Load Cells

Load cells are transducers capable of measuring tensional or compressive loads or forces (and in some cases shear or transverse loads) in terms of some other quantity (usually an electrical signal). The main application of load cells is in weight measurement by using pressure transducers or, for measurement with slung weight systems, tension transducers.

For weight measurement the three basic types of transducers are:

- Hydraulic—converts force into pressure, which can be indicated directly by a pressure gauge calibrated in units of force or weight. Such a system is relatively inexpensive, robust, flexible, and capable of accommodating high forces. It’s temperature- and pressure-dependent because both parameters can affect the compressibility of the fluid.

- Pneumatic—converts force to air pressure either in a closed system or an open (continuous-flow) system. Pressure generated is again a measure of the applied force, but in this case the fluid (air) is highly compressible. Accuracy and consistency of a pneumatic load cell system thus tends to be inferior to a hydraulic system, although it may be eminently suitable as a control system monitoring a pneumatic controller. Generally, the working principle adopted is similar to that of a pneumatic relay.

- Electrical—normally either based on piezoelectric transducers or on resistance strain gauges. In such systems the force is converted to a proportional electrical signal in a bridge circuit that can be fed, via an amplifier if necessary, to an indicating meter calibrated directly in terms of weight, a recorder, digital readout, or a similar device.

ELECTRONIC WEIGHING

Electronic weighing is rapidly becoming an alternative to mechanical weighing, especially in industrial systems. In small-scale applications involving low weight capacities (such as domestic weighing machines), it’s inevitably more costly, but it can have specific advantages. e.g., because weight is increased in terms of an electrical signal (conversion to power), a digital display is relatively simple. Also, because of the high sensitivity of piezoresistive strain gauges, load cells can be designed to measure quite small weights (under 1 lb) with excellent accuracy.

On the industrial side the cost of mechanical weighing increases substantially with increasing capacity; that of an electrical load cell, only marginally so.

There will be a crossover point at which electronic weighing becomes less costly than mechanical weighing; this generally occurs between 500 to 1000 lb, depending upon the particular application ( ill. 8-1).

ill. 8-1. Electronic weighing is more cost effective than mechanical weighing when the mass is very large.

The main advantages of electronic load cell weighing are

- The output is in the form of an electrical signal; this offers far greater flexibility than mechanical signals or pointer movements.

- Electrical load cells have no moving parts and can be hermetically sealed. Such cells are compact, robust, and normally need little or no maintenance.

- It’s adaptable to situations or environments where mechanical weighing devices are impractical or impossible to use.

- The size of the load cell is largely independent of its load capacity. Larger and “stronger” elements are not needed to accommodate very large weights as in mechanical systems.

- Response is much faster than with mechanical systems; this enables it to be used under dynamic conditions, such as fluctuating loads, if necessary.

- Accuracy is at least comparable with mechanical weighing and may be better in particular circumstances. It does not follow that electronic weighing automatically gives greater accuracy, how ever. Under particular circumstances requiring high accuracy of measurement, mechanical weighers may achieve the accuracy required at lower cost. However, on a cost basis, electronic weighing is generally more favorable as the weighing capacity required in creases.

ill. 8-2. Arrangement of strain gauges in an electrical load cell. The horizontally oriented gauges are designated R1 and R2; the vertical gauges, R3 and R4.

ill. 8-3. Bridge circuit for interconnection of strain gauges shown in ill. 8-2.

HIGH-CAPACITY LOAD CELLS

The most common physical form of electrical load cell for high- capacity weighing is a short cylindrical column or tube of steel with resistance strain gauges bonded to it. Normally, four gauges are used in the configuration shown in ill. 8-2. These are connected to a conventional bridge circuit ( ill. 8-3). The bridge is balanced for no-load conditions, where the bridge output is zero. Under load the resultant minute deformation of the gauges results in changes of electrical resistance and corresponding unbalance of the bridge, which produces a small output signal voltage. The value of this volt age is usually of the order of 5-30 mV at full capacity, but it can be appreciably higher. The bridge circuit itself needs only low volt age dc excitation; it may be as low as 4-5 V, and it seldom exceeds a maximum of 20 V but, again, this depends on the design of transducer.

A particular advantage of using a bridge circuit is that this renders the system inherently insensitive to temperature. Any change in resistance through temperature will, theoretically, equally affect all four resistances. In practice, however, specific temperature-compensating elements may be included in the bridge circuit if maximum insensitivity to temperature is required.

Other possible sources of error are nonlinearity, hysteresis and no repeatability, and lack of stability of the input supply. By suitable material selection and design, one can reduce the total of such errors to 1 percent or better of the full capacity in high-capacity cells; for lower-capacity cells it can be reduced even more.

Table 8-1 shows typical performance characteristics quoted for a range of (strain-gauge) load cells designed for accurate measurements in the 1 - 1000-ton range (the definitions are those conforming to SAMA and ASM standards).

Table 8-1. Typical Performance Characteristics of Load Cells.

Nonlinearity: 0.50 percent

Hysteresis (at 50 percent of range): 0.2 percent

Repeatability: 0.1 percent

Rated Output (nominal): 1.6 mV/V

No-Load Output: ± 2 percent

Temperature effect:

On no-load output: 0.05 percent °C.

On rated output (typical): 0.05 percent °C.

Proprietary load cells will also normally become maximum load ratings quoted with a safety factor. They can be expected to take an overload of up to one and one-half times maximum rating with out suffering a permanent zero shift and up to three times the maximum rating specified without electrical failure. So a load cell rated for 1000 tons, for instance, could safely take a load of up to 1500 tons without upsetting the calibration of the weighing system.

Other physical forms of load cell designed to be loaded in compression include hollow rings or toroids and other special sections. Cells designed to be loaded in tension almost invariably take the form of simple solid or hollow cylindrical elements. Deflection at maximum load is not likely to exceed 0.01 in or 0.25 mm (similar to the deflection expected in ordinary structural components under similar loads). Tension cells are inherently capable of higher ac curacy than compression cells and are coming into more widespread use for force, and even weight, measurement.

LOAD CELL DISPOSITION

The simplest form of readout is to take the bridge circuit dc output (usually on the order of millivolts), amplify this, and then feed it directly to a moving-coil voltmeter. The voltmeter scale is then calibrated to indicate weight directly.

This is accurate enough for most general-purpose applications, but generally far less accurate than the performance of the load cell itself. To provide better accuracy, scientists developed a servo potentiometric measurement system, using a self-balancing potentiometric indicator or recorder. In turn, this has now been replaced by its solid-state equivalent.

A typical schematic circuit is shown in ill. 8-4. The principle of a servo-potentiometric indicator is that a voltage from a potentiometer is continuously compared with the input (that is, the volt age from the load cells). The potentiometer slider is driven by a small servomotor and fed via an amplifier from the “difference volt age” until the potentiometer voltage equals the input. At this point, called the balance, the position of the slider is an accurate measure of the unknown input voltage and is indicated by the pointer, which is coupled to it.

ill. 8-4. A servo-potentiometric circuit for use with a load cell.

Voltage variations in mains are rendered harmless by the simple expedient of supplying the potentiometer from the same source as the load cells, so that the effects of fluctuations on the two cancel out.

The potentiometric indicator can be given dials of almost any size and can be made accurate enough to satisfy most requirements. When it’s used with load cells, the power supply for these, the sum mating networks where several cells are used and the necessary calibrating circuits are always built into it.

The potentiometric indicator can be given multiple scales and measuring ranges, and microswitches, actuated by cams attached to the pointer spindle to make or break at any point of the scale, can be fitted into it, for initiation of a variety of alarm control or supervisory functions.

As an alternative to the circular-dial potentiometric indicator, a pen recorder, which marks a chart that may be either circular or in the form of a strip, may be used.

Where an indicator is used, often because it’s usually more readable and accurate, a record may be obtained by another means. An encoding disc is coupled to the pointer and translates its angular position into a corresponding digital signal. This is then further processed in special electronic decoding circuits until it’s in a form where it can actuate electrical printing (or adding) machines and also numerical readouts for digital conversion. The numerical readout has advantages over an analog pointer, because it’s not subject to an operator’s reading or interpretation error, and it can also be read at a greater distance.

Although potentiometric indicators and recorders are very satisfactory in many applications and have been developed to a state of high reliability and variety, they have one weakness. They are electromechanical devices and rely on moving components, such as potentiometers, drives, or motors, which may be affected by severe environmental conditions encountered in steelworks and other heavy industrial environments. Strong atmospheric pollution—vapor, dust, noxious fumes—and vibration, particularly, are liable to interfere with their proper functioning or lead to the need for frequent maintenance.

SOLID-STATE INDICATORS

These limitations can be overcome by the use of a solid-state indicator, which is fundamentally an exact electronic equivalent of the potentiometric indicator but employs solid-state circuits to take the place of the various mechanical or electromechanical elements, and it operates digitally throughout. Having thus no moving components, it’s almost totally unaffected by vibration and other environmental hazards while still retaining the voltage-fluctuation insensitivity of the potentiometric principle. The advantages of digital output (which include its own built-in numerical weight display) are available without the need for costly conversion equipment.

HYDRAULIC LOAD CELLS

Although they are only of industrial interest a brief description will also be given of a modern electro-hydraulic load-measuring system.

The load cell in this case is in the form of a shallow cylinder having its piston or platen bonded to the cylinder wall by a flexible rubber joint. Under the platen the space, which is circular in a compression capsule and annular in a tension capsule, is filled with a fluid such as water, which is incompressible for practical purposes if all the air is extracted.

The load cell is sensitive down to zero loadings. Under load the rubber joint is stressed in shear; it not only acts as a frictionless seal but also helps to constrain the platen to axial motion.

Also because of its construction, the perfect electrohydraulic load cell would be completely free from creep, because no metal parts are stressed to produce the signal. In practice, some small effect, typically 0.005 percent / 5 mins., is measurable.

NONAXIAL LOADS

The electrohydraulic load cell is essentially free from errors introduced by nonaxial loading. However, the main advantage (com pared to strain-gauge load cells) occurs when these nonaxial loads are transmitted. The electrohydraulic capsule, however, does not generate any signal but that due to true axial force.

A pressure sensor is sealed onto the body of the load cell. The sensor converts fluid pressure into an electrical output.

Various options on the performance of the sensor are available, together with a comprehensive range of recording methods, from simple and precise analog indication in pounds or kilograms to systems employing four-channel switching, summation, high/low alarm trips, peak hold, 0-10 mA, and BCD output.

Splash-proof enclosures housing the transmitter are available with the above facilities. Other systems, such as 16-channel signal conditioning units with auto- or manual scan employing printed out put, are offered.

Differential and portable battery digital indicators are also available for use with modern hydraulic load cells.

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