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1. Transducers based on strain gauges

Strain gauge (SG) based force transducers are widely used and offer reliable operation even in unfavorable ambient conditions. The mechanical installation has already been dealt with in the article on 'Installation of force transducers'. Here, we want to address the electrical connection. Strain gauge based sensors use the so-called Wheatstone bridge. This bridge circuit comprises four resistors connected as shown below: It is essential that all strain gauge sensors are fed with an adequate supply voltage or bridge excitation voltage Ub. This excitation voltage Ub is provided by the amplifier systems. Typical values range between 

2.5 and 10 V. The right value for your force transducer can be found in the data sheet; see 'Operating range of the excitation voltage'. The above circuit diagram shows that four leads are sufficient to operate a Wheatstone bridge. Two leads supply the sensor with electric voltage, the other two leads feed the amplifier with the measuring voltage.

knowledge, resource center, articles, electrical connection of force transducers
knowledge, resource center, articles, electrical connection of force transducers

INFO:

The reference excitation voltage is the supply voltage with which the sensors were supplied during determination of the technical data. The operating range of the excitation voltage is the bridge excitation voltage at which your force transducer can be operated while complying with the technical specifications. With the bridge excitation voltage exceeding the specified limit, the strain gauges and other resistors in the force transducer heat up excessively so that individual parameters (sensitivity, temperature coefficient of the sensitivity) will change. When the operating range of the excitation voltage is not exceeded, these changes can be neglected in experimental and production applications. The lower limit of the excitation voltage range results from experimental data: tests at 'zero' excitation voltage cannot be carried through. With high-precision measurements (reference measuring chains), it is generally recommended to choose the bridge excitation voltage given in the sensor's calibration certificate. It is even more advisable to have the measuring chain, i.e. the transducer and the electronics, calibrated together as a system.

2. What is the exact value of a strain gauge transducer's output voltage?

A sensor's exact nominal (rated) sensitivity can be found in the test certificate. In most cases, this sensitivity is 2 mV/V, with the nominal (rated) force being applied. As already mentioned above, the amplifier systems feed the measuring bridge with a bridge excitation voltage. This  bridge excitation voltage often is 5 V. If your force transducer's nominal (rated) sensitivity is 2 mV/V, 10 mV are available at the amplifier's input stage when the force transducer is loaded with nominal (rated) force. For example, when you use an S2M/100 N (100 N nominal (rated) force, 2 mV/V sensitivity at nominal (rated) force) and load it with 100 N you obtain 10 mV. In general, the measured value is not always 100 N; in fact, it should also be possible to detect smaller values. If you want to have a measurement signal resolution of 0.1 N, 10 µV are available at the input stage. If your amplifier has a resolution of 100,000 divisions, which is still less than a high-precision instrument such as HBM's DMP41 offers, this corresponds to the relationship between the height of the Eiffel tower (321 m) and the thickness of a CD case. Requirements are significantly higher with increasingly complex measurement tasks. The better the connection corresponds to the measurement task the more reliable are aour measurement results.

3. Sensors in four wire circuit

knowledge, resource center, articles, electrical connection of force transducers

Strain gauge measurement technology requires a minimum of four wires to be connected. Many sensors work according to this principle, which is called four wire circuit. This configuration is shown below. The lead resistances are drawn in the supply leads. These resistances are to show that the lead resistance must not be neglected.

INFO:

A force transducer is operated in four wire circuit. The following parameters are given:
  • Bridge resistance: 350 Ω
  • Copper cable: 0.14 mm² cross-section
  • Spec. resistance: ρ = 0.0178 Ω⋅mm²/m
This allows calculation of the copper cable's resistance:
The lead resistance is 1.272 ohms with a 5-m-long connection cable (line and return line) and 12.72 ohms with 50 m respectively. The measuring bridge and the lead form a voltage divider, with a partial voltage dropping off in the cable as a result. Therefore, the voltage applied to the bridge circuit is reduced and, as a result, the output signal is smaller. This results in a loss of sensitivity. This sensitivity loss is 0.36% for 5 m and increases to 3.6% for 50 m. These variations are allowed for in the calibration, i.e. the sensitivity specified in the test report or calibration certificate is always valid for the sensor including the mounted cable. The resistance of copper cables is temperature dependent. The resistance increases with increasing temperature so that the voltage applied to the measuring bridge is reduced. This has been taken into account during the balancing of the transducer, even for sensors are available with different cable lengths.

HBM force transducers with a four wire circuit have been calibrated including the sensor cable, i.e. the sensitivity is correct at the cable ends; cutting the cable results in a change in sensitivity. We recommend that you do not cut the cable for the calibration to be maintained. Enter the sensitivity as specified in the test report:  

knowledge, resource center, articles, electrical connection of force transducers

A test report provides a lot of information. The sensitivity is an important value enabling you to set up the amplifier. Example: U93/1kn force transducer The cables running to the amplifier input need not be taken into account. Since modern amplifiers use high-resistance input stages, the voltage drop in these lines can be neglected.

4. Sensors in six wire circuit

knowledge, resource center, articles, force glossary

Many sensors use the six wire circuit. They use two additional leads controlling the excitation voltage at the bridge. Should the cable resistance vary as a result of temperature influence or a change in cable length, the amplifier makes internal readjustments until the setpoint value is reached again. The advantage of this circuit is that you can use very long cables (up to 500 m) without the sensors' sensitivity being affected. Lead resistance variations caused by temperature variations also do not impact the measurement result. This is particularly advantageous when the cable temperature and the temperature of the force transducer are not identical.

INFO:

As already mentioned above, the cable length should not be modified with force transducers in four wire circuit. If changing the cable length is necessary, it is advisable to use a six wire extension cable. When extending the cable connect two sense leads in addition to the supply leads to enable the amplifier to control the impacts of cable extension.

5. Shielding

With HBM force transducers, the cable shield is always connected to the housing. The resulting Faraday cage blocks electromagnetic fields, thus avoiding interference. If sensor cables need to be extended, connect the sensor cable's shield to the extension's shield to keep up the Faraday cage. Connect the plug such that the shield is brought extensively into contact with the plug. If the transducer and amplifier have different potentials, compensating currents can flow via the cable shield that cause major interference. Ideally, you should make a low-resistance connection (potential equalization line). We recommend using a cable with 16 mm2 cross-section. Should this not be experimentally possible, the shield in the plug may be cut. In any case, this is the 'second-best' solution. HBM offers a wide range of measuring cables that have proven their suitability. Varied requirements are made on a measuring cable: low-capacitance, temperature-stable, symmetrical.

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