Load cell

Strain gauge load cells are the kind most often found in industrial settings. It is ideal as it is highly accurate, versatile, and cost-effective. Structurally, a load cell has a metal body to which strain gauges have been secured.  The body is usually made of aluminum, alloy steel, or stainless steel which makes it very sturdy but also minimally elastic. This elasticity gives rise to the term "spring element", referring to the body of the load cell.  When force is exerted on the load cell, the spring element is slightly deformed, and unless overloaded, always returns to its original shape. As the spring element deforms, the strain gauges also change shape. The resulting alteration to the resistance in the strain gauges can be measured as voltage. The change in voltage is proportional to the amount of force applied to the cell, thus the amount of force can be calculated from the load cell's output.

Strain Gauges

A strain gauge is constructed of very fine wire, or foil, set up in a grid pattern and attached to a flexible backing. When the shape of the strain gauge is altered, a change in its electrical resistance occurs. The wire or foil in the strain gauge is arranged in a way that, when force is applied in one direction, a linear change in resistance results. Tension force stretches a strain gauge, causing it to get thinner and longer, resulting in an increase in resistance.  Compression force does the opposite. The strain gauge compresses, becomes thicker and shorter, and resistance decreases. The strain gauge is attached to a flexible backing enabling it to be easily applied to a load cell, mirroring the minute changes to be measured.

Since the change in resistance measured by a single strain gauge is extremely small, it is difficult to accurately measure changes. Increasing the number of strain gauges applied collectively magnifies these small changes into something more measurable. A set of 4 strain gauges set in a specific circuit is called Wheatstone bridge.

Wheatstone Bridge

A Wheatstone bridge is a configuration of four balanced resistors with a known excitation voltage applied as shown below:

Excitation voltage ${\displaystyle V_{\text{EX}}}$ is a known constant and output voltage ${\textstyle V_{o}}$ is variable depending on the shape of the strain gauges. If all resistors are balanced, meaning ${\displaystyle {\frac {R1}{R2}}={\frac {R4}{R3}}}$ then ${\textstyle V_{o}}$is zero. If the resistance in even one of the resistors changes, then ${\displaystyle V_{0}}$will likewise change. The change in ${\textstyle V_{o}}$ can be measured and interpreted using Ohm's law. Ohm's law states that the current (${\textstyle I}$, measured in amperes) running through a conductor between two points is directly proportional to the voltage ${\textstyle V}$ across the two points. Resistance (${\displaystyle R}$, measured in Ohms) is introduced as the constant in this relationship, independent of the current. Ohm's law is expressed in the equation ${\displaystyle I=V/R}$.

When applied to the 4 legs of the Wheatstone bridge circuit, the resulting equation is:

${\displaystyle V_{o}=\left({\frac {R3}{R3+R4}}-{\frac {R2}{R1+R2}}\right)V_{\text{EX}}}$

In a load cell, the resistors are replaced with strain gauges and arranged in alternating tension and compression formation. When force is exerted on the load cell, the structure and resistance of the strain gauges changes and ${\textstyle V_{o}}$ is measured. From the resulting data, ${\textstyle V_{o}}$ can be easily determined using the equation above.[1]

Common types of load cells

There are several types of strain gauge load cells:

• Bending beam: uses strain gauges to monitor the stress in the sensing element when spring element is subjected to a bending forces.[2]
  

  HBBG bending beam load cell (bellows type)
• Load Button: low-profile load cells often used where the load is concentrated in one single point (button).

• Pancake: low-profile load cells often used in vessel weighing; can be tension or compression.

Pancake load cell (tension and compression type)

• Single point shear beam load cell: spring element fixed at one end and loaded on the other.

Single ended shear beam load cell

• Double-ended shear beam: spring element fixed at both ends and loaded in the center.

Double-ended beam load cell

• Canister load cell: Cylindrical shaped spring element; can be used in both tension and compression.
  

  Canister load cell
• S-type load cell: S-shaped spring element; can be used in both compression and tension.
  

  S-type load cell
  
  
• S-type miniature load cell: similar to the S-shaped load cell, but in smaller geometry (height of only 0.75 inches): [3]

Miniature S-beam Load Cell

• Wire rope clamps: an assembly attached to a wire rope and measures its tension; commonly used in crane and hoist applications.

Wire rope clamp- on load cell

• Tension link load cell: often used in crane and hoist weighing systems. Measures tension force only.

Tension link load cell

• Load pin: replaces pulleys and shivs, typically on cranes.

Load pin load cell

[4]

The Load cell is designed to automatically regulate the balancing pressure. Air pressure is applied to one end of the diaphragm and it escapes through the nozzle placed at the bottom of the load cell. A pressure gauge is attached with the load cell to measure the pressure inside the cell. The deflection of the diaphragm affects the airflow through the nozzle as well as the pressure inside the chamber.

The hydraulic load cell uses a conventional piston and cylinder arrangement with the piston placed in a thin elastic diaphragm. The piston doesn't actually come in contact with the load cell. Mechanical stops are placed to prevent over strain of the diaphragm when the loads exceed certain limit. The load cell is completely filled with oil. When the load is applied on the piston, the movement of the piston and the diaphragm results in an increase of oil pressure. This pressure is then transmitted to a hydraulic pressure gauge via a high pressure hose.[5] The gauge's Bourdon tube senses the pressure and registers it on the dial. Because this sensor has no electrical components, it is ideal for use in hazardous areas.[6] Typical hydraulic load cell applications include tank, bin, and hopper weighing.[7] By example, a hydraulic load cell is immune to transient voltages (lightning) so these type of load cells might be a more effective device in outdoor environments. This technology is more expensive than other types of load cells. It is a more costly technology and thus cannot effectively compete on a cost of purchase basis.[8]

Vibrating load cell

Vibrating wire load cells, which are useful in geomechanical applications due to low amounts of drift, and capacitive load cells where the capacitance of a capacitor changes as the load presses the two plates of a capacitor closer together.

Every load cell is subject to "ringing" when subjected to abrupt load changes. This stems from the spring-like behavior of load cells. In order to measure the loads, they have to deform. As such, a load cell of finite stiffness must have spring-like behavior, exhibiting vibrations at its natural frequency. An oscillating data pattern can be the result of ringing. Ringing can be suppressed in a limited fashion by passive means. Alternatively, a control system can use an actuator to actively damp out the ringing of a load cell. This method offers better performance at a cost of significant increase in complexity.

Load cells are used in several types of measuring instruments such as laboratory balances, industrial scales, platform scales[12] and universal testing machines.[13] From 1993 the British Antarctic Survey installed load cells in glass fibre nests to weigh albatross chicks.[14] Load cells are used in a wide variety of items such as the seven-post shaker which is often used to set up race cars.

Load cells are commonly used to measure weight in an industrial environment. They can be installed on hoppers, reactors...etc... to control their weight capacity, which is often of critical importance for an industrial process. Some performance characteristics of the load cells must be defined and specified to make sure they will cope with the expected service. Among those design characteristics are :

• Combined error
• Minimum verification interval
• Resolution

The electrical, physical, and environmental specifications of a load cell help to determine which applications it is appropriate for. Common specifications include:

• Full Scale Output (FSO): Electronic output expressed in mv/V. Measured at full scale.
• Combined Error: % of the full scale output that represents the maximum deviation from the straight line drawn between no load and load at rated capacity. Often measured during decreasing and increasing loads.
• Non-Linearity: The maximum deviation of the calibration curve from a straight line drawn between the rated capacity and zero load. Measured on increasing load and expressed as % of full scale output.
• Hysteresis: Maximum difference between load cell output signals for the same applied load. The first measurement can be obtained by decreasing the load from rated output and the second by increasing the load from zero.
• Repeatability: Maximum difference between output measurements for repeated loads under identical conditions. Measured in % of rated output.
• Zero Balance (Offset): Output reading of the load cell with rated excitation under no load. The deviation in output between a true zero measurement and a real load cell under zero load expressed as a percentage of full scale output.
• Compensated Temperate Range: The temperature range over which a load cell is compensated so that it can ensure zero balance & rated output within specified limits. Expressed as °F or °C.
• Operating Temperature Range: Temperature range extremes in which a load cell can operate without permanent, adverse effects on any of its performance characteristics. Expressed as °F or °C.
• Temperature Effect on Output: Modification of output readings caused by load cell temperature. Expressed as % of full scale output per degree of °F or °C.
• Temperature Effect on Zero: Change in zero balance caused by ambient temperature changes. Expressed as % of full scale output per degree of °F or °C.
• Input Resistance: Input resistance of the load cell's bridge circuit. Measured at the positive & negative excitation leads with no load applied. Measured in Ohms.
• Output Resistance: Output resistance of the load cell's bridge circuit. Measured at the positive & negative excitation leads with no load applied. Measured in Ohms.
• Insulation Resistance: The resistance measured along pathways between the: bridge circuit and transducer element, bridge circuit and the cable shield, and the transducer element and the cable shield. Typically measured at fifty volts under standard test conditions.
• Recommended Excitation: Maximum recommended excitation voltage of the transducer for it to operate within its specifications. Expressed in VDC.
• Cable Length: Length of the standard cable for which the load cell is calibrated. Cable length affects how the load cell is calibrated.
• Safe Overload: The maximum load that can be applied to a load cell without causing permanent effects to its performance specifications. Measured as a % of full scale output.
• Ultimate Overload: Maximum load that can be withstood without causing structural failure.
• Material: Substance that comprises the spring element of the load cell.

[15]

Load cells are an integral part of most weighing systems in industrial, aerospace and automotive industries, enduring rigorous daily use. Over time, load cells will drift, age and misalign; therefore, they will need to be calibrated regularly to ensure accurate results are maintained.[16] ISO9000 and most other standards specify a maximum period of around 18 months to 2 years between re-calibration procedures, dependent on the level of load cell deterioration. Annual re-calibration is considered best practice by many load cell users for ensuring the most accurate measurements.

Standard calibration tests will use linearity and repeatability as a calibration guideline as these are both used to determine accuracy. Calibration is conducted incrementally starting working in ascending or descending order. For example, in the case of a 60 tonne load cell, then specific test weights that measure in 5, 10, 20, 40 and 60 tonne increments may be used – A five step calibration process is usually sufficient for ensuring a device is accurately calibrated. Repeating this five-step calibration procedure 2-3 times is recommended for consistent results.[17]

• Spring scale
• Stretch sensor