|Animating a mechanical arm
using Forward Kinematics © Giulio Porta 2001
For this project you'll build a mechanical hand/arm, but not necessarily like the one here to the right. It could have less than five fingers and it does not have to emulate the human hand in all of its details. The model that you'll build for this project must be segmented. If you are planning to hide the spaces between the joints with knuckles rings or something like that, these extra pieces should be Union permanently to the finger' phalange they belong to. The reason for it is to avoid complicating the hierarchy that you'll set up to animate the hand.
|The assumption that I am making
here is that you already have the model built.
The Forward Kinematics set up is very simple, and yet because it may involve many repetitious steps, mistakes can be made. It involves as the name says working forward, from the extremities of each finger (one at the time), by setting up an hierarchy or "chain" by gluing forward toward the wrist each hand's piece. From this "chain" each link can be undone so to be (in the case of this hand), rotated, but the hierarchy remain intact.
Begin by selecting the tip of a finger, which one does not matter,
1) Click on the Axis icon and move the axis to the knuckle around which that phalange will rotate.
|2) Once the Axis is
relocated, deselect it by clicking on it again.
3) Select the Glue as a Child icon in the undo drawer on the left side of the menu bar, do not use Glue as Sibling. Go on to glue the finger tip to the second phalange.
4) Click on the Axis icon again, and move the new axis to the second knuckle, as you see here to the right. >>>>>>>>>>>
|Notice that these new axis are
growing in size as you keep adding new pieces to the hand's
hierarchy. The reason being: they include more information. It is also an
indication that you are doing things right.
Now that you have completed one finger, test it out, see if each phalange will rotate correctly.
|You should have anywhere between
2 and 3 view open at this point. Select a side view to rotate the finger
in. To move up and down the hierarchy so you can free each phalange or
group of phalanges at the time, use the arrow keys on the key board.
The ones pointing up and down.
Hit the down arrow key on your keyboard, the base of the finger becomes highlighted, click on the two orange colored phalanges, they will turn white. Hit the down arrow key again so only the tip of the finger is highlighted in white. In a side view R+ click and rotate the finger tip. Hit the up arrow key on your key board to go up to the mid phalange and the finger tip, rotate those, hit the up arrow again to move up to the base of the finger and rotate that. Try to create a simple gesture.
|All you'll have to
do now is to repeat the same operation for each finger. By gluing each
finger phalanges starting from its tip to its base. Each finger should be
set up with identical hierarchies.
After each finger is completed, glue each finger as a child to the palm, select the palm's axis and move it to the base of the palm, deselected it, and glue the entire group to the wrist, again move the axis to the the base of the wrist so the palm will rotate around it. Glue the entire group to the arm and move the axis of this new group to the base of the arm.
|This completes the hierarchy set
up. You are now ready to animate the hand.
There are so many possibilities insofar as which finger will move first and of how much, that it may not matter exactly what you do.
|Having the hand playing a piano, or a keyboard can be challenging animation to create. I'm not going to take you through making the keyboard, what is important about it are the relocation of the keyboard's keys axis, they should be moved back to the keys pivot point, where the keys will rotate up and down.|
the hand to start playing the keys, its fingers need to be in the right
position to begin with: Hit the down arrow key on your keyboard,
the hand will turn orange and the arm white, basically you are going down
backward the hierarchy.
Click on orange part of the hand the hand will turn white, Hit the down arrow key
|The pinky is now white, it means the it can be manipulated. In a side view as you can see here to your right >>>>>> R + click and rotate the pinky downward, but without making it touch the keyboard, we'll do that later when the fingers are in place.|
|Click on the hand
again the palm and the remains of the fingers are white, hit the down
arrow again and the next finger will be highlighted white.
Click on the palm again hit the down arrow again
|Your goal is to get
to the thumb, which is the last piece of your hierarchy, when the thumb
turns orange click on it to highlight it and rotate it downward to line up
its tip with that of the pinky.
Make sure to use the side view and R + click for all your rotations.
idea is to have the thumb and pinky hit the keyboard at the same time, as
that happens the two keys will rotate downward under the pressure of the
fingers. In the animation panel type 15 as the current frame
|Select, one at the
time, the two keys that will come into
play and at frame 15 rotate them downward.
Next take the entire hand (you'll have to go back up the whole hierarchy with up up key on your keyboard) and move, or rotate it so the thumb and pinky will touch the two respective keys.
play it action, click on the play button, you will notice that the keys
are moving downward starting at frame 0, before the fingers touch them.
To remedy that problem we'll need to retard the action of the keys so they will not move before the fingers its them.
Click on the Key Frame Editor to open it
F Editor is the scene bookkeeper, in it, you'll see a list of all the
objects in the scene and their actions over time represented by a time
line (a gray straight line with a square at each end.
The objects, (unless you have name them), will have generic names, you can rename them here by double clicking on their name followed by the F2 key, in the blue box you can retype their name.
The object in questions the keyboard key that move too soon, will have a + in front of their name, click on the + to open the object tree, the next branch is Animation, click on the + in front of it to see what kind of animation the object has, it should be Rotate, everything check out.
Notice that two diamond shapes appear below the timelines for Cube 7 and Cube 11 the two keys on the keyboard.
What you'll need to do now is to group the timelines and keyframes and drag them to the point in time when the fingers touch the two keys. There is no marquee tool to select before making a selection, just drag around what needs to be selected and drag it to about frame 7 at that point the fingers should start coming in contact with the keys, activate the side view to see exactly when.
Now you can render the scene to file as an .AVI
|Sound can be added to an .AVI by using Platypus Animator, a shareware, that among other things will ad sound to an .AVI easily. Look for an upcoming tutorial adding sound with Platypus.|
|The articulation and the actions performed by the hand can be more complex, there will be two more tutorial coming up up soon dealing with those possibilities.||The sound I added to this .AVI is one of the MS sounds: the ring. You'll need to turn your speakers on to hear it, it will only play 15 times and stop.|