Data Acquisition

Data Acquisition and Storage

Sampling

The data acquisition circuits of HSDL are based on National Instruments Compact RIO real-time modular system. Standard sampling rates are:

  • 100 Hz
  • 200 Hz
  • 500 Hz
  • 1 kHz
  • 2 kHz
  • 5 kHz
  • 10 kHz

These sampling rates are per channel. All channels are sampled with the same sampling rate except for camera and audio channels. The sampling rate is factory-configured and cannot be changed by the user due to the large impact on data files structure and fixed anti-aliasing filters in signal conditioning circuits.

Some sensors due to their physical properties (RTDs, thermocouples) have slow response and therefore limited bandwidth. For these sensors the A/D conversions can actually occur at lower sampling rates and the “missing” samples to full sampling rate are interpolated using zero padding and digital filter techniques.

In standard HSDL configuration the sampling circuits perform multiplexed sampling, where samples from channels are taken at slightly different times, depending on channel index within a data acquisition module. For applications where preservation of phase relations between channels is critical, optional simultaneous sampling hardware components can be provided. Basic ADC resolution of A/D converters is 16 bit.

Storage

The acquired signals are stored in 16 bit data types with internal precision of 0.0015 %. The stream of data saved by HSDL can be calculated as:

Data_stream_in_bytes_per_second = 2 * sample_rate_in_Hz * number_of_channels

There is a small fixed overhead regardless of the sample rate and number of channels, caused by disk files format.

The total amount of data stored by HSDL is limited by available disk space, which is typically in order of several terabytes. If image frames from camera(s) are also stored, the disk space available for analog and digital data can be significantly reduced. For more details please see the Camera Feature section.
After the HSDL disk is nearly full (95 %), the oldest continuous data is overwritten by the newest data, so the HSDL works like a FIFO memory, where the “first out” data is discarded. Thus the HSDL remembers the last N days of data at any point in time, except for frozen event-related pre- and post-trigger data, which are never discarded, unless an authorized user does so in HSDL maintenance software application. The amount of data stored on HSDL disk(s) can be determined as:

Total_disk_space_in_bytes = Data_stream_in_bytes_per_second * 86 400 * number_of_days

Practical required size of HSDL disk space is larger than suggested by the above formula due to additional space necessary for the operating system, program files, configuration files, log files, etc.

In our standard HSDL systems, the disks are organized into RAID1 (mirroring) configuration, which requires at least 2 physical hard drives, each of the same capacity.

Storage Case Study

For HSDL with 200 input channels and 1 kHz sampling rate the data stream, the data stream to disk would be:

2 Bytes per sample * 1000 Hz* 200 channels = 400 000 Bytes per second

The amount of data stored on HSDL disk(s) when the HSDL remembers the last 30 days of data is:

400 000 Bytes per second * 86 400 seconds per day * 30 days = 1 036 800 000 000 Bytes = 1 TB

Taking into account the operating system, HSDL software overhead and RAID1 disk configuration, the HSDL server would need a total of 2,5 TB of disk space in 2 hard drives (min. 1,25 TB each).