155 lines
6.3 KiB
HTML
155 lines
6.3 KiB
HTML
<html>
|
|
<head>
|
|
<title>The Scan Command </title>
|
|
</head>
|
|
<body>
|
|
<h1>The Scan Command </h1>
|
|
<hr size=4 width="66%">
|
|
<p>
|
|
An important concept in neutron scattering instrument control is a
|
|
"scan". For a simple scan a range of instrument positions is divided
|
|
into equidistant steps. The instrument then proceeds to drive to each
|
|
of these points and collects data at each of them.
|
|
</p>
|
|
<p>
|
|
The general idea of the scan object is, that you configure the
|
|
scan by typing commands at the command line. Once, the configuration is
|
|
finished the requested scan is started. A data file will be written
|
|
automatically to the default location. The scan command can not only scan
|
|
over motors but also about some variables which relate to motors. For
|
|
instance lamda for the wavelength. Scan can scan over more then one variable.
|
|
The syntax of the scan command in some detail:
|
|
<DL>
|
|
<DT>scan clear
|
|
<DD> Clears current scan parameters.
|
|
<DT>scan list
|
|
<DD> lists current scan parameters.
|
|
<DT> scan var <i>name start step</i>
|
|
<DD> Defines a parameter (motor) to be scanned. The name of the parameter, a
|
|
start value and a step width need to be given. More then one scan variable
|
|
can be specified.
|
|
<DT> scan modvar <i>name start step</i>
|
|
<DD> Modifies the scan parameters for scan variable name to the new values
|
|
given.
|
|
<DT>scan getvars
|
|
<DD> Returns a list of currently active scan variables terminated with the
|
|
string -END-.
|
|
<DT> scan np <i>num</i>
|
|
<DD> Sets the number of scan points.
|
|
<DT> scan preset <i>[val]</i>
|
|
<DD> Sets the Preset value for the scan. Without a parameter, inquires the
|
|
current value.
|
|
<DT> scan mode <i>[val]</i>
|
|
<DD> Sets the count mode for the scan. Without a parameter, inquires the
|
|
current value. Possible values are <i>timer</i> or <i>monitor</i>.
|
|
<DT> scan run
|
|
<DD> Executes the scan.
|
|
<DT>scan cinterest
|
|
<DD> This call enables automatic printing of scan counts to your connection
|
|
when new values arise. This command is primariliy of interest for status display
|
|
clients.
|
|
<DT>scan pinterest
|
|
<DD> This function makes the scan command send a notification (the string
|
|
ScanVarChange) to you whenever the scan variables get modified. This command
|
|
is primarily of interest for status display clients.
|
|
</DL>
|
|
</p>
|
|
<h2>Center Scan </h2>
|
|
<p>
|
|
Center scan is a convenience command which starts a scan around a specified
|
|
center value. This mostly used for centering purposes. The syntax is like this:<BLOCKQUOTE>
|
|
<b>cscan</b> <i>var center delta np preset</i>
|
|
</BLOCKQUOTE>
|
|
All parameters must be specified. The parameters and their meanings:
|
|
<UL>
|
|
<LI><b>var</b> is the variable which is to be center scanned.
|
|
Only one can be specified.
|
|
<LI><b>center</b> is the value to use as center of the scan.
|
|
<LI><b>delta</b> is the step width to use for the scan.
|
|
<LI><b>np</b> is the number of points to scan in each direction.
|
|
<LI><b>preset</b> is the preset to use for the counter. As the counter mode,
|
|
the mode currently configured active in the scan object is used.
|
|
</UL>
|
|
<BLOCKQUOTE>
|
|
example: <b>cscan</b> <i>d2hr 0.5 0.01 10 10000</i>
|
|
</BLOCKQUOTE>
|
|
</p>
|
|
<h2>Simple Scan </h2>
|
|
<p>
|
|
Simple scan is a convenience command which starts a scan for one to several
|
|
variables with a simplified syntax. The syntax is like this:
|
|
<BLOCKQUOTE>
|
|
sscan <i>var1 start end var2 start end ... np preset</i>
|
|
</BLOCKQUOTE>
|
|
All parameters must be specified. The parameters and their meanings:
|
|
<UL>
|
|
<LI><b>var1 start end</b> This is how the variables to scan are specified. For
|
|
each variable scanned the name of the
|
|
variable, the start value and the end value of the scan must be
|
|
given. More then one triplet
|
|
can be given in order to allow for several scan variables.
|
|
<LI><b>np</b> is the number of points to scan.
|
|
<LI><b>preset</b> is the preset to use for the counter. As the counter mode,
|
|
the mode currently configured active in the scan object is used.
|
|
</UL>
|
|
</p>
|
|
<h2>Fastscan</h2>
|
|
<p>
|
|
At some instruments there is a fastscan facility. This scan starts a
|
|
motor and the counter. As often as possible or configured, the scan
|
|
module will calculate the difference to the previous count, normalize
|
|
it to the monitor difference and print it. This is usually faster then
|
|
doing a step scan. There are limitations, though:
|
|
<ul>
|
|
<li>Fastscans are restricted to one motor. There is no way that a
|
|
coordinated movement of several motors can be enforced by fastscan
|
|
without mechanical coupling.
|
|
<li>Due to the volatile nature of the beam at SINQ, this type of scans
|
|
come into problems when the beam is off. A warning is printed, but...
|
|
<li>If the motor is driving to fast, there might not be enough
|
|
neutrons around even in the peak to be registered.
|
|
</ul>
|
|
Basically, <b>if you see a feauture in fastscan, it is there, if not,
|
|
that does not mean anything!</b>. Fastscan comes into its own for
|
|
alignment scans or when searching peaks. The syntax is:
|
|
<dl>
|
|
<dt>fastscan motor start stop speed
|
|
<dd>Runs a fastscan on motor between start and stop. Speed is the
|
|
speed with which the motor is run. The motors speed is reset to the
|
|
original value when fastscan finishes.
|
|
<dt>diffscan skip val
|
|
<dd>If fastscan produces to much output on to short intervalls,
|
|
increasing the skip parameter allows to control that. Skip is the
|
|
number of SICS cyles to skip between measurements. Thus this value is
|
|
highly dependent on the overall performance of SICS.
|
|
</dl>
|
|
</p>
|
|
<h2>Peak And Center</h2>
|
|
<p>
|
|
These two commands are related to the scan command insofar as they act upon
|
|
the results of the last scan still in memory. The command <b>peak</b> prints
|
|
the position, FWHM and maximum value of the peak in the last scan. The
|
|
command <b>center</b> drives the first scan variable to the peak center of the
|
|
last scan. Both peak and center use a rather simple but effective method for
|
|
locating peaks. The prerequisite is that the peak is approximatly
|
|
gaussian shaped. The
|
|
algorithm first locates the peak maximum. Then it goes to the left and
|
|
right of the maximum and tries to find the points of half maximum peak height.
|
|
The two points are interpolated from the data and the peak position
|
|
calculated as the middle point between the two halfheight points.
|
|
</p>
|
|
<h2>PSD scans</h2>
|
|
<p>
|
|
Some instrument wish to perform scans with position sensitive
|
|
detectors. In such cases the scan mode can be switched between PSD and
|
|
normal mode. This is done through the commands:
|
|
<dl>
|
|
<dt>scan2d
|
|
<dd>Switches PSAD detetcor scanning on.
|
|
<dt>scan1d
|
|
<dd>Switches back to normal scan mode on a single counter.
|
|
</dl>
|
|
</p>
|
|
</body>
|
|
</html>
|