Control and Data System
The 10m prototype interferometer needs many sensors to be read out and actuators to be controlled. For this purpose (digitising and collecting the data of the sensors, controlling the interferometer) we use the CDS (control and data system) software developed by our colleagues at the LIGO sites. The CDS is also used as data acquisition system for the science data and to collect data from ancillary systems like environmental monitoring devices (room temperature, humidity etc). After recording CDS provides access to all data collected. Additionally all data have to be time-stamped with an accuracy of about 1µs for later analysis. The CDS also has a responsibility to protect the interferometers by avoiding damage from hazardous control signals.
The 10m prototype interferometer contains at least 500 channels forming multiple-input multiple-output control loops. Digital-to-analog conversion (DAC) and analog-to-digital conversion (ADC) are performed using 16bit fully differential PC expansion cards with either PCI-X or PCIe bus connection and a maximum sampling frequency of 256kS/s per channel (the standard sampling frequency is 64kS/s though). The frontend server machines are standard Operton or Xeon based servers.
As all controlling and data acquisition has to be performed in real-time, special care has to be taken to choose low-latency hardware. Standard gigabit ethernet can only be used for tasks that are not time-critical because of the possibly high latency involved. For low-latency communication we use fibre-based Myrinet connections.
CDS schematic overview
A schematic overview of the CDS is given in Figure 1. Besides environmental monitoring (e.g. seismics, magnetic fields, temperature, humidity, air preassure), input data for the control loops will mainly come from photodiodes, most of the outputs will be connected to voice-coil actuators. In the initial configuration the CDS will have to handle about 500 channels that belong to the following groups:
• The Automatic beam alignment maintains the overlap of the cavity eigenmode with the lngoing laser beam (frequency reference cavity, arm cavities, Khalili cavities), as well as the alignment of the Michelson interferometer
• All relevant mechanical resonances of the suspension systems are damped by local control loops
• The length degrees of freedom of the main interferometer will be controlled by fast voice-coil actuators
• As part of a suspension platform interferometer (SPI), differential movements of the isolated tables are compensated after detection by a LISA Pathfinder phasemeter with a newly designed interface to the CDS
The CDS software is based on a real-time enabled Linux operating system. For the operators control screens and channel access are realised with the experimental physics and industrial control system (EPICS).
The workflow for a digital control circuit involves designing the circuit with the aid of Matlab/Simulink tools, creating a real-time kernel module from this circuit model, loading the module on a real-time enabled frontend computer and controlling the module via EPICS control channels and screens.
Figure 2 provides a flow chart of signals going in and out of the CDS.
To achieve the desired timing accuracy of about 1µs for the data acquisition time-stamping we use a custom timing system. This is based on a 2²²Hz master clock phase-locked to a GPS 1Hz clock. The timing information is distributed via fibres to all real-time enabled data collection units.