# Chopper Control

FOCUS is equipped with a Dornier Chopper system running two choppers: a disk chopper and a fermi chopper. In most situations the diskchopper is in slave mode. This means his speed is a predefined ratio of the speed of the fermichopper. Furthermore, there is a phase difference between the two choppers in order to allow for the fligh time of neutrons between choppers.

The program handling RS-232 requests at the chopper control computer is rather slow. This would slow the SICS server to unacceptable levels, if any request would be handled through the RS-232 interface. In order to cope with the problem, the SICS server buffers chopper information. This information is updated any minute if not set otherwise.

The chopper system control is divided into several distinct objects: There is the actual chopper controller which mainly serves for answering status requests. Then there are a couple of virtual motors which represent the four modifiable parameters of the chopper control system. These can be driven through the normal drive command. The commands understood by the chopper controller object are:

choco list
  prints a listing of all known chopper parameters.
choco name
  print only the value of parameter name. Possible values for name can be extratcted from the list printed with choco list.
chosta
  This command procedure prints a status listing of the chopper system in a nicely formatted stefan-happy form.

The following virtual motor variables exist for the chopper system.

fermispeed
  fermi chopper speed
diskspeed
  disk chopper speed. Note, that driving this parameter while the chopper system is in synchronous mode will throw an error condition.
phase
  The phase difference between the two choppers.
ratio
  The ratio of fermi to disk chopper speeds.
updateintervall
  The update intervall for the buffering of chopper data. Units are seconds. Setting to low values here will compromise the responsiveness of the SICS server.

Each of the variables kindly prints its current value when given as a command. Modifying values happens through the normal drive command. For instance the command:

    drive fermispeed 10000

will drive the fermi chopper to 10000 RPM eventually and if no problem occurs. Please check your input carefully for all chopper commands. Dornier has provided no way to stop an erraneous command in its software. So, if you intend to run the chopper to 1000 RPM and mistyped it as 10000 then you'll wait for 20 minutes until the chopper is at speed!