Patent Publication Number: US-2016238501-A1

Title: Falling weight deflectometer

Description:
The present invention relates to falling weight deflectometers. 
     A falling weight deflectometer (FWD) is a non-destructive testing device used by civil engineers to evaluate the physical properties of pavement structures. Data from the FWD is primarily used to estimate the load carrying capacity, of inter alia pavements comprising Portland cement concrete (PCC) or asphalt concrete (AC) surfaces. Use includes roads, airport pavements, and railway tracks. The falling weight deflectometer is typically integrated in a trailer that can be towed to a test location by another vehicle, but can also be integrated in the vehicle itself. 
     The falling weight deflectometer is designed to impart a load pulse to the pavement surface which simulates the load produced by a rolling vehicle wheel. The load is produced by dropping a large weight on a set of rubber buffers. The resulting force is transmitted to the pavement through a circular load plate. A load cell or other load sensing transducer is mounted atop the load plate and measures the applied load on the pavement surface. A series of deflection sensors such as geophones are mounted linearly along a beam extending from the centre of the load plate and measure the resulting pavement surface deflection in response to the load at a variety of distances from the centroid of the applied load. The FWD data may be used to calculate stiffness-related parameters, e.g. Young&#39;s modulus of the layers of a multi-layered pavement structure, such as the pavements of roads or airports comprising PCC or AC. 
     In prior art FWDs the weight is typically raised hydraulically along a vertical guide rod or column and held at a predetermined height by a catch mechanism until measurement is to be performed and a drop command is issued by the FWD operator. The catch mechanism is then released and the weight drops onto a force transmitting means, which includes a circular load plate of known diameter, which prior to the dropping of the weight has been, lowered to make contact with the pavement surface at the desired measurement location. Between the force transmitting means and the weight a damping means, e.g. buffers in the form of rubber blocks, is provided in order to transmit an ideally half-sinusoidal impact force to the pavement. The force transmitting means normally comprises a force sensing means so as to measure the actual impact force. Since the drop-weight mass is substantial, typically between 50 and 750 kg, the drop-weight is normally picked up and lifted back to the latched position by a hydraulic lifting mechanism or the like. Prior art FWDs as described above are e.g. known from JP-A-2007-205955 and DE-U-9305327. Examples of prior art FWDs are also described in the article “Faldloddets historie”, Bohn, Axel O., Asfalt, pages 4-11, Sep. 1989, ISSN 0004-4318, describing the development of the falling weight deflectometer from 1964 to 1989. 
     One major drawback with such prior art FWDs is that a measurement cycle takes relatively long time, in particular if successive measurements are to be made at the same location. A major reason for this is that the process of the hydraulic lifting means being lowered to pick up the weight, raising the weight to the latch, and disengaging weight from the hydraulic lifting means is relatively slow. Moreover, after a few repetitive drops at one location, the load plate must be raised to the transport position and secured, and lowered again at the next measurement position. In the prior art the latter, is a major contributor to the long test cycles. When thousands of consecutive measurements are to be made along a road to be surveyed, accumulated durations of the processes reduce the efficiency of the overall survey. 
     On this background it is the object of the present invention to provide a falling weight deflectometer, which allows consecutive measurements to be made at shorter intervals and thereby enhance the testing rate and the overall productivity of pavement deflection measurements. 
     According to a first aspect of the present invention this object is achieved by a falling weight deflectometer comprising a load plate adapted to engage a test surface, a force transmission means adapted to transmit a force to the load plate, a buffer means, a weight adapted to impact said force transmission means via said buffer means so as to provide a force to be transmitted to said load plate via said force transmission means, a guide means for guiding said weight towards said force transmission means, and a lifting means for lifting said weight to a predetermined height above said force transmission means, characterized in that said lifting means comprises an electric motor driving a threaded spindle in engagement with the weight. 
     By using a threaded spindle in engagement with the weight, the lifting mechanism is integrated with the falling weight, making it possible to raise the weight back to the top position immediately after it has been dropped to impact. In particular, because the lifting mechanism is integrated with the falling weight, separate action of attaching it to a lifting mechanism is necessary in order to lift it back to the top position. Moreover the use of the threaded spindle allows the load plate and force transmitting means to be quickly and efficiently moved to a raised transport position. 
     This is in particular the case when, according to a preferred embodiment, the threaded spindle is in permanent engagement with the weight. In that case, the return motion, when the weight bounces back upon impact due to the buffer means and other elasticity in the overall system, may simply be continued by controlling the motor suitably. 
     According to another preferred embodiment, the threaded spindle is in permanent engagement with the electric motor. With the threaded spindle in permanent engagement with both the weight and the electric motor no couplings or the like need to engage, and the return motion may simply be directly continued by controlling the motor suitably. Even though this involves the falling weight forcing the electric motor to rotate and work as a generator, experience has shown that this does not impede the fall of the weight, and that a sufficiently forceful impact may still be achieved. Integrating all three parts, i.e. electric motor, threaded spindle, and falling weight, in one kinematic train substantially simplifies the construction of the falling weight deflectometer. 
     According to a further preferred embodiment, the threaded spindle is directly driven by the electric motor. Modern permanent magnet motors are fully capable of providing sufficient torque directly on the spindle to lift the weight, even if the threaded spindle has a high lead angle. This further simplifies the construction of the falling weight deflectometer. 
     In this respect, the lead angle is according to a further preferred embodiment so high that the threaded spindle is in a non self-locking engagement with the weight. Having a high lead angle is advantageous as it allows the falling weight to fall almost freely, i.e. with very little resistance, thus providing an adequate impact force. Thus, according to yet another preferred embodiment, the threaded spindle has a lead angle exceeding 40°, preferably exceeding 50°. This allows the weight to fall without being influenced too much by losses caused by rotating the electric motor which at this point acts as a generator. The electric motor has been carefully chosen so as to not generate too much counter electromotive force, i.e. less than 600 V in order not to risk damage of the control electronics. 
     According to another preferred embodiment, the engagement between the threaded spindle and the weight is selected among leadscrew, roller screw, or ball screw engagement. This provides low friction, and the falling of the weight is thus even less impeded. 
     According to a further preferred embodiment, falling weight deflectometer further comprises a motor controller for the electric motor, said motor controller being adapted to control the speed of the falling weight. By having a motor controller adapted to control the speed of the falling weight it becomes possible to influence the falling weight in numerous advantageous ways, inter alia allowing more controlled impacts as compared to the simple drop of the prior art. 
     According to yet a further preferred embodiment, falling weight deflectometer further comprises a motor controller for the electric motor, said motor controller being adapted to control the position of the falling weight. By having a motor controller adapted to control the position of the falling weight it becomes possible to influence the falling weight in numerous advantageous ways, inter alia allowing more controlled impacts as compared to the simple drop of the prior art. 
     According to a further preferred embodiment, the falling weight deflectometer comprises a frame and means for securing the drop weight with respect to the frame. Securing the drop weight allows the use of the threaded spindle to lift and lower the force transmission means, more specifically lowering it into engagement with the test surface an lifting it into a transport position by means the electric motor. 
    
    
     
       The present invention will now be described in greater detail with reference to the drawings on which: 
         FIG. 1  shows a schematic perspective view of the core parts of a falling weight deflectometer according to the invention, 
         FIG. 2  a schematic perspective view of a different embodiment of the falling weight deflectometer including parts of the frame, 
         FIG. 3  shows a schematic perspective view of the falling weight deflectometer of  FIG. 1 , 
         FIG. 4  shows a schematic diagram of elevation versus time for a drop and lift sequence in the prior art, 
         FIG. 5  shows a schematic diagram of elevation versus time for a drop and lift sequence according to the present invention, and 
         FIGS. 6 a -6 f    schematically show the lowering of the FWD load plate onto the surface and a subsequent drop of the weight. 
     
    
    
     Turning first to  FIG. 1 , a perspective view of core parts of the falling weight deflectometer  1  according to the invention is shown. For mobility in practical use the falling weight deflectometer  1  is mounted on a suitable carriage such as a trailer or a vehicle, known per se and shown schematically in  FIGS. 6 a   - 6   f.  Cf. also the various illustrations in the above-mentioned article “Faldloddets historie” incorporated herein by reference. 
     The falling weight deflectometer  1  comprises a drop weight  8  adapted to be dropped onto and impacting a loading transfer plate  6 , from which the impact is transferred to a test surface  30  (shown in  FIGS. 6 a  to 6 f    only), such as a pavement of e.g. Portland cement concrete or asphalt concrete, via force transmission means comprising inter alia a support column  5 , and a load plate  2  adapted to engage the test surface  30 . 
     The load plate  2  is preferably circular and may, as can be seen from  FIG. 1 , be segmented. In the preferred embodiment there are four segments of which three segments  2   a,    2   b,    2   c  are visible. Evidently, any suitable number of segments including one, i.e. a single circular load plate  2 , may be used. As best seen in  FIG. 3 , the segments  2   a,    2   b,    2   c  of the load plate  2  are connected via spacers  3  to an intermediate plate  4 , allowing each of the segments  2   a ,  2   b,    2   c  to have a small degree of freedom in order to allow the overall load plate  2  to accommodate irregularities the test surface  30 , even if the test surface  30  is not entirely plane or horizontal. The intermediate plate  4  preferably also has some degree of freedom with respect to the support column  5  in order to adapt to inclinations in the test surface  30 . 
     The intermediate plate  4  is connected to loading transfer plate  6  via further support means  21 ,  22 ,  23  forming part of the support column  5 . The further support means also includes a force transducer  7 , such as a load cell, adapted to measure the forces in the impact of the falling drop weight  8  onto the loading transfer plate  6 . 
     The drop weight  8  comprises a base plate  9  onto which a number of individual removable weights  10  may be placed in order to achieve a desired amount of weight for a test to be performed. The weight of the each of the removable weights  10  are preferably adapted to manual handling, and the drop weight  8  may thus comprise a substantial number of removable weights  10  stacked onto each other. In order to secure the stacks of removable weights  10  on top of the base plate  9 , securing means  11 ,  12  are also provided. For the manual handling the removable weights  10  are preferably provided with carrier handles  13 . 
     Below the base plate  9  a number of elastic buffers  19  are provided in order to cushion the impact and obtain a suitable sinusoidal half-wave in the impact. The elastic buffers  19  are preferably interchangeable, allowing their number, their size, elastic properties etc. to be varied in accordance with a given desired amount of weight of the drop weight  8 , and pulse width of the sinusoidal half-wave. 
     At two sides of the base plate  9  linear bearings  20 , e.g. comprising ball bearings, are provided in order to allow smooth sliding motion of the base plate  9  carrying the weights  10  along two vertical columns  14  secured to the loading transfer plate  6 , thus guiding the up and down motions of the drop weight  8  when it is dropped or lifted. 
     As for the lifting, the present invention employs an electric motor  15 , mounted on a horizontal cross-beam  16  at the upper end of the two vertical columns  14 . The electric motor  15  is preferably connected directly to a threaded spindle  17 . The threaded spindle  17 , in turn, is in a permanent engagement with a corresponding means in the base plate  9 . This corresponding means is adapted to provide low friction, and may e.g. be a leadscrew, roller screw, or ball screw engagement. Low friction is important as the engagement between the threaded spindle  17  and the base plate  9  should not be self-locking. For the same reason the lead of the thread  18  of the spindle  17  is high, e.g. having lead angle exceeding 40°, preferably exceeding 50°. 
     Having such a high lead and low friction allows the drop weight  8  to be dropped in almost free fall. Evidently, there will be friction and some braking effect from having to turn the electric motor  15 , but this may either be neglected or compensated for by supplying the electric motor  15  with sufficient current to counteract the braking effect. 
     Conversely, the lead of the thread  18  of the threaded spindle  17  should not be so high that the maximum torque that the electric motor  15  may provide is insufficient to lift the drop weight  8 . 
     The preferred motor is preferably a permanent magnet torque motor such as an ETEL TMB Torque Motor, available from ETEL S. A., Zone Industrielle, CH—2112 Môtiers, Switzerland. Such motors allow very high torque which, in turn, allows the use of a direct drive of a spindle with a very high lead. Moreover, such motors provide excellent positional control over the spindle  17 , in turn allowing the impact to be controlled very accurately, e.g. 
     when using a robotic controller, robotic controllers typically providing both a torque control mode and a position control mode. Evidently, an electric motor  15  providing lower torque could be used in connection with a gear mechanism, but in order to keep the movable parts to a minimum and the mechanism simple, a direct drive is preferred. 
     With this drive arrangement it becomes much faster to perform the measurements, because consecutive impacts at the same location may be made in more rapid succession. 
     Turning now to  FIG. 4 , a diagram of a drop sequence with the prior art falling weight deflectometer is shown. For easy comparison with  FIG. 5  later the sequence starts with the drop weight lifted to and held in a predetermined height h 1  from which the drop weight is to be dropped, e.g. 40 cm for a 750 kg drop weight. When the drop weight is released it will fall in virtually free fall, impact the loading transfer plate, and compress the elastic buffers at height h 0  at time of impact t i . Due to the elastic properties of mainly the buffers the drop weight will bounce back and repeatedly impact the loading transfer plate at times t b1 , t b2  etc. until it comes to rest on the load transfer plate. Having come to rest the drop weight is manually attached to the lifting means again, and at t l  the hydraulic lifting process may start again. It should be noted that  FIG. 4  is schematic and that the duration between t b2  and t l  is in fact several seconds as to compared to the approximately 0.285 seconds involved in the free fall from 40 cm. 
     In  FIG. 5  a drop using the present invention is illustrated in a diagram similar to that of  FIG. 4 . The drop weight  8  is dropped from the predetermined height h 1 , falls in a virtually free fall, and impacts the loading transfer plate  6 , at t i . Since the free fall corresponds to that of the prior art FWDs, the fall is represented by similar parabolas in the time vs. elevation diagrams of  FIG. 4  and  FIG. 5 . As compared to the prior, the free fall is, however, not entirely free because base plate  9  of the drop weight  8  is in permanent engagement with the threaded spindle  17 . When dropped, the drop weight  8  will therefore inevitably rotate the electric motor  15 , which even with open electrical supply terminals will tend to brake the fall, i.e. resist the spindle rotation caused by the falling drop weight  8 . This can be compensated for by supplying the motor with sufficient current to turn the motor  15  and the threaded spindle  17  in accordance with the free fall. This is quite easily done due to the ability to accurately control the motor positioning or the torque. The theoretical position of the drop weight  8  through a free fall is easily calculated, and converted to corresponding positions of the threaded spindle  17 , and, in turn, the motor  15 . In order to achieve a truly free fall it would evidently also be possible to incorporate a clutch mechanism between the drop weight  8  and the motor  15 , but for the lifting purposes it is preferred to have a permanent engagement. Alternatively the motor could be set to provide zero torque. 
     However, in order to avoid any influence of the motor controller on the impact, the motor controller is preferably disabled at a predetermined height h 2  before impact, i.e. at t i -Δt, so that the motor basically has open supply terminals at impact. This short time Δt, where the braking is not counteracted can be considered negligible for the impact. In any case the actual force of the impact is measured using the force transducer  7 . 
     Upon impact the drop weight  8  bounces back due to elastic properties inter alia in the buffers  19 . Since the threaded spindle  17  is in permanent engagement with the base plate  9  of the drop weight  8 , it is not necessary to wait until the drop weight  8  settles on the loading transfer plate  6  in order to reattach a lifting means. Instead, all it takes to lift the drop weight  8  back to the drop height h 1  is to start the supply of the electric motor  15  at a suitable lift time t l , e.g. during the first bounce back of the drop weight  8 . The duration of the lifting itself is very short, and within a few seconds at the time t n  the drop weight  8  is back at height h 1  for the next drop. This is long before the next drop would be possible with the prior art hydraulic lifting mechanism. 
     In  FIG. 2  a falling weight deflectometer according to the invention is mounted on a frame  24 . Though not shown in  FIG. 2 , the frame may be mounted on or form an integral part of a carriage with wheels  25  as schematically illustrated in  FIGS. 6 a -6 f   , such as a trailer or a self-propelled vehicle. On the carriage  24 , first securing means  26  and second securing means  27  are provided. The first securing means  26  comprises an arm which may at least partially be swung below the loading transfer plate  6 , allowing the loading transfer plate  6  to be secured thereon, and thus on the frame, by its own weight. The swinging motion is preferably performed by an electric actuator  28  acting on a common shaft  29  on which the arms are mounted. As can be seen from  FIG. 2  the first securing means  26  preferably comprises two shafts  28  each common to a pair of arms, so as to support the loading transfer plate  6  at four corners. The actuators of course need not be electrical but could be hydraulic or driven by other suitable means. Instead of simply supporting the weight of the loading transfer plate  8 , the securing means  26  could of course also engage the loading plate  6  in a locking manner. 
     The second securing means  27  is in a similar manner adapted to be swung below the base plate  9  of the drop weight  8 . Also here, one or more arms are swung by means of an electric actuator turning a shaft to which the arms are secured. Preferably also for the second securing means a double system is provided, so as engage and secure either side of the drop weight as schematically shown in  FIGS. 6 a  to 6 c   . Likewise, as indicated in  FIGS. 6 a  to 6 c   , the second securing means  27  may engage the drop weight  8  in a locking manner rather than just supporting the weight thereof. 
     As will now be explained, the use of the second securing means  27  allows the use of the electric motor  15  and the threaded spindle  17  for quickly moving the support column  5  with the load plate  2  into engagement with the test surface  30 , and upon completion of a series of measurement at the same location quickly moving the support column  5  back into a secured storage position for transport to the next test location. Typically the transport distance to the next location is very short, and any time saved on transport between locations will substantially benefit the overall time necessary for testing of given stretch of pavement. 
     Turning now to  FIG. 6 a    the falling weight deflectometer  1  is shown in a situation with the drop weight  8  secured by the second securing means  27 . The column  5  lifted free of the test surface  30  and the loading plate is in engagement the buffers  19  of the drop weight  8 . If the electric motor  15  is suitably energized it can hold the spindle in a fixed position and thus carry the weight of the loading plate  6  and the support column  5 . In this situation it is possible to disengage the first securing means  26 . Upon disengagement of the first securing means  26  but with the drop weight  8  still secured the support column  5  forms a travelling nut arrangement with the spindle  17 , and may thus be moved up and down by rotation of the spindle. Simply by energizing the electric motor  15  suitably, the support column  5  may be lowered into engagement with the test surface  30  as illustrated in  FIGS. 6 b  and 6 c   . Once engaging the test surface  30  the support column  5  rests firmly thereon. In this situation, as illustrated in  FIG. 6 d   , the second securing means  25  may be disengaged from the drop weight, the drop weight  8  now being held by the suitably energized electric motor  15 . Currently a robotic controller is preferred. Such a robotic controller provides both a position mode and a torque mode for the control of the motor. During lifting and holding the drop weight  8 , the position mode would be used, whereas the torque mode would be used during the drop sequence. 
     From the situation of  FIG. 6 d    the drop weight  8  may be lifted to the drop position shown in  FIG. 6 e   , i.e. the height h 1  as explained in conjunction with  FIG. 5 , by the electric motor  15  turning the spindle  17 . Upon that the drop is performed and the drop weight impacts on the loading plate  6  as illustrated in  FIG. 6 f    and as explained in conjunction with  FIG. 5  above. When a suitable number of drops have been made at one location, the process may be reversed, i.e. the drop weight  8  positioned at the height illustrated in  FIG. 6 d    where it can be secured as illustrated in  FIG. 6 c   . Again acting as a travelling nut the support column may be lifted from the test surface  30  and secured in a transport position. All of the above may be controlled electrically and therefore renders itself to automation, without other manual intervention than e.g. the pushing of a few buttons. Both the lowering and the lifting of the support column  5  may be performed very quickly, e.g. in the range of ½ to 2 seconds, which is a substantial improvement over the prior art setup time. Apart from the possibility of making impacts in rapid succession, the present invention provides additional benefits. Because a torque motor with suitable control has a very good position in addition to the high torque it can produce, it can provide much more than the lifting action. Basically, it can move the drop weight  8  to any desired position between h 0  and h 1  through controlled acceleration and deceleration and hold it at that desired position. This means that unlike the prior art falling weight deflectometers the falling weight deflectometer according to the present invention is in fact not restricted to free falls. Instead of free fall a higher or lower downward acceleration than the acceleration in free fall may be provided. 
     One use for this could be shaping the impact, if rather than disabling the motor controller just before impact, as suggested above, the motor controller maintains engaged, the good position control allows the motor controller to control the force during an impact. Impacts with the desired half-sinusoidal impact force characteristics but long durations, e.g. up to 10 seconds may thus be achieved. Another use could be removing the load of the carriage of the FWD on the ground in impact. If in the last instances before impact the falling drop weight  8  is accelerated beyond the free fall acceleration, inertia will lift the carriage off the ground, thus allowing influences from the weight of the carriage on the measurements to be avoided. 
     The present invention has now been described based on exemplary embodiments. The skilled person, however, will realise that numerous other embodiments and variations as well as applications of the invention are possible without deviating from the scope of the claims. In particular, he will understand that the construction of the force transmission means, including the column  5  and the intermediate plate  4 , or of the load plate  2  may differ from the examples. Moreover, the skilled person will realize numerous other ways of using the falling weight deflectometer in terms of shaping of the impact and measurements for which it may be used.