Guralp Systems Limited
Fortis - Technical Manual

PreviousNext

1. Preliminary Notes 2. Introduction 3. Fortisᴾᴴ ‑ the Post‑hole Fortis 4. Getting started 5. Installation 6. Accessories 7. Calibration 8. Connector pin‑outs 9. Revision History

Section Index: 4.1. Unpacking and packing 4.2. Initial testing 4.3. Setting the gain 4.4. Output polarity

Chapter 4. Getting started

4.1 Unpacking and packing

The Fortis is delivered in environmentally-friendly, flat-packable, suspension packaging. The packaging is specifically designed for the Fortis and should be re-used 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, the calibration pack and either a connection cable, if ordered, or a 19-pin line-socket, to which you can terminate your own cabling.

Caution: Although the Fortis is a strong motion instrument, it contains sensitive mechanical components which can be damaged by mishandling. If you are at all unsure about the handling or installation of the device, you should contact Güralp Systems for assistance.

4.2 Initial testing

Pre-installation testing is most easily accomplished with a Güralp Hand-held Control Unit (HCU) (see section 6.2), which allows access to each individual output and input, but you can quickly assemble a test-rig using a spare connector and a screw-clamp connector block. You will need a DC power supply capable of providing over 100 mA at between 10 and 36 Volts and, if you are not using an HCU, a voltage meter which can measure in ± one-Volt and ± ten-Volt ranges.

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 the unit for trial recording tests. In many cases, there will be a slight offset to the readings – this can be disregarded.

4.3 Setting the gain

The gain of the Fortis can be controlled by a rotary switch, recessed into the base of the instrument, by a configuration setting on the web page of a Güralp Minimus digitiser, or by selectively grounding none, one or both of two control lines, exposed on the main instrument connector.

Pin-out details for the connector are given in Section 8.

Note: The Fortis is delivered from the factory with the gain-setting switch in position 2. This results in a full-scale output corresponding to ±2 g.

4.3.1 To adjust the gain using the switch

Switch position

Amplifier gain

Full-scale (g)

0

8

0.5

1

4

1

2

2

2

3

1

4

4.3.2 To adjust the gain using a Güralp Minimus digitiser

Ensure that the switch is set to position three and then open the web page of the Minimus. Click the Setup tab:

Ensure that the Instrument Type drop-down menu is set to Fortis…

… then select the desired gain from the Instrument Gain drop-down menu:

The new setting takes effect immediately.

Note: This technique only works correctly if the physical gain-control switch on the instrument is set to position three (3).

4.3.3 To adjust the gain using the control lines

Select 0

Select 1

Amplifier gain

Full-scale (g)

Equivalent switch position

low

low

8

0.5

0

low

high

4

1

1

high

low

2

2

2

high

high

1

4

3

Pin-out details are given in Section 8.

4.3.4 Gain interaction

Güralp do not recommend attempting to use more than one gain control method simultaneously. The following information is provided for completeness only.

The gain control switch acts directly on the two gain control lines, “Select 0” and “Select 1”, grounding neither, one or both as appropriate. The following table shows the effect of using both the physical switch on the instrument and the control lines at the same time.

Switch

Select 0

Select 1

Amplifier Gain

Full-scale (g)

?

3

high

high

1

4

high

low

2

2

low

high

4

1

low

low

8

0.5

2

high

ignored

2

2

?

low

8

0.5

?

1

ignored

high

4

1

?

low

8

0.5

?

0

ignored

ignored

8

0.5

?

The following table shows the effect of using the physical switch on the instrument and the web page of a Güralp Minimus digitiser simultaneously:

Switch

Minimus (web) setting

Amplifier Gain

Full-scale (g)

?

0

ignored

8

0.5

?

1

Unity

4

1

×2

8

0.5

×4

4

1

×8

8

0.5

2

Unity

2

2

×2

2

2

×4

8

0.5

×8

8

0.5

3

Unity

1

4

×2

2

2

×4

4

1

×8

8

0.5

4.4 Output polarity

The polarity of output from each component of the instrument is as follows:

Direction of ground acceleration

Polarity of Z output

Polarity of N/S output

Polarity of E/W output

Upwards

positive

zero

zero

Downwards

negative

zero

zero

Northwards

zero

positive

zero

Southwards

zero

negative

zero

Eastwards

zero

zero

positive

Westwards

zero

zero

negative

If the ground accelerates northwards, this moves the casing of the instrument northwards and the N-axis inertial mass is left behind. From the instrument's frame of reference, the mass appears to have been deflected southwards. The feedback system then needs to provide a balancing force to accelerate it northwards and this, by design, will result in a positive output signal from the N/S component.

If the instrument is mounted with the 'N' arrow pointing downwards, gravity will try and pull the inertial mass in the direction of the instrument's N-axis. The feedback system then needs to provide a balancing force to accelerate it upwards which, from the instrument's frame of reference, is now southwards. This is the opposite of the situation described above, so the output from the N/S component will now be negative.

The converses are also true: if the ground accelerates southwards, the instrument will produce a negative output signal from the N/S component and if the instrument is orientated with it's 'N' arrow pointing upwards, it will produce a positive output signal from the N/S component

The polarity of the output signals with respect to acceleration can be demonstrated by selecting a sensitivity of 1 g, 2 g or 4 g and orientating the instrument as shown in the following diagram:

PreviousNext

1. Preliminary Notes 2. Introduction 3. Fortisᴾᴴ ‑ the Post‑hole Fortis 4. Getting started 5. Installation 6. Accessories 7. Calibration 8. Connector pin‑outs 9. Revision History