Selecting an IMU

[Bill O]

Leo,
I forgot about all the HATs that can easily be added for the pi4 that have an imu built in to them. This would be the easiest solution.

Just look up "raspberry pi 4 hat imu" and you will have lots of choices.

[Adelie]

Originally Posted by Frankly

A inertial measurement unit typically refers to a sophisticated /expensive unit that along with dead reckoning navigation determines the vehicle heading by measuring the rotational motion of the earth. Most of us poorer marine autopilot users still rely on good old magnetics to determine the vehicle heading. Newer units use accelerometers to correct the mag heading for vehicle motion and rate sensors to make for better autopilot performance.





Frankly

After a bit of research to figure out why there are 9 axes, or more properly 9 degrees of freedom when I only knew about 6 I found that they are calling magnetometers oriented in the 3 axes the extra 3 degrees of freedom. So even the new units are still sensing the earth’s magnetic field. And a 2 or 3 direction magnetometer is also known as a fluxgate.

Any AP is going to need a fluxgate for long term directional stability. Inertial systems drift over time in the 3 translational degrees of freedom and in the yaw rotational degree.

Pitch and yaw can be constrained from drift if vertical can be regularly established. In a weightless environment they will drift too.

The inertial parts of the sensor will provide turn rate feedback and the fluxgate will keep the heading stable over time.

[Leolotowycz]

Bill o hello
AP IS Raymarine EV-100 Evolution Wheel Pilot.
I was unaware that Pi 4 had built in IMU. SO THERE ISN'T need for a Imu top hat. Thank you
Leo

[Leolotowycz]

Frankly hello
Was reading tonite about sensors with the issue of drift and lack of accuracy. One solution was putting three sensors on the board and average the reading to get back on course.would be done by code.
Thanks for your help.
Leo

[cal40john]

Northbound post #15 shows a Raymarine part. It is a just a rate gyro. MEMS gyros only measure rate of rotation. You have to write software to integrate that to turn it into a heading. The IMU chips mentioned have computers onboard to do this for you. My son had to do the integrating his first year in the FIRST robotics program because that was the only gyro allowed. The IMUs allowed later made his life easier.

Bill O post #16, I believe you misinterpreted his post. I do not read his post as saying the PI 4 has an imu, it says you can buy a hat. I do not see an IMU in any PI 4 specs that I am finding.

The PI sense hat uses an LSM9DS1 chip. Here is a link comparing some IMU chips available. They like the BNO055 over the LSM9DSI. (They actually like a third chip better, but put the BNO055 in second place.)
The BNO055 is the chip I suggested in post #6. The link provided in that post shows how to wire and program (they have a library) the chip for PI.

Adelie in post #17 mentioned the drift problem. A lot of these chips also have a compass. The BNO055 has several fusion modes one of which from the spec sheet is -
3.3.3.5 NDOF
This is a fusion mode with 9 degrees of freedom where the fused absolute orientation data
is calculated from accelerometer, gyroscope and the magnetometer. The advantages of
combining all three sensors are a fast calculation, resulting in high output data rate, and high
robustness from magnetic field distortions. In this mode the Fast Magnetometer calibration
is turned ON and thereby resulting in quick calibration of the magnetometer and higher
output data accuracy. The current consumption is slightly higher in comparison to the
NDOF_FMC_OFF fusion mode.

[mglonnro]

Built-in IMU?

I use this outer space compatible piece of equipment: https://www.raspberrypi.org/products/sense-hat/

[cal40john]

Originally Posted by cal40john

Northbound post #15 shows a Raymarine part. It is a just a rate gyro. MEMS gyros only measure rate of rotation. You have to write software to integrate that to turn it into a heading. The IMU chips mentioned have computers onboard to do this for you. My son had to do the integrating his first year in the FIRST robotics program because that was the only gyro allowed. The IMUs allowed later made his life easier.

Bill O post #16, I believe you misinterpreted his post. I do not read his post as saying the PI 4 has an imu, it says you can buy a hat. I do not see an IMU in any PI 4 specs that I am finding.

The PI sense hat uses an LSM9DS1 chip. Here is a link comparing some IMU chips available. They like the BNO055 over the LSM9DSI. (They actually like a third chip better, but put the BNO055 in second place.)
The BNO055 is the chip I suggested in post #6. The link provided in that post shows how to wire and program (they have a library) the chip for PI.

Adelie in post #17 mentioned the drift problem. A lot of these chips also have a compass. The BNO055 has several fusion modes one of which from the spec sheet is -
3.3.3.5 NDOF
This is a fusion mode with 9 degrees of freedom where the fused absolute orientation data
is calculated from accelerometer, gyroscope and the magnetometer. The advantages of
combining all three sensors are a fast calculation, resulting in high output data rate, and high
robustness from magnetic field distortions. In this mode the Fast Magnetometer calibration
is turned ON and thereby resulting in quick calibration of the magnetometer and higher
output data accuracy. The current consumption is slightly higher in comparison to the
NDOF_FMC_OFF fusion mode.

I should add that I have not researched the PI sense hat beyond the one article that indicates it might have a higher drift rate. It could have all the capabilities of the chip I'm familiar with.

[Bill O]

Originally Posted by Bill O

Leo,
I forgot about all the HATs that can easily be added for the pi4 that have an imu built in to them. This would be the easiest solution.

Just look up "raspberry pi 4 hat imu" and you will have lots of choices.

I did not misinterpret the original post, since I know the basic pi4 doesn't come w/an IMU w/it and the OP was asking for an IMU to add on.

Thought it was fairly clear that I was suggesting finding/adding a HAT w/an IMU, but see that I need to word my responses better/clearer so they are not misinterpreted.

Will agree IMU accuracy/drift is an important issue and the IMU needs to be calibrated. Again will go back to the pypilot which has built in calibration software. Once calibrated it can be locked or left to continuously update the calibration.
One of the largest influences in IMU accuracy is the change in the magnetic field as one moves about compared to the the local magnetic field where the IMU was calibrated. So, if you stay in one general area you will see less error/drift than if you are a world globe trotter and do not recalibrate your IMU often.

Here is a scientific paper discussing drift correction.

[cal40john]

Originally Posted by Bill O

I did not misinterpret the original post, since I know the basic pi4 doesn't come w/an IMU w/it and the OP was asking for an IMU to add on.

Thought it was fairly clear that I was suggesting finding/adding a HAT w/an IMU, but see that I need to word my responses better/clearer so they are not misinterpreted.

Will agree IMU accuracy/drift is an important issue and the IMU needs to be calibrated. Again will go back to the pypilot which has built in calibration software. Once calibrated it can be locked or left to continuously update the calibration.
One of the largest influences in IMU accuracy is the change in the magnetic field as one moves about compared to the the local magnetic field where the IMU was calibrated. So, if you stay in one general area you will see less error/drift than if you are a world globe trotter and do not recalibrate your IMU often.

Here is a scientific paper discussing drift correction.

I thought the OP misinterpreted Bill's post. This below from a previous post by the OP looked to me that the OP is thanking you, Bill, to let the OP know that the PI 4 doesn't need a hat.
I was unaware that Pi 4 had built in IMU. SO THERE ISN'T need for a Imu top hat. Thank you
Leo

I had a look at the pypilot. He recommends the MPU-9255 IMU. This is in the list of compared IMUs that I linked earlier. It looks like the 9255 only outputs rate information, so pypilot is doing the calibration/corrections to come up with heading. The BNO055 also can output out rate, so you could use either one with pypilot.

The BNO055 also does a fusion mode putting out heading, so it is doing what pypilot does, but internally calibrating/corrections/compensation as a function on the chip. It would be interesting to see who does it better.

[Bill O]

With our pypilot/tiny pilot set up, we are running a Pololu AltIMU-10 v4 (now discontinued replaced w/v5) instead of the one Sean recommended and works fine. Sean did look at our IMU and thought his might react faster, but there wasn't a head to head comparison for performance.
While the OP indicated he was using a Raymarine Evo-100 wheel pilot, not certain how they are interfacing the pi4 (opencpn only?) into the system. If the OP downloaded openplotter (the easy way to down load ocpn for a pi), they recommended a Moitessier HAT 2 (the v3 will be out soon) for their IMU add on.

[Frankly]

One bit of info for the Raymarine crowd. Years back I was tinkering with my X-5 on the bench before installing on the boat. The X-5 has the rate sensor on the circuit board and then the standard gimballed fluxgate. If I rotated the fluxgate 90 degrees (leaving the rate sensor fixed) then it was nearly a minute before the readout settled on the new heading. Reversing the process and rotating the rate sensor 90 degrees while leaving the fluxgate fixed the heading changed immediately and over a period of a minute slowly reversed to the original heading. I read into that observation over a 20 to 30 second time frame the vehicle heading comes from integrating the rate sensor, and fluxgate removes the longer term bias error from that integration.

Couldn't repeat the experiment with the Evo as it is all in one unit.


Now I no longer disengage the AP when passing under a bridge.


Frankly