Patent Publication Number: US-6336007-B1

Title: Printer that facilitates detection of deteriorated component

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to judgment methods, and more particularly to a method for detecting. when an image-forming device is inspected for maintenance purposes, deterioration in each device component. The method of the present invention is suitable, for example, for maintenance inspection of an electrophotographic printing device, such as a copier. 
     2. Description of the Related Art 
     A conventional maintenance inspection for an electrophotographic printer has required a maintenance man to use a manual operation and consider each component to be normal if it is currently working, except for components that may be inspected with eyes such as toner in a transparent plastic vessel. A maintenance man would consider a component to be abnormal and exchange/repair it only when it is completely inoperable or very hard to operate. In other words, the conventional maintenance inspection has only judged whether a target component is normal or abnormal, and considered the component to be normal if judging it not to be abnormal. 
     The conventional maintenance inspection cannot help considering a component to be normal which is not completely normal but it has not reached the apparently abnormal state (referred to as “a deteriorated state” hereinafter). However, the subsequent continuous use of the deteriorated state is likely to bring about a near-future failure or very bad operation. Therefore, the conventional maintenance inspection would often result in inoperativeness shortly after the maintenance inspection, annoying customers. In addition, it has been disadvantageously difficult for the conventional maintenance inspection to easily judge whether a component is in the deteriorated state. 
     The deterioration in a printer component may disable printing in the near future as well as gradually lower the print quality. A print operation depends upon components having various print functions as a whole. One deteriorated component would possibly lower the entire print quality. However, the conventional maintenance inspection has a difficulty in quantitatively evaluating the print quality. Even a detection of the image-quality deterioration cannot easily trace the causative component. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an exemplified general object of the present invention to provide a novel and useful evaluation method and printer in which the above disadvantages are eliminated. 
     Another exemplified and more specific object of the present invention is to provide an evaluation method and printer without additional complex detecting means that may enable a maintenance mall to easily identify a deteriorated component in addition to a component that is broken down or is remarkably hard to operate. 
     Still another exemplified object of the present invention is to provide an evaluation method and printer that enable a maintenance man to quantitatively evaluate the current print quality. 
     Another exemplified object of the present invention is to provide a judgment method and printer that may easily localize a causative component after the image quality is considered deteriorated. 
     In order to achieve the above objects, a printer of a first aspect of the present invention comprises a mechanical part which feeds a printing paper and in order to print predetermined information on the printing paper, a mode switch which switches plural modes, and a control part which controls the mechanical part and determines quantitatively in each mode which state among normal, deteriorated, and abnormal states each component in said mechanical part is located in. 
     A printer of a second aspect of the present invention comprises a photosensitive body, a pre-charger which charges the photosensitive body, an optical part which exposes the charged photosensitive body, a developer which applies a bias voltage to toner and develops the exposed photosensitive body, forming a toner image with a desired concentration, a transfer unit which transfers the toner image onto a printing paper by applying a transfer current to the printing paper, and a control part which enable printing by setting to a printable upper and lower limits one or more set values among electric parameters including a surface potential in the photosensitive body, an exposure power in the optical part, the bias voltage in the developer, and the transfer current in the transfer unit. 
     An evaluation method of the present invention comprises the steps of setting a mode used to test a print quality margin, setting to an upper or lower value one or more set values among electric parameters including a surface potential in said photosensitive body, an exposure power in the optical part, a bias voltage in the developer, and a transfer current in the transfer unit, printing a predetermined pattern in accordance with the set electric parameters. 
     Other objects and further features of the present invention will become readily apparent from the following description of the embodiments with reference to accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a block diagram of a printer of a first embodiment according to the present invention. 
     FIG. 2 is an enlarged section showing an exemplified paper feed system that is applicable to mechanical part  10  shown in FIG.  1 . 
     FIG. 3 is a timing chart for detecting that something is wrong with the paper feed motor shown in FIG.  2 . 
     FIG. 4 is a flowchart showing an exemplified evaluation method of the present invention that is applicable to the paper feed system, shown in FIG.  2 . 
     FIG. 5 is a rear view of a motor in an exemplified device applicable to the printer shown in FIG.  1 . 
     FIG. 6 is a timing chart for detecting that something is wrong with an Mg roller and/or Mg roller detector shown in FIG.  5 . 
     FIG. 7 is a front view of the motor in the device shown in FIG.  5 . 
     FIG. 8 is a rear view of a sensor and a photo-interrupter in the device shown in FIG.  5 . 
     FIG. 9 is a timing chart for detecting that something is wrong with a pinch-roller adhesion motor, a pinch roller UP position detector a separation tab set position detector, a pinch-roller position down  1  detector and/or a pinch-roller position down  2  detector. 
     FIG. 10 is a partial block diagram of the printer mechanical part shown in FIG.  1 . 
     FIG. 11 is a detailed block diagram of a control part shown in FIG.  1 . 
     FIG. 12 is a relationship between a grid voltage and toner concentration that affect the print quality. 
     FIG. 13 is an exemplified flowchart of an evaluation method of the present invention. 
     FIG. 14 is a block diagram showing an exemplified high-voltage power unit shown in FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A description will now be give of printer  100  of a first embodiment according to the present invention, with reference to the accompanying drawings. Those elements in each drawing which are designated by the same reference numerals denote the same elements, and a duplicate description thereof will be omitted. 
     The printer  100  of the present invention includes. as shown in FIG. 1, mechanical part  10 , control part  20 , monde switch  30 , detection level input part  40 , and display part  50 . FIG. 1 is a block diagram of a printer of the first embodiment according to the present invention. 
     The mechanical part  10  generalizes components in the printer  100 , and thus includes paper feed, print, and other systems. The control part  20  controls the mechanical part  10 , and includes RAM  22  and ROM  24 . The RAM  22  stores a detection level entered from the detection level input part  40 , and the ROM  24  stores a control program by which the control part  20  controls the mechanical part  10 . 
     The mode switch  30  switches an operation mode of the printer  100  between a normal mode and a maintenance mode, by a manual operation or an automatic operation in accordance with a host device connected to the control part  20 . In the manual input, the mode switch  30  would be equipped with an input device having various types of keys. As described later, the mode switch  30  may be comprised of display part  50  and a touch-key operation panel. The normal mode allows the printer  100  to conduct a normal operation and print predetermined information on a printing paper. The maintenance mode is used for a maintenance man to inspect each component in the printer  100 . The printer  100  of the present invention displays its unique effects when the mode switch  30  is set to the maintenance mode and works similar to known printers when the mode switch  30  is set to the normal mode. 
     The detection level input part  40  may store detection levels corresponding to a completely normal state. a deteriorated state, and an abnormal (or unusual) state. The detection level input part  40  may be used for both the normal and maintenance modes. 
     The display part  50  indicates various messages to a maintenance man and an operational status of the printer  100 , such as “no paper”, “replace toner”, etc. The display part  50  may use both the normal and maintenance modes. 
     With reference to FIG. 2, a description will be given of exemplified concrete components of paper feed motor  66  and paper sensor  69  in the mechanical part  10  that is applicable to the evaluation method of the present invention. FIG. 2 is an enlarged section showing the exemplified paper feed system in the mechanical part  10 . As illustrated, the paper feed system in the mechanical part  10  includes height sensor  61 , “no paper” detecting sensor  62 , pick roller  64 , paper feed motor  66 , feed roller  67 , reverse roller  68 , paper sensors  69  and  71 , central rollers  70 , and eject rollers  72 . 
     The amount of printing paper stock is detected by the height sensor  61  that detects the height of a printing paper. The “no paper” detecting sensor  62  detects that stacker  63  has no printing papers. The height sensor  61  and the “no paper” detecting sensor  62  are connected to the controller  20  shown in FIG. 1, and the display part  50  indicates the information on “no paper”. However, these components are known in the art, and a description of their detailed structures and operations will be omitted. 
     The pick roller  64  is engaged with motor shaft  65  in the paper feed motor  66 , and the paper feed motor  66  is controlled by the controller  20 . The pick roller  64  serves to pick up a printing paper. Other rollers, such as the roller  67 , are connected to motors (not shown) by a well-known method and controlled by the control part  20 , and a detailed description of their structures and operations will be omitted. 
     The reverse roller  68  rotates revere to the feed roller ( 67 , and separates, if two sheets of papers are erroneously fed, the uppermost paper from the remaining papers so as to feed this to feed path  73 . The sensors  69  and  71  may each use a light-emitting element and a light-receiving element, for example. The ejected paper from the eject roller  72  is fed to the print system (not shown). 
     A fault associated with the pick roller  64 , such as a paper jam, is detectable by measuring a paper feed period (“t1”) from time when an ON signal for starting the pick roller  64  is supplied to time when the paper sensor  69  detects a paper edge, and comparing t1 with a reference value. A fault associated with the central rollers  70  at both sides of the feed path  73  is detectable by measuring a paper feed period (“t2”) between the sensors  69  and  71 , and comparing t2 with a reference value. 
     With reference to FIGS. 3 and 4, a description will now be given of the evaluation method of the present invention that is applied to the paper feed system. FIG. 3 is a timing chart showing the paper feed period t1. FIG. 4 is a flowchart that the controller  20  performs and is stored, for example, as a control program in the ROM  24 . Alternatively, the evaluation method of the present invention may be loaded as a printer driver onto a general-purpose personal computer connected to the printer  100 . In that case, the personal computer CPU may perform the following operation of the control part  20 . 
     Advantageously, the evaluation method of the present invention classifies t1 into three groups of “normal state”, “deteriorated state”, and “abnormal state”, and assigns “100 ms or shorter”, “101 ms to  119  ms”, and “120 ms or longer”, to them respectively. 
     The control part  20  initiates the feed motor  60  by supplying a drive circuit in the feed motor  66  (not shown) with an ON signal (step  1001 ), and clears, by setting this time to be time 0, a timer (not shown) which measures the paper feed period t1 (step  1002 ). The timer is provided in the controller  20 , and may employ any structure known in the art. For example, the timer includes a pulse generator that provides a pulse having a 1 ms period, and a counter that counts the number of pulses. Alternatively, the timer may use an electronic clock in a personal computer when the evaluation method of the present invention is loaded as a printer driver onto a general-purpose personal computer connected to the printer  100 . The leading edge time of the feed motor  66  in FIG. 3 is the time when the ON signal is supplied. Thereby, the paper feed motor  66  picks up a printing paper and feeds it to the feed path  73 . When this feeding paper reaches the sensor  69  through the feed roller  67 , a detection signal of the sensor  69  is fed to the control part  20  and consequently the control part  20  may know, using the timer, the detection time by the sensor  69 . The leading edge time of the paper sensor  69  in FIG. 3 is the time when the paper reaches the sensor  69 . 
     The control part  20  thus obtains the paper feed period t1 (steps  1003  and ( 1004 ). When the paper feed period t1 is between 0 and 100 ms, it is normal. The control part  20  then considers the current operational mode to be normal whether it is set to a normal mode or a maintenance mode (steps  1006  and  1007 ), and will or will not display that fact and/or a value of t1 on the display part  50 . 
     When the paper feed period t1 is between 101 and 109 ms, it is deteriorated. The control part  20  then considers the current operational mode to be normal when it is set to the normal mode (steps  1009 ,  1006 , and  1007 ), and considers it to be abnormal when it is set to the maintenance mode (steps  1009 ,  1010 , and  1011 ). The display part  50  indicates the resultant judgment by the control part  20 . 
     According to this embodiment, the control part  20  considers the deteriorated stated to be normal in the normal mode. This is because in the normal mode in the instant embodiment the longer paper feed than the usual does not deteriorate the image quality and a user in general is presumed to be satisfied with the obtained result. On the contrary, the deterioration in a print system component disadvantageously lowers the image quality, as described later, and may be considered to be abnormal even in the normal mode. Optionally, the control part  20  may consider a component to be abnormal in the normal mode when the component belongs to a system other than the print system. Such setting may be preinstalled as a program in the ROM  24  in the factory before the printer  100  is shipped, or may be input by a provider as a result of conference with a customer at an initial setup when he/she provides the customer with the printer  100 . 
     When the paper feed period t1 exceed 120 ms, it is abnormal. The control part  20  then considers the current operational mode to be abnormal (steps  1012  and  1013 ), and displays the fact and/or a value of t1 on the display part  50 . 
     The control part  20  may indicate a conceivable countermeasure instead of or in addition to displaying the normality or abnormity. For example, it is a letter or symbol that recommends an adjustment of the paper feed motor  66 , the paper sensor  69  and/or an engagement state between the pick roller  64  and the motor shaft  65 , or that identifies a component that should be replaced. Needless to say, a deteriorated component might possibly be the sensor  69  or the like rather than tile feed motor  66 . 
     Optionally, the control part  20  may recognize and display the deteriorated state as an independent state, instead of assigning it to the normal or abnormal state. In this case, a maintenance man may inform a customer that something will possibly be wrong with the feed motor  66  etc. in the near future, and give a spare to the customer without replacing the feed motor  66  etc. if the replacement is easy for the customer. Instead, the maintenance man leaves the conclusion to customer&#39;s decision. If the customer would like the maintenance man to come again and replace it when the breakdown occurs, the maintenance man may avoid making repairs this time. Such a measurement may take into consideration the frequency of customer&#39;s use and a deterioration level, e.g., a case where t1 is close to the normal state, such as 101 ms. Such options are common to the following embodiments. 
     The evaluation method of the feed system in this embodiment considers the deteriorated state to be abnormal or at least informs a customer of the deterioration, whereas the deteriorated state has been considered to be normal in the conventional maintenance mode. As a consequence, the instant evaluation method may prevent a breakdown shortly after the maintenance inspection or evade customer&#39;s disgust even if a breakdown occurs shortly after the maintenance inspection. Therefore, the evaluation method of this embodiment may improve the maintenance service. 
     The evaluation method of the present invention changes a detection level between the normal and maintenance modes, but does not change its operation, facilitating easy and inexpensive maintenance inspection. For example, a method that requires the maintenance mode to increase a printing-paper feed speed for a test mode needs an independent drive unit etc., making the entire device expensive. The evaluation method of the present invention eliminates such a problem. 
     The evaluation method of the present invention is applicable to Mg motor  80  and Mg roller rotation detector  81  when the mechanical part  10  is designed as shown in FIG.  5 . The structure shown in FIG. 5 is known as a rear view of a motor in Fujitsu F6760 page printer, and a description of detailed structures and operations of components will be omitted. 
     The control part  20  in FIG. 5 is set so that it considers abnormal a case where 2 seconds after the developer Mg motor  80  starts no output changes from the Mg roller station detector  81  continues for 300 ms or longer. Referring to FIG. 6, according to the evaluation method of the present invention the control part  20  considers t1 to be abnormal when t1 is 300 ms or longer, and t1 to be normal when t1 is less than 300 ms. FIG. 6 is a timing chart for use with the Mg roller rotation detector  81  to detect that something is wrong with the Mg roller motor  80  and Mg roller rotation detector  81 , but the timing chart is usually used to detect that something is wrong with the Mg roller motor  80 . Illustrated t1 is expected to be about 150 ms in the normal state, and thus is classified into three groups of “normal state”, “deteriorated state”, and “abnormal state” which are assigned “shorter than 225 ms”, “225 ms or longer but shorter than 300 ms”, and “300 ms or longer”. The control part  20  considers the deteriorated state to be normal in the normal mode, and considers it to be abnormal in the maintenance mode. 
     It is understood that such an evaluation method requires the step  1005  in FIG. 4 to be replaced with 0 through 224 ms, the step  1008  with 225 through 300 ms, the step  1012  with 300 ms or longer, and these steps  1006  and  1010  with output changes from the Mg roller rotation detector  81 . 
     As described, the evaluation method of the present invention quantitatively detects, using a detector in the printer  100 , which status among the normal, deteriorated and abnormal states each component is located in. The present invention is thus clearly applicable to other detectors. For example, the present invention is applicable to pinch-roller adhesion motor  82 , pinch-roller UP position detector  83 , separation tab set position detector  84 , pinch-roller position down  1  detector  85 , and pinch-roller position down  2  detector  86  shown in FIGS. 7 and 8. Hereupon, FIG. 7 is a front view of a motor in Fujitsu F6760 page printer shown in FIG. 5, and FIG. 8 is a rear view of a sensor and a photo-interrupter in the printer. 
     The control part  20  in FIGS. 7 and 8 is set so that it may considers abnormal a case where no outputs from the separation tab set position detector  84  are generated within 1 sec after the pinch-roller adhesion motor  82  is started. Referring to FIG. 9, the evaluation method of this embodiment makes the control part  20  consider t1 that is 1 sec or longer, to be abnormal and evaluate t1 that is less than 1 sec, to be normal in the normal mode. FIG. 9 is a timing chart for detecting that something is wrong with the pinch-roller adhesion motor  82  and/or detector  83  etc., but is usually used to detect that something is wrong with the pinch-roller adhesion motor  82  (and its engagement with another component). Illustrated t1 is expected to be about 260 ms if normal, and is classified in the maintenance mode into three groups of “normal state”, “deteriorated state”, and “abnormal state” which are assigned “shorter than 630 ms”, “630 ms through 1 sec” and “longer than 1 sec”, respectively. The control part  20  evaluates the deteriorated state to be normal in the normal mode, and to be abnormal in the maintenance mode. 
     The control part  20  terminates and then reactivates the pinch-roller adhesion motor  82  after detecting the separation tab set position. The control part  20  is also set so that it considers abnormal a case where no outputs are generated from either the pinch-roller position down  1  or  2  detector within 1 sec which is determined by an output of a paper thickness indicator (not shown). Referring to FIG. 9, the evaluation method of this embodiment makes the control part  20  consider t2 or t3 that is 1 sec or longer, to be abnormal, and t2 and t3 that are both less than 1 sec, to be normal in the normal mode. However, t2 shown in FIG. 9 is expected to be about 280 ms in the normal state, and thus is classified in the maintenance mode into three groups of “normal state”, “deteriorated state”, and “abnormal state” which are respectively assigned “shorter than 640 ms”, “640 ms through 1 sec”, and “longer than 1 sec”. The control part  20  considers the deteriorated state to be normal in the normal mode, and to be abnormal in the maintenance mode. Similarly, t3 shown in FIG. 9 is expected to be about 320 ms in the normal state, and thus is classified in the maintenance mode into three groups of “normal state”, “deteriorated state”, and “abnormal state” which are respectively assigned “shorter than 660 ms”, “660 ms through 1 sec”, and “longer than 1 sec”. The control part  20  considers the deteriorated state to be normal in the normal mode, and to be abnormal in the maintenance mode. 
     The control part  20  terminates and then reactivates the pinch-roller adhesion motor  82  after detecting the pinch-roller position down  1  or  2 . The control part  20  is also set so that it considers abnormal a case where no outputs are generated from the pinch-roller UP position detector  83  within 2 sec after reactivating the pinch-roller adhesion motor  82 . Referring to FIG. 9, the evaluation method of this embodiment makes the control part  20  consider t4 that is 2 sec or longer, to be abnormal, and t4 that is 2 sec or shorter, to be normal in the normal mode. However, t4 shown in FIG. 9 is expected to be about 940 ms in the normal state, and thus is classified in the maintenance mode into three groups of “normal state”, “deteriorated state”, and “abnormal state” which are respectively assigned “shorter than 1470 ms”, “1470 ms through 2 sec”, and “longer than 2 sec”. The control part  20  evaluates the deteriorated state to be normal in the normal mode, and to be abnormal in the maintenance mode. 
     The evaluation method of this embodiment may be realized using FIG. 4, and a description thereof will be omitted. 
     Next follows a description of the inventive evaluation method applied to the print system. The conventional maintenance inspection cannot recognize the image quality quantitatively. The deteriorated image quality includes white printing (i.e., a phenomenon that a portion that should be colored in black becomes white), white bands, entirely pale or dark color, blotching, being unable to obtain the image quality corresponding to a desired print mode (for example, the image quality is coarse even in a fine mode), black blobs on a paper, etc. It is also difficult for the conventional inspection with eyes to identify a component that becomes deteriorated or broken down. The print operation is composed of a plurality of processes including charging, exposure, development, transferring with a photosensitive drum, and the image quality is a synthetic quality result of these processes. 
     The present invention has addressed, as electric parameters that affect the image quality, a photosensitive-drum surface potential, exposure power, development magnetic-brush bias voltage, and transfer current which depend upon the printer  100 &#39;s environment (such as temperature and humidity), and has intended to improve the entire image quality by changing these parameters singularly or in combination and evaluating the resultant image quality. Concretely, this embodiment judges, as a quality guarantee test at the time of manufacturing and/or maintenance inspection, whether the printer  100  may operate properly, while changing singularly or in combination up and down within a printable range, standard (or current) values of the above electric parameters which activate the printer  100 . 
     With reference to FIGS. 5,  7 ,  8  and  10 , a brief description will be given of the photosensitive-drum surface potential, exposure power, development magnetic-brush bias voltage, and transfer current. 
     The printer  100  includes photosensitive drum  102 , transfer unit  104 , pre-charger  106 , optical part  108 , and developer  110 . A printing paper passes between the photosensitive drum  102  and the transfer unit  104 . The pre-charger (e.g., corona charger)  106  charges the photosensitive drum  102 . The photosensitive drum  102  is made, for example, of an aluminum drum onto which an about 20 im thickness of function-separation type organic photosensitive member is applied. The photosensitive drum  102  has a diameter, for example, of 30 mm and rotates in an arrow direction at a rotational speed of 70 mm/s. The corona charger is made, for example, of a Scorotron charger, and charges uniformly the photosensitive drum  102  surface by about −500 V. The corona charger  106  has a high-voltage wire (not shown) that may apply 8 through 12 kV, and applies a potential by corona discharge onto a grid screen (“grid”) spaced from this wire. The grid is connected directly or close to the photosensitive drum  102 , while the grid voltage and the drum surface potential are controlled to be equal. Surface-potential detector  120  which may employ any structure known in the art detects the surface potential of the photosensitive drum  102 . 
     Next, the optical part  108  exposes the uniformly charged photosensitive drum  102  by a laser and forms a latent image with −50 through −100 V on the photosensitive drum  102 . The exposure power thus determines the latent image quality. 
     Then, the latent image is developed by the developer  110  having development roll  112 , and thereby converted into a toner image on the photosensitive drum  102 . The development roll  112  rotates in arrow direction P in FIG. 10, and a fixed magnetic pole member having a plurality of magnetic poles, and a sleeve that rotates around the magnetic pole member. This sleeve rotates in the arrow direction P as illustrated, and feeds the development agent to a development area that faces the photosensitive drum  102 . Toner retains an electric charge opposite to the electric charge pattern on the photosensitive drum  102 , and is absorbed by the electrostatic force onto the photosensitive drum  102  surface for development. A bias voltage that is applied to the magnetic brush formed on the development roll  112  adjusts charging to toner, and toner concentration. 
     The transfer unit  104  faces the photosensitive drum  102  via the printing-paper feed path. The transfer unit  104  adopts a known transfer unit having a corona (discharge) wire, and applies the transfer current to a printing paper using the corona discharge. The current flowing from the corona (transfer) wire to the photosensitive drum  102  is transfer current. When a printing paper reaches the transfer position, the transfer unit  104  applies a voltage to the corolla wire  282  from a surface opposite to the printed surface of the printing paper. As a consequence, the toner image on the photosensitive drum  102  surface is transferred by absorbing and attaching the toner image onto the printing paper. 
     A description will be given of a margin test of the present invention regarding a controls over the photosensitive-drum surface potential, exposure power, development magnetic brush bias voltage, and transfer current. These are controlled by mechanical-part control circuit  150  that will be described below. Hereupon, the control part  20  shown in FIG. 1 specifically includes, as shown in FIG. 11, controller  140  and mechanical-part control circuit  150 . 
     The controller  140  is connected via an interface (not shown) which is provided at the rear surface etc. of the printer  100 , to a computer, a network, such as a LAN, and other external devices (not shown) (hereinafter simply “host computer”). 
     The controller  140  converts print information sent from the host computer into, for example, bit map video data, and send it to the mechanical-part control circuit  150 . The controller  140  may employ any structure known in the art, and a description thereof will be omitted. 
     The mechanical-part control circuit  150  controls high-voltage power unit  160  and exposure power control part  170 , and generates a switching signal for them. The switching signal is changed by a selection of a double-side or single-side print unit or by a parameter, such as a regular paper and a thick paper. The mechanical-part control circuit  150  controls other mechanical components (units) in the body, such as a main motor. It is conceivable that the mechanical-part control circuit  150  includes pre-charger control part  162 , development magnetic brush bias control part  164 , and transfer current control part  166  shown FIG.  10 . 
     The mechanical-part control circuit  150  includes PROM  152  corresponding to the ROM  24 , various sensors ( 120 ,  122  aid  124 ), RAM  154  corresponding to RAM  22 , and MPU  156 . The mechanical-part control circuit  150  is connected to the controller  140 . and receives bit map video data (print data) from the controller  140 . The display part  50  indicates an operation of the mechanical-part control circuit  150  connected to the display part  50 . The display part  50  may be comprised of the mode switch  30  and a touch-key operational panel. The mechanical-part control circuit  150  is connected to a main motor (not shown), the high-voltage power unit  160 , and the exposure power control part  170 . For illustration purposes, FIG. 11 shows only signal lines that which supply control signals (s 1  through sN), and any desired number of signal lines may be provided depending upon the number of parameters to be changed (i.e., controlled). FIG. 11 generalizes such other signal lines as sN. 
     The PROM  152  stores, as a program, an instruction to each unit and set data, and necessary data is loaded onto and executed by the RAM  154 . The MPU  156  operates in accordance with the program stored in the PROM  152 . Various sensors include or are connected to outputs of surface-potential detector  120 , temperature sensor  122 , and hygrometer sensor  124 , and their outputs are supplied to the MPU  156  in any event. Those sensors such as the temperature sensor  122  and the hygrometer sensor  124  may employ any structure known in the art, and a description of their structures and operations will be omitted. 
     The MPU  156  receives print data and resultant outputs from the various sensors. including information such as the paper size (width), paper type (such as a thick paper, a regular paper, or a printing paper made by other manufacturers), resolution etc., and generates a variety of control signals in accordance with the program stored in the PROM  152 . More specifically, the MPU  156  controls logic for each signal line, thereby controlling a print operation. 
     The signal s 1  switches a speed of the main motor (not shown), a voltage for the pre-charger in the high-voltage power unit  160 , and a resolution in the optical part  108 . The signal s 1  is branched in the mechanical-part control circuit  150 , and output to each unit. The signal sN includes a switch signal to regulate the high-voltage power unit  160 &#39;s transfer current and the development magnetic brush&#39;s bias voltage. 
     For example, (the pre-charger control part  162  in) the high-voltage power unit  160  in response to the signal s 1  switches the voltage in the pre-charger  106 . The pre-charger control part  162  controls the grid and photosensitive drum  102  so that their surface potentials are equal to each other. For example, in an attempt to set the grid and the photosensitive drum  102  to 500 V, the control part controls the voltage to be applied to the wire within the range from 8 to 12 kV. However, the dirty grid and other reasons often prevent the grid and the drum surface potential from being equal to each other. For example, even when the grid is 500 V, the drum surface potential may possibly be 400 V. In this case, the mechanical-part control circuit  150  controls the pre-charger control part  162  so that the drum surface potential becomes 500 V. 
     The exposure power control part  170  controls, using a clock and a counter (not shown), a laser emitting time in response to the signal s 1  (so that the laser emitting time becomes the time set by the clock x the control signal s 1 ). 
     The printer  100  of the present invention advantageously has a print mode which may set one or all of the drum surface potential, the exposure power, the development bias voltage, and the transfer current, to printable upper and/or lower limits. 
     A description will be given of the evaluation method of the present invention applied to the printer  100 , with reference to FIG.  12 . FIG. 12 is the image quality depending upon a relationship between the grid voltage and the toner concentration, and allows the current print status (i.e., margin) to be confirmed in FIG. 12 by adjusting the grid voltage and the toner concentration. For example, suppose that the development carrier is adhered to a printed surface when the drum surface potential is set to the upper limit and the exposure power, the development bias, and the transfer current are set to the current values. That is, when the carrier that is black powder is adhered to the printing paper surface, graining the surface and/or blanching a portion that should be originally colored in black (while this state is referred to as “carrier leakage” in FIG.  12 ), a gap between the developer  110  and the photosensitive drum  102  is not presumably proper and may become an inspection object for maintenance purposes. Similarly, when a portion that is not printed and thus should become white becomes gray or black when the drum surface potential is set to the lower limit and other set values are set to the current values (although this printing state is referred to as “fog” in FIG.  12 ), the abnormally deteriorated development agent or unusual toner concentration are presumably causative and may become an object for maintenance inspections. 
     Next follows a description of the evaluation method of the present invention with reference to FIG.  13 . The (MPU  156  in the) mechanical-part control circuit  150  confirms whether the operational mode is transferred to the test (or maintenance) mode by switching at the mode switch  30  or a key input to the operational panel comprising the mode switch  30  and the display part  50  (step  1002 ). When the test mode is set, the display part  50  then prompts a user to select either a single-set or combination-set printing. Then, the mechanical-part control circuit  150  confirms whether either the single-set or combination-set printing is selected (step  1004 ). Hereupon, the “single-set printing” means a printing where one of the surface potential, the exposure power, the development bias, and the transfer current is set to an upper or lower limit and the other parameters remain to use the current (or standard) set values. The “combination-set printing” means a printing where two or more of them are set to upper and/or lower limits. When the single-set printing is selected, the display part  50  requires the user to select one of the above four objects and an upper or lower value for the selected object. The user, may input that data by any means. For example, the display part  50  may indicate an option of object selection in the order from the surface potential, and then display an option of value selection (i.e., upper or lower limit) for the selected object. Alternatively, the user may input using keys a specific object and a set value. In any event, an object and a set value are consequently selected (step  1006 ). 
     For example, the transfer current set to the lower limit would generate a bad transferring or drum&#39;s evasion. The transfer current set to the upper limit would generate an uneven potential and dust/discoloration on the drum. 
     Hereupon, the “bad transferring” means that a necessary amount of a toner image on the photosensitive drum is not transferred to a paper. The “evasion” means that too much toner remains on the photosensitive drum to be completely removed by a cleaning part. 
     The “uneven potential” means that the photosensitive drum has an uneven potential even after a charge removal or uniform charging is performed for the photosensitive drum. The “dust” means that toner is transferred not to a desired position but to another position. The “discoloration” means that toner is not transferred to a recording paper and no toner is adhered into an image. 
     When the user selects a combination-set printing at step  1004 , the display part  50  prompts the user to select the combination. Similarly, the user may use any input manner. For example, the display part  50  may indicate an option of object selection in the order from the surface potential and then display an option of value selection in the order from the current value. 
     After the steps  1006  and  1008 . the display part  50  prompts the user to designate a print pattern, and the user in response designates the print pattern (step  1010 ). Alternatively, the display part  50  may indicate a plurality of print-pattern candidates that have been previously stored in the PROM  152  etc., and prompt the user to designate one of the candidates. Optionally. such a step may be omitted by always using a fixed print pattern, or a step for confirming whether the selected value is within the actually printable range may be added. For example, when the current value of the grid voltage is 700 V and 150 V is selected to define the lower limit, i.e., the lower limit is set to 750−150=550 V, a step for confirming whether 550 V is more than the printable lower limit (for example, that is 450 V). Such a step is especially useful when the user arbitrarily sets the upper and lower limits using the operational panel. However, such a step is unnecessary if the MPU  156  automatically executes an operation, and ascertains that the set current value may always be between the printable upper and lower limits. The printable upper and lower limits associated with temperature, humidity etc. may be stored as simulation data in the ROM  24 . 
     Suppose that the surface potential and the development bias voltage are varied to their upper and/or lower limits. When the surface potential and the development bias voltage are set to their lower limits, the toner concentration on the photosensitive drum would decrease and fade print. The surface voltage that is set to the upper limit and the development bias voltage that is set to the lower limit would cause the carrier leakage and/or fog (charge injunction) in addition to the faded print. When the surface voltage is set to the lower limit and the development bias voltage is set to the upper limit, a bad fixation, void, (true) fog, and/or dullness occurs. When the surface voltage and the development bias voltage that are set to their upper limits, the toner concentration on the photosensitive drum increases, generating dullness and void. 
     The “faded print” means that the small amount of toner on the photosensitive drum lowers the toner concentration on a paper, exhibiting a pale printing. The “carrier leakage” means that the carrier in the developer is pulled out by the photosensitive-drum surface voltage, and adhered to the paper. The “fog (charge injection)” means that the photosenisitive-drum surface voltage is high enough to absorb the toner on the development roller, developing a portion that should not be originally developed on the photosensitive drum and causing an entirely dark printing. The “(true) fog” means that toner&#39;s electric charges are pulled onto the photosensitive drum and develop a portion that should not be originally developed, generating an entirely dark printing. As to the phenomenon, the (true) fog is similar to the fog (electric charge injection). The “bad fixation” means that too much toner is transferred onto a paper to fix on the paper even after the paper passes through the fixation device, causing peeling off of the toner. The “void” means that toner transferred onto a paper is popped at the fixation stage, generating an uneven toner surface. The void causes an image of unequal brilliance, and the popped toner spreads and results in collapse. This void is likely to occur in the fixation device that uses a flush fixation using light to fix toner. The “dullness” means that too much toner transferred onto a paper causes an image to be fixed outside a portion that defines the original image, causing a blurred line and a crushed letter. 
     Next follows a printing (step  1012 ). The number of prints is preferably, for example, three on end for each kind while all the objects and their set values are printed at an upper left portion of the print pattern, thereby, improving reliability. 
     If necessity arises, the above operation continues for a different control object (step  1014 ). When the normal mode is selected after the test (step  1016 ), a set value for each control object is returned to the current value (step  1018 ). 
     The upper and lower values of the drum surface potential, the exposure power, the development bias voltage, and the transfer current may be set by configuring the high-voltage power unit  160  with rectifier  161 , switching part  162 , transformer  163 . DC output part  164 , voltage control part  165 , and variable DC voltage part  166 . The variable DC voltage part  166  includes resistors R 1  through R 4 , analog switches  167  and  168  connected to R 2  and R 3 . As control signals sn 1  and sn 2  from the MPU  156  opens and closes switches  167  and  168 , the output voltage Vout becomes {(R 1 +R 2 +R 3 +R 4 )/(R 3 +R 4 )}Vf when the switches  167  and  168  both turn off, {(R 1 +R 3 +R 4 )/(R 3 +R 4 )}Vf when the switch  167  turns on and the switch  168  turns off, and {(R 1 +R 2 +R 4 )/R 4 }Vf when the switch  167  turns off and the switch  168  turns on. R 1  through R 4  may be adjusted so that the above values become respectively a standard value, a lower value, and an upper value. The rectifier  161 , switching part  162 . transformer  163 , DC output part  164 , and voltage control part  165  may use any structure known in the art, and a description thereof will be omitted. 
     The PROM  152  has previously stored upper and lower values as simulation data suitable for printer&#39;s environment (such as temperature and humidity), and the MPU  156  may preferably calculate the optimal upper and lower values using such data. Optionally, the upper and lower values may be arbitrarily set from the input means such as the operational panel. Then, the display part  50  and/or all alarm (not shown) may alarm by indication or sound in response to an input which exceeds the printable upper or lower limit. 
     The present invention enables a user to arbitrarily select one or more the above drum surface potential, the exposure power, the development bias voltage, and the transfer current, by means of the display part  50  etc. The printing under such dividable process conditions facilitates a margin confirmation and preventive maintenance. The image quality is commonly composed of a plurality of processes, and the prior art image-result inspection with eyes cannot identify a deteriorated component. On the other hand, the inventive printing may easily identify the deteriorated component by using the electric parameters singularly or in combination. The upper and lower values that are quantified improve simplicity and reliability in comparison with the conventional inspection with eyes. 
     Further, the present invention is not limited to these preferred embodiments, but various variations and modifications may be made without departing from the scope of the present invention. 
     A printer of a first aspect of the present invention may quantitatively recognize deterioration of a component that has not been recognized in the prior art, and predict a possible drawback that would happen in the near future by evaluating the deteriorated state to be abnormal. A printer of a second aspect of the present invention may print while setting one or more of those electric parameters which include photosensitive drum&#39;s surface potential, optical part&#39;s exposure power, developer&#39;s bias voltage, and transfer unit&#39;s transfer current, to printable upper and/or lower limits, thereby confirming the image quality margin easily and improving the reliability on a printed result. The image quality is commonly composed of a plurality of processes, and the prior art image-result inspection with eyes cannot identify a deteriorated component. On the other hand, this invention may easily identify the deteriorated component by using the electric parameters singularly or in combination. The upper and lower limits that are quantified improve simplicity and reliability in comparison with the conventional inspection with eyes. Similarly the evaluation method of the present invention enables printing by setting the electric parameters to upper and/or lower limits, thereby confirming the image quality margin easily and improving the reliability on a printed result.