To ensure that the readings on a scale designed to weigh aircraft are accurate, the test system used to calibrate the scale needs to be even more accurate in applying test loads. The oldest method for testing scales was to use known dead weights, but handling them was expensive and cumbersome. An alternative was to use a manually operated hydraulic pump, similar in concept to the hydraulic jack that one would use to lift a car. But this method is prone to pressure leaks that make it virtually impossible to achieve a high degree of accuracy. A better method is to use an automated system that employs closed-loop control of a hydraulic press to apply a very precisely controlled load. In addition, an automated test system has the ability to measure key parameters electronically and easily group and communicate this data in order to produce documentation of test results, something that is critical when government certification of the testing is involved.
Hydraulics is the power of choice when applying and holding heavy loads, so it makes sense to use hydraulics to exert the test loads on the aircraft scales, and the hydraulics should be controlled by an electro-hydraulic motion controller that can precisely apply the test loads. In the case of scale testers manufactured by General Electrodynamics Corp. (GEC) of Arlington, TX, a manufacturer of advanced portable aircraft weighing and balancing equipment, an RMC75 motion controller manufactured by Delta Computer Systems Inc. of Battle Ground, WA, is used. The RMC75 (shown in Figure 1) has special capabilities for hydraulic motion control, including the ability to perform closed-loop control of the pressure or force that a cylinder produces as well as the position of the rod.