Guralp Systems Limited
MAN-6TF-0002 - CMG-FLUTE H/OBS - Operator's Guide

Chapter 3. Sensor operation.

3.1 Set-up

3.1.1 Orientation

The correct orientation of the instrument is identified by a post protruding from one end of the instrument. The end of the instrument with the post should points Northwards and the post itself should be aligned vertically.

3.1.2 Connections

The supplied cable, OEM-CAB-0005, should be attached to the single connector on the instrument.

3.2 Deployment

3.2.1 Installation

The instrument should be connected to suitable recording equipment using the supplied cable. This cable is suitable for direct connection to a CMG-DM24S3EAM/OBS, which can then be connected to customer equipment as required.

The sensor should be aligned as described in section 3.1.1 and then provided with a 9-36 V DC power supply.

Caution: Moving the sensor while power is applied will cause the auto-centring mechanism to attempt to centre the instrument every ten minutes. There is a small chance that this could damage the instrument so it is recommended that the instrument is only moved when powered down.

3.2.2 Sensor operation

The instrument should be powered from a 9-36 V DC power supply. The quiescent power consumption is less than 300 mW at 12 V.

The sensor provides some status information by pulsing pin 8. This is referred to as the “busy line” and is used to signal a busy condition to external equipment, such as a CMG-DM24S3EAM/OBS.

When first powered up, the sensor will perform an auto-centre and then enter a hibernation mode. The instrument will be recording ground movement but no control functions will be active.

During the centring process:

If centring is not achieved as above, the automatic centring feature will be re-activated after around ten minutes. It is not necessary to wait for ten minutes during initial installation: manual centring can be initiated at any time (see section 3.3.2)

Note: If the instrument is tilted more than 30° from the horizontal, the horizontal components will not centre. If tilted more than 60° from the horizontal, the vertical component will not centre.

3.3 Operation

3.3.1 Control lines

The instrument is equipped with a number of control lines. These are TTL-level inputs which are floated to 5V by internal resistors. To assert one of these lines, it should be grounded to pin 9 ('Digital ground') as described in the following text.

Caution: Never connect any of these control lines to any voltage or current source. They must either be left floating (“high”) or connected to digital ground (“low”).

3.3.2 Centring

To wake and centre the sensor, ground pin 5 ('CENTRE') to pin 9 ('Digital ground') for between one and four seconds. The sensor will switch into one-second response mode and check the mass positions. If any of the mass positions are over 25% of full-scale, the instrument will be automatically re-centred.

If the centre line is grounded for over five but less than fifteen seconds, the sensor will check the mass positions in which ever long period mode is currently selected and centre the instrument if the mass positions are over 25%.

3.3.3 The BUSY line

During centring, the busy line (pin 8) will be driven high periodically. If a suitable LED is connected it will flash repeatedly once when one component is centred, twice when two components are centred and three times when all three components are centred.

3.3.4 Response modes

On some build variants, the low-frequency corner of the instrument's response can be remotely configured by grounding pin 6 and/or pin 7 to pin 9 ('Digital ground'). Four frequencies are available (expressed as periods): one, two, five and ten seconds.

To select the required frequency response, ground pins 6 and/or 7 to pin 9, according to the following table. If not grounded, the pin should be left floating.

Desired response

Pin 6
(Response 0)

Pin 7
(Response 1)

1 Second, Short period

grounded

grounded

2 Second, Medium Short Period

floating

grounded

5 Second, Medium Long Period

grounded

floating

10 Second, Long Period

floating

floating

Note: The instrument's processor can also set these modes under software control. When software control is used, the relevant pins are pulled low by the processor. The actual mode used will be determined from the combination (logical OR) of the pins pulled low by the software and those pulled low by external ground connections. If complete control using pins 6 and 7 is desired, the software setting must first be changed to 10 second mode (as described in section 3.3.8) so that both lines are floating.

3.3.5 Calibration enable

To enable a calibration signal to be injected into the sensor, pin 3 ('Calibration Enable') must be grounded to pin 9 ('Digital ground'). The calibration signal should be injected via pin 4. The signal passes though a 51 KΩ resistor, then through the 64 Ω feedback coil to pin 18 ('Signal ground').

3.3.6 Auto-centre

The instrument will continually monitor its mass positions and, if any mass position stays at or beyond 75% excursion for ten minutes, auto-centring is initiated.

3.3.7 Inclination monitoring

If the instrument is tilted over 30° from the vertical, the corresponding horizontal component will not centre.

If the instrument is tilted over 60°, neither the corresponding horizontal nor the vertical component will centre. If the sensor is maintained at such an angle and fails to centre, it will recheck the inclination every 12 hours to see if the tilt has changed. Auto-centring will then be initiated provided at least one mass position is over 75%.

3.3.8 Serial communication

Serial communication with the instrument allows configuration of the frequency response and initiation of centring and calibration.

The serial port runs RS232 in 8-bit mode (8-N-1) at 38,400 Baud. When used with a CMG-DM24S3EAM/OBS, the instrument's serial port is connected to the EAM's PortSA1 and can be accessed from the EAM's command line using the command

minicom PortSA1

The minicom terminal emulator is described in the EAM manual, MAN-EAM-0003, which can be downloaded from the GSL web site. To exit the emulator, type +, then .

The instrument's processor is normally in a low-power “sleep” mode in order to reduce total power consumption. To wake the processor, ground pin 5 ('CENTRE') to pin 9 ('Digital ground') for between one and ten seconds. This can also be done by using the CMG-DM24S3EAM/OBS' web interface: either issue the CENTRE command using the FORTH terminal emulator or click the CENTRE button in the Control→Port A Instrument.. page.

On waking, the following output is produced:

Wake Centre

--------------------------------
MPE ROM PowerForth for Cortex-M3
v7.10 [build 0447] 6 Mar 2013, 11:44:19
================================
STM32L15x
Guralp Systems Ltd - 6TFSoH v1.0 mgs 06/03/13 (Build 0_59)
--------------------------------
13072 T6J77 UDID 0028:0021:3234:3232:0B47:3233 RS232
ESC for manual control 5 4 3 2 1 0
MEMS ADXL345
N/S 0.8' E/W 0.3' (Z -86.8' )
Short Period
5Volt ON
Mass Positions : -7% -3% 6% Centred
5Volt OFF
BroadBand : SP
Sleep.

The inclinometer output and mass positions are displayed amongst other useful details. If the user keys within five seconds of the prompt “ESC for manual control”, a menu is displayed as below:

SENSORS : A_ll, V_ertical, N_orth, E_ast – centre
MODE : S_hortperiod, B_roadband
CONFIG : R_esponse, C_alenable
M_ass positions : I_nclination : O_ffset null ADXL : e_X_it :- KEY ?

The possible inputs are shown in the table below:

Input

Function

Initiate centring of All components.

Initiate centring of the Vertical component only.

Initiate centring of the North/South component only.

Initiate centring of the East/West component only.

Place the instrument into one-second (Short-period) mode.

Place the instrument into Broadband (long-period) mode, as configured by the control lines (see section 3.3.4). The actual period selected will be reported to the operator after this command has taken effect.

Allows the operator to select the default frequency Response of the sensor: i.e. the response that will be selected on power-up and after each centre is completed (but see section 3.3.4). Note that if the default is changed to MSP (medium-short period), Pin 7 will be low so pulling Pin 6 low will therefore result in a short period response and vice versa. DO NOT attempt to drive these lines high.

Toggle between enabling and disabling Calibration.

Print the current Mass position values.

Print the current Inclinometer outputs (in three axes).

Reset the inclinometer outputs to zero. This command is primarily for factory use and should only be issued when it is known that the instrument is accurately vertical.

Exit the command menu. The processor will check the mass positions, perform centring if required and then enter the low-power “sleep” mode.

If is not keyed, the processor returns to sleep mode.

Note: Response modes set using software can be modified by grounding connector pins 6 and/or 7. The software will ground the relevant pins itself, according to the selected configuration. The actual mode used will be determined from the combination (logical OR) of the pins pulled low by the software and those pulled low by external ground connections. If complete control using pins 6 and 7 is desired (as described in section 3.1.1), the software setting must first be changed to 10 second mode so that both lines are floating. If complete control using software is desired, pins 6 and 7 must both be left floating.