In modern scales the equal arm principal has largely disappeared because the electromagnetic force motor can lift the unknown weight either directly or with mechanical leverage.  The electrical current required to create the force necessary to hold the weigh pan with the unknown weight on it in a fixed (zero) position is proportional to the unknown weight (when the weight of the pan and structure has been zeroed out by tare).  The current thru the force motor coil is converted to a digital form by an analog to digital chip.  The computer can then handle all of the mathematical manipulations required and do corrections for temperature changes and the basic non-linearity associated with the magnetic circuits.  The computer also provides all of the capability for various applications, like percent weighing, parts counting, check weighing, totalizing, compounding, statistics, etc.


The electromagnet force motor starts to become a limitation when the max load approaches 10. kilograms.  The restorative forces require large magnetic structures and generate a lot of heat, which create drift and linearity issues.  Above this range most people shift to strain gauges which can be made to very large capacities, but with reduced accuracy.  Fortunately most weighing in these higher ranges is industrial in nature and does not require the high accuracy demanded in the lab ans analytical areas.


On the other end of the spectrum, when very light loads are to be analyzed in the semi-microgram (10 microgram resolution) and the microgram (1 microgram resolution) things get tough for the flexure based Roberval structure.  When the flexures are necked down to the thicknesses required to produce motion with very small incremental loads the structures become very delicate and are easily damaged.

It took a lot of technology to get the force motor to where it is today, and while not the cure-all, it certainly suits the laboratory and analytical markets quite well.  There does not appear to be a technology offered that would replace the force motor on the near horizon.  People are working to solve some of the strain gauge problems, and thereby, replace the force motor.  People are working on tuning fork based scales as a possible replacement.