Benutzer:Dirk Huenniger/mordredUser

Introduction

This is the users manual of the Mordred program. Mordred is a tool for mechanical engineering. Mordred calculates the movement as well as the forces of a mechanism. The largest part of this texts explains how to define a mechanism using the graphical user interface.

What is a Mechanism?

A Mechanism is a two dimensional Model of the machine (or more generally the mechanical system) you wish to investigate. In our model our machine consists of beams connected hinges. In Mordred a beam is called Line, and depicted as a straight line. A hinge is called Node and depicted as a circle. Furthermore there are bearing in your machine. In Mordred they can be fixed in x and y direction separately. A bearing fixed in both directions, is depicted as a triangle. A bearing fixed in only one direction is depicted as a rectangle. Sometimes two beams are welded together, so the angle between them is fixed. The connection between the two beams is called Fusion and depicted as a circular arc connecting the two lines welded together. To make your machine move you will need some kind of motor, in Mordred you can promote a Line a kind of hydraulic cylinder, such a Line is depicted with a E next to it. And we say that it has got an Elongation, that is it has got an extra length, that is added gradually to it initial length in the course of time. Furthermore there are lines that have got a so called Telescope, that is they do not have a predetermined length. They fit them selfs to the space they have available. They may become shorter or longer than their original distance during the movement of the mechanism. The force in the direction of there length is always zero, but it might be nonzero in the perpendicular direction. Such lines are marked with a T next to them. An other point to mention is the matter of input forces. There can be external forces action upon your mechanism, like the gravitational force caused by a weight your machine might want to lift. Forces can only act on nodes. A node on a which a force is acting is marked with an F close to it.

Making Things Move

One you got a working Mechanism, by moving the move over the plot on the right or by selecting the Animate Radiobutton in the main Window. To get started you may load a standard mechanism from the StandartMechanism menu on the top of the main window.

Drawing Lines

In order to draw lines you need to select the Draw Radiobutton. Move the cursor to the position where you want to start your line, press the left mousebutton, move mouse the position where you want to end your line and release the left Mousebutton. A new line will appear. Usually two new nodes will be created along with the new line, you can also draw a line between to existing node. To do that you must choose start and end positions close to a existing node. When the curser is close to a node, the node will turn red to tell you that the line would snap to this node, if you began or ended it there. Each line you draw starts and ends on a node, this is enforced by Mordred, so you can't do anything wrong in this respect.

Zooming and Moving

You can move and zoom your view on the mechanism. To do that select the Zoom/Move RadioButton in the main window. Click press the left mousebutton and move the mouse, you will see that you can move around the whole mechanism this way. Release the left mousebutton the keep the mechnism in its new postion, you can also abort this action by clicking the right mousebutton once. To Zoom you need to press the right mousebutton an move the mouse up and down. You can also keep the mechnism in its new zoom by releasing the right mousebutton or abort the action by clicking the left mousebutton.

Inspecting and Changing Coordinates of Nodes

You can move the cursor to a node. When you are close to it the node will turn red. Now you can press the left mousebutton and drag it around by moving the mouse. You can also click on a node in order to set the coordinates to particular values. A window will pop up which displays the current coordinates of the node, offering you to change them. You may click Ok to accept the new coordinates of chose cancel to keep the old coordinates.

Bearings

To add or remove bearings select the Bearing Radiobutton. If by clicking on a node you can open a window in which you can define the bearing to be attached to this node. You may click Ok to add the bearing, or cancel to abandon it. To remove a bearing click on the node it is attached to and uncheck both checkboxes, then click Ok. You can also modifly a bearing by clicking on it and modifying the checkboxes according to your needs.

Forces

To add or remove Forces select the Force Radiobutton. Click on a node and a window will pop up, which shows the current force applied to this node. You may modify it according to your need and click Ok to commit the modification, or you may choose cancel to abandon your changes. In order to remove a force set its x and y components to zero and click Ok.

Removing Lines

To remove lines select the Remove Radiobutton. Move the cursor close to a line, until it turns red. Now you can remove it by clicking the left mousebutton. In some situations you might want to remove a property connected to line, but to keep the line itself. Sometimes the only way to do this is by removing the line and redrawing it.

Elongations

To add or remove elongation select the Elongation Radiobutton. Move the cursur close to a line until it turns red. Now you can open a window by clicking the left mousebutton. You can define the new or modify the existing Elongation by modifying the value and clicking the Ok button, or abandon your changes by clicking Cancel. To remove an elongation set its value to zero and click Ok.

Fusions

To add or remove fusions select the Fusion Radiobutton. Move the cursor close to a line until it turn red. Then press the left mousebutton and move the mouse to an other line that has got one Node in common with the first line. Move the cursor close to the new line until it turn red also. Then release the mousebutton. A new fusion will be created connecting both selected lines.

Telescopes

To add or remove a telescope select the Telescope Radiobutton. Move the cursor close to a lines until it turns red. Then click the left mousebutton to add the telescope, or to put it in an other way, to make the line telescopic. You can remove telescopes using the same procedure to remove telescopes.

Plotting

You can plot various variables. Do do that click the Plot button an select the quantities you are interested in, then click Ok.

Getting Static Solution

For each point of time the mechanism moves to a different position. You can pick it by moving the mouse over the plot. You can click the solve button now to get the full solution of the mechanism as a static system at that point of time. If there is not Solution you will be informed about it.

What is a Convergence Exception?

Well it means that Mordred failed to find a solution to the problem you provided. It is very likely that there is no solution to it. Unfortunately Mordred can not say what the exact reason is. You may try to start from a working mechanism, and to change it carefully to the mechanism that caused the problem, somewhere in between you will likely see what is happening and why the problem occured.

What is a Determination Exception?

Technically speaking it means that the system is statically indeterminate. The system can only be solved if the number of equations is equal to the number of variables. The Exception means that Mordred failed to find a solution to the problem you provided. It is quite unlikely that there is a solution, but it happens sometimes. If there are more variables then equations, there is usually a degree of freedom which is not fixed by the constraint, so the is more than one solution and Mordred can not find out which one to take. It often means that there is a loose peace of the machine. If there are more equations then variables, on of two things likely occurred. Firstly it is possible that you have constained a degree of freedom twice. So there could be a node that can only move in x direction because of the rest of the mechanism requires it, but there is an additional bearing that also fixes the direction of motion to x. In such a case you have to remove this bearing or more generally to remove the superfluous constraint. On the other Hand it is also possible that there is something I like to call ring of force. Imagine a square of beams, fused together at the edges. If you replace one of the beams with a one of the beams with a longer one, but keep the others as they are, there will be a tension in the whole square. The problem is that Mordred does not know the pretension of the system, so it can not determine the forces. In the geometric view it is very similar to a superfluous constraint.