Image Acquisition and Processing of Remotely Sensed Data (ImAP RSD). Dec08-01: Inertial Measurement Unit (IMU) Team: Luis, Julian, Amar , Matt Client: Matthew Nelson - Space Systems and Controls Lab (SSCL) Advisor: Dr. Basart. Presentation Outline. Background/History
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Dec08-01: Inertial Measurement Unit (IMU)
Team: Luis, Julian, Amar, Matt
Client: Matthew Nelson - Space Systems and Controls Lab (SSCL)
Advisor: Dr. Basart
The ISU SSCL requires an Inertial Measurement Unit (IMU) and data logging system for the ImAP RSD project.
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An accelerometer coupled with a rate gyro can efficiently be used for attitude determination purposes. Rate gyros measure angular rotation rates. By subtracting out known linear accelerations, an accelerometer can be use as a tilt measurement device. These two angles can be combined in an optimal fashion to accurately determine attitude.
The HABET balloon and payload system has been modeled as a simple, 2-D rigid pendulum. From this model we can determine angular rates, as well as the normal and tangential components of acceleration that the payload will experience.
The equation of motion on the left can be numerically integrated to obtain rotational rates. This model is only for roll/pitch rates.
These rotational rates will help us choose the appropriate rate gyro for our project. We have simulated this model on Simulink. The results follow.
Fig. Model for determining roll/pitch rates.
Roll/pitch rates under 75°/sec.
From past data, we have determined that yaw rates typically range from 20°- 50°.
FFT results suggest a sampling rate greater than 90Hz.
Rotational rates and sampling rate obtained from math model meet functional requirements. Rate gyro used in this project, MLX90906, measures 300deg/sec, which satisfies both functional requirements and math model.
By assuming a simple pendulum, the acceleration equation reduces to the one boxed in red. This equation measures tangential and normal components of acceleration.
These acceleration values will help us choose the appropriate accelerometer for our project. We have simulated this model on Simulink. The results follow.
Greatest magnitude of acceleration expected is under 1.5g.
FFT results suggest a sampling rate greater than 80Hz.
Acceleration and sampling rate obtained from math model agree with our functional requirements. Accelerometer used in this project, MMA7260Q, measures ±2g’s, which satisfies both functional requirements and math model.
We are required to log for a maximum of 3 hours. A 1 GB SD Card will be used for data storage
Using a baud rate of 19200 symbols/sec, we can log for approximately 28 hrs (maximum) at this rate
The power budget for the IMU components totals at .5034 Amp-Hours and will be powered by a 4.8 Amp-Hour battery leaving 4.2966 Amp-Hours for other systems.
Encoder test platform
Rate gyro angular rate
We compare it by differentiate and angular rate
p,q,r are angular rates measured by the rate gyro in the body frame. To transform into the inertial frame, we utilize the transformation matrix, T. We run this through RK4 and produce the desired angles, and thus the payload attitude.
Vout = Output of Accelerometer
Voffset = 0g offset of Accelerometer
1g = Earths Gravity
= Angle of tilt
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but also measures more than the required specifications.
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Power and weight
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