A lot of people have the morning routine of stepping on a scale to check their weight. What most people don’t know is that they are using a very simple “load cell” to find out their weight. Old scales used to use weights to try and level out the two sides of the scale; now we have methods that measure weight automatically.
The first thing we need to know about a load cell is a definition of what we are talking about. 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. They are pneumatic, hydraulic, strain gauge, and capacitance.
Let’s begin by looking at how a pneumatic load cell works. Since it is pneumatic, we know that it will 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 weights. The pressure gauge can convert the air pressure reading into an electrical signal.
Next, let’s talk about a hydraulic load cell. The word hydraulic should let us know that this load cell will work by using fluid, whether water or oil. These load cells are similar to pneumatic load cells but instead of air, they use the pressurized liquid. A hydraulic load cell is consisting of an elastic diaphragm, a piston with a loading platform on top of the diaphragm, oil or water that will be inside the piston, and 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 can 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.
The next type of load cell we will discuss is the strain gauge. This is the most popular style of the load cell. A strain gauge load cell is a transducer that changes in electrical resistance when under stress or strain. The electrical resistance is proportional to the stress or strain 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 measures unknown electrical resistance by balancing two legs of a bridge circuit, one of the legs contains the unknown component. The “Wheatstone bridge” circuit provides incredibly accurate measurements. The strain gauges that are in the Wheatstone bridge are bonded onto a beam which deforms when weight is applied.
The last type of load cell we are going to discuss is a capacitive load cell. Capacitive load cells work on the principle of capacitance, which is 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 giving us a change in the capacitance which can be calculated into a weight.
Now that we have discussed the different types of load cells lets discuss some applications. The first application we are going to discuss is a salt bag filling process. In this application, empty bags are loaded into a machine where arms will swing down and pick up an empty bag and place it underneath a funnel. Above the funnel, there is a fill bin that will dispense salt onto a conveyor belt with a built-in load cell in order to dispense the correct amount of salt into the bags. As the fill bin is dispensing salt, the load cell is giving an analog input to a plc which is the current weight on the load cell. Once the load cell is reading a weight close to the full bag weight the fill bin will close to a trickle until the correct weight is determined. Once the load cell has the full bag weight on it, the conveyor will start dropping the salt into the funnel and down to the waiting bag. The bag will be sealed and removed from the machine so another empty bag can be loaded.
Next, let’s discuss how a load cell can be used in a pressing application. In this example, we will be looking at door panel press. Sheets of aluminum will be rolled into a die which will be closed down onto the aluminum creating a pattern on the door panel. As the die closes, a load cell is sensing the amount of force applied on the die and the aluminum. Once the applied force has reached a predetermined limit the die will open and the panel will now be removed. If the applied force is too light or too heavy the panel could be damaged or not pressed to the correct pattern.
Determining which load cell your application requires depends on how sensitive and accurate your application needs to be. The accuracy and sensitivity are very high with capacitive. A strain gauge type of load cell would be the next in line when it comes to accuracy and sensitivity. While still useful in certain applications, pneumatic and hydraulic load cells would be the less sensitive and accurate types.
In closing, we discussed the four different types of load cells. They were pneumatic, hydraulic, strain gauge, and capacitive. Also, we discussed how a load cell can be used in different industrial applications. In closing remember that the determining factor in choosing a load cell comes down to how accurate and sensitive your application requires.