Sensors - June 2002 - Simplifying the Electronic Balance Load Cell

A new way to manufacture the load cell that forms the core of an electromagnetic force restoration balance reduces production costs, shortens time to market, and achieves a high level of product reliability.

The most accurate electronic balances are based on electromagnetic force restoration (EMFR), also called electromagnetic force compensation. These weigh scale sensors comprise a parallel guidance mechanism that ensures the accurate introduction of the object to be weighed, one or more levers, and an electromagnetic system that assumes the role of the weights in a two-pan scale balance. Equilibrium is maintained by a control system incorporating an optical position sensor.

EMFR Overview
In an EMFR balance (see Figure 1), the triangular knife edges of a pan balance are replaced by flexible bearings.

Figure 1. Electromagnetic force restoration (EMFR) balances can be thought of as sophisticated descendents of the classical pan balance. Instead of the pan balance's knife-edges, flexible bearings compensate the gravitational force exerted by the mass to be weighed. An optical sensor detects the zero position that indicates a state of equilibrium.

When the coil’s force compensates the gravitational force exerted by an unknown mass, an optical sensor detects a stable predefined zero position that indicates a state of equilibrium. Changing the ratio of the levers allows forces smaller than 1 N to balance much bigger ones. Today it is common to have a system with one, two, or even three levers, depending on the load range.

The electric current that measures the applied mass is digitized, temperature compensated, filtered, and finally displayed. For greater convenience of use, modern EMFR balances integrate a Roberval mechanism (see Figure 2) like that in industrial scales.

Figure 2. Modern EMFR balances incorporate a Roberval mechanism such as built into industrial scales. The balance must hold its parallelogram shape to maintain a state of equilibrium regardless of where the load is placed on the platform.

The balance must retain a perfect parallelogram shape so that the system remains in equilibrium no matter where the load is placed on the platform.

These sensors typically offer a displayed resolution of between 300,000 and 3 million digits (1 million digits =1 ppm), depending strongly on the application. In the analytical market segment, resolutions of up to of 50 million digits are required as well as sub-ppm precision (e.g., 10 µg increments over a 200 g range). Some mass comparators, called window comparators, even have resolutions of 1 billion digits over a limited range. The certified temperature range of a typical EMFR varies from 10°C to 30°C or –10°C to 40°C, depending on the precision rating of the particular model; the operating RH range is 5%–85%. Each load cell or balance is individually identified and tested during a two-day cycle in a temperature chamber under different loads, and the data are stored for run-time compensation.

Weigh scale sensors based on the EMFR principle are typically 10–1000 times more accurate than strain-gauge-based industrial load cells and other devices based on the measurement of deformation. Signal strength also compares favorably with strain gauge output. An ideal EMFR load cell has no spring constant, shows no deformation under load, and self-compensates for the thermal expansion of the levers. In reality, some elastic deformation due to the load must reproduce within a few nanometers in order to guarantee the required precision, and the coil must typically be positioned to a tolerance within 10 nm (~³/₁₀₀₀ mil) to eliminate the effects of the unwanted but residual spring constant of the lever system.

Figure 3. The load cell forming part of an EMFR weigh scale consists of more than 100 parts that must be fabricated by various methods and then assembled.

The stability and reproducibility of other components such as the magnet are also crucial to overall system performance. The thermal

Photo 1. This two-lever, 30 kg load cell is an example of conventional construction.

coefficient of the induced magnetic field typically lies between –50 pm/°C and –350 ppm/°C. To precisely compensate the most accurate sensors, temperature effects of <1 mK must be taken into consideration. Using

conversion, an A/D converter can achieve up to 30 bits resolution and must be temperature compensated in software, as is true of the mechanical parts of the system.

The complexity of EMFR load cells (see Figure 3) has historically required the fabrication and assembly of more than 100 parts (see Photo 1) by methods including die casting, stamping, machining, and rolling. The finished unit generally represents an appreciable

Photo 2. Mono Bloc technology integrates all the cell's mechanical functions into a single block of aluminum. It is quite streamlined compared to the conventional cell in Photo 1.

portion of the total cost of a precision laboratory balance or industrial scale. Moreover, the sheer number of components can adversely affect the robustness of the cell.

The MonoBloc
As the use of EMFR weighing equipment began to expand, so did the desirability of reducing the costs and increasing the efficiency of load cell manufacture. Production variations arising from material mismatching or microslippage needed to be eliminated, and one

Photos 3 and 4. The parts reduction achieved by Mono Bloc technology can be clearly seen.

obvious—but not easy—way to do this would be to greatly reduce the parts count.

One solution is MonoBloc technology, achieved by means of wire electrical discharge machining (WEDM). The MonoBloc integrates the cell’s various mechanical functions such as the hanger, coupler, guiding element, flexible bearing, and lever system into a single monolithic structure in the form of a compact block made of a high-performance aluminum alloy (see Photo 2).

The most immediately apparent benefit of MonoBloc technology is parts reduction, as can be observed by comparing Photos 3 and 4. Of course, as is true of a classical load cell, the coil, magnetic system, and optical sensor must be fixed on the lever system (see Photo 5).

Photo 5. In addition to the load cell, an EMFR weigh scale incorporates a coil, magnetic system, and optical sensor.

Wire Electrical Discharge Machining
WEDM is a sophisticated manufacturing technique of cutting complex structures into a metal structure as if with a fret saw. The process is as follows. Two metal parts submerged in an insulating liquid are connected to a current source that can be switched on and off. Switching the current on creates an electric field between the two parts; when they are brought to within a fraction of an inch a spark jumps from one to the other. By repeating this process one spark at a time (never a burst of sparks), the metal workpiece gradually

Photo 6. Wire electrical discharge machining is used to carve a aluminum blocks into what will become load cells. This computer-guided method is so accurate and precise that it can create flexible bearings smaller in diameter than a human hair (vertical dark line). The bearing's position in the load cell is indicated in the photo on the right.

assumes the desired configuration in accordance with the shape of the electrode. Several hundred thousand sparks must fly per second before metal erosion takes place.

For shapes that are relatively simple, a template is stored in a computer that cuts them automatically by guiding the wire along each assigned path (see Photo 6). On high-end machines working with angled, conical, or other complex surfaces, the upper and the lower wire guide system each carries out its own movements. Although the technique is well established for working steel and other hard metals in

Photo 7. Computer-guided wire electrical discharge machining is precise enough to create flexible bearings smaller in diameter than a human hair (vertical dark line). The bearing's position in the load cell is indicated in the photo on the right.

tool manufacture, using it to machine aluminum is rather revolutionary. The precision of WEDM can be seen in Photo 7. The detail on the left is a flexible bearing smaller in diameter than a human hair (the vertical dark line); its position in the load cell is shown in the photo on the right.