Patent Publication Number: US-6219496-B1

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates generally to an image-forming apparatus designed to form multi-colored images on a print sheet without any color shifts. 
     2. Background Art 
     Full color printers are known which are designed to form a multi-colored image by overlaying yellow, magenta, cyan, and block toner images in registration to a print sheet on a conveyer using a plurality of image-forming units placed in the direction of transport of the print sheet. In a printer of this type, misregistation between toner images will cause outlines off the toner images to be shifted from each other, resulting in reduction in image quality. In order to compensate for this shift, the printer prints the toner images on a conveyer belt and adjusts the interval between adjacent two of the image-forming units based on the images on the conveyer belt. It is, however, difficult to eliminate the shift between the toner images completely due to rises in temperature within the printer, of the conveyer belt itself, and of a belt drive shaft. Particularly, a fixing device usually generates a large quantity of heat, impinging upon the movement of the conveyer belt. Specifically, a print operation for a long time will cause the heat to be stored in a frame installed near the fixing device, thereby resulting in radiation of the heat to elevate the temperature of the conveyer belt and the belt drive shaft. When the print operation is turned off, the fixing device is placed in a standby mode so that the temperature thereof will be decreased. The on and off operations of the printer, thus, causes the temperature of the conveyer belt to be changed, which leads to a change in movement of the conveyer belt. The correction of a shift between toner images due to these thermal problems results in consumption of time in each print operation which will lead to a decrease in throughput. 
     SUMMARY OF THE INVENTION 
     It is therefore a principal object of the present invention to avoid the disadvantages of the prior art. 
     It is another object of the present invention to provide an image-forming apparatus designed to correct a belt speed for forming a multi-color image without any color shift and decrease in throughput. 
     According to one aspect of the invention, there is provided an image-forming apparatus which comprises: (a) a conveyer belt transporting a print sheet; (b) image-transferring units provided one for each of a plurality of primary colors, the image-transferring units being arrayed in a direction of transport of the print sheet to transfer primary-colored images to the print sheet on the conveyer belt, respectively; (c) a timer measuring an operating time of the apparatus; and (d) a control means for controlling positions where the images are formed by the image-transferring units on the print sheet conveyed by the conveyer belt based on the time measured by the timer. 
     According to the second aspect of the invention, there is provided an image-forming apparatus which comprises: (a) a conveyer belt transporting a print sheet; (b) image-transferring units provided one for each of a plurality of primary colors, the image-transferring units being arrayed in a direction of transport of the print sheet to transfer primary-colored images to the print sheet on the conveyer belt, respectively; (c) a timer measuring either or any more than two of an elapsed time from turning on of power or release of a sleep mode, a print-off time, and a print time; and (d) a control means for controlling a belt speed based on either or any more than two of the times measured by the timer. 
     According to the third aspect of the invention, there is provided an image-forming apparatus which comprises: (a) a conveyer belt transporting a print sheet; (b) image-transferring units provided one for each of a plurality of primary colors, the image-transferring units being arrayed in a direction of transport of the print sheet to transfer primary-colored images to the print sheet on the conveyer belt, respectively; (c) a timer measuring either or any more than two of an elapsed time from turning on of power or release of a sleep mode, a print-off time, and a print time; and (d) a control means for controlling an image-transferring cycle of each of the image-transferring units based on either or any more than two of the times measured by the timer. 
     In a tandem printer structure, the number of lines between the image-forming units (i.e., an interval between the image-forming units expressed in number of lines which can be drawn therebetween) is equal to a passage time between the image-transferring units/an image-transferring cycle (a time lag when each color image starts to be printed). The passage time between the image-transferring units is equal to an image-transferring unit-to-image-transferring unit interval/a belt speed. When a color shift is occurring, an apparent number of lines between the image-forming units is changed. Thus, the color shift may be corrected by changing any of the above terms. 
     According to the fourth aspect of the invention, there is provided an image-forming apparatus which comprises: (a) a conveyer belt transporting a print sheet; (b) image-transferring units provided one for each of a plurality of primary colors, the image-transferring units being arrayed in a direction of transport of the print sheet to transfer primary-colored images to the print sheet on the conveyer belt, respectively; (c) a timer measuring either or any more than two of an elapsed time from turning on of power or release of a sleep mode, a print-off time, and a print time; and (d) a control means for controlling intervals between the image-transferring units based on either or any more than two of the times measured by the timer. 
     According to the fifth aspect of the invention, there is provided an image-forming apparatus which comprises: (a) a conveyer belt transporting a print sheet; (b) image-transferring units provided one for each of a plurality of primary colors, the image-transferring units being arrayed in a direction of transport of the print sheet to transfer primary-colored images to the print sheet on the conveyer belt, respectively; (c) a timer measuring either or any more than two of an elapsed time from turning on of power or release of a sleep mode, a print-off time, and a print time; and (d) a control means for controlling an image transfer start timing of each of the image-transferring units based on either or any more than two of the times measured by the timer. 
     In the preferred mode of the invention, when a temperature of a fixing device is lower than a set temperature, the elapsed time from turning on of power or release of the sleep mode is set to zero (0), the print-off time is set to infinity, the previous print time is set to zero (0), and control of the controlling means is performed. The controlling means may also drive the conveyer belt for a constant period of time prior to a color shift detecting operation. The driving of the conveyer belt may be performed intermittently. 
     When the temperature of a fixing device is lower than the set temperature, a color shift detecting operation may be performed as is. 
     The timer has a clock circuit supported at least by a backup battery or a structure reading therein information on an external time. Based on this structure, after the power is turned on or the release of the sleep mode, the controlling means calculates the print-off time in a power-off condition from a final printing end time stored in a nonvolatile memory and performs each control. 
     According to the sixth aspect of the invention, there is provided an image-forming apparatus which comprises: (a) a conveyer belt transporting a print sheet; (b) image-transferring units provided one for each of a plurality of primary colors, the image-transferring units being arrayed in a direction of transport of the print sheet to transfer primary-colored images to the print sheet on the conveyer belt, respectively; (c) a timer measuring at least a print-off time and a print time; (d) a storage means for storing therein a first belt speed change curve for obtaining a belt speed estimated as a function of the print time and a second belt speed change curve for obtaining a belt speed estimated as a function of the print-off time when a print operation is started again after a lapse of the print-off time; and (e) a controlling means determining a belt speed corresponding to the previous print time from the first belt speed change curve based on the print-off time and the print time measured by the timer, sets a point on the second belt speed change curve matching the determined belt speed to a count start point of the print-off time to further determine a belt speed on the second belt speed change curve after a lapse of the print-off time, and considers the determined belt speed as one after a lapse of a print time corresponding to the belt speed on the first belt speed change curve in the current print operation to control the belt speed. 
     In the preferred mode of the invention, the storage means has a table defining first belt speed correction values and second belt speed correction values corresponding to the first belt speed change curve and the second belt speed change curve. The controlling means determines the first belt speed correction value from the table which corresponds to the previous print time, determines a point on which the second belt speed correction value matching the first belt speed correction value is located as the count start point to further determine the second belt speed correction value on the table after a lapse of the print-off time, and considers the determined second belt speed correction value as one after a lapse of a print time corresponding to the first belt speed correction value on the table in the current print operation to perform control from that time. 
     According to the seventh aspect of the invention, there is provided an image-forming apparatus which comprises: (a) a conveyer belt transporting a print sheet; (b) image-transferring units provided one for each of a plurality of primary colors, the image-transferring units being arrayed in a direction of transport of the print sheet to transfer primary-colored images to the print sheet on the conveyer belt, respectively; (c) a timer measuring at least a print-off time and a print time; (d) a storage means for storing therein a first belt speed change curve for obtaining a belt speed estimated as a function of the print time and a second belt speed change curve for obtaining a belt speed estimated as a function of the print-off time when a print operation is started again after a lapse of the print-off time; and (e) a controlling means determining a belt speed corresponding to the previous print time from the first belt speed change curve based on the print-off time and the print time measured by the timer, sets a point on the second belt speed change curve matching the determined belt speed to a count start point of the print-off time to further determine a belt speed on the second belt speed change curve after a lapse of the print-off time, and considers the determined belt speed as one after a lapse of a print time corresponding to the belt speed on the first belt speed change curve in the current print operation to control an image-transferring cycle of each of the image-transferring units. 
     In the preferred mode of the invention, the storage means has a table defining first image-transferring cycle correction values and second image-transferring cycle correction values corresponding to the first belt speed change curve and the second belt speed change curve. The controlling means determines the first image-transferring correction value from the table which corresponds to the previous print time, determines a point on which the second image-transferring correction value matching the first image-transferring correction value is located as the count start point to further determine the second image-transferring correction value on the table after a lapse of the print-off time, and considers the determined second image-transferring correction value as one after a lapse of a print time corresponding to the first image-transferring correction value on the table in the current print operation to perform control from that time. 
     According to the eighth aspect of the invention, there is provided an image-forming apparatus which comprises: (a) a conveyer belt transporting a print sheet; (b) image-transferring units provided one for each of a plurality of primary colors, the image-transferring units being arrayed in a direction of transport of the print sheet to transfer primary-colored images to the print sheet on the conveyer belt, respectively; (c) a timer measuring at least a print-off time and a print time; (d) a storage means for storing therein a first belt speed change curve for obtaining a belt speed estimated as a function of the print time and a second belt speed change curve for obtaining a belt speed estimated as a function of the print-off time when a print operation is started again after a lapse of the print-off time; and (e) a controlling means determining a belt speed corresponding to the previous print time from the first belt speed change curve based on the print-off time and the print time measured by the timer, sets a point on the second belt speed change curve matching the determined belt speed to a count start point of the print-off time to further determine a belt speed on the second belt speed change curve after a lapse of the print-off time, and considers the determined belt speed as one after a lapse of a print time corresponding to the belt speed on the first belt speed change curve in the current print operation to control intervals between the image-transferring units. 
     In the preferred mode of the invention, the storage means has a table defining first image-transferring unit-to-image-transferring unit interval correction values and second image-transferring unit-to-image-transferring unit interval correction values corresponding to the first belt speed change curve and the second belt speed change curve. The controlling means determines the first image-transferring unit-to-image-transferring unit interval correction value from the table which corresponds to the previous print time, determines a point on which the second image-transferring unit-to-image-transferring unit correction value matching the first image-transferring unit-to-image-transferring unit correction value is located as the count start point to further determine the second image-transferring unit-to-image-transferring unit correction value on the table after a lapse of the print-off time, and considers the determined second image-transferring unit-to-image-transferring unit correction value as one after a lapse of a print time corresponding to the first image-transferring unit-to-image-transferring unit correction value on the table in the current print operation to perform control from that time. 
     According to the ninth aspect of the invention, there is provided an image-forming apparatus which comprises: (a) a conveyer belt transporting a print sheet; (b) image-transferring units provided one for each of a plurality of primary colors, the image-transferring units being arrayed in a direction of transport of the print sheet to transfer primary-colored images to the print sheet on the conveyer belt, respectively; (c) a timer measuring at least a print-off time and a print time; (d) a storage means for storing therein a first belt speed change curve for obtaining a belt speed estimated as a function of the print time and a second belt speed change curve for obtaining a belt speed estimated as a function of the print-off time when a print operation is started again after a lapse of the print-off time; and (e) a controlling means determining a belt speed corresponding to the previous print time from the first belt speed change curve based on the print-off time and the print time measured by the timer, sets a point on the second belt speed change curve matching the determined belt speed to a count start point of the print-off time to further determine a belt speed on the second belt speed change curve after a lapse of the print-off time, and considers the determined belt speed as one after a lapse of a print time corresponding to the belt speed on the first belt speed change curve in the current print operation to control an image-transferring start timing of each of the image-transferring unit. 
     In the preferred mode of the invention, the storage means has a table defining first image-transferring start timing correction values and second image-transferring start timing correction values corresponding to the first belt speed change curve and the second belt speed change curve. The controlling means determines the first image-transferring start timing correction value from the table which corresponds to the previous print time, determines a point on which the second image-transferring start timing correction value matching the first image-transferring start timing correction value is located as the count start point to further determine the second image-transferring start timing correction value on the table after a lapse of the print-off time, and considers the determined second image-transferring start timing correction value as one after a lapse of a print time corresponding to the first image-transferring start timing correction value on the table in the current print operation to perform control from that time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only. 
     In the drawings: 
     FIG. 1 is a cross sectional view which shows an image-forming device according to the invention; 
     FIG. 2 is a block diagram which shows a circuit structure of an image-forming apparatus; 
     FIG. 3 is a graph which shows a change in speed of a conveyer belt when idled prior to a color shift detecting operation following initialization; 
     FIG. 4 is a graph which shows a change in speed of a conveyer belt when a print operation is started from a standby mode; 
     FIG. 5 is a graph which shows a change in speed of a conveyer belt when corrected; 
     FIG. 6 is a graph which shows a change in speed of a conveyer belt when a print operation is terminated before the speed of the conveyer belt becomes stable, and a subsequent print operation is started; 
     FIG. 7 is a flowchart of a program performed after power supply until a print operation is performed; 
     FIG. 8 is a flowchart of a program performed when a conveyer belt is idled until a color shift detecting operation is performed; 
     FIG. 9 is a flowchart of a program performed to correct speed of a conveyer belt after a color shift detecting operation when a print operation is started again from a print standby mode; 
     FIG. 10 is a graph which shows a relation between a belt speed correction and a print time; 
     FIG. 11 is a graph which shows a relation between a belt speed correction and a print time when a print-off time is short; 
     FIG. 12 is a graph which shows how to determine approximate values of belt speed correction values; 
     FIG. 13 is a graph which shows how to determine approximate values of belt speed correction values when a print operation is started again; 
     FIG. 14 is a graph which shows a change in speed of a conveyer belt after a print operation is resumed within a relatively short time; 
     FIG. 15 is a graph which shows a change in speed of a conveyer belt when corrected after a print operation is resumed within a relatively short time; 
     FIG. 16 is a flowchart of a program performed to initialize a print time count; 
     FIG. 17 is a flowchart of a program performed to determine a correction value; 
     FIG. 18 is a flowchart of a program performed when a print operation is terminated; 
     FIG. 19 is a flowchart of a program performed to handle a print-off time count; 
     FIG. 20 is a block diagram which shows an image-forming device according to the second embodiment of the invention; and 
     FIG. 21 is an illustration which shows how to determine positions of image-transferring units. 
     FIG. 22 is Table  1  showing speed correction values corresponding to elapsed time from start of belt movement. 
     FIG. 23 is Table  2  showing elapsed time from start of belt movement corresponding to print-off time. 
     FIG. 24 is Table  3  showing first belt speed correction values corresponding to print time and print-off time. 
     FIG. 25 is Table  4  showing print-off time counts corresponding to print-off time group numbers. 
     FIG. 26 is Table  5  showing print time initial values corresponding to print time group numbers and print-off time group numbers. 
     FIG. 27 is Table  6  showing print time counts corresponding to print time group numbers. 
     FIG. 28 is Table  7  showing correction information corresponding to print time group number. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like numbers refer to like parts in several views, particularly to FIG. 1, there is shown an image-forming device  10  according to the invention which will be discussed below as a full color printer. 
     The image-forming device  10  has four print assemblies  20 Y,  20 M,  20 C, and  20 K arranged in line and an endless conveyer belt  22  traveling through the print assemblies  20 Y to  20 K. The conveyer belt  22  is made of a synthetic resin material such as fluorine-contained polymers (e.g., Polyvinylidene Fluoriden) which has high durability and ability to hold a print sheet P statically. The conveyer belt  22  is wound around four rollers  24   a ,  24   b ,  24   c , and  24   d . The roller  24   a  is a drive roller which also works as an AC discharge roller to remove a charge from the conveyer belt  22 . The roller  24   b  is a driven roller and also works as an electrifying roller to electrifies the conveyer belt  22 . The rollers  24   c  and  24   d  are guide rollers. The roller  24   d  also works as a tension roller which provides a given degree of tension to the conveyer belt  22 . 
     A hopper  26  is disposed beneath the conveyer belt  22  which stores therein a stack of print sheets P. A pickup roller  28  picks up one of the print sheets P from the hopper  26  and carries it to the conveyer belt  22  through a pair of paper feed rollers  30 . The print sheet P is transported by the conveyer belt  22  to the print assemblies  20 Y,  20 M,  20 C, and  20 K in which full color images are printed, fed to a fixing device  32 , and ejected through a guide roller (not shown) installed on an end cover  16  toward a stacker provided on an upper surface of a top cover  14 . 
     The conveyer belt  22  is charged by the driven roller  24   b  to hold thereon the print sheet P when it reaches the conveyer belt  22  through the driven roller  24   b . The print sheet is, thus, moved to a printing station in a given positional relation to the conveyer belt  22 . The drive roller  24   a , as described above, works as the discharge roller which removes charges from the conveyer belt  22 , thereby causing the print sheet P to be separated from the conveyer belt  22  when it reaches the drive roller  24   a . The print sheet P is next carried to the fixing device  32 . 
     The print assemblies  20 Y,  20 M,  20 C, and  20 K are identical in structure. The primary distinction between them is the color of toner particles contained therein. Specifically, the print assembly  20 Y stores therein developer containing yellow toner particles to form a yellow toner image on the print sheet P on the conveyer belt  22 . The print assembly  20 M stores therein developer containing magenta toner particles to form a magenta toner image on the print sheet P on the conveyer belt  22 . The print assembly  20 C stores therein developer containing cyan toner particles to form a cyan toner image on the print sheet P on the conveyer belt  22 . 
     When the print sheet P enters the print station at the driven roller  24   b  and passes through the print assemblies  20 Y,  20 M,  20 C, and  20 K, yellow, magenta, cyan, and black toner images are overlaid to form a full color image on the print sheet P. The print sheet P is next transported from the drive roller  24   a  to the heat roller type fixing device  32  in which the full color image is thermally fixed on the print sheet P. 
     Each of the print assemblies  20 Y,  20 M,  20 C, and  20 K is designed as an image transferring unit  20 , as shown in FIG. 2, which includes a photosensitive drum  34 , a charging roller (not shown), a developing device (not shown), and a toner cleaner (not shown). Each image-forming unit  20  is installed detachably on a frame  12 . 
     Prior to describing an operation of the image-forming device  10 , how it was developed will be discussed in detail below. 
     The conventional tandem color printers wherein a print sheet is transported by a conveyer belt to a print station in which a plurality of image transfer units are placed in line have problems in that a belt drive shaft is heated by radiation of heat from a fixing device located near the belt drive shaft, leading to a local rise in temperature of the conveyer belt, and the conveyer belt is cooled as moving away from the belt drive shaft, which results in a considerable change in temperature of the conveyer belt, thereby causing the speed of the conveyer belt to be changed. The change in speed of the conveyer belt will cause the timing where each of Y, M, C, and K toner images is transferred to the print sheet to be shifted from the timing where the print sheet passes a corresponding one of the image transfer units, thereby resulting in a shift between colors. 
     Specifically, the main factor of the change in speed of the conveyer belt is thermal expansion of the conveyer belt caused by transfer of the heat from the belt drive shaft. The greater a difference in temperature between the conveyer belt and the belt drive shaft, the higher will be the speed of the thermal expansion of the conveyer belt. As the difference in temperature reaches zero, the speed of the thermal expansion becomes low. Specifically, the thermal expansion of the conveyer belt occurs at an exponential rate. 
     Upon turning off of the printer, the conveyer belt is cooled by surrounding air so that the temperature thereof is decreased at an exponential rate until a difference in temperature between the conveyer belt and the surrounding air reaches zero. The temperature of the conveyer belt at the start of a print operation, thus, depends upon the temperature of the conveyer belt at the end of a previous print operation and the elapsed time between the end of the print operation and resumption thereof (which will also referred to as a belt rest time or print-off time below). 
     As will be apparent from the above discussion, the speed of the conveyer belt depends upon the temperatures within the printer and of the conveyer belt and the belt drive shaft each of which is determined as a function of the time interval between the end of the print operation and resumption thereof plus the elapsed time from the resumption. A change in speed of the conveyer belt will result in a shift between colors of toner images on the print sheet. 
     The image-forming device  10  also includes, as shown in FIG. 2, a controller  36  and a print engine controller  40 . The controller  36  has an operator panel  38  and communicates with a host computer to separate a color image to be printed into primary color images (i.e., toner images) and to control the whole operation of the image-forming device  10 . The print engine controller  40  controls mechanisms of the image-forming device  10  and includes a main control circuit  42 , an LED control circuit, and a motor control circuit  44 . The main control circuit  42  consists of a timer  40   a , a memory  40   b , and a speed control circuit  40   c . The timer  40   a  measures the elapsed time from turning on of the image-forming device  10  or from release of a sleep mode of operation, the print off-time that is the time interval between the end of the print operation and resumption thereof, and the printing time. The memory  40   b  stores correction tables listing first belt speed correction values and second belt speed correction values, as will be described in detail later. The speed control circuit  40   c  controls the speed of the conveyer belt  22  as a function of the times measured by the timer  40   a.    
     The timer  40   a  is made up of a clock generator (not shown) having a crystal oscillator and a counter (not shown) counting the number of clocks generated by the clock generator. 
     The memory  40   b  is made of a ROM or an EPROM and may alternatively be a memory card for facilitating ease of rewriting of the correction tables stored therein. 
     The speed control circuit  40   c  is made of a central processing unit which receives a signal indicative of the time measured by the timer  40   a  and looks up the table in the memory  40   b  to output a control signal indicative of a target speed to the motor control circuit  44 . The motor control circuit  44  controls the electric motor  70  to bring the speed of the drive roller  24   a  of the conveyer belt  22  into agreement with the target speed. 
     The LED control circuit  46  controls an LED head installed in each of the image transfer units  20 . 
     The speed control circuit  40   c  controls the speed of the conveyer belt  22  in two modes of operation: a belt idle mode and a belt speed correction mode as will be described below in detail. 
     In an initial mode of printer operation following turning on of the image-forming device  10 , before the top cover  14  is closed, or in a sleep mode of printer operation, it is impossible for the timer  40   a  to determine the print off-time, so that the belt speed correction mode, as will be described later in detail, cannot be entered even after the top cover  14  is closed or the sleep mode is released. Therefore, when the temperature of the fixing device  32  is higher than a set value (e.g., 80° C.), the speed control circuit  40   c  idles the conveyer belt  22  for a given period of time (e.g., at the same speed as that in a monochrome print mode for 60 sec.) to stabilize the speed of the conveyer belt  22  prior to a typical color shift detecting operation in which toner images are printed on the conveyer belt  22  to detect a shift between colors which is to be corrected when toner images are transferred in registration to the print sheet P. This is because when the fixing device  32  is warmed, it seems that the temperature of a drive shaft of the conveyer belt  22  and the drive roller  24   a  is elevated, so that the speed of the conveyer belt  22  is slower than a saturated speed. Alternatively, when the temperature of the fixing device  32  is lower than the set value, the speed control circuit  40   c  does not idle the conveyer belt  22 . 
     FIG. 3 shows a change in speed of the conveyer belt  22  during the belt idle mode following initialization, as will be described later in detail, prior to the color shift detecting operation. At the start of the initialization, the speed of the conveyer belt  22 , as indicated by broken and solid lines, depends upon the print off-time after the last print operation and the history of print on and off operations before the last print operation, but it becomes stable after the conveyer belt  22 , i.e., the drive roller  24   a  is idled for the given period of time required for the conveyer belt  22  to be saturated in speed after the print off-time reaches infinity. 
     Additionally, it is also impossible for the speed control circuit  40   c  to enter the belt speed correction mode when the print engine controller  40  is turned off to stop controlling the mechanisms in the print standby mode, as will be described later in detail, so that the timer  40   a  is deactivated. Therefore, when the temperature of the fixing device  32  is higher than the set value (e.g., 80° C.), the speed control circuit  40   c  enters the belt idle mode to idle the conveyer belt  22  for the given period of time to stabilize the speed of the conveyer belt  22  prior to the color shift detecting operation. Alternatively, when the temperature of the fixing device  32  is lower than the set value, the color shift detecting operation is performed without idling the conveyer belt  22 . Afterwards, when the timer  40   a  becomes activated, the print-off time and the print time measured and held in the timer  40   a  are reset, and the print operation is started. 
     When the timer  40   a  starts in the print standby mode, the speed control circuit  40   c  enters the belt speed correction mode to correct the speed of the conveyer belt  22 , as will be discussed below in detail. 
     Usually, upon start of the print operation following the print standby mode, the speed of the conveyer belt  22  is changed, as shown in FIG. 4, as a function of changes in temperature of the drive shaft of the conveyer belt  22 , the drive roller  24   a , and the conveyer belt  22 . An exponential approximate curve which is approximate to the change in speed of the conveyer belt  22  is defined and used to make a correction table listing speed correction values for correcting the speed of the drive roller  24   a  (i.e., the speed of the motor  70 ). The correction table is stored in the memory  40   b . The speed control circuit  40   c  reads the print time (i.e., the elapsed time from the start of the print operation) out of the timer  40   a  to correct the speed of the conveyer belt  22  by look-up using the correction table in the memory  40   b  and keeps it, as shown in FIG. 5, constant. 
     The initial speed of the conveyer belt  22  at the start of the belt speed correction mode, in fact, depends upon the print off-time after the last print operation is completed, the print time in the last print operation, and the history of print on and off operations before the last print operation. Particularly, in a case where the print operation is resumed within a short period of time following termination of the last print operation before the speed of the conveyer belt  22  becomes stable, the speed of the conveyer belt  22  upon resumption of the print operation is almost equal to that upon termination of the last print operation. The longer the length of time the print operation is paused, the greater will be a change in speed of the conveyer belt  22  upon resumption of the print operation. A plurality of correction tables, thus, need to be prepared as a function of the length of time the print operation is paused, but it results in a greatly consumed storage area of the memory  40   b . In order to alleviate this problem, the image-forming device  10  of this embodiment uses correction tables one in each print mode regardless of the print-off time and changes, as discussed below in detail, an initial value to be read out of each of the correction tables in view of history of print on and off operations. 
     In the following discussion, it is premised that changes in speed of the conveyer belt  22  when the conveyer belt  22  is at rest and moving are approximate to exponential curves, respectively. 
     The first belt speed curve f 1 (t) used to estimate the speed of the conveyer belt  22  as a function of the print time and the second belt speed curve f 2 (t) used to estimate the speed of the conveyer belt  22  upon resumption of the print operation as a function of the print off-time may be represented as 
       f   1 ( t )= v   0 ·{1−α·Exp(− t /τ0)}  (1) 
     
       
           f   2 ( t )= v   0 ·{1−α+α·Exp(− t/τ 1)}  (2) 
       
     
     where v 0  is the saturated speed, α is a maximum value of a change in speed, τ0 is a time constant indicating a rate of change in temperature of the drive roller  24   a  during the print operation, and τ1 is a time constant indicating a rate of change in temperature of the drive roller  24   a  during the standby mode of print operation. 
     In a case where the print operation is resumed the print off-time T after the last print operation is terminated before the speed of the conveyer belt  22  becomes stable, changes in speed of the conveyer belt  22  may be, as shown in FIG. 6, expressed using the first and second belt speed curves f 1 (t) and f 2 (t). 
     In FIG. 6, the following relations are met at the end of the last print operation and at the start of the current print operation. 
     
       
           v   1 = f   1 ( tn −1)= f   2 (δ t ) 
       
     
     
       
           v   0 ·{1−α·Exp(− tn −1/τ0)}= v   0 ·{1−α+α·Exp(−δ t/τ 1)}  (3) 
       
     
     
       
           v   2 = f   1 ( tn )= f   2 (δ t+T ) 
       
     
     
       
           v   0 ·{1−α·Exp(− tn/τ 0)}= v   0 ·{1−α+α·Exp(−δ t+T/τ 1)}  (4) 
       
     
     where v 1  is the speed upon termination of the last print operation, v 2  is the speed upon start of the current print operation, tn is the first value (sec.) to be read out of the correction table at the start of the current print operation (i.e., the elapsed time from the start of the print operation), tn−1 is the last value (sec.) read out of the correction table at the end of the last print operation (i.e., the elapsed time between the start and end of the print operation), and T is the print off-time from the end of the last print operation. 
     From the equations (3) and (4), the relation between the first value tn to be read out of the correction table at the start of the current print operation and the last value tn−1 read out of the correction table at the end of the last print operation is 
     
       
           tn =−τ0·ln{1−Exp(− T/τ 1)·[1−Exp(− tn −1/τ0)]} 
       
     
     When the temperature of the fixing device  32  is lower than the set value (e.g., 80° C.), it hardly changes, as described above, the temperature of the drive shaft of the conveyer belt  22  and the drive roller  24   a . Therefore, in this case, the speed control circuit  40   c  sets the elapsed time from turning on of the image-forming device  10 , closing of the top cover  14 , or release of the sleep mode to zero (0), the print-off time to infinity, and the print time in the last print operation to zero (0) and controls the speed of the conveyer belt  22  in the above described manner. 
     FIG. 7 shows a flowchart of a program or sequence of logical steps performed by the image-forming device  10  after turned on. 
     After entering the program, the routine proceeds to step S 11  wherein peripheral circuits, i.e., the LED control circuit  46  and the motor control circuit  44  are initialized. The routine proceeds to step S 12  wherein a memory test is performed to test the memory  42   b , a control program storage ROM (not shown), and a RAM installed in the print engine controller  40 . The routine proceeds to step S 13  wherein parameters indicating the print-off time and the print time stored in the memory  42   b  and parameters indicating a paper feed outlet and a print mode (a color or monochrome mode) stored in the RAM are reset. The routine proceeds to step S 14  wherein the printer mechanisms are initialized to check the presence of the image-transferring units  20 , the remaining amount of toner, a paper jam, etc. 
     The routine proceeds to step S 15  wherein it is determined whether the temperature of the fixing device  32  is higher than 80° C. or not. If a YES answer is obtained, then the routine proceeds to step S 16  wherein the conveyer belt  22  is idled at the same speed as that in the monochrome print mode for 60 sec. in the belt idle mode. Alternatively, if a NO answer is obtained, then the routine proceeds directly to step S 17  wherein the color shift detecting operation is performed. The routine proceeds to step S 18  wherein it is determined whether a print request has been made or not. If a NO answer is obtained, then the routine repeats step S 18 . Alternatively, if a YES answer is obtained, then the routine proceeds to step S 19  wherein the print operation is started. 
     If an alarm signal is outputted, the routine proceeds to step S 21  wherein the presence of a print sheet left in a paper transport path, for example, due to a paper jam occurring in the previous print operation is checked. The routine proceeds to step S 22  wherein it is determined whether the print sheet left in the paper transport path has been retrieved or not. If a NO answer is obtained, then the routine repeats step S 22 . Alternatively, if a YES answer is obtained, then the routine proceeds to step S 14 . 
     FIG. 8 shows a sequence of steps performed in FIG. 7 prior to the color shift detecting operation in brief. 
     When the cover  14  is closed after the power is turned on, so that the alarm is turned off or when the sleep mode is released (step S 101 ), the speed control circuit  40   c  determines whether the temperature of the fixing device  32  is higher than 80° C. or not (step S 102 ). If the temperature of the fixing device  32  is higher than 80° C., then the speed control circuit  40   c  idles the conveyer belt  22  at the same speed as that in the monochrome mode for 60 sec. (step S 103 ). Alternatively, if the temperature of the fixing device  32  is lower than 80° C., then the color shift detecting operation is performed without idling the conveyer belt  22  (step S 104 ). 
     FIG. 9 shows a flowchart of a subprogram performed in the belt speed correction mode immediately before step S 18  following the color shift detecting operation in step S 17  or the print operation in step S 19  in FIG.  7 . 
     In step S 201 , the timer  40   a  is activated to measure the print-off time upon termination of the color shift detecting operation or the print operation. The routine proceeds to step S 202  wherein it is determined whether the power supplied to the fixing device  32  is turned off or not. If a NO answer is obtained meaning that the power is being supplied to the fixing device  32 , then the routine proceeds to step S 203  wherein the speed control circuit  40   c  determines whether there is a print job or not. If a YES answer is obtained, then the routine proceeds to step S 204  wherein the print operation is started, and the above described belt speed correction is performed. Alternatively, if a NO answer is obtained, then the routine returns back to step S 202 . 
     If a YES answer is obtained in step S 202  meaning that the power is not supplied to the fixing device  32 , then the routine proceeds to step S 205  wherein it is determined whether the speed control circuit  40   c  is stopping controlling the printer mechanisms or not. If a NO answer is obtained meaning that the control of the print mechanism is being performed, then the routine proceeds to step S 206  wherein there is a print job or not. If a NO answer is obtained, then the routine repeats step S 206 . Alternatively, if a YES answer is obtained, then the routine proceeds to step S 207  wherein it is determined whether the temperature of the fixing device  32  is higher than 80° C. or not. If a NO answer is obtained meaning that the image-forming device  10  is in a cold condition, then the routine proceeds to step S 208  wherein the print-off time and the print time stored in the timer  40   a  are reset. Alternatively, if a YES answer is obtained, then the routine proceeds directly to step S 209  wherein the print operation is started. 
     If a YES answer is obtained in step S 205  meaning that the speed control circuit  40   c  is stopping controlling the printer mechanisms, then the routine proceeds to step S 210  wherein there is a print job or not. If a NO answer is obtained, then the routine repeats step S 210 . Alternatively, if a YES answer is obtained, then the routine proceeds to step S 211  wherein it is determined whether the temperature of the fixing device  32  is higher than 80° C. or not. If a YES answer is obtained, then the routine proceeds to step S 212  to perform the conveyer belt  22  is idled in the manner as described above. Alternatively, if a NO answer is obtained meaning that the image-forming device  10  is in a cold condition, then the routine proceeds directly to step S 213  wherein the color shift detecting operation is performed. The routine proceeds to step S 214  wherein the print-off time and the print time stored in the timer  40   a  are reset. The routine proceeds to step S 215  wherein the print operation is started. 
     If, in the belt speed correction mode, the speed control circuit  40   c  calculates a correction value of the speed of the motor  70  using a table listing a change in speed of the conveyer belt  22 , as shown in FIG. 6, stored in the memory  40   b  and then controls the speed of the motor  70  to regulate the speed of the drive roller  24   a , it will result in a decrease in control speed. In order to avoid this problem, this embodiment prepares, as described above, the correction tables for correcting the speed of the conveyer belt  22  in the memory  40   b.    
     A method of making the correction tables will be described below. 
     First, a method of correcting a color shift will be discussed. A shift in actual speed of the conveyer belt  22  from a nominal speed in design may be expressed as 
     
       
         speed shift=− A   0   e   ct   
       
     
     where −A 0  is a value, as will be described later in detail, determined based on the length of time the conveyer belt  22  is left as is (i.e., the print-off time) after the print operation or the initialization. 
     The correction of the speed shift may be achieved by controlling the speed of the conveyer belt  22 , as shown in FIG.  10 . Specifically, when the conveyer belt  22  begins to be moved, the speed thereof increases gradually and then stabilizes. Thus, a correction of the speed of the conveyer belt  22 , as shown in the drawing, is to made until it reaches a saturated value. 
     However, if an initial value of the speed shift is smaller than one shown in FIG. 10, that is, if the print-off time is short, the correction control is started using as an initial value a correction value A 1  (A 1 &lt;A 0 ), as indicated in FIG. 11, which is defined below A 0  on the correction curve expressed in an exponential function. A required correction of the speed of the conveyer belt  22  is increased as the print-off time is prolonged, but it is saturated in time. The required correction when saturated is thus defined, as shown in FIGS. 10 and 11, as the upper limit A 0 . 
     It is usually difficult to calculate the above belt speed correction every print operation. Thus, approximate values, as shown in FIG. 12, are calculated and listed in table  1 . The table  1  (FIG. 22) is stored in the memory  40   b  and looked up at regular intervals of one sec. In practice, the table  1  is prepared for each of different controlled speeds of the conveyer belt  22  or each of print modes: monochrome print mode, color print mode, thick paper print mode, and OHP print mode. 
     When the print operation is started again after completion of the previous one, the correction value of the speed of the conveyer belt  22  is determined in the following manner. The speed of the conveyer belt  22  after completion of the print operation is gradually decreased, as described above, under the thermal influence of the fixing device  32 . The correction value is, therefore, determined so as to increase as a function of the print-off time. Specifically, one of values, as indicated by a solid line in FIG. 13, approximating a curve, as indicated by a broken line, which increases from zero (0) to A 0  as the print-off time increases from zero (0). 
     The correction values in FIG. 13 bear a one-to-one correspondence to the elapsed time (i.e., an offset that is the thermal influence of the previous print operation expressed in time) from the start of the conveyer belt  22  defined along the abscissa axis of FIG.  12 . Therefore, each of the correction values is, as shown in table  2  (FIG.  23 ), defined as the elapsed time from the start of the conveyer belt  22  corresponding to the print-off time. The elapsed time is, however, counted from a value other than zero (0) at the start of the conveyer belt  22 , as shown in FIG.  11 . 
     The operations described so far are performed in the case where a color shift is saturated at the beginning of the print-off time, that is, where the speed of the conveyer belt  22  has been saturated in a print job performed immediately before the print-off time is measured. The print job is, however, often finished, as shown in FIG. 14, before the speed of the conveyer belt  22  is saturated (broken lines in FIG. 14 indicate the speed of the conveyer belt  22  when the print operations are performed continuously). In such a case, if the second print job is started again immediately after completion of the first print job (e.g., within several minutes), the speed of the conveyer belt  22  when the second print job is started is almost equal to that when the first print job is finished, but a change in speed of the conveyer belt  22  when the second print job is started increases as the print-off time increases. Therefore, a count value of the print-off time is, as shown in FIG. 15, corrected in view of a condition at the end of the first print job. Specifically, an initial count value of the print time at point A in FIG. 15 is set to a time corresponding to a read-out position in a print time initial count value index table  3  which is determined by a count value of the print time at point B and the time intervals between A and B (i.e., the print-off time). 
     The print time initial count value index table  3  (FIG. 24) will be explained below by means of example. In table  3 , the print time indicates the time during which a previous print job is performed. The print-off time count indicates a time interval between the previous print job and a subsequent print job. 
     If a previous print job was executed in the color print mode for 20 sec., one of the print-off time counts in a row of print time group No. 5 is selected. In this case, even after he print-off time until the current print job exceeds 90 minutes, a print-off time count of zero (0) is selected. Next, one of the print times in the color print mode listed in a row of the print time group number equal to the selected print-off time count is selected as an initial value of a print time count when the current print job is started. In this case, three (3) sec. is selected. If a previous print job was executed in the color print mode for 54 sec., one of the print-off time counts listed in a row of print time group No. 11 is selected. If the print-off time until the current print job is one (1) minute, a print-off time count of seven (7) is selected. Next, a maximum one of the print times in the color print mode listed in a row of the print time group number of 7, i.e., 32 is selected as an initial value of a print time count when the current print job is started. The same operation is performed in each of the monochrome print, thick paper print, and OHP print mode. 
     FIG. 16 shows a flowchart of a program to initialize the print time count based on the print-off time listed in table  3  which is performed upon turning on of the motor  70 . 
     After entering the program, the routine proceeds to step S 301  wherein it is determined whether the temperature of the fixing device  32  is higher than 80° C. or not. If a YES answer is obtained, then the routine proceeds to step S 302  wherein the print-off time group number is set to one (1). The routine proceeds to step S 303  wherein it is determined whether the print-off time group number is sixteen (16) or not, that is, whether the end of table  3  is reached or not. If a NO answer is obtained, then the routine proceeds to step S 304  wherein it is determined whether the print-off time count is identical with that indicated in table  4  (FIG. 25) by the print-off time group number or not. If a NO answer is obtained, then the routine proceeds to step S 305  wherein the print-off time group number is incremented by one (1) and returns back to step S 303 . Subsequently, steps  303  to  305  are repeated until the print-off time group number reaches 16 or the print-off time count is identical with that indicated in table  4  by the print-off time group number. 
     If a YES answer is obtained in step S 303  meaning that the print-off time group number has reached 16 or if a YES answer is obtained in step S 304  meaning that the print-off time count has become identical with that indicated in table  4  by the print-off time group number, then the routine proceeds to step S 306  wherein the print-off time group number is decremented by one (1). 
     If a NO answer is obtained in step S 301  meaning that the temperature of the fixing device  32  is higher than 80° C., then the routine proceeds to step S 307  wherein the print time count is reset to zero (0). The routine proceeds to step S 308  wherein the print-off time count is reset to zero (0). Specifically, the speed control circuit  40   c  determines that the image-forming device  10  has started in a cold condition and resets the print-off time count and print time count measured by the timer  40   a  to zero (0). The routine proceeds to step S 309  wherein the print-off time group number is set to zero (0). 
     After step S 306  or S 309 , the routine proceeds to step S 310  wherein an initial value of the print time count is set to one of values listed in a print time correction table  5  (FIG. 26) which is located by the print-off time group number and the print time group number. 
     FIG. 17 shows a flowchart of a program performed at regular intervals of one (1) sec. to determine the correction value for correcting the speed of the conveyer belt  22 . 
     After entering the program, the routine proceeds to step S 401  wherein the print operation is being performed or not. If a NO answer is obtained, the routine terminates. Alternatively, if a YES answer is obtained, then the routine proceeds to step S 402  wherein the print time count is incremented by one (1). The routine proceeds to step S 403  wherein the print time group number is set to zero (0). The routine proceeds to step S 404  wherein it is determined whether the print-off time group number reaches sixteen (16) or not, that is, whether the end of table  3  is reached or not. If a NO answer is obtained, then the routine proceeds to step S 405  wherein it is determined whether the print time count is identical with that indicated in table  6  (FIG. 27) by the print time group number or not. If a NO answer is obtained, then the routine proceeds to step S 406  wherein the print time group number is incremented by one (1) and returns back to step S 404 . Subsequently, steps  404  to  406  are repeated until the print time group number reaches 16 or the print time count is identical with that indicated in table  6  by the print time group number. 
     If a YES answer is obtained in step S 404  meaning that the print time group number has reached 16 or if a YES answer is obtained in step  405  meaning that the print time count has become identical with that indicated in table  6  by the print time group number, then the routine proceeds to step  407  wherein the print time group number is decremented by one (1). The routine proceeds to step S 408  wherein one of correction values listed in table  7  (FIG. 28) which is indicated by the print time group number is selected. 
     FIG. 18 shows a flowchart of a program to terminate the print operation. 
     First, in step S 501 , it is determined whether an emergency stop request is made or not. If a NO answer is obtained, then the routine proceeds to step S 502  wherein all motors are stopped or not. If a YES answer is obtained in step S 501  or step S 502 , then the routine proceeds to step S 503  wherein the print time count is reset to zero (0). The routine proceeds to step S 504  wherein a 60-second counter is reset to zero (0) to initiate the print-off time count. If a NO answer is obtained meaning that all the motor are not stopped, then the routine terminates. 
     FIG. 19 shows a flowchart of a program to handle the print-off time count which is performed by timer interrupt at regular intervals of 5 ms. 
     First, in step S 601 , it is determined whether the print operation is at rest or not. If a NO answer is obtained, then the routine terminates. Alternatively, if a YES answer is obtained, then the routine proceeds to step S 602  wherein the 60-second counter is incremented by one (1). The routine proceeds to step S 603  wherein it is determined whether 60 seconds have passed or not. If a NO answer is obtained, then the routine terminates. Alternatively, if a YES answer is obtained, then the routine proceeds to step S 604  wherein the 60-second counter is reset to zero (0). The routine proceeds to step S 605  wherein the print-off time count is incremented by one (1). 
     By performing the above described operations, the timer  40   a  measures the elapsed time from the turning on of the power or release of the sleep mode, the print-off time, and the print time. The speed control circuit  40   c  monitors the times measured by the timer  40   a  and estimates and corrects the speed of the conveyer belt  22 . This enables formation of color images without any color shifts and decrease in throughput. 
     FIG. 20 shows an image-forming device according to the second embodiment of the invention which is different from the first embodiment in that intervals between image-transferring units are corrected instead of the correction of the belt speed. The same reference numbers as employed in the first embodiments refer to the same parts, and explanation thereof in detail will be omitted here. 
     The image-transferring units  53 Y,  53 M,  53 C, and  53 K are supported by the positioning members  52 Y,  54 Y,  52 M,  52 C,  54 C,  52 K, and  54 K. The positioning members  52 Y,  54 Y,  52 M,  54 M,  52 C,  54 C,  52 K, and  54 K are installed on a device casing (not shown) so as to be movable only in a lateral direction, as viewed in the drawing. Portions of the positioning members opposite portions supporting the image-transferring units are provided with gears connected mechanically to image-transferring unit drive motors  50 Y,  56 Y,  50 M,  56 M,  50 C,  56 C,  50 K, and  56 K through intermediate gears  51 Y,  55 Y,  51 M,  55 M,  51 C,  55 C,  51 K, and  55 K. 
     When the image-transferring unit drive motor  50 Y is rotated in a counterclockwise direction, as viewed in the drawing, and the image-transferring unit drive motor  56 Y is rotated in a clockwise direction through the same angular steps as those of the image-transferring unit drive motor  50 Y, the positioning members  52 Y and  54 Y are moved in parallel in the left direction to shift the image-transferring unit  53 Y away from the image-transferring units  53 M,  53 C, and  53 K. In other words, intervals between the image-transferring unit  53 Y and the other three image-transferring units  53 M,  53 C, and  53 K are increased, respectively. Conversely, when the image-transferring unit drive motor  50 Y is rotated in the clockwise direction, and the image-transferring unit drive motor  56 Y is rotated in the counterclockwise direction, the positioning members  52 Y and  54 Y are moved in parallel in the right direction to shorten the intervals between the image-transferring unit  53 Y and the other three image-transferring units  53 M,  53 C, and  53 K. 
     The image-transferring device of this embodiments measures, similar to the first embodiment, the print-off time, the print time, etc. through the timer  40   a  and estimates the belt temperature to estimate the belt speed based on the belt temperature. The image-transferring device determines target positions of the image-transferring units  53 Y,  53 M,  53 C, and  53 K based on the estimated belt speed using speed-to-position relations, as shown in FIG.  21  and moves the image-transferring units  53 Y,  53 M,  53 C, and  53 K in the above manner to the target positions. In the shown case, the location of the image-transferring unit  53 K is defined as a reference position. Only the image-transferring units  53 Y,  53 M, and  53 C are, thus, moved. 
     While the present invention has been disclosed in terms of the preferred embodiments in order to facilitate better understanding thereof, it should be appreciated that the invention can be embodied in various ways without departing from the principle of the invention. Therefore, the invention should be understood to include all possible embodiments and modifications to the shown embodiments which can be embodied without departing from the principle of the invention as set forth in the appended claims.