The objective behind this software is to design and analyze a four-bar mechanism, visualize the motion, and relate motion to instant centers, velocity and acceleration.
II- Software Features:
This software allows the user to create, modify, and analyze a four-bar mechanism either numerically or interactively with the help of the keyboard or the mouse.
The following steps will show how to design, modify and analyze a mechanism:
(1) Choose "Four-Bar" from the menu "Mechanism" and a default four-bar will appear on the screen. The coupler curve is automatically drawn and updated.
(2) Editing the existing mechanism can be accomplished in two ways:
(a) With the mouse: click on a pivot or a pin joint, indicated with a number near it, hold the mouse button down while moving that node to a new location. Release the mouse button and the mechanism will be re drawn in the new position and a new coupler curve will be drawn.
(b) With the keyboard: choose "Dimensions..." from the menu "Input Data" and a dialog box will appear. The user can edit any value and then click "OK" to draw the new configuration.
(3) The analysis part has a variety of options:
(a) Switching the input link between the left and the right links (R2 and R4). The importance of this option is that the motion of the mechanism varies depending on which is the driving link. One link might cause the coupler to make a circular path while the other link might result in a path that is only a small arc. This is very clear in the case of a crank-rocker mechanism where one link goes through a full rotation and the other one is constrained to an arc.
(b) The results of a kinematics analysis can be seen numerically or graphically.
(i) The Numerical results are made available by choosing the appropriate item in the menu "Output". A window named "console" will appear with the numbers. If the user chooses "Save Output", a copy of the results will be placed in a file. The input angular velocity and acceleration can be changed by choosing menu "Input Data", item "Kinematics ...". The input angle increment for the numerical results is independent from that of animation or graphical analysis. It can be edited by choosing menu "Input data", item "Input Angle Increment ...".
(ii) The graphical representation of the results can be made to appear by choosing the appropriate item in the menu "Analysis". If these windows are pushed into the background, they can be brought to the front by choosing the appropriate item from the menu "Analysis". The windows can be removed by clicking the close box in the upper left corner of the window.
(iii) The numerical coupler velocity and acceleration are readily available from the menu "Output" item "Kinematics". Their directions can be drawn on the screen by choosing "Show Coupler Velocity" or "Show Coupler Acceleration" from the menu "Analysis". There directions (drawn as vectors) can be viewed for the whole motion of the coupler if the mechanism was animated.
(c) The instant centers can be drawn by choosing the appropriate item in the menu "IC". The instant center (IC13 or IC24) for the current position is drawn as a big dot, while the IC's for the rest of the motion are drawn as small dots. To get rid of the instant centers choose the same item again where it is now "Hide IC xx".
(4) Visualization of the mechanism motion is accomplished by choosing menu "Mechanism", items "Animate", or "Animation Variables ...". The latter item allows the user to specify the number of cycles and the input angle increment to be used during the animation. The visualization of the mechanism can be enhanced by zooming in (closer) or zooming out (farther). A special item in the menu "Zoom" is the fourth item where the user can select a specific portion of the screen and blow it up to fill the whole screen. To do that choose "Zoom Rectangle" and the cursor will change to cross hairs. Push the mouse button and move the mouse to create a rectangle, and as soon as the button is released that section will fill the screen.
(5) To get help choose the item "Help" from menu "File".
(6) If the user is satisfied with the design and wants to save it, or replace it with another design, the menu "File" contains items that will save current settings as an input file, which later could be opened via the "Open Input File..." item.
(7) To print the current mechanism along with a kinematics plot like velocity or accelerations, choose "Print..." and choose the appropriate item from the dialog box. The output will look like this if the "Displacement" option was selected:
(8) To exit the application choose "Quit", or command-Q, from the menu "File", or close the main window by clicking on the go-away button in the upper left hand-side of the window.
III- Examples on how to use the software:
* Start the program by double-clicking on the application icon.
* If no mechanism appears choose "Update Four-Bar", and the screen should looks like this:
The red link will represent the driver link, as if there is a motor at the ground link. The blue link will denote the follower link. It is possible to have different linkage motion if the roles of driver and follower are switched as we will see later.
The next step is to alter the default mechanism to suit our design. This can be accomplished in two ways. One is to "drag" the pivots with the mouse as explained earlier in this manual, or the second method is to use the dialog box by choosing the appropriate item in menu "Input Data".
1- Item "Dimensions ...":
2- Item "Kinematics ...":
3- Item "Input Angle Increment ...":
* The results can be viewed numerically or graphically by choosing the appropriate item(s) in menu "Analysis" (graphical) or menu "Output" (numerical).
Graphical representation example:
The vertical line on the plot indicates the current position of the mechanism. If the current value is desired, click the mouse button in the plot window and the cursor will display the value of the selected point in the window.
Numerical representation example:
* The input link can be made to switch between R2 and R4 to view the changes in the range of motion that might result:
* The velocity and acceleration directions can be made to appear at the tracer points. To view these vectors for the whole coupler curve just animate the mechanism (command-A).
* If the screen gets too crowded or some details are too small to see, zooming will be of great help especially the zooming rectangle demonstrated below on a slider mechanism:
* The instant centers for the four-bar mechanism can be displayed using the menu "IC":
The big dot represent the current IC's and the smaller ones are for the rest of the motion.
