How to define spacecraft equivalent circuit?
This page describes how the spacecraft
equivalent is modelled in SPIS, and how the user inputs allow defining it.
Introduction
Spacecraft electrical behaviour can be modelled
by an electric circuit collecting and emitting currents from and to the plasma.
Some electric components of the equivalent circuit are just a way to mimic
materials (e.g. a dielectric coating is equivalent to a capacitor, possibly
with a resistor in parallel), while others are real physical devices (e.g. a
potential supply between SC ground and a Langmuir probe, a resistor between SC
and solar array grounds). Some components are also uniform over a subsystem of
spacecraft (a coated area), while others are unique. Both notions most often
coincide: uniform areas of coating equivalent to resistors and capacitors (we
can call them continuous components),
and a few real discrete devices (we can call them discrete components). The type and values of the continuous
components can be derived from the material properties (and dynamically from
particle flux for radiation induced conductivity), while the discrete
components have to be defined by user through extra data not deriving from
coating or any local data.
It was thus decided to have an automated
derivation of the continuous components from coating data, while the discrete
data have to be defined in an extra file. Of course another reason of that
choice is that defining a huge number of local values for continuous components
was not reasonable.
Continuous components
Continuous components are thus derived from the
user-defined material properties (mostly local fields). Note
also that they are affected by the switching on and off of conductivities (volume, induced in volume,
and surface) through three global parameters. Their equivalent circuit is
the following:
Discrete components
Discrete components are not plugged on top of
coated surfaces, but between conductive parts of sub-systems, which may be
considered as a sub-system ground. The notion of electric (super) node was
defined to represent such local grounds (a piece of conductor indeed as the
thick line on the figure above). So, an electric super node can be the ground
of SC (predefined Id 0), a solar array ground, the box/ground of a system, the
biased part of a device
Electric (super) nodes must first be defined.
This is done through the local field elecNodeId. The
(super) node Id is mapped by the code on each elementary surface of the mesh
from the definition by the user at group level. For each elementary mesh
surface coated by a dielectric an extra elementary node and extra capacitors
and resistors are automatically generated by the code (left hand side of the
figure above), while no extra node or component is added if uncoated or coated
by a conductive coating (right hand side of the figure above).
NB: node 0 is considered as spacecraft ground,
with the only real consequence that it always exists (other nodes only exist if
their Id appear somewhere on the spacecraft).
Discrete devices then have to be plugged
between the electric (super) nodes. They are defined by the user in a file of a
user-specified
name (default name is "circuit.txt"). The syntax of the circuit
file is described in the UI description page. It supports capacitors, resistors
and active bias. An example of global circuit, including continuous and
discrete components is given here:
s
Initialisations
At simulation start, potentials have to be
initialised (and capacitor charges consistently). They may then remain unchanged
or be modified if electric circuit integration is switched on (electricCircuitIntegrate
parameter).
There are 2 different types of initialisations:
-
the initPot local field
defines the potential on top of coatings
-
the definition ob an active
bias between two super nodes initialises (but permanently!) the potential
between the nodes. Electric super nodes or first initialised to 0 potential.
Then the rule is that V Node1 Node2 deltaV
forces node2 potential to be node1 potential + deltaV.
For a coated surface the potential on top of the coating
("upper" capacitor electrode) is thus defined by initPot, while the
potential on ground below ("lower" capacitor electrode) is defined
the circuit definition (discrete device). For a conductive surface, the two
initialisations may collide: if an active potential is imposed to a conductive
node, the local values obtained from initPot are overridden by the potential
imposed by the bias defined in the circuit file.
A richer panel of initialisation capabilities
may be offered in future if needed (e.g. not-permanent non-zero initialisation
of super node grounds), but it was not found very useful now.