Measurement-computing CIO-EXP-GP Manuel d'utilisateur télécharger pdf (Page 36)

Referring back to Figure 7-6, the legs of the bridge are labeled A, B, C and D. Table 7-3 below matches

the legs of the bridge to the resistor number nomenclature that appears on the CIO-EXP-GP.

Table 7-3. Bridge Completion Resistor Identities

RX59RX48RX46RX45RX44RX437

RX58RX42RX40RX39RX38RX376

RX57RX36RX34RX33RX32RX315

RX56RX30RX28RX27RX26RX254

RX55RX14RX22RX21RX20RX193

RX54RX18RX16RX15RX14RX132

RX53RX12RX10RX9RX8RX71

RX52RX6RX4RX3RX2RX10

ArmNull PotBridge DBridge CBridge BBridge AChannel

Some values of precision resistors are available from Measurement Computing Corp.

7.8.2 Nulling Potentiometers & Arm Resistor

Each circuit has a position for a nulling potentiometer and associated arm resistor. The purpose of the

nulling arm is to allow you to zero the reading of strain at a given strain position. There is no formula to

use to select the nulling potentiometer and arm resistor. Bridge resistor values and total gain selected for

the CIO-EXP-GP will affect adjustability for a given nulling circuit. An average value for the arm

resistor is 10k ohms. Start with that and adjust as required.

7.8.3 Strain Gauge Bridge Configuration Examples

Following are three typical strain gauge bridge configurations. They are by no means the only way to

connect a strain gauge to the CIO-EXP-GP. For example, there is no rule that says the ‘A’ leg must be

the strain gauge on a ¼ bridge implementation.

The examples below show how to translate strain to input voltage for the strain gauge

configuration used to measure simple bending strain. Other types of stress and strain: axial,

torsion, shearing , etc. are beyond the scope of this description.

These examples can be used to as a

guide for calculating the bridge voltage in your own application, and thus help you select the proper

amplifier gain and excitation voltage.

The use of quarter bridge, half bridge and full bridge strain gauge configurations are described.

The Application:

In these examples, imagine a beam extending out from a fixed point on a wall. Force is applied to deflect

the end of the beam downward. We know that the maximum strain to be measured will be 250µε (250

micro strain). Knowing the amount of force required and the size of the beam is not necessary, since

strain relates to the change in length of the surface of interest.

The Strain Gauge will be a metal foil type, 350 ohms resistance, Gauge Factor = 2. Refer to the

Appendix for information on these specifications.

The following example shows a bending strain measurement example. It can be used to calculate

the bridge voltage, and thus help the user select the proper amplifier gain and excitation voltage.

The use of one, two and four strain gauges will be examined.

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Measurement-computing CIO-EXP-GP Manuel d'utilisateur Page 36