C. Bona

Computer-aided automatic design

Abstract After a survey of the design process to underline the different stages in which the use of a computer can be utilized with profit, some programs developed to assist the mechanical designers in their job are discussed. These are organized into an integrated system made up of a supervisor (MLINK), two basic languages (MEDES and TEADI), and several post-processors which are briefly described. Further improvements of the system are also discussed concerning new programs to be developed and new types of hardware to be adopted.

The Supervisor (“MLINK”)

This program can be considered as a switch to jump to and from any program of the system. As it gains the control of the CPU the page shown in Fig. 1 appears on the display screen; this is a list of the programs available, any of which can be executed by indicating its name with the light pen. Any name can be added or deleted at any time by hitting the command with the light pen and typing the name by the display keyboard. Any program written with the standard MLINK exit routine can be aborted by hitting any key of the functional keyboard. This action returns the control to MLINK.

The Language for Describing Plane Mechanisms (MEDES)

This language is used to introduce into the computer the description of the mechanism to be studied. Any type of plane mechanisms composed with the basic building blocks shown in Fig. 2 can be very easily described with this program. The description of the mechanism is represented in the computer by a data structure in which all the elements of the mechanism, as well as their functional dimensions and how they are interconnected each other are recorded.

The Language for Describing Plane Shapes (TEADI)

This language is used to introduce into the com-pater the description of plane bodies whole contours are described with straight lines and circles. The user operates as at a drafting board with the difference that, instead of taking a ruler or a compass to draw a line or a circle, he indicates in a menu on the screen the corresponding symbols. This activates the appropriate part of the program, which asks to indicate with the light pen or to type throuth the keyboard the elements or the informations needed to complete the definition. For instance, having defined the two circles of Fig. 3, if one wants to draw the common tangents, he must indicate with the light pen the line in the menu and the two circles in the central area of the screen. Fig. 4-9 illustrate some of the possibilities of the language to help the definition of figures composed of repetitive parts. The possibility to record in a date bank the data structure of any figure to be added to that in course of definition gives a very pleasant flexibility and power to the language which is very appreciated in the case of complex figures with repetitive parts.

Programs for the Design and the Production of Cams

This is the simplest problem of mechanism synthesis as there are infinite degrees if freedom and the problem can be reduced to the computation of the envelope generated by a simple form, the follower in its motion relative to the cam plane. For the most usual follower geometries standard programs have been prepared that give directly the design of the cam, the pressure angle and curvature diagrams, and even a paper tape to drive a N/C machine (Fig. 18-20). This can also be done when the follower is not a simple lever but any kind of mechanism which can be described by MEDES. This is done by inverting the movement of the mechanism (that is driven elements become motors) analyzing the movement by a postprocessor of MEDES to find the locus of the center of the follower seen on a plane which rotates at constant speed around the can shaft (Fig. 21-22).

Synthesis by Mathematical Programming (OTMEC)

In the design environment in which we were working we have found that the synthesis problem rarely is expressed as a precision point problem,.but precision point techniques, intuition or a systematic analysis of the act of movement are often the only means to give a starting point for synthesizing the real mechanism by optimization techniques. A search of the optimization algorithms available and some practical experiments have shown direct search techniques as beeing very attractive. A program is beeing written which used MEDES to compute the new configuration of the mechanisms for every new set of values of its functional dimensions and a pattern search algorithm to drive the search in the parameter’s space.

Construction of Automatic Handbooks with Graphical and Procedural Informations: Mechanism Data Bank (TEXTE)

Fig. 9-15 are some possibilities of a text editor program. This program has been conceived to give the possibility of building and updating with great easyness pages containing alphanumeric, graphic or procedural informations to create a data bank. This program will allow to compose a text and memorize it on an. appropriate file. The text can be composed by written information, which can be taped in from display console, or by graphical information like the structure of mechanisms of the shape of mechanical parts. These will rebuilt by retrieving the associated data structures scanning the corresponding files. In the case the desired figure is not yet prepared, the user can link directly to the program to build it and then back to the supervisor. Any formula, procedure name, or graphic item inserted in the text could be defined as a message entity and the program links directly to the program associated with it by pointing the light pen on it. In this way, the TEXT EDITOR could also be used as a supervisor by requesting the page containing the name of all programs or all the data files inserted in the program. Fig. 10 gives a list of the classes of typical mechanisms in the file. By using the TEXT EDITOR program, this page as well as the corresponding file could be easily made more and more complete. Fig. 11 appears after having selected the straight line generator type. Here again the list of typical straight line generators could be easily updated. Fig. 12 shows the trajectories generated by the mechanism of Fig. 11, while Fig. 13 has been taken with the mechanisms were in movement. Fig. 14, which appears after having selected the files straight line generator, gives information on this type of mechanism recalling the programs provided by the system which will accept as input the data structure associated with it.

Overall Layout Analysis

Fig. 38-40 show an application of both languages MEDES and TEADI together. Starting with a description of the mechanism by MEDES, the shape of each part memorized in the TEADI file is associated to the corresponding element of the skeleton. As both programs are active, the skeleton position can be modified and this causes the redrawing of the parts according to the new position computed by MEDES. Every new position can in its turn be passed to TEADI, where can be manipulated with the usual options (that is scaled, measured, etc.).

Computation of Geometric and Dynamic Properties of “21/2D” Bodies

Fig. 34 shows another program which may be considered as a TEADI postprecessor. Starting with a figure described by TEADI and defining the thickness and density of every layer of which it is composed the program visualizes the center of mass of the body and of its boundary, the principal inertia axes and computes the inertia moments with respect to any predefined axis.

Synthesis by Trial & Errors — Kinematic Analysis by Medes

Fig. 26-30 show different phases of a kinematic analysis program based on the data structures created by MEDES. After having described the mechanism structure, the OPTION lightbutton selects the page of Fig. 26. From here different possibilities are envisaged, one of which is to add some members and another is kinematic analysis. In this kinematic analysis program there is the possibility of changing any dimension of mechanism and see the new configuration or position or ask for a step by step changement of any input quantity or other dimensions (even more than one at the same time) (Fig. 27-28). Every position is recorded, therefore at the end of the movement any quantity of the mechanism (angles, pivots trajectories, distances, etc.) can be visualized as a diagram (Fig. 29-30). The synthesis process can be carried on by utilizing the speed by which the effect of any dimensional variation on the mechanism reflects on any trajectory or other function performed by the mechanism.