Controlling a Sprite with Ultrasound

Transcription

1 Controlling a Sprite with Ultrasound How to Connect the Ultrasonic Sensor This describes how to set up and subsequently use an ultrasonic sensor (transceiver) with Scratch, with the ultimate aim being to control a sprite with ultrasound for application within a game. An ultrasonic transceiver is easy to use, robust and relatively inexpensive. It emits ultrasound waves and measures the time taken between sending a signal and receiving it. The theory behind ultrasound, in particular both its application in industry and within nature, can be found on page 9. Required Components: 1 ExperiSense board, Arduino (UNO) + USB cable (type A to B) 4 x double-ended crocodile clip cables 4 x M/F breadboard jumper cables (see image) An ultrasonic transceiver, e.g. HC-SR04 sensor (see image) Tape or blue-tack 1 ruler (optional: for configuration) Ultrasonic Transceiver in Action. To obtain the most consistent results it is important to ensure the ultrasonic detector is fixed in place. Below you can see the sensor in action.

2 Step 1: Carefully secure the ExperiSense board onto the Arduino UNO; the prongs of the ExperiSense board should fit perfectly into the holes on the Arduino (see left image below). Step 2: The ultrasonic sensor needs to be connected to the ExperiSense board, which is achieved using the four double-ended crocodile clip cables and the M/F breadboard jumper cables; one crocodile clip end attached to the ExperiSense board, the other end attached to the male hookup pigtail, and finally female hookup pigtail connected to the ultrasonic module via one its 4 pins, Gnd, Vcc, Trig and Echo : The Ground ( Gnd ) and Vcc pins of the device need be connected to the GND and the 5V rails on the ExperiSense board, respectively (see green and blue cables in images below). [N.B. The GND pin must be connected first, before the Vcc pin.] The Trig and Echo pins should be connected to the two EXT ports EXT1 and EXT2, respectively (see red cables in images below). Step 3: Secure the ultrasonic device so that it lies horizontal (we used blue-tack here). Step 4: Finally, connect the Arduino to the computer via (type A to B) USB cable.

3 Controlling a Sprite with Ultrasound How to Use the Ultrasonic Sensor Sensor boards that work with Scratch, such as the PicoBoard, are limited by their input-only approach, centred around resistance readings of 0 to 100. As a result, it was previously not conceivable to use a complex device like an ultrasonic transceiver with Scratch. However, with the development of ScratchX, this is now possible through the employment of an Arduino. ScratchX, which can only be accessed online, grants you the ability to play with experimental extensions to Scratch, allowing the creation of Scratch projects that connect with external hardware (such as electronic devices and robotics) and online resources (including web data and web services). There are three types of ultrasonic transducer: transmitters, receivers and transceivers. Transmitters convert electrical signals into ultrasound, receivers convert ultrasound into electrical signals, and transceivers can both transmit and receive ultrasound. Therefore, if a transceiver is used, by measuring the time between sending a signal and receiving an echo the distance of an object can be calculated and illustrated in ScratchX. We feel the best way to visualise the results from the ultrasonic transceiver is to control a sprite s movements; a game has been created in which the player helps a bat navigate back to its nest, avoiding obstacles as it does so, using ultrasound as the method of control. The following instructions will initially explain how to convert information from the ultrasonic device into a sprite s movements (steps 1-5), and then describe how to use this to create a game in the style of Flappy Bird (steps 6-13). [N.B. you can choose to represent this data in any way you see fit]. Step 1: Before you can start using the ultrasonic sensor you ll first need to get the Arduino UNO, and hence the ExperiSense board, communicating with ScratchX see How-to Guide for the ExperiSense board. Step 2: Create a new sprite (we chose a bat since they use a variety of ultrasonic ranging techniques, called echolocation, to detect their prey and avoid obstacles see page 9 for further information).

4 Step 3: Create a variable called Distance / cm. To create a variable go to the Data tab, click on Make a Variable, type in a name and make sure For all sprites is selected, then click OK. Tick the box next to this variable so its value displays on screen (this can be unchecked later on). Step 4: Create the following sequences of script for this sprite. You will find the blocks in Events, Control, Motion, Looks, Data, Operators and More Blocks tabs. The block that reads ultrasound echo time (µs) is a custom block that has been built by us that reads the time measured by the ultrasound transceiver this will be in µs (10-6 s). Some things to note: You want the sprite to show when the green flag is clicked, and also want to push it to the front so that it appears in front of every subsequent backdrop sprite. Converting time into distance. Distance can be calculated using the following formula: speed = distance / time. The ultrasonic signal emitted from the transceiver travels at the speed of sound, which is roughly 340 m/s, or cm/µs. The time measured by the ultrasonic device is the time taken for the signal to reach an object and then return to the detector, hence this value will need to be divided by 2. Therefore, the distance (in cm) between the ultrasonic device and an object is obtained by multiplying the measured time by and dividing by 2. Mapping ultrasound distance to sprite y-position. To map the distance between the ultrasonic transceiver and an object to the y-position of a sprite you will need to set a maximum and minimum distance; we chose 0 and 30 cm since these are the limits of a 30 cm ruler. Since the y-axis in Scratch extends from -180 to 180, to map the measured distance to the y-axis we need to multiply the distance by 12 and subtract 180. Step 5: Click the green flag and you should now be able to control the y-position of the sprite by vertically moving an object (e.g. your hand) with respect to the ultrasonic transceiver. You are now ready to make your game the following steps will guide you through how to create a game in the style of Flappy Bird, whereby the player s character is controlled using ultrasound.

5 Step 6: Draw your title screen as the stage. You should write the title of your game and any instructions to the player, such as Press Spacebar to start (an example is shown below). Next, introduce a blank screen as a secondary stage backdrop. Now create the following sequence of script for the stage; you will find the required blocks under Events and Looks since it not possible to hide the stage, you want to switch to a blank backdrop when the game is started. [N.B. since it is not possible to add motion to a stage all future backdrops will have to be sprites.] Step 7: Now create a 2 nd sprite using the same image as the stage but without the text you will have to save the image to your computer and then upload the image when creating a new sprite. Then, create the following sequences of script for the stage; you will find the required blocks under Events, Control, Looks, Motion and Operators. Things to note: You want this sprite to be hidden until the game is started, hence the hide and show blocks. You want this sprite to start in the centre of the screen, at (0,0), in the same position as the stage backdrop. The speed at which the sprite moves across the screen can be adjusted by changing the value in the change x by (-2) block. Once this sprite has just gone off of screen to the left you want the 3 rd backdrop sprite to begin moving. This is achieved using the broadcast (3 rd Forest) block.

6 Step 8: Duplicate the 1 st backdrop sprite and edit the obstacles and colours therein. Then copy the previous two sequences of script, changing the starting x-position of this backdrop sprite to 480 instead of 0, which is just offstage to the right, as well as the message that is broadcasted you want to trigger the movement of the 4 th backdrop sprite (e.g. 1 st Cave). Step 9: Create a 3 rd backdrop sprite and again copy the sequence of script from the 2 nd backdrop sprite. You will need to use the When I receive (3 rd Forest) block from Events in place of the When (space) key is pressed, which means that this sprite begins to move once the 1 st backdrop sprite has just passed off screen to the left. You will also need to change the broadcasted message within this script, so that the backdrop two away from the current one is activated, i.e. the 3 rd backdrop sprite broadcasts to the 5 th backdrop sprite, the 4 th to the 6 th etc. (see image below). Step 10: You can create as many backdrop sprites as you like, the more you create the longer your game will be. Remember to name your sprites to make things easier, such a broadcasting messages. For each new sprite you create, copy the sequence of script from the 3 rd backdrop sprite changing only the messages that are broadcasted. Both the received and broadcasted messages need to be changed, with the broadcasted message intended for two backdrops ahead (see images below for further clarification). Step 11: You will need to establish an ending for the game, such as a final location or an item to collect, which should be incorporated into one of the backdrop sprites (we chose the bat s nest as a final location that the player must reach to win). It is a good idea to make an additional backdrop sprite after the one containing the end goal, which can t be progressed beyond, just in case the player misses the item they have to collect or the place they must reach.

7 Step 12: There should be consequences for reaching the end goal, and for hitting obstacles that you have drawn. To accomplish this, along with adding some realism, create the following sequences of script for the player (bat) sprite; you will find the required blocks under Events, Control, Looks and Sensing. Things to note: Frequently changing the bat s costume will make it look like it is flapping its wings as it flies (see below left image). Once the player touches an obstacle a Game Over message is broadcasted (see step 13), the sprite disappears and all scripts are stopped for this sprite. The colour is that of each obstacle (e.g. green for the grass on the forest floor), thus you want to copy this sequence as many times as there are different obstacle colours. Once the player touches the end goal a Game Won message is broadcasted (see step 13), the sprite disappears and all scripts are stopped for this sprite. The colour is that of the colour present in the end goal. Step 13: Next, make two more sprites that indicate that the game is over, one for when the game is won and another for if the player dies. You will need these two simple lines of code; the required blocks can be found under Events, Control, Looks and Motion.

8 Extension Task Adding to your game 1. Try to incorporate difficulty levels into your game; have easy, medium and hard difficulty settings whereby the backdrop movement speed increases as the difficulty increases. [Hint: make use of variables see images below.] 2. Maybe add some collectables, such as insects or fruit (these are the typical foods of bats), for the players to pick up as they fly towards their nest you could include a score that increases as more food is collected, or perhaps a constantly decreasing life meter that refills as food is collected? [Hint: create sprites that appear at specific times that move across the screen at the same speed as the backdrops.]

9 Theory behind Ultrasound What is Ultrasound? Ultrasound = sound waves with frequencies higher than the upper audible limit of human hearing, from 20 khz (20,000 hertz) up to several gigahertz (GHz). Ultrasonic is an adjective referring to ultrasound. Ultrasonic devices are used to detect objects and measure distances. Uses of Ultrasound Motion sensors to detect movement/distances, e.g. automated doors, air fresheners, car parking sensors, burglar alarms. Non-destructive testing to detect invisible flaws in products and structures. Underwater range finding (Sonar) to detect movement/distances, e.g. submarine navigation. Medical sonography the imaging of internal body structures, or babies during pregnancy. Physical therapy for treating connective tissue like ligaments and tendons. Biomedicine to break up stony deposits or tissue, or accelerate the effect of drugs. Also; cleaning, mixing, humidification, (sono)chemistry, and wireless communication. Animal Echolocation Echolocation = the locating and identification of objects using reflected sound. Echolocating animals emit calls (ultrasound) out to the environment and listen to the echoes of those calls that return from various objects near them, then use these echoes for locating objects in various environments ( ranging ). Ranging is achieved by measuring the time delay between the animal's own sound emission and any echoes that return from the environment. Echolocation is used by several kinds of animals, e.g. bats, dolphins, whales and shrews. Bats (microbats) Microbats use a variety of ultrasonic ranging techniques to navigate and forage (hunt), generally detecting frequencies between 14 khz and 100 khz. Individual bat species echolocate within specific frequency ranges that suit their environment and food source. Ultrasound is generated via the larynx and emitted most commonly through the open mouth. They mostly emerge from their roosts in caves, attics, or trees at dusk to search for food. They can be found living in very different environments, and their food sources can differ between insects, frogs, nectar, fruit, and even blood.