The Raspberry Pi uses a variety of input/output devices based on protocols such as HDMI, USB, and Ethernet to communicate with the outside world. In this class you will learn how to use these protocols with other external devices (sensors, motors, GPS, orientation, LCD screens etc.) to get your IoT device to interact with the real world. Most physical devices use analog signals; however computer hardware is digital so in this class you will learn how these signals are converted back-and-forth and how this must be considered as you program your device. The basic design of a sensor-actuator system will also be covered. You will also learn how to build more sophisticated hardware systems using Raspberry Pi expansion boards to create fun and exciting IoT devices. Please note that this course does not include discussion forums.
Lecture 3.1 - Servo Code
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來自 University of California, Irvine 的課程
树莓派（Raspberry Pi）接口
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Module 4
In this module we show how to use the Raspberry Pi to interface with more complicated sensors and actuators. We explore the use of the Raspberry Pi camera module and the use of a servo. The Raspberry Pi camera module is used through the picamera library, which we describe. Servos are controlled by generating pulse width modulated signals and varying their pulse width using library functions.
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Ian HarrisProfessor
Department of Computer Science
[MUSIC]
So I'm just going to walk through an example of some code
that manipulates a servo from the Raspberry Pi.
I'll go through it step by step so you can see how it works.
So this is going to be some actually relatively simple code.
It's dumb code, but
what it would do is make the servo scan from left to right back to left.
That's all it'll do.
Over a period it's kind of slow scan.
Slower than that actually, but that's all this code is going to do, so
I just want to show you this example, just walk through each step.
Okay, so we start at the top.
We start by, now this is a throwback actually to
last course when we were talking about the general purpose IO and the Raspberry Pi.
We have to import Raspberry Pi.GPIO.
So we import the GPIO library just so we can manipulate the general purpose pent.
Now first thing we do is we say GPIO.setmode(GPIO.BOARD).
Remember, that's the board pin numbering.
So the pin numbers are two pin numbers you can use, and you gotta select which one.
There's the board pin numbering, which we're using, but there's also the broad
com pin numbering, which we're not using, and that would be GPIO.BCM.
We're not doing that.
We're doing .BOARD.
So we set the mode, then we set up.
We call GPIO.setup making pin 12 an output.
12,GPIO.OUT, because that's where we're going to drive the post with modulating
signal.
That's going to be connected to the signal input of the server.
We're assuming that we're using the wiring that we saw in the last lecture.
Now, then we say pulse with modulation.start.
That start method it just starts a pulse with modulated signal,
starts generating one on the appropriate pin.
In this case pin 12.
Now, we gave it an argument 0.
That's the duty cycle at the beginning, so the duty cycle is from 0 to 100.
So 0 duty cycle means it's low all the time.
So actually this just puts out a 0 all the time but we'll change that.
So then we've got two loops.
For each i in range 100 is the first loop.
So what this is doing is it's iterating through the duty cycle.
So duty cycle goes from 0 to 100.
Actually in this case instead we're saying i in range 10,
it's going to go from 0 to 99, so it won't quite reach 100, but that's close enough.
Maybe i should range 101, but whatever.
So for i in range 100, so each pass through i is going to be 0,
then 1, then 2, and so on.
So pwm.GhangeDutyCycle i, so that just
changed the duty cycle to i, which means in the servo it rotates it a little bit.
So it starts off at 1, then it goes to 2, then 3, then 4 rotating it all the way
through once it gets to 100 or 99 it's basically at a 180 degree turn.
So and I put a small sleep time in between in each iteration right, so
I change the duty cycle rotate a little bit wait 0.1 seconds then rotate it again,
wait 0.1 seconds and so on.
So this is going to do a sweep that first loop just as a sweep from left to right.
Now, the next loop is actually very similar except this time the range is
different ranged from 100 down to 0, and I gave this third argument negative 1.
So negative 1 that third argument to the range function is the step, okay?
It means what change you're going to make to the number every pass throw.
So in the first range in that first loop I just say range 100.
With that it just assumes you want to increment.
You want to add one, but in the second loop I want to decrement.
I want to subtract 1.
So I have the save that, so I say 0 comma, so I say 100 comma 0.
So from 100 to 0 and the third argument I say negative 1 to tell it look,
every pass through you should decrement this.
So it starts off at 100, then 99, then 98, and so on.
So this a range, same range, but in the opposite order, and
this time I just say post modulation.change duty cycle i.
Again, same thing.
So the duty cycle starts off at 99, which is basically 180 degrees, sorry.
It started at 100, which is a 180 degrees, then it goes to 99, 98, 97.
I put the same sleep in between, so
it slowly sweeps the other way, and that's all that this code does.
Now, if I wanted to when you write embedded code you almost
always put it in an infinite loop because what this would do is it would scan to
the left then scan to the right and then it would be done, right?
And it would never do anything again.
It is much more common with embedded systems to have them live, right?
Continually doing something.
So if I were to do that I could take this and
put a wild true around both for loops, right?
So I could say wild true and indent the for loops into that,
and then it would just scan left, scan right, scan left,
scan right forever as long as it received power.
I didn't do that so the way this code is is it will scan left once,
scan right once and that'd be the end of that.
Thank you.
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