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.
Jean-Christophe Emery, Mettlet Toledo GmbH
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.
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.
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.
The stability and reproducibility of other components such as the magnet are also crucial to overall system performance. The thermal
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
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).
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
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
Jean-Christophe Emery is R&D Manager, Load Cell Business Area, Mettler Toledo GmbH, 8606 Greifensee, Switzerland; 41(0)1 944 23 21, fax 41(0)1 944 25 80, firstname.lastname@example.org.