GeoSIG AC-23 Accelerometer User Manual

GeoSIG AC-23 Accelerometer

DOCUMENT REVISION

2

| ****

05.07.01

| Update pictures and connector description for new 12 poles version.

Added principle of operation.

| ****

SR

| ****

JG

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3| 30.05.2003| Changes for new housing| LG| JG|
4| 06.06.2003| Corrected axis polarity for new housing. Movement in axis- direction gives positive response.| LG| JG|
5| 05.12.2003| Updated| SR| JG|
5| 21.04.2004| Reformatted| JG| JG|
6| 05.05.2006| Adapted to new DH housing| SR| JG|
7| 12.12.2007| Adapted to new sensor housing File name and title change| TB| TB|
8| 13.12.2007| Detailed sensor full scale & offset adjustment| TB| TB|
9| 18.03.2011| Board connector pinning added| THL| TAB|
10| 05.08.2013| Update PCB to V3 and orientation of axis| JLT| MAE|
11| 15.10.2014| Adjusted formatting of figures.| SER| MAE|
12| 29.07.2015| Picture update| PAT| JON|
13| 07.05.2019| Mounting section updated| VAG| ALB|
14| 27.05.2019| P/N correction on item 1.2| JOG| TAB|
15| 20.03.2020| Correction of typo on page 9| VAG| JOG|
16| 24.02.2022| Update housing drawing| KEC|  |
17| 05.08.2022| Updated chapter 2, electrical connector Document formatting changed| ALM| ALB| KEC

Disclaimer

• GeoSIG Ltd reserves the right to change the information contained in this document without notice. While the information contained herein is assumed to be accurate,
• GeoSIG Ltd assumes no responsibility for any errors or omissions.

Copyright Notice

No part of this document may be reproduced without the prior written consent of GeoSIG Ltd. Software described in this document is furnished under a license and may only be used or copied in accordance with the terms of such a license

Trademark
All brand and product names mentioned are trademarks or registered trademarks of their respective holders

Warnings and Safety

The sensor housing provides no protection against explosive atmosphere. It must not be directly operated in area where explosive gases are present

Basic Specifications

output OR

2.5 ± 2.5 Volt single-ended output OR 0 – 20 mA Current-loop (OPTION)

Frequency range| from 0.1 Hz to 100 Hz

(optional 200 Hz)

Protections| All connectors pins protected by Transzorb diodes and VDR
Power supply| 10 – 15 VDC
Current drain| Average 90 mA @ 15 VDC

Electrical Connector

All the AC-4x accelerometers are supplied as standard with a 2 m connection cable. Based on the intended use, the 12 pin metallic style connectors will be supplied in one of the following options: Binder Serie 623 or Binder Serie 423

Binder Serie 623

Figure 1, Binder Serie 623 Connector
Cable gland nut has to be determined as per cable external diameter and must be separately ordered. It has also to provide the cable shield connection to connector case.

Binder Serie 423

Figure 2, Binder Serie 423 connector
Cable gland nut has to be determined as per cable external diameter and must be separately ordered. It has also to provide the cable shield connection to connector case

Connector Pin Description

The connector pin assignment and cable colour code can be observed in the table below

Mounting

Figure 2, AC-2X housing

Small size and single bolt attachment allow the AC-2X to be easily installed saving installation time. Levelling is accomplished via three-point levelling screws

Do not overtighten the three-point levelling mechanism. This may damage the sensor

Figure 3, Sensor housing dimensions

The accelerometer must be firmly mounted to a surface and levelled, as the application requires. Check to be sure that the accelerometer is aligned to produce the desired output signals. Acceleration in the direction indicated on the case will produce a positive output signal. The orientation definitions as shipped are: X = East, Y = North and Z = Vertical (Up). The accelerometer has single-bolt, 3-feet-levelling mechanism The surface should have a scribed north/south orientation line accurately surveyed from reliable markers. The X-axis of the sensor must be pointed to East or to any other main direction of the structure to monitor. One M8 expanding nut rock anchor must be used for the sensor fixation

Theory of operation

Introduction

The AC-23 sensor package is a triaxial accelerometer designed for free field and industrial applications regarding STRONG-MOTION earthquake survey, monitoring and research. This sensor is well suited for applications where a high sensitivity is required. The AC-23 sensor can be optionally installed into a rugged protective housing. This optional protective housing is in stainless steel for optimal environmental resistance. As option, the protective housing could be executed with an IP68 grade for free field location where the possibility exists of housing submersion. The sensor could be installed on floor or wall with a modification of the axis organization. With the help of the TEST LINE, the complete sensor can be very easily completely tested. Full scale can be field selected by the user with jumpers.

Principle

The accelerometer is based on a geophone mass-spring system with electronic correction. This type of sensors gives a very good stability in temperature and aging because of the very simple principle. It uses a damped mass spring oscillator called “Geophone” to convert seismic movement into electrical value proportional to the velocity. In a graphic with constant acceleration, the geophone response will present a maximum at the frequency called “Natural Frequency” which is the resonant frequency of the mass-spring oscillator. Above and below this point, the response will decay with one pole slope (±20 dB / decade). The corrector will over-damp the geophone by applying a voltage with opposite polarity over the geophone and the output response will be flat and proportional to the acceleration in this frequency band.

Figure 4 AC-23 Sensor block diagram

The geophone is connected in a resistor bridge, driven by a feedback amplifier, which applies the amplified bridge differential signal in opposite polarity. The bridge is balanced during calibration. The test-line shifts the voltage at one side of the bridge, which produces a current flow in the geophone. This current flowing in the Geophone will move the seismic mass. The movement of the mass generates a voltage across the Geophone, which is detected by the differential amplifier and induces an output signal

Figure 5 TEST INPUT configuration

Electrical Configuration

The full scale can be adjusted without gain re-calibration by means of jumpers with fixed 0.1% precise amplifiers

Figure 6, Full scale setting

The full scale adjustment can be:
Full scale Jumper position

• 0.1 g 1-2
• 0.2 g 3-4
• 0.5 g 5-6
• 1.0 g 7-8
• 2.0 g 9-10
• 4.0 g 11-12

Offset Adjustment

Figure 7, Offset potentiometer location

After the new full scale has been selected, the offset must be checked and eventually the potentiometer  must to be re-adjusted to remove any small offset change at output. In such case, be sure to identify correctly the offset potentiometers and don’t touch any other potentiometers as it would void its calibration. Connect a Digital Voltmeter (DVM) as shown on the above figure and adjust the offset potentiometers so that the DVM readings stay within the ranges indicated in the following table, according to the sensor output range given in Section Error! Reference source not found

Mounting (downhole sensor)

The sensor must be installed in a 3-inch inclinometer tube. At least a 100 mm borehole must be drilled. Depending on the soil condition, it could be required to drill a higher dimension hole and to implement a 120 mm PVC casing to insure a free path when the inclinometer tube is inserted in the borehole.

Borehole preparation

Note: Do not scale the drawing

Do not allow concrete mix from casing sealing to enter the casing

Inclinometer tube installation

Note: Do not scale the drawing. The number of sections is only an example

Figure 9 – Inclinometer tube installation

Sensor installation

Note: Do not scale the drawing. The number of sections is only an example

Figure 10 – Sensor Installation

Inclinometer casing assembly

The borehole must have a casing or the soil must insure that a free path for the inclinometer tube is warranted. It is recommended to insert the inclinometer tube as soon the borehole is ready. The free path for the inclinometer tube should be 10 to 15 cm, 12 cm typically. It could be required to insert some water in the casing to sustain the water pressure at the bottom of the borehole. The inclinometer tube should be mounted with a maximum deviation of ±1° / 3 meters and with a maximum deviation from vertical at sensor location of ±3°. The functional limit for the sensor is ±9°. The water level in the inclinometer tube should be maximum 50 meters, including fast elevation due to heavy rain. It is recommended to use the optional assembly kit that GeoSIG can provide (optional) with the inclinometer tube. It will insure a perfect sealing of the tube elements and would avoid concrete mix to enter the tube.

The dimensions of the inclinometer tube are

Figure 11 – Inclinometer tube dimensions

INCLINOMETRIC CASING (3 m section)

A Inner diameter 76.1 mm

B Groove outer diameter 86.4 mm

C Thickness         2.2 ±0.1 mm

D Groove inner diameter 82.0 mm

Length 3 meters

Weight 1.4 Kg/m

Borehole diameter         > 120 mm

| COUPLING ELEMENT

A Inner diameter 81.0 mm B Outer diameter 92.0 mm C Thickness 2.2 mm

D Groove inner diameter 87.6 mm

Length 300 mm

Weight 0.5 kg

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Table 3 – Inclinometer tube dimensions

The following elements will be inserted in the borehole

Axis orientation Figure 15, Down hole axis orientation

Before the sensor is inserted in the inclinometer tube, the guiding system must be mounted bellow it. The guiding system must be orientated before the insertion.
The engraved mark on bottom cover is showing the positive direction of X axis:

Installation Verification

Please note that temperature compensation device is mounted for each axis inside the sensor and that the temperature in the sensor has to stabilize before accurate measurement can be done. Allow at least half an hour for temperature stabilization

GeoSIG Ltd, Wiesenstrasse 39, 8952 Schlieren, Switzerland Phone: + 41 44 810 2150, Fax: + 41 44 810 2350 info@geosig.com, www.geosig.com

References

• GeoSIG | Seismometer, accelerometer, SHM, and other earthquake monitoring and measuring solutions

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