need help with simple robotic mechanism

[brunoscience]

This is a Halloween project. I want to make two life size human legs on a table  to move by remote control. I am an experienced builder, but I have no experience in robotics. Can someone please explain to me what exactly I need to make it work ? 

See picture for details. In the drawing the gray is the table, the red parts are the legs. (I want it to go from 1 to 4).

I want the legs to be fixed on the blue line and I only want the knee of one leg to fold by a push of a button on a remote.

(What I know is, that I need a robotic part at the knee, that will connect the upper and lower half of the leg, and at the push of a button make it bent. And than straight again.)

I would appreciate any help or ideas.

Thank you,
Bruno

[waltr]

If the heel is always on the table it could ride in a slot and is pulled/pushed bu a number of methods. One is a cable loop that is moved by a gear motor. This would be much simpler than trying to build a knee joint.

[brunoscience]

thank you for the idea walt.
I am trying to make it as realistic as possible, because the legs will be on display at a halloween exhibit.

I want all the mechanism to be where the hips are (the blue line) or where the knee is.

I saw a video of a lego crane on youtube and I got an idea to apply a rotation at the hip (blue line = axis or rotation).

What do I need to make it move in two directions (up and down) at the hips and how to make it remote controlled ?

Is there a motor that has different speed at different directions ? For example, I want the leg to bent slowly (in diagram pictures 1 to 3), but to straighten really fast (in diagram picture 4). I want to create the effect as if the person is in pain, so I want to make it look as if he is kicking or jerking his legs. 

[Cristi_Neagu]

That could be accomplished with a standard DC motor. All it needs is good timing. You send a pulse to it long enough for the knee to rise as high as you want, and then cut the power. The leg will just fall down.

If you want it automated, it will look awkward if some kind of random pulse generator isn't used. But if you'll be pushing a button on a remote to do this, then it should work ok.

Good luck.

[waltr]

To put the whole mechanism in the knee would require some design skill.

But to move the hip the mechanism could be hidden under the table. An extension of the femur past the joint could be used as a level. Pull this lever down to raise the leg.