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#1 to #140 (Updates)

 

WATER ROCKET - FLIGHT COMPUTER
This section describes the details of the water rocket flight computer (FC) as designed by the Air Command team.

The purpose of the FC is to co-ordinate various events during the flight of a water rocket. One of its responsibilities is to deploy the parachute at the desired time.

Version 1.2
V1.2 Circuit diagram
FC Prototype (front)
FC Prototype (back) Motor & gearbox at the top and launch detect switch at the bottom
FC assembled
Power supply.
FC ready for its maiden flight.

 

Version 1.2 of the FC was successfully flight tested on 11th November 2006 with 100% parachute deployment reliability over the 5 missions flown that day.

Update: As of 3rd February 2007 the FC had flown 17 missions.
16 successful deploys
1 failed deploy. - Suspect grain of sand in the motor gearbox.

This version of the FC was intended to test the ability of the system to function at water rocket type G forces and to test the entire system from launch detect, parachute deployment to the recovery phase. Future versions of this FC will have more capability such as apogee detection for parachute deployment and many other experiments.

Following is a detailed description of the FC V1.2 design.

Circuit Description

The circuit is based around an inexpensive PIC16F628A microcontroller. Weight and reliability played a significant roll in choosing the type of microcontroller. The PIC 16F family of microcontrollers was considered a good compromise between capability and minimum necessary external components.

Please refer to the circuit diagram at left.

You will notice that the PIC has no external clock source in this design. We are using the internal 4Mhz clock. (This was another reason this PIC was chosen)

The PIC has three external inputs, the "Program" button, the "Arm" button, and the "Launch Detect" switch.

The capacitor and resistor on the "Program" button are used for debouncing the switch contacts.

The launch detect switch is simply a microswitch with a weight glued to the armature. The switch must be oriented in such a way that it will activate when the rocket is launched. (All other switches are oriented in such a way that the predominant G-forces do not activate them - this includes the power switch!)

The PIC uses 7 of its output lines to directly drive the segments on the LED display with 330 Ohm current limiting resistors in series with each LED. An 8th output drives the "Armed" LED.

The last PIC output drives the deploy motor. The motor is driven via an opto-coupler in order to isolate the two power supplies. This design feature may be retained for future designs in order to allow the microcontroller to run from a different voltage to the drive actuators or sensors.

The opto-coupler is a 4N33 (mostly because that's what we had on hand) and drives a small signal transistor BC548 to provide enough current for the tiny motor. A more powerful transistor was not necessary.

The two power diodes are placed in series in order to reduce the voltage across the motor.

The power switch is a DPDT type switch and isolates both power supplies.

Assembly Code

Here is the PIC assembly code listing for V1.2:

Flight1_2.asm

Here is the assembled HEX version of the code ready for downloading to the microcontroller.

Flight1_2.HEX

Power Supply

The power supply consists of two separate batteries. This was necessary to isolate the processor supply from the noisy motor power supply. Both batteries are small 6V 150mA batteries housed end to end in a single AA battery holder. A small piece of double sided PCB is placed between the two batteries to provide the other two contacts.

In future designs it is hoped that better filtering of the noise generated by the motor will eliminate one of the batteries.

The motor draws about 80mA when running, but it only runs for about 250ms for each deploy, so the battery should be good for quite a few launches.

The microprocessor and LED displays draw about 8-60mA from the other battery depending on how many LEDs are on. This means that the FC can be on continuously for about 2 hours before needing a battery change. A typical launch event has the FC on for only about 5 minutes (usually less) so quite a few launches can be achieved with one battery too.

Operation

Once the FC is turned on, it waits for the user to either select a time delay, or arm the system. The following settings are available in V1.2: (This range can easily be changed in the code)

LED Display Deploy Delay
(Seconds)
0 3
1 3.25
2 3.5
3 3.75
4 4
5 4.25
6 4.5
7 4.75
8 5
9 5.25
A 5.5
b 5.75
C 6
d 6.25
E 6.5
F 6.75

To change the delay simply press the "Program" button until the desired delay is displayed.

Once the delay is chosen, press the "Arm" button. This will cause the FC to go into a loop waiting for the "Launch Detect" switch to activate. In this mode the time cannot be changed. If you wish to change the delay after the system is armed, simply turn the FC off and then on again.

When the system is armed the "ARMED" LED will light and the LED display will continue to indicate the time delay chosen.

When the FC detects a launch the time delay starts and the LED display indicates "L" for launched. This is mostly used for testing as you are unlikely to be close enough to see the display when you launch the rocket.

After the preset time delay the FC drives the deploy motor and displays "P" for parachute briefly. (Again useful for testing)

The computer then goes back to the beginning waiting for the user to set a new delay.


water rocket parachute deployment

 

Hardware Design

The V1.2 prototype was built on two circular "strip" type PCBs to fit the housing in the rocket. The two PCBs were attached together with nylon screws to reduce overall weight. The "Front" PCB contained the displays and switches while the back PCB mounted the deploy motor and launch detect switch. See photos on left.

The battery holder was kept separate in order to be able to place the weight in another part of the nosecone in order to keep the rocket more balanced.

If you would like further information please contact us.

Improvements

  • The computer did not have an aerodynamic cover, but that is something that can be easily added.
  • The LED display is hard to see in full sunlight, but with a small shroud around the display that should solve the problem.


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