Weighing, Balancing

General Electrodynamics Corp. (GEC), Arlington, TX, is the world's leading manufacturer of advanced portable aircraft weighing and balancing equipment, heavy-duty portable truck scales, and electronic portable test equipment. Use of the company’s aircraft scales are by aircraft manufacturers, fleet operators, and research and testing labs worldwide.

To ensure that the company’s products produce accurate results, they need calibrating. In the case of GEC’s scales that weigh commercial and military transport aircraft, calibrating involves using a press to apply precise loads from 0 to 100,000 lb to the scales while comparing the scale readings with the known applied force. To meet the company’s specifications, the readings produced by GEC’s scale testing systems must be accurate to at least 0.025% of the applied load when testing a scale with 100,000 lb.

To ensure that the scale readings are accurate, the test system needs to be even more accurate in applying the test loads. GEC’s scale tester (Figure 1) employs a hydraulic press that pushes down on the scale while monitoring feedback from a load cell mounted between the hydraulic cylinder and the scale. Hydraulic power is ideal for this application because it typically takes less energy to apply and hold large amounts of force with a fluid power system than with an electric motor and gearing. However, achieving the accuracy needed by the scale tester requires precise closed-loop electrohydraulic controls.

The oldest method for testing scales was to use known dead weights, but handling them is expensive and cumbersome. As an alternative, GEC used to test its scales using a manually operated hydraulic pump, similar in concept to the hydraulic jack that one would use to lift a car. However, this system was prone to pressure leaks that made it virtually impossible to achieve the desired level of accuracy. With an automated hydraulic system that employs closed-loop control, there is elimination of such errors. In addition, the automated motion control system has the ability to measure key parameters 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.
 

Handling the Task
What type of electro-hydraulic controller can handle the task and meet the stringent accuracy requirement? In the case of GEC’s scale testers, an RMC75 motion controller manufactured by Delta Computer Systems Inc., Battle Ground, WA, is used. The RMC75 (see Figure 2) 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. This is important in press applications because the typical scenario is for the controller to use position control to extend the rod to place it into position for applying pressure, and then switch into pressure/force control mode to apply the desired force in a smooth and precise manner. Delta describes this feature as dual loop control, allowing two control loops to run simultaneously and seamlessly make decisions about which control loop should control the actuator. Accomplishing this with the Delta controller is 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.

In the case of the GEC system, providing cylinder position information to the RMC75 is via a Temposonics linear magnetostrictive displacement transducer (LMDT) 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 to 10V range, and a filter to eliminate noise in the signal before 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" bore and 8" stroke) that has a very high oil column resonance for quick response to closed-loop controls. Provided by the Wilson Co., Addison, TX, were all the key system components. Figure 3 is a block diagram of the system showing how the components connect.
 

During operation, the RMC75 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. Then the controller switches smoothly to closed-loop force control mode to ensure application and maintenance during the scale test of the proper force. Since different scales have different dimensions, the RMC75 needs to sense, in an active manner, when the cylinder rod comes in contact with the scale in order to know when to switch to pressure/force control. The controller does this by watching for the increase in the load cell’s force reading that occurs when contact with the scale occurs.

Needed are both fast control loop times and pressure sensors in order to ensure repeatability and accuracy of measurement. Forces will increase rapidly during the transition from position to force control. If the control system does not respond quickly enough as the applied force on the load cell approaches its force set point, the system will overshoot its force set point. The RMC includes features such as eight times oversampling for more accurate reading and detection of rates of change. Detecting the rate of change along with derivative gain is important because they allow the controller to have more braking, reducing the rate of change before reaching the force set point. This minimizes or eliminates overshoot. Additionally, predicative gain values (feed forwards) can estimate the control output faster than the PID gains can respond and thus provides more accurate control under dynamic situations such as the transition from position to force control.
 

Exceeding Expectations
Amith Kalaghatagi, engineering manager, GEC, explains that, “With the Delta controller and its high-speed control, we have been able to exceed our minimum testing accuracies. In fact, we have been able to hold up to 100,000 lb pressure on the scale being tested to within 1 lb.”

The GEC system uses Keithley multimeters, which are capable of reading data from the load cell in the nanovolt range, in order to verify and attain accuracy specifications, proving application of the exact amount of force.

To program the Delta motion controller, GEC’s Kalaghatagi uses Delta’s RMCTools programming software.

“The most useful tool was the Tuning Wizard,” Kalaghatagi says. “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 achieve the desired force within about half that amount of time.”

One of the challenges in ensuring accuracy of the scale measurements was to take into consideration variations in load cell readings that occur due to environmental factors such as temperature. To enable the system to be self-correcting as conditions change, programming of the motion controller by GEC engineers was to use two control loops. An inner loop responds to readings from the load cell to handle the bulk of the control function, and an outer loop 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, even though I had never used a motion controller before,” Kalaghatagi states.

To control the test operations, GEC engineers wrote a human interface application for a notebook PC using Visual C (Figure 4.). The PC sends commands to the motion controller via direct connection to the RMC75.

The keys to this successful application are achieving a high degree of control over accuracy, precision, and repeatability so that aircraft owners, as well as manufacturers and testing labs, can 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 is no longer necessary. Customers like Lufthansa, Thai Airways, and U.S. military testing labs can test their scales quickly and easily in their own facilities.
 

Delta Computer Systems Inc.
Battle Ground, WA
www.deltamotion.com