December 09, 2020
Load cells are one such instrument that are commonly used to measure weight. They can measure things as small as a needle to as heavy as big drilling machines. You can find load cells everywhere, even in your nearby grocery stores to weigh your items, though, they are inside the instruments.
Geotechnical instrumentation and monitoring is a vast field and it encompasses several sensors that aid in structural health monitoring, landslide monitoring, excavation monitoring, etc.
Load cells are available in different shapes, types, and sizes. If you’re curious about what a load cell is and how it works, keep reading to be enlightened.
Let us discuss more about them along with their types, working principle, advantages, and much more.
A load cell is a transducer that converts the mechanical force into readable electrical units, similar to our regular weighing scales. Their main purpose is to weigh or check the amount of load transferred.
The load cell sensors are always bonded along with elastic material, known as strain gauges.
To understand more on them, it is necessary that you must know about strain gauges, their types, working principle, as well as, areas of application.
| Read More: Strain Gauge: Principle, Types, Features and Applications |
The type of instruments used in the geotechnical field depends on the scope of work. Load cells can be used at the initial stage i.e. during the research and development or even at a later stage while monitoring a structure.
Geotechnical instruments are used both pre and post-construction to ensure the safety of structures, dams, tunnels, bridges etc. Proper geotechnical monitoring ensures the long-term safety of these structures.
Load cells find their application in the geotechnical field and, they are commonly used to monitor:
Figure 1: Compression load cell in between the strut at an under construction metro station
The I-beam struts are usually installed in the framework of underground metro stations. The strain gauge solid load cells are widely used to measure the amount of load being transferred to the struts. The load cells are installed in between the struts.
The anchor-back load cells are designed to test and measure the loads in tie-backs, rock bolts, and ground anchors. The load test is carried out by applying a load to the tie-back anchor with an in-line hydraulic jack.The load cell is placed between the retaining wall and the hydraulic jack. Once the hydraulic jack opens up, the load transferred to the tie-back is measured by the load cell.
There are various types of retaining walls viz. sheet pile, anchored retaining walls, bored pile retaining walls, cantilevered. The load cells are installed in these walls to monitor the changes behind it and study the effectiveness of different kinds of restraining systems at the same time.
The load cells measure and indicate the excessive load beforehand.
Figure 2: Anchor Load cell in the diaphragm wall at an under construction metro station
To measure the stability of the diaphragm or sheet pile wall, load cells are installed in the tie-back anchor systems. However, the position of the load cell is in line with the struts installed.
Underground cavities need to be monitored closely because they are under constant threat of roof collapse or sidewall falling.
Slope failures and landslides have been a major concern for the geologists as they are the most common form of the natural disaster. Hence, pre-stressed grouted anchors are used to provide active support to the walls and roofs.
Load cells measure the amount of load transferred and warn the respective authorities on time to avoid major mishaps.
Figure 3: Anchor Load cell in the cavity in a dam.
The tie-back anchors are installed on the upstream side of a concrete dam to overturn the moments caused by very high flood conditions. Load cells are installed to monitor the anchor for the stability of the dam and provide early warnings.
Figure 4: Compression load cell used in pile testings
Before constructing a structure, static load testing is carried out. The deep excavations and foundations are filled with piles to investigate their load-bearing capacity.
The load is applied by placing the hydraulic jack against the reaction piles and beam or directly by compressing.
If you’ve ever wondered how load cell sensors work, here’s your answer!.Load cell principle involves the use of many specific geotechnical instruments.It can’t work without being paired up with sensors, one of them being Strain Gauges.
Strain Gauges are thin elastic materials made up of stainless steel and are fixed inside the load cells using proprietary adhesives. The strain gauge has a specific resistance that is directly proportional to its length and width.
When a force is applied on the load cell, it bends or stretches causing the strain gauge to move with it. And, when the length and cross-section of the strain gauge changes, its electrical resistivity also gets altered, thereby changing the output voltage.
There is another concept involved with the working principle of load cells. Let’s have a look aset it.
Whenever the change in the resistance of strain gauge takes place, it is displayed as an electrical output. But, have you ever wondered how the resistance change of a strain gauge is measured?
A load cell works only when the strain gauge has some change in its resistance and we use the Wheatstone Bridge to measure this change.
Load cell circuit is also known as a Wheatstone Bridge Circuit.
Let us assume that a load cell sensor has four internal strain gauges i.e. A, B, C, and D as shown in the image above.
The input voltage supplied by a signal conditioner or digital display is attached to the two opposite corners of the bridge i.e. C and D whereas, the output voltage is measured by joining the A and B resistors to the signal side of the digital display.
When no load is applied to the load cell (Load=0), the circuit is said to be balanced. As soon as the load is applied to it, the strain gauge resistors will witness a change in its resistance, thereby altering the voltage flowing through the circuit.
Hence, the voltage across A and B will change which will be displayed as the weight on the readout unit or the digital display.
The output of the Wheatstone bridge or a load cell is an analogue data which is converted to readable units using an interpreter.
This is a common question that we get from a lot of people. Since both these sensors are intricately involved with each other, people sometimes fail to find the difference between the two. The most obvious difference is that a strain gauge is a single resistive element while a load cell is an arrangement of four strain gauges in a Wheatstone bridge arrangement as stated above.
On the other hand, using a load cell needs the application of voltage to opposite nodes whereas to make use of a strain gauge, you need to arrange two to four strain gauges to experience opposite forces.
When it comes to the many types of load cells, there are many that offer different applications and principles. Let’s see the various types of load cell, their working and specifications.
One of the first types of load cell is the Encardio Rite Model ELC-150S-H high capacity compression load cell that is extensively used for compressive load measurement during testing of piles. For testing of piles at loads greater than 12,500 kN, more than one load cell can be used.
The Encardio Rite model ELC-150S-H is a resistive strain gage type precision engineered, high capacity load cell sensor designed to measure large compressive load or axial forces. It is specially designed for civil engineering applications. It is available in capacities ranging from 5000 kN to 12500 kN.
If you’re wondering how a strain gauge load cell works, let’s take a look at it.
The compression strain gauge load cell comprises a columnar element of high strength martensitic stainless steel. The sensor utilizes sixteen 350 Ohm resistance strain gauges, wired to form a 1400 Ohm bridge.
To minimize the effect of uneven and eccentric loading, the strain gauges are equally spaced along the circumference.
The load applied to the cell can be measured by using any standard digital read-out unit suitable for resistance strain gauge applications. The load cell has great resistance to extraneous forces and is protected against dust, moisture and adverse environmental conditions.
The sectional area of the element is varied in the different capacity load cells to give approximately the same millivolt output for a variation of zero to full load.
The Encardio Rite Model ELC-210S resistive strain gauge type compression load cell is a heavy duty precision load cell. It is specially designed to meet the increasing demand in load measurement with a high degree of accuracy and reliability.
The load cell sensor is ideally suited for measurement of compressive load or forces in struts application. It is available in capacities ranging from 1000 kN to 3500 kN.
The internal construction of the load cell is columnar. The element of the load cell is made of martensitic stainless steel. The element is hardened to give better linearity and hysteresis. The strain gauges used are of foil type and are bonded to the elements using special epoxy cements which are very reliable.
The working of the load cells involves the use of strain sensors circuit.The electron beam welded sensor utilizes precision bonded foil strain gages connected in a simple Wheatstone bridge circuit. The output is derived from imbalances in the bridge circuit as load is sensed by the sensor.
Typically the bridge circuit is excited by 10 V DC to give a full scale output of around 1.5 mV/V. Load applied to the cell can be measured by using any standard digital read-out unit suitable for resistance strain gage applications.
This type of load cell is the Encardio Rite Model ELC-30S resistive strain gauge type centre hole load cell which is extensively used to determine the load in rock bolts, tiebacks, foundation anchors, cables or struts depending on the application. It is also used for compressive load measurement between structural members i.e. tunnel supports or at the junction between a beam and the top of a pile strut.
The Encardio Rite model ELC-30S is a precision engineered resistive load cell with a central cylindrical hole. The cylindrical load cell sensor is specially designed for civil engineering applications and is available in capacities ranging from 200 kN to 2000 kN.
The centre hole load cell comprises an element of high strength martensitic stainless steel. It uses eight 350 Ohm resistance strain gauges, wired to form a 700 Ohm Bridge. To minimize the effect of uneven and eccentric loading, the eight strain gauges are mounted at 45° to each other in a groove at the base of a stainless steel columnar element.
The load applied to the cell can be measured by using any standard digital read-out unit suitable for resistance strain gauge applications. The strain gauges are mounted in a groove at the base of a stainless steel columnar element.
A stainless steel diaphragm is an electron beam welded to the element to cover the groove resulting in a vacuum of around 1/1000 Torr inside the sensor.
All this helps in the sensor becoming immune to atmospheric corrosion and the effect of ingress of water. The sectional area of the columnar element and the depth of the groove in it is varied in different capacity load cells to give approximately the same mV/V output for a variety of zero to full load.
A load cell is used to determine the load in rock bolts, tiebacks, foundation anchors, cables or struts depending on the application. Encardio Rite Model ELC-30SH is also used for compressive load measurement between structural members i.e. tunnel supports or at the junction between a beam and the top of a pile strut.
The Encardio Rite model ELC-30S-H is a precision engineered high capacity load cell with a central cylindrical hole. It is specially designed for civil engineering applications. It is available in capacities ranging from 3500 kN to 10000 kN.
The centre hole load cell comprises a cylinder of high strength martensitic stainless steel. It generally uses eight 350 Ohm resistance strain gages, wired to form a 700 Ohm Bridge for capacity 3500 kN and 5000 kN. For capacities above 5000 kN, sixteen 350 Ohm resistance strain gages are used to form 1400 Ohm Bridge.
To minimize the effect of uneven and eccentric loading, the strain gauges are equally spaced along the circumference. The load applied to the cell can be measured by using any standard digital read-out unit suitable for resistance strain gage applications.
The Encardio Rite Model ELC-31V is a precision engineered vibrating wire hydraulic centre hole load cell, specially designed for civil engineering applications. It is fluid filled and is constructed from stainless steel. It is available in capacities ranging from 250 kN to 2000 kN.
The model ELC-31V vibrating wire load cell is made of a sensitive pressure pad which is formed by joining together two very stiff steel discs at their periphery. The space inside the cell is filled with de-aired fluid. When the load is applied to the cell, the pressure on the fluid changes.
This change in fluid pressure is used to record the variation in the load being applied to the cell. The load is distributed equally over the loading area of the cell by a thick, distribution plate. Load distribution plates can be used both above & below the load cell sensor to ensure an even distribution of load on to the cell.
Bottom load distribution plates are not required if an adequate bearing plate has been incorporated into the proposed installation arrangement.
The pressure in the load cell is measured by a vibrating wire pressure transducer. The vibrating wire pressure transducer is of stainless steel construction and incorporates the latest vibrating wire technology to provide electrical read-out. A glass to metal seal is provided for easy cable connection.
The data from the vibrating wire pressure transducer can be read by Model EDI-51V read-out data logger. The data can also be remotely read by the Model EDAS-10 automatic data acquisition system.
The Encardio Rite Model ELC-32V is a precision engineered load cell sensor with a central cylindrical hole. It is specially designed for civil engineering applications.
It is available in capacities ranging from 250 kN to 3500 kN. Higher capacity load cells and those having an internal diameter different from the standard specified range are available on request.
The centre hole load cell sensor comprises a cylinder of high strength martensitic stainless steel. It generally uses three vibrating wire strain gages mounted at 120° to each other to minimize the effect of uneven and eccentric loading.
The load applied to the cell is measured individually by the vibrating wire strain gauges using Encardio Rite model EDI-51V digital read-out unit for vibrating wire load cells. The average of the three readings is taken.
Alternatively, the load cell can be connected to Encardio Rite model EDAS-10 data acquisition system for continuous monitoring. If specifically required, the load cell can be provided with four or six vibrating wire strain gages.
Pneumatic Load cells are designed as such that the balancing pressure is automatically adjusted by them. The pneumatic load cell working is based on the following concept.When air pressure is applied to one end of the diaphragm, it releases through the other end/nozzle, at the bottom of it. A pressure gauge attached to the load cell sensor measures the pressure inside the cell. The deflection of the diaphragm changes the airflow through the nozzle as well as the pressure inside the chamber or diaphragm.
Piezoelectric load cells work on the same principle as that of the strain gauge load cells but, the voltage output is generated by piezoelectric material which is proportional to the deformation of the load cell.
The Piezoelectric load cells find their application in the areas of dynamic loading where strain gauge load cells often fail with high dynamic loading cycles. The piezoelectric effect is dynamic, which means the electrical output of a gauge is a non-static impulse function. The voltage output is useful only when the applied force or strain is changing and does not measure static values.
Let’s have a look at the piezoelectric Load Cell advantages and disadvantages. Some of the pros are that they are compact in size and hence, extremely easy to handle. They also have a good frequency response and rugged construction for harsh applications. On the downside, it has high temperature sensitivity and is not suitable for static conditions.
The applications of load cells include:
The %RO (Rate Output) of all the load cells that Encardio Rite has is mentioned in its specifications. The accuracy of a load cell sensor is determined through its %RO.If a 10,000 kg load cell has an error of ± 0.5%RO, it means the best resolution of the load cell would be ±50Kg
Load Cell Sensitivity is a very important factor for the instrument. A sensitivity of 3mV/V means that the load cell produces an output signal of 3mV at nominal force when the input voltage is 1V.
Over time, load cells tend to age and misalign. Hence, there’s a need to calibrate them regularly. The comparison of actual load cell outputs against the test loads is known as calibration.
A strain gauge is a single transducer used to convert the mechanical deformation into readable electrical output. Whereas, a load cell comprises an array of strain gauges that convert the mechanical load into readable units.
The axis along which the load cell is designed to be loaded is known as the primary axis.
It is the load applied along the primary axis.
The difference between the output at a specific load and the corresponding point on the straight line drawn between minimum load and maximum load is termed as Non-Linearity. It is usually expressed in units of %FS.
The maximum difference between output readings for repeated loadings while keeping the loading and environmental conditions identical is known as Non-Repeatability.This was all about the Load Cells and its different types. If you have any queries, feel free to comment below.
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