Accurate Aircraft Testing
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.
These functions are important in applications that apply controlled amounts of force because the typical scenario is for the controller to use position control to extend the cylinder rod to place it into position for applying pressure, and then switch into pressure/force control mode to apply the desired force smoothly and precisely. With the Delta controller this is accomplished simply by executing sequences of commands that deal with position/velocity values or pressure/force values directly. Other motion controllers require complex programming as they attempt to make a smooth transition between modes.
The key system components were provided to GEC by the Wilson Company of Addison, TX. As illustrated in Figure 3, cylinder position information is provided to the motion controller by a Temposonics linear magnetostrictive displacement transducer (MDT) that is mounted inside the cylinder. For high-precision and reliability, the sensor connects directly to the RMC75 via SSI (the industrial synchronous serial interface standard). Force information comes from an Interface Inc. load cell that is mounted between the cylinder rod and the scale to be tested. To improve the accuracy and provide immunity against signal noise, the feedback signal coming from the load cell goes through an amplifier to raise the signal level to the 0-10 volt range and a filter to eliminate noise in the signal before it is provided to the motion controller. The controller operates the hydraulics via commands to a high-speed servo-quality proportional valve by Parker, connected to the large Parker cylinder (8-inch bore and 8-inch stroke) that has a very high oil column resonance for quick response to closed-loop controls.
The typical testing operation consists of a sequence of steps involving different control modes. At the start of the sequence, the motion controller positions the cylinder rod close to the scale being tested using open-loop control, and then switches to closed-loop position-control mode, monitoring position feedback at 1,000 times per second as it moves the cylinder into contact with the scale to start applying force. Since different scales have different dimensions, the RMC75 needs to actively sense when the cylinder rod has come in contact with the scale in order to know when to switch to pressure/force control. The controller does this by watching closely for the increase in the load cell’s force reading that occurs when the rod and scale first come into contact. Then the controller switches smoothly to closed-loop force control mode to ensure that the proper force is applied and maintained during the scale test.
Because force increases rapidly in this application during the transition from position to force control, the control system must be able to respond quickly as the applied force on the load cell approaches its force setpoint. At this time, the control system employs “braking” techniques to reduce the rate at which the force changes, thus avoiding overshooting the setpoint. To ensure accurate readings and detection of rates of change, the RMC includes features to filter out noise such as eight times oversampling of input levels. Additionally, predictive gain values (feed forwards) can cause control outputs to change rapidly, allowing the system to respond more accurately under dynamic conditions.
To verify that the accuracy specs are attained, the GEC system uses Kiethley multimeters which are capable of reading data from the load cell in the nanovolt range. “With the Delta controller and its high-speed control, we have been able to exceed our minimum testing accuracies,” says Amith Kalaghatagi, GEC’s engineering manager. “In fact, we have been able to hold up to 100,000 pounds pressure on the scale being tested to within one pound.”
Delta Computer Systems’ RMCTools programming software was used to program the motion controller. “The most useful tool was the Tuning Wizard,” says Kalaghatagi. “Without it, finding the optimal control loop parameters would be very tedious. In one case we wanted to achieve the target force within 20 seconds which would have been very difficult to accomplish at all without the Tuning Wizard. Using the tool, we were able to tune the system to reach the desired force within about half that amount of time.” To control the test operations, GEC engineers wrote a human interface application for a notebook PC using Visual C. The PC sends commands to the motion controller via direct connection to the RMC75.
A necessary design goal was to ensure that a high level of measurement accuracy is maintained even as environmental factors change. For example, changes in temperature can affect load cell readings. To enable the system to be self-correcting as conditions change, GEC programmed the motion controller to use two control loops, an inner loop that responds to readings from the load cell to handle the bulk of the control function, and an outer loop which receives inputs from the multimeters to tweak the control output provided by the inner loop in order to adjust it as environmental conditions change. “Learning the Delta system took only a couple of days and implementing the entire project took us less than a month,” says Kalaghatagi, “even though I had never used a motion controller before.”
The goal of the project was to produce a test system with a high degree of accuracy, precision and repeatability that can enable aircraft owners as well as manufacturers and testing labs to generate equivalent results. In the old days, it was necessary to send scales back to the manufacturer for calibration. With the GEC system and the RMC75, that’s no longer necessary. Customers like Lufthansa, Thai Airways, and the U.S. military testing labs can test their scales quickly and easily in their own facilities.