Abstract:
A target object can be vibrated using actuation that exploits the piezo-electric (“PE”) property. Under combined conditions of vibration and centrifugal acceleration, a centrifugal load of the target object on PE vibration actuators can be reduced by using a counterweight that offsets the centrifugal loading. Target objects are also subjected to combinations of: spin, vibration, and acceleration; spin and vibration; and spin and acceleration.

Description:
This application claims the priority under 35 U.S.C. §119(e)(1) of provisional application Ser. No. 60/759,912, filed Jan. 18, 2006 and incorporated herein by reference. 
    
    
     This invention was developed under Contract DE-AC04-94AL85000 between Sandia Corporation and the U.S. Department of Energy. The U.S. Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to the simulation of mechanical operating conditions and, more particularly, to the simulation of vibration conditions. 
     BACKGROUND OF THE INVENTION 
     The need for simulation of vibration conditions that will be experienced by a given entity is well known in the art. Such vibration simulation is useful in the design, development, and qualification of myriad products employed in consumer, commercial, industrial, aerospace, and military applications. Shakers can sometimes be used to simulate vibration conditions in combination with centrifuges that simulate acceleration conditions. 
     Such mechanical shakers simulate vibration conditions by employing mechanical drive arrangements to drive mechanically movable components. It is therefore apparent that, by their very nature, mechanical shakers have complicated designs, tend to be physically large, heavy and cumbersome, and have considerable potential to exhibit reliability problems. These problems can be particularly acute when the shakers are subjected to other environments such as centrifuge acceleration. 
     It is therefore desirable to provide for improvements in the simulation of vibration environments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 2  are perspective views of commercially available piezo-electric actuators. 
         FIG. 3  diagrammatically illustrates a vibration simulation system according to exemplary embodiments of the invention. 
         FIG. 4  diagrammatically illustrates a payload assembly according to exemplary embodiments of the invention. 
         FIG. 5  illustrates the payload assembly of  FIG. 4  in more detail according to exemplary embodiments of the invention. 
         FIG. 6  illustrates the payload assembly of  FIG. 4  in more detail according to further exemplary embodiments of the invention. 
         FIG. 7  diagrammatically illustrates a combined vibration and acceleration simulation system according to exemplary embodiments of the invention. 
         FIG. 8  diagrammatically illustrates the system of  FIG. 7  in more detail according to exemplary embodiments of the invention. 
         FIG. 9  diagrammatically illustrates a system that simulates a spin condition in combination with acceleration and/or vibration according to exemplary embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the invention simulate vibration conditions by exploiting materials that possess the piezo-electric (also referred to herein as “PE”) property. If a positive voltage is applied across a material having the PE property, the length of the material increases. Reversing the polarity of the applied voltage causes the length of the material to decrease. Exemplary embodiments of the invention use these characteristics to produce a vibration condition. 
       FIGS. 1 and 2  are perspective views of commercially available piezo-electric actuators, with dimensions shown in mm. The actuators  11  and  21  include respective steel frames  13  and  23  that hold respective stacks  15  and  25  of PE ceramic cells. The PE stacks  15  and  25  will extend in length in response to the application of a positive voltage. This causes the both sides of the steel frames  13  and  23  to be displaced vertically toward the respective PE stacks  15  and  25 , so the actuators  11  and  21  contract vertically. The PE stacks  15  and  25  will contract in length in response to a reversal in the polarity of the applied voltage. This causes both sides of the steel frames  13  and  23  to be displaced vertically away from the respective PE stacks  15  and  25 , so the actuators  11  and  21  expand vertically. The reciprocal displacements described above are also referred to herein as frame displacement strokes. Frame displacement strokes occur along a displacement axis, such as shown generally at  17  and  27  in  FIGS. 1 and 2 . The PE actuators  11  and  21  are essentially “solid state” devices, with no internal moving parts. 
     Piezo-electric ceramic materials are inherently dense, so even compactly sized PE actuators such as  11  and  21  can deliver a relatively strong actuation force. Piezo-electric actuators are commercially available in a wide variety of actuation force (load) and displacement stroke ranges. As set forth below, PE actuators such as  11  and  21  are conventionally used to perform various functions in a variety of applications:
         Optics &amp; Vision Applications—Positioning of mirrors or lenses, Micro-scanning, Dithering, Focusing, Laser cavity tuning, Alignment or deformation of fibers, Deformation of FBG, Scanners, Choppers, Interferometers, Modulators, PDP glass cutting;   Mechanics Applications—Positioning of tools, Pick &amp; Place, Clamps, Active Wedges, Damping, Active control, Generation of ultrasonic or sonic vibrations, Health monitoring;   Fluids Applications—Proportional valves, Pumps, Measuring, Injections, Ink jet, Droplet generators, Flow mass meter;   Electronics Applications—Positioning of masks, wafers or magnetic heads, Non-magnetic actuation, Circuit breakers, Chip testing;   Air &amp; Space Applications—Active flaps, Shape control, Active wing; and   Electrical Energy Applications—Piezoelectric generator, Energy harvesting, Electric switch.       

     According to exemplary embodiments of the invention, the operation of PE actuators, such as described above with respect  FIGS. 1 and 2 , is used to simulate vibration conditions. Repeated voltage polarity reversals will produce correspondingly repeated reciprocal displacements of the actuator frame (e.g.,  13  or  23  in  FIG. 2 ). If the PE actuator is suitably mounted to a target object, then the target object will experience vibration corresponding to the reciprocal displacements of the actuator frame. 
       FIG. 3  diagrammatically illustrates exemplary embodiments of a vibration simulation system according to the invention. In the system  31  of  FIG. 3 , a vibration control system (VCS)  32  provides one or more voltage signals at  33 . An amplifier arrangement  34  receives the one or more voltage signals at  33  and produces therefrom one or more respectively corresponding and suitably amplified drive voltage signals  35 . The drive voltage signaling at  35  drives a PE actuator arrangement  36  mounted to a target object (also referred to interchangeably herein as a “test unit”)  37 . In response to the drive voltage signaling at  35 , the frames of the PE actuators at  36  exhibit reciprocal displacement as described above with respect to  FIGS. 1 and 2 , thereby imparting vibration to the target object  37 . The voltage signaling  33  and corresponding drive voltage signaling  35  are produced, respectively by the VCS  32  and the amplifier arrangement  34 , such that the target object  37  will experience a vibration condition in accordance with a desired vibration profile. In some embodiments, the PE actuator arrangement  36  consists of a single PE actuator driven by a single drive voltage signal at  35 . In various embodiments, each voltage signal  33  from the VCS  32  may provide input to one or more amplifiers at  34 , and each amplifier may in turn drive one or more PE actuators at  36 . 
     Piezo-electric actuators, such as shown in  FIGS. 1 and 2 , are significantly smaller than the conventional mechanical shakers mentioned above. This provides for great flexibility in the design of the PE actuator arrangement  36 . For example, the PE actuator arrangement  36  provides scalability, because the number of constituent PE actuators therein can be easily increased as the size of the target object  37  increases. Also, each of the constituent PE actuators of the arrangement  36  can be mounted relatively easily in any desired physical orientation, so that the displacement axis of any PE actuator can be oriented as desired. In various embodiments, groups of two or more PE actuators are mounted together in adjoining relationship in a chain-like configuration with their respective displacement axes aligned. This chain-like configuration produces a composite actuator whose frame displacement stroke is some desired multiple of the nominal frame displacement stroke of a single PE actuator. Thus, various embodiments employ any desired number of actuators and/or composite actuators, each having any desired frame displacement stroke, and each having its displacement axis oriented as desired. 
     In some embodiments, shown by broken line in  FIG. 3 , a sensor arrangement  38  including one or more sensors is mounted to the target object  37  to provide one or more feedback signals  39  to the VCS  32 . These feedback signals  39  are indicative of the vibration condition(s) at one or more locations on the target object  37 . The VCS  32  adjusts the voltage signaling at  33  based on the feedback signals  39  and the desired vibration profile. In some embodiments, the sensors are accelerometers. 
       FIG. 4  diagrammatically illustrates exemplary embodiments of a payload assembly  41  according to the invention. The payload assembly  41  includes the PE actuator arrangement  36 , assembled on a mounting structure  43  that mounts the PE actuator arrangement  36  to the target object  37 . The payload assembly  41  includes another mounting structure  42  that is connected to the PE actuator arrangement  36  opposite the mounting structure  43 . The mounting structure  42  mounts to a base platform  44 . The base platform  44  provides a reaction mass that permits the PE actuator arrangement  36  to impart vibration to the target object  37 . The amplifier arrangement  34  is mounted on the base platform  44  in some embodiments. In feedback control embodiments, such as shown by broken line in  FIG. 3 , the payload assembly  41  also includes the sensor arrangement  38 , mounted to the target object  37 . In some embodiments, the sensor arrangement  38  is mounted to the PE actuator mounting structure  43 . In some embodiments, part of the sensor arrangement  38  is mounted on the target object  37 , and part is mounted on the PE actuator mounting structure  43 . 
       FIG. 5  illustrates the payload assembly of  FIG. 4  in more detail according to exemplary embodiments of the invention. In the payload assembly  41 A of  FIG. 5 , the mounting structures  42 A and  43 A are rigid plates. In some embodiments, the plates  42 A and  43 A have provided therein respective patterns of holes arranged to provide a plurality of possible PE actuator mounting sites, thereby providing flexibility in the positioning of the PE actuator arrangement  36 A relative to the test unit  37 A. The PE actuators can be interposed between and bolted (or fastened in any other conventional manner) to the mounting plates  42 A and  43 A at any position where a hole in mounting plate  42 A is aligned with a hole in mounting plate  43 A. In some embodiments, each feedback sensor (not explicitly shown in  FIG. 5 ) is mounted in proximity to a respectively corresponding PE actuator or a respectively corresponding grouping of PE actuators. In some embodiments, accelerometer sensors are readily mounted where desired using, for example, conventional adhesion techniques. 
       FIG. 6  illustrates the payload assembly of  FIG. 4  in more detail according to further exemplary embodiments of the invention. In the payload assembly  41 B of  FIG. 6 , the test unit  37 B is connected to a mounting ring that includes a plurality of flexible legs  62 . These flexible legs extend from the test unit  37 B and terminate in a mounting flange  63  to which the PE actuator mounting structure  43 B is fastened. In the payload assembly  41 B, both of the mounting structures  42 B and  43 B are provided in the form of mounting plates. In the same fashion described above with respect to  FIG. 5 , the mounting plates  42 B and  43 B of  FIG. 6  can be provided with respective hole patterns arranged to provide a plurality of possible PE actuator mounting sites, thereby providing flexibility in positioning the PE actuator arrangement relative to the test unit  37 B. In some embodiments, the mounting plate  42 B has an annular structure with a central opening (not explicitly shown in  FIG. 6 ). In various embodiments, feedback sensors such as accelerometers are mounted on some or all of the flexible legs  62 . 
     In the examples shown in  FIGS. 5 and 6 , the PE actuators are positioned peripherally around the respective test units in a generally symmetrical arrangement. In some embodiments, the PE actuators are positioned as necessary to produce vibration conditions in accordance with predetermined vibration profiles. 
     Referring again to  FIG. 4 , in some embodiments, the base platform  44  is a movable platform, which permits the target object (test unit) to be subjected to both vibration and acceleration conditions simultaneously. This is useful, for example, in the simulation of aircraft flight conditions, including launch conditions and atmospheric re-entry conditions experienced by a spacecraft. The payload assembly embodiments  41 A of  FIG. 5  are useful, for example, in the simulation of spacecraft launch conditions, and the payload assembly embodiments  41 B of  FIG. 6  are useful, for example, in the simulation of spacecraft re-entry conditions. 
       FIG. 7  diagrammatically illustrates exemplary embodiments of a system for simulating both vibration and acceleration conditions simultaneously according to the invention. In  FIG. 7 , the payload assembly  41  is mounted on a movable base platform that is designated as  44 A, and is embodied as a centrifuge arm supported for rotation about an axis  71 . Signals are routed between the VCS  32  and the payload assembly  41  in conventional fashion via a slip ring  72  disposed in alignment with the axis  71 . 
       FIG. 8  illustrates the system of  FIG. 7  in more detail according to exemplary embodiments of the invention. In particular,  FIG. 8  shows the payload assembly  41 B of  FIG. 6  mounted on the base platform  44 A, embodied as a centrifuge arm assembly.  FIG. 8  also illustrates additional features of the centrifuge arm assembly. In  FIG. 8 , the centrifuge arm assembly  44 A includes a cylinder  80  mounted on a mounting plate  81  that is carried at the radially outer end of the centrifuge arm assembly  44 A by two parallel centrifuge arm portions  85 . The payload assembly  41 B is received in the cylinder  80 , with the mounting plate  42 B bolted (or otherwise fastened) onto a radially inwardly facing surface on the radially inner end of the cylinder  80 . 
     In some circumstances, the combination of the weight of the test unit and the acceleration that the centrifuge imparts to the test unit can produce an actuator overload condition wherein the load limit of the PE actuators is exceeded. For example, at 100 g&#39;s of steady acceleration, even a 7 lb. test unit can produce the actuator overload condition. The heavier the test unit, the lower the acceleration required to produce the actuator overload condition. 
     Exemplary embodiments of the invention can avoid the aforementioned actuator overload condition by employing an accelerated counterweight that compensates for the centrifugal load of the test unit. As shown in  FIG. 8 , some embodiments provide a counterweight  82  that is attached to one end of a strap  83  whose other end is attached to the payload assembly  41 B. The strap  83  extends radially inwardly from the payload assembly  41 B to a reaction beam  84  that is mounted on and extends between the parallel centrifuge arm portions  85 . The strap  83  is looped over a sheave or pulley  87  that is mounted on a radially outwardly facing surface  88  of the reaction beam  84 , such that a center axis of the sheave  87  is oriented generally parallel to the reaction beam  84 . The strap  83  loops over the sheave  87  and passes downwardly through a space between the surface  88  and the sheave  87 . When the centrifuge arm  44 A is at rest (this condition is not explicitly shown in  FIG. 8 ), the counterweight  82  hangs on the strap  83  straight downwardly below the aforementioned space between the surface  88  and the sheave  87 . When the centrifuge arm assembly is in motion, centrifugal force causes the counterweight  82  to swing radially outwardly. At high enough g levels, the counterweight  82  will ultimately assume the fully radially extended position illustrated in  FIG. 8 . In some embodiments, the mass of the counterweight  82  is set such that the centrifugal force on the strap  83  directly cancels the centrifugal load of the test unit  37 B on the PE actuators. In this way, the counterbalancing force varies automatically with the g level of the centrifuge. 
     The weight of counterweight  82  is determined in some embodiments by multiplying the weight of the test unit  37 B by the ratio of the radial distances of the test unit and the counterweight from the rotational axis of the centrifuge. For example, if the counterweight  82 , in its fully radially extended positioned as shown in  FIG. 8 , is located 24 feet from the rotational axis, and if the test unit  37 B is located 28 feet from the rotational axis, and if the test unit  37 B has a weight of 10 lbs., then the counterweight  82  should have a weight of 10×28/24=11.7 lbs. 
     In some embodiments, the strap  83  is a nylon cable or rope having formed at each end thereof loops that engage respective eyebolts (not explicitly shown) mounted on the counterweight  82  and on the mounting plate  43 B of the payload assembly  41 B (see also  FIG. 6 ). To reach the mounting plate  43 B, the strap  83  passes through the central opening  89  in the annular mounting plate  42 B, and also passes through the PE actuator arrangement  36  mounted circumferentially around the opening  89  (see also  FIG. 6 ). In some embodiments, the counterweight  82  is implemented by a base plate on which any desired combination of weights can be stacked in generally the same manner as exercise weights are placed on a barbell. In some embodiments, the strap  83  is rigged with a load sensor  86  positioned between the counterweight  82  and the payload assembly  41 B, in the load path of the strap  83 , as shown in  FIG. 8 . The load sensor  86  can be used to verify that the load canceling technique is working properly and the PE actuators are not being overloaded. 
     Referring again to  FIG. 4 , some embodiments of the invention are configured to spin (rotate) the payload assembly  41  relative to the base platform  44  in order to subject the target object  37  to spin testing.  FIG. 9  diagrammatically illustrates examples of such embodiments in more detail. A spin drive  91  (e.g., a suitable rotary drive motor) is rotationally coupled to one end of a drive shaft  93  by a shaft coupler  92 . The drive shaft  93  is supported for rotation by a bearing housing  94  that includes radial bearings to fix the drive shaft  93  radially, and thrust bearings to fix the drive shaft  93  axially. The end of the drive shaft  93  opposite the shaft coupler  92  is fixedly mounted (e.g., bolted) to the mounting plate  42 B of the payload assembly  41 B. In some embodiments, the spin drive  91  is mounted on the bearing housing  94  by mounting legs  95 . 
     In the arrangement of  FIG. 9 , the exterior of the bearing housing  94  is configured as a plate or lid, for mounting on a cylinder  80 A in generally the same fashion that the mounting plate  42 B is mounted on the cylinder  80  of  FIG. 8 . The entire payload assembly  41 B is thus received within the cylinder  80 A, as shown in  FIG. 9 . 
     In some embodiments, signals are routed between the VCS  32  and the PE actuator arrangement  36 B in conventional fashion via a slip ring  96  positioned on the drive shaft  93 . In some embodiments, the target object is subjected to vibration, acceleration, and spin simultaneously, so both the slip ring  96  and the slip ring  72  (see also  FIG. 7 ) are utilized to route signals between the VCS  32  and the payload assembly  41 B. In some embodiments, the target object is subjected to spin and acceleration simultaneously, but without vibration, so the PE actuator arrangement  36 B can be omitted. In some embodiments, the target object is subjected to vibration and spin simultaneously, but without acceleration, so the cylinder  80 A does not move relative to the VCS  32  (as does the cylinder  80  of  FIG. 8 ), and the slip ring  72  is therefore not required. In embodiments wherein the target object is subjected to both spin and acceleration simultaneously, operating power is routed to the spin drive via the slip ring  72  (as shown in  FIG. 9 ). In embodiments where the aforementioned centrifugal load counterbalancing is used, the spin drive  91  can be suitably fitted with an exteriorly accessible eyebolt (not shown) to engage the counterweight strap  83  of  FIG. 8 . 
     In some embodiments, the drive shaft  93  weighs at least about 10 times the weight of the test unit  37 B. In this way, the drive shaft  93  provides a low noise reaction mass that reduces undesired vibrational noise that can be produced by operation of the bearings in the bearing housing  94 . 
     Referring again to  FIGS. 1-3 , the steel frames  13  and  23  preload the PE stacks  15  and  25 . However, PE actuators of the type shown in  FIGS. 1 and 2  often have an operating voltage range that is not symmetric about zero volts. In such cases, the amplifiers used at  34  (see also  FIG. 3 ) can be chosen to include a conventional DC offset feature. With this DC offset feature, the output of the amplifier can be adjusted to have a DC offset that “centers” the PE actuators, thereby permitting the VCS  32  to control the PE actuators, via the amplifiers, using a VCS operating voltage range that is symmetric about zero volts. For example, for a conventional PE actuator having an operating voltage range from −20V to +150V, the amplifiers can be adjusted to a DC offset of 65V. With this DC offset, and assuming an amplification factor of 20 for the amplifiers (as is common in the art), a voltage range of −4V to +4V at  33  will produce a corresponding range of −15V to +145V at  35 . If the amplifiers can provide up to 2.4 A of drive current (as is common in the art), the capacitive load of a PE actuator can be quickly driven for full stroke displacement about the mean position of the actuator. In some embodiments, the displacement of the PE actuators covers about 47.5 microns in each direction, for a full stroke of about 95 microns. 
     Still referring to  FIG. 3 , in some embodiments, the VCS  32  has a plurality of inputs available for sensor feedback signals  39  from several types of conventional sensors, including, for example, accelerometers that include integrated electronics. The VCS  32  digitizes feedback signals received from sensors, performs spectral content calculations in real time, and updates the voltage signals  33  as necessary to achieve a the overall vibration condition (profile) that is desired. In some embodiments, the VCS  32  has a plurality of outputs available to provide a plurality of voltage signals  33  that can each range from −10V to +10V. 
     In various embodiments of the invention, the VCS  32  includes modular control software with a variety of functionalities. For example, individual software modules can be used to control swept sine, random, shock, and mixed-mode testing, and software modules for signal analysis and transient capture can be provided. In any of the above-described embodiments that implement vibration-only testing, combined vibration and acceleration testing, combined vibration and spin testing, and combined vibration, acceleration, and spin testing, the VCS  32  can implement any of the following types of vibration test control: discrete open loop control; single input, single output (SISO) closed loop control; or multiple input, multiple output (MIMO) closed loop control. 
     In some discrete open loop control embodiments, the VCS  32  uses a single voltage signal at  33  to drive one or more PE actuators at  36  in parallel, via respectively corresponding amplifiers at  34 . In various discrete open loop embodiments, sinusoidal voltage signals with frequencies that range from about 1 Hz to about 3000 Hz are used. Various discrete open loop control embodiments use either of the test items  37 A and  37 B. 
     In some SISO closed loop control embodiments, the VCS  32  operates in the random or swept sine mode, and uses a single voltage signal at  33  to drive a plurality of PE actuators at  36  in parallel, via respectively corresponding amplifiers at  34 . A single sensor (e.g., accelerometer) is used at  38  to provide feedback information indicative of measured vibration. In some embodiments, the single sensor is positioned generally centrally with respect to the PE actuators. Various SISO closed loop embodiments use either of the test items  37 A and  37 B. 
     In MIMO closed loop control embodiments, the VCS  32  employs spectral density matrices to control: the power spectral density (PSD) magnitude associated with each of a plurality of drive control channels (where a drive control channel refers to an amplifier/PE actuator combination); the phase relationship between any two control channels; and coherence between any two control channels. These VCS capabilities are generally known in conventional vibration simulation systems that vibrate test items by mechanical shakers with multiple drives. However, the drive control channels in conventional systems use mechanical shakers instead of PE actuators. In such conventional operation: each of the drive control channels has associated therewith a respectively corresponding one of a plurality of accelerometer feedback channels; each drive voltage signal from the VCS has an affect on every drive control channel; and an undesired resonant response associated with a given drive control channel can be reduced by increasing the magnitude of an out-of-phase drive voltage on another drive control channel at the resonant frequency. 
     In some MIMO closed loop control embodiments of the invention, each of a plurality of pairs of drive control channels is driven in parallel by a respectively corresponding one of a plurality of voltage signals produced at  33  by the VCS  32 , and each pair of drive control channels has associated therewith a respectively corresponding one of a plurality of sensor feedback channels. Various ones of these MIMO closed loop control embodiments use three pairs of drive control channels, three voltage signals  33 , three sensor feedback channels  39 , and either of the test items  37 A and  37 B. 
     In some MIMO closed loop control embodiments, each of a plurality of drive control channels is driven by a respectively corresponding one of a plurality of voltage signals produced at  33  by the VCS  32 , and each of the drive control channels has associated therewith a respectively corresponding one of a plurality of sensor feedback channels. Various ones of these MIMO closed loop control embodiments use six drive control channels, six voltage signals  33 , six sensor feedback channels  39 , and either of the test items  37 A and  37 B. 
     MIMO closed loop control is useful, for example, in situations where natural frequency modes of the payload assembly tend to cause discrepancies between the desired vibration profile and the vibration condition that would be produced by the PE actuator arrangement without MIMO closed loop control. As one example, without MIMO closed loop control, the flexible legs  62  of the payload assembly  41 B ( FIG. 6 ) can contribute to undesired frequency modes. MIMO closed loop control can also be useful, for example, in compensating for unbalanced rotation of the drive shaft  93  in  FIG. 9 . Such unbalanced rotation can result in undesired rotational noise. However, the noise typically occurs at constant frequencies, so MIMO closed loop control can be effective in eliminating the noise. 
     In any or all of the above-described embodiments, the VCS  32  can be implemented, for example, by a conventional Spectral Dynamics Jaguar vibration control system. 
     Although exemplary embodiments of the invention have been described above in detail, this does not limit the scope of the invention, which can be practiced in a variety of embodiments.