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Find the Right Connection Cable for Your Strain Gauge Application

Which Connection Cables to Use for Electrical Foil Strain Gauge Applications Structural Tests/Durability Measurements

There are a lot of cables available in the market which may be used for strain gauge applications.

The success of a measurement depends on the right connection cables. Not only do they have to transfer the measurement signals from the sensor to the DAQ system, but they also have to avoid interference signals and resist stress during their use.

Ideally, the cable should not have influence on the strain measurement. In reality, however, cables/wires could have an influence on the measurement signal. The effects of the wires can be minimized to an acceptable level. HBK offers a wide choice of different measurement cables and small-scaled stranded wires for a wide range of applications. There are some important points to consider when selecting the right cable for your application:

Multi-stranded wires with tin coatings are mostly used for strain gauge applications. Usually, copper conductors are used as wires (most common standard because of good price-to-conduction ratio).

The strain measurement signal in a quarter-bridge configuration is very sensitive:

  • A typical excitation voltage for a strain gauge quarter bridge is 2.5 V
  • Strain applied to the strain gauge creates a bridge voltage output that is relatively low!

(0.000125V for 100µm/m strain respectively 0.0025V for 2000µm/m strain). This is visualized in the graphs below for a typical quarter-bridge application.

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This measurement voltage signal must not be interfered with by external signals. This is the reason why the right measurement cable is absolutely necessary!

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Bridge voltage output signal with 100µm/m strain signal. The bridge excitation voltage (red) is very small in comparison to the excitation voltage (blue):

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1 Function of wires

Connection between the sensor and the amplifier

  • Provide power to excite the strain gauge circuit
  • Transmit the measurement signal from the sensor to the DAQ system
  • Protective function against external interferences

2 Interferences/Impacts on the measurement cable

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3 The requirements on a cable for strain gauge applications at a glance

  1. Low resistance (generally) and low-capacitance
  2. Good insulation
  3. Good mechanical protection
  4. Good handling (flexibility)
  5. Suitable working temperature range and low influence of temperature changes
  6. Good solderability of the wires
  7. Safety requirements (flame-retardant,etc.)
  8. Mechanical robustness
  9. Robustness against different media (water, oil, solvents,etc.)

4 Environmental requirements on cables/wires

The coating and the wire insulation influence the temperature range to which they can be exposed. The following diagram shows the typical temperature range of cables depending on their coating.

  • For most applications, PVC-insulated cables are suitable; they offer an excellent price-to-performance ratio (up to 80 °C)

  • For medium temperatures, TPE cables are a good choice (up to 150 °C)

  • For higher temperatures, we recommend using PFA cables (up to 250 °C) or polyimide-coated cables (>300 °C)

  • At low temperatures, standard cables can get brittle. This is especially critical when they are used in dynamic test environments. Using cables with a PTFE, PI, or glass fiber sheath is recommended in this case.

There are many more requirements on cables such as resistance against different fluids and flammability. The following table provides an overview of typical cable jackets/insulation materials and their temperature range:

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5 Diameter of conductor

  • The diameter of the wire has a huge influence on the resistance. The excitation voltage of a strain gauge bridge generates a current which heats up the conductor. The smaller the diameter the higher the temperature increase on the wire.
  • To minimize errors, the largest possible wire diameter shall be used to minimize the effect of lead-wire resistance in strain gauge applications and the thermal effects of the cable.
  • In some applications, thin cables are necessary to reduce inertia/weight or allow for a small bending radius.
  • Long sensor cables generally require bigger wire diameters.

A small-diameter cable shall be used at the strain gauge to reduce the solder amount and parasitic stress on the gauge.However, it must be taken into account that these thin wires affect the circuit stability and sensitivity.

  • A solder terminal can be used as the point of intersection between the measurement cable and the connection wire of the strain gauge. This method enables a transition from a small-diameter cable to a cable with a thicker diameter:

Strain gauge connected with HBM's patented 4-wire technology:

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What is HBK's patented 4-wire configuration?

Only the 4-wire circuit, or HBM's patented Kreuzer circuit, enables different cable resistances to be compensated for. A known electric current flows through the resistor via two of the leads. The voltage drop at resistor RKab1 is corrected (at high impedance) via two additional leads.

The Kreuzer circuit measures the voltage across resistor RKab2 and adds it to the excitation. The voltage and thus the current through completion resistor Rerg are independent of the cable resistance. Zero point and sensitivity errors resulting from cable effects are electronically compensated for.

 

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Tips for stripping strain-gauge wires

1 Thermally strip 5mm of insulation off the wire to be attached to the strain gauge.

Thermal stripping prevents damage which can occur from mechanical stripping with pliers.

2 Tin the wire end with solder.

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3 Trim the tinned conductor so that it does not extend the gauge carrier after soldering (1-3mm depending on the strain-gauge geometry).

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6 Number of conductors

1-wire: Connections between strain gauges and solder terminals

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3-/4-wire: For quarter-bridge applications (only 4-wire shown) or full bridges:

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5-wire: Half-bridge applications

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6-wire: Full-bridge applications

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7 Cable length

  • In strain gauge measurement, cable lengths range from a few centimeters to hundreds of meters.
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  • Use twisted and shielded cables to minimize electromagnetic interference.
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  • Generally, keep the length as short as possible to minimize thermal and electromagnetic interference.
  • For long distances, choose bigger conductor diameters to keep the influence of resistance low.
  • If signals are transmitted with high frequency and DC also a low-capacitance wire is recommended.

What is the difference between a DC and a carrier-frequency amplifier?

DC amplifier

  • Contains a generator providing a stabilized DC voltage for feeding the bridge circuit
  • Amplifies static and dynamic signals up to high frequencies
  • In practice: usually max. 10 kHz; higher frequencies result from interference pulses which should not influence the measurement signal

Disadvantage: Interference (caused by electric or magnetic fields as well as thermoelectric and galvanic voltages in the measuring circuit) is fully amplified.

  • Error in the measurement result
  • Electrical or magnetic shielding is required
  • Or mathematical correction of thermoelectric voltages

Carrier-frequency amplifier

  • Generator supplies a voltage- and frequency-stabilized alternating voltage to supply the bridge circuit
  • Output voltage = alternating voltage whose amplitudes are proportional to a bridge unbalance (amplitude modulation)
  • Frequency selection so that only the frequency of the supply voltage is amplified (interferences have no influence)
  • Common carrier frequencies:
    • 225 Hz: Measurement of static and quasistatic processes (up to 9 Hz)
    • 5 kHz: Measurement of static and quasistatic processes (up to 1 kHz)

Disadvantage: Limited bandwidth

8 Cable protection around the strain gauge for harsh conditions

  • Moisture-proof connection between the wire and the protective coating.  Therefore, maximum adhesion between the covering agent and the connection cable and the material surface is required
  • Fluoropolymer cables shall be etched previously to allow proper sealing of the measurement cable
  • Special cables with water-blocking tape are recommended for submersion under water (please contact the HBM Service Team)
  • Ensure a minimum length of the protective agent around the cable access to maximize the creep distance and ensure that critical points are sealed
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9 Test dynamics

  • For high-dynamic measurements, jumper wires should be used. Jumper wires consist of many thin stranded single wires which are surrounded by really flexible insulation.
  • Solid wire shall only to be used on static objects (e.g. for bridge interconnections)
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10 Conclusion

  1. Use the maximum cable diameter
  2. Use low resistance and low capacitance cable
  3. Minimize cable lengths if possible
  4. Use minimum diameter on the strain gauge directly
  5. Choose the right cable depending on your test scenario
  6. Use flexible cables
  7. Use cables with a conductive the shield
  8. Ensure proper grounding for shield
  9. Ensure Faraday cage for the measurement signal chain
  10. Careful routing of wires
  11. Twisted wires
  12. Do not lay main power cables close to the measurement cable in one cable (90° crossing of power lines and measurement signal lines)
  13. Remove noise sources
  14. Use carrier frequency amplifiers
  15. Use the correct filters

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