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mattallen37 wrote:You should also note, that some "drift" is actually very correct. The earth rotates 360*/24 hours (once a day). That means that you should see a "drift" of 1 degree every 4 minutes. However, if the axis of the gyro is perfectly parallel to the axis of the earth, you should see no "drift".

mattallen37 wrote:Linus, good points you bring out. I didn't really mean for it to have anything to do specifically with the HT Gyro, but more for an absolute position gyro.

mattallen37 wrote: BTW, is there any simple (or pre-made) NXC solution to convert from the rotation speed measured by the HT Gyro, into an absolute position? I don't really need anything to precise, but it would be interesting to examine some NXC code.

phi = integral(omega(t) dt)

float phi = 0;
int lastTicks = GetTickCount();
int curTicks;
while(true) { 

  // convert omega to unit "degrees per ms" here.
  float omega = GetGyro(sensorPort) / 1000.0;
  curTicks = GetTickCount();
  int deltaTTicks =  curTicks - lastTicks;
  phi += omega * deltaTTicks;
  lastTicks = curTicks;

  // use phi here...
}//end while

linusa wrote:If this pseudo-NXC-code above is right (I hope so), it should do simple numeric integration. Note how it only uses GetTickCount() once per iteration to get maximum accuracy (change the name of the function if it's called differently in NXC, this is out of my head). Also you either have to convert the angular velocity to degrees per millisecond, or the ticks to seconds... Please check the code for yourself to verify I didn't make any mistakes...

#define offset 1.72

float phi = 0;
int lastTicks;
int curTicks;

task main(){
  SetSensorHTGyro(S2);
  lastTicks = CurrentTick();
  while(true){

    // convert omega to unit "degrees per ms" here.
    float omega = (SensorHTGyro(S2) - offset) / 1000.0;
    curTicks = CurrentTick();
    int deltaTTicks = curTicks - lastTicks;
    phi += omega * deltaTTicks;
    lastTicks = curTicks;

    ClearScreen();
    NumOut(0, LCD_LINE1, SensorHTGyro(S2));
    NumOut(0, LCD_LINE2, omega);
    NumOut(0, LCD_LINE3, deltaTTicks);
    NumOut(0, LCD_LINE4, phi);
    Wait(20);
   // use phi here...
  }//end while
}

linusa wrote:Great, really cool that it works

I just noticed that you're doing the slow LCD painting and a Wait(20) inside the polling loop. During this time you don't get any "rotation information", so if the gyro moves, this movement is lost to you. You should consider doing multi-tasking, and just have one loop that reads and updates phi as often as possible. On the other hand, not too often (or the gyro might get unstable or whater it is I do vaguely remember). It could be this: if you poll more often than the firmware updates its registers (I think that might be every 3ms for analog sensors), then you get "double" values, and if you integrate them, that's biased.

So, for a better test, introduce this extra polling tast, but keep a Wait(4) inside the loop for safety...

float offset;

float phi = 0;
int lastTicks;
int curTicks;

task main(){
  SetSensorHTGyro(S2);
  lastTicks = CurrentTick();
  offset=0;
  repeat(500){
    offset += SensorHTGyro(S2);
    Wait(3);
  }
  offset/=500;
  lastTicks = CurrentTick() + 4;
  while(true){

    // convert omega to unit "degrees per ms" here.
    float omega = (SensorHTGyro(S2) - offset) / 1000.0;
    curTicks = CurrentTick();
    int deltaTTicks = curTicks - lastTicks;
    phi += omega * deltaTTicks;
    lastTicks = curTicks;

    ClearScreen();
    NumOut(0, LCD_LINE1, SensorHTGyro(S2));
    NumOut(50, LCD_LINE1, offset);
    NumOut(0, LCD_LINE2, omega);
    NumOut(0, LCD_LINE3, deltaTTicks);
    NumOut(0, LCD_LINE4, phi);
    Wait(2);
  }
}

linusa wrote:As to your other questions: later

mattallen37 wrote:You should also note, that some "drift" is actually very correct. The earth rotates 360*/24 hours (once a day). That means that you should see a "drift" of 1 degree every 4 minutes. However, if the axis of the gyro is perfectly parallel to the axis of the earth, you should see no "drift".

aswin0 wrote:

mattallen37 wrote:You should also note, that some "drift" is actually very correct. The earth rotates 360*/24 hours (once a day). That means that you should see a "drift" of 1 degree every 4 minutes. However, if the axis of the gyro is perfectly parallel to the axis of the earth, you should see no "drift".

This is only true on the poles. On the equator you won't measure an angular rate because of earth rotation.