For many structural engineers, the VTI Instruments EX1403 Precision Bridge and Strain Gauge Instrument from AMETEK Programmable Power is the go-to solution for complex test applications. One reason why is that it’s very easy to programmatically configure the built-in strain gauge excitation sources. So there’s no need to manually reconfigure hardware to make measurement changes. In addition, the VTI EX1403 can internally convert the voltage and current measurements to strain or resistance measurements.
The VTI EX1403 provides regulated bridge excitation on a per-channel basis. The excitation voltages and currents are independently programmed as follows:
- Excitation voltages:
- 5 V, +1 V, +2 V, +5 V, and +10 V
- 30 mA drive
- Customizable to up to 10 V
- Excitation currents:
- 1 mA, 5 mA, and 6 mA
- 10 V compliance
- Customizable to up to 6 mA
You can program the excitation sources to provide balanced or imbalanced excitation. Remote sense lines on each input connector are available to improve half- and full-bridge performance.
Configuring a strain gauge channel
When configuring a channel to make strain gauge measurements, set the following parameters:
- Strain.ExcitationType specifies the type of excitation applied to the channel – Voltage, Current, or External. If you select External, the instrument turns off its internal excitation supplies and allows you to supply a custom excitation externally. Voltage excitation should be used for most strain or bridge measurements. Current excitation is primarily used for 2 or 4 wire resistance measurements and 2 wire dynamic strain gauge measurements.
- Strain.ExcitationLevel specifies the voltage or current level. When using an external excitation source, set this parameter to specify the magnitude of the excitation. This is the value that the instrument will use when calculating the resistance or strain value that it returns. When Strain.Excitation is set to Current, reading this parameter returns the last measured or calibrated value of the current selection. A related function, Strain.MeasureExcitationLevel, will cause the instrument to perform an internal measurement of the voltage-excitation level and return that value.
- Strain.DynamicExcitationEnabled allows you to continuously monitor a channel’s voltage-excitation level. When enabled, a single auxiliary analog-to-digital converter scans the channels for which this parameter is true at approximately 20 readings per second. (If all 16 channels are enabled, it will take about 800 ms to measure them all once.)
- Strain.BridgeType allows you to select the type of bridge to use, configuring the channel hardware and the engineering unit (EU) conversion appropriately. For the TwoWireOhm and FourWireOhm values, the nominal gauge resistance must be specified via the Strain.OhmGaugeResistance property. When an input channel is programmed as a strain gauge channel, and Strain.BridgeType is set to one of the instrument’s predefined types, the instrument automatically configures the channel’s internal circuitry and the way it processes the data, allowing the instrument to return data as either resistance or strain measurements.
- Strain.ShuntEnabled specifies whether-or-not the bridge will be shunted. If so, the shunt resistor is specified by the Strain.ShuntSource parameter. The available resistors on the VTI EX1403 are 50 kΩ (InternalCustom1), 100 kΩ (InternalCustom2), and External. Two front panel connection ports are provided on the front of the instrument to allow the user to easily insert their own shunt calibration resistors. To take advantage of this, the user must select External to access those connection ports.
- Strain.RemoteShuntEnabled selects how the shunt will be applied, either internally or via the Remote Shunt pins (pins 4 and 5 of the channel’s RJ45 connector). For quarter-bridge types, it’s impossible to internally shunt the bridge. Thus, this parameter must be set to true when shunting channels configured in this mode.
- Strain.GaugeFactor specifies the sensitivity of the strain gauge. StrainUnstrainedVoltage is the bridge voltage when the gauges are at rest. The function Strain.MeasureUnstrainedVoltage can be called to cause the instrument to measure this.
- Strain.PoissonRatio specifies the Poisson’s Ratio for use in the EU conversion of bridge types HalfPoisson, FullPoisson, and FullBendingPoisson.
- Strain.LeadWireResistance specifies the resistance of the wiring from the instrument to the gauge. This should always be used in quarter-bridge or TwoWireOhm modes to compensate for desensitization caused by the resistance of the lead wires. It can also be used with half- and full-bridge modes if the external excitation sense lines are not utilized to bypass this effect. You can use the Strain.MeasureLeadWireResistance function to actually measure the strain lead-wire resistance. In the TwoWireOhm mode, the resistance of the bridge at the time of measurement must be known precisely for this measurement to work; it should be specified before measuring via the Strain.OhmGaugeResistance property.
For more information on the VTI EX1403 or making strain gauge measurements, contact one of our sales representative by visiting powerandtest.com/sales. You can also email us at firstname.lastname@example.org or call 800-733-5427 or 858-450-0085.