Load cells directly measure force or weight. These transducers convert mechanical force into electrical signals by measuring deformations produced by the force or weight.
A common application of these devices is measuring dry or liquid materials in a hopper. A measure of the weight through a load cell yields a measure of the quantity of the material in the hopper.
Technology behind working of load cell
Most commonly used load cell is the strain gauge load cell. When force is applied to one side of the piston or diaphragm, the amount of pressure (pneumatic or hydraulic) applied to the other side to balance that force is measured.
The mechanical component of a load cell or strain gauge transducer is the structure (spring element). The structure reacts to the applied load and focuses that load into an isolated, uniform strain field where strain gauges can be placed for load measurement. The three common load cell structure designs—multiple-bending beam, multiple column and shear web—form the basic building blocks for all possible load cell profiles and configurations.
- Multiple-bending beam load cells are low capacity (20 to 22 KN) and feature a wheel-shaped spring element that is adaptable to low-profile transducers. It contains four active gauges or sets of gauges per bridge arm, with pairs subjected to equal and opposite strains (tension and compression).
- Multiple-column load cells consist of multiple columns for higher capacity (110 K to 9 MN). In this arrangement, each bridge arm contains four active strain gauges, with two aligned along the principal axis of strain and the other two in the traverse direction to compensate for Poisson’s effect.
- Shear-web load cells have a medium capacity (2 K to 1 MN) and use a wheel form with radial webs subject to direct shear. The four active strain gauges per bridge arm are bonded to the sides of the web, 45 degree to the axis of the beam.
How to choose the right load cell
Load cells operate in two basic modes, first the compression mode, during which the weighing vessel sits on one or more load cells and the tension mode, during which the weighing vessel hangs from one or more load cells. You can design the different load cell structure by using any of these configurations for compression-only forces or you can design them to measure both a tension and compression force.
Beyond the principal measurement, you select a load cell primarily based on capacity, accuracy, and physical mounting constraints or environmental protection. You cannot determine expected performance by any one factor. You must pinpoint it through a combination of different sensor parameters and the way you designed the load cell into your system. Refer to the table to compare the range, accuracy, sensitivity, and price of different load cell types.
|Load Cell Sensors||Price||Weight Range||Accuracy||Sensitivity||Comparison|
|Beam style||Low||10 – 5k lb||High||Medium||
|S Beam||Low||10 – 5k lb||High||Medium||
|Canister||Medium||Up to 500k lb||Medium||High||
|Pancake/Low Profile||Low||5 – 500k lb||Medium||Medium||
|Button and Washer||Low||Either 0 – 50k lb or
0 – 200k lb
What should be the supporting capabilities
Define your minimum and maximum capacity requirements. Be sure to select the capacity over the maximum operating load and determine all extraneous load and moments before selecting a load cell. The load capacity must be capable of supporting the following:
- Weight of the weighing structure (dead load)
- Maximum live load that can be applied (including any static overload)
- Additional overload arising from external factors such as wind loading or seismic activity
Load cells are designed for general-purpose use or are fatigue-rated to withstand millions of load cycles with no effect on performance. General-purpose load cells are designed for static or low-cycling frequency load applications. They typically survive up to 1 million cycles depending on the load level and transducer material. Fatigue-rated load cells are typically designed to achieve 50 million to 100 million fully reversed load cycles, depending on the load level and amplitude.
Physical and environmental constraints
One of the key characteristics to consider is how you are integrating the load cell into your system. Identify any physical restrictions that limit size (width, height, length, and so on) or the way the load cell can be mounted. Most tension and compression load cells feature center female threads on top and bottom for fixturing, but they also may have male threads or a mixture of both. Consider how the system will operate and what the worst-case operating conditions may be—the widest temperature range, the smallest weight change required to be measured, the worst environmental conditions (flood, tempest, seismic activity), and the maximum overload conditions.
Measurement of torque by different sensors
Torque is the tendency of a force to rotate an object about an axis. Torque sensors are composed of strain gauges that are affixed to a torsion bar. As the bar turns, the gauges respond to the bar’s shear stress, which is proportional to the torque.
Reaction torque sensors
Reaction torque is the turning force, or moment, imposed on the stationary portion of a device by the rotating portion as power is delivered or absorbed. If the load source is held rigid while the drive source is trying to rotate, the torque is sensed. Reaction torque sensors are restrained so they cannot rotate 360 degree without the cable wrapping up because the housing or cover is fixed to the sensor element. These sensors are commonly used to measure torque of a back-and-forth agitating type motion. Because these sensors do not use bearings, slip rings, or any other rotating elements, their installation and use can be very cost-effective.
Rotary torque sensors
Rotating torque sensors are similar in design and application to reaction torque sensors except that the torque sensor is installed in line with the device under test. Since the shaft of a torque sensor is rotating 360 degree, these sensors must have a way to transfer the signals from the rotational element to a stationary surface. You can accomplish this by using slip rings or rotary transformer method.
Slip ring method
In this method, the strain gauge bridge is connected to four silver slip rings mounted on the rotating shaft. Silver graphite brushes rub on these slip rings and provide an electrical path for the incoming bridge excitation and the outgoing signal. You can use either AC or DC to excite the strain gauge bridge.
Rotary transformer method
For the transformer method, the rotating transformers differ from conventional transformers only in that either the primary or secondary winding is rotating. One transformer is used to transmit the AC excitation voltage to the strain gauge bridge and a second transformer is used to transfer the signal output to the nonrotating part of the transducer. Thus, two transformers replace four slip rings, and no direct contact is required between the rotating and stationary elements of the transducer.
Signal conditioning for load and torque sensors
Load and torque sensors can be either conditioned or unconditioned. You can connect conditioned sensors directly to a DAQ device because they contain the required components for filtering, signal amplification, and excitation leads along with the regular circuitry for measurement. If you are working with unconditioned sensors, you must consider several signal conditioning elements to create an effective bridge-based load and torque measurement system. You many need one or more of the following:
- Excitation to power the Wheatstone bridge circuitry
- Remote sensing to compensate for errors in excitation voltage from long lead wires
- Amplification to increase measurement resolution and improve signal-to-noise ratio
- Filtering to remove external, high-frequency noise
- Offset nulling to balance the bridge to output 0 V when no strain is applied
- Shunt calibration to verify the output of the bridge to a known, expected value