Guralp Systems Limited
Fortis - Technical Manual


1. Preliminary Notes 2. Introduction 3. Getting started 4. Installation 5. Accessories 6. Calibration 7. Connector pin-outs 8. Revision History

Section Index: 5.1. The Break-out Box 5.2. The hand-held control unit

Chapter 5. Accessories

5.1 The Break-out Box

The Fortis can be supplied with an optional Break-out Box (BoB). This is a small, passive device which simplifies the operation of the Fortis when used with third-party digitisers or when frequent local centring is required, as with some mobile structures.

The Fortis uses the same BoB as Güralp 5TC, 5TB and 40T instruments.

The Break-out Box has three 26-way bayonet connectors wired identically. These are labelled, SENSOR, CONTROL and RECORDER. A fourth connector, a 10-way bayonet labelled POWER, is wired in parallel with the power supply lines of the other connectors and can be used to provide power for the instrument in situations where the digitiser does not provide power.

The Break-out Box is equipped with two buttons, marked ENABLE and CENTRE. Pressing both simultaneously for more than five seconds initiates the centring function (offset-nulling) in the instrument. They are wired in series so that the ENABLE button functions as a safety interlock to prevent accidental activation of the CENTRE button.

The Break-out Box can be permanently connected in-line between the Fortis (SENSOR connector) and the digitiser (RECORDER connector). This leaves the CONTROL connector free. A hand-held control unit (see section 5.2) can be temporarily connected here in order to measure the instrument's outputs using the built-in analogue volt-meter, if required.

The pin-out for the SENSOR, CONTROL and RECORDER connectors is identical to that for the Fortis itself and is given in Section 7.1.

The pin-out for the POWER connector is given in Section 7.2.

5.2 The hand-held control unit

The Fortis can be supplied with an optional Hand-held Control Unit (HCU). This is a portable device which provides easy access to the accelerometer's outputs and calibration controls as well as displaying the output acceleration on an analogue meter. It can also be used as a convenient way to power the instrument when used with a digitiser which does not provide a suitable power supply.

The Fortis utilises the same HCU as Güralp 3-series instruments but there are a few differences in the way the device is operated.

The output from the Fortis is acceleration, not velocity, and the instrument does not have mass position outputs. To view the acceleration output, put the main selection switch in one of the three 'VEL' positions.

Note: The Fortis does not have mass position outputs so the meter will not be active if the switch is in any of the 'MASS POS' positions.

Because the Fortis does not have mass-locks or mass position outputs, some parts of the HCU should be ignored.

5.2.1 Signal meter

The upper section of the HCU contains a simple voltmeter for monitoring the signals from the instrument.

To monitor the outputs, the dial should be set to V, N/S or E/W in the VEL range.

You can set the range of the meter with the RANGE switch. When switched to 10 V, the meter ranges from –10 to + 10 V (as marked.) When switched to 1 V, the range is between –1 and +1 V.

Note: The V, N/S and E/W MASS POS settings are not applicable to the Fortis. The meter will not be active if any of these 'MASS POS' positions are selected.

5.2.2 Calibration

You can calibrate a Fortis through the HCU by connecting a signal generator across the yellow and green CAL SIG inputs and setting the CAL ENABLE switch to the “V” position. The sensor's response can now be monitored or recorded and calibration calculations carried out. See Section 6 for more details.

Note: All three components will be calibrated simultaneously when the CAL ENABLE switch is in the “V” position.

Caution: Do not set the CAL ENABLE switch to either the N/S or the E/W positions. Depending on the setting of the gain switch (see section 3.3.1), doing so may change the instrument's gain.

After calibrating, switch the CAL ENABLE switch back to the OFF position.

5.2.3 Centring (offset-nulling)

The next section of the HCU contains three toggle switches, marked ENABLE, UNLOCK/LOCK and CNTR. The Fortis has an automatic offset-nulling facility which can be operated by holding both the ENABLE switch and the CNTR switch down for five seconds. If the signal meter select switch is in any of the VEL positions and the RANGE switch is in the 1 V position, a series of centring pulses will be visible until the operation is complete complete.

Note: The red LED marked BUSY (on the right, beneath the meter) is not used by the Fortis and should be ignored.

5.2.4 Open-loop response

The Fortis can be temporarily placed into open-loop mode by holding the ENABLE switch down while simultaneously holding the UNLOCK/LOCK switch in the UNLOCK position. Releasing either switch immediately returns the instrument to normal (feedback) mode. When in open-loop mode, the instrument will react much more quickly to changes in its orientation during, e.g., physical levelling.

Note: The LOCK position of the UNLOCK/LOCK switch has no effect on the Fortis and should be ignored.

5.2.5 4 mm terminal posts (“banana” sockets)

Beneath the switches are 4 mm terminal post connections, also known as “banana sockets” for each of the signal output lines from the instrument, for attaching to your own equipment as necessary.

Only the left-hand two columns of blue connectors, marked “VELOCITY”, “+” and “-” and the green “SIGNAL GROUND” connectors are relevant to the Fortis. The connectors marked “VELOCITY”, “+” and “-” are connected to the differential acceleration outputs. For single-ended use, use only the “+” connectors, together with a green SIGNAL GROUND connector, (all of which are equivalent).


1. Preliminary Notes 2. Introduction 3. Getting started 4. Installation 5. Accessories 6. Calibration 7. Connector pin-outs 8. Revision History