Autopilot with XMEGA128A1 and ADXRS453 gyros

Alex Zarenin November 29, 2013 at 11:52am

Did you try the highest oversampling setting for MPL3115? With the oversampling set at 128 I obtained a rate of ~3Hz of altitude measurements which were rather stable from sample to sample (a few centimeters). However the problem I encounter with this sensor is that it requires about 2-3 minutes of running at full speed before it actually settles on stable measurement - for the first 2-3 minutes the measurements slowly "float" up or down until they settle.

Attached is the chart from a test flight - MPL3115A2 was "warmed up" for 2 minutes before the flight. For this flight the sensor is configured for oversampling of 32 measurements for the effective rate of about 10 Hz (light blue line) which then smothed by the IIR filter with alpha=1/32. My data logger takes measurements every 10 msec while sensor data changes about every 100 msec, so on the chart you may see "flat" lines between the moments I obtain the new measurements. Dark blue line represent the "smoothed" sensor data.

Sergiu November 8, 2012 at 9:09am

Hi

This is the code I used a while ago:

Rem Absolute pressure sensor
Dim Twi_start As Byte
Dim Alt_static As Single , Alt_temp As Single , Tmp_frac As Byte
Dim Mpl_dev_id As Byte , Mpl_status As Byte , Mpl_int As Integer
Dim Mpl_alt_msb As Byte , Mpl_alt_csb As Byte , Mpl_alt_lsb As Byte
Dim Mpl_temp_msb As Byte , Mpl_temp_lsb As Byte
Dim Byte1 As Byte At Alt_static Overlay , Byte2 As Byte At Alt_static + 1 Overlay
Dim Byte3 As Byte At Alt_static + 2 Overlay , Byte4 As Byte At Alt_static + 3 Overlay
Dim Timer_static As Single , Dt_static As Single
'***************************
Open "twic" For Binary As #3
Config Twi = 400000
I2cinit #3
'***************************
Rem Read MPL3115A2 dev Id which should be &C4
I2cstart #3
    I2cwbyte &HC0 , #3                                      'write addr.
    I2cwbyte &H0C , #3
I2crepstart #3
    I2cwbyte &HC1 , #3
    I2crbyte Mpl_dev_id , Nack , #3
I2cstop #3
Print #2 , Hex(mpl_dev_id)
Rem Set to Altimeter with OSR=128
I2cstart #3
    I2cwbyte &HC0 , #3
    I2cwbyte &H26 , #3
    I2cwbyte &HB8 , #3
I2cstop #3
Rem Enable Data Flags in PT_DATA_CFG
I2cstart #3
    I2cwbyte &HC0 , #3
    I2cwbyte &H13 , #3
    I2cwbyte &H07 , #3
I2cstop #3
Rem Set Poling active and start device
I2cstart #3
    I2cwbyte &HC0 , #3
    I2cwbyte &H26 , #3
    I2cwbyte &HB9 , #3
I2cstop #3
'********************
Declare Sub Altitude_static()

......................................................................

Sub Altitude_static()
   Timer_static = Timer_static + Dt_static    ' I read sensor at every 1.3 seconds
If Timer_static >= 1.3 Then
      Timer_static = 0

       I2cstart #3
          I2cwbyte &HC0 , #3                                'write addr.
          I2cwbyte &H01 , #3
   I2crepstart #3
    I2cwbyte &HC1 , #3
    I2crbyte Mpl_alt_msb , Ack , #3
    I2crbyte Mpl_alt_csb , Ack , #3
    I2crbyte Mpl_alt_lsb , Ack , #3
    I2crbyte Mpl_temp_msb , Ack , #3
    I2crbyte Mpl_temp_lsb , Nack , #3
   I2cstop #3
     Tmp_frac = Mpl_alt_lsb
     Shift Tmp_frac , Right , 4                             'use bit7-4
     Mpl_int = Makeint(mpl_alt_csb , Mpl_alt_msb)
     Alt_static = Tmp_frac / 10
     Alt_static = Alt_static + Mpl_int
     Tmp_frac = Mpl_temp_lsb
     Shift Tmp_frac , Right , 4                             'use bit7-4
     Alt_temp = Tmp_frac / 10
     Alt_temp = Alt_temp + Mpl_temp_msb
     'Print #2 , Alt_static ; " " ; Alt_temp
End If
Reset Watchdog
End Sub

''''''''''''''''''''''''''''''''''''''''''''''''''''''''

However i came to the conclusion that no accuracy can be obtained from this digital absolute sensors. The result is jumping +/- tens of centimeters.

The only reliable way to use in multicopters is using of something like MP3h6115A and some Sallen Key op-amp filter.

Regards,

Sergiu

Sergiu March 11, 2012 at 5:30am

Eugenijus

So far I am one man show. So it takes longer time to finish one project than team working. On the other hand it have only my mistakes...For more info on two MCU's AP see my other post :

http://diydrones.com/profiles/blogs/xmega-ap-and-esc

Regards,

Sergiu

Sergiu March 11, 2012 at 4:03am

Euginijus,

At the beginning I made Ap's with even 4 MCU's. All of them ATmega. One was doing R/C reading, another GPS and compass,etc. I did a big step forward when XMEGA came out. With it, I made my first AP with one MCU only. It was possible not because of higher MIPS but because of DMA. Later, keeping navigation (R/C input, PWM output, GPS, Sonar) in one MCU and AHRS ( gyro,accelerometer,compass) in another MCU, given me more processing power and more MCU peripherals. However is nothing about " do not keep all eggs in same basket " because any of the MCU will fail, the copter will crush. Having only one MCU with enough MIPS and DMA (like STM32) it is also OK. You don't need to bother anymore about MCU's data exchange.

Regards,

Sergiu

Sergiu March 4, 2012 at 5:44am

Right now I'm working on the code for SPI comm. with gyros.

I use (for many years ) Bascom AVR compiler.

Developer

Sandro Benigno March 4, 2012 at 4:00am

Yeah... there go the nice and expensive gyros from your other post!
Their "neighborparts" are also great, including those low heat switching power suppliers.

Looking forward to know about the funniest tricky part: the code. =)

Blogs

Autopilot with XMEGA128A1 and ADXRS453 gyros

Comments