This is a tutorial about sensing the environment using a Raspberry Pi. measure a digital input (from a button) output a digital signal (to an LED)

Transcription

1 Practical 2 - Overview This is a tutorial about sensing the environment using a Raspberry Pi and a GrovePi+. You will learn: digital input and output measure a digital input (from a button) output a digital signal (to an LED) analogue sampling and synthesis measure an analogue input (the Rotary Angle Sensor potentiometer, the light sensor) control an analogue output convert an analogue value to a physical value bus-communicated devices measure range to an object (ultrasonic ranging) We will continue with pair programming in these practical sessions. You will need to be in a group of two people (with no more than one group of three people). Each person in the group will alternate between the different roles of driver and navigator. Connecting your Pi Each pair will need to sit at a workstation which is fitted with a KVM switch (keyboard, video display, and mouse) and:

2 a desk mounted Raspberry Pi 3 a green box containing: a GrovePi+ board, a set of GrovePi+ peripherals, and a Raspberry Pi 2 model B (which you won t use) First, shut down the Pi by clicking on the power symbol (top right corner from the login screen), then choose shut down. Once it has completed the shutdown process you should unplug the micro USB power lead and HDMI cable. Now carefully connect the blue GrovePi board, then reattach the HDMI cable, then the micro USB power lead. The Pi will now power up. Press the KVM switch once if you don t see anything. You can log into the Pi using your campus credentials. Before you can use your GrovePi board you must load the driver module. Open a terminal and enter the following two commands: sudo modprobe i2c-dev sudo modprobe i2c-bcm2708 The jedit editor

3 Your Pi comes pre-installed with jedit, a simple text editor: 1. Run jedit from the application menu > Programming > jedit. 2. Save the default file into a new folder at the path /home/practical2 and name the file Button.java You can choose your preferred way of developing: you can continue to use jedit, or switch back to nano. Either way you will need to run javac and java from the command line, so you must set the CLASSPATH environment variable: export CLASSPATH=.:/usr/local/lib/jgrove.jar Digital I/O Individual digital signal lines are two levels, binary (high/low voltage) such as the input from a switch or the output to turn on a buzzer. Pins can be configured for input (where we are measuring their value), or output (where we control their value). Digital inputs effectively have a threshold for the incoming voltage, and result in a binary value. Digital outputs are usually ground (0V) for low 0 and the reference voltage level for high 1.

4 Switched digital input We will write some Java code to interface through the GrovePi+. Start with a class Button, which has a public static void main(string[] args) method. We will need to add imports for grovepi.grovepi, grovepi.pin, and all sensor classes with grovepi.sensors.*. In the main() method, we need to obtain an instance of a GrovePi class: GrovePi grovepi = new GrovePi(); then, from that instance, get its device factory, and use it to create a ButtonSensor instance on pin D4. ButtonSensor button = grovepi.getdevicefactory(). createbuttonsensor(pin.digital_pin_4); Write a loop to continuously poll the state of the button using button.ispressed(), which returns a boolean (true if a press is detected and false if no press it detected). When a press is detected print a message to the screen using: System.out.println("Button press detected").

5 Now try compiling your code (if compilation fails, fix the problem and re-run the last command from your history by pressing <kbd>up</kbd> then <kbd>enter</kbd>). javac Button.java Once your code compiles succesfully, run it using: java Button With your code running, connect a Grove Button to port D4 with a cable. Pressing the button should trigger your message. If you are at the terminal, press <kbd>ctrl</kbd>+<kbd>c</kbd> to stop. You can get the current system time in milliseconds with long System.currentTimeMillis(). Change your loop to measure and display the duration between each button press. hint: create two variables, one to store the time of current button press, and one for the previous button press (you will need to initially set this to the current time). When a button press is detected store the time of this press, then subtract the time of the previous press and print this result (the elapsed time) to the screen. Don t forget to transfer the current button press time into the previous button press time variable before the next iteration of the loop!

6 More interactions Even though the button only gives us basic click information, we can create code to identify different types of interactions, for example, we could distinguish single clicks from double clicks Modify your code to compare the elapsed time between clicks to a threshold (say, 500 milliseconds), if the time between presses is greater than this threshold print single click detected, whilst if the time is shorter print double click detected. Switched digital output Last time, we made an LED turn on with this code: import grovepi.sensors.led; //... Led led = grovepi.getdevicefactory().createled(pin.digita L_PIN_3); led.setstate(true); Connect a Grove LED to port D3 with a cable. If there is no LED installed, put one in: the negative (-ve, cathode) side of an LED typically has a larger plate inside the LED (it sometimes also have a flat edge on the LED dome, and a shorter leg), and the positive (+ve, anode) side of an LED typically has a smaller plate inside the LED (it sometimes has the rounded edge on the LED dome, and a longer leg).

7 Modify your loop so that a button press toggles the LED on or off. Run the code to check. Next, modify your loop so that a single click will turn the LED off, but it can only be turned on with a double click. Again, run the code to check. Analogue I/O Analogue values form a continuous spectrum such as a measure of the amount of light, or the brightness of an LED. So a computer can make use of them, analogue inputs and outputs are converted between a voltage level and a fixed digital representation. Analogue Input: Light sensor Analogue inputs require sampling, a process called analogue to digital conversion (ADC). They are quantized (turned into a fixed representation, effectively an integer number), in both value (e.g. a number of bits resolution over some physical range), and time (e.g. at a given sample rate). In the GrovePi+, ADC sampling is to a 10-bit value (giving 2^10=1024 states) numbers between 0 and 1023 representing the voltage range 0 to 5V. Start a new file ~/practical2/light.java (perhaps begin with a copy of the previous file: cp Button.java Light.java, but be sure to rename the class from Button to Light ).

8 Add an import for grovepi.pinmode. From the grovepi instance, you can perform an ADC reading. Attach a light sensor to the first an analogue input ( A0 ). Write a loop to print the ADC value from the light sensor. You can poll the state of the light sensor with: int grovepi.analogread(pin.analog_pin_0); Analogue Output The inverse of ADC is digital to analogue conversion (DAC), turning a digital signal into a varying voltage. The GrovePi+ kit doesn t include a true DAC, but does support Pulse-Width Modulation (PWM) on three of the digital pins ( D3, D5 & D6 ), which is suitable for controlling the brightness of LEDs. PWM works by having a high frequency on/off signal, and adjusting the ratio of the on/off time (the frequency remains the same). You will need to set the pin as an output, e.g. for D3 : grovepi.pinmode(pin.digital_pin_3, PinMode.OUTPUT);. Now you can use grovepi.analogwrite(pin.digital_pin_3, int value); to control the proportion of the time on (8-bit range, values 0-255). Change your program to act like a torch by providing more light when dark, less when light.

9 Now modify your program to act like an automatic LCD backlight, by converting a range of light readings into a corresponding range of brightnesses: the lighter it is, the brighter the screen (but none to be completely switched off). Converting samples to physical values The GrovePi Rotary Angle Sensor is a potentiometer a knob that varies resistance as it s turned. This change in resistance can be measured as a corresponding change in voltage, by performing an ADC reading.

10 Start a new file ~/practical2/rotary.java (perhaps from a copy of the last program: cp Light.java Rotary.java, changing the class name to Rotary ). This potentiometer is linear, the ADC will give a reading that is proportional to the resistance, and the resistance is proportional to angle. Write your program to read the raw ADC values from the device. Use it to work out the minimum and maximum values for the rotation, store these as constants in your code. Modify your code to calculate the percentage level of maximum rotation. Given that the angular range is 300 degrees, and the change in values is linear with rotation, we can turn this voltage in to physical value. Modify your program to calculate and display the current rotation angle. Range sensing Ultrasonic ranging works by transmitting a sound pulse at a frequency beyond human hearing, then timing how long it takes to appear at the receiver. Create a new source file Distance.java. Make sure you import grovepi.grovepi, import grovepi.pin and import grovepi.sensors.*. Create a new GrovePi object called grovepi,

11 and a range sensor object: UltrasonicRangerSensor rangesensor = grovepi.getdevic efactory().createultrasonicsensor(pin.digital_pin_4); Write a sampling loop to measure and display the distance to the nearest object ( int rangesensor.getdistance() ). Connect an ultrasonic range sensor to pin D4 and run your program. Move your hand to different distances to see the range change. Only if you have time, modify your code to map the distance range to fading LEDs. As the distance decreases to zero, fade up firstly the blue LED ( D6 ) to maximum, then also fade up the green LED ( D5 ) to maximum, then finally fade up the red LED ( D3 ) to maximum. Turning off your Pi Once you are finished, you can safely power down your Pi by pressing the power symbol at the bottom right of the screen then selecting the power off option You should now unplug the USB power lead, then carefully remove the GrovePi board. Next, reattach the power USB power lead. The Pi should re start ready for other students to use.