Breakout Box Terminal Guide for Strain Gage Measurement on Spider-80SG

A. Introduction
Strain gages setup on our Spider-80SG is one of the most common questions we received. The Spider-80SG is designed to be used with strain gages; wonderfully, software interface is still within the same easy-to-use and intuitive platform of our EDM software. The connection to Spider-80SG breakout box is the final piece of the puzzle and it is a lot simpler than it looks.

The idea of having the breakout box is to provide users with flexibility to have multiple configuration types available to users. Depends on applications, users are free to choose whichever configuration suitable. We will be explain a few primary configurations here. To measure strain value, the resistance to be measured must be configured by a complete full-bridge circuit. By using super-position technique, users will have the ability to complete the full-bridge circuit, and select partial of the equation to take measurement.

B. Hardware Connection
Breakout box primarily has the following pins:

Name Description
EXC+ Positive Excitation (+2.5V/ +5V)
EXC- Negative Excitation (-2.5V/-5V)
SEN+ Positive excitation sensing
SEN- Negative excitation sensing
IN+ Positive input
IN- Negative input
350 350 Ω terminal
120 120 Ω terminal
GND Ground

Positive and Negative Excitation sensing are used when the strain gage is located at a certain distance from the Spider, such that there is a significant drop in the Excitation voltage that is fed to the Strain gage. In this case, precise value of the excitation voltage is needed to measure the strain accurately.

EXE + from the bridge needs to be connected to SEN + and EXE – from the bridge needs to be connected to the SEN – to measure the excitation voltage at the bridge.

Strain gage usually comes with 2 or 3-wire terminal. The 3-wire configuration one cable come from one terminal, and 2 cables attached to the remaining second terminal. Strain Gages can be connected in the following five basic types of configurations: (For simplicity, we recommend configuration 1 and 2)

1. Quarter Bridge Type I, 2-wire configuration:
In this configuration, there is only one Strain Gage is connected to measure the strain from the test object.

The strain gage is connected between the EXC+ and IN- terminals as shown. The two terminals of the Strain Gage are interchangeable for this configuration.

Resistors RW1 and RW2 denote the wire resistances of the two wires originating from the strain gage and connected to the Spider-80SG terminals.

Breakout Box Connections:

Strain Gage Terminals Spider-80SG Break Out Box
Strain Gage Terminal 1 EXC+
Strain Gage Terminal 2 IN-

2. Quarter Bridge Type I, 3-wire Configuration
Most Strain Gages have 3 wires (2 wires originating from one of the terminals and 1 wire from the other terminal), this is usually done to compensate for the resistance added by the wire.

One disadvantage of the Quarter Bridge Type I (2-Wire Configuration) is that the resistances caused by the lead wires from the terminals of the strain gage add a small additional value of resistance to the arm of the bridge to which it is connected to. For example, if 120 Ω strain gage is connected, the actual resistance would be 120 + RW1 + RW2 Ω. The other internal resistor in the arm would be 120 Ω making the bridge slightly out of balance. Since strain gage works with very minute changes in the resistances, the value of RW1 + RW2, although small, can affect readings.

With the 3-wire configuration, as shown below, the third wire is connected in such a way that the wire resistance is added to both resistors of the arm.

The third wire needs to be connected to pin marked either 120 Ω or 350 Ω on the breakout box depending on the resistance of the connected strain gage.

It should be ensured that the terminal of the strain gage with 2 wires needs to be connected to IN-and 120/350Ω. The other terminal of Strain gage with one wire should be connected to EXC+.

With the above configuration, assuming 120 Ω strain gage is connected.

Resistance between EXC+ and IN- = 120 Ω + RW1 + RW2

Resistance between IN- and EXC - = 120 Ω(Internal) + RW2 + RW3

The resistances RW1, RW2, RW3 primarily depend on their length (as they are made of same material and operated at the same temperature), so when the length is ensured to be about the same, then RW1 = RW2 = RW3 making the above bridge, with 3-wire configuration, balanced.

Strain Gage Terminals Spider-80SG Break Out Box
Strain Gage Terminal 1 EXC+
Strain Gage Terminal 2 IN-
Strain Gage Terminal 2 120 Ω / 350 Ω

3. Quarter Bridge Type II
Quarter Bridge Type II is typically used for temperature compensation. Due to the changes in the temperature, the resistance of the strain gage will not be constant making the bridge out of balance even when there is no external strain applied.

To compensate for the changes in temperature, a second dummy strain gage is attached between IN- and EXC- similar to a Half Bridge configuration. However, the dummy strain gage does not measure strain of any kind and should not be connected to the object undergoing stress. As the dummy strain gage is placed in the same environment as the strain gage measuring strain, both strain gages are subject to the same temperature. Any change in the temperature would influence change the resistances of both strain gages equally and hence making the bridge balanced.

Strain Gage Terminals Spider 80SG Break Out Box
Strain Gage 1 Terminal 1 EXC+
Strain Gage 1 Terminal 2 IN-
Strain Gage 2 Terminal 1 (Dummy / Passive) IN-
Strain Gage 2 Terminal 2 (Dummy / Passive) EXC-

Each strain gage connected also has RW1 and RW2 associated with it making the bridge balanced.

4. Half Bridge
For both Half Bridge Type I and Type II configurations, there are two active strain gages completing one arm of the bridge.

Strain Gage Terminals Spider 80SG Break Out Box
Strain Gage 1 Terminal 1 EXC+
Strain Gage 1 Terminal 2 IN-
Strain Gage 2 Terminal 1 IN-
Strain Gage 2 Terminal 2 EXC-

The other arm of the bridge in internal. Since, both strain gages are operated in the same environment with similar wire lengths, the bridge is balanced by default.

5. Full Bridge
For both Full Bridge Type I and Type II configurations, all 4 resistors (strain gages) of the bridge are external.

Strain Gage Terminals Spider 80SG Break Out Box
Strain Gage 1 Terminal 1 EXC+
Strain Gage 1 Terminal 2 IN-
Strain Gage 2 Terminal 1 IN-
Strain Gage 2 Terminal 2 EXC-
Strain Gage 3 Terminal 1 EXC-
Strain Gage 3 Terminal 2 IN+
Strain Gage 4 Terminal 1 IN+
Strain Gage 4 Terminal 2 EXC+

C. Input Channel Table Setup
Test Configuration is similar to our standard DSA test setup. There are only a few items to look in Input Channels setup for the strain gage on Spider-80SG.

To set the input channel table, navigate to menu item Setup > Input channels.

Set the required Measurement Quantity for Spider 80X channels and the Spider 80SG channels. For non-strain channels, set the Sensitivity and the Input mode. For Strain channels, set the Input range and then click on the tab Strain Gage parameters for further strain gage related settings.

Strain gage parameters are unique and are very different from your normal accelerometers.

For the enabled channels, select Bridge type (depends on your configuration), Nominal gage resistance and the Excitation voltage. It is recommended to enable the Calibration at all times (Input channel must be calibrated before taking any measurements).

Enter the value of Gage factor depending on the type of strain gage being used.

Click on OK to complete the setup, and we are ready to take our measurements. Just be sure to calibrate and re-zero the strain gage before every measurement as standard practice for best data.