FAA uses electrohydraulic controls for runway testing
Hydraulic actuators excel at applying real-world forces in testing applications, so manufacturers and research labs doing product strength and endurance testing often apply hydraulics. When the hydraulics are controlled by programmable closed-loop motion controllers, the tests can be made flexible and very precise—saving time and money, and yielding high-quality results, according to test engineers at the Federal Aviation Administration (FAA) National Airport Pavement Test Facility (NAPTF) in Atlantic City, N.J.
New airport runways are occasionally being built, but old ones are being completely rebuilt or overlaid for many good reasons beyond wear and tear, such as the availability of new materials and changes in aircraft landing gear design, wheel loading, and traffic levels. Consider the problem of setting guidelines for airport runway construction to support the newest wide-body jets. To keep up with the latest requirements on pavement construction, airport managers consult with the FAA standards and guidelines contained in documents called Advisory Circulars.
The FAA’s Atlantic City test facility (see Figure 1) incorporates a 1,200-ft pavement section and a movable carriage with wheeled load modules that apply the stress of a lifetime of aircraft operations. The structure can simulate 30 years’ worth of aircraft traffic in a few months. The runway has different segments to simulate pavement over different types of soils, as would be found in different parts of this country or the world. Some segments are concrete, and some are asphalt.
The aircraft impart most damage during taxiing from the gate to the takeoff end of the runway. A wide-body jet such as the new Airbus A380 can exert between 55,000 and 65,000 pounds of weight on the pavement per wheel. To simulate an aircraft of a particular type, the NAPTF team uses load modules extending down from the carriage that put the desired number of wheels and the desired amount of weight against the simulated runway pavement.
For the tests to yield correct results, the wheel load or the force on the pavement needs to remain constant, even though the pavement surface can contain irregularities (bumps and ruts). This fact, plus the requirement that the testing system be easily reconfigurable to simulate many aircraft, drives the need for a hydraulic load management system that is controlled by a programmable motion controller.
PID too slow for motion
Originally, FAA engineer Ryan Rutter was using a PLC to control the hydraulics, but he found that it was difficult to precisely regulate force exerted by the wheels. The PLC was running a PID (proportional, integral, derivative) control loop, but the slow loop time for the PLC plus the effects of compliance of the nitrogen-filled tires made it difficult to correctly track the irregularities in the asphalt. Rutter needed a more responsive control system. The hydraulics distributor Advanced Fluid Systems of York, Penn., recommended the RMC75 electrohydraulic motion controller manufactured by Delta Computer Systems Inc. of Battle Ground, Wash.
Using the Delta RMC (Figure 3) gave Rutter a 10x improvement in the time that it takes to close the control loop. The controller computes the applied force by using pressure readings from sensors on either end of the pistons in the load module actuators. These pressures are multiplied by the piston area, and then the cap and rod forces are subtracted. The PID gains, along with feed-forward terms in the loop equation, generate the drive signal required to close the loop and make the actual force equal the force setpoint.
This worked well as long as the wheels were not moving. When the wheels started to move up and down over ruts in the runway, the PID gains could not respond fast enough because of the compliance of the gas-filled tires. The controller gains could only be increased so far before oscillations occurred.
Change in pressure
To rectify the problem and keep the force and pressure constant, the rate of change in pressure must be zero. The formula that governs the rate of change in pressure is:
is the rate of change in pressure with respect to time in the cylinder
is the bulk modulus of oil
Q is the flow
v is the velocity of the actuator relative to the piston
A is the area of the piston
V is the trapped volume of oil between the valve and the piston
To keep the rate of change of pressure constant at zero, the numerator of the equation must be zero. To do that requires knowing the speed of the actuator at all times when trying to maintain pressure or force. As the wheels move up and down in the runway ruts, the velocity of the wheel motion causes pressure changes. Since the motion controller is controlling the hydraulic fluid flow, it was clear that by adjusting the flow to match the velocity of the actuator times the area of the piston, the motion controller can maintain a constant pressure between the wheels and the runway.
In the testing, for every 0.1 in. the actuator was moved down relative to the runway surface, the force between the runway and a pair of wheels increased about 1,800 pounds. The 1,800 pounds per 0.1 in. approximation wasn’t linear but was close enough. If the target test weight for the combined tire pair is 36,000 pounds, then the tires need to be pushed into the runway 2 in. Maintaining that force through the duration of the test required knowing or “mapping” the surface profile of the runway as a function of distance along the runway, so that the motion controller’s cubic spline function could be used to control the position of the tires. To obtain this information, the design team moved the machine slowly down the simulated runway over the length of a test area that was about 160 ft long and recorded the elevation between the carriage and the runway every 6 in. using a string potentiometer attached to the load module.
Next, the runway surface profile was entered into the motion controller in a long spline array. The controller was then put in position control mode, executing its cubic spline function, with its output to the hydraulic servo valve geared to the current runway depth plus an offset, to generate what is approximately the necessary force. Spline functions are used by the Delta controller to “connect the dots” of known target axis positions with smooth motion curves that do not exhibit any bumps or jerks due to step changes in acceleration.
Since the force-to-tire compression distance relationship is nonlinear, more precise control of the force exerted on the runway could be maintained using cascaded control loops. In the cascaded loop arrangement, the output of the outer force loop PID is used to offset the inner loop’s target position generated from the runway rut depth spline profile. This cascaded loop arrangement worked very well because the inner loop could be tuned up relatively tightly. Since it only cared about errors in position, tire compliance made little or no difference. The outer loop’s primary value is to compensate for the nonlinearity of the force-to-compression distance.
Cascade loops, nonlinear
Using cascade loops makes it easier to keep a nonlinear system such as this under control. If the inner loop that is based on following the runway profile can be accurate to within 10%, then the outer loop that does closed-loop control of force only needs to supply a correction factor for the last 10% to fully reject any unpredictable disturbances in the system. By reducing changes in force required to compensate for the difference between target and actual values in the weight that the wheels exert on the runway, changes can be accomplished more smoothly and quickly, and factors such as oscillations can be eliminated.
The data that the FAA is obtaining via the taxi tests is being incorporated into a software program called Faarfield, which airport pavement engineers use to help them design airport pavements.
“As a result of tests that we have performed at the test facility, we have determined that, overall, airports will save $167 million per year in overlay design,” said Dr. Satish Agrawal, manager of the National Airport Pavement Test Facility. “We also expect that our design software will save approximately 3% in the Airport Improvement Program (AIP) funding each year.”
– Peter Nachtwey is president – engineer, Delta Computer Systems Inc. Edited by Mark T. Hoske, CFE Media, Control Engineering, www.controleng.com.
Also see the Machine Control Channel: