Technology

Shear-Beam Load Cells vs. Self-Leveling Pressure Transducers

The self-leveling load cell is a very sophisticated tool of metrology. There are several advantages to using the SL load cell compared to using a much more crude and simple design such as the shear beam load cell. The first advantage is inherent in its name. The load cell self levels which allows it to accurately measure weight on inclined surfaces. It also has very low profile which is mandatory for being conducive of designing a low height device. SL load cells truly excel at the point where they bridge a mechanical force into an electrical signal. These load cells use precision pressure transducers utilizing piezoresistive semiconductor sensors, which delivers a number of advantages including a high rating for fatigue life.

In many applications, a load cell only needs to measure accurately along one line of force. This is how most load cells are designed including shear beam load cells. This design will suffice in an application where the environment is controlled to only apply force along an axis completely perpendicular to the load cell. Any slight angle that this force varies will result in error. This becomes a fatal problem when an object is being weighed. Accurate weight cannot be measured unless the load cell is perfectly oriented to be perpendicular to the direction of gravity. However, this requirement is usually impossible to control in a portable weighing device. The top piston of a self-leveling load cell automatically orients itself to be perpendicular to whatever force is applied to it. It can do this at an angle up to 5 degrees off of the perpendicular angle. This occurs due to the most simple of laws in hydrostatics. The fluid pressure under the piston must equalize throughout its volume. This means that the fluid is exerting a reaction force against the piston, and it is pushing with the same force at all areas of the piston. This instantly drives the system to reach equilibrium where the surface of the top piston is perfectly perpendicular to the direction of force. It is similar to the way you can tilt your glass of water and watch the top surface of the fluid instantly move to maintain an orientation perpendicular to the direction of gravity.

Self-leveling load cells have a very low profile for the weight they can measure. Compared to shear beam load cells, SL load cells are usually ½ of the height of a shear beam load cell with the same capacity. This is because a SB load cell functions by bending a piece of metal. Therefore, the metal must be thick in the direction it is being bent, in order to support a heavy load. SL load cells transmit a force into a fluid pressure. The pressure is the same no matter what volume of fluid is in it. Therefore, it allows a design which is extremely thin for the amount of force it can handle.

After the force is transmitted into a fluid pressure within the self-leveling load cell, it is turned into an electrical signal by a pressure transducer. These transducers use a piezoresistive semiconductor sensor chip in an oil isolated housing with an integral ceramic for temperature compensation and calibration. This design provides exceptional accuracy, stability, repeatability, durability, and temperature performance. The technology put into these transducers is far advanced from the strain gauge technology used on typical shear beam load cells. SB load cells are also very difficult to compensate for temperature because they have a large mass of steel which will expand and contract as temperature changes. When repeatedly bending a metal object, there is always a risk of developing fatigue cracks over time. Fatigue occurs when a load is applied and removed a certain number of times. For this reason, most manufacturers provide no rating or guarantee on the life cycle of a standard shear beam load cell. The pressure transducers used in the self-leveling load cells have a rated life cycle that is greater than one-million cycles. This gives the user a huge margin of confidence that the load cell will perform accurately longer requiring less frequent calibrations.

The sophistication of the self-leveling load cell has given it advantages across the board compared to older technologies such as the shear beam load cell. At first glance, one could appreciate that the SL load cell has about half the height of other load cells. Within further inquiry we realize that as long as we are within 5 degrees the force is automatically applied in a manner that is exactly perpendicular eliminating all possible errors from off center loading or inclines. Then the electrical signal is generated with the most advanced transducer technologies available. This equates to a simple and precisely delivered mechanical load in a low profile package and a conversion into an electrical signal that utilizes the best of advantages in accuracy, stability, repeatability, temperature compensation, and fatigue resistance.