http://dasl.mem.drexel.edu
µNAV Tutorial
Last Updated Monday, June 25, 2007 by Tony Le.

  • Contents

  • Overview

    • Keywords: µNAV, Crossbow, Zagi

    The MNAV100CA is a calibrated digital sensor system with servo drivers, designed for miniature ground and air robotic vehicle navigation and control. All sensors required for complete airframe stabilization and navigation are integrated into one compact module. The MNAV100CA includes 3-axis accelerometers, 3-axis angular rate sensors, and 3-axis magnetometers; static pressure (altitude) and dynamic pressure (airspeed) sensors; and a GPS receiver module.

    The MNAV is the sensor suite for the Zagi UAV. The sensor data is used for autonomous control. As a result, most of the problems that may occur during autonomous flight can be localized to the MNAV.

  • Parts/Components
    • Table 1. Parts List

    Part Description
    Vendor
    Part#

    Price (2007)

    µNAV Sensor Suite Crossbow MNAV100CA $1495.00
    GPS Antenna PC-Mobile GAS76  $29.90
  • Calibrate MNAV

    • Calibrating the MNAV is time consuming so be patient. If not, you can use the scaling values that came with the MNAV. The DASL MNAV scaling values can be found here. You will use the included Micro-VIEW software. The program is helpful to debug and verify the performance of all the individual sensors on the MNAV.
      The XYZ coordinates are defined below:

      • X-axis – along the top pointing to the side of the relative pressure sensor
      • Y-axis – from the side of LEDs to the side of connectors
      • Z-axis – from the top to the bottom

    • Make sure to use the "Check Cali Results" to verify that the values are valid. The information below is found on page 36 or the MNAV100CA Users Manual.
    • When you use the MICRO-VIEW software for calibration, the software gathers data from the MNAV100CA via RS232, and then processes this data to compute the parameters.
      The calibration procedure involves several steps:

      1. Place the MNAV100CA with the x-axis pointing up on a level surface, connect RS232 cable to PC, power the MNAV100CA and then wait for 10 minutes.
      2. Start “Micro-VIEW”, select COM Port, Update Rate and set the MNAV100CA in “Volt Mode”. Click on the “Calibration” button to enter the calibration interface.
      3. Click on “Keep Still”, keep the unit still and wait for 10 seconds. Then click on “Finish this Step”.
        Step 4 ~ 7: Calibrate Scale Factors of Gyroscopes
      4. In the gyroscope section, select the angle you want to rotate through when you calibrate the scale factors of the gyros.
      5. Click on “X-Rotate”, rotate the MNAV100CA through the angle specified in step 4 around the positive x-axis direction, and then click on “Finish This Step”.
      6. Click on “Y-Rotate”, rotate the MNAV100CA through the angle specified in step 4 around the positive y-axis direction, and then click on “Finish This Step”.
      7. Click on “Z-Rotate”, rotate the MNAV100CA through the angle specified in step 4 around the positive z-axis direction, and then click on “Finish This Step”.
        Step 8 ~ 13: Calibrate Bias and Scale Factors of Accelerometers
      8. In the accelerometers section, place the MNAV100CA with the x-axis pointing up on the level surface, click on “X-Up”, keep the unit still for 10 seconds, and then click on “Finish this Step”.
      9. Place the MNAV100CA with the x-axis pointing down on the level surface, click on “X-Down”, keep the MNAV100CA still for 10 seconds, and then click on “Finish This Step”.
      10. Place the MNAV100CA with the y-axis pointing up on the level surface, click on “Y-Up”, keep the MNAV100CA still for 10 seconds, and then click on “Finish This Step”.
      11. Place the MNAV100CA with the y-axis pointing down on the level surface, click on “Y-Down”, keep the MNAV100CA still for 10 seconds, and then click on “Finish This Step”.
      12. Place the MNAV100CA with the z-axis pointing up on the level surface, click on “Z-Up”, keep the MNAV100CA still for 10 seconds, and then click on “Finish This Step”.
      13. Place the MNAV100CA with the z-axis pointing down on the level surface, click on “Z-Down”, keep the MNAV100CA still for 10 seconds, and then click on “Finish This Step”.
      14. To calibrate magnetometers for relative field estimate, in magnetometers section, repeat steps 8~13.
        Step 15~17: Calibrate the zero bias of the pressure sensors:
      15. In the pressure section, click on “Ground Level Zero”, keep the MNAV100CA still for 10 seconds, and then click on “Finish This Step”.
      16. Keep the port of the pressure sensor open to the static air, click on “Static Zero”, keep the MNAV100CA still for 10 seconds, and then click on “Finish This Step”.
      17. The default scale factors of absolute pressure and pitot pressure have been shown in relevant sections, and stored in EEPROM.
      18. Click on “Check Cali Result”, if “Pass” is displayed, the calibration result is valid; if “Failure” with some help message is shown, please follow the instructions in the help message.
      19. Click the little button “R” next to the text frame if you want to read the parameter of a certain sensor; click on “Read all Parameters from EEPROM” if you want to read the parameters of all sensors from EEPROM.
      20. The Zero Bias and Scale Factor of gyros, accelerometers and magnetometers are shown in the text frames. Click the little button “W” next to the text frame if you want to write a parameter of a certain sensor; click the button “Write all Parameters to EEPROM” if you want to write the parameters of all sensors to EEPROM.
      21. Cycle the power to the MNAV100CA, and set MICRO-VIEW in ‘Scaled Mode’. The graphs of the sensor outputs are shown. You can verify if the calibration you performed is working well.

  • Updating Firmware
    • On the MNAV Sourceforge website, you can find the MNAV firmware. The archived zip files come with source code and a pre-built .hex firmware files. As of June 2007, the latest firmware file can be found here.
    • To upload the .hex firmware file to the MNAV, hook the included serial cable connector for the MNAV to pins 3, 18, and 33.
    • The use the MNAV PC Loader executable that came with the MNAV software CD (also found here).

    • The program use is rather inconsistent so if it fails just start from the beginning again.
    • Make sure the MNAV is OFF. Run pcloader.exe and then select the "Connect Target" button
      and then plug in the MNAV. A pop-up message will confirm that the MNAV is connected.
    • Select the "Browse" button to navigate and select the .hex firmware file.
    • Finally, select the "Start Loading" to begin uploading the file. It should be a rather continuous
      load and it may freeze partially through the upload. If that happened repeat the upload process.

  • Add to Zagi

    • The MNAV has been rather reliable outside the GPS. The MMCX connector on the MNAV is rather prone to bend and snap off. I had to have it re-soldered on numerous occasions. The best thing to do is to use some hot glue and glue the connector on.

    • The GPS Antenna that comes with the MNAV is also unreliable. Due to it sheer size and length, finding a GPS signal is difficult. The GPS Antenna that I used to replace it was at least 3 feet long and was more
      of a high-power antenna then the one that comes with the MNAV.
    • The last thing is to make sure that there is a secure connection to the Stargate. I recommend a combination of a rubber band and machine screws or zip-ties.
    • All these additions are mainly to minimize damage in the event of a crash.

  • Final Thoughts & Useful Links

    There have been a lot mixed reviews of the MNAV. For IMU purposes the combination of the MNAV and Stargate work well. However, adding navigation and GPS control has caused many problems. The GPS takes time to warm up from a cold start and sometimes a signal doesn't come up at all. Make sure to test the system before going out on any test flights. As stated, the main issue lies in the GPS. Originally, the problem was the antenna and then it was interference from the wireless antenna on the Stargate. Using a 5.5 dBi high gain antenna with the Stargate created interference due to its close proximity with the MNAV. Therefore, to avoid the problem try to use anything below a 2 dBi rubber ducky antenna and the recommended GPS antenna. The last step is to put all the electronics together on the Zagi and begin test flights.