The measurement of force using force transducers ensures maximum measurement accuracy. It is, however, beneficial to measure force in the force shunt. For these cases, special sensors accurately matched to the application are available. This article presents three methods used to measure in the force shunt.
The following three methods can be used:
- Installation of strain gauges
- Utilization of screw-on type strain transducers, sometimes even with integrated electronics.
- Utilization of force washers, based on a strain gauge or piezo-technology.
Table 1 shows the key advantages and disadvantages of the methods presented:
| Installation of strain gauges |
Utilization of strain transducers |
Utilization of force washers |
| Advantages |
- Well suited for highly filigree structures with a small force applied which do not enable the other methods to be used because of excessive force shunt.
- Minimum space required.
|
- Easy installation by screwing onto existing structures, rapidly deployable.
- Compensation of the effects of strains that are to be suppressed when connecting several sensors in parallel.
- Versions with integrated electronics can be directly calibrated in the application Ideal for use on bolts or screws.
|
- High degree of protection
- Delivered ready for installation.
|
| Disadvantages |
- Installation effort (bonding, wiring, protective coating).
- Calibration in the force shunt required.
|
- Calibration in the force shunt required (reduced effort with variants with integrated electronics).
|
- Calibration in the force shunt required.
|
Using Strain Gauges
Figure 1 shows a VY41 strain gauge:
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Figure 1. VY41 strain gauge. The strain gauge is ready for connection in a full bridge circuit to minimize wiring effort. Observe the 45 degree alignment marks that are essential for installation in this case. Image credit: Hottinger Baldwin Messtechnik GmbH
There are a number of advantages in direct installation of strain gauges to measure force:
Bonded strain gauges (SG) do not have any influence on the object structure being tested. The dynamic behavior and stiffness of the structure remains unchanged. Strain gauges offer clear benefits in the case of filigree structures as they require only very small forces to be deformed.
In this installation, strain gauge full bridges are used which, when selected carefully compensate for parasitic effects such as bending moments or torsion or, alternatively, measure these effects.
The output signal of such a measuring bridge is based only on the gauge factor of the strain gauge that is used and on the strain level as well as the material's Poisson's ratio; it can be calculated as follows:
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In this:
| U/U0 |
: Output signal of the measuring bridge |
| k |
: Gauge factor of the strain gauge |
| ε |
: Strain level on the strain gauge |
| μ |
: Poisson's ratio |
A disadvantage of bonded strain gauges is that to enable the structure to attain a defined stiffness, the output signal that can be attained is very small. The strain gauge needs to be installed on site. A protective coating is required. Also the strain gauges need to be connected with great care which increases the time required for installation.
Using Strain Transducers for Force Measurement
Figure 2 shows a strain-gauge based transducer.
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Figure 2. Strain gauge-based SLB700 strain transducer. Image credit: Hottinger Baldwin Messtechnik GmbH
Strain transducers are sensors that can be installed on the existing structure. These transducers are based on a spring element onto which a strain gauge full bridge has been installed.
These transducers have a silicone coating (white area on the transducer) as shown in Figure 2 which, in addition to moisture protection, offers some degree of mechanical protection. Strain transducers are based on the principle of strain transformers. Figure 3 shows the SLB spring element.
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Figure 3. SLB spring element: The picture clearly shows the area of strain where the strain gauge has been installed. Image credit: Hottinger Baldwin Messtechnik GmbH
The approximate excessive increase in strain can be calculated as follows:
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Where:
| εSG |
: Strain present under the strain gauge. |
| εObject |
: Strain between the screwed connections. |
| lStrain sensor |
: Distance between the screwed connections. |
| lStrain zone |
: Length of the area of structural weakening. |
The idealizations assumed are:
- The zone of strain application is strain-free. In principle, a very high sensitivity can be attained, however, in practical use, an output signal of 1.5mV/V at 500µm/m has proved favorable. This results in a sensitivity increase of 230% compared with a strain gauge full bridge as described above.
- The temperature dilatation of components has been compensated for by taking appropriate circuitry measures.
- In addition, strain transducers with integrated electronics are available that can be calibrated in the application, therefore providing an extremely efficient measuring chain.
- The sensors without electronics have a high bridge resistance of 700Ω. This enables several strain transducers to be connected in parallel without requiring an excessively high amplifier supply current.
- SLB strain transducers from HBM can be mounted on structures using 4 M6 screws. It needs just a plane surface, free from paint or other coatings. Upon screw-mounting of the strain transducer with the recommended torque, we recommend applying corrosion protection, for example ABM75 foil.
Using Force Washers for Force Measurement
Figures 4, 5 and 6 show force washers used for measurement.
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Figure 4. KMR force washer, smallest design, 20kN nominal (rated) force. Image credit: Hottinger Baldwin Messtechnik GmbH
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Figure 5. CFW piezoelectric force washers, available with 20 to 700kN. Here: 330 and 700kN. Image credit: Hottinger Baldwin Messtechnik GmbH
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Figure 6. For tool monitoring, a force washer measures the forces acting on a screwed connection. Image credit: Hottinger Baldwin Messtechnik GmbH
Force washers can be based on both strain gauge or piezo technology. Force washers can be used on both bolts or screws. The force shunt results from the bolt or screw on which the force washer has been installed functioning as a parallel spring element and thus reducing the measurement system's sensitivity. In order to ensure good repeatability, it is essential that a pre-stress is applied to the force washer.
Force washers offer excellent protection and are delivered ready for mounting; in this regard, they offer the same advantages as strain transducers. Force washers, too, provide sufficient sensitivity that, with piezoelectric transducers, is independent of the nominal (rated) force.
Calibrating Measuring Chains in the Force Shunt
All the three methods are related in terms of the measuring chain that requires calibration after installation. This means that measurements have to be taken at two known points of force. The sensor output signal is then assigned to the forces. Since sensor behavior is linear within strictly defined error limits, these methods cannot be used for high-precision measurements; two-point calibration is sufficient in general.
The integrated electronics of the strain transducer follows this line of reasoning. Calibration requires measurement at the zero position and a control impulse sent to the electronics. Another control impulse is required when the maximum force has been applied, followed by automatic adjustment of the electronics. Zeroing is, of course, separately possible, without changing the gain factor.
Conclusions
This paper shows a number of useful methods for the measurement of forces in the force shunt. The common point in all the methods is that they marginally impact the mechanical behavior of the structure as a whole. However, piezo- or SG-based force transducers is the first choice when high accuracy is required .
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This information has been sourced, reviewed and adapted from materials provided by HBM, Inc.
For more information on this source, please visit HBM, Inc.