TECHNICAL FIELD The present invention relates to the control of a camera aperture using a motor, preferably a stepping motor. By means of the invention, quicker and more energy-efficient aperture control is achieved in comparison to what has previously been possible.

BACKGROUND ART In mirror reflex cameras of the SLR (Single Lens Reflex) type, the photographer views the subject that is to be photographed through the lens of the camera, by means of a mirror arrangement that directs the image up into the viewfinder. The camera's aperture is usually fully open in this mode, in order for the photographer to be able to obtain as bright a view of the subject as possible.

In association with an exposure in an SLR camera among other things-on account of the above-the following must take place : the mirror arrangement is moved. aside, the aperture is changed from fully open to the value that the photographer has selected, the shutter is closed, makes an exposure and opens again, the aperture returns to being fully open and the mirror arrangement returns to its initial position. The whole of this process must be carried out as quickly as possible, so that the time from the photographer pressing the button until the exposure takes place does not jeopardize the quality of the picture, and so that the total time taken is not perceived as a problem by the photographer.

This means that the time for changing the aperture from fully open to the value required by the photographer must be made as short as possible, which is also the case for the time to change the aperture back from the value required by the photographer to fully open. The controlling of the

aperture must, of course, also be carried out in such a way that the value required for the aperture is obtained very accurately.

A known way of controlling the aperture in a camera is with a so-called stepping motor, which is known, for example, from the American patent 4,998, 129. The arrangement in this document aims to overcome problems that can cause the stepping motor to lose power in certain positions, and thereby not be able to operate the aperture.

In addition, the American patent 4,453, 816 describes the control of an aperture in a camera using a stepping motor, but in this document a device is described that requires two separate stepping motors or one stepping motor that has two different types of gearing, depending upon which of two different control inputs in the motor is used.

The control of an aperture in a camera by means of a stepping motor provides a relatively simple control as the motor is normally controlled by means of pulses, but this is associated with a number of problems. Among other things, the motor cannot receive the control pulses at an arbitrary speed, as at a particular pulse frequency the motor will not be able to recognize all the pulses. This means that the pulse frequency (corresponding to the speed of the motor) must be chosen in the light of the motor's required movement.

Another important factor concerning the movement of the motor is the energy supply, which must of course be minimized, but at the same time must be chosen in the light of the speed and accuracy of the movement.

DISCLOSURE OF INVENTION There is thus a need for a method by means of which it is possible to control an aperture in a still-photography camera using a stepping motor in such a way that the control is quick, energy-efficient and as accurate as possible.

This need is fulfilled by the present invention by the disclosure of a method for use in a still-photography camera for changing the aperture from a first aperture opening to a second aperture opening using a stepping motor, where the movement of the stepping motor during the control of the movement of the aperture from the first aperture opening to the second aperture opening is controlled so that the motor's movement comprises at least a first interval of time with varying acceleration.

This first interval of time with varying acceleration can suitably be combined with a second interval of time with constant acceleration, or alternatively with a second interval of time with varying acceleration.

During its movement, the aperture will offer different amounts of resistance (inertia) depending upon the position that it is in. For example, the inertia will be greatest in the case of greatest aperture opening and will be less in the case of smaller aperture openings. The intervals of time according to the invention are therefore suitably arranged so that the first interval of time, in other words the interval of time with the varying acceleration, is used for the part of the movement of the aperture that comprises the greatest opening of the aperture during the whole of the movement. In this way, the motor is able to be made to move with a small acceleration at the aperture position that gives the greatest load, and thereafter with increasing acceleration as the load reduces.

At the positions with smaller loads from the aperture, it is sufficient for the motor to be operated with a constant acceleration, which according to the invention is carried out by the second interval of time being used for the part of the movement of the aperture that comprises such aperture positions.

However, in such positions, a second interval of time with varying acceleration can also be used.

If required, there can also be an interval of time with constant speed, in other words with no acceleration, between the first and the second intervals of time.

BRIEF DESCRIPTION OF DRAWINGS The invention will be described in greater detail below, with reference to the attached drawings, in which: Figure 1 shows the motor speed as a function of control pulses according to the invention for a closing movement of an aperture, and Figure 2 shows the motor speed as a function of control pulses according to the invention for an opening movement of an aperture, and Figure 3 shows the motor speed as a function of control pulses for an alternative opening movement according to the invention of an aperture.

MODES FOR CARRYING OUT THE INVENTION The present invention relates, as mentioned, to the control of the movement of an aperture in a camera, preferably a mirror reflex camera, using a so- called stepping motor. The stepping motor is controlled using control pulses that are sent to the motor in pairs, so-called pulse pairs. The pulse pairs are sent to the motor's field windings, and for each pulse pair the rotor turns through a well-defined angle, which means that this type of motor is well- suited for precision control.

The movement of the motor is to a certain extent dependent upon the frequency at which the pulse pairs are sent to the motor. If the pulse pairs are sent to the motor at a low frequency, the movement of the motor will be uneven, as the motor then has time to brake between each movement and must then start from a standstill at the next pulse pair. On the other hand, if the pulse pairs are sent at too rapid a rate, the motor will not have the time to carry out the movements at a rate that corresponds to this increased pulse frequency, and certain pulses will be"lost". In other words, these effects must

be taken into account in the design of the pulse pattern for the control of the stepping motor in question.

The aperture that is to be controlled using the stepping motor according to the invention will also affect how the pulse pattern for the control of the motor should be designed. The aperture will namely offer the greatest load at large aperture openings, while the inertia thereafter decreases with decreasing aperture opening. In other words, the load on the stepping motor will be greatest when the closure of the aperture commences in the exposure process in an SLR camera described initially.

On account of the effect of the abovementioned factors-the characteristics of the motor and the varying inertia of the aperture-a supply of pulse pairs to the motor at a constant frequency will not be optimal. According to the invention, the pulse frequency (and thereby the speed of rotation of the motor) is therefore matched to the current position of the aperture, as shown in Figure 1. Figure 1 shows the motor in a position where it controls the aperture in an exposure process as described above. Thus at the start of the process, the aperture is at its maximal opening, in other words in the position that offers the greatest inertia and thereby the greatest load on the motor.

The horizontal axis in the diagram in Figure 1 shows the number (N) of pulse pairs sent to the motor, and the vertical axis shows the pulse frequency (f).

As the speed of the motor is directly dependent upon the pulse frequency, the vertical axis can also be said to show the speed of the motor.

In the process that is depicted in Figure 1, stopping down is to be carried out to an aperture position that corresponds to Y pulse pairs being sent to the stepping motor. During stopping down, the pulse frequency is initially increased the whole time, which is shown by the appearance of the segment 1 in Figure 1, as during a first interval this is not linear but is curved, which corresponds to the motor being given increasing acceleration the whole time.

After a certain time, the speed has reached a maximal value, for which

reason this speed can be maintained for a certain time, which is shown by the change in the graph to a horizontal line 2. The aperture has now left its greatest value, and has reached an opening that provides relatively little inertia. The control of the stepping motor and thereby the aperture during the remaining movement can therefore be carried out with a constant retardation (negative acceleration), so that the movement of the aperture is completed at a low speed, in other words there is a"soft braking to a halt". This is shown by the change in the graph to the segment 3 with constant inclination, corresponding to a constant reduction in the frequency of the pulse pairs sent.

As has been mentioned above, the maximal value for the speed is determined by factors in the motor, for example the quickest pulse speed that the motor or its control circuits can utilize, and friction in the system.

Figure 2 shows the converse of the process in Figure 1, in other words the process that takes place when the aperture is to be returned to fully open after the actual exposure of the film has taken place. In other words, at the commencement of the process in Figure 2, the aperture has a small opening and is to be changed back to a large opening, which means that initially the aperture offers little load for the motor, and that the movement will be concluded with maximal load from the aperture on the motor. The opening movement is therefore commenced by an interval of time with a constantly increasing frequency of pulse pairs sent to the motor (constant positive acceleration), corresponding to the linearly inclined segment 5 in Figure 2.

When a certain optimal speed is reached, the motor is sent pulse pairs at a constant frequency for a certain interval of time, in other words corresponding to a constant speed (no acceleration) of the motor, which is shown by the horizontal line 6 in Figure 2.

At the end of the movement in Figure 2, the opening movement of the aperture, the aperture will offer the greatest inertia to the motor, in other

words the greatest load. The opening movement of the aperture is therefore concluded by an interval of time with reducing retardation (negative acceleration) of the motor, so that, at the end of the movement, the motor has the lowest retardation, which is shown by the curved segment 7 in Figure 2.

Figure 3 finally shows a variant of the invention. Like Figure 1, Figure 3 shows a closing movement of an aperture. As with the closing movement in Figure 1, the closing movement that is shown in Figure 3 comprises a first interval of time 1 with varying acceleration and an interval of time 2 with constant speed. Unlike the closing movement in Figure 1, the movement in Figure 3 comprises a second interval of time 3'with varying acceleration, which in this case concludes the closing movement. In another alternative variant of the invention, this can be applied to an opening movement, which thus comprises both a first and a second interval of time with varying acceleration. In this variant of the invention, the first and the second intervals of time can have acceleration graphs that vary in different ways or in the same way.

If required, the horizontal segments of the graphs in 1 and 2 can be omitted, in other words the movement with varying positive or negative acceleration can be directly linked in time to the movement with constant positive or negative acceleration.

The lengths of the intervals of time during which the different pulse frequencies are used as above, are determined experimentally or by the use of mathematics for each type of aperture.

In a variant of the invention, the amplitude of the pulses that are sent to control the motor is varied during the control procedure, for example in order to save energy. This can, for example, be carried out on the basis of the fact that in certain positions (for example, with a constant speed of the motor) the

energy that the motor requires in order to maintain/achieve the required speed will be less than the energy that is required, for example, for acceleration or retardation movements of the motor.

The invention is not limited to the examples of embodiments described above, but can be varied freely within the framework of the following patent claims. For example, the principle with intervals of varying and constant acceleration can be used to control motors that are controlled by other types of pulse patterns than pulse pairs, for example three pulses at a time or one pulse at a time.