Abstract:
A control system for reducing flicker in an electrical resistance heater comprising: a source of AC (alternating current) current for supplying AC current to an electrical resistance heater; a bidirectional solid state switching device connected between said source and said 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 said heater which effectively ramps heater power up and down in response to a binary control signal which randomly turns on said switching device.

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 resistive electrical 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 then 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 useable 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 deducible, 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 logitudinal 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 thermographic 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. 
     U.S. Pat. No. 5,580,478, issued Dec. 3, 1996, inventors Tanamachi et al., discloses a temperature controlled, electrically heated drum for developing exposed photothermographic media. 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. A temperature control mechanism, rotatably mounted in conjunction with the cylindrical drum and electrically coupled to the electrical heater, controls the temperature by controlling the flow of electricity to the electrical heater in response to control signals. A temperature sensor is thermally coupled to the surface of the cylindrical drum. A temperature sensor mechanism, rotatably mounted in conjunction with the cylindrical drum and electrically coupled to the temperature sensor, senses 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 the microprocessor means, optically couples the temperature signals from the rotating temperature sensor means to the non-rotating microprocessor means and optically couples the control signals from the non-rotating microprocessor means to the rotating temperature control means. 
     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. 
     Although this technique is useful for the purpose for which it was intended, new flicker requirements of regulatory authorities in Europe (EC 65000-3-3) make this control technique unacceptable. 
     It is therefore desirable to provide a temperature control system for electrical resistor heaters that satisfy the new flicker requirements. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a solution to the problems discussed above. 
     According to a feature of the present invention, there is provided a control system for reducing flicker in an electrical resistance heater comprising a source of AC (alternating current) current for supplying AC current to an electrical resistance heater, a bidirectional solid state switching device connected between said source and said electrical resistance heater; and a control circuit for controlling said bidirectional solid state switching device to supply a varying, phase controlled duty cycle of current to said heater which effectively ramps heater power up and down in response to a binary control signal which randomly turns on said switching device. 
     ADVANTAGEOUS EFFECT OF THE INVENTION 
     The invention has the following advantages. 
     1. New flicker requirements of a European agency are met without any internal software changes to the temperature control algorithms and with only minor changes to the circuit board. 
     2. The control technique is simple, cost efficient and effective. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective 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 system incorporating the present invention. 
     FIG. 4 is a block diagram of a rotating board shown in FIG.  3 . 
     FIG. 5 is a diagrammatic view illustrating the known heater control system. 
     FIG. 6 is a diagrammatic view illustrating the heater control system of the present invention. 
     FIG. 7 is a schematic diagram of the system of FIG.  5 . 
     FIG. 8 is schematic diagram of the system of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 diagnostic quality dry silver film. Cylindrical drum  10 , mounted on frame  11 , is rotatable around axis  12 . Optionally, exterior surface  14  of drum may be coated with silicone layer  15 . Also optionally, exterior surface  14  of drum  10  is divided into zone separately controlled heating zones. Since the edges of surface  14  of drum  10  may cool faster 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  of drum  10  over a portion of the circumference of drum  10 . With a known temperature of exterior surface  14  of drum  10 , typically 255 degrees Fahrenheit, a known rotational rate, typically 2.5 revolutions per minute, 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, cooling rollers ( 22 ,  24 ,  26 ,  28 ,  30  and  32 ) cool the photothermographic media to a temperature below development temperature. 
     As an example, cylindrical drum is constructed from aluminum having a diameter of 6.25 inches (15.9 centimeters) and with a hollow interior and shell thickness 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  18 ,  16  and  20 , respectively. Exterior surface  14  of drum  10  may have a very delicate coating, so temperature measurement of the drum is done internally in order not to damage the surface coating. Mounted on the interior surface  34  of drum  10  are temperature sensors  42 ,  44  and  46  adapted to sense the temperature of zones  18 ,  16  and  20 , respectively. 
     Since drum  10  is rotating, communication to electrical resistance heaters  36 ,  38  and  40  is done by way of rotating circuit board  48  mounted on one end of cylindrical drum  10  which rotates at the same rate as drum  10 . Circuit board  48  is controlled by stationary mounted communications circuit board  50  positioned to optically cooperate with rotating circuit board  48 . Communication occurs over an optical communications link. 
     The temperature of exterior surface  14  is typically maintained across drum  10  and from sheet to sheet of photothermographic media to within ±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 . Rotating circuit board  48  rotates with drum  10  to communicate heater control information to drum  10  and to communicate temperature information to software located on system controller board  52  (stationary). Communications board  50  (stationary) converts serial data from system controller board  52  to optical data rotating board  48 , and vice versa. Machine interface board  54  supplies an ACCLOCK signal  56  which is used to synchronize serial communications between system controller board  52  and rotating board  48 . System controller board  52  provides memory  58  in which the temperature control software resides. Microprocessor  60 , time processing unit  62  and I/O unit  64  are used by the software to monitor and regulate the temperature of exterior surface  14  of drum  10 . 
     In general, software on system controller board  52  loads heater control data indicating which electrical resistance heaters  36 ,  38  and  40  to turn on or off into I/O unit  64  to be shifted serially to communication boards  50 . Communications board  50  converts the data to an optical signal which is sent to rotating board  48  over optical link  66 . Rotating board  66  interprets this data into signals which are used to switch power on or off independently to electrical resistance heaters  36 ,  38  and  40 . In response to the heater control data, rotating board  48  reads data from temperature sensors  42 ,  44  and  46  and sends this data via optical link  66  to communications board  50 . Communications board  50 , in turn, sends this data to system controller board  52 . In system controller board  52 , temperature data is read by time processing unit  62 . Software can then read this data and convert the temperature data into temperatures and react accordingly to turn electrical resistance heater  36 ,  38  and  40  on or off 
     FIG. 4 illustrates a block diagram of rotating board  48  attached to rotating drum  10 . Optical transmitter  92  is mounted on the rotational axis of drum  10  facing communications board  50 . Optical detector  94 , an infrared photosensor, is mounted next to optical transmitter  92  as close as possible to optical transmitter  92  and facing communications board  50 . All optical transmitters and sensors face each other across the space between communications board  50  and rotating board  48  at a distance of 0.6 inches (1.5 centimeters). 
     Control signals for electrical resistance heaters  36 ,  38  and  40  are received via optical link  66  by optical detector  94 . The control information is passed to shift register  96  through heater control bit latch  98  to solid state relay  100  for electrical resistance heater  36 , to solid state relay  102  for electrical resistance heater  38  and to solid state relay  104  for electrical resistance heater  40 . Watchdog timer  106  watches an interruption in the receipt of the serial data from optical link  66 . Received data is also passed from shift register  96  through framing detector  108  received serial data for validity and performs control functions. Temperature data is received from temperature sensors  42 ,  44  and  46  by RTD signal conditioner  112  and passed to an analog multiplexer  114  under control from state machine  110 . Provided the synchronization bits in the serial data received by optical detector  94  are correct, state machine  110  then transmits temperature data through V to F converter  116  to optical transmitter  92  for transmission across optical link  66  to communications board  50 . AC power is received by electrical resistance heaters  36 ,  38  and  40  through slip rings  67 . Transformer  118 , power supply  120  and AC clock generator  122  (HI  111 ) provide overhead functions. 
     Referring now to FIG. 5, there is shown a diagrammatic view illustrating a known heater control system. As shown, photothermographic processor drum  200  has electrical resistance Zone  1  heater  202 , Zone  2  electrical resistance heater  204  and Zone  3  electrical resistance heater  206 . AC power from power slip rings  208  is supplied over bus  210  to Zone  1  solid state relay with zero crossing triggering circuit  212 , to Zone  2  solid state relay with zero crossing triggering circuit  214  and to Zone  3  solid state relay with zero crossing triggering circuit  216 . Circuits  212 ,  214  and  216  supply switched AC power respectively to heaters  202 ,  204  and  206  over respective power links  218 ,  220  and  222 . Circuits  212 ,  214  and  216  receive heater control signals from signal decode and heater control bit latch  224  over control links  226 ,  228  and  230 . Latch  224  receives optically coupled control signals from the system control board (arrow  132 ). 
     FIG. 7 is a schematic diagram of relevant components of the Zone  2  heater system. Latch  224  is a MC74HC173, whose pin  4  supplies the heater control signal over control link  228 . Circuit  114  includes zero crossing optocoupler  240  (IS 02  type MOC  3033 ) and triac  242 . The control link  228  from latch  224  pin  4  turns on optocoupler  240  which turns on triac  242  (and thus Zone  2  heater  204  (FIG.  5 )) at the next AC line voltage zero crossing and maintains triac  242  in the on state until control link  228  goes low. At this time, the triac  242  will turn off the Zone  2  heater  204  current at the next AC line zero crossing. 
     The heater control system of FIGS. 5 and 7 has been found not to satisfy the new European flicker standards. 
     According to the present invention, the system of FIGS. 6 and 8 obviates the limitations of the FIGS. 5 and 7 system. As shown in FIG. 6, the Zone  2  heater control signal on link  228  from latch  224  is supplied to a microprocessor  250  which delays the heater control signal over link  252 . The Zone  2  solid state relay circuit  254  operates with random turn-on triggering. FIG. 8 shows microprocessor  250  to be PIC  12 C 508  and circuit  254  to include IS 02  optocoupler  256  and triac  242 . 
     By changing the optocoupler to a type MOC3022, the triac  242  can be turned on at any time (random turn-on). This allows us to turn on the triac  242  with a narrow pulse and the triac will then stay on until the next zero crossing of the AC line. 
     The program in the PIC microprocessor  250  operates by having two inputs. One is a square wave generated from the AC line and has it&#39;s transitions synchronized to the AC line zero crossings. The other input is the digital control line from latch  224  pin  4 . When the control input is high, a pulse is generated to the triac  242  after a variable delay time measured from the next AC line zero crossing. This delay time decreases in a linear manner until the delay time goes to zero at which time the triac trigger pulse occurs immediately after the AC line zero crossing. This effectively allows the triac  242  to conduct for the full line cycle and applies maximum power to the heater  204 . When the control line goes low the microprocessor  250  increases the delay time in a linear manner until the point is reached where the delay time is greater than the time for ½AC cycle. When this happens, the delay time is restarted and no trigger pulse is generated. This effectively applies no power to the heater  204 . 
     During the time when the delay is increasing or decreasing between these two extremes, the heater  204  is supplied with a varying, phase controlled duty cycle which effectively ramps the heater  204  power up and down in response to the binary control signal. This softens the turn-on and turn-off of the heater  204  and spreads the charge in line current over a longer time, which allows the unit to pass the new European flicker requirements. Moreover, the large expense of hardware and software design and re-qualification of a new design is mitigated, production is not impacted and resources for new product designs are available. 
     It will be understood that the random turn-on triggering circuit used to control the temperature of Zone  2  heater  204  could also be used to control the temperature of Zone  1  heater  202  and/or Zone  3  heater  206 . 
     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 
                 heated drum 
               
               
                   
                 11 
                 frame 
               
               
                   
                 12 
                 axis 
               
               
                   
                 14 
                 exterior surface 
               
               
                   
                 15 
                 silicone layer 
               
               
                   
                 16, 18, 20 
                 edge zones 
               
               
                   
                 22, 24, 26, 28, 30, 32 
                 rollers 
               
               
                   
                 34 
                 interior surface 
               
               
                   
                 36, 38, 40 
                 resistance heaters 
               
               
                   
                 42, 44, 46 
                 temperature sensors 
               
               
                   
                 48 
                 rotating circuit board 
               
               
                   
                 50 
                 mounted circuit board 
               
               
                   
                 52 
                 controller board 
               
               
                   
                 54 
                 interface board 
               
               
                   
                 56 
                 signal 
               
               
                   
                 58 
                 memory 
               
               
                   
                 60 
                 microprocessor 
               
               
                   
                 62 
                 processing unit 
               
               
                   
                 64 
                 I/O unit 
               
               
                   
                 66 
                 optical link 
               
               
                   
                 92 
                 optical transmitter 
               
               
                   
                 94 
                 optical detector 
               
               
                   
                 96 
                 shift register 
               
               
                   
                 98 
                 bit latch 
               
               
                   
                 200 
                 processor drum 
               
               
                   
                 202 
                 zone 1 heater 
               
               
                   
                 204 
                 zone 2 heater 
               
               
                   
                 206 
                 zone 3 heater 
               
               
                   
                 208 
                 slip rings 
               
               
                   
                 210 
                 over bus 
               
               
                   
                 212, 214, 216 
                 triggering circuit 
               
               
                   
                 218, 220, 222 
                 power links 
               
               
                   
                 224 
                 latch 
               
               
                   
                 226, 228, 230 
                 control links 
               
               
                   
                 240 
                 optocoupler 
               
               
                   
                 242 
                 triac 
               
               
                   
                 250 
                 micoprocessor 
               
               
                   
                 252 
                 overlink 
               
               
                   
                 254 
                 relay circuit 
               
               
                   
                 256 
                 ISO2 optocoupler