Method for controlling lens of image forming apparatus

Abstract: In a method for controlling a lens (24) of an image forming apparatus (1), the lens (24) is moved at a high speed from a present position till passing a home position, and thereafter the lens (24) is moved at a low speed in the reverse direction to stop at the home position. Waste of time can be decreased and rapid control of the lens (24) can be achieved by high speed movement of the lens (24) to a position near the home position, and the lens (24) can be precisely stopped at the home position by low speed movement of the lens before stopping.

Description: The present invention relates to methods for controlling a movable lens of an image forming apparatus, such as a copying machine, so as to stop the lens at a home position and so as to move the lens from a present position to a specified position. Recently, some image forming apparatus such as copying machines has been provided in which image forming magnification is changeable. In some examples of such apparatus, image forming magnification is changed stepwise or continuously by moving a lens, such as a zoom lens, a fixed focal length lens and the like. In such image forming apparatus having a movable lens, lens positions is corrected to a home position (life-size position) when turning on a power switch. For example, when the power switch is turned off in an enlargement mode, the lens remains at a position of the enlargement mode. In order to reset the apparatus in a life-size mode when the power switch is turned on, it is necessary to return the lens to the home position. As a method for correcting a lens position to a home position more precisely, Japanese Unexamined Patent Publication No. 60-513 shows one in which irrespective of a lens being in a position of the enlargement mode or reduction mode, the lens position is corrected to a home position by moving the lens in one direction always, whereby any irregularity in lens stop positions which is caused by backlash or friction of gears is reduced. However, conventional methods including the above mentioned method execute access of a lens to a home position at the same speed as the speed at which usual movement of the lens is executed and this is rather high. Accordingly, it is liable to happen that the lens passes the home position during a time that a microcomputer or the like detects the home position arrival of the lens and then stops the lens, which consequently causes a difference between the home position and the actual lens stop position. It will be seen that the above problem is eliminated by reducing the moving speed of lens when correcting lens position to the home position. However, the reduction of the moving speed of the lens disadvantageously causes vibration of the lens due to resonance and the like. If resonance is prevented by further reducing the moving speed of the lens, waste of time will be involved. Especially, when there is a long distance between a present position of the lens and the home position, a very long time is required to move the lens to the home position. As methods for moving a lens from a present position to a specified position, for example, there is known a method in which a lens is moved from a present position directly to a specified position (see Japanese Unexamined Patent Publication No. 60-114847), and another method in which a lens is firstly returned to a home position and then moved to a specified position. However, it will be seen that a position error inevitably occurs when actually moving the lens. Accordingly, in the abovementioned former method in which a lens is directly moved to a specified position, the positional error is accumulated on repeating the movement of the lens. Consequently, it is difficult to precisely control the lens position. On the other hand, in the latter method in which a lens is once returned to a home position and then moved to a specified position, such a defect as abovementioned can be eliminated. However, if a present position of a lens is near the specified position and at the same time these two positions are far from a home position, it takes much time to move the lens to the specified position. Accordingly, rapid control of lens cannot be achieved. The avoidance of positional error is also a feature of GB-A-2133566, in which a zoom lens always approaches the specified position in the same direction to avoid the effects of backlash. If the present position is sufficiently far from the specified position the adjustment begins at an increased speed but this drops to a slow speed during the return to the home position and a subsequent reversal to the specified position. Therefore, in this case also the control of lens position cannot be achieved rapidly. An object of the present invention is to provide a method for controlling a lens of an image forming apparatus in which the lens may be controlled with a reduced waste of time and an increased precision. In one aspect of the invention, the lens is moved from a present position at a first, higher speed to pass the home position and the lens is then moved at a lower speed in the reverse direction to stop at the home position. In another aspect of the invention the lens is moved directly to a specified position when a present position and the specified position of the lens are on the same side of a home position, that is, when the lens does not pass the home position and, in respect of movements from a present position on one side of the home position to a specified position on the opposite side of the home position, the lens is moved to the home position to execute positional correction and is then moved to the specified position. Embodiments of the invention are described below by way of non-limitative example, with reference to the accompanying drawings, in which: Fig. 1 is a view showing an internal construction of a copying machine in which methods of the present invention are carried out; Fig. 2 is a plan view showing a mechanism for driving a zoom lens of the copying machine of Fig. 1; Fig. 3 is a diagrammatic view showing a circuit for driving the zoom lens of Fig. 2; Figs. 4a,4b are views showing output signals of a control device in the driving circuit of Fig. 3; Figs. 5a, 5b views showing movements of a zoom lens according to methods of the present invention; Figs. 6 and 7 are flow charts showing control of a zoom lens in a first embodiment of the present invention; Fig. 8 is a diagrammatic view showing a circuit for driving a zoom lens in a second embodiment of the present invention; Fig. 9 is a view showing output signals of the driving circuit of Fig. 8; Figs. 10 and 11 are flow charts showing control of the zoom lens by the driving circuit of Fig. 8; Fig. 12 is a flow chart showing control of a zoom lens according to a third embodiment of the present invention; Fig. 13 is a plan view showing a mechanism for driving a zoom lens in a fourth embodiment of the present invention; Fig. 14 is a flow chart showing control of a zoom lens in a fifth embodiment of the present invention; Figs. 15a, 15b, 15c are views showing movements of a zoom lens according to methods of the present invention. Fig. 1 shows an internal construction of a copying machine 1 (image forming apparatus) in which a method of the present invention is carried out. A contact glass 2 on which an original is to be put and an original cover 3 are provided in an upper portion of the copying machine 1. A paper feed tray 4 is provided in one side of the copying machine 1. A paper feed cassette 5 is mounted. A paper discharge tray 6 is provided in the other side thereof. A photosensitive drum 7 is provided in the copying machine 1 and a main charger 8, a blank exposure lamp 9, a developing device 10, a transfer device 11, a separating device 12, a cleaning device 13, and a discharger 14 are arranged around the photosensitive drum 7. Above the photosensitive drum 7 is provided an optical system 15 for focusing an original image on the photosensitive drum 7. Further, a conveying belt 16, a fixing device 17 and a pair of paper discharging rollers 18 are provided on the downstream side of paper conveying direction of the photosensitive drum 7. The optical system 15 comprises an exposure lamp 19, various kinds of mirrors 20, 21, 22, 23, and a lens or a zoom lens 24. The exposure lamp 19 and the mirrors 20, 21, 22 are integratedly fitted to movable frames a, b. An original is exposedly scanned by a predetermined reciprocating movement of the movable frames a, b. A desirable magnification is set by moving the zoom lens 24. A mechanism for driving the zoom lens 24 is shown in Fig. 2. The zoom lens 24 is fixed on an upper surface of a movable lens frame 25. On a forward side of the movable lens frame 25 is fixed a wire 26 which is passed over pulleys 27, 28 and wound around a driving pulley 29a of a stepping motor 29. Accordingly, the zoom lens 24 and the lens movable frame 25 are integratedly moved by driving the stepping motor 29. Further, a photosensor 30 which includes a light-emitting element and a light-receiving element and constitutes position correcting means is provided behind the zoom lens 24. On a rear side of the movable lens frame 25 is provided a detection projection 25a which constitutes the position correcting means. The detection projection 25a is provided in such a position that the projection is apart from the photosensor 30 in an enlargement mode (100% to 141% in this embodiment), that is, when the movable lens frame 25 is on the left side of a position shown in full lines in Fig. 2, and the detection projection 25a interrupts light of the photosensor 30 in a reduction mode (62% to 99% in this embodiment), that is, when the movable lens frame 25 is on the right side of the position shown in a full line in Fig. 2. The photosensor 30 is so constructed as to be turned from OFF to ON by the interruption of light. Consequently, it is detected by ON or OFF state of the photosensor 30 whether the zoom lens 24 and the movable lens frame 25 pass the life-size position or home position. Fig. 3 shows an example of a circuit for driving the stepping motor 29. Coils A, B, C, D of the stepping motor 29 are respectively connected through buffer drivers 31 to four ports of a control device 32 (which constitutes a part of position correcting means). The coils are applied with voltage in accordance with output of the control device 32. The control device 32 receives a detection signal (positional signal) from the photosensor 30. An output pulse signal of the control device 32 is determined according to positional signals. According to output pulse signals of the control device 32, the stepping motor 29 is driven either at a high speed (first speed) or at a low speed (second speed). When the stepping motor 29 is driven at a high speed, such pulse signals (167Hz here) as shown on the left side portion of Fig. 4a and Fig. 4b are issued from the control device 32 by interruption of a timer provided in the control device 32, whereby 2-2 phase excitation of the stepping motor 29 is performed. On the other hand, when the stepping motor 29 is driven at a low speed, such pulse signals (10Hz here) as shown in the center portion of Fig. 4a are issued from the control device 32, whereby 1-2 phase excitation of the stepping motor 29 is performed. In other words in the low speed driving of the stepping motor 29 which is performed by 1-2 phase excitation, the rotation angle of the stepping motor 29 corresponding to one pulse is half as large as in the high speed driving. Also, the pulse frequency is about 1/17 as many as in the high speed driving. Accordingly, the rotation speed of the stepping motor 29 is very low compared to the high speed driving. By providing such low speed driving vibration is eliminated that occurs when the zoom lens 24 and the movable lens frame 25 are moved. Further, such signals as shown in the right side portion of Fig. 4a are issued as shown when stopping the zoom lens 24 so that low voltages (5V here) are applied to the coils A, B, C, D of the stepping motor 29. Consequently, the zoom lens 24 is assuredly stopped. When the power switch is turned on, the control device 32 executes the home position correction of the zoom lens 24 in the following manner. Movements of the zoom lens 24 are shown in Fig. 5a. In this figure, full lines, dotted lines and chain lines indicate the high speed driving, the low speed driving, and the home position respectively in this order. (A) If the positional signal is in OFF state when the power switch is turned on (Fig. 5a, (A)): 1. The zoom lens 24 is moved in the direction of reduction by driving the stepping motor 29 at the high speed, and stopped when the stepping motor 29 completes a predetermined number of steppings (5 steps here) after the positional signal comes into ON state. 2. Next, the zoom lens 24 is moved in the reverse direction (in the direction of enlargement) by driving the stepping motor 29 at the low speed, and stopped at the moment when the positional signal comes into OFF state. (B) If the positional signal is in ON state when the power switch is turned on (Fig.5a, (B)) 1. The zoom lens 24 is moved in the direction of enlargement by driving the stepping motor 29 at the high speed, till the positional signal comes into OFF state, that is, till the zoom lens 24 passes the home position. 2. Next, the zoom lens 24 is moved in the direction of reduction by driving the stepping motor 29 at the high speed, and stopped when the stepping motor 29 complete a stepping of five after the positional signal comes into ON state. 3. Further, the zoom lens 24 is moved in the reverse direction (in the direction of enlargement) by driving the stepping motor 29 at the low speed, and stopped at the moment when the positional signal comes into OFF state. The control of the control device 32 is shown in the flow charts of Figs. 6, 7. Fig. 6 shows a routine for judging an initial position of the zoom lens 24 when the power switch is turned on. Firstly, the positional signal is confirmed when the power switch is turned on (step S₁). If the positional signal is in ON state, IC flag (flag for confirming the initial position of the lens) is set at 1 (step S₂). If the positional signal is in OFF state, IC flag is set at 0 (step S₃). A step counter of the stepping motor 29 is reset at 0 (step S₄). The interruption period of the timer provided in the control device 32 is set at 6 milliseconds so as to produce a pulse signal of 167 Hz is issued (step S₅). After the initial position of the zoom lens 24 is determined in the abovementioned manner, the program advances to a drive routine. Firstly, when the step counter of the stepping motor 29 is determined to be 0 at step S₆, the program advances to step S₇. At step S₇, when IC flag is 1, that is, when the zoom lens 24 is in a position of the reduction mode, the zoom lens 24 is driven at a high speed in the direction of enlargement by 2-2 phase excitation of the stepping motor 29 (step S₉) till the positional signal comes into OFF state (YES at step S₈). When the positional signal comes into OFF state, IC flag is set at 0 (step S₁₀). The program advances to step S₁₁. when IC flag is 0 at step S₇, the program advances directly to step S₁₁. In other words, in this embodiment, if the zoom lens 24 is in a position of the reduction mode when the power switch is turned on, the zoom lens is once moved to a position of the enlargement mode and the home position correction is always started from a position of the enlargement mode. The true home position correction is started at step S₁₁. Since the zoom lens 24 is always in a position of the enlargement mode at this starting point as abovementioned, the positional signal is in OFF state. Accordingly, the zoom lens 24 is driven at a high speed in the direction of reduction (step S₁₃). Such high speed driving of the zoom lens 24 is continued till the position signal comes into ON state. Thereafter, the step counter of the stepping motor 29 is added with 1 by 1 (step S₁₂) till the step counter reaches 5 (NO at Step S₆). At this time, the interruption period of the timer is turned to be 100 milliseconds so that the control device 32 issues a pulse signal of 10Hz (step S₁₄). At this time, since the positional signal is in ON state (YES at step S₁₅), the zoom lens 24 is driven at a low speed in the direction of enlargement by 1-2 phase excitation of the stepping motor 29 (step S₁₆). When the positional signal comes into OFF state (NO at step S₁₅), the zoom lens 24 is stopped and at the same time the voltage applied to the coils A, B, C, D of the stepping motor 29 is kept at 5V in order to hold the zoom lens 24 at a fixed position (step S₁₇). The method of this embodiment executes the home position correction by moving the zoom lens 24 at a high speed from a position of the enlargement mode to a position of the reduction mode till the zoom lens 24 passes the home position, and thereafter moving it at a low speed in the reverse direction and finally stopping it when the photosensor 30 comes into OFF state, as abovementioned. In other words, the zoom lens 24 is moved at low speed immediately before being stopped. Accordingly, it will be seen that the home position correction can be attained with extremely smaller error than the conventional methods. Besides, since the zoom lens 24 is moved at a high speed to a position near the home position, loss of time can be decreased. It will be noted that a lens driving mechanism and a position detecting means of the present invention is not limited to the abovementioned embodiment, and the abovementioned advantages can be obtained by using other known devices. In Fig. 8 is shown another lens driving mechanism which uses a digital servomotor. Fig. 8 is a view showing an arrangement of a second embodiment of the present invention in which the home position correction of the zoom lens 24 is executed by using a servomotor. In this Figure, indicated at 30 is the abovementioned photosensor for issuing a positional signal after the power switch is turned on, a positional signal is sent to a CPU 33 for use in control. The CPU 33 receives a positional signal, and issues a signal for driving and stopping the zoom lens 24 and also issues a signal for controlling the rotation direction of the servomotor 34, namely, the driving direction of the zoom lens 24 and a driving reference clock pulse so that the drive of the zoom lens 24 is controlled. A rotary disk having a plurality of rectangular teeth at a predetermined interval on its periphery is connected to a rotary shaft of the servomotor 34. Also, a photosensor is provided near the rotary disk. The rotary disc and the photosensor constitute a rotary encoder 35. The rotary encoder 35 issues a rotary pulse having a cycle in proportion to the rotation speed of the servomotor 34. The rotary pulse is sent to a PLL 36a which compares the phase of the rotary pulse with that of the clock pulse so that the servomotor can rotate accurately in synchronism with the driving reference clock. The driving circuit 36 receives the abovementioned signals from the CPU 33 and the PLL 36a to drive the servomotor 34. Fig. 9 is a wave form view explaining driving of the servomotor 34. A signal (A) is a positional signal whose OFF state indicates a position of enlargement mode and ON state indicates a position of reduction mode. A signal (B) is a driving signal and gives a driving command to the servomotor 34 during ON state. A signal (C) is a signal for determining the moving direction of the zoom lens 24. At high level of the signal (C), the zoom lens 24 is moved from enlargement to reduction, and at low level the zoom lens 24 from reduction to enlargement. A signal (D) indicates a clock pulse for driving the servomotor 34. The left side half indicates a pulse of 1000 Hz for driving the servomotor 34 at a high speed and the right side half a pulse of 100 Hz for driving the servomotor 34 at a low speed. The control of the zoom lens 24 by the use of the servomotor 34 is shown in flow charts of Fig. 10 and 11. Fig. 10 corresponds to Fig. 6 of the first embodiment. If the positional signal is in ON state when the power switch is turned on, IC flag is set at 1. If it is in OFF state, IC flag is set at 0 (steps S₂₁ to S₂₃). A time up value (equivalent to 20 milliseconds, see the signal (D) in Fig. 9) is reset in a timer provided in the CPU 33 (step S₂₄). A driving reference pulse of 1000 Hz (the left side half of the signal (D) in Fig. 9) is issued (step S₂₅). After the intial position of the zoom lens 24 is determined in the abovementioned manner, the program advances to a true driving routine shown in Fig. 11. The movement of the zoom lens 24 is shown in Fig. 5a. Fig. 11 corresponds to Fig. 7 in the first embodiment. Firstly, at step S₂₆, the timer has yet not started. Consequently, the determination of step S₂₆ is NO and the program advances to step S₂₇. When IC flag is 1 at step S₂₇, the zoom lens 24 is in a position of the reduction mode. Also, the determination of step S₂₈ is NO. Accordingly, the zoom lens 24 is moved at a high speed till the positional signal comes into OFF state (step S₂₇ to S₂₉). When the positional signal comes into OFF state, IC flag is set at 0 (step S₃₀). The program advances to step S₃₁. If when flag is 0 at step S₂₇, the program advances directly to step S₃₁. Since the zoom lens 24 is in a position of the enlargement mode at this stage, the positional signal is in OFF state. Consequently, the determination of step S₃₁ is NO and the zoom lens 24 is moved at a high speed in the direction of reduction (step S₃₃). Such high speed driving of the zoom lens 24 is continued till the positional signal comes into ON state (YES at step S₃₁). At this time, the timer is started (step S₃₂). When 20 milliseconds have passed (YES at step S₂₆), a driving reference clock pulse of 100 Hz is issued (step S₃₄). Since the positional signal is ON state at this time (YES at step S₃₅), the zoom lens 24 is driven at a low speed in the direction of enlargement (step S₃₆). When the positional signal comes into OFF state (NO at step S₃₅), the zoom lens 24 is stopped (step S₃₇). Since the zoom lens 24 is always moved to the home position in the same direction, the irregularity in lens stop position can be eliminated and the home position correction can be carried out with an increased preciseness. In the abovementioned two embodiments, the home position correction is executed in the same direction. However, the home position correction can be precisely executed by moving the zoom lens 24 to the home position in different directions with using two kinds of driving speed, namely, a high speed and a low speed. This movement of the zoom lens 24 is shown in Fig. 5b. Fig. 12 is a control flow chart showing a third embodiment of the present invention in which a stepping motor is used as shown in Fig. 7. However, this embodiment can be sufficiently carried out by using a servomotor. In Fig. 12, since the step counter of the stepping motor 29 is 0 at step S₄₁, the program advances to step S₄₂. When IC flag is 1 at step S₄₂, that is, when the zoom lens 24 is in a position of the reduction mode, th positional signal is in ON state (NO at step S₄₃). Accordingly, the zoom lens 24 is driven at a high speed in the direction of enlargement (step S₄₅). Such high speed driving of the zoom lens 24 is continued till the positional signal comes into OFF state (YES at step S₄₃). At this time, the step counter of the stepping motor 29 is added with 1 by 1 (step S₄₄) till the step counter reaches 5 (NO at step S₄₁). At this time, the interruption period of the timer is changed to 100 milliseconds (step S₄₆) so that the driving reference clock pulse of 10 Hz is issued from the control device 32. Since IC flag is 1 at this time (NO at step S₄₇) and at the same time the positional signal is in OFF state (YES at step S₄₈), the zoom lens 24 is driven at low speed in the direction of reduction (step S₄₉). When the positional signal comes into ON state (NO in step S₄₈), the zoom lens 24 is stopped and kept at the position (step S₅₀) (Fig. 5b (B)). On the other hand, when IC flag is 0 at step S₄₂, that is , when the zoom lens 24 is in a position of the enlargement mode (NO at step S₄₂), the zoom lens 24 is driven at a high speed in the direction of reduction (step S₅₂). Such high speed driving of the zoom lens 24 is continued till the positional signal comes into ON state (YES at step S₅₁). At this time, the step counter is added with 1 by 1 (step S₅₃) till the step counter reaches 5. At this time, the driving reference clock pulse of 10 Hz is issued to start the low speed driving of the zoom lens 24 (NO at step S₄₁, step S₄₆). At this time, since IC flag is 0 and the positional signal is ON (YES both at S₄₇ and S₅₄), the zoom lens 24 is driven at a low speed in the direction of enlargement (step S₅₅). When the positional signal comes into OFF state (NO at step S₅₄), the zoom lens 24 is stopped (step S₅₀) (Fig. 5b (A)). In the abovementioned third embodiment in which only one photosensor 30 is used, the home position correction of the zoom lens 24 is executed with less increased preciseness than in the case in which the zoom lens 24 is always moved to the home position in one direction. In other words, even if a command that the zoom lens 24 be stopped is given at the home position, a stopped position of one direction is liable to be a little different from a stopped position of the other direction. This is because the zoom lens 24 is moved to the home position in the opposite directions. Fig. 13 shows a mechanism for driving a zoom lens which is used in a more preferred fourth embodiment to eliminate the slight stopped position difference caused in the abovementioned third embodiment. The arrangement at this embodiment is the same with that shown in Fig. 2 with an exception that two photosensors 30a, 30b are provided as shown in Fig. 13. In Fig. 13, the photosensors 30a, 30b are arranged near each other so as to detect the detection projection 25a. The positions of the photosensors 30a, 30b are determined so that both a position at which the zoom lens 24 is stopped when the zoom lens 24 is moved at a predetermined low speed from a position of the reduction mode to a position of the enlargement mode and then the photosensor 30a detects an end of the detection projection 25a (the output changes from ON state to OFF state) and a position at which the zoom lens 24 is stopped when the zoom lens 24 is moved at a predetermined low speed from a position of the enlargement mode to a position of the reduction mode and then the photosensor 30b detects an end of the detection projection 25b (the output changes from OFF state to ON state) coincide with the home position. The flow chart of Fig. 12 is basically applicable to this fourth embodiment. However, it will be noted that there are the following differences: It is detected by the two photosensors 30a, 30b whether the positional signals are in OFF or ON state at step S₄₃ and S₅₁; After the interruption of timer (step S₄₆), ON state determination of the positional signal at step S₅₄ is executed by only the photosensor 30a and at the same time OFF state determination of the positional signal at step S₄₈ is executed by only the photosensor 30b. In other words, in Fig. 12, when IC flag is 1 at step S₄₂, that is, when the zoom lens 24 is in a position of the reduction mode, both positional signals of the photosensors 30a, 30b are in ON state (NO at step S₄₃). Accordingly, the zoom lens 24 is moved at a high speed to a position of the enlargement mode (step S₄₅). Such a high speed driving of the zoom lens 24 is continued till both positional signals of the photosensors 30a, 30b come into OFF state (YES at step S₄₃). At this time, the step counter of the stepping motor 29 is added with 1 by 1 (step S₄₄). When the step counter reaches 5 (NO at step S₄₁), the interruption period of the timer is changed to 100 milliseconds (step S₄₆) so that the driving reference clock pulse of 10 Hz is issued from the control device 32. At this time, since IC flag is 1 (NO at step S₄₇) and at the same time the positional signal of the photosensor 30b is in OFF state (YES at step S₄₈), the zoom lens 24 is moved at a low speed in the direction of reduction (step S₄₉). When the positional signal of the photosensor 30b comes into ON state (NO at step S₄₈) the zoom lens 24 is stepped and held at the position (step S₅₀) (Fig. 5b (B)). Consequently, the zoom lens 24 is stopped at the home position. On the other hand, when the zoom lens 24 is in a position of the enlargement mode (NO at step S₄₂), the zoom lens 24 is moved at a high speed in the direction of reduction (step S₅₂) till both positional signals of the photosensors 30a, 30b come into ON state (YES at step S₅₁). Such a high speed driving of the zoom lens 24 is continued till both positional signals of the photosensors 30a, 30b come into ON state, At this time, the step counter is added with 1 by 1 (step S₅₃). When the step counter reaches 5, the driving reference clock pulse of 10 Hz is issued so as to start the low speed driving of the zoom lens 24 (NO at step S₄₁), step S₄₆). Since IC flag is 0 at this time and the positional signal of the photosensor 30a is in ON state (YES both at steps S₄₇ and S₅₄), the zoom lens 24 is moved a low speed to a position of enlargement (at step S₅₅). When the positional signal of the photosensor 30a comes into OFF state (NO at step S₅₄), the zoom lens 24 is stopped and held at the position (step S₅₀) (Fig. 5b (A)). The zoom lens 24 is stopped at the same home position as the reverse direction movement. According to the abovementioned first to fourth embodiments, the zoom lens 24 is moved at a high speed from the present position till passing the home position, and thereafter moved at a low speed in the reverse direction to stop at the home position. Accordingly, the zoom lens 24 can be stopped at the home position with decreased waste of time by the high speed movement to a position near the home position. Also, the zoom lens 24 can be stopped at the home position with increased preciseness by the low speed movement immediately before stopping. The home position correction of the zoom lens 24 is executed by the control device 32 and the photosensor 30 in the abovementioned manner. Further, a fifth embodiment of the present invention will be described in which movement of the zoom lens 24 from a present position to a specified position (a position in accordance with a magnification given by an operator) is controlled by the stepping motor 29 or the servomotor 34. This control of the lens movement is carried out according to the following scheme. In the following embodiments, a home position is referred to as a life-size position. (A) When the present position of the zoom lens 24 and a specified position are on the same side of the life-size position: A distance between the present invention of the zoom lens 24 and the specified position is computed and the zoom lens 24 is moved so as to cover the distance. (B) When the present position of the zoom lens 24 and the specified position are on the opposite sides of the life-size position: Firstly, the zoom lens 24 is moved to the life-size position at which position correction is then executed. Thereafter, a distance is computed which is between the life-size position and the specified position and the zoom lens 24 is moved the distance to reach the specified position. Such control operation will now be described with reference to the flow chart of Fig. 14. In Fig. 14, firstly, it is checked whether the magnification designated by operating keys or the like has changed from a previous designated magnification (step S₆₁). When the designated magnification has not changed, it is not necessary to move the zoom lens 24. However, when an auto clear key is turned on (YES at step S₆₂), the copying machine comes into a life-size mode and the life-size position correction of the zoom lens 24 is executed (step S₆₃). On the other hand, when the designated magnification has changed (YES at step S₆₁), the present position of the zoom lens 24 is detected. The following control is executed based on the present position. (a) When the present position is the life-size position (YES at step S₆₄) the zoom lens 24 is directly moved by a predetermined number of pulses from the present life-size position to the specified position in accordance with a designated magnification (step S₆₅). This movement of the zoom lens 24 is shown in Fig. 15a. (b) When the present position is in the reduction mode and the designated magnification is in the enlargement mode or the life-size mode (YES at both of step S₆₆ and step S₆₇), or when the present position is in the enlargement mode and the designated magnification is in the reduction mode or the life-size mode (NO at step S₆₆ and YES at step S₆₈), the life-size position correction of the zoom lens 24 is executed in accordance with flow chart of Fig. 7, 11 or 12, and is indicated as step S₆₉ in Fig. 14. Thereafter, the zoom lens 24 is moved from the life-size position to the specified position (step S₆₅). This movement of the zoom lens 24 is shown in Fig. 15 b. In other words, Fig. 15 b (b₁) shows the life size position correction of the zoom lens 24 executed by moving the zoom lens 24 in the same direction at a high speed and a low speed (Fig. 7 and 11). Fig. 15 b (b₂) shows the life-size position correction of the zoom lens 24 executed by moving the zoom lens 24 in two directions at a high speed and a low speed (Fig. 12). When the specified position is the life-size position, tile zoom lens has reached the specified position by executing the life-size position correction. (c) When the present position is in the reduction mode and the designated magnification is in the reduction mode (YES at step S₆₆ and NO at step S₆₇), or when the present position is in the enlargement mode and the magnification is in the enlargement mode (NO at step S₆₆ and NO at step S₆₈) , the life-size position correction is not executed but the zoom lens 24 is directly moved to the specified position. This movement of the zoom lens 24 is shown in Fig. 15 c. According to this method, when the zoom lens 24 is moved from a position of the enlargement mode to a position of the reduction mode or from a position of the reduction mode to a position of the enlargement mode, the life-size position correction of the zoom lens 24 is once executed in the way and then the zoom lens 24 is moved to a specified position. Accordingly, an error in position of the zoom lens 24 caused in the preceding movement can be eliminated by this position correction and position of the zoom lens 24 can be precisely controlled at all times. Further, when the zoom lens 24 does not pass the life-size position, in other words, when the zoom lens 24 is moved from a position of the enlargement mode to another position of the enlargement mode or from a position of the reduction mode to another position of the reduction mode, the life-size position correction of the zoom lens 24 is not executed but the zoom lens 24 is moved directly to a specified position in the same manner as used in conventional copying machines. Accordingly, there is little waste of time. Furthermore, when the life-size position correction of the zoom lens 24 is once executed and the zoom lens 24 is moved to a specified position, the distance between the life-size position and the specified position is computed and the zoom lens 24 is moved so as to cover the distance by sending a driving motor pulses corresponding to the distance as conventional copying machines. Accordingly, the movement of the zoom lens 24 can be controlled more rapidly than conventional method in which the zoom lens 24 is moved stepwise to a specified position with confirming the position of the zoom lens 24. In the abovementioned fifth embodiment of the present invention, the life-size position correction of the zoom lens 24 is executed at a high speed and a low speed and thereafter the zoom lens 24 is moved to a specified position. In the abovementioned embodiments, the position correction of a zoom lens is described with a home position being set at a life-size position. However, according to the present invention, it will be apparent that same control is applicable in the case that position correction of a zoom lens is executed with a home position being set at a position of the enlargement mode or a position of the reduction mode. Also, it will be apparent that a method of the present invention is not limitedly applied for a zoom lens but is applicable for other kinds of lens used in image forming apparatus, such as fixed focal length lens to assure the same effects. Furthermore, in the abovementioned embodiments of the present invention in which the position correction of a zoom lens is executed in one direction, the zoom lens 24 is once moved to a position of the enlargement mode and then moved in the direction of reduction. However, It will be apparent that position correction of a zoom lens may be executed by firstly moving the zoom lens to a position of the reduction mode and then moving it in the direction of enlargement.

Claims: A method for controlling a lens of an image forming apparatus comprising the steps of moving the lens at a first speed from a present position until it passes a home position, and moving the lens at a second speed lower than the first speed in the reverse direction to stop at the home position. A method for controlling a lens of an image forming apparatus as defined in claim 1 wherein the movement of the lens at the second speed is always executed in a fixed direction. A method for controlling a lens of an image forming apparatus as defined in claim 1 wherein the movement of the lens at the second speed is executed in opposite directions when moving from present positions on opposite sides of the home position. A method for controlling a lens of an image forming apparatus as defined in claim 3 wherein the stopping of the lens being moved at the second speed at the home position is executed by two separate means for detecting a home position in accordance with the movement direction of the lens. A method for controlling a lens of an image forming apparatus as defined in any one of claims 1 to 4 wherein the lens is driven by a stepping motor. A method for controlling a lens of an image forming apparatus as defined in claim 5 wherein the rotation speed of the stepping motor is controlled by interruption of a timer. A method for controlling a lens of an image forming apparatus as defined in claim 6 wherein the rotation speed of the stepping motor is controlled by interruption of a timer and by changing the excitation phases of the stepping motor. A method for controlling a lens of an image forming apparatus as defined in any one of claims 1 to 4 wherein the lens is driven by a servomotor. A method for controlling a lens of an image forming apparatus comprising the steps of moving the lens directly to a specified position when a present position of the lens and the specified position are on the same side of a home position, and in respect of movements from a present position on one side of the home position to a specified position on the opposite side of the home position, moving the lens to the home position to execute position correction of the lens and moving the lens from the home position to the specified position. A method for controlling a lens of an image forming apparatus as defined in claim 9 wherein the position correction of the lens is always executed in a fixed direction. A method for controlling a lens of an image forming apparatus as defined in claim 10 wherein the position correction of the lens is always executed at a constant speed. A method for controlling a lens of an image forming apparatus as defined in claim 10 wherein the position correction of the lens is executed by moving the lens at a first speed from a present position till passing a home position, and moving the lens at a second speed lower than the first speed in the reverse direction to stop at the home position. A method for controlling a lens of an image forming apparatus as defined in any one of claims 9 to 12 wherein the lens is driven by a stepping motor. A method for controlling a lens of an image forming apparatus as defined in claim 13 wherein the rotation speed of the stepping motor is controlled by interruption of a timer. A method for controlling a lens of an image forming apparatus as defined in any one of claims 9 to 12 wherein the lens is driven by a servomotor. A method for driving a lens of an image forming apparatus as defined in any one of claims 9 to 15 wherein the lens is moved in opposite directions when moving from present positions on opposite sides of the home position. A method for controlling a lens of an image forming apparatus as defined in any one of claims 9 to 16 wherein a distance from the home position to a specified position is computed and the lens is moved to the specified position in accordance with the computer distance.

Inventors: ANDO HIRONORI, YAMADA SHUJI, ANDO, HIRONORI, YAMADA, SHUJI
Applicants: MITA INDUSTRIAL CO LTD, MITA INDUSTRIAL CO. LTD.
Application year: 1988
Main classification: G03G15
IPC codes: G03B27, G03G15