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Elisa

    Elisa-3


    1 Overview

    Elisa-3 is an evolution of the Elisa robot based on a different micro-controller and including a comprehensive set of sensors:

    The robot is able to self charge using the charger station, as shown in the previous figure. The following figure illustrates the position of the various sensors:

    1.1 Useful information

    • The top light diffuser and robot are designed to lock together, but the diffuser isn't fixed and can thus be removed as desired; the top light diffuser, as the name suggests, helps the light coming from the RGB led to be smoothly spread out, moreover the strip attached around the diffuser let the robot be better detected from others robots. Once the top light diffuser is removed, pay attention not to look at the RGB led directly. In order to remove the top light diffuser simply pull up it, then to place it back on top of the robot remember to align the 3 holes in the diffuser with the 3 IRs emitters and push down carefully until the diffuser is stable; pay attention to not apply too much force on the IRs emitters otherwise they can bend and stop working.
      1

     

    • When the top light diffuser is fit on top of the robot, then in order to change the selector position you can use the tweezers; the selector is located near the front-left IR emitter, as shown in the following figure:

    • If you encounter problems with the radio communication (e.g. lot of packet loss) then you can try moving the antenna that is a wire near the robot label. Place the antenna as high as possible, near the plastic top light diffuser; try placing it in the borders in order to avoid seeing a black line on the top light diffuser when the RGB led is turned on.

    2 

    1.2 Robot charging

    Elisa-3 can be piloted in the charger station in order to be automatically self charged; there is no need to unplug the battery for charging. The following figures shows the robot approaching the charger station; a led indicates that the robot is in charge:

    3  

    The micro-controller is informed when the robot is in charge and this information is also transferred to the PC in the flags byte; this let the user be able to pilot the robot to the charger station and be informed when it is actually in charge. More information about the radio protocol can be found in the section Communication.

    Moreover the robot is also charged when the micro USB cable is connected to a computer; pay attention that if the USB cable is connected to a hub, this one need to be power supplied.

    Following video shows the Elisa-3 piloted through the radio to the charging station using the monitor application:

    1.3 Top light diffuser

    From February 2013 onwards the Elisa-3 is equipped with a new top light diffuser designed to fit perfectly in the 3 IRs emitters of the robot. The diffuser is made of plastic (3d printed), it is more robust and it simplifies the removal and insertion. Here is an image:

    2 Hardware

    Following figures show the main components offered by the Elisa-3 robot and where they are physically placed: 45

    2.1 Power Autonomy

    The robot is equipped with two batteries for a duration of about 3 hours at normal usage (motors run continuously, IRs and RGB leds turned on).

    2.2 Detailed Specifications

    Feature Technical information
    Size, weight 50 mm diameter, 30 mm height, 39 g
    Battery, autonomy LiIPo rechargeable battery (2 x 130 mAh, 3.7 V). About 3 hours autonomy. Recharging time about 1h e 30.
    Processor Atmel ATmega2560 @ 8MHz (~ 8 MIPS); 8 bit microcontroller
    Memory RAM: 8 KB; Flash: 256 KB; EEPROM: 4 KB
    Motors 2 DC motors with a 25:1 reduction gear; speed controlled with backEMF
    Magnetic wheels Adesion force of about 1 N (100 g) depending on surface material and painting Wheels diamater = 9 mm Distance between wheels = 40.8 mm
    Speed Max: 60 cm/s
    Mechanical structure PCB, motors holder, top white plastic to diffuse light
    IR sensors 8 infra-red sensors measuring ambient light and proximity of objects up to 6 cm; each sensor is 45° away from each other 4 ground sensors detecting the end of the viable surface (placed on the front-side of the robot)
    IR emitters 3 IR emitters (2 on front-side, 1 on back-side of the robot)
    Accelerometer 3D accelerometer along the X, Y and Z axis
    LEDs 1 RGB LED in the center of the robot; 8 green LEDs around the robot
    Switch / selector 16 position rotating switch
    Communication Standard Serial Port (up to 38kbps) Wireless: RF 2.4 GHz; the throughput depends on number of robot: eg. 250Hz for 4 robots, 10Hz for 100 robots; up to 10 m
    Remote Control Infra-red receiver for standard remote control commands
    Expansion bus Optional connectors: 2 x UART, I2C, 2 x PWM, battery, ground, analog and digital voltage
    Programming C/C++ programming with the AVR-GCC compiler (WinAVR for Windows). Free compiler and IDE (AVR Studio / Arduino)

    3 Communication


    3.1 Wireless

    radio base-station is connected to the PC through USB and transfers data to and from the robot wirelessly. In the same way the radio chip (nRF24L01+) mounted on the robot communicates through SPI with the micro-controller and transfers data to and from the PC wirelessly. The robot is identified by an address that is stored in the last two bytes of the micro-controller internal EEPROM; the robot firmware setup the radio module reading the address from the EEPROM. This address corresponds to the robot id written on the label placed under the robot and should not be changed.

    3.1.1 Packet format - PC to radio to robot

    The 13 bytes payload packet format is shown below (the number in the parenthesis expresses the bytes) :

    6

    • Command: 0x27 = change robot state; 0x28 = goto base-station bootloader (this byte is not sent to the robot)
    • Red, Blue, Green leds: values from 0 (OFF) to 100 (ON max power)
    • IR + flags:
      • first two bits are dedicated to the IRs:
        • 0x00 => all IRs off
        • 0x01 => back IR on
        • 0x02 => front IRs on
        • 0x03 => all IRs on
      • third bit is reserved for enabling/disabling IR remote control (0=>disabled, 1=>enabled)
      • fourth bit is used for sleep (1 => go to sleep for 1 minute)
      • fifth bit is used to calibrate all sensors (proximity, ground, accelerometer) and reset odometry
      • sixth bit is reserved (used by radio station)
      • seventh bit is used for enabling/disabling onboard obstacle avoidance
      • eight bit is used for enabling/disabling onboard cliff avoidance
    • Right, Left motors: speed expressed in 1/5 of mm/s (i.e. a value of 10 means 50 mm/s); MSBit indicate direction: 1=forward, 0=backward; values from 0 to 127
    • Small green leds: each bit define whether the corresponding led is turned on (1) or off (0); e.g. if bit0=1 then led0=on
    • Flags2:
      • bit0 is used for odometry calibration
      • remaining bits unused
    • Remaining bytes free to be used

    3.1.1.1 Optimized protocol

    communication between the pc and the base-station is controlled by the master (computer) that continuously polls the slave (base-station); the polling is done once every millisecond and this is a restriction on the maximum communication throughput. To overcome this limitation we implemented an optimized protocol in which the packet sent to the base-station contains commands for four robots simultaneously; the base-station then separate the data and send them to the correct robot address. The same is applied in reception, that is the base-station is responsible of receiving the ack payloads of 4 robots (64 bytes in total) and send them to the computer. This procedure let us have a throughput 4 times faster.

    3.1.2 Packet format - robot to radio to PC

    The robot send back to the base-station information about all its sensors every time it receive a command; this is accomplished by using the "ack payload" feature of the radio module. Each "ack payload" is 16 bytes length and is marked with an ID that is used to know which information the robot is currently transferring. The sequence is the following (the number in the parenthesis expresses the bytes):

    7

    Pay attention that the base-station could return "error" codes in the first byte if the communication has problems:

    • 0 => transmission succeed (no ack received though)
    • 1 => ack received (should not be returned because if the ack is received, then the payload is read)
    • 2 => transfer failed

    Packet ID 3:

    • Prox* contain values from 0 to 1023, the greater the values the nearer the objects to the sensor
    • The Flags byte contains these information:
      • bit0: 0 = robot not in charge; 1 = robot in charge
      • bit1: 0 = button pressed; 1 = button not pressed
      • bit2: 0 = robot not charged completely; 1 = robot charged completely
      • the remainig bits are not used at the moment

    Packet ID 4:

    • Prox4 contains values from 0 to 1023, the greater the values the nearer the objects to the sensor
    • Ground* contain values from 512 to 1023, the smaller the value the darker the surface
    • AccX and AccY contain raw values of the accelerometer; the range is between -64 to 64
    • TV remote contains the last interpreted command received through IR

    Packet ID 5:

    • ProxAmbient* contain values from 0 to 1023, the smaller the values the brighter the ambient light
    • Selector contains the value of the current selector position

    Packet ID 6:

    • ProxAmbient4 contains values from 0 to 1023, the smaller the values the brighter the ambient light
    • GroundAmbient* contain values from 0 to 1023, the smaller the values the brighter the ambient light
    • AccZ contains raw values of the accelerometer; the range is between 0 and -128 (upside down)
    • Battery contains the sampled value of the battery, the values range is between 780 (battery discharged) and 930 (battery charged)

    Packet ID 7:

    • LeftSteps and RightSteps contain the sum of the sampled speed for left and right motors respectively (only available when the speed controller isn't used; refer to xpos, ypos and theta when the speed controller is used)
    • theta contains the orientation of the robot expressed in 1/10 of degree (3600 degrees for a full turn); available only when the speed controller is enabled
    • xpos and ypos contain the position of the robot expressed in millimeters; available only when the speed controller is enabled

    Click on ELISA SOFTWARE

    5 Odometry

    Odometry of Elisa-3 is quite good even if the speed is only measured by back-emf. On vertical surfaces the absolute angle is given by the accelerometer measuring g... quite a fix reference without drifting 😉 A fine calibration of the right and left wheel speed parameters might give better results. However the current odometry is a good estimate of the absolute position from a starting point. The experiments are performed on a square labyrinth and the robot advances doing obstacle avoidance. The on-board calculated (x,y,theta) position is sent to a PC via radio and logged for further display. Details about the code can be found in the advanced-demo project, in particular the motors.c source file. The PC application used for logging data is the monitor.

    5.1 Autonomous calibration

    Since the motors can be slightly different a calibration can improve the behaviour of the robot in terms of manoeuvrability and odometry accuracy. An autonomous calibration process is implemented on-board: basically a calibration is performed for both the right and left wheels in two modes that are forward and backward with speed control enabled. In order to let the robot calibrate itself a white sheet in which a black line is drawed is needed; the robot will measure the time between detection of the line at various speeds. The calibration sheet can be downloaded from the following link calibration-sheet.pdf. In order to accomplish the calibration the robot need to be programmed with the advanced firmare and a specific command has to be sent to the robot through the radio module or the TV remote; if you are using the radio module you can use the monitor application in which the letter l (el) is reserved to launch the calibration, otherwise if you have a TV remote control you can press the button 5. The sequence is the following:

    1. Put the selector in position 8 2. Place the robot near the black line as shown below; the left motor is the first to be calibrated. Pay attention to place the right wheel as       precise as possible with the black line   3. once the robot is placed you can type the l (el) command (or press the button 5); wait a couple of minutes during which the robot will do various turns at various speed in the forward direction and then in the backward direction 4. when the robot terminated (robot is stopped after going backward at high speed) you need to place it in the opposite direction in order to calibrate the right motor, as shown below.

    5. once the robot is placed you can type again the l (el) command (or press the button 5) 6. when the robot finish, the calibration process is also terminated.

    The previous figures show a robot without the top diffuser, anyway you don't need to remove it!

    6 Tracking


    6.1 Assembly documentation

    You can download the documentation from here tracking-doc.pdf. Have a look also at the video:

     

    6.2 SwisTrack

    Some experiments are done with the SwisTrack software in order to be able to track the Elisa-3 robots through the back IR emitter, here is a resulting image with 2 robots:

    Pre-compiled SwisTrack software (Windows) can be downloaded from the following link SwisTrack-compiled.

    Following video shows the tracking of 5 robots:

     

    The SwisTrack software lets you easily log also the resulting data that you can then elaborate, here is an example taken from the experiment using 5 robots:

    The following video shows the test done with 20, 30 and 38 Elisa-3 robots, the tracking is still good; it's important to notice that we stopped to 38 Elisa-3 robots because are the ones we have in our lab.

     

    6.3 Position control

    We developed a simple position control example that interacts with Swistrack through a TCP connection and control 4 robots simultaneously; the orientation of the robots is estimated only with the Swistrack information (delta position), future improvements will integrate odometry information. The following video shows the control of 4 robots that are driven in a 8-shape.

     

    All the following projects require the Elisa-3 library, for building refer to the section Multiplatform monitor.

    One of the characteristics of the Elisa-3 robot is that it can move in vertical thanks to its magnetic wheels, thus we developed also a vertical position control that use accelerometer data coming from the robot to get the orientation of the robot (more precise) instead of estimating it with the Swistrack information, you can download the source code from the following link:

    We developed also an example of position control that control a single robot (code adapted from previous example) that can be useful during the initial environment installation/testing; you can download the source code from the following link:

    Another good example to start playing with the tracking is an application that lets you specify interactively the target point that the robot should reach; you can download the source code of this application from the following link:

    6.4 Utilities

    In order to adjust the IR camera position it is useful to have an application that turn on the back IR of the robots. The following application back-IR-on-4-robots-rev182-30.06.14.zip is an example that turn on the back IR of 4 robots, their addresses are asked to the user at the execution.

    7 Local communication


     

    The advanced firmware is needed in order to use the local communication. You can find some examples on how to use this module in the main, refers to demos in selector position from 11 to 14. Here are some details about the current implementation of the communication module:

    • use the infrared sensors to exchange data, thus during reception/transmission the proximity sensors cannot be used to avoid obstacles; in the worst case (continuous receive and transmit) the sensor update frequency is about 3 Hz
    • bidirectional communication
    • id and angle of the proximity sensor that received the data are available
    • the throughput is about 1 bytes/sec
    • maximum communication distance is about 5 cm
    • no reception/transmission queue (only one byte at a time)
    • the data are sent using all the sensors, cannot select a single sensor from which to send the data. The data isn't sent contemporaneously from all the sensors, but the sensors used are divided in two groups of 4 alternating sensors (to reduce consumption)

    8 ROS

    This chapter explains how to use ROS with the elisa-3 robots; the radio module is needed here. Basically all the sensors are exposed to ROS and you can also send commands back to the robot through ROS. The ROS node is implemented in cpp. Here is a general schema:

    First of all you need to install and configure ROS, refer to http://wiki.ros.org/Distributions for more informations. Alternatively you can download directly a virtual machine pre-installed with everything you need, refer to section virtual machine; this is the preferred way.

    • This tutorial is based on ROS Hydro.
    • If you downloaded the pre-installed VM you can go directly to section Running the ROS node.

    ROS Elisa-3 node based on roscpp can be found in the following repository https://github.com/gctronic/elisa3_node_cpp.

    8.1 Initial configuration

    Following steps need to be done only once after installing ROS:

    1. If not already done, create a catkin workspace, refer to http://wiki.ros.org/catkin/Tutorials/create_a_workspace. Basically you need to issue the following commands:
      mkdir -p ~/catkin_ws/src
      cd ~/catkin_ws/src
      catkin_init_workspace
      cd ~/catkin_ws/
      catkin_make
      source devel/setup.bash 
    2. You will need to add the line source ~/catkin_ws/devel/setup.bash to your .bashrc in order to automatically have access to the ROS commands when the system is started
    3. Clone the elisa-3 ROS node repo from https://github.com/gctronic/elisa3_node_cpp; you'll have a directory named elisa3_node_cpp that is the repo local copy
    4. Copy the repo directory elisa3_node_cpp (this is the actual ros package) inside the catkin workspace source folder (~/catkin_ws/src)
    5. Install the ROS dependencies:
    • sudo apt-get install ros-hydro-slam-gmapping
    • sudo apt-get install ros-hydro-imu-tools
    6. Open a terminal and go to the catkin workspace directory (~/catkin_ws) and issue the command catkin_make, there shouldn't be errors
    7. The USB radio module by default requires root priviliges to be accessed; to let the current user have access to the radio we use udev rules:
    • plug in the radio and issue the command lsusb, you'll get the list of USB devices attached to the computer, included the radio:
    Bus 002 Device 003: ID 1915:0101 Nordic Semiconductor ASA
    • issue the command udevadm info -a -p $(udevadm info -q path -n /dev/bus/usb/002/003), beware to change the bus according to the result of the previous command. You'll receive a long output showing all the informations regarding the USB device, the one we're interested is the product attribute:
    ATTR{product}=="nRF24LU1P-F32 BOOT LDR"
    • in the udev rules file you can find in /etc/udev/rules.d/name.rules add the following string changing the GROUP field with the current user group:
    SUBSYSTEMS=="usb", ATTRS{product}=="nRF24LU1P-F32 BOOT LDR", GROUP="viki"
    To know which groups your user belongs to issue the command id
    • disconnect and reconnect the radio module
    8. Program the elisa-3 robot with the last advanced firmware (>= rev.221) and put the selector in position 15

    8.2 Running the ROS node

    First of all get the last version of the elisa-3 ROS node from github:

    • clone the repo https://github.com/gctronic/elisa3_node_cpp and copy the elisa3_node_cpp directory inside the catkin workspace source folder (e.g. ~/catkin_ws/src)
    • build the driver by opening a terminal and issueing the command catkin_make from within the catkin workspace directory (e.g. ~/catkin_ws).

    Now you can start the ROS node, for this purposes there is a launch script (based on roslaunch), as explained in the following section. Before starting the ROS node you need to start roscore, open another terminal tab and issue the command roscore.

    8.2.1 Single robot

    Open a terminal and issue the following command: roslaunch elisa3_node_cpp elisa3_single.launch elisa3_address:='1234' where 1234 is the robot id (number on the bottom).

    If all is going well rviz will be opened showing the informations gathered from the topics published by the elisa ROS node as shown in the following figure:

    The launch script is configured also to run the gmapping (SLAM) node that let the robot construct a map of the environment; the map is visualized in real-time directly in the rviz window. Here is the video:

     

    8.3 Virtual machine

    To avoid the tedious work of installing and configuring all the system we provide a virtual machine which includes all the system requirements you need to start playing with ROS and elisa. You can download the image as open virtualization format from the following link ROS-Hydro-12.04.ova (based on the VM from http://nootrix.com/2014/04/virtualized-ros-hydro/); you can then use VirtualBox to import the file and automatically create the virtual machine. Some details about the system:

    • user: gctronic, pw: gctronic
    • Ubuntu 12.04.4 LTS (32 bits)
    • ROS Hydro installed
    • Webots 8.0.5 is installed (last version available for 32 bits linux)
    • git-cola (git interface) is installed
    • the catkin workspace is placed in the desktop

    9 Videos


    9.1 Autonomous charge

    The following videos show 3 Elisa-3 robots moving around in the environment avoiding obstacles thanks to their proximity sensors and then going to the charging station autonomously; some black tape is placed in the charging positions to help the robots place themselves thanks to their ground sensors. The movement and charging is independent of the gravity. It works also vertically and up-side-down.

    9.2 Remote control

    Following video shows 38 Elisa-3 robots moving around with on-board obstacle avoidance enabled; 15 of them are running autonomously, the remaining 23 are controlled from one computer with the radio module.

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