1. Field of the Invention
The present invention relates to a lens controlling apparatus suitable for use in a still camera, a video camera or the like.
2. Description of the Related Art
It has heretofore been known that stepping motors are employed in many precision machines which require highly accurate position control.
Stepping motors are also employed in, for example, video cameras for the purpose of lens control. This is primarily because such a stepping motor can achieve a high positioning accuracy and can be controlled easily and at high speed because of its open-loop control.
FIG. 1 shows the arrangement and operation of a stepping motor. Referring to FIG. 1, the central magnetic poles constitute a rotor, while the peripheral magnetic poles constitute a stator. Each of the magnetic poles of the rotor is formed by a permanent magnet, while each of the magnetic poles of the stator is formed by the magnetization of the stator by the current supplied to the stepping motor.
In the stepping motor shown in FIG. 1, if the polarity of the stator is varied in the order of from (a) to (d), the rotor moves as indicated by the respective arrows:
(a) Magnetic poles A and -A become a south pole and a north pole, respectively, and magnetic poles B and -B have no polarities; PA1 (b) The magnetic poles A and -A have no polarities, and the magnetic poles B and -B become a south pole and a north pole, respectively; PA1 (c) The magnetic poles A and -A become a north pole and a south pole, respectively, and the magnetic poles B and -B have no polarities; and PA1 (d) The magnetic poles A and -A have no polarities, and the magnetic poles B and -B become a north pole and a south pole, respectively.
As can be seen from FIG. 1, to rotate the stepping motor, it is only necessary to vary the polarity of the stator in the above-described manner.
In some existing stepping motor driving apparatus, to cope with the problem of noise or the like, the stator is magnetized by using a sinusoidal, driving current waveform such as shown in FIG. 2. In the current waveform shown in FIG. 2, the positions indicated at (a) to (d) respectively correspond to the positions indicated by the arrows in FIG. 1.
In the case where the stator is magnetized by using the driving current waveform shown in FIG. 2, if the stepping motor stops at a phase position other than the phases indicated by the respective arrows in FIG. 1, the relationship between the magnetic poles of the stator and those of the rotor becomes imperfect and the rotor becomes extremely unstable. As a result, the rotor moves by a particular magnetic pole of the rotor being attracted by either one of two adjacent magnetic poles of the stator opposite to the particular magnetic pole. For this reason, in the case of the driving current waveform shown in FIG. 2, the stepping motor must not be stopped at any phase position other than predetermined phases (the positions indicated by the respective arrows in FIG. 1, i.e., the positions indicated at (a) to (d) in FIG. 1).
After the stepping motor has been stopped, to hold the stepping motor at that stop position, it is necessary to continue to supply current having a phase identical to the phase of the driving current waveform supplied to the stepping motor immediately before the stop of the stepping motor. The magnitude of the current may be far smaller than that of the driving current supplied during the driving of the stepping motor.
FIG. 3 diagrammatically shows one example of the stepping motor driving apparatus, and FIG. 4 shows in detail the output current waveform of the driving apparatus of FIG. 3 which corresponds to either one phase.
The driving apparatus shown in FIG. 3 includes a control device 50 for performing control of the apparatus in which a stepping motor 52 is disposed, the control device 50 being formed by a microcomputer, and a motor driving device 51 for driving the stepping motor 52 on the basis of a control signal supplied from the control device 50, as well as the stepping motor 52. The internal arrangement of the motor driving device 51 will be described below. Input/output lines 501 to 505 are disposed between the control device 50 and the motor driving device 51. The line 501 is a clock input line for inputting a clock signal to the motor driving device 51, the line 502 is a driving-direction input line for inputting a driving-direction signal to the motor driving device 51, the line 503 is a monitor output line for outputting a signal indicating that the phase of the motor driving circuit 14 is a predetermined phase, the line 504 is a stop instruction input line for inputting a stop instruction to the motor driving device 51, and the line 505 is a power saving instruction input line for inputting a power saving instruction to decrease the amount of current to be supplied to the stepping motor 52.
The motor driving device 51 includes a counter 511 for counting the number of clocks which are supplied from the control device 50 via the clock input line 501, a current waveform forming circuit 512 for forming a current waveform in accordance with a count value provided by the counter 511, a switch 513 disposed between the clock input line 501 and the counter 511 and operated via the stop instruction input line 504, a constant current source 514 for supplying a constant current to the stepping motor 52 during a power saving operation, and a switch 515 for selecting current to be supplied to the stepping motor 52 by performing switching between the output of the current waveform forming circuit 512 and the output of the constant current source 514. The switch 515 is operated via the power saving instruction input line 505.
Clocks supplied via the clock input line 501 are counted by the counter 511 included in the motor driving device 51, and a current value which is set by the current waveform forming circuit 512 advances step by step in accordance with the count value of the counter 511, thereby forming a motor driving current waveform. A logic level supplied via the driving-direction input line 192 determines whether the stepping motor 52 is to be driven in the forward or backward direction.
Each time the current waveform reaches a predetermined phase, a low-level (L) signal is outputted to the monitor output line 503 as shown in FIG. 4. If the stop instruction input line 504 goes to a high level (H), the switch 513 is opened to disconnect the clock input line 501 and the counter 511 from each other. Accordingly, even if a clock is inputted, the counting of the counter 511 does not advance any more so that the stepping motor 52 stops. If the level of the power saving instruction input line 505 is "H", the output of the current waveform forming circuit 512 is connected to the stepping motor 52, while if the level of the power saving instruction input line 512 is "L", the output of the constant current source 514 is connected to the stepping motor 52.
The control device 50 controls the motor driving device 51 in the following manner:
(1) When the stepping motor 52 is to be driven, the control device 50 sets a direction in which to drive stepping motor 52 via the driving-direction input line 502. Then, the control device 50 inputs a clock signal to the clock input line 501 and causes the current waveform forming circuit 512 to output a motor driving current waveform, thereby driving the stepping motor 52.
(2) When the stepping motor 52 is to be stopped, after the control device 50 confirms through the monitor output line 503 that the stepping motor 52 is positioned at any one of the predetermined phases, the control device 50 sets the level of the stop instruction input line 504 to "H".
(3) When the direction of driving of the stepping motor 52 is to be changed, the control device 50 temporarily stops the stepping motor 52 in the manner stated above in the paragraph (2), and inverts the level of the driving-direction input line 502. Then, the control device 50 sets the level of the stop instruction input line 504 to "L" and causes the current waveform forming circuit 512 to output the motor driving current waveform, thereby driving the stepping motor 52.
(4) If the stopped state of the stepping motor 52 is to be continued, the control device 50 sets the level of the power saving instruction input line 505 to "L" to decrease current consumption, and switches over the switch 515 to select the output of the constant current source 514 as current to be supplied to the stepping motor 52, thereby decreasing power consumption.
If the above-described stepping motor driving apparatus is applied to the control of a zooming lens and a focusing lens in a video camera, the control device 50 operates in the following manner. In operation, to identify the mechanical position of the zooming lens or the focusing lens, the control device 50 calculates an address each time the stepping motor 52 reaches one of the predetermined phases, and the motor driving device 51 outputs the low-level signal "L" to the monitor output line 503 when the stepping motor 52 reaches one of the predetermined phases. Specifically, each time the control device 50 detects that the low-level signal "L" is outputted to the monitor output line 503, the control device 50 increases or decreases the value of an address according to the direction of driving of the stepping motor 52, thereby identifying the respective positions of the zooming lens and the focusing lens. Thus, the control device 50 performs lens control during an autofocus or zooming operation on the basis of such an address.
However, the above-described motor driving apparatus having the above-described arrangement has the following problem.
For example, when the control device is to stop the lens, if the stepping motor is not positioned at any of the predetermined phases, the control device fails to output a stop instruction, and must output the stop instruction after the stepping motor reaches any one of the predetermined phases.
In a video camera or the like which uses the control device, not only does the control device drive the lenses, but also performs many other functions. Accordingly, it is a substantial burden on the control device to oversee the timing of outputting a stop instruction.
If the rotor of the stepping motor fails to stop at a predetermined phase and stops at a halfway phase position relative to the magnetic poles of the stator, the rotor becomes extremely unstable and moves by a particular magnetic pole of the rotor being attracted by either one of two adjacent magnetic poles of the stator opposite to the particular magnetic pole. If the rotor moves, a discrepancy occurs between an actual position of the lens and an address calculated by the control device, thereby impeding an autofocus or zooming operation.
Similarly, the power saving instruction must not be executed except when the relative position between the magnetic poles of the rotor and the magnetic poles of the stator corresponds to the predetermined phase. Accordingly, in this case as well, a phase deviation occurring between the magnetic poles of the rotor and the magnetic poles of the stator becomes a serious problem in terms of control.