Invention and Development of the "Synergistic" Motion System

Klaus L. Cappel, P.E.


Introduction: Processes of Invention

Evidence is accumulating to show that the processes of invention and innovation can be analyzed and classified. Implementation of the insights thus gained may lead to a more effective use of mental capabilities in the pursuit of these activities.

Organizations such as commercial and government laboratories still practice activities tracing back to the example set by Thomas Edison, although such pursuits may no longer be led by a single, all-encompassing intellect. An early example of what may be termed the democratization trend of invention was the brain-storming session. Participants were urged to voice their free associations, while at the same time being enjoined to withhold all criticism of ideas brought forth by their colleagues. This was a fundamental misunderstanding of the process of creation: The prohibition of critique was meant to preserve harmony by suppressing personal attacks that would inhibit the very expression of ideas the meeting was intended to foster, but it often prevented constructive criticism that that might have moved the process forward. All invention has an iconoclastic component; the breaking of eggs precede the preparation of the omelet.

Individual, solitary invention differs from the scenario above in two important ways: First, there is usually no record of the process itself, such as might be preserved by written minutes of the group’s meeting, or at least from the composite memory of the participants. The inventor is usually too busy with his objective to practice simultaneous introspection, and only in retrospect may he be able to reconstruct the sequence of images, thoughts and emotions that accompanied the work.

The second important difference lies in the necessity for constant self-critique which must alternate with the flow of ideas in order to move the process forward. However much one may later be ashamed of some of the ideas one has produced in the search for the ideal, one must continue to criticize oneself for good reasons while refraining from self-detraction.

The intellectual parallel to this two-sided approach to invention is a long-known phenomenon called figure-ground reversal. It is illustrated by a picture of a complex white vase on a black background. A mental switch now displays a pair of black profiles facing each other against a white background.

There are undoubtedly many paths towards invention, some characteristic for different people, some pursued by the same person as determined by the nature of the problem itself. The work to be described in the following was based on "accentuating the negative": Instead of "improving" an existing construction, one looks at its worst features and imagines them removed and tries to come up with a substitute. The "ground" is now bare and calls for a new "figure". Only the second step can, strictly speaking, be called "invention", but, at least in this case, breaking the eggs is a prerequisite for the omelet being cooked up.

Of course, no invention springs forth entire and perfect, like Athena from the head of Zeus. It must usually be based on some technology to become practical. Some inventors were unfortunately so far ahead of their time that their concepts could not be implemented, as was true of Charles Babbage and his computing engine. But usually, an invention, to be useful, must make the transition to its close relative, design, which, in turn, must be rooted in current practice (though that, too, may require innovation). Design by itself can be innovative and ingenious, but it is more rule- and practice-based. If some part of a design is judged inadequate on some grounds - strength, stiffness, space occupied - the designer attempts to modify it, perhaps by changing its shape or material properties or geometry, or the entire scheme as a last resort. It is when these attempts fail that the environment is ready for an inventive step, but that may be taken in only a small number of cases.

It may go without saying that a thorough knowledge of the underlying physical principles is a prerequisite for a successful invention. One of the saddest stereotypes in popular culture is the crazy inventor, a figure of fun with his Rube Goldberg nightmares. This is not really funny when one considers the wasted lives based on the chimera of future fame and wealth. When I was working for a research institute I occasionally received letters from inventors who disagreed with current technological trends. One came from an engineer who was convinced that the switch to airplane jet engines was a step in the wrong directions: The flapping wing, used for millions of years by birds, was the way to go. And another strongly held opinion was expressed by a strange man who reasoned that rocket propulsion was not a good method of space propulsion. A propeller like a motor boat’s was more logical. When I gently pointed out the difference - a propeller acted against a medium that provided the required reaction - he replied that such a medium existed in space - the ether. What could one say?

The following attempts to relate the background and invention of a widely used device, as far as it can be reconstructed, in the framework of the thoughts expressed above.



Vibration testing is a procedure used in a wide range of technical areas for evaluating the performance of objects subjected to acceleration. Such testing is used by the automobile industry to investigate the dynamic behavior of cars ("shake and rattle"). Builders of satellites vibrate their products to determine their ability to remain functioning after exposure to the high-g launch environment. Civil engineers and manufacturers reproduce the shaking typical of earthquakes to assess the integrity of critical structural and mechanical components essential for public safety following such natural disasters. Finally, pilots and astronauts are trained in so-called flight simulators to respond to motion cues, which are accelerations perceived by the equilibrium organs of the inner ear and the proprioceptors imbedded in their muscles (the origin of the "seat-of-the-pants" perception). It has been found that seasoned pilots are disturbed by the absence of such motion cues in a purely visual simulation environment, hence the use of the moving base to provide the necessary realism.

While the frequencies and amplitudes of the applied accelerations differ widely among the dynamic regimes mentioned above, most of the equipment used to reproduce them is the same, within the range of the motions typical of each. Differences in use are reflected in the terminology: Shake tables excite equipment, while moving-base simulators carry people.

This essay recounts the origin of a major departure from the conventional design of vibration producers, made about 35 years ago, which eventually led to its adoption all over the world, after the initial resistance died down. In order to grasp the extent of the innovation, it seems desirable to explain the environment from which it sprang, and in which it is still embedded.


Kinematics (or the Geometry of Motion)

A rigid body can move in space in just six ways: three of them along linear axes: fore-and-aft, sideways, and up-and-down; and three rotational, about the three axes - roll, pitch and yaw (following ship terminology). It follows that six restraints are needed to maintain the body immobile in space, against forces applied to it. When thus immobilized, the body is seen as a structure, capable of resisting the three forces and three moments tending to displace it in the six directions, referred to technically as degrees of freedom. There are a few obvious restrictions on the arrangements of the restraints: For example, no group of three can be at once parallel and coplanar, but that detail need not concern us here. However, when the number of restraints is less than six, the configuration becomes unstable

Any one or all of these restraints can be replaced by an element capable of changing its length in response to a command, such as that supplied by a hydraulic actuator under servo valve control. Each such substitution provides an additional degree of freedom for the object to move in, the number (referred to the axes) depending on the geometry of the restraints.



Providing more than six restraints - active or passive - makes this arrangement redundant, a term arising from structural engineering. It is also called indeterminate, a term that does not mean that it cannot be analyzed, but only that the laws of statics are nor sufficient to determine the loads or moments, but merely that additional considerations must be applied. In structures, it is necessary to include the elastic properties of the restraints that determine the extent to which they change length under load. This type of analysis has been routinely performed for many years.

Handling redundancy in actively controlled restraints subject to continual rapid changes in forces and lengths, typical of hydraulic actuators on shake tables, is much more difficult, and no satisfactory method is so far available. (The author is developing one which is presently being implemented.) A common experience is one in which the actuators "fight each other", that is, they apply opposing forces to the table, which is thus subjected to high bending moments. This not only detracts from the forces intended to accelerate the platform, but in one case has actually fractured a magnesium table. Thus, from the point of the control engineer, a determinate system, with six actuators, is preferred because of its greater simplicity and freedom from the undesirable effects mentioned.


Conventional Six-Degree-of-Freedom Systems

Motion in all six degrees of freedom (6-DOF) provides the most realistic dynamic regime and is thus preferred where the ultimate in fidelity of simulation is required. The simplest and most frequently used determinate system comprises three vertical actuators, arranged on a triangular base, and three horizontal actuators, two attached to one side of a square or rectangular table, with a space between them to restrain yaw moments, and a third at the center of an adjacent side. The three vertical actuators produce vertical displacement and rotation about the two horizontal axes, and the three horizontal ones, horizontal movements in the plane, and rotation about the vertical axis.

The greatest advantage of this configuration is its relative freedom from kinematic coupling between degrees of freedom. Consider the consequence of raising the platform by a distance small compared to the length of the horizontal actuators. These will rotate about their fixed attachment points at the foundation and will therefore pull the platform towards the foundation by a distance proportional to the actuator length and to one minus the cosine of the angle subtended by the actuator. Not only is this angle small, but, since in most shaking regimes the displacement is inversely proportional to the acceleration, this small error is in most cases negligible. In other words, the slight coupling between the vertical and horizontal motions (and between other degrees of freedom) is usually acceptable.

On the other hand, the configuration does have features considered disadvantageous. The first is the required floor space. Each actuator must be backed up by a reaction mass, usually made of concrete, that prevents excessive recoil motion in response to the reaction of the actuator to the force it applies to the table. The need for horizontal reaction masses may quadruple the area required for such an installation. Skimping on the depth of the reaction mass, which makes it recoil more, will create a discrepancy between the commanded motion and the actual length change of the actuator, leading to a degradation in the fidelity of the desired acceleration.

The required floor space can be decreased by the length of the actuators, by arranging the horizontal ones in a cyclic (swastika) pattern along the table edges, each attached to the corner far from the reaction mass. However, this requires four actuators, so in the horizontal plane the configuration is again redundant, with the disadvantages mentioned before.

Another undesirable feature of the conventional configuration is the indeterminacy of its position when control is lost. This can be a serious problem when the table carries a tall specimen that should not be tilted or exposed to impact when the table is stopped by some of the bottomed actuators.


A Different Approach

In 1962, I responded to a request for improving an existing, conventional 6-DOF vibration system that exhibited many of the problems described above: It had seven actuators, which had fractured the table. Lack of space limited the thickness of the horizontal reaction masses, which were so thin that they could be seen to bend when the actuators were in service. As expected, the table slumped in an unpredictable way when the power was shut off.

In proposing a remedy, I recall setting three aims: Eliminate the horizontal reaction masses (since they couldn’t be enlarged), use six actuators to eliminate the antagonism created by the seventh, and provide a stable rest position to protect the human subjects whose response to vibration was to be determined by the vibration system.

With a background in structures, I visualized a space frame, a stable configuration composed of triangles. A triangle is a figure that cannot collapse (as can, for example, a square with hinged connections at the corners). A familiar example can be seen in the trusses used in many bridges. All that would be required to convert such a structure to a shake table would be the presence of six "struts" capable of changing their lengths, i.e. substitution by hydraulic actuators.

The configuration would thus consist of two equilateral triangles, one of which formed the base, and the second, turned 180 degrees so that its vertex was over the base of the bottom triangle, became the moving platform. The vertices of the triangles would then be connected, in zigzag sequence, by the six actuators, any adjacent two of which became the sides of a triangle, the whole thus forming an octahedron, a stable space frame resting only on a bottom reaction mass. The tyranny of the right angle had been abandoned, and my objectives appeared capable of being reached.

It may be objected that the concept greatly increased the extent of coupling between degrees of freedom. In fact, most or all of the actuators must change length to produce motion in one degree of freedom, and changing the length of a single actuator may produce motion in more than one degree of freedom. However, the large increases in computing speed made it feasible to control the motion of the platform in the face of this increased level of kinematic coupling, at least for the low frequencies required of flight simulators. Much higher frequencies can now be handled.

The reception of the concept was, to put it mildly, cool. The official who headed the project, a friend of mine, was reported to have stated that he would have liked me to do the work, if only I had not come up with such a crazy idea. Having other things to do, I laid it aside.



A couple of years later, The Franklin Institute Research Labs, where I was then employed, was approached by the corporate office of the Sikorsky Aircraft Division of United Technologies, with a request to design and build a 6-DOF flight simulator. We proposed, designed and built such a system based on the new concept , and a patent was applied for, and later granted. Then, towards the late sixties, the Boeing 747 was approaching its introduction, and the airlines were anxious to obtain better simulator training for their pilots, in order to reduce training flights which occasionally led to fatal crashes. Link, then the prime producer of flight simulators, obtained a license. Other manufacturers entered the field, and my institution sued them successfully. (I had to settle for $100, according to the terms of my employment.)

At present, there must be thousands of these machines in use, built by a number of manufacturers for practically every airline and air force in the world. We built one for Daimler-Benz in Germany for automobile simulation. I proposed the idea of using two such platforms in tandem, one of them on rails, to simulate in-flight refueling which often led to accidents. This concept was later adopted by the US Air Force (the organization that had initially rejected the idea), and also by Daimler-Benz, which put its simulator on rails to be able to produce larger linear motions than the platform actuators could supply. Among the latest embodiments are driving simulators being built by the University of Utah, and Delft Technical University in The Netherlands.

For some time now, one could not pick up a trade publication without seeing a picture of my baby every couple of months. One unexpected result of the concept was the ability to be transported in large trailers, which made it possible to ship a simulator to a distant site where flight training was needed but funding for a permanent installation was not available.

Other applications have since been found. Cincinnati Milacron, a manufacturer of machine tools, has developed the configuration of the concept to perform complex machining tasks that in former times required a complex superposition of slides and gimbals, each providing a single degree of freedom. I have also heard that the realistic motions of the dinosaur head and neck in the movie Jurassic Park were produced by a pair of 6-DOF systems. It is evident that the mounting of all actuators on a single plane made such applications possible.

My last contribution to this concept suspended a helicopter from an upside-down version of the system, which was used to determine experimentally its response to gunfire in flight, relieved of the risks of such a test in actual flight.

Overall, though, the initial response to the new concept was unexpectedly negative and remained so for many years. In the end, wide acceptance provided vindication, though that was not accompanied by rewards other than psychic. To quote a bit of doggerel:

"Whoever thinks of something new
Gets scorn and laughter for his due.
At last they see the reason why:
`Twas obvious` is then the cry."


Summing Up

It might be relevant to attempt to sum up the development of the synergistic motion system in both its technical and human contexts. In the narrow field of vibration of a 6-DOF system one eliminates most of the disadvantages of an existing technology, such as large space requirements, redundant actuators and unpredictable rest position, by appealing to a structural concept: the determinate space frame, which is based on simple statics, a branch of mechanics. At the same time, one had to make the decision to accept an obvious disadvantage: the increased coupling between degrees of freedom. However, advances in computers made this tradeoff acceptable.

The elements coming together in the concept are a perception of a need (motion in 6-DOF, in a range of disciplines), the disadvantages of existing solutions, and the potential means available in existing technology, not necessarily exploited in existing devices.

Quite possibly, other solitary inventors have had similar experiences: They are unrecog-nized in their environment with respect to a particular novel concept, but thoroughly embedded in an applicable technology (except where it was not available - see Charles Babbage). The concept of the synergistic shaker elicited no interest among the staff and management of the organization and was rejected by the potential user. Even demon-stration of the prototype to the Board of Managers brought forth no response. The isolation was social, but not technical.

In the absence of objective records detailing the origin of a concept, introspective accounts can be enlightening and encouraging: One thinks of the memories the French mathematician Poincare, and of the German chemist’s Kekule’s discovery of the ring-shaped benzene molecule which came to him in a dream of a snake biting its own tail. However, there are other paths to innovation, specifically by a close-knit group of technical people with different but overlapping skills and different ways of approaching problems, perhaps mixing predominantly critical minds with more freely associating ones. The process can benefit from the presence of a group leader who acts mainly as an integrator and conciliator, the provider of a watering can rather than soil and fertilizer. An autocratic group leader will inevitably stifle the process of innovation.

Perhaps a better insight into the creative process in its various manifestations will bring benefits to humanity at a lesser cost than is now being paid by its practitioners, most of whom will always remain undeterred by the obstacles in their path.