Abstract:
A temperature control system for reducing flicker in an electrical resistance heater comprising: an electrical resistance heater divided into at least two electrical loads and; a source of electrical power for repetitively supplying electrical power to each of the at least two electrical loads at successive different time sub periods within a predetermined time period, wherein the sum of the power supplied to the at least two electrical loads is equal to the total power supplied to the electrical resistance heater during the predetermined time period.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to apparatus for controlling temperature and, more particularly, to apparatus for controlling the temperature of a temperature controlled heater to reduce flicker. 
     BACKGROUND OF THE INVENTION 
     Photothermography is an established imaging technology. In photothermography, a photosensitive media is exposed to radiation to create a latent image which can be thermally processed to develop the latent image. Devices and methods for implementing this thermal development process are generally known and include contacting the imaged photosensitive media with a heated platen, drum or belt, blowing heated air onto the media, immersing the media in a heated inert liquid and exposing the media to radiant energy of a wavelength to which the media is not photosensitive, e.g., infrared. Of these conventional techniques, the use of heated drums is particularly common. 
     A common photosensitive media usable in these imaging processes is known as a photothermographic media, such as film and paper. One photothermographic media has a binder, silver halide, organic salt of silver (or other reducible, light-insensitive silver source), and a reducing agent for the silver ion. In the trade, these photothermographic media are known as dry silver media, including dry silver film. 
     In order to precisely heat exposed photothermographic media, including film and paper, it has been found to be desirable to use electrically heated drums. In apparatus employing this technique, a cylindrical drum is heated to a temperature near the desired development temperature of the photothermographic media. The photothermographic media is held in close proximity to the heated drum as the drum is rotated about its longitudinal axis. When the temperature of the surface of the heated drum is known, the portion of the circumference around which the photothermographic media is held in close proximity is known and the rate of rotation of the drum is known, the development time and temperature of the photothermographic media can be determined. Generally, these parameters are optimized for the particular photothermographic media utilized and, possibly, for the application in which the photothermographic media is employed. 
     In order to achieve a high quality-image in the photothermographic media, very precise development parameters must be maintained. Generally, the circumference of the drum over which the photothermographic media travels will not vary significantly. Also, the rate of rotation of the drum, or the transport rate of the photothermographic media through the thermal processor, can be rather precisely maintained. However, it is generally more difficult to control and maintain the temperature of the surface of the drum. 
     In addition, other factors also contribute to inaccurate processing. The closeness of the proximity which the photothermographic media is held to the drum partially determines the temperature at which the emulsion in the photothermographic media is heated. Further, the presence of foreign particles between the drum and the photothermographic media can interrupt the flow of heat from the drum to the photothermographic media which can affect image quality. 
     Because many factors affect image quality, one of which is the temperature at which the photothermographic media is developed, the preciseness at which the surface temperature of the drum can be maintained is important to thermal processing of photothermographic media. 
     The temperature of the drum depends upon many factors. These include the rate at which heat is delivered to the drum, the thermal conductivity and the thermal mass of the drum, the thermal mass of the photothermographic media, the rate, i.e., the number of sheets (if sheet photothermographic media is used) of photothermographic media being processed, the ambient temperature, whether thermal processing is just beginning or whether the thermal processing is in the middle of a long run. 
     In addition, heated drums are used extensively in various other material processing applications. Examples include calendaring, laminating, coating and drying. 
     Typically, heat is delivered to such drums through the use of electrical resistance heating elements. Since the heated drum is rotating during thermal processing and since it is a desirable to deliver electrical power to the electrical resistance heating elements during rotation of the drum, is desirable to be able to deliver electrical power from a stationary power source, e.g., the standard AC line, to the moving, rotating drum. Electrical power may be delivered to the drum through the use of slip rings coupled to the drum. 
     In addition, to precisely control the temperature of the electrically heated drum there should be a means to sense the temperature of the drum and a means to control the electrical power applied to the electrical resistance heaters in response to the signal from the temperature sensor. 
     U.S. Pat. No. 5,580,478, issued Dec. 3, 1996, inventors Tanamachi et al., discloses such a heated drum processor where separate electrical resistance heaters heat a central heat zone and contiguous edge zones. Temperature control of the electrical heaters is obtained through duty cycle modulation. Solid state relays in the power circuit to the electrical heaters are turned on and off with zero crossing triggering. 
     Power transients cause flickering light in the lighting systems that share the affected power grid. New flicker suppression standards have become law in recent years in Europe. The common practice in flicker suppression has been to add suppression electronics to an apparatus thus increasing manufacturing costs. A common solution has been to use the AC input power and its zero crossing as a reference as to when to adjust throughput power to load. U.S. Pat. No. 4,908,956, issued Mar. 20, 1990, inventor Grund, U.S. Pat. No. 5,907,743, issued May 25, 1999, inventor Takahashi, U.S. Pat. No. 6,188,208, issued Feb. 13, 2001, inventors Glaser et al., are examples. They differ in how to adjust the power on/off timing relative to the zero crossing. All use electronic circuits to implement. Another invention, U.S. Pat. No. 5,818,208, issued Oct. 6, 1998, inventors Othman et el., uses electronics to measure the AC power voltages and load currents at the voltage source converter terminal and calculate active and reactive current loads in an effort to minimize flicker. 
     U.S. Pat. No. 6,420,685B1, issued Jul. 16, 2002, inventor Tanamachi discloses a control system for reducing flicker in an electrical resistance heater, a bidirectional solid state switching device connected between the source and the electrical resistance heater; and a control circuit for controlling the bidirectional solid state switching device to supply a varying, phase controlled duty cycle of current to the heater which effectively ramps heater power up and down in response to a binary control signal which randomly turns on the switching device. 
     There is thus a need for a way to control flicker in an electrical heater system that is efficient and cost effective. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a solution to the needs and problems discussed above. 
     According to a feature of the present invention, there is provided a temperature control system for reducing flicker in an electrical resistance heater comprising: 
     an electrical resistance heater divided into at least two electrical loads and; 
     a source of electrical power for repetitively supplying electrical power to each of said at least two electrical loads at successive different time sub periods within a predetermined time period, wherein the sum of the power supplied to said at least two electrical loads is equal to the total power supplied to said electrical resistance heater during said predetermined time period. 
     ADVANTAGEOUS EFFECT OF THE INVENTION 
     The invention has the following advantages. 
     1. Flicker suppression is achieved without reliance on additional electronic circuitry. 
     2. Flicker suppression is efficient and cost effective. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of a portion of a thermal processor utilizing a rotatable, electrically heated drum. 
         FIG. 2  is a cross-sectional view of the drum shown in FIG.  1 . 
         FIG. 3  is a high level block diagram of an electronic temperature control apparatus incorporating the present invention. 
         FIG. 4  is a block diagram of a processor communication board utilized in the temperature control apparatus of FIG.  3 . 
         FIG. 5  is a block diagram of a rotating board utilized in the temperature control apparatus of FIG.  3 . 
         FIG. 6  is a diagram useful in explaining the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In general, the present invention provides a temperature controlled, electrically heated drum with flicker suppression. A cylindrical drum has a surface and is rotatable on an axis. An electrical heater is thermally coupled to the surface of the cylindrical drum. Solid State Relays (SSRs), non-rotatably mounted in conjunction with the cylindrical drum and electrically coupled to the electrical heaters through slip rings, controls the temperature by controlling the flow of electricity to the electrical heaters in response to control signals from the non-rotatably mounted microprocessor. A temperature sensor mechanism, rotatably mounted in conjunction with the cylindrical drum sense the temperature of the surface of the cylindrical drum and produces temperature signals indicative thereof. A microprocessor, non-rotatably mounted with respect to the cylindrical drum, controls the temperature of the electrically heated drum by generating the control signals in response to the temperature signals. An optical mechanism, coupled to the temperature control means, the temperature sensor means and rotating microprocessor means, optically couples the temperature signals from the rotating temperature sensor means to the non-rotating microprocessor means. 
     A portion of a thermal processor utilizing a rotatable electrically heated drum  10  is illustrated in  FIGS. 1 and 2 . Such a thermal processor may be used to process medical diagnostic quality dry silver film. Cylindrical drum  10 , mounted on frame  1 , is rotatable around axis  12 . Optionally, exterior surface  14  of drum  10  may be coated with silicone layer  15 . Also, optionally, exterior surface  14  of drum  10  is divided into separately controlled heating zones  16 ,  18 ,  20 . Since the edges of surface  14  of drum  10  may cool more than the central portion of surface  14 , a central zone  16  is controlled independently of edge zones  18  and  20 . Photothermographic media (not shown) is held in close proximity of exterior surface  14  and drum  10  over a portion of the circumference of drum  10  by means of holding down rollers (not shown). With a known temperature of exterior surface  14  of drum  10 , typically 252 degrees Fahrenheit, a known rotational rate, and a known portion of circumference of surface  14  over which the photothermographic media passes, a known development temperature and dwell time can be achieved. After heated development, a cooling system  22  cools the photothermographic media to a temperature below development temperature. The cooled media is then transported to an output tray. 
     As shown in  FIG. 2 , cylindrical drum  10  is constructed from aluminum having a diameter, for example, of 8 inches (20.32 centimeters) and with a hollow interior and shell thickness for example, of 0.25 inches (0.635 centimeters). Mounted on the interior surface  34  of drum  10  are electrical resistance heaters  36 ,  38  and  40  adapted to heat zones  16 ,  18 ,  20 , respectively. Exterior surface  14  of drum  10  may have a very delicate silicone coating  15 , so temperature measurement of the drum is done internally in order not to damage the surface coatings. Mounted on the interior surface  34  of drum  10  are temperature sensors (RTDs)  42 ,  44  and  46  adapted to sense the temperature of zones  16 ,  18  and  20 , respectively. 
     The temperature of exterior surface  14  is maintained across drum  10  and from sheet to sheet of photothermographic media to within .+−0.0.5 degrees Fahrenheit in order to produce diagnostic quality images. 
     A high level block diagram of the major components of the temperature control circuitry is illustrated in FIG.  3 . Since drum  10  is rotating, communication to electrical resistance heaters  36 ,  38  and  40  is done by way of slip ring assembly  67  which is mounted on one end of cylindrical drum  10  and which rotates at the same rate as drum  10 . As shown in  FIG. 3 , circuit board  48  is optically coupled by stationary mounted optical receiver  50  positioned to optically cooperate with rotating circuit board  48 . One way communication occurs over optical communications link  66  from the rotating board to the non-rotating processor communication board  52  through optical receiver  50 . Rotating circuit board  48  rotates with drum  10  to communicate temperature information from the three drum heated zones  16 ,  18 ,  20  to software located on processor communications board  52  via link  66  to optical receiver  50 . Processor communications board  52  contains a microprocessor whose software interprets the coded temperature information from the three heater zones  16 ,  18 ,  20  and converts it to actual zone temperatures. The software then closes the control loop by calculating via a heater control algorithm whether the heater corresponding to the sensed temperature in a particular zone should be turned on or off. The microprocessor then turns on a solid state relay to apply power to the appropriate heater through slip ring assembly  67  A-E. 
     More detail of the function of the processor communication board  52  is shown in  FIG. 4.  120 Vac from source  70  of the imager in which drum  14  is mounted is brought in to the board  52  to power the processor heaters and supply 12 Vac to power the rotating board. The 12 Vac is supplied via step down transformer  100 . There are three solid state relays  101 ,  102  and  103  which control power to each of the three drum heaters  36 ,  38  and  40  under control of microprocessor  104 . Coded  12  bit digital temperature data is supplied to the microprocessor  104  from each of the three temperature sensors  42 ,  44 ,  46  via optical link  66  and optical receiver  50 . Communication to the rest of the imager is through the  12 C interface  105 . New software can also be downloaded via the communications system. Interface  105  also includes an RS232 communications port for service of the processor control system. 
     Referring now to  FIG. 5 , there is shown in greater detail the electrical components disposed on the rotating drum  10 . Slip rings  67  A-D supply controlled 120 Vac power to resistance heaters  36 ,  38  and  40 . 12 Vac power is also supplied via slip ring  67 E to bridge rectifier and filter  200  to produce a dc voltage supplied to +5V regulator  202 . +2.5V precision voltage reference  204  and precision voltage divider chain  206  provide d. c. voltages to Analog to Digital Converter  208  and current sources  210 - 216 . Current sources  210 ,  212  and  214  are respectively coupled to temperature sensors  46 ,  44 ,  42 . The temperature signals from sensors  42 ,  44 ,  46  are applied to analog mux  218  which is controlled by rotating microprocessor  220 . Mux  218  supplies the temperature signals serially to A to D converter  208  which converts them to digital signals which are read and processed by microprocessor  220  and then communicated over optical communications link  66  by microprocessor  220  and infrared LED  222 . 
     According to the invention, flicker is suppressed by distributing an approximately constant level of power to multiple loads over time. There are several components to this flicker suppression solution: 
     1). Divide the load into two or more smaller loads. 
     2). Switch the power on to these loads at different times within a set period of time. This period of time continuously repeats. The duration of power applied for any specific load within its period of time is what we call the duty cycle for that load&#39;s power consumption. A 50% duty cycle means power is applied for a half of a period. Ideally, the loads have been chosen so that each load&#39;s maximum duty cycle is proportional to the time difference between switching loads. So load A should switch off before or at the time load B switches on. 
     3). The time between switching loads is proportional to a load&#39;s power consumption as a ratio of the whole power consumption of the apparatus. This timing is referred as the phase. For example, if power consumption is divided into 4 equal loads, then each load is switched on at 90 degrees out of phase with the previous (360 degrees to a circle). If there were 3 loads and the third load consumed twice the power of the other two, then one could switch at 0 and 90 degrees for the two equal loads, and 180 degrees for the third load. Once the third load switches on, there will be no additional loads switching on for half a period. To further this example, lets say the period is 2 seconds. In this case a load switches on at 0 seconds, the other equal load at ½ second, and the large load at 1 second. Then the whole process repeats again at the 2-second mark, and for every 2 seconds thereafter. 
     In the configuration of the thermal processor described above, the invention to suppress flicker can be carried out in an exemplary way as follows. AC power from source  70  is supplied to electrical resistance heaters  36 ,  38 ,  40  to heat zones  16 ,  18  and  20 ; respectively by way of solid state relays  101 ,  102 ,  103  and slip rings  67 A,  67 B,  67 C. Relays  101 ,  102 ,  103  are controlled by microprocessor  104  to feed power to electrical resistance heaters  36 ,  38 ,  40  at different times within a set period of time. In the example given above, assuming power supplied to the middle zone  16  heated by heater  38  is twice the power supplied to end zones  18 ,  20  heated by heaters  36  and  40  respectively, at time 0 seconds., X watts of power would be supplied to heater  36  by switching on relay  103 . At time 0.5 seconds, relay  103  would be switched off, ending power supply to heater  36  and relay  101  would be switched on to supply X watts of power to heater  40 . At time 1.0 seconds, relay  101  is switched off and relay  102  switched on to 2× watts to heater  38 . At 2 seconds, power to heater  38  is stopped by switching relay  102  off. The, the process is repeated again during successive 2 second periods. This power cycle is illustrated in  FIG. 6  illustrates these signals. Signal A represents the power supplied to heater  36 , signal B represents the power supplied to heater  40 , and signal C represents the power supplied to heater  38 . 
     A time period of 2 seconds was used for this example to allow some thermal migration of the applied power to reach the temperature sensing RTDs. This period is dependent on the thermal design of the application, but in practice can vary from 1 to 2 seconds. The temperature of a zone is measured at the end of the period, and used to calculate the duty cycle for that zone&#39;s next period. The duty cycle calculation is that of a Proportional-Integral-Difference (PID) temperature control algorithm. The flicker control discussed reduces the magnitude of this application&#39;s peak current load by a factor of 3. In practice, the duty cycles were less then that shown in  FIG. 6  which show maximum load conditions. Shorter duty cycles still fall within the flicker control&#39;s reduction of the peak load draw of current. 
     For the condition of warming up the drum from a cold start condition, all the load current will be continuously. There is no temperature control in effect for this condition, so there will not be any current switching/flicker. Once the drum is within an operational temperature range, the PID temperature and flicker control begins. 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 PARTS LIST 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  10 
                 drum 
               
               
                   
                  11 
                 frame 
               
               
                   
                  12 
                 axis 
               
               
                   
                  14 
                 exterior surface 
               
               
                   
                  15 
                 silicone coating 
               
               
                   
                  16, 18, 20 
                 controlled heating zones 
               
               
                   
                  22 
                 cooling system 
               
               
                   
                  34 
                 interior surface 
               
               
                   
                  36, 38, 40 
                 electrical resistance heaters 
               
               
                   
                  42, 44, 46 
                 temperature sensors 
               
               
                   
                  48 
                 rotating circuit board 
               
               
                   
                  50 
                 optical receiver sensor 
               
               
                   
                  52 
                 processor communication board 
               
               
                   
                  66 
                 link 
               
               
                   
                  67 
                 slip ring assembly 
               
               
                   
                  70 
                 120 Vac power 
               
               
                   
                 100 
                 transformer 
               
               
                   
                 101 
                 solid state relay 
               
               
                   
                 102 
                 solid state relay 
               
               
                   
                 103 
                 solid state relay 
               
               
                   
                 104 
                 microprocessor 
               
               
                   
                 105 
                 12C and RS232 communication interfaces 
               
               
                   
                 200 
                 bridge rectifier and filter 
               
               
                   
                 204 
                 precision voltage reference 
               
               
                   
                 206 
                 precision voltage divider chain 
               
               
                   
                 208 
                 A to D converter 
               
               
                   
                 210 
                 current source 
               
               
                   
                 212 
                 current source 
               
               
                   
                 214 
                 current source 
               
               
                   
                 216 
                 current source 
               
               
                   
                 220 
                 microprocessor 
               
               
                   
                 222 
                 infrared LED