170 lines
6.2 KiB
Text
170 lines
6.2 KiB
Text
Specifying and Configuring and Aircraft Electrical System
|
|
=========================================================
|
|
|
|
Written by Curtis L. Olson <curt@flightgear.org>
|
|
|
|
February 3, 2003 - Initial revision.
|
|
|
|
|
|
Introduction
|
|
============
|
|
|
|
The FlightGear electrical system model is an approximation. We don't
|
|
model down to the level of individual electrons, but we do try to
|
|
model a rich enough subset of components so that a realistic (from the
|
|
pilot's perspective) electrical system may be implemented. We try to
|
|
model enough of the general flow so that typical electrical system
|
|
failures can be implimented and so that the pilot can practice
|
|
realistic troubleshooting techniques and learn the basic structure and
|
|
relationships of the real aircraft electrical system.
|
|
|
|
An electrical system can be built from 4 major components: suppliers,
|
|
buses, outputs, and connectors. Suppliers are things like batteries
|
|
and generators. Buses collect input from multiple suppliers and feed
|
|
multiple outputs. Outputs are not strictly necessary, but are
|
|
included so we can name generic output types and provide a consistent
|
|
naming scheme to other FlightGear subsystems. Finally connectors
|
|
connect a supplier to a bus, or a bus to an output, and optionally can
|
|
specify a switch property (either a physical switch or a circuit
|
|
breaker.)
|
|
|
|
At run time, the structure specified in the electrical system config
|
|
file is parsed and a directional graph (in the computer science sense)
|
|
is built. Each frame, the current is propagated through the system,
|
|
starting at the suppliers, flowing through the buses, and finally to
|
|
the outputs. The system follows the path of connectors laid out in
|
|
the config file and honors the state of any connector switch.
|
|
|
|
|
|
Suppliers
|
|
=========
|
|
|
|
A supplier entry could look like the following:
|
|
|
|
<supplier>
|
|
<name>Battery 1</name>
|
|
<prop>/systems/electrical/suppliers/battery[0]</prop>
|
|
<kind>battery</kind>
|
|
<volts>24</volts>
|
|
<amps>60</amps> <!-- WAG -->
|
|
</supplier>
|
|
|
|
<name> can be anything you choose to call this entry.
|
|
<prop> is the name of a property that will be updated with the state
|
|
of this supplier.
|
|
<kind> can be "battery", "alternator", or "external".
|
|
<volts> specifies the volts of the source
|
|
<amps> specifies the amps of the source
|
|
|
|
Currently <volts> and <amps> are not really modeled in detail. This
|
|
is more of a place holder for the future.
|
|
|
|
For alternators, you must additionally specify:
|
|
|
|
<rpm-source>/engines/engine[0]/rpm</rpm-source>
|
|
|
|
The value of the rpm source determines if the generator is able to
|
|
produce power or not.
|
|
|
|
|
|
Buses
|
|
=====
|
|
|
|
A bus entry could look like the following:
|
|
|
|
<bus>
|
|
<name>Essential/Cross Feed Bus</name>
|
|
<prop>/systems/electrical/outputs/bus-essential</prop>
|
|
<prop>/systems/electrical/outputs/annunciators</prop>
|
|
<prop>/systems/electrical/outputs/master-switch</prop>
|
|
</bus>
|
|
|
|
<name> is whatever you choose to call this bus
|
|
|
|
You can have an arbitrary number of <prop> entries. Each entry is the
|
|
name of a property that will be updated with the value of the current
|
|
at that bus. This allows you to wire devices directly to the bus but
|
|
does not allow you to insert a switch or circuit breaker in between.
|
|
See "Outputs" and "Connectors" if you want to do that.
|
|
|
|
|
|
Outputs
|
|
=======
|
|
|
|
An output entry could look like the following:
|
|
|
|
<output>
|
|
<name>Starter 1 Power</name>
|
|
<prop>/systems/electrical/outputs/starter[0]</prop>
|
|
</output>
|
|
|
|
An output isn't entirely unlike a bus, but it's nice conceptually to
|
|
have a separate entity type. This enables us to specify a common set
|
|
of output property names so that other subsystems can automatically
|
|
work with any electrical system that follows the same conventions. An
|
|
output lives on the other side of a switch, so this is how you can
|
|
wire in cockpit switches to model things like fuel pump power,
|
|
avionics master switch, or any other switch on the panel.
|
|
|
|
<name> is whatever you choose to call this bus
|
|
|
|
You can have an arbitrary number of <prop> entries. Each entry is the
|
|
name of a property that will be updated with the value of the current
|
|
at that bus. This allows you to wire devices directly to the bus but
|
|
does not allow you to insert a switch or circuit breaker in between.
|
|
See "Outputs" and "Connectors" if you want to do that.
|
|
|
|
Other FlightGear subsystems can monitor the property name associated
|
|
with the various outputs to decide how to render an instrument,
|
|
whether to run the fuel pump, whether to spin a gyro, or any other
|
|
subsystem that cares about electrical power.
|
|
|
|
|
|
Connectors
|
|
==========
|
|
|
|
An connector entry could look like the following:
|
|
|
|
<connector>
|
|
<input>Alternator 1</input>
|
|
<output>Virtual Bus 1</output>
|
|
<switch>/controls/switches/master-alt</switch>
|
|
<initial-state>off</initial-state> <!-- optional tag -->
|
|
</connector>
|
|
|
|
A connector specifies and input, and output, and any number of
|
|
switches that are wired in series. In other words, all switches need
|
|
to be true/on in order for current to get from the input to the output
|
|
of the connector.
|
|
|
|
<input> specifies the <name> of the input. Typically you would
|
|
specify a "supplier" or a "bus".
|
|
|
|
<output> specifies the <name> of the output. Typically you would
|
|
specify a bus or an output.
|
|
|
|
You can have an arbitrary number of <switch> entries. The switches
|
|
are wired in series so all of them need to be on (i.e. true) in order
|
|
for current to pass to the output.
|
|
|
|
Note: by default the system forces any listed switches to be true.
|
|
The assumption is that not every aircraft or cockpit may impliment
|
|
every available switch, so rather than having systems be switched off,
|
|
with no way to turn them on, we default to switched on.
|
|
|
|
This is a problem however with the starter switch which we want to be
|
|
initialized to "off". To solve this problem you can specify
|
|
<initial-state>off</initial-state> or
|
|
<initial-state>on</initial-state> Switches default to on, so you
|
|
really only need to specify this tag if you want the connector's
|
|
switch to default to off.
|
|
|
|
|
|
Summary
|
|
=======
|
|
|
|
The electrical system has a lot of power and flexibility to model a
|
|
variety of electrical systems. However, it is not yet perfect or
|
|
finished. One major weakness is that it doesn't yet model degraded
|
|
battery or generator power, and it doesn't model the "charge" of the
|
|
batteries in case of a generator failure.
|