Surveyors Wanted to Explore Mars and its Moons¶

Exercise¶

1. For each path below, program your robot to drive as indicated (if needed use gyro sensor). When it’s done use if statement and/or loop and program robot to repeat behaviour continuously

2. For each path below, program your robot to drive as indicated. Use if statements and/or loops. Complete the following activities with a touch sensor attached to your robot:

   1. Drive forward, stop when the touch sensor is pressed.

   2. Drive (forward / backward), change direction to (backward/forward) when the touch sensor is pressed (repeat behaviour continuously).

   3. Drive forward, turn around when the touch sensor is pressed, continue driving (repeat behaviour continuously).

   4. Wait for the touch sensor to be pressed, drive backward ~50cm (repeat behaviour continuously).

3. For each path below, program your robot to drive as indicated. Use if statements and/or loops. Complete the following activities with both the touch sensor and the ultrasonic sensor connected to your robot:

   1. Drive forward, stop when there is an obstacle within 30 cm of the robot.

   2. Drive forward, turn around when there is an obstacle within 30 cm of the robot, continue driving (repeat behaviour continuously).

   3. Wait until the touch sensor is pressed, then drive forward until an obstacle is within 30 cm of the robot.

   4. Drive forward until either the touch sensor is pressed OR the robot is within 30 cm of an obstacle, then turn to a new random direction and continue driving (repeat behaviour continuously).

Theory¶

The if statement

In order to write useful programs, we almost always need the ability to check conditions and change the behavior of the program accordingly. Conditional statements give us this ability. The simplest form is the if statement, which has the general form:

if BOOLEAN EXPRESSION:

STATEMENTS

A few important things to note about if statements: The colon (:) is significant and required. It separates the header of the compound statement from the body. The line after the colon must be indented. It is standard in Python to use four spaces for indenting.

All lines indented the same amount after the colon will be executed whenever the BOOLEAN_EXPRESSION is true.

Here is an example:

The boolean expression after the if statement is called the condition. If it is true, then all the indented statements get executed.

The while statement

The general syntax for the while statement looks like this:

Like the branching statements and the for loop, the while statement is a compound statement consisting of a header and a body. A while loop

Notice that if the number is set to 42 on the first line, the body of the while statement will not execute at all. Here is a more elaborate example program demonstrating the use of the while statement:

The flow of execution for a while statement works like this:

1. Evaluate the condition (BOOLEAN EXPRESSION), yielding False or True.

2. If the condition is false, exit the while statement and continue execution at the next statement.

If the condition is true, execute each of the STATEMENTS in the body and then go back to step 1.

The body consists of all of the statements below the header with the same indentation.

The body of the loop should change the value of one or more variables so that eventually the condition becomes false and the loop terminates. Otherwise the loop will repeat forever, which is called an infinite loop.

An endless source of amusement for computer programmers is the observation that the directions on shampoo, lather, rinse, repeat, are an infinite loop.

In the case here, we can prove that the loop terminates because we know that the value of len(name) is finite, and we can see that the value of pos increments each time through the loop, so eventually it will have to equal len(name). In other cases, it is not so easy to tell.

What you will notice here is that the while loop is more work for you — the programmer — than the equivalent for loop. When using a while loop one has to control the loop variable yourself: give it an initial value, test for completion, and then make sure you change something in the body so that the loop terminates.

Short help on programming¶

Commands/functions needed for the exercise

will use any button to stop script

#!/usr/bin/env python3

btn = Button()

Connect large motor to port B

mL = LargeMotor('outB');

Run to a position relative to the current position value. The new position will be current position + position_sp. When the new position is reached, the motor will stop using the action specified by stop_action.

#!/usr/bin/env python3

mL.run_to_rel_pos(position_sp= 500, speed_sp = 250)

speed_sp is between 0 and 1000 position_sp >0 Motor will run forward position_sp <0 Motor will run backwards stop_action - can be "coast"or "brake"

mL.stop(stop_action='brake')

Use If you want to make your robot speak

#!/usr/bin/env python3

Sound.speak('Forward').wait()

Plug a gyro sensor into any sensor port

#!/usr/bin/env python3

gyro = GyroSensor()

Read the value from gyro sensor

#!/usr/bin/env python3

gyro.mode = 'GYRO-ANG'

start_angle=gyro.value()

Plug a touch sensor into any sensor port

#!/usr/bin/env python3

ts = TouchSensor()

Read the value from touch sensor

#!/usr/bin/env python3

if ts.value()==0: # touch sensor NOT pressed

Plug an ultrasonic sensor into any sensor port

#!/usr/bin/env python3

us = UltrasonicSensor()

Read the value from an ultrasonic sensor

#!/usr/bin/env python3

us.mode='US-DIST-CM'

Distance=us.value()/10 # convert mm to cm

a ‘bump’ is a release of the touch sensor button

#!/usr/bin/env python3

def WaitForBump():

This function is built into the ev3dev2 library.

  PreviousState=0

  while True:

    CurrentState=ts.value()

    if PreviousState==1 and CurrentState==0: #button was released

      mL.run_forever(speed_sp=400)

      mR.run_forever(speed_sp=400)

      PreviousState=CurrentState # Ready for next loop

      sleep(0.01) # don't read the sensor too frequently

  while True: # forever

    WaitForBump()

    Distance=us.value()/10 # convert mm to cm

    if Distance<30:

      mL.stop(stop_action='brake')

      mR.stop(stop_action='brake')