The seismometer used in the NARS Baja project is the Streckeisen STS-2 broad band sensor. The STS-2 is an electronic force-feedback sensor that provides an output signal proportional to ground velocity over a broad frequency range. Three identical obliquely-oriented mechanical sensors are used and standard vertical and horizontal outputs are derived by summing the raw sensor signals within the STS-2. The housing is vacuum-tight and designed to minimize the distortion of the package by barometric pressure changes.
The STS-2 is designed for quick and simple installation, wide temperature range operation, and secure transport, while resolving minimum earth noise levels over the frequency range.
|Key features of the STS-2:|
|Generator constant:||1500 V*s/m|
|Electronic self-noise:||Approx. 6 dB below USGS low-noise model between 5 mHz and 15 Hz|
|Response:||Ground velocity between corners 8.33 mHz (120 sec) and 50 Hz|
|Seismic signal output:||+/- 20 V differential range|
|Power input:||10 - 30 V DC|
|Power consumption:||Average 1.0 W|
|Temperature range:||+/- 10 C without mass recentering|
For more information:
G.Streckeisen AG, Messgeraete, Daettlikonestrasse 5, CH-8422 Pfungen, Switzerland
For precise timing we use the Trimble Acutime 2000 GPS smart antenna. It generates a pulse-per-second (PPS) output synchronized to UTC within 50 nanoseconds (one sigma), outputting a timing and position packet for each pulse.
|Key features of the Acutime 2000|
|Accuracy:||UTC 50 nanoseconds (static)|
|PPs (puls per second output):||10 microseconds to 500 milliseconds (user-programmable)|
|Interface:||Serial RS422/485 or RS232 (cable length up to 300 meter)|
|Serial protocols:||TSIP, NMEA|
|Prime power:||+8 V DC to +36 V DC|
|Power consumption:||110mA @ 12 Volts, 1.3 Watt (typical)|
|Housing:||Waterproof, pole mounting|
For more information:
The NARS data logger
The Seismology group of Utrecht University together with the Instrumental group of the Physics department, recently developed a new seismic datalogger. To avoid many drawbacks of current dataloggers which are mainly due to several processors needing to communicate, a new design concept was chosen. The new datalogger consists of only two main components: (1) a data acquisition module, which does the AD conversion and timing. (2) an off-the-shelf computer, for all further data processing. Both modules are connected via a standard printer port interface, assuring that the end user has the maximum freedom in computer selection. As technology advances rapidly, upgrading of the datalogger will become possible by simply replacing the computer by a newer type. For the same reason, service of the system will be more cost effective and less dependent on the manufacturer.
Data logger concept
The data acquisition module is connected via a printer port interface to the computer. It provides 6 analog input channels, 6 I/O lines, and controls the seismometer, temperature sensor, pressure sensor and GPS receiver (figure 5). Each analog input channel is converted with a separate ADC. A high precision clock, independent from GPS, time stamps all ADC data. The data from the GPS is also time stamped by the same clock. Both data streams are kept separate throughout the entire data acquisition. Data processing and time correlation between GPS and ADC samples are postponed until the data are processed by the computer. After further filtering and decimation, the data are written in mini-SEED format to disk.
The NARS datalogger has been optimized for the use with the STS-2 seismometer. Three analog channels of the data acquisition module are used for seismic signal logging while one channel is used to measure the individual boom positions of the sensor. This is important for monitoring the drift of the seismometer. One of the I/O lines is connected to the "Autozero" input of the STS-2, allowing, if needed, an automatic zero leveling procedure of the sensor. The two analog inputs left are used for barometric pressure (Vaisala PTB101) and temperature logging (LM35 National Semiconductor). The GPS receiver is the Acutime2000 of Trimble. This receiver has been factory optimized for time synchronization and will deliver time pulses with an accuracy of UTC +/- 100 nsec. Antenna and receiver are built in one housing and the RS_422 interface allows long cable to be used. The computer is a Compaq Armada 100s with DFT screen. This laptop model is quite small and has relatively low power consumption. Computer and data acquisition module are connected via the printer port. Both use the EPP (Enhanced Parallel Port) mode for high speed bi-directional data transfer. The operating system of the computer is Real Time Linux.
Data acquisition module Summary
The data acquisition module consists of 3 electronic boards (PCB's):(1) The main board, with all ADC's, high precision clock and control logic, (2) a DC/DC board with the voltage converters, and (3) an analog multiplexer board. All boards are housed in a robust water tight cylinder. External connections are made of quality MIL-5015 connectors.
The DC/DC board converts any direct current input voltage between 9 and 34 Volt to the appropriate voltage levels used by the data acquisition module. The module also provides the power for connected sensors like the STS-2, pressure sensor and temperature sensor. The main input voltage as well as the 5 Volt output voltage can be monitored by computer software.
All controller functions, FIFO communication and EPP interface are implemented in programmable logic (FPGA) which makes the design highly robust. One single high precision clock (OCXO) is provided for the system logic operation and timestamping of all acquired data (figure 6). Signal from each analog channel is converted with a 24 bits Delta Sigma ADC to an output rate of 1000 Hz. The 6 ADC's are working in parallel. Each sample taken by these ADC's is time tagged with the local clock and stored in the ADC FIFO. Writing to the FIFO's will set status flags like FIFO empty, half full and full. Depending on the FIFO status, the computer will read out the ADC data. GPS is used for absolute timing. In addition to position and time, the receiver outputs a timing pulse (PPS) which is synchronized to UTC. On reception of this pulse, GPS data is time tagged, stored in the GPS FIFO and also read by the computer.
This small board has been added for use in conjunction with the STS-2 seismometer to measure the boom position of the sensors. One analog channel and 3 I/O lines are used for this. Signal changes are so slow that one measurement every 20 or 30 seconds is enough. This allows for the possibility to use only one analog input channel and measure in turn all 3 boom positions. Channel selection is done by computer software by activating one of the 3 I/O lines.
To avoid corrosion in the field, the housing consists of a water tight cylindrical pot with lid, both made of stainless steel. To obtain sufficient delay for environmental temperature changes, the wall and lid are made 5 mm thick. For easy maintenance the PCB's and connectors are mounted directly on the lid (figure 4).
The operating system of the computer is Real Time Linux. Real Time Linux is a hard real-time operating system that can coexist with Linux OS on the same computer. With Real Time Linux it is possible to create real-time POSIX.1b threads that will run at precisely specified moments of time. The computer software is split in two parts, a time critical real-time thread and a normal Linux task (figure 7). Communication between the real-time thread and Linux task is accomplished by reading and writing of software FIFO's. All communication with the data acquisition module is handled by the real-time thread.
The interface with the user is a graphical X_window based program. The user can change system settings and monitor in real-time the different analog signals. Settings can be changed on the flight and other programs may be run without stopping or affecting the logging process. Some screen dumps are shown in (figure 8 to 11).
For the ADC's we chose the ADS1210 from Burr-Brown. For high precision applications, Burr-Brown recommends an external reference. Detailed analysis shows that the accuracy of the external reference voltage fully determines the analog quality of the design. None of the proposed voltage references by Burr-Brown deliver sufficient accuracy for our system. With a small modification of the system design, we could improve the quality of the reference to the necessary accuracy. A known noise source in delta-sigma converters is the idle tone. Usually a very low level and long periodic signal at the output, produced by the modulator of the converter. With some extra hardware we have overcome this problem and significantly improved the system accuracy for the longer periods. A dynamic range of more than 140 dB at a period of 25 seconds was obtained (figure 12, 13).
|Analog input channels:||6 differential|
|3 for seismometer STS2 X,Y,Z|
|1 for boom position STS-2 X,Y,Z|
|1 for barometric pressure|
|1 for temperature sensor|
|Analog input voltage:||+/- 20 Volt. Total 40 Volt swing|
|Digital input/output lines:||6|
|1 for STS-2 auto zero pulse|
|3 for STS-2 boom position multiplexer|
|1 for GPS power switch|
|Sample periods/stream:||1000, 200, 40, 20, 4, 2, 1 Hz|
|Dynamic range at 20 Hz:||Closed input, >= 146 dB at 1 sec, >= 140 dB at 25 sec|
|STS-2 connected, >= 142 dB at 1 sec, >= 140 dB at 25 sec|
|Data storage format:||Mini-SEED|
|Data sample format:||Integer, Steim-1 compression|
|Input voltage:||9.....34 Volt DC|
|Power consumption:||4.0 Watt|
|With GPS and STS-2 < 6.5 Watt|
|Aging < 1.10-9/day, < 1.10-7/year|
|Computer interface:||EPP 1.9|