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Measuring Vibrating Wire Strain Gauges

Vibrating wire sensors are used in the mining, civil and hydrological engineering, and other geophysical disciplines.

There are two versions of the dataTaker data loggers which support the vibrating wire strain gauges as follows

dataTaker 515 Geologger

dataTaker 615 Geologger

The Geologgers are functionally the same as the dataTaker 505 and dataTaker 605 data loggers, with the addition of an internal vibrating wire strain gauge support module and associated commands for reading vibrating wire sensors.

The Geologgers support all the signal types and functions available in the dataTaker 505 and the dataTaker 605 data loggers.

Vibrating wire strain gauges are essentially taut wires which change their natural resonant frequency in proportion to the square root of tension placed on them.

In practice these elements are used in various sensors designed to measure soil pore pressure, strain in structure, rock stress, overburden pressure, etc.

The Geologgers support vibrating wire strain gauges with resonance between the range of 600 Hz and 4.5 KHz.

Vibration Wire Support by the Geologgers

The Geologgers generate a 'current pulse' to excite or 'pluck' the wire in the vibrating wire gauge. Immediately following excitation, the resonant frequency of the vibrating wire is measured. The advantage of the plucking pulse method is that a fixed pulse is able to stimulate a wide range of gauges. This greatly simplifies channel programming for the user.

The balanced plucking pulse is approximately 200 µS long and up to 36 Volts in amplitude. The pulse has a current source characteristic that provides automatic cable length compensation. Sensors on long cables will be pulsed with the similar energy as those on shorter cables.

The Geologger has a high gain low noise signal amplifier with transformer coupling on the input.

The amplified signal is filtered using band pass filters (500Hz to 5KHz) and a phase lock loop to reduce frequency noise before the frequency is measured by a precision frequency counter.

Signals of the order of tens of microvolts can provide useful readings. Transformer coupling ensures very high common mode rejection, a characteristic that is needed to reject 50/60 Hz mains noise and other interfering noise.

A block diagram of the pulse pluck method is illustrated in below

 

 

Figure 106 - Block Diagram of Vibrating Wire Strain Gauge Support

Differential Measurement of Vibrating Wire Strain Gauges 

The preferred method for connecting vibrating wire strain gauges to the dataTaker is differential connection, where the sensor is connected between the +ve and –ve terminals of the analog input channels.

Differential connection of a Vibration Wire sensor is illustrated below

 

 

Figure 107a - Differential Vibrating Wire Strain Gauge Connection

 

 

Figure 107b - Differential Vibrating Wire Strain Gauge Connection with Shield

 

A shielded signal cable is optional, however it will often be found necessary when noise pick-up is a problem. The preferred shield connection point is either one of the Geologger ground GND terminals or a case ground terminal strip.

If the Analog Return terminal of the channel is not used for other purposes, it can be used as a shield terminal. However because the Analog Return terminal is internally connected to ground via a 100 Ohm resistor, its effectiveness is not as great as a direct connection to ground. (Also if lightning strike is possible, then the resistor may be destroyed).

Vibrating wire strain gauges which are directly connected to the analog input channels as differential inputs are sampled, and the data is returned to the host computer when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

BEGIN
 RA3M
  1FW
  5FW
END

instructs the Geologger to measure the vibrating wire strain gauges 

connected between the +ve and ñve terminals of analog input channel 1

connected between the +ve and ñve terminals of analog input channel 5

The FW specifies a vibrating wire strain gauge input, and configures the channel to pluck the sensor and measure the frequency returned.

Using DeLogger, vibrating wire strain gauges that are connected differentially can be measured by the following Program Builder program.

The differential connections are selected from the Vibrating Wire Configurations dialog which opens when you have selected the analog input channel.

 

 

Data is returned in units of Hz, and can be scaled to engineering units using either Polynomials or Spans (See Section III ñ Scaling Data, Polynomials, Spans and Functions), or using calculations. (See Section III ñ Channel Variables and Calculations).

The dataTaker reads the inputs every 3 minutes, and readings are stopped by a H (Halt) command.

Single Ended Measurement of Vibrating Wire Strain Gauges

Vibrating wire strain gauges can also be connected to the Geologger as single ended analog inputs. The signal from the sensor is connected between the +ve, ñve or Tterminal, and Analog Return terminal.

Single ended connection for vibrating wire strain gauges is illustrated below

 

 

Figure 108 - Single Ended Vibrating Wire Strain Gauge Connection

 

The connection of vibrating wire strain gauges as single ended inputs referenced to Analog Return is best used where

cable lengths are relatively short (< 100 meters)

vibrating wire strain gauges have good sensitivity (high signal to pluck ratio)

Because of the great range in vibrating wire strain gauge sensitivity, it is difficult to predict the operating limits. It is suggested that where cable lengths are in excess of 100 meters, a test be conducted with the gauges to be deployed.

Single ended connection of vibrating wire strain gauges allows up to three sensors to be connected to each analog input channel, and up to 30 sensors to be connected to a Geologger 515 or Geologger 615.

Vibrating wire strain gauges which are directly connected to the analog channels as single ended inputs are sampled and the data returned when a Schedule containing the channel is executed.

Using DeTransfer, the command for example

BEGIN
 RA3M
  1 ]FW  1+FW  1-FW
END

instructs the Geologger to measure 3 vibrating wire strain gauges connected between channel 1+ and Analog Return, channel 1ñ and Analog Return, and channel 1 ] and Analog Return.

The FW specifies a vibrating wire strain gauge input, and configures the channel to pluck the sensor and measure the frequency returned.

Using DeLogger, vibrating wire strain gauges that are connected as single ended inputs can be measured by the following Program Builder program. Single ended connections are selected from the Vibrating Wire Configurations dialog which opens when you have selected the analog input channel.

 

 

Data is returned in units of Hz, and can be scaled to engineering units using Polynomials or Spans (See Section III ñ Scaling Data, Polynomials, Spans and Functions), or by using calculations. (See Section III ñ Channel Variables and Calculations).

The dataTaker will read the inputs every 3 minutes, and readings are stopped by entering a H (Halt) command.

Speaker Output

The Geologger has a built in speaker, and 3.5mm mono or stereo headphone jack for 8 Ohm headphones. The speaker or headphones are used for fault diagnosis.

The speaker output is optional, and is determined by the Speaker Switch as follows

   /V         Enable output from the speaker

   /v         Disable output from the speaker (Default)

The Speaker Switch defaults to /v when the Geologger is initially powered up, hardware reset, or executes a RESET command.

Using DeTransfer, the speaker can be turned on by the command

/V

and off by the command

/v

Using DeLogger, the speaker can be turned on by including the Speaker Switch Enabled command in the Pre Schedule Initialization Commands under Settings : Special Commands in the Program Builder.

 

 

The speaker can be turned off again here if required by placing a Speaker Switch Disabled command in the Post Schedule Initialization Commands

The speaker is connected to the high gain amplifierís output. The frequency response of the small speaker is not flat, and the use of headphones is preferred.

Diagnosing Faults with the Speaker

The speaker or headphones output can be used to diagnose faults with the vibrating wire strain gauges, the sensor cabling and sensor connection.

A good vibrating wire strain gauge which is correctly installed will produce a clean "ping" sound decaying over a period of a few seconds, when the sensor is sampled.

Note : The full decay can only be heard for a single channel, or the last channel of a channel sequence. Other channels can be heard but only for approximately half a second.

If a clean pinging sound is not heard when the vibrating wire strain gauge is sampled, then the following trouble shooting guide will help diagnose the problem

If there is only random noise, check the channel type, wiring and resistance (see below).

If a ping can be heard but it is faint or buried in random noise, then the cable is too long or is "leaky", or the gauge sensitivity is too low.

If the ping is not clean and pure, then the gauge is possibly faulty. The gauge may have been mechanically damaged during installation.

If you can hear a low frequency hum, then noise pick is a problem. If the gauge is placed near a transformer, electric motor, high current power cables, etc, then relocate or reorient the gauge for minimum pickup. Ensure that the cable is shielded to prevent capacitive pickup.

Measuring Gauge Resistance

The vibrating wire strain gauge and cable integrity is best determined by measuring the circuit resistance.

This can be done using a multimeter, or using the Geologger's analog input channels to measure the resistance (See Section II - Measuring Resistance).

The vibrating wire strain gauge and cable resistance should be stable, and should not drift with time.

Measurement Delay 

The Geologger inserts a delay period between stimulating or plucking the vibrating wire strain gauge, and reading the resonant frequency of the wire.

If the data returned from vibrating wire strain gauge is unstable, to the extent that it varies by ±20Hz even though the speaker indicates a strong signal, then the gauge signal may contain harmonics.

The harmonics generally decay more rapidly than the fundamental frequency, and so increasing the time between stimulation and the frequency measurement can improve the results.

The measurement delay can be adjusted by setting the channel factor which specifies a delay in milliseconds (see Section III - Channel Options). The default measurement delay is 200 mS, which is suitable for most gauges.

Using DeTransfer, the measurement delay is set by the command for example

1FW(500)

will increase the delay from the default 200 mS to 500 mS.

Using DeLogger, the measurement delay can be set for each channel in the Vibrating Wire Configurations dialog which opens when you have selected the analog input channel.

 

Extra Samples 

The Geologger normally measures a vibrating wire frequency over a period of 10 line periods (167mS at 60Hz, or 200mS at 50Hz). The Geologger samples the vibrating wire strain gauge frequency 10 times, and returns data which is the average of these readings.

Sampling the vibrating wire strain gauge inputs 10 times has been found optimal for most gauge types. However for gauges with a rapid signal decay, this period can be reduced so that the measurement window does not extend into the noise which may be present.

The Extra Samples (ES) channel option can be used to control the number of samples taken by the Geologger to determine the vibrating wire strain gauge frequency.

Using DeTransfer, the number of samples is set by the command for example

1FW(ES4)

specifies that 5 samples (the original sample + 4 extra samples) are to be taken when the vibrating wire strain gauge is read.

The ES channel option and the measurement delay channel option can be used in the same channel specification to optimise gauge measurement.

Using DeLogger, the number of extra samples can be set for each channel by clicking Channel Options:SpecialÖ to open the Channel Properties dialog. Select the Special tab to enter the number of Extra Samples.

 

The measurement delay and the number of extra samples can be combined.

Using DeTransfer, the command for example

3FW(ES6,100)

specifies a measurement delay of 100 mS between plucking the sensor and beginning sampling of the frequency, and specifies that 6 extra samples are to be taken.

Measuring Gauge Temperature

Most vibrating wire strain gauges are sensitive to temperature fluctuations. Where a vibrating wire strain gauge temperature is likely to change significantly, the gauge temperature should be measured.

The vibrating wire strain gauge temperature can be measured using IC temperature sensors, RTDs or thermistors supported by the logger.

Depending on the internal wiring of the vibrating wire strain gauge, it is often possible to measure the vibrating wire frequency, and a resistive temperature sensor (RTD or thermistor), on a single analog input channel.

High Resistance Nickel RTDs

High resistance RTDs can be wired as single ended inputs, and the vibrating wire strain gauge sensor wired as a differential input.

There is no compensation for cable wire resistance on the measured RTD temperature. Therefore the RTD sensor should be a relatively high resistance type (>1000 Ohm), so that temperature measurement errors due to cable resistance are minimised.

However the differential input for the gauge allows maximum accuracy.

The following configuration is recommended

 

 

Figure 109 - Connecting a Vibrating Wire Gauge and 2 Wire RTD

 

The RTD temperature is read as a single ended input, and the vibrating wire frequency is read as a differential input.

Using DeTransfer, the command for example

BEGIN
 RA10M
  1-NI  1FW
END

instructs the Geologger to read the nickel RTD connected as a single ended input to channel 1, and the vibrating wire frequency connected differentially to channel 1.

Using DeLogger, the vibrating wire sensor and RTD can be measured by the following Program Builder program.

 

 

Low Resistance Platinum RTDs

If the RTD temperature sensor is a low resistance platinum type, then it can be measured by the more accurate 3 wire method, and the vibrating wire sensor measured by the single ended method. This compromise still allows the gauge temperature and gauge signal to be measured on the same channel.

However this configuration has the disadvantage that the single ended vibrating wire strain gauge connection will be a little less accurate than a differential connection.

The following configuration is recommended

 

 

Figure 110 - Connecting a Vibrating Wire Gauge and 3 Wire RTD

 

The temperature channel is read as a 3 wire RTD, and the vibrating wire strain gauge is read as a single ended input.

Using DeTransfer, the command for example

BEGIN
 RA10M
  1PT385  1-FW
END

Using DeLogger, the vibrating wire sensor and low resistance RTD can be measured by the following Program Builder program.

 

 

Copper RTDs

It is possible to use the copper coil in the vibrating wire strain gauge as a temperature sensor, provided that a three wire connection is used. The vibrating wire strain gauge is measured as a differential connection.

The following configuration is recommended

 

 

Figure 111 - Connecting a Vibrating Wire Gauge and 3 Wire Copper RTD

 

Using DeTransfer, the following command for example

BEGIN
 RA15M
  1CU(135)  1FW
END

uses the copper coil of the vibrating wire strain gauge as a copper RTD to measure the gauge temperature, and the gauge signal is measured as a differential connection.

The channel factor for the copper RTD is 135 Ohms of coil resistance at 0°C.

Using DeLogger the vibrating wire sensor, and the sensor coil used as a copper RTD, can be measured by the following Program Builder program.

 

Page Content


Home

Title and Waranty

Go to: Section 2 | Section 3

Section 1


Construction of the dataTaker 50

Construction of the dataTaker 500 600

Construction of the CEM

Getting Started

 

Section 2


Interfacing

Powering the dataTaker

Powering Sensors from the dataTaker

The Serial Interfaces

The RS232 COMMS Serial Interface

The NETWORK Interface

Analog Process

Connect Analog

Analog Chns

Measuring Low Level Voltages

Measuring High Level Voltages

Measuring Currents

Measuring 4-20mA Current Loops

Measuring Resistance

Measuring Frequency and Period

Measuring Analog Logic State

Measuring Temperature

Measuring Temperature with Thermocouples

Measuring Temperature with RTDs

Measuring Temperature with IC Temperature Sensors

Measuring Temperature with Thermistors

Measuring Bridges and Strain Gauges

Measuring Vibrating Wire Strain Gauges

The Digital Input Channels

Monitoring Digital State

The Low Speed Counters

The Phase Encoder Counter

The High Speed Counters

The Digital Output Channels

The Channel Expansion Module

Installing The Panel Mount Display

 

Section 3


Programming the dataTaker

Communication Protocols and Commands

Entering Commands and Programs

Format of Returned Data

Specifying Channels

The Analog Input Channels

The Digital Input Channels

The Counter Channels

The Digital Output Channels

The Real Time Clock

The Internal Channels

Channel Options

Schedules

Alarms

Scaling Data - Polynomials, Spans and Functions

CVs Calcs and Histogram

Logging Data to Memory

Programming from Memory Cards

STATUS RESET TEST

Switches and Parameters

Networking

Writing Programs

Keypad and Display

Error Mess Text

Appendix A - ASCII

Appendix B - ADC Timing