Roll Control Experiments

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Project Goal

Previously, I experimented with using canards to control the attitude of a rocket in flight. The testing that was performed using the Totally Stable Rocket showed that canards were not useful in controlling the roll of a rocket. This project is an addition to the Totally Stable project and the Attitude Stability Unit (ASU) project to enable experimenting with roll control.

Project Description

The project is still in its early phases of development. The project web page has been created to enable sharing large data files with interested parties. Improvements will be made to the page when the project becomes better defined.

The characteristics of the project are:

  • Reuse the ASU, but have it manipulate Trailing Edge Control Surfaces (ailerons) on the rear fins of a rocket.
  • Design a small rocket (using a 54 mm motor") to use the trailing edge controls and accommodate the ASU.

Test rocket specifics:

  • Name: tbd
  • Size: ???" long (with 54 mm boat-tail adapter) and 4" dia.
  • Mass without motor: ?? kg.
  • CP position: ??" from nose.
  • CG position: Varies, but has a 1 static margin with the worst case motor.

Key Components

  • 6-Degree of freedom (6-DoF) sensor. We have upgraded to the Analog Devices ADIS16365 sensor. This sensor contains three MEMS gyroscopes and three accelerometers that are oriented appropriately to measure acceleration and angular velocity in three dimensions. It communicates using Serial Peripheral Interface (SPI).
  • Servomotor shaft position sensors. The angular positions of the aileron drive shafts are directly measured. We use a Avago Technologies HEDS 5500 sensor on each canard shaft. These sensors communicate via A & B quadrature output waveforms. Some models of the sensors also provide an index output, which is active when the shaft is positioned at zero degrees.
  • Aileron deflection servomotors. Currently, we are using a Futaba BLS153 (same dimensions as the older S9650) digital servomotor to control each canard. These motors are controlled by pulse width modulation.The input control signal period is 20 ms. Repeatedly applying a 1.5 ms pulse to this signal will make the servo seek its neutral position (0 degrees). Repeatedly applying a .9 ms pulse will make the motor seek to the 45 degree counter clockwise position. Repeatedly applying a 2.1 ms pulse will make the motor seek to the 45 degree clockwise position. Note that the canard gear wedge in the ASU will reverse the direction of rotation, multiply torque by 4.5, and reduce total movement by a factor of 4.5. Therefore, .9 ms pulses will make the canard seek a deflection of -10 degrees and 2.1 ms pulses will make the canard seek a deflection of +10 degrees.
  • CPU board. The CPU board receives data from the sensors, computes the rocket's angular and linear accelerations, velocities, and positions. It then uses this information to compute a set of canard deflections that would correct the rocket's attitude. We have selected the BeagleBoard (beagleboard.org) for our CPU broad. The BeagleBoard features a 720 MHz OMAP3 processor.
  • Sensor Interface Board. The sensor interface board interfaces the CPU board to the sensors and servomotors.
  • The rear fin assembly. The rear fin assembly contains four fixed fins. Each fin features an inboard trailing edge control surface (aileron) that can be deflected +/-10° by high tensile strength wires attached to the ASU aileron drive shafts.

Mechanical Design Drawing

Structural components are made from 6061-T6 aluminum unless otherwise specified.

#D drawing of the Rear Fin Assembly
3-D Drawing of the Rear Fin Assembly
Click to see hi-rez 3-D drawing. (Needs Acrobat Reader Ver. 8+.)

Machining Sequence of the rear fin can (Click to see hi-rez.)
Notes :                                                                                                  
Fin can is drawn upside down.                                                    
4th-axis rotary table chuck grips material from bottom of drawing.
Step 1
Step1
Step 2
Step2
Step 3
Step3
  • Initial material is 3" OD, 2" ID thick walled tube.
  • Faces are trued and needed ID is bored on a lathe. All other operations are performed on a 4-axis mill.
  • Registration marks are drilled near tailstock.
  • Walls are thinned between fin tabs.
     
  • First waterline milling operation to form edge of fin tabs and rear centering ring mounting tabs.
  • Second waterline milling operation to form rest of fin tabs, rear centering ring tabs, and aileron bearing boss.
  • Fin attachment holes are drilled and tapped.
  • Bearing holes are reamed.
Step 4
Step4
Step 5
Step5
Step 6
Step6
  • Walls between fin tabs are thinned to .060".
  • Slots are cut into the fin can walls to reduce its mass.
  • Fin can is cut off of chuck.
  • Rear centering ring mounting holes are drilled and tapped.

CAM files being examined:

CAM project file for Step 5 - Cutting Lightening Slots

Tormach PCNC110 G-code file

Electrical Design Drawings

TBD

Software

The same ASU software is reused with adjustments to accommodate trailing edge controls instead of canards.

Flight Testing the Device

TBD.

Gallery

TBD.

Status

Early stages of development. Manufacturabilty of the concept is being evaluated.