Taking measurements from Compass Module whilst moving

Mightor · 2007-06-30 18:56

Hi there,

I'm finding that the compass module is quite slow when it comes to returning a measurement; in fact even the pdf mentions this, between 30-40 ms. During this time the bot is quite likely to have moved since the measurement command was given. During those 30-40ms when does the actual measurement take place? Does it take a whole bunch of measurements and then calculate some kind of average or is it just r e a l l y s l o w to respond to commands? There was no additional info in the Hitachi datasheet.

Gr,
Mightor

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
| What the world needs is more geniuses with humility, there are so few of us left.

nowhoha · 2007-07-03 14:53

Mightor,

Could you create an inertial nav system using the H48C 3-Axis Accelerometer, and just read the compass for occasional corrections? The spec sheet says 200 samples/second.

Mightor · 2007-07-03 18:51

Hiya Nowhoha,

I am not familiar with that particular type of accelerometer, I only have a 2D one. The idea was to count the number of times my bot revolves and how many times the wheel encoders have fired so I can calculate the number of pulses per 360 degrees of turn on my Boebot [noparse]:)[/noparse] This post is related to the Source code for encoder calibration software thread also in this sub forum. I basically want to come up with an alternative method to calibrate the encoders.
So far I have come up with this to count the number of times I've made a full turn:

' ============================================================================
' CountFullTurns.bs2 - This Hitachi HM55B Compass Module test program
' counts the number of times the bot has turned 360 degrees.
'
' Author.... (C) 2007 Xander Soldaat -- All Rights Reserved
'
' {$STAMP BS2}
' {$PBASIC 2.5}
' ============================================================================

' -----[noparse][[/noparse] Pins/Constants/Variables ]-------------------------------------------

DinDout        PIN      2                    ' P2 transceives to/from Din/Dout
Clk            PIN      0                    ' P0 sends pulses to HM55B's Clk
En             PIN      1                    ' P2 controls HM55B's /EN(ABLE)

Reset          CON      %0000                ' Reset command for HM55B
Measure        CON      %1000                ' Start measurement command
Report         CON      %1100                ' Get status/axis values command
Ready          CON      %1100                ' 11 -> Done, 00 -> no errors
NegMask        CON      %1111100000000000    ' For 11-bit negative to 16-bits

Measuring      VAR      Nib

x              VAR      Word                 ' x-axis data
y              VAR      Word                 ' y-axis data
status         VAR      Nib                  ' Status flags
angle          VAR      Word                 ' Store angle measurement

' -----[noparse][[/noparse] Main Routine ]-------------------------------------------------------

Measuring = 0
x = 0
y = 0

DO                                           ' Main loop

  IF Measuring = 0 THEN
    GOSUB Compass_Start_Measure
    GOSUB Compass_Get_Status
    Measuring = 1
  ELSEIF (status = Ready) THEN
    GOSUB Compass_Get_Axes
    Measuring = 0
  ELSE
    GOSUB Compass_Get_Status
  ENDIF

  angle = x ATN -y                           ' Convert x and y to brads
  angle = angle */ 360                       ' Convert brads to degrees

  PULSOUT 13, 730
  PULSOUT 12, 730
  IF (angle < 5) OR (angle > 355) THEN
    FREQOUT 7, 20, 3000
  ENDIF
LOOP                          ' Repeat main loop


' -----[noparse][[/noparse] Subroutines ]--------------------------------------------------------

Compass_Start_Measure:
  HIGH En: LOW En                            ' Send reset command to HM55B
  SHIFTOUT DinDout,clk,MSBFIRST,[noparse][[/noparse]Reset\4]
  HIGH En: LOW En                            ' HM55B start measurement command
  SHIFTOUT DinDout,clk,MSBFIRST,[noparse][[/noparse]Measure\4]
  status = 0                                 ' Clear previous status flags
  RETURN

Compass_Get_Status:
  HIGH En: LOW En                          ' Measurement status command
  SHIFTOUT DinDout,clk,MSBFIRST,[noparse][[/noparse]Report\4]
  SHIFTIN  DinDout,clk,MSBPOST,[noparse][[/noparse]Status\4]  ' Get Status
  RETURN

Compass_Get_Axes:                            ' Compass module subroutine
  SHIFTIN  DinDout,clk,MSBPOST,[noparse][[/noparse]x\11,y\11]   ' Get x & y axis values
  HIGH En                                    ' Disable module

  IF (y.BIT10 = 1) THEN y = y | NegMask      ' Store 11-bits as signed word
  IF (x.BIT10 = 1) THEN x = x | NegMask      ' Repeat for other axis

  RETURN

I occasionally get a missed count for a turn (no beep when the bot's made a full turn), but I am reluctant to increase the angle gap. Right now this is between 355 and 5 degrees.

I still have to put the encoder code in there, this was basically just for testing.

Gr,
Mightor

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
| What the world needs is more geniuses with humility, there are so few of us left.

Taking measurements from Compass Module whilst moving

Comments