Bill Atkinson Inertia Measurement Unit P09233
Airframe Measurements
1) DEVICE Name:
An Inertial Measurement Unit (IMU) is a device that uses accelerometers and gyroscopes to measure axial and rotational accelerations as well as orientation based upon initial conditions. A six degree of freedom IMU has 3 accelerometers and 3 gyroscopes to measure x, y, and z accelerations and roll, pitch, and yaw. This sensor is important for UAV’s because it will indicate the aircrafts roll, pitch, and yaw rates, accelerations in x, y and z. The IMU will also indicate the aircrafts orientation in space. It is needed to give this information to a flight control system to allow an aircraft maintain flight in a particular direction, altitude, and orientation.
2) Design Specification:
The requirements for the IMU are to know the orientation of the aircraft while it is in flight as well as its horizontal, lateral, and vertical accelerations. The device that we picked out is the ADIS16350 IMU from Analog Devices. The min/max voltage required for the IMU is between 4.75-5.25 volts.
This IMU satisfies the requirements given to it because it is a six degree of freedom
IMU that is designed to be used in guidance, control, and stabilization. The six degrees of freedom allows it to measure roll, pitch, yaw, and axel accelerations. These are all the accelerations that act on an aircraft.
There are several ways to measure orientation and axel accelerations however they all revolve around the same type of devices: IMU’s, gyros, and accelerometers. A six degree of freedom IMU uses 3 gyroscopes and 3 accelerometers to measure roll, pitch, yaw, and accelerations in x, y, and z. Many IMU’s also contain a sensor that tells it in which direction gravity is pulling it so that it knows its orientation at all times. The IMU is the simplest solution to this problem because it combines all the sensors needed into one device. It would be possible to build your own IMU by buying 3 gyroscopes and 3 accelerometers and have them measure the required information. This method would require more work, in terms of setup time and programming. It might cost less in terms of dollars but it would cost more time to build your own IMU. For our project it made the most sense to buy an IMU.
3) Pugh Analysis:
The IMU’s that we looked the most promising were the MicroStrain’s and the
Analog Devices. MicroStrain is one of the brands that NASA’s payload directed flight program uses to measure roll, pitch, yaw, as well as movement in x, y, and z. There were three IMU’s from MicroStrain we looked at the Inertia-link, 3DM-GX1, and 3DM-GX2.
All three of these IMU’s were very similar in their capabilities with the main differences being price, power usage, and extra features that were not essential to our project such as wireless communication and magnetometers. The advantage of using one of the
MicroStrain products is that it is more familiar and it is a proven technology though a
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Bill Atkinson Inertia Measurement Unit P09233
Airframe Measurements team member. The main disadvantage with this brand is that all of their IMU’s are very expensive around $1400 and higher. This is where Analog Devices fits in. All of their
IMU’s are relatively inexpensive at around $300. Like MicroStrain all of the Analog
Devices IMU’s look the same in there specs and it was not really clear what the major differences were between them based upon their data sheets. The Analog Devices IMU that looked the best was the ADIS1650 do to its lower price. The advantages of the
ADIS1650 to the MicroStrain IMU’s is that it is smaller, has about the same accuracy, it uses less power, and it is a lot less expensive. The main disadvantage of the Analog
Devices is lack of experience with the product, and it has fewer input method options.
The Pugh analysis was designed to help compare similar products to one another to compare chosen qualities against each other to see which one is the best. The first step of the analysis was to pick a sensor to be the baseline and compare several of the other makes and models against it, using +’s to indicated something better and –‘s to indicate areas where it was worse. I chose 6 models of IMU from three brands. I chose three from
Analog Devices because all of them were in our price range, two from MicroStrain because NASA uses that brand for their UAV’s and one from Gladiator Technologies to see how it compared. All three Analog Devices and both MicroStrain’s made it through the first screening. Next we weighted the earlier used criteria to compare the products directly to see which got the best score. The weights were as follows: cost 20%, Size 3%, weight 3%, Resolution 18%, interface 29%, Power Consumption 7%, and Sampling Rate
20%. It was found that the ADIS16350 got the highest score due largely to its low cost.
The second best was the ADIS16354. The third best was the Inertia-link thought we will not follow up on this one because of its high cost. It receive third position do the way numbers are given in this comparison chart.
4) Components Specification:
The IMU interfaces with the MCU through a serial interface and breakout board. It is mounted in the box though two 1.588mm holes on its base. The IMU will be vibration isolated using a rubber dampening sheet attached to its base. Vibration dampening is very important for IMUs in order to read there orientation and accelerations as accurately as possible.
Detailed Specs of the IMU
The IMU requires between 4.75 and 5.25 Volts
There are three sensitivity setting for this IMU ±300 o
/s, ±150 o
/s, and ±75 o
/s.
The data output rate is 350Hz.
The IMU costs about $500, the one selected for this project was inexpensive compared to most others. The IMU will likely need to be calibrated/ zeroed every time it is used to increase its accuracy; this is to prevent errors in its calculations.
There is no/very little maintenance needed for the IMU other than to keep it away from too much moisture and not to drop it or through against things. It is a robust device but it is a very expensive piece of hardware to be treated without care.
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Bill Atkinson Inertia Measurement Unit P09233
Airframe Measurements
5) Test Plan:
We will come up with a test cycle with the MAV team that will test both of our
IMU’s. This test cycle will allow for a direct comparison between our two IMU’s. The
IMU’s will be tested on a test stand that is being designed by the MAV team that will be able to rotate along two axes. The angular rotation rates of what the IMU’s read will be compared to that of the test stand cycle as well as against each other.
6) Test Results:
Discuss the Results of the testing, reference plots and diagrams, and discuss what they mean, how well the device performed, and make a recommendation whether to implement the device or to look for a better alternative.
7) Final Design:
If the original device does not meet the Design Specifications then repeat the above until a final device is chosen. Otherwise talk about the Final Design.
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