Guralp Systems Limited

Chapter 1. Introduction

The CMG-5T accelerometer is a three-axis strong-motion force-feedback accelerometer in a sealed case. The sensor system is self-contained except for its 10 – 36V power supply, which can be provided through the same cable as the analogue data. An internal DC–DC converter ensures that the sensor is completely isolated.

5T cylinder edge

The 5T system combines low-noise components with high feedback loop gain to provide a linear, precision transducer with a very large dynamic range. In order to exploit the whole dynamic range two separate outputs are provided, one with high gain and one with low gain. Nominally, the high gain outputs are set to output a signal 10 times stronger than that from the low gain outputs.

The 5T sensor outputs are all differential with an output impedance of 47 Ω. A single signal ground line is provided as a return line for all the sensor outputs.

Full-scale low-gain sensitivity is available from 4.0g down to 0.1g. The most common configuration is for the 5T unit to output 5 V single-ended output for 1 g (≈ 9.81 ms-2) input acceleration. The standard frequency pass band is flat to acceleration from DC to 100 Hz (although other low pass corners from 50 Hz to 100 Hz can be ordered.) A high frequency option provides flat response from DC to 200 Hz.

Each seismometer is delivered with a detailed calibration sheet showing its serial number, measured frequency response in both the long period and the short period sections of the seismic spectrum, sensor DC calibration levels, and the transfer function in poles/zeros notation. Installation is simple, using a single fixing bolt to fix the sensor onto any surface. No sensor levelling is required.

Optionally, you can use a Güralp Control Unit (CU) and Breakout Box (BB) to distribute power and calibration signals to the sensor and to receive the signals it produces. The CU can also trim DC offsets during installation, if required. It is available in both rack-mounted and water-resistant portable formats. The accelerometer housing itself is completely waterproof, with a hard anodised aluminium body and “O”-ring seals throughout. The photograph below shows two instruments under test by long-period total immersion in a tank of water.

5T immersion test