Guralp Systems Limited

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1. Introduction 2. Installation 3. Calibration 4. Inside the 5T 5. Calibration information 6. Connector pinouts 7. Specifications 8. Revision history

Section Index: 3.1. Absolute calibration 3.2. Relative calibration 3.3. Calibrating accelerometers

Chapter 3. Calibration

The 5T is supplied with a comprehensive calibration document, and it should not normally be necessary to calibrate it yourself. However, you may need to check that the response and output signal levels of the sensor are consistent with the values given in the calibration document.

3.1 Absolute calibration

The sensor's response (in V/ms-2) is measured at the production stage by tilting the sensor through 90 ° and measuring the acceleration due to gravity. Local g at the Güralp Systems production facility is known to an accuracy of 5 digits. In addition, sensors are subjected to the “wagon wheel” test, where they are slowly rotated about a vertical axis.

The response of the sensor traces out a sinusoid over time, which is calibrated at the factory to range smoothly from 1g to –1g without clipping.

3.2 Relative calibration

The response of the sensor, together with several other variables, is measured at the factory. The values obtained are documented on the sensor's calibration sheet. Using these, you can convert directly from voltage (or counts as measured in Scream!) to acceleration values and back. You can check any of these values by performing calibration experiments.

Güralp sensors and digitizers are calibrated as described below.

In this diagram a Güralp digitizer is being used to inject a calibration signal into the sensor. This can be either a sine wave, a step function or broad-band noise, depending on your requirements. As well as going to the sensor, the calibration signal is returned to the digitizer on a full rate channel (older digitizers used one of the 4 Hz auxiliary (Mux) channels). The calibration signals and sensor output all travel down the same cable from the sensor to an analogue input port on the digitizer.

The signal injected into the sensor gives rise to an equivalent acceleration (EA on the above diagram) which is added to the measured acceleration to provide the sensor output. Because the injection circuitry can be a source of noise, a Calibration enable line from the digitizer is provided which disconnects the calibration circuit when it is not required. Depending on the factory settings, the Calibration enable line must be either allowed to float high (+5 to +10 V) or held low (0V, signal ground) during calibration: this is specified on the sensor's calibration sheet.

The equivalent acceleration corresponding to 1 V of signal at the calibration input is measured at the factory, and can be found on the sensor calibration sheet. The calibration sheet for the digitizer documents the number of counts corresponding to 1 V of signal at each input port.

The sensor transmits the signal differentially, over two separate lines, and the digitizer subtracts one from the other to improve the signal-to-noise ratio by increasing common mode rejection. As a result of this, the sensor output should be halved to give the true acceleration.

CMG-5T instruments are tuned at the factory to produce 1 V of output for 1 V input on the calibration channel. For example, a sensor with an acceleration response of 0.25 V/ms-2 should produce 1 V output given a 1 V calibration signal, corresponding to 1/0.25 = 4 ms-2 = 0.408 g of equivalent acceleration.

3.3 Calibrating accelerometers

Both the DM24 digitizer and Scream! software allow direct configuration and control of any attached Güralp instruments. For full information on how to use a DM24 series digitizer, please see its own documentation. If you are using a third-party digitizer, you can still calibrate the instrument as long as you activate the Calibration enable line correctly and supply the correct voltages.

Input calibration signal strength (MB)

697,221 counts

Returning signal strength (Z3)

701,512 counts

Sensor acceleration response

0.254 V/ms-2

Equivalent accel. from 1V calibration

1.968 ms-2

Digitizer input port sensitivity

3.507212 µV/Bit

Calibration channel sensitivity

3.491621 µV/Bit

If you prefer, you can inject your own signals into the system at any point (together with a Calibration enable signal, if required) to provide independent measurements, and to check that the voltages around the calibration loop are consistent. For reference, a DM24-series digitizer will generate a calibration signal of around 16000 counts / 4 V when set to 100% (sine-wave or step), and around 10000 counts / 2.5 V when set to 50%.

PreviousNext

1. Introduction 2. Installation 3. Calibration 4. Inside the 5T 5. Calibration information 6. Connector pinouts 7. Specifications 8. Revision history