The GaiaCode Sigma Ʃ digital force feedback accelerometer is based on a truly rectilinear suspension system, integrated with an 8-channel 24-bit high resolution digitiser. The three axis strong motion accelerometer is housed in an ‘O’ ring sealed hard anodised waterproof aluminium case. The system is self-contained, except for the power source. Three separate digital data outputs ports are provided, these being Ethernet, USB and serial RS-232. An optional external GPS module time synchronises the digital accelerometer data or, as a backup NTP time synchronisation is available.
Figure 1 Strong motion accelerometer with Mil spec connectors.
The SIGMA is a compact and robust forced analogue feedback triaxial accelerometer with an integral 8-channel 24-bit high resolution acquisition system, with an exceptional performance, suitable for local and regional recording, for earthquake and engineering applications.
An isolated dc-dc converter ensures galvanic isolation of the system and operates from +9VDC to +36VDC.
The analogue feedback accelerometer has an extremely large dynamic range and the 8-channel 24-bit acquisition system is incorporated to exploit the full dynamic range of the sensor as high gain and low gain digital outputs.
The low and high gain outputs are set digitally using the (PGA) Programmable Gain Amplifiers of the 8-channel Acquisition system.
Nominally the high gain outputs are set to have a 12 times larger output than the low gain outputs.
The sensor's analogue differential outputs are interfaced to the differential inputs of the digitiser. The digital part of the circuity is optically isolated from the front end analogue to digital converter circuitry.
Figure 1 shows the sensor's connector turret, allowing easy connection to the sensor. All the connectors are waterproof with O ring seals to a depth of 2 meters of water. The display provides the state of health information of the feedback sensor and the digitiser.
Full-scale low and high gain sensitivity is digitally user-adjustable from ±4.0 g to ±0.33 g on individual channels of the digitiser.
The standard frequency pass band is flat to acceleration from DC to 350 Hz.
Detailed sensor calibration information is provided with every sensor, including sensor dc calibration levels, the frequency response of the instrument and the transfer function in poles/zeros notation. The digitiser calibration values are also provided and the calibration values are stored within the digitiser.
It is extremely simple to install the digital sensor, a single point slotted base bolt point ensures that the sensor can be fixed firmly to the installation point. Three leveling feet ensure that the sensor is leveled and locked to the installation bolt.
The North/South ordination points are machined on the sensor base and the sensor component fixing screws are fixed with dowel pins to achieve the highest possible orientation accuracy. Any errors in pointer to sensor orientation are less than ± 0.1 degrees.
After installation, the sensor output offsets are nulled electronically, without needing to expose the inside of the accelerometer.
The digital sensor noise performance is better than 0.15 µm/s2/√ Hz. The sensor dynamic range exceeds 145 dB.
The seismic accelerometer is provided with different connector options, either the Gaiacode D type waterproof connectors or Mil-spec. connectors.