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Signals - Week 1

For all of our activities and code running, we'll be using the ARM mbed Simulator

Basic Signals

A Signal can be defined as a time-varying quantity that relays some sort of information.

In robotics and electrical engineering, the “quantity” we are referring to here is most often voltage. (And if not that, then current).

  • For the duration of this training program, a signal can be effectively thought of as a voltage that is changing over time.

Signals are passed between devices to send and receive information. We will mostly be working with primitive data types, integers, strings, etc.

Digital Signals

Digital signals must have a finite set of possible values, but in our case we will be primarily working with binary digital signals, either high or low. In software the high is a 1, or true, and the low is a 0, or false.

This signal is 1100111101101001100100000111111101110010010000011001111100000001111110001.

It is very much so garbage random data, but we will be working with data that hopefully isn't.

Analog Signals

Analog signals are signals that are meant to be interpreted in terms of a range.

For example, at time 4.34s, we can see that this voltage is 1.935V. The board can read this to a certain level of accuracy. If we want to read a non-binary value from the environment, this is how that is done sometimes.

PWM (Pulse Width Modulation)

PWM is a type of protocol used with binary digital signals. It is used to simulate an analog output with a digital. We simply control how long the signal is high.

The duty cycle of a PWM is the data you are sending, in the form of a percentage high.

PWM is a crude way to mimic an analog signal using a digital one, however this aspect of PWM is generally only used with leds and motors, as they will work

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This week, you will do a very simple PWM during Exercise 2. It uses a very basic PWM signal generator to control the LED.

PWM as a protocol is also used to control servos and some brushless motor controllers, but we specifically don't use PWM very much anymore, as we have switched to all CAN control for the motors we use. You'll learn about CAN control in week 2.

Exercise 1: Using Digital Signals

The mbed library allows us to very easily manipulate digital inputs and outputs. You can experiment with this using the ARM mbed Simulator.

Creating a digital Input can be done using the DigitalIn Class. It is constructed as such:

DigitalIn input(PinName);

You can get its value the same way you would a variable, or you can use .read()

int x = input; 
int x = input.read(); //alternate usage, both work

Where x will be 1 if the input is high (Button pressed) and 0 if the input is low (Button unpressed).

You can also create a digital output, like an led, simply, with the DigitalOut Class

DigitalOut output(PinName);

like the DigitalIn, the DigitalOut allows you to get the current value of the out by using it as a variable:

int x = output;
int x = input.read(); //again both of these commands will do the same thing

However now you can also set the value like a variable

output = 1; // setting the output high

Assignment

WIth this information, write a simple program in the simulator that binds the LED directly to the button. If the button is pressed, the LED should be on, and if the button is not pressed, the LED should be off. The led pin is LED1 and the button pin is BUTTON1. You can play around with this by pressing the button in the simulator. Once your code is functional, copy it back into the file on github, and then push.

Exercise 2: Analog Input

Now let's mess around with getting our board to input analog signals. This is basically reading the voltage on a pin relative to ground. The AnalogIn has us covered here. AnalogIn functions similarly to DigitalIn (AnalogOut mirrors DigitalOut as well).

AnalogIn anInput(PinName);

double x = anInput;
double x = anInput.read(); //you can use either of these

The nucleo board works on 3.3v logic, that is, when reading and analog input it will map (0, 3.3) volts to a (0, 1) float.

You can add a potentiometer using the simulator, by clicking Add Component, then selecting Potentiometer.

Assignment

Using a potentiometer, have an led flicker with a period of 2s, and be on for that percentage of time (Ex: at 0.3 it will be on 30% of the time, which is 0.6 seconds, and off 70% of the time, 1.4 seconds). wait_ms(milliseconds) might come in handy here. You can create a potentiometer by going to the "Add Components" button and going through the dropdown to potentiometer. Again, once you're done, put this on gituhub and push.