The Motion Base Platform Requirement (As seen through the eyes of a Feedback Control System Engineer)

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This white paper was originally written by Mr. Bruce Baker. I have taken the liberty of republishing my father's works for all to learn from. Enjoy - Rachel

Over the last several years, there has been much discussion about the need for motion base platforms for aircraft simulators. Several times, an attempt has been made to prove or disprove the need for a motion base platform, and indeed, the need has been both proved and disproved. From all these studies and from personal experience, a few fundamental truths have emerged:

  1. Air combat does not require and motion base platform
  2. Hovercraft simulation (Helicopters VSTUL) does require a motion base platform
  3. Map of the Earth flight, particularly under IFR conditions, using a system such as the AH-64 PNVS, does require a motion base platform
  4. Manual terrain following (200’ altitude, 300 knots)does require a motion base platform
  5. Carrier landings require a motion base platform
  6. Realistic pilot response to wind gusts, turbulence, shear and the like require a motion base

In general, any pilot task which requires frequent, rapid control inputs requires a motion base platform.

The Analytical Approach

And analytical approach can be taken to understand exactly what a motion base platform is doing as far as the pilot is concerned. Figure 1 shows a block diagram of the pilot’s roll control loop for a real aircraft. The pilot gets cues from the aircraft which, in turn, tells the pilot what the aircraft is doing. The roll (or any axis) acceleration and velocity cues are primary through the “seat of the pants” of the pilot, while the roll angle cues are entirely visual. Admittedly, the pilot gets some velocity cue from looking at the visuals. But under transient conditions, it is difficult for the pilot to perceive as he must derive velocity by mentally differentiating position. The pilot cannot get accelerations cues by "looking out the window."

Figure 2 shows the pilot’s roll control loop for a simulator with motion. The roll acceleration and velocity feedback signals are still present although they have been modified by the two filters. The design of these filters is critical in how the system responds.

It has been determined experimentally that pilots normally close the roll control loop in the 1.0-1.5Hz region when they are doing a high work load control task. Technically, this indicates that the pilots are responding to inputs which are below 1.0-1.5Hz and are ignoring inputs above 1.5Hz. It can be shown analytically that the roll control loop transient response will not change significantly provided the motion base filters do not appreciably change the phase of the acceleration and velocity signals in the vicinity of the cross over frequency (1.0-1.5Hz).

Figure 3 shows the phase shift for two different combinations of the motion base platform wash-out filters and actuator servo bandwidths. Both of these were designed to provide zero phase shift at 1.0Hz. The objective is to provide a phase curve that is flat (0 degrees) in the vicinity of 1.0Hz. Increasing the actuator servo bandwidth on the motion base platform and decreasing the wash-out filter frequency improves the flatness of the phase curve. The actuator servo bandwidth is normally limited by the motion base platform and cockpit structure. The wash-out frequency is limited by the actuator stroke since decreasing the wash-out frequency by a factor of 2 requires increasing the stroke by a factor of 4.

Figure 4 shows the pilot’s roll control loop for a simulator without motion. Note that the acceleration feedback is completely missing and any rate information must be derived by differentiating the roll position from the visual display. It can be shown analytically that unless the pilot drastically changes his compensation - i.e. the way he uses cues to control the aircraft - the roll control loop becomes unstable. Furthermore, the pilot’s gut response is completely different without motion than it is with motion.

This argument can be extended to include the other axes of the aircraft.

This is the end of the original white paper. Below is additional information.

In this article, Mr. Baker says that the actuator servo bandwidth can be limited by the motion base platform and cockpit structure. Unlike some motion base manufacturers, all of our motion base designs have been engineered keeping this very problem in mind. It can be a serious problem if the actual structures for both the motion base and the cockpit are not sound. Therefore, we have designed the entire line of motion bases to no affect the servo bandwidth of the actuator so that it isn't limited by the structure. In this way, our customer can rest assured that the system is producing servo bandwidth at it's maximum and the motion base is strong and well-built.

Figure 1 - Pilot's Roll Axis Control Loop for a Real Aircraft

Figure 1 - Pilot's Roll Axis Control Loop for a Real Aircraft

Figure 2 - Pilot's Roll Control Loop for a Simulator with Motion

Figure 2 - Pilot's Roll Control Loop for a Simulator with Motion

Figure 3 - Phase shift curve for two different actuator servo/washout filter combinations

Figure 3 - Phase shift curve for two different actuator servo/washout filter combinations

Figure 4 - Pilot's Roll Control loop for a Simulator without Motion

Figure 4 - Pilot's Roll Control loop for a Simulator without Motion

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