Embedded LINUX in a Soft Real-Time Task:
The Canadian Geological Survey Internet Seismometer
© September 2001
Sensor: Each axis has four ADXL105 iMeMS accelerometer chips connected in parallel (to reduce noise and enhance linearity)
Full-scale: + / - 4g
Noise: 0.5 mg RMS, DC to 42 Hz
Bandwidth: DC to 42 Hz (set by 151 coefficient FIR filter)
Sample Rate: 100 samples per second, optional 50, 60, 75, 150, 300.
Total instrument delay: 685 ms due to a two-stage FIR filter implementation.
Nonlinearity: 0.2% of full scale.
Temperature Sensitivity: less than + / - 0.5% change over the operating temperature range.
Cross-Axis Sensitivity: + / - 1%
Alignment Error: + / - 1 degree
Zero-Adjust: none needed, provided the instrument is installed level.
Operating Temperature: - 20 to + 50 degrees C
Power Consumption: 8.1 W max at + 9 V to + 18 V DC, 5.7 W typical when the internal Battery is fully charged.
Built-in UPS: More than 6 h run time with internal 6 V lead-acid battery.
Computer: National Semiconductor Geode, 266 MHz, 128 Mb RAM, 16 Mb Flash disk. Optional 32 Mb up to 512 Mb Flash disk.
Operating System: Linux, Kernel 2.2.16
Data Format: Standard CNSN 5-minute CA data files, optional 5-minute miniSEED or ASCII data files.
Timing Accuracy: + / - 10 ms, assuming reasonable access to NTP-servers.
Total Data Time Lag: 685 ms, due to FIR filter length.
Data Retrieval: Via FTP or TFTP, the 90 Mb RAM disk stores about 2.5 days of data.
Internet security: password protection for FTP access, secure shell access for maintenance.
Connectors: DB9-Female: Serial port to set up configuration parameters, RJ-45 10baseT Ethernet, 2.1 mm female for 12 V ,1.5 A regulated power supply.
Indicators: Heartbeat / diagnostic LED, Ethernet activity LED.
Physical: Hammond 1590EBK case, 7.4'' x 4.7'' x 3.1'' high.
Mounting: Case should be cemented to the basement floor using "plaster of Paris" (recommended, since it makes leveling easier) or bolted down through a hole drilled in the case.