The Vexmen of Brandywine Robotics

Blog Post

Cool Robot Tests Mimicking Human Motion

Here is an article I came across this week that is trying to get more fluid human like motion in their gestures.

www.engadget.com/2014/10/09/disney-natural-moving-robots

What they are doing is dampening the system to get less jerky movements in the robot arms. If we look at these experiments and compare them to industrial robots we can see a few things about dampening.

First the use of pneumatics and hydraulics give more dampening to the system. What that means is that these types of devices react a wee bit slower than if it were directly driven by a motor and gears. This smooths out the start and stop a bit. As you get more into the math of engineering spring damper systems are a very basic way of controlling a system. The spring part gets you really fast reaction but it tends to overshoot and then oscillate. The damper part holds it back a bit to smooth out the motion and can counteract the springiness.

So in the video you can see the arm with three degrees of freedom being controlled from the guy off camera. As he moves the first arm the sensors tell how much he wants the other arm to move to be like the first one. The tubes run from like motor to like motor on the arms. As the arm in robot one moved the pressure of the air or liquid in the tube changes. This change in pressure tells the other motor to move one way or the other.

This is where dampening comes in. The air or fluid in the line reacts with a slight delay and in less sharp transfer of force. If we were using just electrical signals we would be reading so many times a second and modeling how far we need to go to get robot a to look like b. But with the air pressure telling us how far we are from equilibrium it makes for a smoother motion.

If you compare this motion to typical industrial robot motion you will notice the industrial robot wants to get from point a to point b in the most efficient path. This leads to a fast acceleration and a fairly high velocity and quick deceleration on the way to point b. To the human observer it looks jerky. Dampening smoothes out the acceleration and deceleration. Compare Bruce Lee doing kung fu versus the smoother motion of Ti Chi.

As we get our robots built with vex sensors you will see the values change as you move the arm or wheels or claw that is attached to the sensor. The trick you want to have happen is for your robot to get to the desired position from where your sensor reads right now.

How we do that is a control loop and via a function that is keeping track of where we are, and where we want to be. For those that took the line following class this is proportional control. As we get better at programming we can also add in where we were and where we think we will be going next. This is PID control.

But this video shows how we can do the same thing mechanically via fluid or air dampening. The second video has these little diaphragm based dampers which are as pure of a damper as you can get. It is pretty ingenious to put them on a string on an elbow joint to help control the arm movement. As you move the arm it looks so natural due to the dampening controlling the start and stop. But what they don’t show is the tuning required to get there. Over or under damped systems don’t look quite so natural. They look lethargic or the springiness takes over making the dampening too little to do much of anything..