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Measuring Bridges and Strain Gauges
The dataTaker provides comprehensive support for Wheatstone bridge circuits in full,half and quarter bridge configurations.
Wheatstone bridge circuits are extensively used for measuring the output of straingauges, and for measuring the output of other sensors where a relatively small changein resistance must be detected.
Bridge circuits have the advantage of high measurement sensitivity, and also provide asignificant degree of temperature compensation.
The dataTaker supports two methods of Wheatstone bridge measurement
ï excitation of sensors is by a constant current
ï excitation of sensors is by a voltage
Both of these methods of bridge support can have a number of options, depending thenumber of active arms in the bridge and the number of wires used to connect bridges tothe dataTaker.
Constant Current Excitation of Bridges
The constant current excitation method of bridge measurement has a constant current of2.500 mA or 250.0 µA flowing in the bridge circuit for excitation.
The bridge sensitivity and zero is independent of the length of leads used to connect thebridge to the dataTaker.
In some cases the bridge output can have greater linearity and reducedtemperature sensitivity for constant current excitation, than for voltage excitation.
The Bridge Excitation Current
The bridge excitation current is supplied by the Excite terminal of the analog inputchannel during measurement. The bridge excitation current is 2.500 mA by default, butmay be set for 250.0 µA if required.
If the 250.0 µA excitation current is required, then this is specified as a channel option inthe channel specification.
Using DeTransfer, 250.0 µA excitation current is specified as follows
5BGI(I)
where the channel option I specifies that a 250.0 µA excitation current is output from theExcite terminal during bridge measurement.
The default excitation current of 2.500 mA is equivalent to
5BGI(II)
where the channel option II specifies that a 2.500 mA excitation current is output fromthe Excite terminal during bridge measurement.
Using DeLogger, the excitation current can be selected in the Channel Properties dialogbox of the Program Builder. When the bridge channel has been created, right click onthe Data Use icon and select PropertiesÖ Click on the Excitation tab, and select theexcitation current required.
Page Content
Home
Title and Waranty
Go to: Section 2 | Section 3
Section 1
Construction of the dataTaker 50
Construction of the dataTaker 500600
Construction of the CEM
Getting Started
Section 2
Interfacing
Powering the dataTaker
Powering Sensors from thedataTaker
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 420mA 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
Both the 250.0 µA and 2.500 mA excitation currents can be used within the sameapplication, for different bridges connected to different channels.
The bridge excitation current is supplied from the Excite terminal for a period of 30 mSduring measurement of the bridge circuit.
The Arm Resistance
The data that is returned by the dataTaker from bridge circuits excited by constantcurrent is the ratio of the change in arm resistance to the nominated arm resistance. Thedata is returned in units of ppm.
The arm resistance for the bridge being measured must be known by the dataTaker, andis specified as a channel option in the channel specification. The default arm resistanceis 350 Ohm, which is typical for many types of strain gauges.
Using DeTransfer, the arm resistance is specified as follows
5BGI(120.5)
In this example the bridge circuit connected to analog channel 5 has an arm resistanceof 120.5 Ohm. The arm resistance is specified in Ohms.
Using DeLogger, the arm resistance for constant current excited bridges can be definedin the Resistance Wiring Configurations dialog of the Program Builder, which openswhen you select the Bridge input channel type.
Full Bridge with Constant Current Excitation
The dataTaker can provide excitation and measure the output from a full bridge ofdevices such as strain gauges, pressure cells, load cells, etc. This configuration is a 4wire input, and supports 1, 2 or 4 active arms. Any of the bridge arms can be activearms. The configuration also provides compensation for cable wire resistance, allowinglong cable wires to be used.
Bridge arms which are not active must have bridge completion resistances. These can
Measuring Temperature with ICTemperature Sensors
Measuring Temperature withThermistors
Measuring Bridges and StrainGauges
Measuring Vibrating Wire StrainGauges
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 andCommands
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, Spansand Functions
CVs Calcs and Histogram
Logging Data to Memory
Programming from Memory Cards
STATUS RESET TEST
Switches and Parameters
Bridge arms which are not active must have bridge completion resistances. These canbe an inactive device of the same type as the devices on the active arms, or these canbe a resistor with the same resistance value as the active devices at rest, and ideallyhave a temperature coefficient that is similar to that of the active devices.
The entire bridge circuit is external to the dataTaker ñ the logger does not provide anybridge completion for partial bridges.
The full bridge configuration with constant current excitation is connected to thedataTaker as a 4 wire input as follows
Figure 100 ñFull Bridge with Constant Current Excitation
where the Excite terminal provides current excitation of 250.0 µA and 2.500 mA, whichreturns via the Analog Return terminal. The bridge output is read between the +ve and ñve terminals.
Full bridges with constant current excitation are sampled, and the data is returned whena Schedule containing the channel is executed.
Using DeTransfer, full bridges with constant current excitation are measured by thecommand for example
BEGIN RA5M 1BGI(4W,120) 2BGI(4W,120)END
which instructs the dataTaker to measure the output from full bridges connected toanalog input channels 1 and 2. The full bridge circuits have an arm resistance of 120.0Ohm.
The BGI specifies that the signals applied to these channels are from constantcurrent excited bridge circuits. The 4W channel option indicates that the bridge isconnected in a 4 wire configuration, where the excite terminal provides excitation. Thisoption must be specified for all four wire bridge connections.
The excitation current channel option is not specified, and so the default 2.500 mAexcitation current is used.
Using DeLogger, full bridges with constant current excitation can be measured by thefollowing Program Builder program. The 4 wire connection is selected from the BridgeWiring Configurations dialog which opens when you have selected the analog inputchannel.
Networking
Writing Programs
Keypad and Display
Error Mess Text
Appendix A ASCII
Appendix B ADC Timing
The dataTaker will read the inputs every 5 minutes, and readings are stopped byentering a H (Halt) command.
Interpreting the Data from a Full Bridge with Current Excitation
The data returned from full bridges is the ratio of change in measured resistance to thearm resistance, expressed in parts per million as follows
where
∆R is the sum of changes in arm resistances taking into account sign of the changesRarm is the nominated arm resistance, default is 350 Ohm
Calculating Microstrain for Full Strain Gauge Bridges
When using stain gauges in full bridges, it may be desirable to convert the returned datafrom units of ppm to units of Microstrain. This can be done by the following formula
This full bridge method of strain gauge measurement has a measurement resolution ofapproximately 0.2 Microstrain.
Using DeTransfer, the output from a full bridge with constant current excitation can becalculated to units of microstrain by the program for example
BEGIN RA5M 5BGI(4W,120,=1CV,W) 2CV(ìMicrostrain =ì)=(4/(4*2.0))*1CVEND
which instructs the dataTaker to firstly read the bridge output in ppm, and save this inChannel Variable 1 (1CV), then calculate microstrain from the reading using the formulaabove.
Here it is assumed that the gauge factor is 2.0 for the strain gauges used (check yourstrain gauge supplier or manufacturer for details of the gauge factor).
The use of calculations in the dataTaker are discussed in detail in Section III ñ ChannelVariables and Calculations.
Using DeLogger, the calculation can be entered in the Program Builder as follows
Refer to your DeLogger Manual for details of using calculations in the Program Builder.
Half Bridge with Constant Current Excitation
The dataTaker can also provide excitation and measure the output from a half bridgeconfiguration of strain gauges, pressure cells, etc. This configuration is a 3 wire input,and supports 2 active arms.
This configuration compensates for cable wire resistance and temperature difference.
The half bridge configuration with constant current excitation is connected to thedataTaker as a 3 wire input as follows
Figure 101 ñ Half Bridge with Constant Current Excitation and Two Active Arms
When using stain gauges in half bridges and current, it may be desirable to convert thereturned data from units of ppm to units of Microstrain. This can be done by the followingformula
This bridge configuration can be used over a wide variation of resistance.
The half bridge configuration can be used to measure the position of the wiper of apotentiometer (<5 KOhm) the ends of the potentiometer are connected betweenExcite/+ve terminals and Analog Return, and the wiper is connected to the ñ ve terminal.The arm resistance is set to the total resistance of the potentiometer.
Using DeTransfer, half bridges with constant current excitation can be measured by thecommand for example
BEGIN RA10M 1BGI(250) 3BGI(250)END
which instructs the dataTaker to measure the half bridges connected to analog inputchannels 1 and 3. These half bridge circuits all have an arm resistance of 250.0 Ohm.
The BGI specifies that the signals applied to these channels are from constantcurrent excited bridge circuits. The default configuration for this type of bridge input is a3 wire connection, and so no connection needs to be specified.
Using DeLogger, half bridges with constant current excitation can be measured by thefollowing Program Builder program. The 3 wire connection is selected from the Bridge
Wiring Configurations dialog which opens when you select the analog input channel.
The dataTaker will read the inputs every 10 minutes, and readings are stopped byentering a H (Halt) command.
Interpreting the Data from a Half Bridge with Current Excitation
The data returned from half bridges is the ratio of change in measured resistance to thearm resistance, expressed in parts per million as follows
where
∆R is the sum of changes in arm resistances taking into account sign of thechangesRarm is the nominated arm resistance, default is 350 Ohm
Calculating Microstrain for Half Strain Gauge Bridges
When using stain gauges in half bridges, it may be desirable to convert the data fromunits of ppm to units of Microstrain. This can be done using the standard formula
or
This half bridge method of strain gauge measurement has a measurement resolution ofapproximately 0.2 Microstrain.
Using DeTransfer, the output from a half bridge with constant current excitation can becalculated to units of microstrain by the program for example
BEGIN RA5M 7BGI(=10CV,W) 15CV(ìMicrostrain =ì)=(4/(2*2.0))*10CVEND
which instructs the dataTaker to read the bridge output in ppm, save this in Channel
which instructs the dataTaker to read the bridge output in ppm, save this in ChannelVariable 10 (10CV), then calculate microstrain from the reading using the formula.
Here it is assumed that the gauge factor is 2.0 for the strain gauges used (check yourstrain gauge supplier or manufacturer for details of the gauge factor).
The use of calculations in the dataTaker are discussed in detail in Section III ñ ChannelVariables and Calculations.
Using DeLogger, the calculation can be entered in the Program Builder as follows
Refer to your DeLogger Manual for details of using calculations in the Program Builder.
Quarter Bridge with Constant Current Excitation
The quarter bridge configuration for measuring bridges is a variation of the half bridgeconfiguration, where there is one active device such as a strain gauge, and a bridgecompletion resistance to balance the bridge.
The bridge completion resistance can be an inactive device of the same type as theactive device, or can be a resistor with the same resistance value as the active device,and ideally has a temperature coefficient similar to that of the active device.
The entire bridge circuit is external to the dataTaker ñ the logger does not provide anybridge completion for partial bridges.
Quarter bridges with constant current excitation are connected to the dataTaker asfollows
Figure 102 ñ Quarter Bridge with Constant Current Excitation
This 3 wire configuration provides compensation for cable wire resistance, allowing longcable runs to be used.
The basic quarter bridge configuration can be used for multiple quarter bridges, with ashared bridge completion resistor. The shared bridge completion resistor should beadjacent to the dataTaker to ensure accurate lead wire compensation.
Figure 103 ñ Multiple Quarter Bridge with Shared Bridge Completion
The quarter bridge configuration with constant current excitation is sampled, and thedata is returned when a Schedule containing the channel is executed.
Using DeTransfer, quarter bridges with constant current excitation can be measured bythe command for example
BEGIN RA30S 1..3BGI(120)END
which instructs the dataTaker to measure the output from quarter bridges connected tothe analog input channels 1, 2 and 3. These quarter bridge active arms all have aresistance of 120.0 Ohm.
The BGI specifies that the signals applied to these channels are from constantcurrent excited bridge circuits. The default configuration for this type of bridge input is a
3 wire connection, and so no connection needs to be specified.
The excitation current channel option is not specified, and so the default 2.500 mAexcitation current is used.
Using DeLogger, quarter bridges with constant current excitation can be measured bythe following Program Builder program.
The 3 wire connection is selected from the Bridge Wiring Configurations dialog whichopens when you select the analog input channel.
The dataTaker will read the inputs every 30 seconds, and readings are stopped byentering a H (Halt) command.
Interpreting the Data from a Quarter Bridge with Current Excitation
Quarter bridge data is returned in units of ppm, and is ratio of the change in measuredresistance to the arm resistance as follows
or
where
Ract is the active arm resistanceRc is the bridge completion resistanceDR is the change in bridge resistanceRarm is the nominated arm resistance, defaults to 350 Ohm
The bridge completion resistor Rcmust have a resistance equal to that of the activedevice at rest, for the bridge to be properly balanced.
Calculating Microstrain for Quarter Strain Gauge Bridges
When using stain gauges in quarter bridges, it may be desirable to convert data fromunits of ppm to units of Microstrain. This can be done by the standard formula
or
This quarter bridge method of strain gauge measurement has a resolution ofapproximately 0.2 Microstrain.
Using DeTransfer, the output from a quarter bridge with constant current excitation canbe calculated to units of microstrain by the program for example
BEGIN RA5M 2BGI(=5CV,W) 8CV(ìMicrostrain =ì)=(4/2.0))*5CVEND
which instructs the dataTaker to firstly read the bridge output in ppm, and save this inChannel Variable 5 (5CV), then calculate microstrain from the reading using the formulaabove.
Here it is assumed that the gauge factor is 2.0 for the strain gauges used (check yourstrain gauge supplier or manufacturer for details of the gauge factor).
The use of calculations in the dataTaker are discussed in detail in Section III ñ ChannelVariables and Calculations.
Using DeLogger, the calculation can be entered in the Program Builder as follows
Refer to your DeLogger Manual for details of using calculations in the Program Builder.
Voltage Excitation of Bridges
The alternative method for measuring bridge circuits with the dataTaker is thevoltage excitation with ratiometric measurement. The principal of the method is that thebridge is excited by a constant voltage source, and the bridge output voltage ismeasured as a ratio of the measured excitation voltage.
In practice, resistance of the cable wires connecting the bridge to the logger reduces theexcitation voltage that is actually applied to the bridge, which in turn results in aproportionate loss of output signal voltage from the bridge.
To correct for this error the actual voltage applied across the bridge is measured using asecond channel.
The Bridge Excitation Voltage Source
The bridge excitation voltage, also often referred to as the bridge power supply, can besupplied from a number of sources
the Excite terminal of the analog channel, which can output a nominal 5 Volts (actuallynearer 4.5 Volts)
the Excite terminal of the analog channel, which can output a 2.500 mA or250.0 µA precision current
the switched 5 Volt sensor power supply terminal of the dataTaker, which is limited to100 mA total current draw
an external voltage source
The bridge excitation voltage must be switched on during the period of measurement
if excitation from the dataTaker is used, then excitation can be switched on by the loggerat appropriate times
if an external voltage is used for excitation, the bridges can be either permanentlypowered or can be powered only during measurement by using a digital output channelto control a relay which switches power to the bridges
The default bridge excitation voltage is the 5 Volt supply from the Excite terminal, and isautomatically selected when bridge inputs with voltage excitation are specified.
However if the bridges are powered from external sources, then the Excite terminalvoltage should be disabled.
Using DeTransfer, this is done by the command for example
2BGV(N)
where BGV specifies that a bridge voltage is to be measured, and the N channeloption specifies no voltage excitation from the Excite terminal.
Alternatively if the bridge is to be excited by either of the Excite terminal current sourcesthen channel option I for 250.0 µA current, or channel option II for 2.500 mA current,should be used.
Using DeTransfer, this is done by the command for example
1BGV(I)8BGV(II)
8BGV(II)
where BGV specifies that a bridge voltage is to be measured, and the I and II channeloptions specify current excitation from the Excite terminal.
DeLogger does not directly support the measurement of bridges that are excited by avoltage. However bridge measurements, including controlling the method of powering,can be programmed into the dataTaker via the User channel type (DeLogger Ver 4.2.15or later) in the Program Builder. This procedure is illustrated in the following topics.
Measuring the Bridge Excitation Voltage
In practice the resistance of the cable wires connecting bridges to the dataTaker reducethe excitation voltage that is actually applied to the bridge.
This results in a proportionate loss of output signal from the bridge. To correct for thiserror, the actual excitation voltage across the bridges is also measured.
The bridge excitation voltage is connected as a differential or single ended voltage inputto any analog input channel, and must be measured immediately before the output ofany bridge is measured.
This measurement is referred to as the ‘bridge reference voltageí, and is measured on
the bridge reference channel that is identified to the dataTaker by the BR channeloption for the particular channel.
Using DeTransfer, the command for example
1V(BR)
identifies that the bridge reference voltage is to be measured as a differential voltageconnected to analog input channel 1.
dataTaker 50,500,600 series loggers : The bridge reference channel has a maximuminput voltage of 2.5 Volts for dataTaker 50,500,600 series loggers. Therefore if thebridge excitation voltage is greater than 2.5 Volts, then this must be externallyattenuated (see Section II ñ Measuring High Level Voltages) before input to the bridgereference channel on these loggers.
Whenever the bridge excitation voltage must be attenuated, the attenuation factor is alsodeclared as a channel option to the bridge reference voltage channel as follows
1V(2.0,BR)
which declares an attenuation of 2.0:1, that is appropriate for bridges powered by anexternal 5 Volt supply.
dataTaker 505,605 series loggers : If a dataTaker 505,605 series logger is being used,then the bridge excitation voltage can be measured directly as a High Level Voltage(see Section II ñ Measuring High Level Voltages) on the bridge reference channel, forexample
2HV(BR)
The bridge reference channel does not return any data when it is scanned. The data isretained for subsequent use in bridge measurements and calculations.
However if you want to include the bridge power supply or excitation measurements inyour data, then the bridge reference voltage can be returned by a second command forexample
1V(BR) 1V
where the channel 1V will return the bridge reference voltage.
Note : The bridge reference channel must precede the bridge measurement channel(s)in the dataTaker program, because the bridge reference voltage is used to calculate thebridge data for the subsequent bridge measurement channels.
Note : If bridge measurements are included in more than one Schedule, then the bridgereference channel(s) must be declared in each Schedule.
If a bridge reference channel is not declared, then the bridge reference voltage defaults
If a bridge reference channel is not declared, then the bridge reference voltage defaultsto 5 Volts. This is based on the assumption that most voltage excited bridges will bepowered from the dataTaker 5 Volt sensor power supply.
DeLogger does not directly support the measurement of bridges that are excited by avoltage. However the bridge reference channel, and bridge measurements channelscan be programmed into the dataTaker via the User channel type (DeLogger Ver 4.2.15or later) in the Program Builder. This procedure is illustrated in the following topics.
Full Bridge with Voltage Excitation
The full bridge with voltage excitation configuration is the more traditional method for themeasurement of bridge outputs. However a full implementation of requires moreresources than any of the constant current methods, requiring
two channels for the each bridge, if each bridge has a separate bridge excitation
two channels for the first bridge, and one channel for each additional bridge that isexcited by the same bridge power supply. This configuration is only appropriate if allcable wires are the same length, such that all bridges receive the samevoltage excitation as measured for the first bridge
This configuration supports 1, 2 or 4 active arms. Any of the bridge arms can be activearms.
Bridge arms which do not have active devices must have bridge completion resistancesto balance the bridge. These can be inactive devices of the same type as the activedevices, or can be a resistor with the same resistance value as the active devices at rest,and ideally have a temperature coefficient that is similar to that of the active devices.
Where the bridge power supply and bridge output are measured for each bridge, this isreferred to as a six wire connection as illustrated below
Figure 104 ñ Full Bridge with Voltage Excitation
The entire bridge circuit is external to the dataTaker ñ the logger does not provide anybridge completion for partial bridges.
Full bridges with voltage excitation are sampled, and the data is returned when aSchedule containing the channel is executed.
Using DeTransfer, full bridges with voltage excitation are measured by the commandsfor example
BEGIN RA5M 1V(BR) ëbridge reference channel 2BGV(4W,N) ëbridge measurement channelEND
END
which instructs the dataTaker to measure the bridge excitation voltage connected toanalog input channel 1 (bridge reference channel), and the bridge output connected tothe analog input channel 2.
The BR indicates which analog channel the bridge excitation voltage is connected to formeasurement. Note that the bridge reference channel is measured before the bridgeoutput channel is measured.
The BGV specifies that the signal applied to this channel is from a bridge that is excitedby a voltage. The bridge output data is returned in units of ppm.
The 4W channel option indicates that the 4 wire measurement method is to be used.This option must be specified for all full bridge inputs.
The bridge excitation voltage is supplied from an external source in this example, andso the Excite terminal is disabled by the N channel option.
DeLogger does not directly support full bridges with voltage excitation. However, fullbridges with voltage excitation can still be measured with DeLogger (Ver 4.2.15 or later)by using the User channel as illustrated by the following Program Builder program.
The bridge reference channel could also be entered as a low level voltage channel, andthe Channel Properties set to Bridge excitation voltage channel in the Reference tab asfollows
The dataTaker will read the inputs every 5 minutes, and readings are stopped byentering a H (Halt) command.
Various compromises are possible with full bridges with voltage excitation as follows
the number of cable wires can be reduced from six to four by measuring the bridgeexcitation voltage at the dataTaker, rather than at the bridge. However this does not takeinto account reduction of the excitation voltage at the bridge due to cable resistance.
the bridge reference channel can be shared by a number of bridge measurementchannels. No errors will be introduced provided all of the bridges have the sameexcitation voltage. This can be done by close proximity of the bridges to a sharedsupply, or the use of cable wires of the same type and length.
Interpreting the Data from a Full Bridge with Voltage Excitation
Data returned from full bridges with voltage excitation is calculated as the ratio of thechange in bridge output voltage to bridge excitation voltage, expressed in parts permillion as follows
where
∆V is the change in bridge output voltageVexcite is the bridge excitation voltage, measured by the bridge reference channel
Calculating Microstrain for Full Strain Gauge Bridges
When using stain gauges in full bridges, it may be desirable to convert the returned datafrom units of ppm to units of Microstrain.
This can be done by the following formula
This full bridge method of strain gauge measurement has a resolution of approximately0.2 Microstrain.
Using DeTransfer, output from a full bridge with voltage excitation can be calculated tounits of microstrain by the program for example
BEGIN RA5M 1V(BR) 2BGV(4W,N,=1CV,W) 2CV(ìMicrostrain =ì)=(4/(4*2.0))*1CVEND
which instructs the dataTaker to read the bridge excitation and bridge output voltages,calculate the ratio in ppm and save in Channel Variable 1 (1CV), and calculate themicrostrain from the reading in ppm using the formula above.
Here it is assumed that the gauge factor is 2.0 for the strain gauges used (check yourstrain gauge supplier or manufacturer for details of the gauge factor).
The use of calculations in the dataTaker are discussed in detail in Section II ñ ChannelVariables and Calculations.
Using DeLogger (Ver 4.2.15 or later), the calculation can be entered into the ProgramBuilder as follows
The bridge reference channel could alternatively be entered as a low levelvoltage channel, and the Channel Properties set to Bridge excitation voltage channel inthe Reference tab as illustrated on the previous page.
Refer to your DeLogger Manual for details of using calculations in the Program Builder.
Half Bridge with Voltage Excitation
Half bridges with two active arms and voltage excitation are commonly used when alarge number of bridges need to be located in close proximity.
The dataTaker supports this configuration by using single ended inputs and the singleended reference SE Ref.
Half bridges with two active arms require two bridge completion resistances to balancethe bridge. The two bridge completion resistances can be either inactive devices of thesame type as the active device, or can be a resistor with the same resistance value asthe active devices, and ideally have a temperature coefficient similar to that of the activedevices.
This half bridge configuration with voltage excitation can be used to measure a singlehalf bridge, or to measure a number of half bridges which share the same bridgeexcitation voltage supply, and share the same set of bridge completion resistors.
Multiple half bridges that are excited from a single excitation voltage source and sharebridge completion resistors are illustrated below in Figure 105. The configuration for asingle half bridge is that for the innermost half bridge.
Figure 105 ñ Half Bridges with Voltage Excitation
dataTaker 50,500,600 series loggers : If this bridge configuration is connected to adataTaker 50,500,600 series logger, then the bridge completion resistors must provide a2:1 attenuation of the 5 Volt bridge excitation voltage, to a reduce the signal suitable forinput to the bridge reference channel.
dataTaker 505,605 series loggers : If this bridge configuration is connected to adataTaker 505,605 series logger, then the bridge excitation voltage can be measured asa High Level Voltage (see Section II ñ Measuring High Level Voltages) on the bridgereference channel.
The half bridges should preferably all be in close proximity to the bridge completionresistors. However if this is not possible, then the bridge completion resistors can belocated at the dataTaker, and each half bridge connected by three leads. This willprovide lead compensation for zero, but no scale compensation.
Half bridges with voltage excitation are sampled, and the data is returned when aSchedule containing the channel is executed.
Using DeTransfer, half bridges with voltage excitation are measured by the commandsfor example
BEGIN RA10M 1V(BR,5.0) 2*BGV(N,X) 2+BGV(N,X) 2BGV(N,X)END
which instructs the logger to measure the bridge reference voltage that is differentiallyconnected to analog channel 1, and measure the output from half bridges connected assingle ended inputs to analog channels 2* through 2ñ .
Assuming that the excitation voltage is supplied from an external 10 Volts source, thedefault Excite terminal voltage output is disabled by the N channel option.
The BR channel option indicates the analog channel to which the bridge excitationvoltage is connected for measurement, and is attenuated by a factor of 5.0:1 to reducethe 10 Volt excitation voltage into range for a dataTaker 50,500,600 series logger. The
the 10 Volt excitation voltage into range for a dataTaker 50,500,600 series logger. Thereference channel must be read before the measurement channel.
The BGV specifies that the signals applied to these channels come from voltage excitedhalf bridges. The X channel option indicates that the single ended inputs are to bemeasured with reference to SE REF terminal.
DeLogger does not directly support half bridges with voltage excitation. However, halfbridges with voltage excitation can still be measured with DeLogger (Ver 4.2.15 or later)using the User channel type as illustrated by the following Program Builder program.
The dataTaker will read the inputs every 10 minutes, and readings are stopped byentering a H (Halt) command.
Interpreting the Data from a Half Bridge with Voltage Excitation
Data returned from half bridges with voltage excitation is calculated as the ratio of thechange in bridge output voltage to bridge excitation voltage, expressed in parts permillion as follows
where
∆V is the change in bridge output voltageVexcite is the bridge excitation voltage, measured by the bridge reference channel
Calculating Microstrain for Half Strain Gauge Bridges
When using stain gauges in half bridges, it may be desirable to convert the data fromunits of ppm to units of Microstrain. This can be done by the standard formula
or
This full bridge method of strain gauge measurement has a measurement resolution ofapproximately 0.2 Microstrain.
Using DeTransfer, output from a half bridge with voltage excitation can be calculated to
Using DeTransfer, output from a half bridge with voltage excitation can be calculated tounits of microstrain by the program for example
BEGIN RA10M 1V(BR,5.0) 2*BGV(N,X,=1CV,W) 2CV(ìMicrostrain 1 =ì)=(4/(2*2.0))*1CV 2+BGV(N,X,=3CV,W) 4CV(ìMicrostrain 2 =ì)=(4/(2*2.0))*3CV 2BGV(N,X,=5CV,W) 6CV(ìMicrostrain 3 =ì)=(4/(2*2.0))*5CVEND
which instructs the dataTaker to
read the bridge excitation and bridge output voltages
calculate the ratio in ppm and save in Channel Variables
calculate the microstrain from the readings in ppm using the formula above
Here it is assumed that the gauge factor is 2.0 for the strain gauges used (check yourstrain gauge supplier or manufacturer for details of the gauge factor).
The use of calculations in the dataTaker are discussed in detail in Section II ñ ChannelVariables and Calculations.
Using DeLogger (Ver 4.2.15 or later), the calculation can be entered into the ProgramBuilder as follows
etc, etc
Refer to your DeLogger Manual for details of using calculations in the Program Builder.
Converting Bridge Outputs to Engineering Units
This chapter has provided methods to convert measured bridge output in ppm to units ofMicrostrain for the various bridge configurations. However units of Microstrain apply tostrain gauge bridges which are measuring deformation.
Many sensors available today employ a bridge circuit to sense the parameter they aredesigned to measure. For example some pressure cells, load cells, microdisplacementtransducers, etc. in fact contain a diaphragm or similar structure which has a full straingauge bridge bonded to one surface. The diaphragm is mechanically distorted by thepressure or load, which is measured by the strain gauge bridge. This distortion iscalibrated to units of pressure, or load, etc. by the manufacturer.
Supporting these types of sensors with the dataTaker is quite simple, as shown by the
Supporting these types of sensors with the dataTaker is quite simple, as shown by thefollowing examples.
Pressure Transducer
A pressure transducer that is constructed as a full bridge device with a 4 wireconnection, is connected to the dataTaker as a full bridge with constantcurrent excitation (type BGI) as illustrated in Figure 100.
The transducer has an output of 0.05 V full scale at 10 VDC excitation. The dataTakerwill measure
and
Therefore 1ppm = 100 kPa/5000 ppm = 0.02 kPa. This transducer calibration can beused in a dataTaker program to return the data in units of kPa.
Using DeTransfer, the program will be similar to
BEGINY1=0,0.02îkPaî RA1S 1BGI(Y1)END
Using DeLogger, the calibration for the pressure transducer must be entered as apolynomial into the Polynomials dialog under the Settings tab of the Program Builder asfollows
The polynomial is then attached to the bridge input channel in the program to convertthe raw data to units of lbs as follows
For further discussion of polynomials, see Section III ñ Polynomials and Spans of thismanual, and the DeLogger Users Manual.
Load Cell
A load cell that is constructed as a full bridge device with a 4 wire connection, isconnected to the dataTaker as a full bridge with constant current excitation (type BGI) asillustrated in Figure 100.
The load cell measures a load of 100 lbs full scale, and has an output of 2.0006 mv/V atfull scale. The dataTaker will measure
and
Therefore 1ppm = 100 lbs / 2000.6 ppm = 0.049985 lbs. This transducer calibration canbe used in a dataTaker program to return the data in units of lbs.
Using DeTransfer, the program will be similar to
BEGINY1=0,0.049985îlbsî RA1S 1BGI(Y1)END
Using DeLogger, the calibration for the load cell must be entered as a polynomial into
Using DeLogger, the calibration for the load cell must be entered as a polynomial intothe Polynomials dialog under the Settings tab of the Program Builder as follows
The polynomial is then attached to the bridge input channel in the program to convertthe raw data to units of lbs as follows
For further discussion of polynomials, see Section III ñ Polynomials and Spans of thismanual, and the DeLogger Users Manual.
Measurement Ranges and Accuracy
Measurement Ranges and Accuracy
The dataTaker measures all bridge inputs as a low level voltage, with a resolution of 1µV, and a nominal accuracy of 0.1%.
The accuracy for particular applications can be calculated from this information, and theexcitation current or voltage used.
Error Messages
There are no specific error messages for bridge inputs. However input voltage signalswhich fall outside the voltage range of the dataTaker will produce an overrange readingof ñ99999.9 ppm or +99999.9 ppm.
The dataTaker also reports the error condition with the error message ëE11ñinput(s) outof rangeí if the Messages Switch /M is enabled.
