Abstract:
An electrically isolated temperature sensor for use with a printer, copier, or all-in-one fuser. The fuser includes an AC driven heater to which a thermistor is mounted for sensing the temperature of the fuser heater. A resistance of the thermistor controls the period of a periodic signal generated by an astable multivibrator. An optical isolator isolates the printer fuser from down line processing circuits, and transfers the periodic signal to such processing circuits. The printer fuser employs a separate floating ground that is not connected to other DC circuits of the printer. With this arrangement, any AC power that is inadvertently coupled from the heater to the DC circuits of the fuser is isolated thereto. The AC power is isolated to the fuser and cannot be propagated through the fuser to other down line circuits of the printer.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     None. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to laser printers, copiers and all-in-one devices, and more particularly to toner fusers for use with such printers. 
     2. Description of the Related Art 
     In laser type printers, toner particles are electrostatically attracted to a print media, such as paper, to produce text, symbols or other images. The toner particles must be fused to the paper in order to make the text or image permanent and resistant to smudging or smearing. Once the toner particles are electrostatically attracted to the print media in the pattern of the text or image, the toner is fused to the print media through the use of high temperatures and pressure applied to the toner in order to permanently imbed the toner into the print media, or on the surface thereof. As can be appreciated, the temperature to which the toner is subjected must be controlled in order to assure a consistent, quality print job. The fusing temperature can change as a function of changes in parameters such as paper weight, duration of printing, etc. Fusing temperatures higher than necessary can cause the toner to fuse to some of the printer apparatus, such as the fuser belt. A fusing temperature that is too low will result in inadequate fixing of the toner to the print media, thus allowing smearing of the text or image. The fusing temperature of the toner must thus be automatically adjusted in order to maintain an optimum print quality. 
     Several types of fusers are known in the art for fixing the toner particles to the print media. One type of fuser employs an axial lamp to generate the power necessary to fuse the toner to the print media. Another type of fuser includes a flat ceramic slab heater with a heating resistor on one side of the ceramic member and a thermistor on the back side of the heater member. The thermistor functions to sense the temperature of the ceramic member and through feedback control circuits, control the electrical energy applied to the heating resistor and thereby control the temperature at which toner fusing occurs. Typically, the heater comprises one or more strips of a metallic material that becomes heated when an electrical current passes therethrough. Generally, the heater strip is driven by a source of AC electricity that is controlled in some manner in order to control the amount of thermal energy generated by the heater strip. 
     The use of the ceramic slab type of fuser heater is advantageous as there is a high speed transfer of heat from the heater, through the ceramic slab, to the print media. The faster the heat can be transferred to the print media, the more quickly the printer can commence printing. The time many printers must wait until the fuser is sufficiently hot to commence printing the first sheet of print media can be 10-20 seconds. The faster the printer can start printing, the more efficient the printer becomes. The ceramic slab heater technology is often referred to as “instant on” fusing technology. This is desirable because the time to first print is on the order of 10 seconds, or less. 
     In order to efficiently transfer thermal energy, the ceramic slab of the heater is constructed as a thin member, on the order of about 0.5 mm to about 2.0 mm. This construction facilitates a low thermal mass and thus can be quickly heated to the desired operating temperature. The thin construction of the ceramic slab permits the temperature sensor to be located in close proximity to the heated side of the slab heater, thereby permitting more accurate control of the temperature, and shorter response times to reach desired fuser temperature. 
     As noted above, the heating element located on one side of the ceramic slab heater is normally driven by the AC line voltage having a magnitude of either 120 Vrms or 240 Vrms. The thermistor is located on the opposite side of the ceramic slab heater, and is connected to low voltage circuits (5VDC) of the printer fuser. Accordingly, both the AC power line voltage circuits and the low DC voltage circuits exist in close proximity to each other in the fuser assembly. The thin ceramic slab member functions as an electrical insulator between the AC and DC circuits. As can be appreciated, it is highly desirable to maintain adequate electrical isolation between the AC power line circuits and the DC circuit of the printer, otherwise substantial damage can occur if the AC line current is allowed to be imposed on the DC circuits of the printer. A failure in the ceramic slab, such as cracking thereof, can allow the AC energy of the heater apparatus to be effectively connected to the low voltage DC circuits of the fuser. Unless isolated, the AC energy can propogate from the DC circuits of the fuser to the other down line circuits of the printer. As noted above, the AC and DC circuits of the fuser are separated from each other by only 1 mm, or so, i.e., the thickness of the ceramic slab. 
     Various AC/DC isolation schemes for fusers have been proposed in the laser printer field. One electrical isolation technique involves the use of a 1:1 transformer to isolate the thermistor from the AC line voltage. See  Electronic Design  magazine, Apr. 22, 2008, article entitled “Circuit Transfer Resistance Value through Isolation Barrier,” by Leo Sahlsten. With this technique, the magnetic coupling between the primary and secondary of the transformer provides the electrical isolation. The disadvantage with this method is that the scope of the resistance change in the thermistor is small, namely about 1.5 decades of resistance change. This obviously limits the accuracy and/or overall range by which temperature changes can be sensed. A much better AC/DC isolation technique would allow detection of 4+ decades of thermistor resistance change, namely from about 2.4E6 Ohms (cold or nearly open) to about 2.1E3 Ohms (over temperature). 
     Another technique for providing electrical isolation between the AC and DC circuits of the fuser of a laser printer involves the use of capacitive coupling therebetween. With this technique, a capacitor, such as a Y-cap, is allowed to be connected between the AC line voltage and the low voltage DC circuits. Again, this method fails to allow 4+ decades of resistance change in the thermistor to be accurately and efficiently detected. 
     Yet another method of providing AC/DC isolation in printer fuser apparatus involves the use of optical techniques to couple light energy to the thermistor. While this provides ideal electrical isolation between the AC circuits and the DC circuits, the problem is the inability to effectively provide sufficient electrical energy to power the thermistor in order to obtain a voltage therefrom representative of the temperature. Stated another way, there is insufficient power available in the optical signals so that when transferred through an opto-isolator, the resulting electrical energy will not adequately energize the thermistor. 
     From the foregoing, it can be seen that a need exists for a printer fuser that can provide sufficient DC power to power the circuits thereof, and provide output electrical signals representative of the fuser temperature, all electrically isolated from other DC circuits of the printer. Another need exists for a printer fuser that is isolated from the other circuits of the printer by a separate ground system. Yet another need exists for an improved printer fuser employing temperature conversion circuits where multiple decades of temperature changes can be converted into multiple decades of resistance values to thereby provide increased temperature measuring accuracy. 
     SUMMARY OF THE INVENTION 
     A fuser assembly is disclosed which provides a temperature sensing system for measuring fuser temperatures and for converting the sensed temperature into a corresponding periodic signal having a period representative of the fuser temperature. The temperature sensing system is isolated from the AC drive system that drives the fuser heater, and is isolated from the other down line DC printer circuits that receive and process the periodic signals to control the AC drive to the fuser heater. 
     According to a feature of the invention, the temperature sensing system is isolated from the AC drive system by magnetic coupling, and the periodic signals representative of the fuser temperature are optically coupled to the down line processing circuits. The temperature sensing fuser circuits are thereby isolated from both the associated AC power and down line DC printer circuits. 
     According to yet another feature of the invention, the temperature sensing system of the fuser is constructed so that four decades of temperature change are converted into about three decades of periods of a digital pulse train. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is an electrical schematic diagram of one embodiment constructed according to the invention; 
         FIG. 2  is a table illustrating the relationship between the fuser temperature, resistance, pulse train period and printer status; and 
         FIG. 3  graphically depicts the correspondence in the conversion between thermistor resistance and the period of digital signals, according to one embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted,” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. In addition, the terms “connected” and “coupled” and variations thereof are not restricted to physical or mechanical connections or couplings. 
     In addition, it should be understood that embodiments of the invention include both hardware and electronic components or modules that, for purposes of discussion, may be illustrated and described as if the majority of the components were implemented solely in hardware. However, one of ordinary skill in the art, and based on a reading of this detailed description, would recognize that, in at least one embodiment, the electronic based aspects of the invention may be implemented in software. As such, it should be noted that a plurality of hardware and software-based devices, as well as a plurality of different structural components may be utilized to implement the invention. Furthermore, and as described in subsequent paragraphs, the specific mechanical configurations illustrated in the drawings are intended to exemplify embodiments of the invention and that other alternative mechanical configurations are possible. 
     The term image as used herein encompasses any printed or digital form of text, graphic, or combination thereof. The term output as used herein encompasses output from any printing device such as color and black-and-white copiers, color and black-and-white printers, and so-called “all-in-one devices” that incorporate multiple functions such as scanning, copying, and printing capabilities in one device. Such printing devices may utilize ink jet, dot matrix, dye sublimation, laser, and any other suitable print formats. 
     The present invention provides a system and method for sensing the temperature of a fuser and providing a corresponding cyclical signal representative of the same. The temperature sensing system is isolated from the AC line that drives the fuser heater.  FIG. 1  illustrates an embodiment of a printer fuser  10  and associated temperature sensing system  12  for sensing the temperature of the fuser heater  14 , and maintaining the heater  14  at a desired temperature. A source  16  of AC power, which may be 120Vrms, 240Vrms, or other AC line voltage source, is employed to supply AC energy to the fuser heater  14 . The AC power source  16  can be the AC power plug of the printer. The fuser heater  14  can be a resistive conductor which is heated when the AC energy passes therethrough. In one embodiment of the invention, the resistive conductor of the heater  14  is located on one side of a ceramic slab  18 , and a temperature sensor  20  is located on the other side of the ceramic slab  18 . The temperature sensor  20  can be a thermistor which provides an output resistance as a function of the temperature of the body of the thermistor device  20 . The thermistor  20  is mounted to the ceramic slab  18  so as to be in intimate thermal contact therewith. 
     As is well known in the art, the heat generated in the ceramic slab  18  fuses toner particles deposited on a print media that passes under the ceramic slab  18 . The principles and concepts of the invention can be employed in color electrophotographic printers of the type illustrated and described in U.S. Pat. No. 6,879,803 by Gogate et al., the disclosure of which is incorporated herein by reference. The amount of AC power applied to the fuser heater  14  is controlled by a control  22  which permits sufficient energy to be coupled to the fuser heater  14  to maintain a desired temperature and thereby provide optimum fusing of the toner particles to the print media. It is to be noted that the desired operating temperature of the fuser heater  14  may change dynamically depending on the print job to be carried out, and changed during the specific print job. The AC control  22  can be a solid state switch controlled by a processor of the printer to apply or remove the AC signal with respect to the heater  14 . Alternatively, the control  22  can be a circuit that controls the duty cycle of the AC signal so that only a portion of each AC cycle is applied to the fuser heater  14 . It is understood that the portion of each AC cycle allowed to be applied to the fuser heater  14  is increased when it is desired to increase the thermal energy imparted to the ceramic slab  18 , and vice versa. Many other conventional AC control circuits can be employed. 
     If the circuits controlling the AC control system  22  become defective, or if the solid state switches in the AC control circuit  22  become short circuited, then the AC energy may be continuously applied to the fuser heater  14 , thereby generating excessive heat. As a result, the ceramic slab  18  to which the fuser heater  14  is attached, can crack or otherwise break. The AC line current can thus be coupled through the broken ceramic slab  18  to the thermistor  20  and to the electrical circuits of the fuser associated therewith. 
     From the foregoing, it can be seen that unless the DC voltage circuits of the printer fuser  10  are isolated from the AC circuits, a failure in the ceramic slab  18  can present the potential of allowing the AC power that drives of the heater  14  to be coupled through the DC fuser circuits to other down line printer circuits and cause overall catastrophic printer damage. According to a feature of the invention, the damage caused by the inadvertent imposition of the AC line power on the DC fuser circuits is limited only to the DC fuser circuits, which are modular in form and can be replaced. As noted above, the ceramic slab  18  may be no thicker than 1 mm, or so, and thus the physical separation between the fuser heater  14  and the thermistor  20  is very small. As noted above, with such a small physical separation between the AC and DC fuser circuits, even if not in physical contact, the 120VAC potential on the fuser heater  14  can arc through a crack in the ceramic slab  18  and place such potential on the thermistor  20  and the other DC circuits electrically connected therewith. 
     Reference is now made to the construction and operation of the fuser assembly of  FIG. 1 . The 120 volt, 50-60 Hz, AC signal from the power source  16  is coupled not only to the control circuit  22  which drives the fuser heater  14 , but to the primary of a transformer  24 . The transformer  24  has a low voltage secondary, such as 5-24VAC. A transformer with a 24VAC secondary winding is selected according to a preferred embodiment of the invention, although other output AC voltages can be employed with equal effectiveness. In accordance with an important feature of the invention, the transformer  24  provides both physical and DC isolation between the primary and secondary. Only a magnetic coupling exists between the transformer primary and secondary. The transformer secondary is connected to a rectifier circuit and filter capacitor, collectively shown as circuit  26 . The output of the rectifier/filter circuit  26  is generally a DC voltage, with some possible ripple. Those skilled in the art can readily appreciate that the rectifier/filter circuit  26  is well known in the art. The output of the rectifier/filter  26  can include a threshold device so that the downstream circuits of the temperature sensing system  12  are not powered until the DC voltage exceeds a predefined magnitude. 
     The filtered output of the rectifier/filter circuit  26  is coupled to the input of a voltage regulator  28  which is of conventional design and available in integrated circuit form, such as integrated circuit LM140. The voltage regulator  28  functions to maintain the DC voltage of the temperature sensing system  12  at a predefined magnitude, and remove the residual ripple, irrespective of changes in AC line voltage. While not critical to the operation of the temperature sensing system  12 , the regulated voltage is chosen in the preferred form of the invention as 5 VDC. It is important to note that the temperature sensing system  12  does not share a ground system with either the AC circuits, or with other downstream DC circuits of the printer. Rather, the DC circuits of the temperature sensing system  12  are provided with a floating common, identified by numeral  30 , that is not connected to the other printer ground circuits, shown by the symbol of reference numeral  31 . 
     The DC voltage output from the voltage regulator  28  powers an astable multivibrator  32 , or oscillator, connected to provide an output digital signal with a frequency that varies as a function of the voltage across the thermistor  20 . Since the thermistor  20  provides a voltage output that varies as a function of the temperature of the ceramic slab  18 , the temperature sensing system  12  effectively functions to provide an output digital waveform having a frequency (or period) that varies with the temperature of the ceramic slab  18 . As will be described in more detail below, this combination is responsive to a large range of temperatures of the ceramic slab  18 , which are accurately converted to corresponding large range of digital signal frequencies. Indeed, it has been found that four decades of resistance changes can be represented with three decades of frequency changes. Accordingly, very small differences in the change in temperature of the ceramic slab  18  can be accurately represented by corresponding different digital signal frequencies. 
     The astable multivibrator  32  can be of many different designs. In the preferred embodiment of the invention, the astable multivibrator  32  is constructed using an LM 555 timer connected for astable operation. To that end, the Reset (not) input of the multivibrator  32  is connected to the VCC output of the voltage regulator  28 . The Control input of the multivibrator  32  is connected through a capacitor  34  to the floating common  30 . The Gnd terminal of the multivibrator  32  is connected to the floating common  30 . The timing of the astable multivibrator  32  is determined by the value of the resistance of resistor  36  and thermistor  20 , as well as the value of the timing capacitor  38 . The resistance of the thermistor  20  has the largest affect on the output frequency of the astable multivibrator  32 , while the resistor  36  is chosen to be of much smaller value so that the output frequency of the astable multivibrator  32  changes in a major way as a function of the resistance of the thermistor  20 , and thus as a function of the temperature of the ceramic slab  18 . The value of the timing capacitor  30  is chosen so that the output period of each cycle of the digital signal is on the order of seconds for high thermistor resistances, such as 2.4E6 Ohms, and the output period will be on the order of milliseconds for low thermistor resistances, such as 2.1E3 Ohms. In the event that the thermistor  20  becomes open circuited, or extremely cold, the output period of the astable multivibrator  32  will exceed about 1.204 s. This information can be used by the processor of the printer during diagnostics to determine a malfunction of the thermistor  20 , namely an open thermistor  20 . On the other hand, if a shorted thermistor  20  or overtemperature condition exists, the output period of the astable multivibrator  32  will be less than about 1.497 milliseconds, which information can be determined during printer malfunction diagnostics to determine that a shorted thermistor  20  exists. These above values assume a thermistor characteristic such as the commercially available Semitec 364 FT, resistor  36  value of 1K ohms, and capacitor  38  value of 0.43 uF. 
     In view of the foregoing, the thermistor  20  is of the type that exhibits a large range of resistances to cover the temperature range of the fuser heater  14 . The table of  FIG. 2  illustrates the related parameters involved in converting the temperature of the fuser heater  14  to a corresponding digital signal period, and the related printer fuser status. As can be seen, the range of fuser temperatures sensed is between −16° C. and 250° C., and the thermistor resistance varies from about 2M Ohm to about 2.4K Ohm, almost a four decade change. In the temperature sensing system  12 , the multivibrator  32  can provide pulse periods that can be distinguished to 1 microsecond, or better, (due to a timer in ASIC  54 ) which allows very small temperature changes to be sensed. 
     The output of the astable multivibrator  32  is coupled through a current limiting resistor  40  to an opto-isolator  42 . The opto-isolator  42  is of the type that is high speed, with fast rise and fall times (less than about 0.1 microseconds) of signals coupled therethrough. This is advantageous so that the period of the digital signals output from the astable multivibrator  32  can be accurately determined, and the corresponding temperature of the ceramic slab  18  ascertained. In the preferred embodiment, the opto-isolator  42  is of the type H11L1, while other high speed opto-isolators can be employed with equal effectiveness. 
     The opto-isolator  42  functions not only to convey the output temperature-related signals from the temperature sensing system  12  to the other down line printer circuits, but also to provide electrical isolation for the temperature sensing system  12 . It is noted that the only coupling between the diode  44  and the output device  46  of the opto-isolator  42  is optical in nature. The output of the device  46  is connected to 3.3 VDC through a load resistor  50 . The output of the temperature sensing system  12  is thus a digital signal train  52  having a magnitude of about 3 volts, where the frequency or period of the pulses is representative of the temperature of the ceramic slab  18 . It can be seen that because of the optical isolation, any AC power line energy present in the DC circuits of the fuser cannot propagate to other down line DC printer circuits. 
     The period of the digital pulse train  52  can be determined in a number of conventional ways. A down line ASIC circuit  54  can be employed to respond to successive rising edges or falling edges of the digital waveform  52  and determine the cyclical period using a table. Alternatively, a programmed processor (not shown) can be programmed with software to respond to the successive rising or falling edges, and with the time period, consult a table to find a corresponding temperature. A look-up table  56  programmed in the ASIC  54  or processor can be readily designed by those skilled in the art by incrementally raising the temperature of the thermistor  20  by known increments (such as 1° C.), and noting the corresponding digital signal period. This information can be stored in the table  56 . Nonlinearities in the temperature conversion process thus become less of a problem. If desired any non-linearity between the fuser temperature and the digital signal period can be compensated for by corresponding software functions that are adapted for removing the same. 
     The circuits of the ASIC  54  can be programmed to store predefined fuser temperatures for different operating conditions. The ASIC  54  can continuously monitor the fuser temperature by processing the periods of the pulse train  52 , and compare the monitored fuser temperature with the predefined temperatures. If the fuser temperature requires changing, the ASIC  54  provides a feedback signal to the AC control  22  to drive the fuser heater  14  in a manner to achieve the desired temperature. 
     With reference to  FIG. 3  of the drawings, there is illustrated the correspondence between the range of resistances of the thermistor  20  and the period of the digital signals output by the astable multivibrator  32 . A low thermistor resistance of 1,654 Ohm is converted to a digital signal with a period of 1.284 ms. A high thermistor resistance of 4,000,000 Ohms is converted to a digital signal having a period of 2,384 ms. Thus, a four decade change in resistance of the thermistor  20  is mapped or converted into a corresponding three decade change in the period of the digital signal output from the astable multivibrator  32 . The particular conversion correspondence between resistance and digital signal period shown in  FIG. 3  is merely exemplary and is not a necessity to the operation of the fuser temperature sensing system  12 . 
     While the preferred embodiment of the invention has been disclosed, other variations are readily possible. For example, the temperature conversion process does not require a digital signal train, but may employ the period of an AC signal generated by a voltage controlled analog oscillator. In addition, while the features of the invention have been described in connection with a printer, the principles and concepts of the invention can be employed as well in copiers, all-in-one fusers and other reproduction equipment. 
     The foregoing description of several methods and an embodiment of the invention has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise steps and/or forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.