Measuring Forces in the Force Shunt
This article discusses the processes and techniques used to measure forces in a force shunt. It explains the concepts of force sensors and transducers and force measurements. It concludes with a brief overview of calibration of these devices.
Force Sensors and Transducers
Force sensors are devices that respond to or detect physical force caused by a physical load, weight or pressure. A force transducer is then a force sensor that is able to transform this physical force into an output electrical signal (voltage or current). Examples include piezoelectric transducers, strain-gauge load cell, pneumatic and hydraulic pressure transducers.
Force transducers are often integrated into process control systems. When these transducers are incorporated into an open-loop control system, they must be calibrated to obtain accurate results (see “Calibrating the Force Measuring System”). Calibration gives a characteristic curve created by graphing the transducer response (output signal) vs. the applied stimulus (force) over a range of values. This curve then serves as a reference for proper calibration even after the device is installed.
Accurate measurements depend not only on a properly calibrated device, but also proper placement or orientation of the object or machine applying the force. Force measurement techniques involve this interface between transducer and object or device generating the force.
In electronics, a shunt is a device that creates a path of low resistance that allows or diverts the flow of electric current through another point in the circuit. Simply put, the electric current will flow through the shunt because it has low resistance.
Similarly, a force shunt can be defined as a device that offers a low resistance to the flow of force such that a portion of the process force flows through it. In this article, the force being considered is a mechanical force created by a physical load, weight, or pressure.
A force shunt is different from an electrical shunt since, with the latter, most of the electricity flows through the shunt; with the former, less of the force is typically diverted. However the underlying concept is the same.
The two main categories of force measurement are direct force measurement and force shunt measurement. They are defined as follows:
- Direct force measurement quantifies the process force that flows through the transducer or force measuring instrument. Most (ideally all) of the process force should flow through this transducer. For an example see the article “Choosing the Right Load Cell For Your Job”. A direct force measurement requires the use of mounting kits fixed through bolts, screws, and nuts. The mounting kits must have flat surfaces and be rigid.
- Force shunt measurement quantifies forces diverted or “shunted” away from the main process force. Safety structures or other external connections to the measured entity may sometimes cause these force shunts. Force shunts can also be inherent in the system and not caused by these auxiliary attachments. By measuring the force along the force shunt, the applied force can be determined in both magnitude and direction.
Direct force measurement’s advantage is high accuracy, but sometimes force shunt measurement’s advantages outweigh this. For these applications, special sensors must be designed.
Force Measurement Methods
Various methods are used for determining force. These include lever-balance methods, force-balance methods, hydraulic and pneumatic pressure measurement, an acceleration measurement and the use of elastic elements. Each of these methods has evolved over the years; however, the most common ones are based on the deformation of the structure inside the instrument that measures forces. That is, they involve the use of elastic elements.
This section discusses two common, closely related instruments that employ deformation: strain gauges and strain gauge transducers.
The Use of Strain Gauges
Two types of strain gauges are bonded and unbonded. For force shunt measurements, a bonded strain gauge is the most appropriate. This is because bonded strain gauges have practically no influence on the structure of the object under test. They do not affect the stiffness and the dynamic behavior of the test object as a whole.
Figure 1 below shows a full-bridge strain gauge.
The gauge must be carefully selected for each application to compensate for parasitic effects of bending moments or torsion, temperature shifts and other undesirable effects (see “Load Cell Mounting and Installation Best Practices”).
When installing the strain gauge on site of the object to be measured, the gauge is carefully attached or bonded to its body. This must be done carefully and properly to ensure that only the tensile and/or compressive forces act on it and not any bending moments.
Furthermore, the output of the strain gauge bridge as shown in Figure 2 below depends on the gauge factor of the strain gauge, the strain level and the supply voltage.
Figure 2 above is a quarter-bridge (single strain gauge) setup and mathematically expressed as the following:
The gauge factor () of the strain gauge should also be obtained during calibration. This value is then used in the expression below for , along with the excitation voltage, , original strain gauge length, , change in strain gauge length, , and the output voltage, .
The bridge output signal can be increased by a controlled weakening of the area on the object’s body surface where the strain gauge is attached. However, this affects the object’s stiffness, dynamic behavior, and stability.
The Use of Strain-Gauge Transducers
Strain gauge transducers include the strain gauge described above, but have other components such as an elastic element (also called the structural member) and a housing unit.
With these devices, one or more strain gauges are fixed onto the elastic element so that the latter acts as a primary transducer converting the force exerted by the test object into the strain. Then each strain gauge acts as a secondary electrical transducer that converts the strain to changes in electrical resistance.
On the transducer device, the strain in the area of the installed strain gauges is greater than the strain value between the two screwed connections or load application points. See Figure 3 below.
This shows that the strain generated by the force applied to the transducer is concentrated on the zone where the strain gauge is attached. The gauge is mounted so that the long lengths of the conductor are aligned in the direction of the force acting on the test object.
The approximate excessive increase in strain can be mathematically expressed as:
This expression is based on ideal conditions with the assumption that the zone around the application point is strain-free. However, in practical terms, this is not entirely correct.
Furthermore, from the mathematical expression, it becomes clear that the strain transducer’s sensitivity can be adjusted by means of the length ratio of strain zone (the ideal strain zone is at the area where the strain gauge is mounted) and the distance between the screw connections. This then means that a very high sensitivity can be achieved theoretically.
Parallel Strain Gauge Transducer Connections
Tacuna Systems strain transducers without inbuilt electronic circuits can be easily connected in parallel since each device has a high bridge resistance of the same value. This high bridge resistance ensures that no excessively high amplifier supply current is needed, reducing amplifier cost. Parallel connection between these transducers allows output measurements to compensate for unwanted strain effects.
An example of this compensation for undesired strain is in the measurement of forces acting on a column. In this situation, the only strain that is relevant is the one resulting from tensile or compressive loading. Assume the measurement system is set up such that two single point load cells are connected in parallel and mounted on a column at the same height, opposite from each other. Now, when a bending load is applied, one transducer will experience a higher strain while the other experiences a lower strain. By determining the difference, only the tensile or compressive portion of the strain is measured and the bending gets compensated for.
Force Shunt Connections
Tacuna Systems offers various load cell types not limited to just single point and double-ended load cells. We also offer load cells that can be mounted in a force shunt setup. So far, this article has introduced the concepts of strain gauges vs. strain gauge transducers. Here we list the advantages and disadvantages of each to force shunt measurements.
The advantages of using strain gauges for force shunt measurements are:
- They do not occupy much space
- They are suitable for highly filigree structures with a small force applied to the gauge as they avoid excessive force shunts created by strain gauge transducer methods.
Some disadvantages include the following:
- The installation process requires bonding, wiring, protective coating etc.; these all increase installation time.
- It requires calibration of the force shunt, also increasing setup time.
Strain Gauge Transducers
The advantages of using strain transducers are:
- They offer easy installation and rapid deployment. This is especially true of Tacuna Systems load cells, which can be screwed readily onto existing structures.
- The load cell output is easily wired to electronics amplifiers and conditioners. Tacuna Systems offers these electronic devices optionally bundled with our load cells.
- They offer temperature compensation and the range can be seen in the load cell datasheet.
The main disadvantage of using strain transducers is that calibration in the force shunt is still required. However, Tacuna Systems calibration services are readily available and this services can be purchased in conjunction with a load cell
Calibration of the System
Calibration is the process of comparing a measuring instrument against an authoritative reference for the same type of measurement. The force measurement system needs to be calibrated before or after installation.
In calibrating a direct measurement system, it is advisable that no force shunts should be presented and it should be noted that the characteristic features of the instrument such as stiffness and dynamic behavior affects the overall design and can require the use of very large structures for measuring large force vectors.
Where force shunts are present, calibration is highly important especially in the case of determining the magnitude and direction of the force in the force shunt. This means it helps to ensure that the instrument is providing an accurate indication of the actual force in the object being tested.
Load cells can be calibrated with very high precision before mounting in the field (also called formal calibration), however, for force shunt measurement, calibration should be done directly on the object in the field right after installation is carried out.
Datasheets for products
A wide range of load cell product types and designs are available to choose from for any of your applications. Each product has a data sheet that shows its uncertainty, accuracy, sensitivity and other important specifications that are needed in designing a force measuring system in the force shunt.
The calibration procedure will involve the measurement of the force when zero-load is applied on the shunt. Also, the output is measured when the maximum rated force is applied to the shunt. The necessary adjustments can then be made to the procedures until the output of the measurement instrument matches the nominal value specified in its datasheet. When calibrating the force shunt, conditions such as temperature, alignment of loading and humidity should be controlled as much as possible.
This article explains the various concepts of force measurement and products in our product line that are readily available for measuring forces in the force shunt. The two methods discussed have very minimal effects on the dynamic mechanical behavior of the structure being monitored as a whole. It also asserts the superiority of strain-gauge-based instrumentation technology for very high accuracy shunt force measurements.