Accelerometers

Hi, this is Richard Mitchell, talking about accelerometers, which we use for finding out whether a robot is going up a hill, and more. So accelerometers. We may want to know if a robot is going up a hill. We might want to know at what angle a robot gripper is, or if a robot has bumped into something. All of these can be determined using accelerometers. But accelerometers have other uses. For instance, modern smart phones have them for setting, for instance, whether you display things in landscape or portrait mode. Cars also have them, triggering an airbag if you crash into something. So what’s the basic idea of an accelerometer? Well, according to Isaac Newton, force equals mass times acceleration.

If you apply a force to an object, it accelerates. That is, it speeds up, or down, or it changes direction. Similarly, if an object decelerates by bumping into something, for instance, a force is generating. Another force that we all experience here on Earth is gravity. An accelerometer is a device which measures either force or acceleration. Details of how they work. Quite often, these contain microscopic crystals which are stressed by forces, and generating voltages using the so-called piezoelectric effect, which we can measure. So in terms of how we detect whether something’s going up a hill, we think about gravity. Here, we’ve got a diagram which represents an object which is on a horizontal surface.

Gravity is acting on that object, pulling it downwards. So the force down is perpendicular to the surface. If however, the object is on a slope, like this, gravity is still operating vertically. But part of that force is operating in this direction, which is perpendicular to the surface, and some of it is going down parallel to the surface. And it turns out, if the slope is a particular length l, and it rises by amount h, then the force parallel to the surface is the normal weight due to gravity multiplied by h/l.

Now, for robots, we have so-called 2D or 3D accelerometers, and these measure forces in directions which are at right angles to one another. They can be used to determine the angle of a robot. Is it going uphill or not. But they are used also on smart phones. For instance, to determine whether or not you display in this mode, portrait mode, or in this mode, which is landscape mode. Let’s demonstrate this with another web page. This web page illustrates accelerometers and how they work on mobile phones. We’ve got an image of a phone like this, and this vertical line here is showing an indication of the gravitational force coming down. Effectively, the phone is vertically.

And what we see is the force in the y direction is 3, due to the weight of the phone. But if I start to rotate the phone to the right, we now see that, not only is there a force in this direction, which is smaller than it was, but there’s also one in the perpendicular direction. And the y-force now has gone down from 3 to 2.6, and now the x-force is 1.5. Go a bit further, the two forces are about the same. Further, still, and we now see that the force in the x direction, 2.6, which is greater than that in the y direction, as a result of which, the phone is now moved to landscape mode.

Put it back, back into portrait mode. You can also tell the angle of your phone, which is what we use in order to find out whether the robot is going uphill. So if I tilt backwards, what we see is, if you at the y-force, it’s gone down from 3 to 2.6. It’s now down to 2.12, and so forth. OK.

So we’ve seen that accelerometers can be used to measure the slope a robot is on, we will use this, as we’ll see next week, to reduce the rocking motion that the ERIC robot has. We could also use them as bump sensors so that, for instance, when the robot hits a wall, it decelerates, and we could measure that force. In fact, we don’t do it that way, because you can also detect whether the robot has bumped into a wall by when you’re commanding it to go forward, but it’s not moving at all. Anyhow, you’re free to investigate the web page which simulates the phone to see what happens as you move it around.