GENERAL INFORMATION
The Spectrometer Instrument Control Environment (SPICE) is a LabVIEW
based program written at ORNL for control of neutron scattering
instruments. The core components of the program (outlined below)
are written to be independent of the type of instrument allowing the
software to be expanded to different instrument types. LabVIEW
was chosen as the language for development for a number of reasons:
1.
The most compeling reason to choose LabVIEW is the ease with which
hardware can be interfaced. Almost all modern hardware comes
shipped with LabVIEW drivers or (at least) drivers which are easily
accessed from LabVIEW. This greatly simplifies the process of
hardware integration in SPICE.
2.
As LabVIEW's programming language, G, is a graphical programming
language, a graphical
user interface (GUI) is easily generated. The presence
of a GUI greatly improves user friendliness.
3.
LabVIEW supports a large number of communications protocols including
TCP/IP, UDP, ActiveX and DataSockets (most of these protocols are used
quite extensively in SPICE - for instance, the Status Display program
is totally driven by LabVIEW DataSockets).
4.
There are numerous mathematical functions and analysis tools
incorporated directly into LabVIEW. This includes linear algebra,
basic calculus (integration, differentiation, etc.), basic statistics,
and curve fitting routines including Levenburg Marquardt algorithms.
5.
Finally LabVIEW is largely platform independent allowing the code to be
written and executed on several operating systems including Windows,
Mac OS, and several Unix platforms.
CORE
COMPONENTS
The core of SPICE consists of 2 components, a command line interpreter
and a hardware interface layer. These 2 components are completely
independent of the type of instrument being controlled.
1. Command Line Interpreter
The command line interpreter sequentially executes commands from a
first-in-first-out (FIFO) stack. This command stack takes the
form of a simple ascii file on the control computer which means that
any process which is capable of writing to this ascii file is able to
issue commands. Currently, commands can be issued through a
provided command-line of the main program window or from the graphical
user interface (see Information about the Graphical User
Interface for more information).
The commands are all executed dynamically. This means that none
of the commands are hard-wired into the program and commands can be
added, removed, or modified without restarting the program. The
available commands are specified in a top-level configuration file.
The command interpreter includes an integrated alias structure which is
similar to the alias capabilities of UNIX. This allows users to
rename commands or combine a frequently used command sequence into a
single, simple command. There are 3 levels of alias definitions
included in SPICE: 1. System
- alias definitions specific to a specific class of instrument 2. Instrument
- alias definitions specific to a specific instrument 3. User -
alias definitions which which the user can specify
Another feature of the command line interpreter is a structure which
allows access to any variable stored in LabVIEW global variable space
from the command line. These variables are accessed using a
syntax @(name) where name and its definition is stored in a variables
configuration file. Detailed information about the use of this
structure is given in the How to use command-line variables
guide.
2.
Hardware Interface Layer
The hardware interface layer consists of a series of classes and
methods following the example of standard object-oriented programming
languages. Any SPICE hardware driver must contain all the methods
required for the specific class of hardware. For instance, to add
a new motor controller and its included motor axes, 2 drivers are
required, one for the controller class and one for the motor class
(which will call the methods present on the controller driver).
Provided that both drivers have the methods required for their
respective classes, entries are added in the proper configuration
files, the main program is re-initialized, and the new hardware is
ready to be used. In this scheme, hardware can be added and
removed dynamically without requiring a restart of the main program.
The currently implemented classes in SPICE are: controller, motor,
counter (includes ratemeters), interfaces (gpib, rs232, etc.), and
pseudomotors (not physical motors but quantities which can be derived
from physical motors).
The triple-axis implementation of SPICE contains several advanced
features designed largely to expand flexibility. These include:
1.
UB matrix: The conversion of angles to and from reciprocal
space coordinates uses a UB matrix formalism. The UB matrix is
typically used in a 4-circle diffractometer but has been modified in
SPICE to work with the standard triple-axis geometry where one usually
has 2 perpendicular arcs and a sample rotation axis to define the
sample orientation. In addition, the formalism has been
generalized to allow inelastic scans to be performed. The
advantages of this formalism are: (a)
The arcs can now be used to make movements in reciprocal space (b)
Any crystal symmetry (including tricilinc) can be used within the program
2.
Tabbed Graphical User Interface - see the Introduction to
the GUI guide for more information.
3. Scanon
- the scanon structure allows any combination of drives and counts to be
converted into a properly formatted scan. For more information,
see the How to create a custom scan guide.
4.
Countfiles - Generally a scan consists of drives followed
by counts to some specified preset (for instance, 30 seconds).
The usual count in a scan can be replaced by a series of commands
specified in a countfile. See the How to use a countfile for more
information.
5.
Status Display - A stand-alone program has been written to
monitor the state of the triple-axis spectrometer. This program
is written using LabVIEW DataSockets and, consequently, can be operated
from any computer with a valid internet connection. More detailed
information about the Status Display can be found in the Introduction
to Status Display guide.
6.
Drive, Scan, and Count - We have implemented a scheme where
all changeable quantities (including physical motors, pseudomotors,
sample enviroment quantities, flipper voltages, etc.) are all treated
as motors. This means that the command to change the magnetic
field strength is the same as the command to drive a physical motor
which is the same as the command which changes the energy value.
A consequence of this is that the majority of experiments can be
performed with a knowledge of 3 basic commands: drive,scan,
and count.
This means that users only need to learn these 3 basic command to
be able to perform many experiments and helps with user friendliness.
