What is a Load Cell?
Load cells are a fairly common mechanism used in our everyday life. Take a standard morning routine of stepping on a scale to check your weight as an example. This mechanism utilizes a load cell to find out our specific measurements/ weight dimensions.
A load cell is a force gauge that consists of a transducer that is used to create an electrical signal whose magnitude is directly proportional to the force being measured.
There are four common types of load cells. Pneumatic, hydraulic, strain gauge and capacitance. We explain this below;
Strain Gauge Load Cell
This is one of the most popular styles of load cells. A strain gauge load cell is a transducer that changes in electrical resistance when under stress. The electrical resistance is proportional to the stress placed on the cell making it easy to calibrate into an accurate measurement. The electrical resistance from the strain gauge is linear, therefore it can be converted into a force and then a weight if needed.
A strain gauge load cell is made up of 4 strain gauges in a “Wheatstone bridge” configuration. A Wheatstone bridge is an electrical circuit that uses four resistive elements in a specific configuration shown in Figure 1. In most strain gauge load cells, all four resistive elements are strain gauges that are positioned in such a way that when a load is applied to the load cell:
One pair of strain gauges opposite each other in the bridge is in placed tension, and increases slightly in resistance
The other pair of strain gauges is placed in compression, and decreases in resistance by the same amount as the other pair
The net effect of one pair of resistors increasing while the other is decreasing is a differential shift in voltage across the output of the bridge. This output shift is proportional to the load applied to the sensor.
Wheatstone bridges are typically used because they produce an incredibly accurate and precise output that minimizes the effect of external influences. For example, if the temperature of the circuit changes, that can significantly affect the resistance of each strain gauge, however since the strain gauges are in a balanced bridge configuration, they all shift by the same amount and it does not affect the accuracy of the measurement.
Figure 1: Wheatstone Bridge Strain Gauge Circuit
Pneumatic Load Cells
Since it is pneumatic, we know that these types of cells deal with air pressure. A pneumatic load cell consists of an elastic diaphragm which is attached to a platform surface where the weight will be measured. There will be an air regulator which will limit the flow of air pressure to the system and a pressure gauge.
Thus, when an object is placed on a pneumatic load cell, it uses pressurized air or gas to balance out the weight of the object. The air required to balance out the weight will determine how heavy the object weighs. The pressure gauge can convert the air pressure reading into an electrical signal.
Hydraulic Load Cells
The word hydraulic is indicative that this piece of machinery works by using liquid, whether water or oil. These load cells are similar to pneumatic load cells, but instead of air , they utilize pressurized liquid.
A hydraulic load cell consists of an elastic diaphragm, a piston with a loading platform on top of the diaphragm, oil or water that will be located inside the piston as well as a bourdon tube pressure gauge.
When a load is placed on the loading platform, the piston applies pressure to the liquid contained inside it. The pressure increase of the liquid is proportional to the applied force or weight.
After calibrating the pressure, you accurately measure the force or weight applied to the hydraulic load cell. The pressure reading can be read as an analog gauge or it can be converted into an electric signal from a pressure sensor.
Capacitive Load Cell
Capacitive load cells work on the principle of capacitance; the ability of a system to store a charge. The load cell is made up of two flat plates parallel to each other. The plates will have a current applied to them and once the charge is stable, it gets stored between the plates.
The amount of charge stored, the capacitance, depends on how large of a gap between the plates. When a load is placed on the plate, the gap shrinks providing a change in the capacitance which can be then calculated into a weight.
Why Use Load Cells?
The fact that such a large proportion of all global sales still relies on precise weight measurement is one reason that load cells play such a vital role in modern commerce.
The primary purpose of load cells is to help users measure the physical quantity or mass, and to convert force or energy into another form (e.g. force, light, torque, motion). In more directly industrial uses, load cells are extremely important in maintaining precise quality control systems for manufacturing consistency and safety processes. They’re also very widely used in all manner of medical and laboratory settings to achieve optimally accurate measurements for a wide range of scientific and research purposes.
The importance of load cells is further exemplified by those industries that are less intrinsically reliant on critically accurate measurements utilizing these pieces of equipment too. An example of this is within general logistics and freight calculation. Due to Load cells compatibility with a wide array of digital and computer-based communications, they are easily integrated into a plethora of industries in a seamless and time-efficient way; allowing them to respond to key diagnostic information in real time.
As this is so much more difficult to achieve using traditional mechanical platform weighing devices, load cell installation often proves to be a far more efficient and cost-effective choice for most companies in the long-run.
Types of Load Cell Applications
Load cells serve a variety of functions and are extremely pivotal pieces of equipment for use in research, prosthetics and healthcare applications, to name a few. Whether used in food processing to precisely measure ingredients and to properly distribute the products during packaging or in the medical and pharmaceutical production to weigh equipment,
hair strand test monitoring, baby scales, stretcher scales, bed scales, medical pump testing, sling/hoist scales, biomedical research and hand force applications.
Due to the intricacies of most of these applications, they tend to rely heavily upon miniature load cells; designed with minimum space and high capacity loads. Micro load cells offer certain benefits such as having a low profile, single point platform, cost-effectiveness, long term stability and low hysteresis. Depending on the applications, most manufactures are willing to make custom micro load cells for their customers.
Examples within the medical setting
Miniature load cells are utilized extensively for testing and calibration of medical equipment, to provide the most accurate treatment devoid of human errors, solving a variety of common issues in hospitals, nursing homes and patient residences. These include;
- Monitoring weight of IV Bags on IV stands to detect when the bag is empty and needs to be replenished
- Monitoring and control of Syringe Pumps to provide a safety switch to cut off pumping in case of clots or occlusions
- Provide force feedback information to surgeons during surgery in order to prevent damage to delicate bones or tissue
- Provide information about condition of muscles after surgery and during rehabilitation
- Remote monitoring and control of inventory to eliminate stock outs of critical parts and drugs at points of use and delivery
- Monitor the pressure inside of fluid-filled tubing to prevent dangerous over-pressure conditions
Load cells are often an overlooked component in modern industrial and manufacturing industries. But their overall importance in accurately providing precise measurements in a timely fashion is undisputable.
With a broad selection of configurations to choose from, market knowledge of what these components are and how they work is thus essential when selecting load cells for your intended application.
Expertise and extensive marketing knowledge is important to ensure you get the right load cell that will work for you and your business. This is where SMD sensors are here to help. SMD seniors is a world leader in specialty sensor designs. Supplying leading edge sensor technology offering superior stability, temperature performance, lower power consumption and flexibility of design. With over 30 years of experience, our clients trust us to provide effective, cost-conscious solutions for the best and most load cells to suit you and your business needs. Call today to find out how we can help!