[psas-avionics] LTC III Design Spec Progress

Tue, 16 Dec 2014 20:52:13 -0800 

I've added more words to the LTC III design spec doc based on the
discussion we had in the rocket room a few weeks back.  Changes have
been committed to the launch-tower repo:

  
https://github.com/psas/launch-tower/blob/master/ltc3/doc/design/LTC3_Design_Spec.adoc
  
Stakeholders and other interested parties, please give it a quick read
and provide feedback.  As soon as the Goals and Requirements are
nailed down, we can get to work on the Design.

HTML copy attached for your convenience.


-- 
Paul
Title: PSAS Launch Tower Computer III Design Specification

FIXME: Feel free to make any additions or corrections.

Abstract

The existing launch tower computer (LTC), the second major design since the group’s founding, is due for an overhaul. It’s only been two years since LTC II was put into service, but there have been rapid advances in available off-the-shelf technology, and our understanding of what we need (and don’t want) in an LTC has continued to evolve.

This document should be considered the source of authority for all questions related to the design and implementation of the LTC III. If the design of the LTC doesn’t conform to this document, either fix the LTC, or update the document.

Definitions

Computer

The single-board computer that comprises the core of the LTC.

Launch Tower Computer (LTC)

The entire system, including the enclosure, the single-board computer, the peripherals, etc.

LTC II Review

The Good

  • Safety interlocks!

  • Solar charging.

  • LiPo ignition battery.

The Bad

  • Phidgets. Expensive (relatively), fiddly, and require their own interface board. Their software stack is thick and troublesome to install.

  • The shore power DC-DC converter integrated with the power board was sorely inadequate.

  • Lead-acid car batteries (LTC II’s main power source) are bulky and heavy.

  • The door covering the side bulkhead (power and arm switches) can’t be closed when the shorting bar is in place. The bar has to be stored elsewhere when the LTC not in use.

  • The PowerPole connectors on the bottom bulkhead are too deeply recessed, located too close together, and poorly labeled.

LTC III Goals

  • Simplify! LTC II has too many circuit boards, too many wires, too many everything. It’s difficult to service most of LTC II’s components without major disassembly. Peripherals (sensors, relays, etc.) should be contained on a single custom PCB.

  • Make it smaller. Reduce the size of the boards, batteries, and potentially even the enclosure.

  • Replace LTC II’s plastic enclosure with a metal enclosure, for durability and RFI protection.

  • Improve the enclosure-to-tower mounting apparatus; make it fast and easy to mount (and dismount) the enclosure to the tower (DIN rail mounting (or something similar).

  • Strive for field-replaceable units (FRU) whenever possible. FRUs can be swapped out with basic tools, in a minimum amount of time.

  • Make it user-friendly (i.e., fool-proof). Any member of PSAS should be able to achieve a high-level understanding of the LTC in a short amount of time. Any member should be able to operate, and if necessary service, the LTC given suitable documentation.

  • Conduct a thermal analysis. Do we need an active cooling system to deal with the heat of a desert launch site?

Requirements

Power Sources

All the ways the LTC can get the juice it needs.

  • 12 VDC rail from main batteries.

  • 12 VDC rail from photovoltaic panels.

Power Supplies

The power rails the LTC provides to its various peripherals.

  • 5 VDC rail to power to computer, etc.

  • 12 VDC rail to ignition battery charger.

  • 24 VDC rail to shore power.

  • Several external expansion power connectors; switchable by the computer.

Networking

  • WiFi connection to Launch Control.

  • Ethernet connection to the rocket; debug; expansion.

Sensors

  • Voltage sensors for each power rail.

  • Temperature sensor for enclosure interior.

User Interface

  • Stand-alone voltmeters on the front panel, for each power rail. (Stand-alone meters have no dependency on the computer being booted.)

  • Front panel LEDs indicating the status of each relay, and the rocket-ready status.

  • Main power switch

  • Ignition arming switch.

  • Ignition shorting bar.

  • Ignition fuse.

  • Label all the things! Components, connectors, switches, oh my!

External Connections

  • Rocket umbilical. Connection state should be detectable by the computer.

  • Away box

  • WiFi coax

  • Distinct, keyed connectors for each external connection. There should be no way to accidentally connect a data cable to a power outlet; or to reverse the polarity of a connector.

  • Several Ethernet connectors that provide external access to the LTC’s internal subnet, for debug and expansion purposes.

  • Several expansion trigger switches, for computer control of cameras, etc. Switches short two external inputs together. Nate proposes 1/8" phono connectors for these.

  • Expansion Ethernet and power connectors can be paired up for use with passive PoE injectors, providing single-cable power and network support for off-board devices (e.g., weather station).

Design

TODO: More words, please.

  • BeagleBone Black SBC

  • BBB flipped upside down and mated to peripheral board via pin headers. A kilt, rather than a cape!

Power Sources

Power Supplies

Ignition Subsystem

Networking

  • Internal Ethernet switch.

References

"BeagleBone Black GPIOs", KiloBaser.

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