactly as you draw by hand views and sections in the rule. The drawings are first made ​​visible on the screen, and then plotted on paper or printed.

The formerly hand-drawn line is the basic element in a CAD system. This made ​​the " prefabricated " basic objects of the system: line, circle, ellipse, polyline, polygon or spline. The internal representation of these objects is vector-oriented, that is, only its parameters are stored (for example the two end points of a straight line or the center and the radius of a circle ). In this way, the storage requirement in the computer small ( contrary pixel graphics ). The object is created from the few data at output.

It adds the objects still be added selectable attributes such as color, line type and line width. With the possibility of so-called tools to edit the objects and to dimension the virtual drawing and writing, in almost all activities on paper and in the CAD system are executable.

Tools enable and facilitate, for example, creating, positioning, modification and deletion of objects, drawing guides, finding of marked points of the objects ( for example, end and middle points of lines, centers of circles, etc. ), drawing from the perpendicular, tangent, and equidistant to property lines and hatching closed lines. The full dimension is created after only the end points (for example, a length dimension ) or the object ( for example, a sheet of a radius ) has been selected. The accuracy of the dimensions is a multiple of those in a classic drawing. Indirectly generated dimensions must not be calculated consuming, it can be read from the CAD drawing.

Repeated objects can be created equal from the beginning " in series". Object groups can be edited as a whole, for example stretch or compress or enlarge only proportional or out.

An organizational tool is done the drawing in parts at different levels ( Layer technology). This corresponds to the making of a classic drawing on multiple transparent papers superimposed represent the whole.

Representations of standard and repeat parts can be stored in a library and related back from there and pasted. Subregions can be zoom in (zoom), so that a small screen resolution (1600 × 1200 pixels for CAD applications, a low resolution) is not a hindrance.

Modern CAD systems also have interfaces to extend the functionality by means of macros.

Drawing in Space: 2 ½ D CAD

By drawing lines in space to allow body suggest. Such lines indicate, for example, the edges of a cuboid. A body is however only then simulates sufficient if it, both with various physical properties has a volume and surfaces. Such defective models are sometimes referred to as 2 ½ D models, in contrast to sufficient models.

An equally sloppy, but more descriptive designation of a body with only 2 ½ instead of 3 dimensions refers to its simplicity. These are bodies whose origin can be imagined by extending planar contours in the third dimension. Makes a thin sheet one ( in a plane approximation ) getting fatter, we obtain first a board and a last column, ie body, in which all parallel to the exit face cuts look the same. As a CAD tool, this approach extrusion.

A precursor to the extrusion ( is a 3D tool ) is drawing with height. For example, to create not only a rectangle, but a rectangular, but which is defined solely by means of two parallel rectangular contours. Its interior and its surface are not fixed. The box is empty and has transparent walls. In addition, the drawing is possible with the survey. One can thus draw a second provided with height object in a plane parallel to and In this way, two 2 ½ D body that are not on the same level. One of possible improvements is to be able to turn the 2 ½ - D body in space.

When drawing with height and survey the objects have gotten only more attributes. Therefore, the progression from 2D to 2 ½ D CAD consists chiefly of the ways to represent the modeled body of a selected view point as spatial objects, that is to draw. In the square were as lines add from a lower corner to the associated upper corner. Lines are incorporeal by definition, but can be thought of as wires. Thus we call this simplest of CAD modeling species next line or edge model and wireframe. In order to distinguish the front from the rear of the massive cube -intentioned, had the computationally intensive tool Hide Hidden edges are developed and added.

A variant for creating wireframes using height and elevation is drawing in several intersecting planes. Each have a surface contour of a 2 ½ D body is located in a respective plane of the drawing. A simple example is the representation of a cuboid in the xy, xz and yz planes in the spatial Cartesian coordinate system.

Pure 2 ½ D CAD systems are no longer available today, but the basic level in most popular 3D CAD systems. From a historical perspective, the 2 ½ D CAD technology was a precursor to the 3D systems. The inherent limitations were primarily due to the slowness and low memory capacity of the computer, less by not existing elaborate software.

3D - CAD

The dissolved with 3D CAD task is much more demanding than in the plane ( 2D CAD) or to draw in space ( 2 ½ D CAD). The computer has a virtual model of a three dimensional object is created. In addition to geometric and physical properties can be simulated. The so-called volume geometric model described is the so-called body - model additionally has physical properties such as density, resilience coefficient, permissible deformation and fracture stress, thermal and electrical conductivity, and thermal expansion coefficient, and others. It has a surface with structure and optical properties. Such a body can be described weigh virtual, elastic, plastic and thermal deformation. Its geometry and material properties, the specifications, for example, a finite element program, with which it is inspected for deformation and breakage. You can light it and recognize its optical properties there.

An intermediate step is the so-called space model. It is used when the surface shape of an object is primarily important. In automobiles it is the determined by the aesthetics and the momentary taste quite arbitrary forms of body panels, for aircraft originating from aerodynamic optimization of the wing and fuselage panels, which usually have no recordable with known surface equations forms. The surface model is envisioned as a sheet metal model, but how the wireframe also no mass. His objects are merely geometric surfaces.

Volume models are composed usually of simple primitives ( cube, pyramid, cylinder, cone, sphere, torus ), which is favored by the possibility of Boolean operators. For example, a lying triangular prism can be combined with a vertical square when a chimney should protrude from a house roof. By moving a flat contour out of the plane (on a straight line: Extrusion | on a circular arc: rotation ) can also gain basic body ( a special case of torus: a circle is moved in a circle ).

Development of modeling techniques

CAD programs

CAD programs are available for many different applications and operating systems. See the list of CAD programs and the list of EDA applications. Unlike Office solutions, there are strong specializations in the field of CAD. So there are often national leader in fields such as electrical engineering, road construction, surveying, etc.

Mechanical Design

Mechanical CAD programs can be found mainly in the following areas:

  • Construction Architecture ( CAAD )
  • Timber
  • Engineering
  • Historical Reconstruction
  • Urban Development
  • Hydraulic
  • Transportation Infrastructures
  • Plant
  • Vehicle
  • Mold and tool Packaging Development and Diecutting
  • Schematics in the hydraulic
  • Wiring diagrams in pneumatics

Electronic circuits

Another field of application is the design of electronic circuits. Such programs are often summarized under the terms eCAD and EDA, in particular for applications in chip design, the PCB design, installation technology and microsystem technology.

For special requirements, special areas with strongly varying development methods have formed. This is especially true for the computer-based chip design, ie the design automation (EDA ) for analog or digital integrated circuits, for example ASICs. This is related to the design of programmable devices such as gate arrays, GAL, FPGA, and other types of programmable logic ( PLD ) using, for example, VHDL and Abel. The automated layout design of integrated circuits is often referred to as a layout synthesis.

In the development of printed circuit boards of the first design of the circuit takes place in the form of a circuit diagram, followed by computer-aided layout design.

Even in the classical installation technology, there are numerous areas of application for computer software, particularly in residential installations for industrial or public buildings or the design and implementation of PLC-based control systems.

In the field of microsystems technology is a particular challenge is to bring together data circuit with the mechanical product design data (CAD ) and to produce directly with such data microsystems.

Data formats

Due to the system can not be transferred all the information in the data exchange. While pure drawing elements no longer pose a problem today, the exchange of writings, dimensions, hatches and complex structures is problematic because there are no standards for it. Exist even at national level in various industries greatly different specifications, which additionally complicates standardization.

Most programs rely on its own file format. This complicates the exchange of data between different CAD programs, which is why there are approaches to standardization. As data exchange format for drawings and archiving of documents the format DXF of the world leader Autodesk is commonly used today.

It is necessary to distinguish between the CAD system is neutral and CAD system-specific data formats. Significant CAD system- neutral data formats are VDAFS, IGES, SAT, STEP and IFC as well as for special applications, the STL interface. The data formats are as follows:

  • The DXF format has become widely established as a data exchange format for drawings, it is the only supported format of all CAD systems and has become the industry standard. Some CAD systems can read DXF files only as 2D data and write frequently go CAD system-specific features such as dimensions, hatches, etc. lost or can not be equivalently represented in the target system.
  • The DWF ( Design Web Format engl. ) was originally conceived by Autodesk for the exchange of data via the Internet, supporting all elements of DXF and is highly compressed. However, it could not prevail. DWF files were displayed with plugins in browsers.
  • VDA -FS - data exchange format for surfaces, developed by the Association of German car manufacturers ( VDA), in the past de facto standard in this area;
  • IGES - Data exchange format for 2D drawings and 3D data (area ), in many CAD applications as an exchange format is common and possible. Raises due to better usability VDAFS from more and more, is more extensive and system-independent as DXF usable, but not as widely used and with the same weaknesses.
  • STEP - a standard file exchange format, which was developed internationally. STEP is considered to be the best interface for geometry data. Where information such as colors, assemblies, structures, views, sheets and model attributes can be passed. Also available for the transfer of drawing data ( there but not as powerful as in the 3D area ). STEP is not supported by all CAD systems.
  • VRML97-ISO/IEC 14772, was originally developed as a 3D standard for the Internet. Most 3D modeling tools allow the import and export of VRML files, which makes the file format has established itself as an exchange format for 3D models. For use as a CAD CAD exchange format, it is not really suitable, but probably for transfer to, for example, animation and rendering software.
  • STL - made ​​up of triangular faces models. Is used primarily for transfer to rapid prototyping systems.
  • IFC - one developed for building open standard. It will not pass any drawings, but specifications and geometries. It was developed by buildingSMART eV ( to April 2010, International Alliance for Interoperability eV). It is a model-based approach for optimizing the planning, execution, and management processes in construction. The Industry Foundation Classes - IFC - are an open standard for building models. The IFC standard is registered under ISO 16739.

With the CAD system- neutral formats succeed usually only the transfer of edge, surface, and solid models. The construction history is lost in the rule so that the transmitted data usually for further processing have only limited usefulness. CAD system- specific data formats enable the transmission of the complete CAD models, but they are only available for a few systems.

For the transfer of PCB exposure data to create films for printed circuit boards, the so-called Gerber format and the newer Extended Gerber format is of great importance (see photographic film ).


The term "Computer Aided Design" originated in the late 50s as part of the development of the programming system APT, which served the computer aided programming of NC machines.

At MIT in Boston Ivan Sutherland 's Sketchpad showed in 1963 with its development that it is possible to create on a computer-controlled radar screen interactive ( light pen, keyboard ) simple drawings (English skit ) and change it.

1965 ( aircraft, USA ), the first attempts for a commercial CAD system for technical drawings (2D) were started at Lockheed. This system, CADAM ( Computer Augmented Design and Manufacturing ) connected based on IBM mainframes, special screens, and with high costs, was later marketed by IBM, and was, at least in the aircraft industry, the market leader until the 1980s. It is partly absorbed into CATIA. In addition, a PC-based version of CADAM was developed and marketed by the name of HELIX, but this is virtually disappeared from the market.

At the University of Cambridge, England, in the late 1960s were recorded the first research that should explore whether it is possible to use 3-D basic body and to use this for mapping complex combinations (eg pipelines in chemical plant ). From this work, the system PDMS (Plant Design Management System), which is now marketed by the company Aveva, Cambridge, UK was born.

Also at the end of the 1960s, the French aircraft manufacturer Avions Marcel Dassault a graphics program to create drawings started programming ( Dassault Aviation today). From this, the CATIA program was created. The Mirage was the first aircraft to be developed with it. At that time such a program still needed the power of a mainframe computer.

By 1974, B -spline curves and surfaces have been introduced for the CAD.

After the first personal computers were in the second half of the 1980s in the company, and CAD programs to get it to market. During this time there were a number of computer manufacturers and operating systems. AutoCAD was one of the first and most successful CAD systems that worked on different operating systems. In order to enable the exchange of data between these systems, AutoDesk defined for its CAD system AutoCAD DXF file format as a "neutral " export and import interface. In 1982 AutoCAD for the DOS operating system. However, the approach in the design remained almost the same as before with the drawing board. The advantage of 2D CAD drawings were very clean, which could simply be changed back. Also, it was quickly possible to draw different versions of a component.

In the 1980s, began a CAD boom because of falling employment costs and better and better software. In industry, the hope was cherished, to be able to solve all pending drawing and design tasks with a system. This approach is, however, failed. Today is used for each specific planning task a special system with very powerful special functions. The move to the third dimension was affordable by the ever-increasing performance of the hardware then towards the end of the 1980s, even for smaller companies. So virtual body could be examined from all sides. Similarly, it was possible to simulate loads and derive manufacturing programs for computer- controlled machine tools (CNC).

Since the early 2000s, the first steps are to make them disappear until then still absolutely necessary drawing. In the increasingly existing 3D models are introduced by the dimension about color and material all the information necessary for manufacturing. If the 3D model is extended to these additional, geometry foreign properties, it becomes the product model, for example, supported by the STEP data format. The single unitary volume objects are to instances of different classes. This allows design rules and references between objects ( eg windows is anchored in the wall) can be realized.