Mood Brightener: ...more from Stay Homas. (Confination VII)
Sensors:
Sensors are how we detect and interact with the world. The word comes from the same root as the senses that we describe ourselves as having. All sensors depend on some characteristic of a material that changes in response to a stimulus. I know that sounds fairly formal but lets explore it first through the sensors we know best.
Vision (eyes):
Without getting into the biological mechanisms we can still notice that light enters our eyes and stimulates particular cells on our retina. Those cells produce an electrical signal that depends on the color and intensity of the light which 'hits' them. Some retinal cells only sense intensity. The stimulation of the light produces electrical signals which travel up the optic nerve where our brain somehow makes sense of them. This is why we can, in crude ways, insert electrodes into the brain to create something like a very low resolution image in the mind of a blind person.
Smell:
Smell works much the same as vision. The molecules which are the 'smell' can stimulate certain receptors in our nose. When those receptors are stimulated they send electrical signals to our brain the we 'interpret' as a smell. Why some things smell good and others less good is complex and will keep neuroscientists busy for several more lifetimes I am sure.
Sound, Taste, and Touch:
Different detection mechanisms but same process.
Physical Sensors:
The sensors we use in the world that are not based in biology work a similar way. There is something in the sensor that produces some measurable change when it is stimulated. We then use some form of technology to detect and quantify that change which we can convert into a measurement.
Sensors are everywhere in our lives, often in ways that we barely notice. What are some examples that you can think of? We will take a moment to discuss this in class.
[ cell signals, themometers everywhere, flame detectors on gas stoves, door lock sensors on microwaves, body weight sensors in cars,.....it goes on and on...]
Passive and Active:
As I was writing these notes I realized that I think of sensors as coming in two broad categories. This is not an uncommon thought as it turns out but, as you might expect, different people use the same descriptors to mean different things. Passive and Active are words that people use to distinguish some kinds of sensors from other. For me this description of passive and active sensors from NASA matches pretty well. In this definition a passive sensor is one that receives a stimulus from the the world as it is. By this definition all of our human sensors are passive sensors. A thermometer is a passive sensor as is your cell phone when it is detecting the signal strength you are receiving. An active sensor in this definition is one that must both create the effect and then detect it. Radar meets this definition. You must send out the radar signal for it to bounce off the clouds and rain to create a weather image. Echolocation (whether by bats or humans) is an active sensor system. In some sense, when you go out in the dark with a flashlight, you and the flashlight together form an active sensor system -- you shine the light on things so it will be reflected back to your eyes to be detected.
Another way to think about this is simple passive sensors are only a detector. More complex active sensors contain both a source AND a detector. A thermometer is only a detector. A weather radar has both as source and a detector. All sensors have a detector but only active sensors have a source!
Our Robots:
Our robot vehicles have two types of sensors on board. There is the Ultrasonic Ranger (the eyes on top) and at least 4 IR sensors in different places. Both these types of sensor systems would be considered active sensors by the NASA definition. The ultrasonic ranger sends out a pulse of sound which is then detected when it comes back (as you saw in the earlier homework problem). The IR sensor acts like a flashlight continously 'shining' at the world and the detector notices if that IR light is reflected back from something.
Both of these sensors use waves as the signal that is detected so we need to spend some time exploring core ideas about waves before we move on to discuss the details of the sensors on our robots.
Wave Basics:
A good place to start is with ocean waves which most people have experienced. There are 3 important characteristics of a wave that we need to have some conceptual clarity about. One is the speed of the wave through whatever medium is 'carrying' the wave. One is the height of the wave which we more formally call the amplitude. The last is the wavelength or frequency of the wave. How are each of these characteristics of a wave illustrated in this slightly dramatic video clip?
If you compare ocean wave to ripples on a pond or waves traveling along a rope you will recognize that different waves travel at different speeds. It turns out that the speed of the wave is determined by the physical characteristics of the medium through which it is moving. Waves in fresh water and salt water travel slightly different speeds. The depth of the water also affects the speed of the wave. Sound waves travel different speeds in different materials which is ultimately why inhaling helium makes your voice sound like Mickey Mouse. Even the temperature of the air affects the speed of sound. Wave speeds are usually measured in m/s though sometimes in km/s if they're fast.
Concept 1: The speed of a particular type of wave depends on the characteristics of the medium through which it moves.
We're also pretty clear that big waves are scarier than small waves. Big and small are terms we use to describe the height of the wave. If you look carefully at the ocean you will realize that the wave is doing two related things as it travels. It's lifting up part of the ocean and pushing down other parts of the ocean. In a general sense the amount of lift or pushing down is called the amplitude. The height of the wave from the bottom of the trough to the crest of the wave is twice the amplitude (the amplitude lifted up + the amplitude pushed down). If you watch the ocean carefully you will discover that the speed of the wave doesn't really care about the amplitude. Big waves and small waves travel at the same speed in the same medium. What is different is that big waves have more energy. Loud sounds have more amplitude. Big earthquakes have waves with greater amplitude. Amplitudes are usually measured in m for physical waves though sometimes this is difficult.
Concept 2: Waves with a greater amplitude have more energy.
Now think about the small waves you see when you toss a rock in a still lake or pond. Have you noticed that the amplitude of the waves (ripples) gets smaller and smaller as they spread out? The entire energy of each wave (ripple) is getting spread out across a bigger and bigger circle as it progesses outwards. That would suggest that while the total energy in the wave might be pretty consistent the amount of energy in each part of the wave is getting less. How does this explain why sounds get fainter and fainter the farther you are from the person shouting at you? Is this consistent with the idea of lights getting dimmer and dimmer the farther away you are? Does this seem to apply to lasers or waves at the beach? What is the difference?
Concept 3: Waves that spread out from a single source rapidly lose their amplitude (proportional to the square of the distance)
When we're at the ocean we are very conscious of a feature of those waves that can be decribed as the distance between successive crests. This distance is small for ripples on a pond and quite large for normal ocean waves. For tsunami this crest to crest distance can be more than 100 km. This crest to crest feature of waves is called the wavelength. The symbol for wavelength is λ. This is the greek character called lambda (lower case) and it is the equivalent of an 'l' which is a thoughtful choice for a length! Even waves that we can't see have a wavelength like sound waves or light. Sound waves have wavelengths that are closely related to the size of the instrument that produces them. You might expect a flute to produce sounds with wavelengths of 30 cm or less while a tuba would produce waves that are 5 m or more in wavelength. Stringed instruments are similar though the string produces a string wave when plucked or bowed which is then converted to a sound wave of a very different wavelength. While we don't see it so much in ocean waves most waves can have a wide range of possible wavelengths. Wavelengths are measured in m.
Concept 4: The wavelength (λ) of a wave is the distance between successive crests.
Here is a representation of the wavelengths of different kinds of light (EM waves).
Here is a very similar graphic for sound waves. You will no doubt notice that it is labeled in different units than a length. We will address that in a moment but the core idea that infrasound has long wavelengths and ultrasound has shorter wavelengths is correct.
The next thing to notice about waves shows up when you stand in the surf. What you will have noticed is that there is a consistent and predictable amount of time that elapses between the arrival of each crest. We call this time the period (T). We're noticing the same thing in a slightly different way if we say that in some period of time, a second, a certain number of crests will arrive where we are. We call this the frequency (f). Conceptually is a relationship between T and f because a big period means a small frequency and vica versa. Mathematically we say f = 1/T or T = 1/f. Frequencies are measured in Hertz (Hz on the plot above) and periods are measured in seconds.
Concept 5: The period (T) and the frequency (f) describe how often crests arrive at a point in the world. They are related by the expression f = 1/T.
Last idea! As you stand in the surf you will notice, if you stop playing in the waves and do a little science for a moment, that the crest of the wave travels 1 wavelength in 1 period on it's way to you. This leads us to the most important mathematical tool we have for waves.
vwave = wavelength/period = λ/T = λ*f
Concept 6: vwave= λf or vwave= λ/T
Here's a quick illustration of how the units in the wave speed equation work out. This is based on the HW question below about the speed of light.
Whew! That's a lot of concepts but now we have the tools to have a better understanding of the sensors in our robot!
Assignment Breadcrumb Reading: Bb Quiz
Can you see/hear?
Refer to the images in the Breadcrumb. Can I see light that has a wavelength of 1 μm and can I hear sound that has a frequency of 10 Hz?
Before Next Class:
Assignment HW: Bb Quiz
Wavelength of Middle C:
The note in the middle of a piano keyboard is called Middle C. The sound it produces has a frequency of 256 Hz. Knowing that the speed of sound in air is 343 m/s calculate the wavelength of this sound. Note that Hz is a unit which is the same as (1/s) which you will need to be sure your units work out.
Assignment HW: Bb Quiz
Speed of light:
Consider the image below which shows the frequency and wavelength of different colors of visible light. Using Concept 6 calculate the speed of light from the data on the image. Notice that the frequency is in THz and the wavelength is in nm (remember those prefixes!)
Looking Ahead:
Look ahead to the next Breadcrumb: Sensor II
Assignment Breadcrumb Reading: Bb Quiz
Why Stealth:
What is the primary reason the B-2 Stealth Bomber is hard for any radar to see?