Purpose: The LV2.3 rocket design is intended to be a testbed for development of our avionics, recovery, and control systems. We have successfully planned, designed, built, and tested systems in each of these areas. This project represents our next progression from simple control systems such as roll control to a considerably more complex six-degree-of-freedom control system. This step in our development process will likely result in some failures because it is more complex (by orders of magnitude) than anything we have done so far. From our failures and successes, we aim to actively guide our rocket.
There will be a final step in this process, which will be guiding our rocket in an unstable configuration (no fins) using only thrust vectoring. With that, we will push toward orbit.
- Rocket must be statically stable (pass RSO eval).
- Control rocket(s) through a set of waypoints.
- Design shall be scalable to the next airframe.
- Fin design must be one which is characterized (lift, drag, etc.) from sub-sonic thru super sonic
- On error, fins lock neutral through apogee, then servos OFF.
- Fins have short mechanical travel to minimize control gain.
- Fins have margin between mechanical travel and software travel limits to avoid travel stop nonlinearities.
- CAN bus servo interface.
- Hardware watchdog timer centers the servo if FC COMM is lost.
- Realtime RF data downlink to ground including raw sensor readings, control outputs, current tracked rocket state (position, velocity, roll rate).
- Self-test routine at startup must be automatic (key for pre-launch testing).
- In-field setup shall be power-on fin self-test, launch detect trigger, and beep sequences or LED status codes.
- Launch detect shall be indicated via pin-header jumper.
- For test flights, a pre-defined flight plan shall be programmed, which will include a variety of control maneuvers such as translation maneuvers, roll, pitch, yaw.
Calculations for control bandwidth based on Ixx for required force to achieve 100Hz, 20Hz, 10Hz, 5Hz, 1Hz swinging rocket +/- 15 degrees.
- from that determine servo speed
“Tube diameter” through which we must fly in order to get into a good enough orbit.
Models and simulation in matlab
- Matlab models decomposed into sub-modules that can be simulated on their own and worked on by individuals
- With accounting for performance aspects of servos (or other actuators)
- Ability to input trajectory and have control simulate following trajectory
- Ability to simulate jet-stream winds aloft
- Ability to define “assertions”, which if violated during any simulation will raise an obvious indicator that an assertion failed for the given system.
- Creation of scripts, which for the given configuration, can calculate the required control authority, control bandwidth and other critical control parameters.
- Scripts such that control bandwidth can be defined, and control forces, actuator speeds, sensor feedback latencies, resolutions etc can be back-calculated.
- Monte-Carlo simulation of upper and lower bounds of various parameters (e.g. too much/too little control authority, too much/too little control bandwidth, etc)
The current fin design is shown below. There are still details to be worked out in the fin construction, travel stops, servos, etc. Those design details are captured in the analysis sections that follow.
*General/Needs to be categorized
Fin equations (relating fin length, airspeed, air-density, and angle-of-attack to lift and drag) - Done
Mass moment of inertia calculations - Postponed
Measured mass moment of inertia in principle axes - Postponed
Determination of performance requirements - A.L.
Estimate of control forces based on performance requirements - A.L.
Control fin servo design - In Progress...
Bump stop characterization - Waiting on gearbox
IFM sensor purchase - Done
IFM sensor test (in final design holder) - C.M.
Characterize Dewalt motor - Done
Re-run shrink fit test calcs - Done
Confirm dims, finish model and compile drawing for test shaft - Done
Machine test shaft - D.C.
Design test fixture w/ insulator for arbor press - ??
Purchase adhesive - Done
Complete Strain Gage Installation - Done
Calibrate Load Cell - D.K. HOW?? Lever arm??
Characterize linkage rod - D.C.
Test linkage interface (ultimate strength) - D.C.
System diagrams describing datums, PC's and MC's positions and forces
Equations of motion describing the plant
State-Space representation of plant
Pole placement and control design
Forces produced by an aerodynamic fin are highly non-linear and depend on many factors. These dependencies are too complex to include in the overall control models, so fin equations are separated to abstract them from the guidance system. At higher altitudes, fin forces will be replaced by thruster forces, so separating them now is a logical choice.
Our design uses an 8 inch fin with a 60 degree sweep and a flat trailing edge. The trailing edge is perpendicular to the airframe centerline (0 notch ratio).
The fin produces a lift force of 62 N (14 lbs) per degree at Mach 1.2, 1855 N (417 lbs) at a 30 degree AOA.
The above equations require known atmospheric conditions, which can be approximated with a standard atmosphere as a function of altitude only.
Mass Moment of Inertia Calculations
Our project requires the full inertia tensor.
This is obtained from solid models used to design the rocket.
- Model audit
- List all parts in model
- Check material properties in each part (steel aluminum, etc)
- Check mass of parts against actual parts
Measured Mass Moment of Inertia
The mass moment of inertia is measured experimentally in (approximately) the principle axes. Inertia measurements in off axis orientations can be made to confirm the principle axis orientations.
Then, the results are checked against the model.
The performance requirements describe how quickly the rocket and control system must track position and orientation commands. This can be expressed in terms of bandwidth. Bandwidth describes how quickly the rocket must track an orientation command, up to a pre-defined level of error in tracking.
Further, the performance requirements describe how accurately a particular position must be reached; possibly in meters or kilometers for our application.
The closed loop bandwidth of the rocket and controller depends on the size of the rocket and our estimate of disturbances that occur during flight. One answer to the bandwidth requirement has been "as high as possible". Since our rocket doesn't need to track a moving or accelerating object, "as high as possible" is a poor choice. A better choice is to determine what control bandwidth would result in reasonable forces on the rocket, considering the size and mass properties of that particular flight.
To do this, we compute the forces required to force the rocket to track a sine wave orientation command of a variety of frequencies. Next, we choose a command frequency (bandwidth) that will result in forces which are within the capabilities of our airframe. This is repeated in each of the principle axes of the inertia tensor.
For our application, position accuracy of the rocket is determined by the goal of the flight. A test flight of a small sounding rocket has position accuracy tolerances only to evaluate the control. An orbital insertion flight, however, will require some analysis to determine positional accuracy requirements.
Estimate of Control Forces
Control forces must be evaluated to determine the following:
- Fin strength
- Servo strength / power
- Control linkage strength / stiffness
Presently assuming a working force of 470 lbs (From analysis above)
Control Rod Forces
From the fin dimensions and aerodynamics, the fin center of pressure is 1.25 inch behind the middle of the root chord. With the fin pivot at the root chord, the 470 lbs max fin force at 30 degree AOA will produce 588 inch lbs of torque. Since the control rods are mounted at a distance of (CHECK THIS) 1.5 inches from the fin pivot, the normal working control rod force requirement is 392 lbs.
For the type of control rod we have selected, we did some analysis of the rods' strength and spring constant so that it could be considered in the dynamic analysis.
Control Fin Servo
Due to the control fin forces expected (above), servos with significant force/torque are required. For the longitudinal rocket dynamics, the servo speed could be very slow in comparison with the roll control system previously designed and flown. For the roll axis, however, we can expect even faster dynamics that previously seen due to the increased size of the fins. The fin size has increased from 4 60 degree fins at 2.5 inch chord, to 3 60 degree fins at an 8 inch chord. This increases the area available for roll by 7.68, but since the fin actuation travel is 5 degrees instead of 15 degrees, the actual increase is only 2.56.
What that means is, our roll axis response will require at least as much processing power as the original roll control, and our servos should remain as fast as they were on roll control. This is a servo capable of producing 588 inch lbs of torque at the fin, and actuating through 10 degrees of rotation in 0.06 seconds. A quick power analysis indicates that 193 watts are required to do this. Assume motor efficiency of 78%, that gives 250 watts.
|Fin Actuation Requirements Summary|
|Torque||66.43 [N m]||588 [inch lbs]|
|Angle of Rotation||0.1745 [radians]||10 [degrees]|
|Transit Time||0.060 [seconds]||60 [ms]|
|Power Required||250 [watts]||0.335 [HP]|
We have chosen to create a custom designed servo using a DC brush motor, a custom motor control, and a custom gearbox.