Mechanical Hardware
The mechanical hardware for control surface actuation was completed and fitted into the dedicated wind tunnel airframe. The fins with inset actuation surfaces are shown in Figure 1.
Figure 1: Fins fitted to the wind tunnel airframe.
High torque digital RC servos were mounted in the configuration shown in Figure 2, to position the servo rods on the outer edge of the airframe so as to run past the solid fuel motor without interference.
Figure 2: Four servos mounted in a compact unit.
The white nylon blocks attached to the servo horns clamp the servo rods in place, and provide a pivot on the servo horn. The hollow aluminium servo rods and control surface lever arms are shown in Figure 3.
Figure 3: Servo rods with the lower linkage that clamps onto the aileron axel.
The fully assembled airframe is shown in Figure 4, ready for use in the wind tunnel. PVC tube was used for the airframe, broken into two sections (motor and controls).
Figure 4: Fully assembled air frame.
Wind Tunnel Testing
An encoder was fitted in the nose cone attached to the wind tunnel support rod. Using an ATMega8 microcontroller, the rocket rotation and fin angle were controlled and monitored over a Bluetooth serial module interfaced with a host laptop. An open loop step response test was carried out to facilitate system identification of the rocket. The raw data obtained is shown in Figure 5.
Figure 5: Encoder data for the step response wind tunnel test.
A closed loop step-hold proportional-integral controller was implemented and tested for various gains and wind speeds. Initially, steady state oscillations resulted for most controllers and wind speeds. Amongst other characteristics identified, by controlling only two of the four fins gave increased stability. Further data collection and simulation is required to fully characterise this phenomenon. Work will continue on the project after the Christmas break.
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