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IMU information

Discussion in 'Standard/4k Discussion' started by Trackman1, Apr 8, 2016.

  1. Trackman1

    Jan 1, 2016
    Likes Received:
    Ontario, Canada
    This may be old news to some and new to others.
    I found this information in a document on this site called "dji phantom 3 summary guide." Thanks to the author for writing.
    This clip from that document explains a lot about the IMU.

    IMU: An inertial measurement unit (IMU) is an electronic device that measures and reports a craft's velocity, orientation, and gravitational forces, using a combination of accelerometers and gyroscopes, sometimes also magnetometers. An IMU is used to manoeuvre aircraft Phantom, among many others, and spacecraft, including satellites and landers. Recent developments allow for the production of IMU-enabled GPS devices. The IMU allows the GPS receiver to work when GPS-signals are unavailable.
    The IMU Inertial Measurement Unit is a small box component on the motherboard containing three accelerometers and three gyroscopes. The accelerometers are placed so that their measuring axes are orthogonal to each other. They measure inertial acceleration, also known as G-forces.
    These are really incredible devices that make miniaturization possible. You probably have one in your smart phone.
    The IMU in the Phantom 3 works by detecting the current rate of acceleration using micro accelerometers, and detects changes in rotational attributes like pitch, roll and yaw using micro gyroscopes. The compass (magnetometer) assists to counter orientation drift.
    The angular accelerometer measure how the Phantom is rotating in space. There is one sensor for each of the three axes: pitch (nose up and down), yaw (nose left and right) and roll (clockwise or counter-clockwise).
    The linear accelerometers measure non-gravitational accelerations of the aircraft. Since it can move in three axes (up & down, left & right, forward & back), there is a linear accelerometer for each axis.
    The Main Controller (MC) of the Phantom continually takes the data feed from the IMU and calculates the aircraft’s current position. First, it integrates over time the sensed acceleration, together with an estimate of gravity, to calculate the current velocity. Then it integrates the velocity to calculate the current position.
    The Main Controller of the Phantom also integrates inputs from the GPS system, barometric pressure sensor for altitude correction, and the magnetic compass.
    IMU’s aren’t perfect and a major disadvantage of using IMU for navigation is that they typically suffer from accumulated errors. Because the guidance system is continually adding detected changes to its previously-calculated positions, any errors in measurement, however small, are accumulated from point to point. This leads to 'drift', or an ever-increasing difference between where the system thinks it is located, and the actual location.
    Because the devices are only able to collect data in a finite time interval, IMUs are always working with averages. So if an accelerometer is able to retrieve the acceleration once per second, the device will have to work as if that had been the acceleration throughout that whole second, although the acceleration could have varied drastically in that time period. Of course modern devices are able to collect data much faster than once per second, but over time that error increases exponentially.

    You can find the whole document by searching for - dji phantom 3 summary guide
    gvsukids, Mark The Droner and LamboH like this.
  2. Mark The Droner

    Aug 26, 2015
    Likes Received:
    Brookeville, MD, USA
    Interesting. That's a lot more detail there than what's in my P2V manual or anywhere else I've seen. What especially caught my eye was this:

    The IMU allows the GPS receiver to work when GPS-signals are unavailable.

    That's wild.

    I had a weird flight on Tuesday, and I didn't quite understand what happened, but this post seems to explain it.

    I was coming back from a run and was in RTH and I wanted to guide my P2V down to me from about a quarter mile out, so I thought I had flicked the S1 switch down and up (which must be done on a P2V to regain control), and then I put the S2 switch in Home Lock and began to guide it home. But instead of coming straight home, it kind of angled to the left about 30 degrees. This puzzled me. With home lock, it should come straight home.

    I then put it in Phantom mode and brought it home.

    Later I looked at the flight on my tablet recorder and saw it was in ATTI mode when it angled to the left. Did I not finish with the S1 switch and mistakenly leave it in ATTI? I am not sure. Apparently I did. But if I did, how would it behave if the S2 switch is in home lock? After all, ATTI would not be reading satellites so it would not know where the home point is located.

    The description of how the IMU works seems to make sense. It apparently disregarded the GPS fix and the satellites, but nonetheless knew from the IMU the general location of the home point, and so attempted to move towards that location even while ignoring the satellites.

    Pretty amazing.
  3. N017RW

    May 2, 2014
    Likes Received:
    Palm Beach Co.- FL
    The IMU measures the Phantom's attitude and motion in free-space. It provides data so the MC can properly interpret control commands and maintain stable flight. It has nothing to do with GPS as that is radio based. The article is a good intro but poorly written.