Mounting of strain sensors

Strain sensors are a clever alternative to force sensors for large forces above 10,000 N and are used in a variety of applications. A few examples are presented below. Additional application examples can be found on the product website of the strain sensors.

Strain measurement in industrial applications:

Strain measurement in rough outdoor applications:

To find the right strain sensor for your application, you must consider a number of criteria. The selection of a strain sensor starts with the application environment. There are strain sensors for rough outdoor applications or indoor industrial applications. The design characteristics of the structure to be monitored then determine whether a screw-on strain sensor can be used or if it is better to measure the strain via a drill hole, for example. Similarly, the available space must be considered; Baumer also offers solutions for confined space conditions.
The measurement range of the strain sensor should be selected according to the expected strain at the intended position. If the expected strain is not specified yet, it is better to choose a strain sensor with a larger measurement range for an initial trial. 
As most strain sensors already have integrated amplifier electronics, standard signals such as +/- 10 V, 4..20 mA, CANopen can generally be selected. For passive sensors without amplifier electronics, there are separate amplifiers available or unamplified output signals in mV/V. 
Strain sensors are fatigue-endurable and are excellently suited for cyclical applications. Short cycle times in the millisecond range are easy to monitor. Strain sensors with suitable measurement mechanisms can also be used well for static applications. It should be noted that strains that are not caused by the stress but by external factors such as temperature changes may distort the output signal. However, there are clever measures that can compensate for these effects. 

Screw-on strain sensors are easy to install and can detect very small strains in the micrometer range on a structure that are caused by the application of force. To find the optimum place for positioning a strain sensor and obtain the best possible results, some points need to be observed.

The strain sensor should be placed in positions where measurable strains in the required direction due to the application of force can be expected on the structure. These are mostly strains or mechanical tensions that are generated by bending as well as pull/compressive stress. The finite element method can be used to determine the expected surface strain and direction for multi-axial strain situations and thus the required measurement range for the intended position. If this option is not possible, the easiest approach is to carry out a trial with a strain sensor that has a larger measurement range, e.g., 750 or 1000 µm/m, and thereby determine the actually occurring strain at the selected point. A comparison to a known force results in a simple correlation to the respective strain on the structure to be monitored. Of course, factors such as the involved construction design, accessibility, or protection of the strain sensor also play an important role in the positioning of strain sensors. 

What strains are favorable for positioning?

Strain through bending:

When strain sensors are positioned, surface strains that are caused by bending are easy to determine and thus favorable. The diagrams below present some examples of the points at which the largest measurable strains can be expected for bending.
 

Strain from pull or compressive stress:

Usually a monitored machine exhibits combined strains from bending, pull/compressive stress, torsion, etc. These can be easily determined by a finite element calculation. In practical application, however, it also becomes apparent that a simple mechanical consideration is sufficient to identify suitable points on the structure where strains occur and are measurable. The precise strains can be determined by comparison with known stresses for the measured strains. Of course, it is also possible to first glue strain gauges on positions of interest to facilitate suitable positioning of the strain sensor.

Usually a monitored machine exhibits combined strains from bending, pull/compressive stress, torsion, etc. These can be easily determined by a finite element calculation. In practical application, however, it also becomes apparent that a simple mechanical consideration is sufficient to identify suitable points on the structure where strains occur and are measurable. The precise strains can be determined by comparison with known stresses for the measured strains. Of course, it is also possible to first glue strain gauges on positions of interest to facilitate suitable positioning of the strain sensor.

Strain sensors from Baumer mostly have mechanisms with limited rigidity. This means that only a small force is required to extend or compress the sensor to its nominal strain. Therefore, the strain sensor has only a limited effect on the structure to be monitored. Furthermore, the installation screws are only stressed in a limited way, which enhances stable signal quality without potential movement of the screws. This may be essential if the strain sensor cannot be cyclically gauged.
This makes Baumer strain sensors suitable for both static and cyclical applications.

For optimal measurement results, strain sensors are screwed in place on the component using the included fastening screws. The strain sensor detects strains in the micrometer range via a frictional connection. Blind holes or clearance holes can be used. See also the section on fastening options. The screwing on of durable strain sensors with consistent quality eliminates the need for the elaborate application of glued strain gauges.

What should be considered when installing strain sensors?

​​​​​​​For strain sensors to precisely detect the surface strains and deliver good measurement results, various external conditions must be observed during installation that can also be found in the operating instructions of each strain sensor. The installation surface plays a major role in this. The strain of the base structure is transferred to the strain sensor with a frictional connection.

What fastening options are available?

Option 1: 
Strain sensors can be attached to the structure to be monitored with fastening screws in clearance holes.
 

Option 2: 
In thinner structures, strain sensors can be fastened in through holes using a nut.

How does the installation surface have to be prepared?

Important:
The sensor delivers imprecise measurement results if the measuring surface is soiled or if it is installed faultily:

• ​​​​​​​Prevent soiling by grease and oil
• Install the sensor on a prepared, level surface
• Observe the surface roughness

The respective installation screws, hole distances, and diameters are listed in the respective operating instructions.


Option: 
If it is difficult to prepare the installation surfaces in the required quality, ball shims can be used to equalize certain unevenness or angular hole deviations. 
 

There are also diamond-coated friction shims available on the market that equalize light unevenness and increase friction.

Strain sensors are precision measurement sensors containing mechanisms that were gauged at the factory. The sensor can be damaged if it is dropped when not packaged.
Please observe the notes on storage and transportation in the operating instructions.

The applicable tightening torque can be found in the respective operating instructions.

The strain sensors are intended to measure strain due to the impact of mechanical force and not be affected by thermal expansion. When affected by heat, a body expands in all directions. The appropriate circuitry of the strain gauges inside the strain sensor compensates for even thermal expansion. The strain sensors are compensated for thermal expansion of steel. Make sure that the thermal distribution at the selected point is as even as possible. This neatly compensates for temperature effects.

Thermal expansion:

Sample thermal expansion coefficients α: 

Chromium nickel steel CrNi 80 20         α = 15.5 x 10-6 [K^-1]
Chrome steel 13 Cr                               α = 11 x 10-6 [K^-1]
Pure aluminum                                      α = 23.8 x 10-6 [K^-1]

Examples:
1.    Expansion with a chromium nickel steel rod CrNi 80 20, from 20°C to 70°C with a length lo of 1 m.
∆l   = lo x α x ∆T  = 1000 mm x 15.5 x 10^-6 [K^-1] x 50 K = 0.775 mm
This is equivalent to 775 µm/m

2.    Expansion with an aluminum rod, from 20°C to 70°C with a length lo of 1 m. 
∆l   = lo x α x ∆T  = 1000 mm x 23.8 x 10^-6 [K^-1] x 50 K = 1.19 mm
This is equivalent to 1190 µm/m

Therefore, if you want to monitor an aluminum structure with a strain sensor that is compensated for steel, you should compensate for the thermal expansion so that you only have the strain due to mechanical effects. However, if it is possible to gauge after each mechanical stress cycle, then the effects of thermal expansion can be neglected.

Other possibilities for temperature compensation: 

• ​​​​​​​90° placement of strain sensors to compensate for thermal effects or via temperature measurement on the component.

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A resulting force for a specific strain can be calculated mathematically or simply in comparison to a specific force. For this purpose, the strain sensor is screwed onto the unstressed structure and then gauged. Then the structure is subjected to a known load or a defined force. This provides the connection between the strain and the force resulting from it. This calibration must be carried out only once per system. Baumer strain sensors are already adjusted to strain during manufacturing and therefore output the same measured values when they are replaced.

Screw the strain sensor in place and allow it to heat up with the power switched on according to the operating instructions. (Measurements with the sensor not screwed on and placed on a table do not produce stable measurement results). Initial settlement can be minimized by exposing the structure to be monitored to the maximum stress 10 times. The sensor should be gauged to the amplifier when the structure is stress-free. This way, signal changes due to the installation can be compensated.

Gauging after screwing on:
Strain sensors should be gauged after installation on the amplifier. This way, signal changes due to the installation can be compensated. 
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Gauging during operation:
In cyclical processes it is recommended to gauge the strain sensor after each cycle while the machine is in a stress-free state. This compensates for temperature effects or any slight movement among the installation screws. Baumer strain sensors mostly feature soft measuring mechanisms that also allow stable static measurements without cyclical gauging. 

Strain sensors with integrated amplifier electronics:

Strain sensors with separate amplifier electronics:
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Passive strain sensors that are directly connected to a control with integrated amplifier:

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Important: The connection or junction cables must have at least the same degree of protection as the strain sensor. For example, IP69K for DST55R with IP69K degree of protection