Abstract:
An electric blanket includes a heating element extending through the blanket so that, upon receiving electric current from a power source, the element heats the blanket. A regulator circuit is connected to the element. It is configured to measure the resistance of the element and to control delivery of the electric current from the power source to the element responsively to the resistance so that the element heats to a desired temperature.

Description:
The present application is based on a provisional application filed on Jun. 3, 1999, and having U.S. Ser. No. 60/137,387, the entire disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Electric blankets typically include a wire element that extends through the blanket and through which electric current passes to generate heat. A desired temperature setting is generally achieved by controlling the amount of current passing through the wire. Furthermore, one or more heat-sensing mechanisms are typically provided in the blanket to detect an undesirable increase in temperature so that current to the heating element can be discontinued. 
     SUMMARY OF THE INVENTION 
     The present invention recognizes and addresses disadvantages of prior art constructions and methods. 
     Accordingly, it is an object of the present invention to provide an improved electric blanket. 
     It is also an object of the present invention to provide an improved electric blanket control. 
     These and other objects are achieved by an electric blanket including a heating element that extends through the blanket so that, upon receiving electric current from a power source, the element heats the blanket. A regulator circuit is connected to the element. It is configured to measure the resistance of the element and to control delivery of electric current from the power source to the element responsively to the measured resistance so that the element heats to a desired temperature. 
     A method according to the present invention includes providing a heating element extending through the blanket so that, upon receiving electric current from a power source, the element heats the blanket. The resistance of the element is measured, and delivery of electric current from the power source to the element is controlled responsively to the resistance so that the element heats to a desired temperature. 
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended drawings, in which: 
     FIG. 1 is a cut away perspective view of a heating element for use in a preferred embodiment of an electric blanket constructed in accordance with the present invention; 
     FIG. 2 is a schematic illustration of an electric blanket according to a preferred embodiment of the present invention; and 
     FIG. 3 is a circuit diagram of a preferred embodiment of an electric blanket constructed in accordance with the present invention. 
     Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made to the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations. 
     Referring to FIG. 1, an electric blanket heating element  10  includes a polymer core  12  about which is wound a wire element  14 . An insulation layer  16  encases the core and wire element. As should be understood by those skilled in this art, heating element  10  extends through the interior of an electric blanket in a predetermined pattern in order to heat the blanket to a predetermined setting. The structure and manufacture of an electric blanket apart from the heating element, and the placement of the heating element in the electric blanket, should be well understood by those skilled in this art and is therefore not discussed in detail herein. It should be understood, however, that any suitable such construction and placement is within the scope and spirit of the present invention. 
     The construction of wire  14  may vary as suitable for the requirements of a given blanket. For example, assume that a blanket is rated for 125 watts, that the blanket is of such a size that a 100 foot heating element (measured as the length of wire element  14 ) is required, and that a 120 volt power source drives the heating element. Power is equal to V 2 /R, and the wire must therefore provide a resistance of approximately 115 ohms. Therefore, the heating element&#39;s resistance must be approximately 1.15 ohms per foot. The resistance estimate is an approximation since, as discussed below, the resistance varies with temperature. 
     Any wire of a suitable material and construction may be used to provide the desired resistance. Exemplary metal materials include cadmium copper alkaloid (CDA 162), KANTHAL 70 or 52, and nickel 270. The choice may depend on factors such as cost and the desires of a particular manufacturer. 
     Given a particular material and wire length, resistance is thereafter determined by the wire&#39;s cross-sectional area. Frequently, however, it is desirable to minimize cross-sectional area to make the heating element less apparent to the user. If the wire length is fixed, restrictions on cross-sectional area may limit the available materials. 
     To permit a broader choice of materials, a wire element having a non-circular, for example rectangular or other polygonal shaped, cross-section may be used to increase the wire&#39;s cross-sectional area without increasing the heating element&#39;s profile. The increase in cross-sectional area decreases the wire&#39;s resistance, thereby allowing the use of a broader range of materials than would otherwise be permitted. 
     In one exemplary embodiment, a 125 watt electric blanket uses an approximately 100 foot heating element made of 37 gauge cadmium copper wire. To prevent the heating element from being too bulky or overly noticeable to the consumer, 30 gauge, or smaller, wire is preferred when using wire having a circular cross-section. 
     Furthermore, a relatively high temperature coefficient is also preferable. Temperature coefficient is the measure of the tendency of the wire&#39;s resistance to vary with temperature. It is generally stated in percent increase in resistance per degree Fahrenheit or Celsius. A higher temperature coefficient facilitates detection of temperature increases in that a larger change in resistance per unit of temperature change is more easily and reliably detected. Preferably, materials having a temperature coefficient at or above that of cadmium copper (0.194%/° F.) are used. It should be understood, however, that this range refers only to certain preferred embodiments of the present invention and that materials with lower temperature coefficients may be used, provided the blanket includes a control system capable of effectively detecting temperature changes in such materials. 
     Core  12  can be made of any suitable polymer, for example polyester. Outer insulation  16  may also be made of a polymer, such as polyvinylcholoride (PVC). In one embodiment, the minimum average thickness of outer insulation  16  is approximately two mills. Heating element  14  measures at 37 gauge, and the outer diameter of heating element  10  is approximately 72 mills. 
     Referring to the schematic illustration in FIG. 2, an electric blanket is indicated in phantom at  18 . The heating element is incorporated in the blanket in a conventional manner and is indicated at  10  as a resistance. The heating element is connected to a  120  volt AC voltage source  20  through a full-wave bridge rectifier  22 , a sampling resistor  24  and a triac switch  26 . As should be understood by those skilled in this art, a triac switch conducts AC current between inputs  28  and  30  in both directions as long as an activating signal is present on a control lead  32 . If the activating signal is discontinued, the triac conducts current until the input signal&#39;s next zero crossing. 
     The activating signal is provided by an optically isolated triac driver  34  that acts as a switch passing current from node  30  to the control lead  32 . Thus, when driver  34  is activated by its control lead  36 , the signal from source  20  drives triac  26 . During this signal&#39;s positive cycle portion, current travels through triac  26  in the direction indicated by arrow  38 . During its negative cycle portion, current travels through the triac in direction  40 . 
     A control circuit  42  controls driver  34 . Control circuit  42 , for example comprising a single integrated circuit (IC), may include a microprocessor and an A/D converter. Through the converter, the IC receives voltage measurements from nodes  44  and  46 . The measurement from node  46  is the voltage across sampling resistor  24 . Thus, the controller may determine the current through heating element  10  by dividing the voltage measured at  46  by the known resistance of sampling resistor  24 . The voltage applied to the system is measured at  44 . Thus, the system&#39;s total resistance is equal to the voltage measured at  44  divided by the current measured at  46 . The resistance of heating element  10  may therefore be determined by backing out the known resistances of the components upstream from the heating element. 
     As discussed above, the temperature of heating element  10  is related to its resistance. Wire manufacturers typically rate wire resistance with respect to a predetermined temperature, generally around 75° Fahrenheit. The manufacturer also typically provides the wire&#39;s temperature coefficient. Thus, given a known length L of heating element  10  having a temperature coefficient TC and a rated resistance X (in ohms per unit length) at Y° Fahrenheit, and given a measured resistance Z (in ohms) between nodes  44  and  46  as discussed above, heating element temperature T=Y+(1/XL)(Z−XL)/TC. 
     The variables Y, TC, X and L are known and may be stored in memory associated with control circuit  42 . Therefore, upon determining the measured resistance Z, the controller may determine the heating element&#39;s temperature T by the equation above. Alternatively, temperature T may be calculated over a range of resistances Z to create a table relating temperature to measured resistance. The table may then be stored in the control circuit&#39;s memory so that the controller, upon determining an actual measured resistance between nodes  44  and  46 , may determine temperature T by reference to the table. 
     The control circuit  42  may be disposed in a suitable housing attached to or within blanket  18  or in a housing remote from the blanket. In either case, the control circuit may be configured for use with several different heating elements, each having a range of possible measured resistances Z that does not overlap the range of any of the other heating elements. Thus, the measured resistance Z identifies which heating element the blanket contains, and the controller can then determine temperature T from the temperature coefficient TC and nominal temperature Y for that heating element or from a lookup table for that heating element. 
     In operation, control circuit  42  may control the heat output of heating element  10  by various methods. Generally, however, the heating element&#39;s heat output varies predictably with current. Since triac  26  controls the amount of current passing through the heating element, the element&#39;s heat output may be determined by controlling the ratio of the triac&#39;s on-time to its off-time based on some predetermined scale. 
     One suitable scale is on-cycle percentage. For example, assume that the controller is programmed or otherwise configured to drive triac  26 , and therefore heating element  10 , in any of five settings one through five, where each setting represents a predetermined desired temperature. Assume also that the temperature for the highest setting is at or less than the temperature achieved if triac  26  is held continuously on. Each setting, then, corresponds to a percentage of cycles of the source  20  input signal during which triac  26  is activated over a given base number of cycles. For example, if the base number of cycles is 100, and setting five corresponds to continuous application of current through heating element  10 , triac  26  is activated during all 100 cycles at setting five. If setting four, however, corresponds to application of  85  percent of the current level of setting five, triac  26  is then activated for 85 of the 100 cycles in each base period. Similarly, the controller under each of the other three settings activates triac  26  for a number of cycles in the base period corresponding to the relationship of the desired current level in that setting to a benchmark current level, in this case the current level under the maximum setting. It should be understood that the base period and settings described herein are provided for exemplary purposes only and that any suitable arrangement may be used. 
     The number of cycles used as the base period may vary as desired, as may the selection of cycles within the base period during which triac  26  is activated under the different settings. For example, assuming that the setting is 80%, the triac may be activated and deactivated for alternating cycle periods at a ratio of 4:1 (for example deactivating for every fifth cycle) or in any other suitable pattern so that the desired activation period is achieved over the base period. 
     In a second variation, the control circuit may be programmed to activate triac  26  based on the heating element&#39;s measured temperature. Each setting corresponds to a predetermined desired temperature for heating element  10 . When control circuit  42  determines that the selected temperature has been achieved through detection of temperature T as described above, it deactivates triac  26  to stop current flow. After a predetermined cooling period, control circuit  42  reactivates triac  26  until the heating element again reaches the predetermined temperature. Alternatively, upon deactivating triac  26 , the controller periodically reactivates the triac for one-cycle periods, and determines temperature T at each activation until T falls to a threshold temperature below the predetermined temperature at which the triac was deactivated. The controller then reactivates the triac. Furthermore, the control circuit may be configured to activate and deactivate triac  26  for predetermined periods to achieve the desired temperature. 
     In yet another variation, the control circuit may intermittently trigger triac  26  at a predetermined phase angle in the input signal from source  20 . After the triggering signal, triac  26  deactivates at the input signal&#39;s next zero-crossing, and the intermittent activating signals on lead  32  therefore intermittently open and close triac  26 . Each user setting corresponds to a point in the input signal waveform prior to the zero-crossing at which the control circuit triggers triac  26 . Each setting thereby corresponds to the percentage of the input waveform that is allowed to pass to the heating element and therefore corresponds to the amount of current provided to the element. 
     In any of these arrangements, control circuit  42  permanently deactivates triac  26  if a resistance across nodes  44  and  46  indicates that an undesirably high current level exists. 
     FIG. 3 provides an electrical schematic diagram of the system in FIG.  2 . Control circuit  42  measures the input voltage waveform at node  44  after the input voltage is divided down to a level at or below five volts through a voltage divider formed by resistors  50  and  52 . The voltage drop across bridge circuit  22  and triac  26  is approximately constant. The control circuit measures current from node  46  across sampling resistor  24 . 
     A user inputs a setting to the control circuit through a single push button switch (not shown) that outputs a pulse to the controller to increment the controller&#39;s present setting within the range of possible settings. Alternatively, a plurality of buttons that correspond to respective settings may be provided. To apply current to heating element  10  in response to the setting, control circuit  42  applies a control voltage to the input of optically isolated triac driver  34  which, in turn, activates triac  26  through lead  32 . 
     The full-wave bridge  22  rectifies current to the heating element, thereby reducing the 60-Hz magnetic field. Further magnetic field reduction can be achieved by adding a filter capacitor  54  across the load. 
     While one or more preferred embodiments of the invention have been described above, it should be understood that any and all equivalent realizations of the present invention are included within the scope and spirit thereof. The embodiments depicted are presented by way of example only and are not intended as limitations upon the present invention. Thus, it should be understood by those of ordinary skill in this art that the present invention is not limited to these embodiments since modifications can be made. Therefore, it is contemplated that any and all such embodiments are included in the present invention as may fall within the literal or equivalent scope of the appended claims.