Load Cells

Working

Load cells works like calibrated spring. It used for measuring forces or for weighing system. It converts force or weight into electrical signal. A load cell usually consists of four strain gauges in a Wheatstone bridge configuration. Load cells of one or two strain gauges are also available. The gages are developed from an ultra-thin heat-treated metallic coil and are chemically bonded to a thin dielectric layer. "Gage patches" are then mounted to the strain element with specially formulated adhesives. The precise positioning of the gage, the mounting procedure, and the materials used all have a measurable effect on overall performance of the load cell. Wheatstone bridge with strain gauges that measure this mechanical flexion hence giving a proportional electrical signal. This electrical signal is amplified by instrumentation amplifier because electrical signal output is very low. Load cells are used for sensing large, static or slowly varying forces with little deflection.

Load cell types

• Double ended shear beam
• Single ended shear beam
• Single column
• Multi column
• Membrane
• Torsion ring
• Bending ring
• Pancake
• Digital Electromotive Force

Strain gauge load cell

Strain gauge load cell consist of a structure that elastically deformed when subjected to a force and Strain gauge network produces an electrical signal proportion to deformation. Example of this type of load cells are: Beam type load cell, Ring type load cell.

Beam type load cell

Beam type load cell are commonly employed for low level loads. Four strain gauge, two on the top surface and two on the bottom surface is used as elastic member for a load cell. The range and sensitivity of this beam type load cells depends upon the shape of the cross section, location of point of application of the load and fatigue strength of the material from which beam type load cell is fabricated.

Ring type load cell

Ring type load cell has a ring as elastic material. The ring element can be designed to cover a very wide range of loads by varying diameter D, thickness T or the depth of the ring. The load P is directly proportional to the output voltage Eo. The range of ring type load cell controlled by the strength of material used in fabricating the ring.

Styles of load cells

Compression/Tension Load Cells

Compression/tension load cells can be used for applications where the load may go from tension to compression and vice versa. They are ideal for space restricted environments.

S-Beam Load Cells

S-Beam load cells get their name from their S shape. S-Beam load cells can provide an output in under tension or compression. Applications include tank level, hoppers and truck scales.

Bending Beam Load Cells

Used in multiple load cell applications, tank weighing and industrial process control. Used in multiple load cell applications, tank weighing and industrial process control.

Platform and Single Point Load Cells

Platform and single point load cells are used to commercial and industrial weighing systems. They provide accurate readings regardless of the position of the load on the platform.

What are the factors to select a load cell?

• Capacity (kgs, pounds)
• Basic type and mode of operation
• Performance or accuracy level -related to system requirements.
• Method of mounting.
• Approval requirements (metrological, safety, environmental)
• Material of construction.
• Cost.

Repeatability

Repeatability is the agreement between the results of successive measurements of the load cell output for repeated applications of a given force applied in the same direction. Repeatability is normally measured by applying the full rated load to the load cell three times under carefully controlled conditions. Environmental conditions must remain constant during the test. The measurements are carried out at the full rated load of the load cell so the rated output is the average of the three output values obtained from the test. Output values can be expressed as millivolts per volt (mV/V) or millivolts (mV) providing they are all in the same units.

What type of errors occurred in load cell outputs?

Non-linearity, hysteresis and creep errors are most commonly considered errors in load cell application.

Non linearity

If measurements are being made in one direction only the non-linearity describes how they reliance upon a linear output, which is normally assumed for scaling on amplifier or meter, can lead to the linearity error being applied to the measurement point over the entire range of the load cell.

Hysteresis

Our hysteresis data is achieved by using the mid load point as the reference point to calculate hysteresis during an incremental and decremental calibration of the load cell.

Creep: Creep is the change in load cell output that occurs with time when a constant load is applied with environmental and other variables remaining constant. Creep is specified over a fixed time period, often 20 minutes.

How Environmental factors affect the load cells?

Temperature

Generally temperature changes will result in a change in a load cell's physical dimensions, shift in the zero balance of the bridge and a small change in the Young's modulus or stiffness of the structure of the load cell causing an output span change.

Pressure

For particular types of load cell, static or dynamic ambient pressure changes can cause error due to a net resultant force exerted on a sensitive form area.

Load cell calibration

Load cell calibrated by using one of the three methods.
• Dead weight
• Lever
• Hydraulic

Application fields

• Automatic dispensers
• Tablet testing
• Pipette control
• Dynamic or fast weighing
• Crystal growing machines
• Ashing and residue analysis
• Tensionmetry
• Thermogravimetry