The Vectornav VN-100 IMU (they call it an Attitude Heading Reference System, for some reason) is out and it looks like the best device on the market in this price range. Here's the description from the site.
The VN-100 combines a 3-axis accelerometer, 3-axis gyro, 3-axis magnetometer, and a high performance processor onto a single surface mountable chip-sized module to create a high performance orientation sensor. Fully calibrated for bias, gain, and misalignment, the VN-100 accurately calculates orientation over the entire 360° range at 200 Hz. Filtered orientation data and inertial measurements are assessible via either a SPI or RS-232 serial interface.
With its small size, high performance, and low cost, the VN-100 has numerous potential applications. A development kit is available for the VN-100 which comes with a sensor pre-installed and gives easy access to all of the sensor's features.
Features and Benefits:
Complete Sensor Package
3-axis accelerometer
3-axis gyro
3-axis magnetometer
Multiple Output Types
Heading , Pitch, Roll
Quaternion / DCM
Acceleration, Angular Rates, Magnetic
High Precision
Heading accuracy < 1.0 (static)
Pitch/Roll accuracy < 0.5 (static)
< 3 deg dynamic
Fast Onboard Processor
Extended Kalman Filter
200 Hz update rate
Surface Mount Package
Hand solderable
30 pads
Ultra-Compact Size
22 x 24 x 3 mm
3 grams
Fully Calibrated
Scale Factor and Gain
Axis misalignment < 0.05 deg
Hard / Soft Iron Compensation
Digital Interface
Serial UART up to 921600 bps
SPI Interface up to 18 MHz
Low Cost
$400 - 500
*Price depends on quantity
I just today submitted my dissertation (mechanical engineering) en which i built a segway style robot. The easiest way to filter a good reading out of a gyro and 2 perpendicular accelerometers is a complementry filter. It is basicly a low pass filter on the accelerometers (measuring component of gravity plus noise (vibrations/movement/etc)) and a high pass filter on the integrated (to sum the change in angle) gyro readings. Summing the gyro*dt readings carries error and so eventually you get drift in the determined angle, the high pass filter removes this very well (although can require gain due to reduced sensitivity). A kalman works better, but is more complicated to implement if it is not your main focus.
I heard (talked to a friend of mine that knows these guys personally) that they are rolling out a temp compensated version VERY soon, and are following that with a version that is the same size, but has a 10Hz GPS on board - If they do both of those things and keep the same price point, push this model down slightly in price - well, I'd be in line to buy one.
I saw this a while back and its useless for aircraft because there is no centriepetal compensation. Also They have no temperature compensation so kind of expensive for a closed source imu that would'nt fly an aircraft.
Regarding the mounting location of the gyros and accelerometers...I am not so sure that we are forced to mount them close to the center of gravity.
The gyros can be mounted anywhere, because the rate of rotation is the same everywhere on the airframe.
Theoretically, the accelerometers should be mounted near the center of gravity, but if they are not, you could compensate for the centrifugal acceleration if you know where they are mounted.
Would be useful if someone made a small magnetometer board for remote placement. Perhaps throwing an ADC and a voltage reference on there to digitize it as close to the sensor as possible, and then have the usual uart/spi adc interface ...
Has anyone tried using magnetic shielding on motor and other high current wiring?
This company makes magnetic shielding in a variety of forms: http://www.lessemf.com/mag-shld.html
Their "MµCord" is made for wiring, and it is $1.75 per foot. Would be interesting to see if this helps magnetometer readings.
Magnetic Shielding Materials
Learn how to Make Your Own Magnetic Field Shields With These Unique Materials
My humble opinion about the magnetic interference is that a really helpful IMU should provide the capability of remotely install the magnetic sensor. Even most of the real avionics solutions are like this.
Why? Because inertial sensors should be as close as possible to the gravity center (where, in case of an electric helicopter the motor sits) while the magnetic sensor should stay as far from any metal assembly or high current wires as possible.
I'd like to see VN-100 providing an alternate port for an external magnetic sensor - it would become then really helpful for an electric UAV.
My project's biggest challenge (a VTOL) is the fixed point flying and I really need a fast reacting IMU and don't care too much about GPS but I've already purchased the UAV DevBoard.
Currently my best option would be replacing the GPS with the magnetometer for the UAV DevBoard but implementing the 7 state EKF on the DSPIC... If someone implemented this in DSPIC pls. let everyone know (and eventually provide the code too :) )
Second option - get the VN-100 with a remote magnetometer.
Comments
Hi Stanford ... now look at this beauty http://www.quantitec.de/produkte/imu-ins/ and its only 400Euro ! 9dof redundant + cortex + usb + bluetooth + lipo battery.
Regarding the mounting location of the gyros and accelerometers...I am not so sure that we are forced to mount them close to the center of gravity.
The gyros can be mounted anywhere, because the rate of rotation is the same everywhere on the airframe.
Theoretically, the accelerometers should be mounted near the center of gravity, but if they are not, you could compensate for the centrifugal acceleration if you know where they are mounted.
Best regards,
Bill
This company makes magnetic shielding in a variety of forms:
http://www.lessemf.com/mag-shld.html
Their "MµCord" is made for wiring, and it is $1.75 per foot. Would be interesting to see if this helps magnetometer readings.
My humble opinion about the magnetic interference is that a really helpful IMU should provide the capability of remotely install the magnetic sensor. Even most of the real avionics solutions are like this.
Why? Because inertial sensors should be as close as possible to the gravity center (where, in case of an electric helicopter the motor sits) while the magnetic sensor should stay as far from any metal assembly or high current wires as possible.
I'd like to see VN-100 providing an alternate port for an external magnetic sensor - it would become then really helpful for an electric UAV.
My project's biggest challenge (a VTOL) is the fixed point flying and I really need a fast reacting IMU and don't care too much about GPS but I've already purchased the UAV DevBoard.
Currently my best option would be replacing the GPS with the magnetometer for the UAV DevBoard but implementing the 7 state EKF on the DSPIC... If someone implemented this in DSPIC pls. let everyone know (and eventually provide the code too :) )
Second option - get the VN-100 with a remote magnetometer.
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