The 5U accelerometer is delivered in a single cardboard box with foam rubber lining. The packaging is specifically designed for the 5U and should be reused whenever you need to transport the sensor. Please note any damage to the packaging when you receive the equipment, and unpack on a clean surface. The package should contain:
the accelerometer
a signal connection cable (if ordered)
a suitable connector.
The sensor serial number label can be found on the lid of the sensor. If you need to request the sensor production history, you will need to quote either the serial number of the sensor or the works order number, which is also provided on the calibration sheet.
To test the 5U before installation, you will need a power source which can deliver 100 mA at 10 to 12 V and a digital voltmeter (DVM) with 1 and 10 V ranges. You should never supply more than 12.5 V to the 5U unit. Also ensure that the supplied cable is connected with the correct polarity (see the Appendices).
To make it easier to measure the output from the sensor, you can use the 5U handheld control unit or a compromised interface box, which can be manufactured from a screw clamp connector block. This will simplify the connections to the appropriate connector pin outputs.
Place the 5U sensor on a flat surface with the POSITIVE ACCELERATION arrow horizontal.
Switch on the power supply.
Connect the voltmeter to pins J and K of the output connector (corresponding to the low gain vertical component.) Measure the output of the low gain vertical component. The steady output voltage should be about zero (± 10 mV).
Now turn the sensor on its side, so that the POSITIVE ACCELERATION arrow points upwards.
The low gain vertical component should now read about –5 V, corresponding to –1g.
If the performance so far has been as expected, the instrument may be assumed to be in working order and you may proceed to install for trial recording tests. Most likely, however, you will need to adjust the mass deflection offset.
NOTE: If you are in any doubt about how to install the sensor, you should contact Güralp Systems.
The sensor can be fixed on to any surface either with three M10 or M12 hexagonal bolt screws and washers (to protect the anodised surface), or with an appropriate fixing clamp. The three holes used to fix the sensor are shown in the mechanical drawing of the sensor base, above. You may find it useful to copy the footprint of the sensor base onto a metal sheet as a template to drill fixing holes into the surface where you want to fix the sensor.
Fix the sensor to the mounting surface, with the POSITIVE ACCELERATION arrow pointing in the correct direction. The accelerometer has no levelling feet, but can use internal simulated level adjustment to compensate as long as it is fixed to a hard, clean surface within 1 degree of the horizontal.
The sensitive direction of the sensor is marked on the outside of the unit, with the arrow pointing towards the positive sensor output orientation. If this arrow is horizontal, the sensor will be responsive to horizontal signals; likewise, if the arrow points upwards, the sensor will respond to vertical ground acceleration (with positive output corresponding to an upwards ground movement.)
Although converting a sensor from horizontal to vertical response involves compensating for the acceleration due to gravity, you do not have to zero the sensor manually. The sensor output offset adjustment is carried out electronically by emulating the shift in mass position. You can then calibrate the offset using a potentiometer within the unit, without any loss of dynamic range.
If required, make a screening box for the sensor, to shield it from draughts and sharp changes of temperature. A suitable box can be constructed from expanded polystyrene slabs (e.g. 5 cm building insulation slabs) with sealed joints between them and a hole drilled for the connector. You can then use high-grade glass fibre sealing tape to fix the leads in position, and fasten the box to the mounting surface. Commercial duct sealing tape is ideal.
Connect the sensor to an analogue input of your digitizer using the cable provided, or to another recording device with a cable made up as described in Section 2.5, page 7.
Installation in Hazardous environments
The fully enclosed aluminium case design of the 5U instrument makes it suitable for use in hazardous environments where electrical discharges due to the build up of static charge could lead to the ignition of flammable gasses. To ensure safe operation in these conditions, the metal case of the instrument must be electrically bonded ('earthing') to the structure on which it is mounted, forming a path to safely discharge static charge.
Where electrical bonding ('earthing') is required during the installation of a 5U instrument, the central mounting hole that extends through the instrument should be used as the connection point. This is electrically connected to all other parts of the sensor case. Connection can be made by either a cable from a local earthing point terminated in a 8mm ring tag or by the mounting bolt itself.
Once installed, you should centre the instrument ready for use. The offset can be as much as the entire output range of the accelerometer, which corresponds to around 1 degree from the horizontal or vertical.
Begin monitoring the output signal of the sensor using a digitizer or digital voltmeter (see Chapter 4, page 13.)
Remove the pressure release cap and insert a thin screwdriver or pot-adjuster into the hole.
Locate the small potentiometer screw head just inside, and turn it in one direction or another until the output voltage is reading zero.
Replace the pressure cap to keep the instrument's electronics protected from water and dust.
After the cover is installed, the accelerometer outputs may drift until the system establishes temperature equilibrium with its environment and the sensor settles down in its position. If required, the offset adjustment can be repeated to achieve a better output offset. With experience, it should be possible to reduce the output level to less than ± 1 mV.
The CMG-5U sensor has two separate outputs, with low gain and high gain. The straight-through cable provided will connect the low gain outputs to the digitizer input (see Appendices). To make use of the high-gain signal, you will need to make up a cable connecting the high-gain pins (C and G) of the sensor to the digitizer input pins (J and K). The sensor outputs have an output impedance of 47 Ω, which is low compared to the input impedance (1 MΩ) of the digitizer.
The low and high gain output lines are differential outputs balanced about signal ground so that either differential drive or single-ended drives of opposite polarity (phase) are available. For a single-ended drive, the signal ground must be used as the signal return path. You must not ground any of the active output lines, as this would allow damaging currents to flow through the output circuits. Also, if single-ended outputs are used, the positive acceleration outputs must be interfaced to the recorder.
If you have two analogue ports available on a connected digitizer, you could make up a three-way cable which connects both high and low gain outputs to the digitizer. You should not attempt to connect both outputs of the 5U to the same digitizer input port.
The sensor is normally powered directly from the digitizer through the 10-way connector, although you can use a separate 12 V DC power supply if you wish (again, a custom cable will be required.) The current consumption from a 12 V supply is about 53 mA. An isolated DC–DC converter installed inside the sensor housing forms the main part of the 5U unit's power supply; its filtered outputs provide the ±12 V required to operate the sensor electronics. The DC–DC converter is protected against polarity reversal.
The calibration signal and calibration enable inputs are referenced to the signal ground. These lines can be connected directly to the digitizer's calibration lines.