Abstract:
An instrumented, adjustable, educational, structural model employs novel sensor housings and free-fall, pendulous, and cantilever-beam test objects made of stiff, hard, compressible material, for calibrating accelerometers; for demonstrating behavior-testing and health-monitoring machinery technology; and for exploring the radiant, vibrant, automatic, communicative nature and behavior of energetically interacting things. Unusual tests confirm Newton&#39;s Laws of Motion, and how energetic interaction causes things to happen and move. Somewhat surprisingly, sensor signals faithfully track the arcing motion of a glider type swing, but not that of a simple pendulous swing. Resulting from a dimensional interference fit, residual stresses in the compressible housing of a plug-in, motion sensor accurately align, secure and intimately clamp together imperfect mounting surfaces; and facilitate sensor assembly without the use of adhesives. Compression and expansion of a fluidic strain sensor converts surface distortion of a vibrating beam structure into an oscillating electrical signal for visual display on a monitor.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to both the field of education and demonstration, and the field of measuring and testing. More specifically, it relates to instrumented educational structural models; plug-in motion-sensing accelerometers; and fluidic structural strain gauges. Operation involves testing the behavior and monitoring the health of familiar structural models: a swing, a beam, and a free-fall rig, instrumented with novel motion and strain sensors. 
     Almost since the dawn of human thought, physical models have been created and employed to help explain, demonstrate, teach, and explore the wonders of nature and creation. Today, in our modern, computerized, information age, such models proliferate. Physical, mental, pictorial, graphical, symbolic, verbal, and virtual, models play a vital role in our highly industrialized society. 
     With the relatively recent advent of computers, and the development of a multitude of electronic sensors, instrumented structural models of such things as building, bridges, bones, machines, cars, and satellites now occupy much space and attract attention in our government, industrial and educational laboratories. A legion of busy scientists and technologists laboriously test and modify the behavior, and monitor the health of these structural models. A typical human task today might involve creating, testing, modeling and modifying the behavior of a structure to improve how it functions, feels, looks, sounds, interacts, or copes. 
     Of course, this enterprising situation also creates a need for educational and demonstration models to help educate and train students and workers, as well as the general public. To do a better job, workers need to further develop their natural testing talents and communicating skills, as well as to learn more about the automatic, interactive nature and behavior of things. Even ancient biblical wisdom encourages them to test all things, and retain what is good. 
     Just to reasonably cope in our modern technical society, people in general need to be more aware of important scientific endeavors, such as the space station, Freedom, which has undergone extensive behavior testing in model form, and to become more familiar with scientific testing methods and the basic universal technology involved. 
     In structural dynamics laboratories at major universities, sophisticated, instrumented, educational models now help students to naturally and interactively learn in a fun way by doing, testing, relating and repeating. Such models often employ various types of commercial sensors, such as capacitance, resistance, inductance and piezo-electric, to measure motion, force, pressure, sound, and other physical variables. But several perplexing problems still plague these structural models and the sensors involved. 
     Employing industrial components and designs, present educational structural models are usually quite expensive, difficult to adjust and operate, and inconvenient to instrument with sensors. 
     Older educational models fail to promote and demonstrate modern, universal, structural technology and terminology. For example, today sensors are structures, not mystical devices. Like humans, sensors employ structures and the natural energetic way that structures interact to sense and communicate information. And all of the technology and terminology taught in the university courses applies to the sensor structure as well as to that of the test object. 
     Expensive commercial and industrial sensors are usually not suitable for educational applications because of difficulties installing and removing them, especially on test objects requiring several sensors or an array of sensors. Bending or twisting the sensors instead of removing them in a prescribed linear way often damages delicate electrical pin connections or electrical cables. 
     Installing commercial motion sensors has always been an arduous, time consuming, precision task. Much time, money and research effort has been invested in developing improved mounting means, including stud, bolt, clamp, pad, adhesive, tape, wax, and magnets. Popular stud mounting, which mechanically clamps imperfect mating surfaces together into intimate contact has proven to be the best behaving method, although not always convenient or practical. Any interface irregularities or added interface structures modify the motion of the sensor, causing errors at higher frequencies of interest. 
     Highly perfected microphone technology offers a promising solution to low-cost, educational motion and strain sensors, especially in applications requiring very high sensitivity. But unsuccessful attempts over the past two decades to modify and convert low-cost, electret microphone structures into motion-sensing accelerometers have failed because of difficulty in attaching a small seismic mass to the flimsy diaphragm, usually made of an extremely thin, metalized plastic film. Any slight inadvertent force during assembly stretches and relaxes the taut diaphragm, rendering it useless for accelerometers. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly, many of the above problems and difficulties are obviated by the present invention, which provides an educational structural model incorporating an easily adjustable, pendulous swing; an impact-actuated, free-fall test rig; and a strain-gauged cantilever beam; all instrumented with a low-cost, easy-to-install, electrostatic motion-sensor assembly, and all partially and economically made of a stiff, hard, compressible, elastic material. 
     In the free-fall rig, a falling actuator mass impacts a junction block to relax a flexible filament suspension line, which allows the instrumented test-object mass to fall freely for a brief interval of time, confirming Newton&#39;s famous Law of Motion, force equals mass multiplied by acceleration, or F=ma. 
     Somewhat surprisingly, in the adjustable swing assembly, sensor signals faithfully track the arcing motion of a glider-type swing, but not that of simple swing. This strange phenomena demonstrates what happens or doesn&#39;t happen when structures do not energetically interact. The compressible material in the swinging mass grips a one-piece, continuous loop, flexible, filament suspension line; holding the transformable swing in adjustable alignment with the frame of the educational structural model. 
     In the cantilever beam model, compression and expansion of a fluidic strain sensor converts the surface distortion of a vibrating beam into an oscillating, decaying signal for visual display on a monitoring instrument. 
     The low-cost, accelerating motion sensor involved conveniently plugs into or onto the test object or a mounting pad accessory, and intimately clamps imperfect interface mounting surfaces together with a residual compressive force, similar to a popular stud mount. Pressing and slightly twisting the sensor into a mounting hole compresses the elastic material in the vicinity of the interface, grips the test object to retain some of the compressive stress, and deflects enough to conform to irregularities in the mounting surface of the test object. After installing the compressible sensor with a slight twisting motion, it&#39;s quite difficult to pull it straight off. 
     Natural damping inherent in the hard-rubber body material reduces the tendency for sensitivity of the motion sensor to increase at higher frequencies. An external O-ring clamping the electrical cable to the body acts as a strain relief against any inadvertent forces tugging on the cable, 
     Therefore, the primary object of this invention is to provide an instrumented, adjustable, educational, structural model for calibrating accelerometers; for demonstrating behavior-testing and health-monitoring of machinery technology; and for exploring the radiant, vibrant, automatic, communicative nature and behavior of energetically interacting things. 
     Another object of this invention is to physically demonstrate with familiar objects how through energetic interactions, which involve a transfer of energy, things function to transfer forces of nature into motion; how the behavior of a structure depends upon the way it transfers, converts, stores, or dissipates the energy involved; and how energetically interacting structures can be employed to sense and communicate information. 
     Still another object of this invention is to provide a low-cost motion sensor that quickly and conveniently installs by plugging into a hole or onto a post on a test object, mounting pad, or calibrator, whose motion is to be measured. 
     Still another object of the present invention is to provide a rugged motion sensor that can withstand student abuse, and will not be damaged by pulling, twisting or bending to remove it from the test object. 
     Still another object of the present invention is to provide a sealed motion sensor not appreciably affected by environmental moisture or humidity. 
     Still another object of the present invention is to convert low-cost, mass-produced, popular microphone cartridges into practical motion-sensing accelerometers by attaching a seismic mass to the diaphragm to enhance the inherent acceleration sensitivity. 
     Still another object of this invention is to provide a compatible, convenient, fluidic strain sensor that mechanically or adhesively installs on a surface of a test object. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features of the present invention are set forth in the appended claims. The structure of the educational model incorporating a swing, a free-fall object, a beam assembly, motion sensors, and a strain sensor; the manner of attaching the sensors to a test object; together with further objects and advantages can be best understood by referring to the following description in connection with the accompanying drawings, in which: 
     FIG. 1 is a drawing showing a side elevation view of the complete educational structural model assembly, with the free-fall rig sectioned to show design details; 
     FIG. 2 is a drawing showing a partial front view of the educational structural model assembly; with two bushings removed to show the geometry of the suspension; 
     FIG. 3 is a perspective view of a motion sensor incorporating the compressive, plug-in mounting means of the present invention; 
     FIG. 4 is a cross-sectional drawing of the construction of a motion sensor according to the present invention for measuring the acceleration aspect of motion along its sensitive axis; 
     FIG. 5 is a partial cross-sectional drawing of a modified microphone having a mass attached to its diaphragm to increase its inherent sensitivity to accelerating motion; 
     FIG. 6 is a cross-sectional drawing of the construction of a fluidic strain sensor; 
     FIG. 7 is a drawing showing an end view of the fluidic strain sensor having a near cylindrical shape. 
     FIG. 8A is an illustrative plot of strain (ordinate) vs. time (abscissa) a vibrating cantilever beam on which the fluidic strain sensor is mounted. 
     FIG. 8B is an illustrative plot of acceleration (ordinate) vs. time (abscissa) for the free-fall motion of the test object mass. 
     FIG. 8C is an illustrative plot of acceleration (ordinate) vs. time (abscissa) for the motion of the glider swing. 
     FIG. 8D is an illustrative plot of acceleration (ordinate) vs. time (abscissa) for the motion of the simple swing. 
     FIG. 8E is an illustrative hybrid plot showing acceleration (left ordinate) vs. time (abscissa) for the motion of a misguided glider swing, and also showing to the right thereof the acceleration (right ordinate) vs. frequency (abscissa) for such misguided glider swing. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG.  1  and FIG. 2, which are different views of the same object, an instrumented structural model is illustrated, which is employed to explore, demonstrate and teach universal structural technology and terminology. 
     The part of the structural model in the general direction of arrow  20 , illustrates a free-fall test rig, which includes a sturdy hardwood frame, a composite elastic suspension assembly, an actuator mass  24 , and a free-fall mass  21  with a motion sensor  50  attached by plugging it into a hole. The free-fall mass  21  is a hollow ball made of a hard, plastic material. The actuator mass  24  is a hard rubber ball with a central hole and a plastic tube  25  installed to guide its decent. The composite elastic suspension consists of a band or loop of elastic, bungee-type cord  27  looped over the support rod  29 , and a length of flexible filament fishing line  22 , connected together by a plastic cable tie  26 . Above a bulky knot retaining a small plastic junction block  23 , the flexible line  22  is threaded through the central hole and plastic guide tube  25  in the actuator mass  24 . Below the knot and junction block  23 , the line  22  is fastened to the free-fall mass  21  by means of another bulky knot wedged into a stepped hole. Two hard rubber bushings, severally indicated at  28 , position and retain the elastic cord  27  by firmly gripping the support rod  29  protruding from the column  15  of the assembled frame. 
     The wood supporting frame consists of a base  11  with two attached columns  12  supporting a fixed beam  13  mounting a spacer  14 , a cantilever beam  41 , the swing pivot rods  34 ,  34  and another column  15 , to which the free-fall assembly is attached. Long flat-head connecting bolts  16  and thread inserts fasten together the various parts of the frame assembly. Rubber feet  17  cushion the base  11 . 
     Manually lifting and dropping the actuator mass  24  causes it to impact the junction block  23  and relax the lower portion of the flexible suspension line  22 , allowing the test mass  21  to fall freely for a brief interval of time, about 50 milliseconds. 
     The motion sensor  50  and associated instruments capture and display the event as a step increase in signal, where the flat plateau following the step represents a constant, one local “G” of acceleration. Adding an additional, auxiliary mass to the test mass and repeating the test does not change the signal. 
     These test results confirm the obvious: the farther an object falls, the faster it goes, and the harder it hits when stopped. They also confirm Newton&#39;s famous Law of Motion, F=ma (force equals mass multiplied by acceleration), since tests prove that the acceleration is the same when the ratio of force to mass is constant, as is the case with gravity. Test results also suggest a way of calibrating accelerometers by simply dropping them, and measuring the output signal for one local “G” of acceleration, which doesn&#39;t vary much from the standard value of 32.14 feet per second increase in speed each second (9.84 meters/second 2 ). Typically, this particular drop test method illustrates testing the behavior of a structure with impact excitation. 
     Referring again to FIG.  1  and FIG. 2, the part of the educational structural model in the general direction of the arrow  30 , illustrates a pendulous swing assembly, which can be configured with parallel suspension arms as a glider-type swing, as illustrated, or as a simple swing by moving together the two separate pivots on each of the pivot rods  34 . In addition, the swing mass  31  can be aligned or adjusted relative to the frame by sliding it on the flexible suspension line  32 , which it firmly grips. 
     The swing assembly illustrated in the direction of arrow  30  includes two pivot rods  34  protruding from the wood frame; a one piece, continuous loop, flexible filament suspension line  32 ; a hard-rubber, compressible swing mass  31 , and two motion sensors  50  plugged into to the swing mass  31 . The “V” shaped, flexible suspension line  32  drapes around the pivot rods  34 , and threads through tiny, near radial, pin holes and large axial holes in the swing mass  31 . Near one face of the swing mass  31 , the line  32  exits the mass  31  at the left axial hole, drapes around the left pivot rod  34 , goes back into and out of the mass  31  at the center axial hole, wraps around the other pivot rod  34 , then goes back into the mass and through the right axial hole to the other face, where the pattern repeats. Eventually the two ends of the one-piece line  32  meet and tie together inside the left axial hole of the mass  31 , where the journey began. At the pivot rods  34 , the line  32  is clamped between hard rubber spacers  35  and bushings  33  tightly gripping the pivot rods  34 . 
     Manually lifting and releasing, or pushing the swing mass  31  to energize it causes the mass  31  to freely swing back and forth at a natural rate, about once per second, while air resistance gradually reduces the excursion. Somewhat surprisingly, adding an additional auxiliary mass to the swing mass  31  does not change this motion nor the sensor signals. 
     During operation and testing, sensor signals faithfully track the arcing motion of the glider-type swing, but not the motion of the simple swing. The simple swing does not generate any signal because there is no energetic interaction, no transfer of energy between the sensor  50  and the swing mass  31 . Through energetic interaction with the Earth, gravity moves both objects. Tracking the arcing motion of the glider swing by displaying the horizontal and vertical motion signals in an X/Y fashion on the monitor builds confidence in the instruments. The arcing motion of the display resembles the mouth of a happy face graphic. Reversing the vertical signal polarity or the mounting of the vertical motion sensor  50  changes the smile into a frown. Misalignment of the glider swing relative to the frame causes uneven peaks and valleys in the vertical motion signal, which can be corrected by adjusting the swing mass  31  on its suspension line  32 . Checking for misalignment is a typical health monitoring task. 
     Referring to FIG. 3, in the general direction of the arrow  50 , a motion sensor is illustrated, which is employed to measure a component of the acceleration aspect of motion along its central, sensitive axis, indicated by an arrow  51 . The device contains an internal, modular, transducer module  53  in the form of an electrostatic, variable capacitance type motion sensor  53 , which generates an electrical output signal faithfully following the vibratory motion of the test object to which it is attached. 
     Referring to FIG. 4, a cross-sectional drawing illustrates the construction of the state-of-art motion-sensing accelerometer  50  pictured in FIG.  1 . This unusual motion sensor  50 , includes a sensing module  53  housed in a stiff, hard rubber body  54  having both a hole  65  and a post-like projection  61  for attachment to a test object or to a mounting pad accessory  58  by gripping either a post  66  or the walls of a hole  62 , and holding compressed mating interface surfaces  63  and  64 , or  67  and  68 , in intimate contact. The behavior of this quick, simple pug-in installation resembles that of a popular, threaded, stud mounting means of the highest integrity. 
     Facilitating convenient plug-in assembly, a slight interference fit of one thousand of an inch between the sensing module  53 , and the stiff, hard rubber body  54  accurately holds the sensing module  53  in alignment with the mounting surfaces  63  and  67 , without the use of adhesives. A collapsible, vent pin-hole  69 , allows air to escape from the inner chamber through a hollow-needle tool during assembly. An undersized hole in the body of the sensor grips and seals the pigtail cable assembly  55 , providing strain relief for the cable. 
     Firmly pushing and twisting the protruding sensor post  61  into a cylindrical hole  62  results in a strong, adequate, holding force for most applications. However, a compressible annular ridge or bead on the end of the sensor mounting post  61  fitting a similar shaped groove near the bottom of the test object hole  62  would lock the two piece assembly together even more securely. 
     Referring to FIG. 4, during installation when the motion sensor  50  is plugged into or onto the test object  58 , the force required to overcome the interference fit between the post  61  and hole  62  generates stresses in the elastic body  54  of the sensor  50 , and in the test object  58 . The resulting radial compressive stress in the post  61  clamps it to the walls of the hole  62 . When the installing force is released and the compressive stresses relax somewhat, axial tensile stress develops in the post  61 , holding the mating mounting surfaces  63  an  64  together in intimate contact with residual compression. During operation, the residual compressive stress at these interface surfaces ensures the faithful transmission of motion from the test object  58  to the sensor structure  50 . In a similar way, residual compressive stresses in the body of the sensor  54  securely lock the sensor  50  to the test object  58 , when using the alternative, hole mounting means  65  to install the sensor on a rigid post  66 . 
     The dynamic behavior of the sensor  50  with its compressive mounting means is adequate for most educational and structural behavior testing applications. With low mass, high stiffness, and good internal damping, the assembly provides a reasonably flat response over a wide frequency range, typically from one to above 1,000 Hertz, which means that within that range the sensor treats all frequencies nearly the same. In other words, within the range of interest, the sensor generates essentially the same output signal amplitude for the same input motion, regardless of frequency. Behavior tests of the sensor with this educational, structural model confirm that the motion sensor  50  adequately follows a one-millisecond rise-time, step increase in acceleration lasting about 50 milliseconds, without appreciable overshoot, ringing, or decay. 
     The motion sensor assembly  50  also conveniently installs in a mounting pad  58  attached to a test object with an adhesive. Commercially available, adhesive-mounted, hard-rubber bumpers with an added central hole  62  provide a convenient installation for many applications. 
     A conventional accelerometer with a threaded hole in its base can be mounted in a similar plug-in way with a hollow cylinder of hard, compressible material bolted to its base, instead of a stud. Tightening the screw expands the compressible cylindrical post to tightly fit in the mounting hole. 
     Referring to FIG. 5, the sensing module  70  is an electrostatic, electret type microphone with a small metallic mass  72  attached to its taut diaphragm  73  with a minute amount of a permanent, stiff, viscous fluid  74 , such as silicon grease. To avoid damaging the delicate, metalized, plastic-membrane diaphragm  73  during assembly, the silicone grease  74  is first applied to the flat interface surface of the mass  72 , before it is delicately dropped onto the diaphragm  73 . Then the assembly is vibrated to firmly seat the mass  72  in place. This mechanical/chemical way of attaching the mating parts together withstands severe transverse shocks as well as moderately high temperatures. 
     Implementing Newton&#39;s famous Law of Motion, force equals mass times acceleration, or F=ma, this motion-sensing accelerometer module is structured with a mass-loaded spring in the form of an elastic diaphragm  73 , and an electrical, variable-capacitance sensing element to measure its deflection. The accelerometer module  70  measures the force required to automatically give its internal seismic mass  72  the same motion as the test object to which its case  71  is attached. Tilting the sensitive axis of the accelerometer from horizontal to vertical essentially weighs its seismic mass on the spring type scale. 
     For measuring accelerations in orthogonal directions, two or three of the modular sensing capsules  70  readily house in a similar hard-rubber body with their sensitive axes aligned perpendicular to each other. Such biaxial or triaxial assemblies attach to the test object in the same way as a single-axis sensor  50 , employing either a plug or hole with an interference fit and residual stresses. 
     Mounting or bonding two such sensor modules  70  together, face-to-face in a common housing creates a well known, differential-mode sensor, whereby one signal increases while the other decreases when the sensor is accelerated. Operating in this sophisticated differential mode doubles the sensitivity, improves linearity, and rejects common-mode noise. A small dab of silicone grease applied between the two masses  72  of a differential mode sensor bonds them together mechanically, and stiffens the structure of the moving assembly to transverse disturbances. Such a fluidic connecting means avoids a difficult dimensional tolerance problem trying to exactly fit a solid mass between two fixed diaphragms, which move only a few ten-thousandths of an inch during operation. Two motion sensors,  50 , one mounted in each face of the swing mass, demonstrate differential-mode operation. 
     Other type modular sensing modules, such as piezoelectric, resistive, or inductive would work just as well in the compressible housing  54  of FIG.  4 . Such sensing modules with internal, microelectronic, integrated-circuit, isolation amplifiers operating over two wires  56  and  57  conducting both signal and power are sometimes called “smart sensors.” 
     Referring to FIG. 6, in the general direction of the arrow  80 , a fluidic strain sensor is illustrated, which is employed to measure the surface strain of a stressed mechanical structure, such as the cantilever beam  41  to which it is adhesively attached. The drawing illustrates the construction of the fluidic strain sensor  80  pictured in FIG.  2 . This strain sensor  80  includes an electronic pressure sensing module  82  housed, clamped, and sealed within an elastic body  81  made of a hard, compressible material, and exposed to the pressure in an elongated, sealed, internal chamber  86 . To prevent a pressure build-up during assembly, a small, collapsible pin hole  87  formed by a hollow needle vents the inner chamber during assembly. A flat, two-wire  84  and  85 , ribbon cable  83  clamped by an undersize hole in the compressible housing  81  connects the sensing module terminals  88  and to an external signal and power conditioning circuit. A conical exit hole or strain-relieving tubing prevents the two wire cable  83  from bending sharply. Referring to FIG. 7, a flat or conformal surface  89  on the elastic body  81  attaches to the test object surface by means of an adhesive or mechanical fasteners. 
     Referring to FIG. 2, distortion of the mounting surface of the cantilever-beam test object  41  in the direction of the longitudinal axis of the sensor  80  shortens or stretches the sensor chamber  86 , which compresses or expands the air in the chamber  86 , changes its pressure, and generates a corresponding electrical output signal. Thus the net result is a very sensitive, low-cost, convenient, fluidic strain sensor  80  compatible with the external electronics of the electrostatic motion sensor  50 , and electrostatic microphones. 
     The strain-instrumented, cantilever beam assembly  40  incorporates and models the basic spring and deflection sensing elements of a conventional force sensor. With an auxiliary mass, such as sensor  50 , attached near its tip, this electromechanical structure also serves as a seismic accelerometer model, sensitive to disturbances such as tapping on the base  11  of the frame. A motion sensor  50  near the tip of the beam  41 , provides a means for calibrating the strain sensor  80  by measuring the tip deflection of the beam and calculating the stress near the base. 
     Therefore, the creative concepts in the present invention provide a practical, useful, low cost, instrumented, educational, desktop structural model incorporating and promoting state-of-the-art technology for exploring, demonstrating, and teaching universal technology in a fun way. This technology explorer serves as a valuable teaching and training accessory for expanded computers acting as virtual oscilloscopes and spectrum analyzers.