Load Cells: Types, How It Works, Applications, & Advantages

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 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.

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.

What is a Load Cell?

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 |

What is load cell used for?

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:

1. Deep foundations: excavation bracing like struts, soldier pile; tiebacks or anchors; retaining walls
2. Tunnels and shafts: Steel liner plate, cast-in-place concrete, segmented precast concrete, shotcrete
3. Dams: Concrete Dams, Underground Power House
4. Piles: Pile load test

Struts

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.

Tie-backs, rock bolts, ground anchors

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.

Retaining Wall

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.

Diaphragm or Sheet Pile Wall

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.

Roof support in underground cavities / Slope stability in landslide zones

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.

Dams

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.

Pile Load test

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.

What is the working principle of a load cell?

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.

What is the Wheatstone Bridge?

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

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.

Difference between Load Cell and Strain Gauge

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.

Types of Load Cells

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 …[Continue reading]