Abstract:
An improved elastic motor is disclosed having a constant torque characteristic. In contrast to earlier motors which use springs wound on spools, the present invention utilizes a reel with elastic stretched to its maximum tension. Each unit length of the elastic is allowed to relax back to its normal state while generating work in a process that will continue until the wheel is fully unwound. The present invention will ease the design of mobile devices and the like by providing a power source having a predictable, stable, and constant torque output characteristic.

Description:
SCOPE OF THE INVENTION 
     This invention relates generally to elastic motors, and more specifically, to an improved elastic motor having constant torque characteristics. 
     BACKGROUND OF THE INVENTION 
     Elastic motors have been employed for centuries and have found particular application in model airplanes for over 150 years. 
     Typical prior art elastic motors, such as those used in a model airplane, generally comprise a rubber loop threaded through a hook on a propeller shaft and is further attached to another hook at the tail of the craft. As the motor is wound up it first twists the elastic into a skein, then a row of knots form and spread along the whole length. A third stage occurs when a row of knots forms in the already knotted skein. When this row is complete the rubber is substantially stretched to its limit. 
     Upon release of propeller, there is a burst of power. When this is spent, a period of slowly declining torque follows for the majority of the motor run and is followed by a decline to zero torque. 
     The torque characteristics of the prior art elastic motor devices, like those described above, make model airplane design a considerable challenge. Clearly, this is because it is difficult for a designer to properly construct design constraints when the source of propulsion has such wavering torque characteristics. Aside from model airplane design, conventional elastic motors have undesirable performance in other applications as well. The knotting of the rubber introduces internal friction in the wound skein that can be eased somewhat by lubrication. Lubrication, however, drastically reduces the useful life of the rubber. 
     Therefore, the shortcomings of the prior art suggest a strong need for an elastic motor design that has a constant torque characteristic and does not damage the elastic material. 
     SUMMARY OF THE INVENTION 
     The present invention is directed towards an elastic motor, more specifically, to an elastic motor having a constant torque characteristic. 
     The motor system of an embodiment of the present invention starts with a reel which is wound with elastic stretched to its maximum tension. Each unit length of the elastic is allowed to relax back to its normal state while generating torque in a process that continues until the reel is fully unwound. The process may be seen as analogous to a steam engine which has a supply of steam at constant pressure. Portions of steam are fed to a cylinder where they expand to generate work (pressure times change in volume) by pushing a cylinder back to turn a wheel. When the expansion is complete the steam is exhausted and the process repeated. In the case of the present invention, a unit length of stretched elastic is connected into a system and allowed to contract to its unstressed state while turning a wheel and developing work. When fully contracted the relaxed elastic is fed to a take up reel and a new unit length of stretched elastic is taken. The process is continued until all the stretched elastic is used up. There is no twisting or knotting of the elastic and no need to lubricate it to prevent binding and wear as in a twisted skein, although some lubrication may assist operation. 
     Hence, it is an object of the invention to provide an improved elastic motor. 
     Furthermore, it is an object of the invention to provide an improved elastic motor having a constant torque characteristic. 
     Further, it is an object of the invention to provide an improved means of powering portable devices. 
     Additionally, it is an object of the invention to provide an elastic motor having increased elastic material life. 
     Further, it is an object of the invention to provide an improved means of driving a dynamo. 
     Furthermore, it is an object of the invention to provide an improved means of powering wind-up toys and devices. 
     These and other objects will become apparent to those skilled in the art upon study of the following drawings and detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 (FIG. 1) depicts an elastic motor typical of the prior art. 
     FIG. 2 (FIG. 2) depicts the torque characteristics of an elastic motor typical of the prior art. 
     FIG. 3A (FIG. 3A) depicts an elastic motor, in accordance with the present invention, in wind up mode. 
     FIG. 3B (FIG. 3B) depicts an elastic motor, in accordance with the present invention, in the start of operating mode. 
     FIG. 3C (FIG. 3C) depicts an elastic motor, in accordance with the present invention, at the end of the first cycle of operation. 
     FIG. 3D (FIG. 3D) depicts the behavior of the elastic material in an elastic motor in accordance with the present invention. 
     FIG. 3E (FIG. 3E) depicts the torque characteristics of an elastic motor in accordance with the present invention. 
     FIG. 3F (FIG. 3F) depicts an intermediate position of the bar when traveling around the periphery of the reel during the operational mode. 
     FIG. 4A (FIG. 4A) depicts an alternate embodiment of an elastic motor in accordance with the present invention. 
     FIG. 4B (FIG. 4B) depicts an alternate embodiment of an elastic motor in accordance with the present invention, in operational mode. 
     FIG. 5A (FIG. 5A) depicts an alternate embodiment of an elastic motor in accordance with the present invention utilizing crossbars in the elastic. 
     FIG. 5B (FIG. 5B) depicts a detail of the elastic used in the device of FIG.  5 A. 
     FIG. 6 (FIG. 6) depicts a simplified two reel arrangement utilizing two rollers. 
     FIG. 7 (FIG. 7) depicts an elastic motor, in accordance with the present invention, being wound by a motor in wind up mode. 
     FIG. 8 (FIG. 8) depicts an elastic motor, in accordance with the present invention, comprising a lock. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 depicts a typical prior art elastic motor in a model airplane. A rubber loop  101  is threaded through a hook  102  on the propeller shaft  103  and is attached to another hook  104  at the tail. As the motor is wound up it first twists into a skein, then a row of knots form and spread along the whole length. A third stage occurs when a row of knots forms in the already knotted skein. When this row is complete the rubber is generally stretched to its limit. 
     Upon release of propeller  105  there is a burst of power, A to B as depicted on the torque characteristic of FIG.  2 . When this is spent, a period of slowly declining torque B to C follows for the majority of the motor run and is followed by a decline to zero torque, points C to D. 
     FIGS. 3A through 3D show a first embodiment of the present invention. FIG. 3A shows an elastic motor in accordance with the present invention in wind up mode. Fully relaxed elastic  301  passes through movable rollers  302  as reel  303  rotates in a clockwise direction  305 . The elastic  304  is wound at constant tension and is fully stretched. 
     In FIG. 3B, an elastic motor in accordance with the present invention at the start of operating mode is depicted. Movable rollers  302  grab the elastic  301  close to the reel  303 . Bar  307  is coupled to the reel  303  and is used to prevent slip of the stretched elastic  304 . The length of the elastic between  307  and  302  is NL where L is a unit length of relaxed elastic. The operation cycle allows this to be relaxed back to its relaxed length L while the tension in the elastic provides torque to turn reel  303  and supply power to a load. Reel  303 , upon release, will begin to rotate in a counterclockwise direction  306 . 
     FIG. 3C depicts an elastic motor in accordance with the present invention at the end of the first operating cycle. Reel  303  is allowed to release, causing counterclockwise rotation  306 . The length between rollers  302  and bar  307  shortens to unit length L as the tension in the elastic  308  decreases until complete relaxation. 
     FIG. 3D depicts an elastic motor in accordance with the present invention that is capable of multiple operating cycles. This is accomplished by introducing an additional bar  309  and disengaging bar  307 . In FIG.  3 D( 1 ), the elastic motor is shown at the end of the first operating cycle, as described above in FIG.  3 C. In FIG.  3 D( 2 ), rollers  302  are first backed up and the elastic  308  is released. Bar  307  remains engaged to maintain secure contact between elastic  308  and reel  303 . Reel  303  continues to rotate in counterclockwise direction  306 . In FIG.  3 D( 3 ) the changeover point between cycles is shown. While the elastic remains released between reels  302 , additional bar  309  is introduced between bar  307  and elastic  308 . Reel  303  is poised to continue rotation in counterclockwise direction  306 . In FIG.  3 D( 4 ) the start of the second cycle is shown. Pinch rollers  307  and  309  are moved to position  302  and new clamping bar  315  is placed in the initial position of  307  as in FIG.  3 B. Clamping rollers  307  and  309  are opened and elastic  308  is released. This cycle is identical to the start of the first operating cycle described in FIG. 3B, with bar  307  replaced with bar  315 . Reel  303  is now ready to continue rotating in counterclockwise direction  306  until bar  315  approaches clamping rollers  307  and  309  in position  302 . At that point, the cycle repeats with bar  315  in position of  307  of FIG.  3 D( 1 ). The pinch rollers  302  and clamping bars  307 ,  315  and  309  recycle their functions with each ensuing cycle. 
     FIG. 3E depicts the output torque characteristics of the elastic motor just described. Torque is plotted on axis  311  versus time on axis  312 . Peak values  310  are equal to the elastic tension multiplied by the reel radius. The average output torque is half that amount and is constant throughout operation. 
     FIG. 3F depicts reel  303  in an intermediate position between those shown in FIGS. 3B and 3C. Here, reel  303  is rotating in counterclockwise direction  306  while stretched elastic  304  is released. Bar  307  moves along the periphery of reel  303  as shown by arrow  320 . Thus, bar member  307  begins on the upper left-hand side of reel  303  (FIG. 3B) and finishes at the top of reel  303  (FIG. 3C) during the operational mode. 
     Another embodiment of the present invention comprises two reels, one for relaxed elastic, the other for stretched elastic. The two reels may be the size, but it is not necessary. Differing diameters or geometries can provide different output characteristics. FIG. 4A depicts an exemplary motor  400  in accordance with this embodiment of the present invention. Motor  400  is shown in wind up mode. Reel  406  is loaded with relaxed rubber  401 , the end of which is connected to opposing reel  407 . Reel  406  rotates in a clockwise direction  403  and reel  407  rotates in counterclockwise direction  403 . The elastic  404  wound on reel  407  is stretched to n times its lengthened, and optimally, is at full tension. Thus reel  407  turns n times as fast as reel  406 . Roller  405  prevents the elastic  401  on reel  406  from slipping over itself and ruining the tension relationship. 
     FIG. 4B depicts the motor  400  in operational mode, wherein the device is producing work. Reel  407  has two bars  408  and  409  pressing against the elastic  404  to prevent slippage. Reel  407  rotates in clockwise direction  410  taking bars  408  and  409  with it. Opposing reel  406  rotates in counterclockwise direction  411 . When bar  408  approaches roller  405 , the elastic  404  between bar  408  and roller  405  is in its relaxed state. At this point, roller  405  is disengaged to allow bar  408  to pass and further allow elastic  404  to wind onto reel  406 . After bar  408  passes, roller  405  is put back in place. Bar  408  is now removed and the elastic  404  between bar  407  and roller  405  undergoes the same process. At this point, bar  408  is replaced and is ready for the next cycle. Motor  400  is capable of multiple operating cycles. 
     Such a device  400  as described can achieve the required operation, however, the need to remove and replace bars  408  and  409  and roller  405  complicates operation. Thus, an alternate embodiment is depicted in FIG.  5 A. An elastic sheet  501  is used comprising integral crossbars that engage in slots  502 ,  507  and  508  on reel end plates  503  and  504  to maintain the high and low tension zones in the elastic  501  and allow automatic transfer between the two reels  505  and  506  without the need for any other moving parts. The two reels  505  and  506  are fitted with slotted end plates  503  and  504 . Reel  505  rotates in counterclockwise direction  510  n times as fast as reel  506  rotates in clockwise direction  511 . Reel  506  has two slots  507  and  508  in its end plates. Reel  505  has  2 n slots  502  in its end plates  503 . The end plates  503  and  504  overlap at point  509  so that the elastic  501  can transfer between the reels  505  and  506  without any slipping and thus maintain the tension relationship. Rotating reel  506  counterclockwise will automatically stretch the elastic  501  to n times its original length. The two reels  505  and  506  could be coupled together by a gear box or a cog belt to have an n to 1 speed ratio and assist in maintaining accurate alignment. Coupling the two reels  505  and  506  together reduces the output torque to [1−(1−/n)] of the maximum. Thus, the higher the value of n, the higher the system efficiency. 
     FIG. 5B depicts a detail of the elastic used in device  500 . Elastic  501  comprises an elastic strip  515  and integral crossbars  516 . Crossbars  516  articulate with slots  502 ,  507  and  508  to constrain the movement of reels  505  and  506  and thus maintain the tension relationship. 
     FIG. 6 depicts a simplified two reel system  600  utilizing two rollers  603  and  604 . Relaxed tension reel  601  rotates in counterclockwise direction  606  at 1/n times the speed the stretched elastic reel  602  rotates in clockwise direction  607 . Two rollers  603  and  604  prevent the elastic  605  from slipping on reels  601  and  602 . The system  600  winds up on reel  602  and the elastic  605  is stretched n times its released length. However, one who is skilled in the art would recognize the need to maintain high tension between rollers  604  and  603  to maintain motive force. 
     FIG. 7 depicts an additional embodiment of the elastic motor in wind up mode. Here, motor  701  is utilized to provide power to rotate reel  303  in direction  305 . Fully relaxed elastic  301  is then wound around reel  303 . As in earlier embodiments, tension rollers  302  stretch elastic  304  such that it wound around reel  303  under tension. Various other means may be implemented in place of motor  701 . Additionally, reel  303  may be manually wound. 
     FIG. 8 depicts an additional embodiment of the elastic motor comprising lock  801 . Lock  801  is utilized to lock the rotation of reel  303  so that stretched elastic  304  may be stored on reel  304 . Furthermore, lock  801  may be used to stop the elastic motor while operating.