topic Re: Precision of MMA8451 in Kinetis Microcontrollers

Precision of MMA8451

michelmeunier — Wed, 25 Nov 2015 18:17:32 GMT

Hello,

I am very surprised by the very bad precision of the MMA8451 on the FRDM-KL25Z.

When the board is fixed, the (x,y,z) values of the 1g is changing quite a lot from one read to an other. For example the z value change between 3931 to 3968. It's about 1%. Why have 14bits precision, when the true precision is not good? I am in 2g mode to have the best precision.

Michel

Re: Precision of MMA8451

bobpaddock — Wed, 25 Nov 2015 18:47:04 GMT

How clean is the power supply?

Re: Precision of MMA8451

michelmeunier — Wed, 25 Nov 2015 19:32:31 GMT

It is powered by the FRDM-KL25 board. And The board is powered by USB of my computer.

Re: Precision of MMA8451

TomasVaverka — Thu, 26 Nov 2015 09:27:46 GMT

Hi Michel,

For the best noise performance, it is recommended to select the lowest possible ODR and use both Low Noise (LNOISE bit = 1) and High Resolution (MODS [1:0] = 0b10) modes. For more information please refer to our AN4075.

Hope it helps.

Regards,

Tomas

Re: Precision of MMA8451

michelmeunier — Fri, 27 Nov 2015 09:18:54 GMT

Hello

I have tried the Tomas' explainations:

error=MMA1_Disable();

error=MMA1_ReadReg8(XYZ_DATA_CFG,&valeur);

valeur&=0xFC; //2g mode

error=MMA1_WriteReg8(XYZ_DATA_CFG,valeur);

error=MMA1_ReadReg8(CTRL_REG1,&valeur);

valeur|=0x3C; //1.56Hz rate plus low noise

error=MMA1_WriteReg8(CTRL_REG1,valeur);

error=MMA1_ReadReg8(CTRL_REG2,&valeur);

valeur&=0xF7;

valeur|=0x10; //High resolution mode

error=MMA1_WriteReg8(CTRL_REG1,valeur);

error=MMA1_ReadReg8(F_SETUP,&valeur);

valeur&=0xFC; //no FIFO

error=MMA1_WriteReg8(F_SETUP,valeur);

error=MMA1_Enable();

And it's only a little bit better:

my board is on a table and the z value is between 4026 and 4080! And, no I am not a the top of a buiding during an earthquake ;-)

I thing I am going to take an other system because the MMA8451 is really to bad and it hasn't enough precision for my goal.

Michel

Re: Precision of MMA8451

bobpaddock — Tue, 01 Dec 2015 13:28:32 GMT

USB is not always the cleanest of power supplies. Take a look at the VTI/Now Murta SC3100 . It can sense me typing from several feet away (at first I thought the part was junk and noisy tell I figured out the typing connection). http://www.murata.com/en-sg/products/sensor/accel/sca3100 Analog Devices makes parts in the aircraft grade with lots of zeros in the price tag if noise is that big of issue. Consider using a Median Filter: http://embeddedgurus.com/stack-overflow/tag/median-filter/