An electronic scale uses a Load Cell (or cells) to sense the weight applied to the load receiving element. A load cell is simply a transducer that converts a force into a signal. The signal from the cell is then sent to an indicating element, often an electronic instrument, which converts the signal into a useable format. Load cells are used for measuring strain in several applications, this page will deal with their use in weighing devices only.
While there are several types of load cells available, by far the most common type is based on strain guages. Typically, four of these strain guages are arranged in a balanced bridge (usually Wheatstone Bridge), bonded to a Flexure Element. Less common are cells which utilize only one or two strain guages. The signal obtained from the cell is relatively small, usually in the order of only a few millivolts. This very small signal will require amplification and further processing before it can be used.
Load cells may be manufactured in a variety of configurations depending upon their intended use. Typical configurations include compression, tension, shear or bending cells. In simple terms, the Digital Indicating Element sends a low voltage Excitation to the cell(s) via the load cell cable. The excitation signal is applied to the strain guage across two of the arms of the Whetstone bridge. When the bridge is balanced (i.e. no load on the load receiving element), there will be no signal. As a load is applied, the bridge will become unbalanced and a signal will be generated.Two more wires pick up this Signal and send it back to the instrument for processing. This is known as a 4-wire system. In a six-wire system, two additional wires are used to sense the actual excitation voltage reaching the cells. This information can then be used to bias the instrument and results in a more stable system eliminating the errors caused by reference voltage changes.
The signal which returns to the indicating element is proportional to the load on the load receiving element. This signal is processed in the instrument through a filter and amplifier, then through the Analog to Digital (AD) convertor. The AD converter is usually of the Dual Slope Integration variety. Dual slope integration is used to overcome the effects of component variations. As the same components are used for the integration on both slopes, any inherent errors are self cancelling. Many integrated circuit manufacturers produce components specially designed for weighing applications.
Recently there has been a move to Digital Load Cells, which are in fact not digital at all. In these systems, the Analog to Digital Conversion is done either completely inside the Load Cell, or in a junction box located close to the load cell. The resulting Digital Signal is then sent back to the instrument. The advantages are many, but foremost is the fact that the digital signal is not subject to EMI/RFI as it travels along the load cell cable.
In addition to picking the correct load cell type for an applications, the manufacturer will have to consider several other load cell parameters. Load cells will be rated in terms of the maximum capacity they are designed for, the minimum dead load that must be present, the number of divisions that the full scale signal may be divided into, the signal to excitation ratio in terms of millivolts per volt and the Class of weighing they are designed to be used for. In addition, there may be parameters that address the environment the cell is designed to be used in.
The following formulas will allow you to choose an analog load cell for a given job:
|Load Cell Capacity||((Live Load) + (Dead Load)) / Multiple [R]|
|Full Scale Output [VoFS]||((Load Cell Sensitivity) * (Excitation Voltage))|
|Actual Load on Cell||((Live Load Capacity) / Scale Multiple)) [R]|
|Load Cell Output [Eo]||((Actual Load on Cell) / (Load Cell Capacity)) * Full Scale Output [VoFS]|
|Meter Sensitivity [Sw]||((Load Cell Output [Eo]) / (Display Counts available))|
Therefore, as long as the weighmeter sensitivity [Sw] is greater than microvolt output [uV] required the output is sufficient to drive the chosen instrument. To increase the scale resolution use a higher mV/V load cell, do not increase the load cell excitation voltage [V(Excitation)]. High Full Scale output [VoFS] = Greater Resolution.
Other Load Cells
While strain guage load cells are by far the most common, there are several other load cell types as well. These include hydraulic, piezo electric, linear variable dsplacement transducers (lvdt), vibrating wire and even radiation emittor/detectors. Most of these are relatively rare, however hydraulic load cells are increasingly used in applications where the somewhat delicate nature of strain guages may not be desireable. High frequency vibrating wire technology is used in some high precision speciality & scientific applications.
Hydraulic load cells are the second most common in weighing applications. They may drive an indicating element directly or they may be connected to an electronic pressure guage. In the latter case, the rest of the scale functions very similarily to a strain guage load cell electronic scale. In the former case, there are no electronic compnents utilized. Fully hydraulic scales are often medium capacity scales. Hydraulic hanging dial scales are fairly common.
High Frequency Vibrating Wire transducers exploit the dependency of a vibrating wire's resonant frequency to the tension in the wire. A wire is driven at a resonant frequency, usually through the use of electro-magnets. Through mechanical means, a load is applied to the wire causing its vibration frequency to change. This change is proportional to the load being applied. The frequency change is detected by the indicating element and is converted into a useable signal. This type of transducer is effected by temperature. Temperature must either be compensated for, or preferably controlled. This is why the technology is often found utilized in laboratory conditions where temperature is relatively stable. The technology is relatively unaffected by other commonly encountered external variables.Return to Top
The most universal measuring device for the electrical measurement of mechanical quantities is the strain gauge. Several types of strain gauges depend on the proportional variance of electrical resistance to strain: the piezoresistive or semi-conductor gauge, the carbon-resistive gauge, the bonded metallic wire, and foil resistance gauges.
The bonded resistance strain gauge is by far the most widely used in commercial weighing applications. These gauges consist of a grid of very fine wire or foil bonded to a backing or carrier matrix. The electrical resistance of the grid varies linearly with strain. In use, the carrier matrix is bonded to the flexure element. The flexure element is the main component of the load cell. Force is applied to the flexure element resulting in minute deformation of the element. Strain is found by measuring the change in resistance of the strain guages. These strain guages are arranged in a bridge formation. The advantages of the bonded resistance strain gauge include low cost, short gauge length, only moderately affected by temperature changes, small physical size and low mass, and relatively high sensitivity to strain.
In a strain gauge application, the carrier matrix and the adhesive must work together to transmit the strains from the flexure element to the guage. In addition, they serve as an electrical insulator and heat dissipator. Failure of either the carrier matrix or the adhesive will result in failure of the cell.
Because of its outstanding sensitivity, the Wheatstone bridge circuit is the most frequently used circuit for static strain measurements. Ideally, the strain gauge is the only resistor in the circuit that varies and then only due to a change in strain on the surface. Unfortunately, the reality is that the load cell cable, junction boxes and other components in the circuit also vary in resistance as conditions change. Temperature changes are the foremost contributor to resistance changes.
Temperature dependent changes of the specific strain gage grid resistance occur in the applied gage owing to the linear thermal expansion coefficients of the grid and specimen materials. These resistance changes appear to the indicating element as mechanical strain in the flexure element. The representation of the apparent strain as a function of temperature is known as the temperature characteristic of the strain gage. In order to keep apparent strain through temperature changes as small as possible, each strain gage is matched during the production to a certain linear thermal expansion coefficient.
Service Temperature Range
The service temperature range is the range of ambient temperature where the use of the strain gages is permitted without permanent changes of the measurement properties. Service temperature ranges are different whether static or dynamic values are to be sensed.
Maximum Permitted RMS Bridge Energizing Voltage
The maximum values quoted are only permitted for appropriate application on materials with good heat conduction (e.g., steel of sufficient thickness) if room temperature is not exceeded. In other cases temperature rise in the measuring grid area may lead to measurement errors.
Indicating elements for an electronic scale generally fall into one of two types. The first is an indicator which contains the A/D convertor and is used for analog load cells, the second is basically a "dumb" indicator and is used only to display the output from a digital load cell(s).
Indicators may be simple display devices or may be complex programmable devices capable of running batching systems, truck weigh in/out systems etc.
Calibration of scales is done using mass standards. In small capacity scales, mass standards are used to the capacity of the scale. In larger capacity scales techniques such as Substitution or Strain Load calibrations are usually employed to reduce the required number of standards. The standards used are usually made of stainless steel, mild steel, brass or cast iron. (See Calibrations Page for more information).
Calibration of older electronic scales was facilitated by placing resistors in series with the signal lines from the load cells (some manufacturers placed the series resistor in line with the excitation voltage). Later scales used potentiometers to make adjustments easier. Several scale manufacturers still use junction boxes to sum multiple cells. These "J"-boxes generally contain a small printed circuit board equipped with potentiometers to balance the cells (or groups of cells - called sections). The latest generation of scale does away with resistors and potentiometers and accomplishes adjustment through software.
Calibrating through software involves placing a known load (at least 10%) of the scales capacity on the load receiving element and then keying in the known weight. The instrument then relates the signal being received to the known weight on the platter. Any subsequent weight value will be related to this known quantity through linear interpolation and/or extrapolation.
Several instruments on the market today are also capable of multiple set point calibration. These allow a calibration that is linear between calibration points, but the slope of the lines between individual calibration points may change. Obviously, these indicators, while more versatile, have the potential of calibrating in errors to the weight reading. They must be calibrated with extreme care.
Load cell balancing may also be accomplished using mechanical techniques. Adding shims or shortening load arms to force a cell to take more load, or removing shims or lengthening load arms to take less load will have the same effect on the output signal as adding or removing resistors has.