Abstract:
Energy-harvesting and -storage devices in conveyor belts and methods for molding those devices integrally into modular belt links and for enhancing energy harvesting through resonance tuning. Piezoelectric materials, electro-active polymers, thermoelectric generators, RF receivers, photovoltaic devices, linear induction generators, and inductive transformer coupling are used to harvest energy to power belt on-board devices.

Description:
BACKGROUND 
     The invention relates generally to power-driven conveyors and more particularly to instrumented conveyor belts having on-board devices powered by energy harvested from ambient sources. 
     Sensors, indicators, transmitters, receivers, and other electrical and electromechanical devices are being installed more often in conveyor belts. Most of these devices require electric power to operate. Individual cells or batteries also installed in the belts power the devices. But the batteries require periodic monitoring and eventually manual intervention to install fresh replacements. 
     SUMMARY 
     This shortcoming is overcome by a conveyor belt embodying features of the invention. Such a conveyor belt comprises a belt body with an energy-harvesting device disposed in or on the belt body. An energy-storage device, such as a capacitor or battery, coupled to the energy-harvesting device stores energy harvested by the energy-harvesting device. An electrical device, such as a sensor and a transmitter, disposed in or on the belt body is powered by energy stored in the energy storage device. 
     Another version of a conveyor belt embodying features of the invention includes a belt body having an outer surface and an inner core. A thermoelectric generator connected to the belt body is arranged to measure the temperature difference between the inner core and the environment external to the outer surface and to generate a voltage proportional to the temperature difference. 
     In another aspect of the invention, a method for injection-molding a conveyor belt module having a p-n junction comprises: (a) injecting an electrically conductive first material that is doped either positively or negatively into a mold having a cavity in the shape of a conveyor belt module to fill the mold from the outside in and form an outside material around an inner void; (b) subsequently injecting an electrically conductive second material doped opposite to the first material into the mold to fill the void and form an inside material and a p-n junction at the interface of the inside and outside materials; (c) applying heat and pressure to the mold to form a conveyor belt module; and (d) installing an electrode in electrical connection with the inside material. 
     In yet another aspect of the invention, a method for co-molding a conveyor belt module having a p-n junction comprises: (a) injecting an electrically conductive first material that is doped either positively or negatively into a first mold having a cavity formed in the shape of a base layer of a conveyor belt module by a bottom half-mold and a first top half-mold; (b) applying heat and pressure to the first mold to form a base of a conveyor belt module; (c) removing the first top half-mold from the bottom half-mold; (d) closing a second top half-mold having a cavity formed in the shape of a top layer of a conveyor belt module onto the bottom half-mold to form a second mold; (e) injecting an electrically conductive second material doped opposite to the first material into the cavity in the second top half-mold; and (f) applying heat and pressure to the second mold to co-mold a top layer onto the base layer and form a p-n junction at the interface of the base and top layers. 
     In another aspect of the invention, a method for making an injection-molded conveyor belt module having an embedded capacitor comprises: (a) injecting a non-conductive material into a cavity in a mold for a conveyor belt module to form an outer insulating layer with an interior surface surrounding a first interior void; (b) injecting a conductive material into the first interior void to form a conductive layer coating the interior surface of the outer insulating layer and confining a smaller second interior void; (c) injecting a dielectric material into the second interior void to form a dielectric layer coating the conductive layer and confining a third interior void smaller than the second interior void; (d) injecting the conductive material to fill the third interior void and form a conductive plate in the interior of the conveyor belt module that forms a capacitor with the conductive layer separated from the conductive plate by the dielectric layer; and (f) applying heat and pressure to the mold to form a conveyor belt module embedded with the capacitor. 
     And in still another aspect of the invention, a method for enhancing the harvesting of energy for powering devices in a link conveyor belt comprises: (a) operating a conveyor system including an advancing link conveyor belt having a vibration-sensitive energy-harvesting device with a frequency response to belt vibration, wherein the frequency response has a peak at a resonant frequency and a bandwidth about the resonant frequency; and (b) matching the frequency spectrum of the belt vibration caused by an operating parameter of the conveyor system to the frequency response of the energy-harvesting device to couple more vibrational energy into the energy-harvesting device. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These features and aspects of the invention, as well as its advantages, are better understood by referring to the following description, appended claims, and accompanying drawings, in which: 
         FIG. 1  is an isometric view of a conveyor system having an instrumented conveyor belt embodying features of the invention; 
         FIG. 2  is a block diagram of an instrumented portion of the conveyor belt of  FIG. 1 ; 
         FIG. 3  is a side elevation view of a conveyor belt as in  FIG. 1  with a piezoelectric energy-harvesting device sensitive to vibrational energy; 
         FIG. 4  is an isometric view of a conveyor belt module in a conveyor belt as in  FIG. 1  with a thermoelectric generator; 
         FIGS. 5A-5C  are side views of three versions of piezoelectric devices embedded as strain generators in modular conveyor belt links usable in a conveyor belt as in  FIG. 1 ; 
         FIG. 6  is a side view of a mold for injection-molding a modular conveyor-belt link with a p-n thermocouple junction for use in a conveyor belt as in  FIG. 1 ; 
         FIGS. 7A and 7B  are side views of a molding process for co-molding a modular conveyor-belt link with a p-n thermocouple junction for use in a conveyor belt as in  FIG. 1 ; 
         FIG. 8  is a side view of a portion of a conveyor belt as in  FIG. 1  with a linear electric generator; 
         FIG. 9  is a side view of a portion of a conveyor belt as in  FIG. 1  with an inductively coupled electric generator; 
         FIG. 10  is a side view of a portion of a conveyor belt as in  FIG. 1  with a radio-frequency energy harvester; 
         FIG. 11  is a side view of a portion of a conveyor belt as in  FIG. 1  with a photovoltaic energy harvester; 
         FIG. 12  is a side view of an injection-molded modular belt link with an embedded energy-storage capacitor usable in a conveyor belt as in  FIG. 1 ; and 
         FIG. 13  is an oblique view of an array of thermistors embedded in a modular conveyor belt link usable in a conveyor belt as in  FIG. 1  shown in phantom. 
     
    
    
     DETAILED DESCRIPTION 
     One version of a conveyor system embodying features of the invention is shown in  FIGS. 1 and 3 . A conveyor, shown in this example as a conveyor belt  10  supported on a carryway  12 , transports articles in a conveying direction  14  on an outer conveying surface  16  along a carryway segment  18  of the belt&#39;s endless conveyor path. At the end of the carryway, articles are conveyed off the conveyor belt. After rounding drive sprockets  20 , the conveyor belt  10  follows a return segment  19  on its way back around idle sprockets  21  to the carryway segment  18 . Both the drive and idle sprockets are mounted on rotatable shafts  22 . A drive motor  24  is coupled to the drive shaft to drive the belt in the conveying direction  14 . In the returnway segment  19 , the belt is supported between a pair of spaced apart rollers  26 ,  27  or shoes to take up sag in the catenary  28  formed between the rollers. 
     As shown in  FIG. 1 , the conveyor belt  10  is instrumented with an electronic-component package  30  disposed on or in the conveyor belt. Included in the electronic-component package, as shown in  FIG. 2 , is an energy-harvesting device  32 , a rectifier  33 , an energy-storage device  34 , a voltage regulator  35 , a sensor  36 , and a transmitter  38 . The sensor  36 , which may sense temperature, humidity, atmospheric pressure, or some other local condition, makes measurements of the local condition, conditions the measurements, and sends them to a processor  40  over a data line  42 . The processor  40  buffers, scales, filters, or formats the sensor measurements before sending them to the transmitter  38  over a data line  43 . The transmitter  38  then wirelessly transmits the measurements remotely over an antenna  44 . Power to operate the electrical device  45  comprising the sensor, transmitter, processor, and other auxiliary circuitry is provided over power lines  46  by the energy-storage device  34 , which may be a capacitor or an electrical cell or battery, through the voltage regulator  35 . The energy-harvesting device  32  scavenges energy from external sources coupled to the conveyor belt. The scavenged energy generates a varying voltage that is rectified to charge the energy-storage device, which supplies the energy as needed by the electrical device  45 . 
     Mechanical vibrations are ubiquitous in conveyor systems. In many industrial applications, the environment in which the conveyor operates vibrates. Within the conveyor itself, chordal pulsation of the drive sprocket, vibration of the catenary in the return, and belt-surge pulses are all transmitted through the conveyor belt. These are just a few examples of the kinds of vibrational energy that can be harvested to power electrical or electromechanical devices embedded in a conveyor belt. A piezoelectric device  48  is especially adaptable to serve as an energy-harvesting device to harvest vibrational energy in the conveyor belt as shown in  FIG. 3 . The piezoelectric device  48  generates an electrical charge when the piezoelectric material is deformed. The cyclic vibrations in the belt cause the piezoelectric device to deform and generate a voltage that can be used to charge the energy-storage device  34 . The piezoelectric device  48  may be made of crystals, such as quartz, magnetostrictive materials, or piezoelectric ceramics, for example, and disposed in or on the conveyor belt  10 . 
     One way to enhance the harvesting of vibrational energy is to tune the frequency response of the piezoelectric drive to the frequency spectrum of the belt vibration to maximize the amount of energy harvested. The frequency response of the piezoelectric device has a peak at a center, or resonant, frequency and drops off on either side of the peak to define a bandwidth over a narrow range of frequencies to which the device is especially sensitive. By selecting a piezoelectric device whose bandwidth covers the expected belt-vibration frequencies, energy harvesting is enhanced. 
     Unlike flat belts, which are frictionally drive by pulleys, modular plastic conveyor belts, which are constructed of a series of rows of fixed-pitch belt modules or links, are positively driven by sprockets. Consequently, like power chains, modular plastic conveyor belts are subject to chordal, or polygonal, action as the fixed-pitch belt is driven by the sprockets. The frequency of the chordal action equals the ratio of the linear speed of the belt to the belt pitch. The efficiency of vibrational-energy harvesting increases if the resonant frequency of the piezoelectric device equals the frequency of the linear speed pulsations in the belt caused by the chordal action. One way to achieve highly efficient harvesting is to tune or select the resonant frequency of the piezoelectric device to match the frequency of belt vibration closely. Another way is to tune the vibration to match the resonant frequency of the piezoelectric device by, for example, adjusting the linear speed of the belt. Alternatively, the amount of vibrational energy available for harvesting can be increased by increasing the amplitude of the chordal action by using smaller-diameter sprockets with fewer teeth. 
     As shown in  FIG. 3 , the tension in the conveyor belt  10  in the returnway segment  19  is low. Consequently, the belt forms a catenary  28  between the support rollers  26 ,  27 . The catenary has a resonant frequency that is a function of belt pitch, catenary depth, belt weight, belt spring constant, and the distance between the two support points of the catenary, i.e., the spacing between the return shoes or rollers  26 ,  27 . When the frequency of belt vibration equals the resonant frequency of the catenary, the catenary vibrates aggressively, as indicated by arrow  50 . Consequently, belts are normally operated at a speed that does not induce in the belt significant vibrational frequencies at the resonant frequency of the catenary. But, to make more vibrational energy available for harvesting, the speed of the belt or one of the other belt variables can be adjusted to induce the catenary to resonate and couple more energy to the piezoelectric device. 
     Surging is another source of resonant vibration in a fixed-pitch belt. A resonant compression and expansion of the belt, surging usually occurs on the carryway segment  18  of the belt path, as indicated by arrow  51 . The resonant frequency is a function of belt pitch, spring constant, and linear belt speed. Energy harvesting is enhanced by adjusting these quantities for a particular application to cause resonance. 
     Instead of a piezoelectric device harvesting the kinetic energy of a vibrating belt, an electrostatic device can be used. An electrostatic energy-harvesting device comprises a charged variable capacitor, or varactor, whose capacitance changes as a function of belt vibration. Changes in capacitance due to vibration cause voltage changes across the plates of the varactor. The changing voltage charges the energy-storage device. The varactor, which is designed to operate in resonance, is selected to have its resonant frequency as close as possible to the vibration frequencies expected to be present. Thus, the vibration-sensitive varactor may be used as an energy-harvesting device disposed in or on the conveyor belt  10 . 
     Yet another version of an energy-harvesting device is illustrated in  FIG. 4 . A thermoelectric generator (TEG)  52  comprises positively doped (p-type) and negatively doped (n-type) semiconductor materials  54  in a layer forming a p-n junction sandwiched between a first heat exchanger  56  and a second heat exchanger  57 . The TEG  52  is embedded in the conveyer belt  10  with its first heat exchanger  56 , which is in contact with one of the semiconductor material, disposed at the outer surface  16  of the belt for exposure to the temperature of the process through which the belt advances. The second heat exchanger  57 , in contact with the other semiconductor material, is disposed in the inner core of the conveyor belt. By the Seebeck effect, a voltage is generated when a temperature gradient is established across the two layers of semiconductor materials. So, as the conveyor belt advances through a process whose temperature varies, the temperature lag between the core of the weakly conductive belt and the temperature of the process establishes a temperature difference between the two heat exchangers  56 ,  57  that generates a voltage charge on the two wires  58 ,  59  connected for the semiconductor materials. The two wires are connected to the embedded electrical device  60 , which includes the rectifier, energy-storage device, and other electronics powered by the harvested thermoelectric energy. 
     Alternatively a TEG can be realized as a thermocouple TC 1 -TC 8  comprising two metal-alloy wires—one a p-type P, the other an n-type N—welded together as shown in  FIG. 13 . The wires are connected electrically in series and thermally in parallel. A voltage V is generated as one side is heated and the other cooled. The changing voltage is rectified and charges an energy-storage device  61 . More energy can be harvested by connecting multiple p-n junctions in series and multiple series of junctions in parallel to form a thermoelectric battery. Thus, a TEG is especially effective at harvesting energy in a conveyor belt that is used in a heating or cooling process in which a plastic or other low-conductivity belt is subjected to varying temperatures. 
     As the conveyor belt  10  in  FIG. 1  transports articles along the carryway  12 , the weight of the belt and the articles, acting through a friction factor between the bottom of the belt and the carryway surface, creates a resisting force to belt motion that exhibits itself as belt tension. The tension increases from the infeed end of the carryway to the discharge end. The conveyor belt has an associated spring constant that describes the strain of the belt under tension. The belt strain is relaxed in the low-tension belt return segment  19 . As shown in  FIG. 5A , a piezoelectric device  62  is embedded in a modular conveyor belt link  64 . As the belt containing the link strains during its advance along the carryway, the piezoelectric material in the device  62  is strained and a voltage generated. As with the other energy-harvesting devices, the voltage is rectified to charge the energy-storage device. Instead of being molded into the belt link  64 , as in  FIG. 5A , the piezoelectric strain generator  62 ′ can be mechanically fastened or bonded to a modular link  64 ′, as in  FIG. 5B . As another alternative, piezoelectric fibers  66  are co-injected with the base polymer during injection molding of a link  64 ″, as in  FIG. 5C . With the fibers aligned and suitable electrodes  68 ,  69  attached to route the voltage to an energy-storage device, the modular link  64 ″ serves as a supporting matrix for the fibers. As the link strains, the fibers are strained and produce a voltage that is used to charge the energy-storage device. Instead of measuring strain caused by belt tension along the carryway, a piezoelectric energy-harvesting device could be used to respond to the drive force of a sprocket tooth  70  pushing against a drive pocket  71  in the belt module, as shown in  FIG. 5B . In this example, a piezoelectric device  62 ″ located close to the drive pocket  71  responds to the force of the tooth  70  against the drive surface  72  as the sprocket rotates in the direction of arrow  74 . The force is greatest when the tooth of the drive sprocket engages the drive pocket at top dead center and decreases as the modular link  64 ′ makes its way around the sprocket to the exit point. 
     A TEG can be a separate device embedded in or attached to a conveyor belt or modular link as previously described, or it can be realized as a custom modular link.  FIG. 6  shows a mold  76  comprising two mold halves  78 ,  79  that, when pressed together, form a cavity  80  for molding a modular plastic belt link. First, an electrically conductive first material doped either positively or negatively is injected into the mold cavity  80  through injection ports  82 . The first material fills the cavity from the outside in and forms an outside material coating  84  around an inner void  86 . An electrically conductive second material doped opposite to the first material is then injected into the cavity to fill the void as an inside material  85  and form a p-n junction with the first outside material along their interface. Heat and pressure are applied to the mold halves to mold the conveyor belt module. After the mold halves are parted and the module ejected, an electrode connection (not shown) is made to the inside material  85 . 
     A belt module having an integral p-n junction can alternatively be manufactured in a co-molding process as shown in  FIGS. 7A and 7B . First, the electrically conductive first material  84  is injected into a base cavity  88  in a two-part mold  90 , consisting of a bottom half  92  and a top half  93 . Heat and pressure are applied to the mold to form a module base  94 . The top half  93  of the mold is removed and replaced with a second top half  95  closing on the bottom half  92  and the base  94 . The second top half  95  has a cavity annex  96  at a top surface  98  of the base  94 . The electrically conductive second material  85  is injected into the cavity annex  96  to fill the annex. Heat and pressure are applied to the closed mold halves to form a top layer  99  atop the oppositely doped base layer  94 . The interface between the two layers forms a p-n junction. Electrodes can be added in a secondary manufacturing step. Thus, a belt module with a molded-in p-n junction to operate as a TEG can be made by either of these injection-molding processes. 
       FIG. 8  shows the harvesting of energy by means of a linear electric generator scavenging energy from the belt&#39;s linear motion. Fixed magnetic fields  100  traversing the conveyor belt  102  are generated by electromagnets or permanent magnets  104  along one or more portions of the belt&#39;s conveying path. The belt&#39;s motion in the conveying direction  14  through the magnetic fields induces a voltage across the terminals  106  of coils  108  embedded in the conveyor belt  102 . The induced voltage is rectified and charges a local energy-storage device. 
     As shown in  FIG. 9 , a transformer  110  is used to couple energy to a conveyor belt  112 . The transformer comprises a primary winding  114  mounted at a fixed location along the belt&#39;s path. A coil  116  in the belt  112  acts as the secondary winding of the transformer. When the primary winding is energized by an ac source  120 , an ac voltage is induced in the secondary winding, i.e., the coil  116 , by transformer action. The ac voltage is rectified in a rectifier  122  and charges an energy-storage device  124 , such as a capacitor or battery, mounted in or on the conveyor belt  112 . The transformer allows the energy-storage device to be charged while the belt is stationary with the coil  116  stopped at the charging station defined by the location of the primary winding  114 . Alternatively, one or more charging stations along the length of the conveying path can be used to charge the energy-storage device while the conveyor belt is advancing or stationary. Other charging stations can be located along the conveyor belt&#39;s path. 
     Another energy-harvesting technique is shown in  FIG. 10 . A conveyor belt  126  has antenna coils  128  connected to a rectifier and energy-storage device as in  FIG. 9 . An external radio-frequency (RF) transmitter  130  is coupled to transmitting antennas, such as directional antennas  132 , to direct the RF energy at the conveyor belt and limit scatter. The belt-borne antenna coils  128  receive the generated RF signal and send it to a rectifier and energy-storage device, as in  FIG. 9  for use by other electrical components disposed in or on the belt. 
       FIG. 11  shows a conveyor belt  134  with photovoltaic devices  136  mounted at an outer surface, such as a top surface  138 , of the belt. The photovoltaic devices convert incident radiation, such as visible light, into a voltage that is rectified and used to charge a local energy-storage device. In this way, artificial or solar light can be harvested to power electronic devices on a conveyor belt. 
     One of the energy-storage devices that can be used in the conveyor belts described is a capacitor, which may be a separate device attached to or molded into the belts. But a storage capacitor can also be formed with the belt in an injection-molding process, as shown in  FIG. 12 . First a non-conductive polymeric material is injected into a mold cavity to form an outer insulating layer  140  of a modular belt link  142  surrounding a first interior void  144 . Then, a conductive polymeric material is injected into the first interior void to form a conductive layer  146  coating the interior surface  148  bounding the first interior void  144 . A dielectric polymer is then injected into a second smaller interior void  150  to form a dielectric layer  152  coating the inner surface  154  of the conductive layer  146  and surrounding an even smaller third internal void  156 . Next, the conductive material is injected into the third internal void to fill it and form a conductive plate  158  in the core of the modular link. Heat and pressure are applied to the mold to form the conveyor belt module  142  with an integrally molded capacitor in which the conductive plate  158  is separated from the conductive layer  146  by the dielectric layer  152 . Electrodes  160 ,  161  can be connected to each of the conductive plate  158  and the conductive layer  146  in a secondary manufacturing step to form terminals of the capacitor that can be accessed for charging by an energy-harvesting device and for powering on-board electrical or electromechanical components. 
     Although various versions of energy-harvesting and -storage devices have been described in detail, other versions are possible. For example, electroactive polymers that change shape with applied voltage will conversely generate a voltage when mechanically strained. Consequently, a modular belt link molded out of an electroactive polymer is usable to harvest energy in a like manner to a piezoelectric material.