VibWire-108 Theory Of Operation, Systems Drawing

System Diagram - Fig 1

Introduction

The following diagram details the operations of the differential input version of the VibWire-108. Figure 1 shows the component parts used to make up VibWire-108. An analogue data acquisition interface built into the instrument is used to read the temperature/analogue input signals. A a singled ended input system is available and works in exactly the same however there is no differential input amplifier

Sensor Excitation

The VibWire-108 initialises any vibrating wire sensor by first sending an excitation pulse of around 200 micro-seconds at 8V amplitude to the sensor coil and forces it to vibrate. The coil is excited and is forced to oscillate at its natural frequency and so produces an echo pulse that is transmitted back to the instrument. The natural frequency of the sensor is determined by the physical properties of the coil and so the physical properties of the sensor used to make an investigation..

The initial start oscillations of the sensor coil is ignored and the echo signal then measured repeatedly over 100 oscillations for a period of 2 seconds and the average value reported. Ensemble averaging the results over a number of measurement cycles reduces false peaks and noise that may contribute errors to the true sensor signal.

The phased locked loop is used to lock the phase and frequency of the sensor excitation signal to that of the echo signal received from the sensor. Each subsequent ping to the sensor are in phase the the previous echo and as such always act to add to the amplitude of the signal, See Fig 3. In this way the VibWire-108 only uses a minimum amount of energy to excite the sensor. The sensor coil oscillates at it’s natural frequency and this depend upon the physical conditions into which it is exerted.

Time

Excitation
F Excit

Excitation Pulse
in phase with echo

Fig 3

There are little unwanted harmonics in the echo signal and these fade quickly after the first few oscillations. The echo signal settles to a sinusoidal signal that is measured by the frequency counter.

As the excitation signal follows the natural frequency there is little added noise from unwanted harmonics that can in worst case distort frequency measurement. Signals in the order of a few millivolts are all that are needed to make a measurement.

Common Mode Rejection

The VibWire-108 uses a high quality differential amplifier to connect the vibrating wire sensor to the precision frequency counter. The differential amplifier removes common mode signals such as 50/60 Hz mains pickup etc.

A digital filter used in collaboration with a low and high pass filter is used to condition the sensor signal before it is measured by the frequency counter. The analogue filters are set at 100 Hz and 10KHz respectively and remove the out of band signals.

Differential Input Sensor Connection

Figure 3 shows how a vibrating wire sensor is connected to the differential input of the VibWire-108. The shield supplied within some cable is used when noise pickup is a problem and is terminated at the connector shown. The shield can also be terminated at any Gnd point within the instrument and the effect will be the same. The vibrating wire sensors are connected to the +Vib and -Vib inputs respectively.

Differential Vibrating Wire Sensor
Connection (Fig 2)

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