Electronic – Gyroscope Bandwidth on LSM9DS1

bandwidthgyroscopeimusensor

I am trying to read data at 119Hz and above from the gyroscope embedded in the LSM9DS1 9dof IMU package and transfer the raw data from the gyroscope to an Android device. On the Android, I plan to use this data along with the accelerometer and magnetometer data to calculate roll, pitch, yaw, and xyz position.

I was wondering about one of the setting on the gyro, called the bandwidth. I read from a few sources that this refers to the read rate of the gyro, but for this sensor the ODR tells me the output data rate. The ODR goes much higher than the bandwidth. Is the bandwidth a lowpass filter applied to the sensor data before being collected by the ADC? That seems to be what is suggested in this data sheet in section 2.6, although this is describing the accelerometer.

Thanks!

Best Answer

Check section 2.7 of the datasheet.

The bandwidth (actually, the cutoff of the low pass filter) is automatically set depending the the output data rate (ODR) that you select. There is a chart (actually two, as it also depends on the power save mode) that shows the cut off frequencies for the various ODRs.

The datasheet is written bit strangely. The text keeps talking about "bandwidth" when the tables are all about "cutoffs."

Since the LSM9DS1 also has a high pass filter, the bandwidth would be the difference between the selected highpass and low pass filters.

Related Solutions

Electronic – Magnetometer dynamic calibration

I like your graphs. They clearly show that roll, pitch, and yaw seem to be working. Congratulations! That's already more progress than most people make.

I'm guessing that the code you presented is calculating "the wrong" MAG_Heading value, different from the MAG_Heading value you expected.

It would be a lot easier for us to help you if you gave us: (This is the "describe the symptoms" section of "How To Ask Questions The Smart Way" )

the AK8975 magnetometer output values m_x, m_y, and m_z at some single point in time.
The pitch and roll values at the same instant
the allegedly wrong MAG_Heading output value calculated from those values
what you expected the correct MAG_Heading to be

So I'm left to speculate that perhaps you're running into the same sorts of problems I create for myself :-).

What angle format do your sin() and cos() and atan2() functions expect? Do you need to do some sort of conversion between the format pitch and roll are stored in to that format? Do you need to convert from that format to what you need MAG_heading? (brads, degrees, or radians? floating-point or fixed-point?)
Is there an offset in the raw m_x, m_y, m_z values that needs to be subtracted off?
Are all the parts lined up in the way assumed by the code? In particular, is the pitch and roll axis lined up with the magnetometer axis? (Is m_x supposed to point forward, along the roll axis? Is m_y supposed to point to the right, along the pitch axis?)
Maybe some sensor value or another -- perhaps m_z -- needs to be negated before feeding into this code?
Is maybe this code being interrupted by one interrupt or another that corrupts its internal values? I seem to recall a different project that, after someone put a "divide" in an interrupt routine, every trig function calculation everywhere else in the program would often give the wrong result.
Is maybe interrupts firing so often that this code never actually finishes running?

There seems to be other people discussing very similar code elsewhere: http://diydrones.com/forum/topics/heading-from-3d-magnetometer ; http://diydrones.ning.com/profiles/blogs/dcm-imu-theory-first-draft ; http://aeroquad.com/showthread.php?1138-REVOLUTION!!!-New-IMU!!! ; http://www.rcgroups.com/forums/showthread.php?t=1436742&page=6 ; http://aeroquad.com/showthread.php?691-Hold-your-heading-with-HMC5843-Magnetometer ; etc.

Electronic – simple IMU – tilt magnetometer

It looks like the Gyro and the Magnetometer are slightly miscalibrated. In excel, it looks like they are about 135 degrees apart. Try changing your formula to be this instead:

=DEGREES(ATAN2(U3,T3))+Dcal

Where Dcal is some calibration value. It looks like the data matches up fairly well when Dcal is about 135 degrees.

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Electronic – Magnetometer dynamic calibration

Electronic – simple IMU – tilt magnetometer

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