SPIRIT II Timer

S.P.I.R.I.T. II Rocket Timer
Team Members: Matt Facchine, Chris Engel, Steve Aumack, Mike Koontz and Travis Teates

Problem

Our project is the design of a main rocket timer for the payload of the SPIRIT II rocket. SPIRIT (Student Projects Involving Rocket Investigation Techniques) is a student-run organization at Penn State in which students gain hands-on experience building a complex payload for the rigorous sounding rocket environment. The objective of the SPIRIT II rocket, which has a tentative launch date of November 2002, will be to conduct wind experiments in the mesosphere. The Technical Supervisor is Timothy F. Wheeler of the Penn State Electrical Engineering Department. The specifications that we must consider when coming up with a design for the Timer controls include:

A 10-15 minute flight time operation
Minimize Timer clocking error
High vibration and low atmospheric pressure sustainability
Output up to 8 user-defined enable levels
Output up to 4 Square timing pulses

Other variable factors that our design considers include:

Designing a circuit to regulate power spikes and outages
Timer remote programmibility
Minimize PC board size

All of the circuits designed for the SPIRIT II timer must fabricated on a PC board and encased in a box to secure the timer inside the payload.

Solution

The main problem was trying to decide which type of microcontroller to use, the P-Brain or the HC11 Trainer Board, to control the Timer.

Trade-off Chart

	P-Brain	HC11 Trainer Board	Advantage
Cost	$136 Each	$68 Each	HC11 Trainer Board
Memory	256 bytes RAM, 2Kbytes EEPROM	256 bytes RAM, 512 bytes EEPROM	P-Brain
Reprogrammibility	Controlled by switches	Controlled by jumper blocks	Push
Size	Small module (50 Pins)	Large board (output pins)	P-Brain
Implementation	Needs 1 PC Board	Needs 1 PC Board	Push
Complexity of PC Board	Multiple Layers	1 Layer	HC11 Trainer Board
Relative Cost of PC Board	More	Less	HC11 Trainer Board
Programming/Testing Platform	486 based PC with Motorola PCBUG11 Software	Any computer with Terminal Program	HC11 Trainer Board

After analyzing the trade-off chart above, we decided the Trainer Board was the best solution. The two areas that the P-Brain outperfromed the Trainer board are in memory and size. However, we were able to write the code for 10-15 minutes of flight time, with 12 outputs and a clock error of 16ms, for less than 512 bytes. Therefore the relatively small memory size of the Trainer Board was adequate. We were able to overcome the size problem of the Trainer Boards by "sandwhiching" the 4" X 6" PC board with the Trainer Boards, thus fitting all components within the specified 4" X 6" box. Since the Trainer Board matched or outperformed the P-Brain in all other areas the choice was clear.

The purpose of power electronics of this project is to provide the microcontroller with a constant five volts regardless of the external power conditions. Another task was to electrically isolate the microcontroller outputs from the experiments. We met the first objective by using a voltage regulator to drop the rocket voltage to five volts. A battery supply, consisting of eight AA batteries, was added to supply power in the event of main power interruption. A diode was added in series between the backup battery line and the main power to keep the power constant and prevent current from flowing in the reverse direction. The outputs were isolated by using IGBT's. The IGBT's provide a path to ground for the experemints when given a voltage from the microcontroller.

Upon implementation of our Rocket Timer,
S.P.I.R.I.T. II will experience...

Success!!!

***A Special Thanks to Contractor Bob Wilson for making this project possible.***