Image stabilizing apparatus for a portable video camera

Abstract: The image stabilization apparatus for a portable video camera is such that for angular variations about the optcial axis (OZ) of the camera, the horizontal and vertical deflec­ tion currents Ix and Iy of the camera are converted as a function of these variations by an angular measurment device integrally connected with the camera prior to being applied to the corresponding horizontal and vertical coils (4 and 5, respectively) of the camera.

Description: The invention relates to an image stabilization apparatus according to the introduction part of claim 1. Such a camera is described in EP-A-0 149 365 A2. The time constant of the integrator 11h shown in Figure 2b of EP-A-0 149 365 is chosen relatively high in order to make sure that in the so-called "panning mode" the constant-angular-velocity DC signal applied to the input of said integrator is converted into a gradually-rising ramp-shaped constant-angular-velocity drive signal which is applied to the yaw driver 12a (see in particular page 41, lines 9-14 of the cited document.) US-A-3,619,500 discloses an image motion stabilization system for an image tube 10, said system having integrator means 22 receiving from rate gyros 18, an angular velocity value  M subtracted from a rate input  C . The output  E from the integrator means 22 is simultaneously fed to the control inputs of motors 14 and of a coil 28 as well thus ensuring mechanical and electronic compensation for image shift. Low frequency components in  E as well as the steady state component actuate a linkage 16 mechanically connected with the image intensifier tube 10 to modify its orientation thereby compensating for an input disturbance angle  B of the body 15. For such components, the electronic image shift is not necessary. As the disturbance frequency increases and since the inertia prevents motors 14 to be effective to compensate for the angular error, such compensation is taken over by the electronic image shift relying on the deflection yoke 28. Accordingly, the electronic integrator 22 here will not, by itself, provide a slow return to zero, since the low frequency error compensation is achieved by a mechanical control imparted by motors 14. It is an object of the present invention to improve the operation of such a camera with respect to suppressing interferences due to undesired rapid angular movements of the camera. This object is solved by the invention as described in claim 1. Advantageous embodiments of the invention are described in the subclaims. Compared with the prior art shown in EP-A-0 149 365, in the subject-matter claimed the time constant of said integrator means is lowered so that it is able to furnish an increasingly reduced fraction of the angle of inclination when the angular velocity of the camera decreases. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustration of a portable video camera; Fig. 2, on a larger scale, shows the target plate of the video camera of Fig. 1; Fig. 3 is a block diagram of an embodiment of the image stabilizer according to the invention; Fig. 4 is a block diagram of another embodiment of image stabilizer; Fig. 5 is a block diagram of a charge-coupled device of a camera. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 schematically shows a portable video camera and its main components, for the sake of better comprehension of the invention. Hence the video camera includes an objective or lens 1, a target plate 2, an electron gun 3 and horizontal deflection coils 4 for scanning in the x direction and vertical deflection coils 5 for scanning in the y direction, although these coils may also be deflection plates. As is well known, a subject 6 outside the camera forms an optical image 7 on the target plate 2 of a video camera, and this optical image is scanned by the cathode reading system of the camera with the aid of an electron beam emitted by the electron gun 3, which is deflected horizontally and vertically by the deflection coils 4 and 5 having vertical and horizontal fields, respectively. Fig. 2, in broken lines, shows the target plate 2 that has undergone a rotation about the optical axis OZ of the camera. It should be noted here that to obtain compensation for the displacements of the position of the image on the target plate, i.e., its stabilization, it is necessary that the target area should be greater than the area of the optical image projected on it by means of the objective or lens 1. The position of the target plate 2 of the camera may be defined in Cartesian coordinates, using a system of axes, x, y and z. First, let it be assumed that between a time 0 and a time t, the camera has undergone an angular variation about its optical axis OZ. A point in the optical image 7 on the plate 2 is defined at time 0 by its coordinates x₀, y₀, z. When this point has rotated by the angle γ about the axis OZ, it is then defined by x t , y t , z (z is unchanged). In order that the angular variation γ will have no effect at all upon the output signal of the camera, that is, will definitively not affect the position of the image with respect to the screen on the receiver, this angular variation must be compensated for and the cathode reading system of the camera must be made to rotate about the same angle γ. Let Ix and Iy represent the currents that allow the vertical field and horizontal field of the deflection coils 4 and 5 to be created; in the absence of angular displacements of the camera, Ix, Iy are thus x and y scanning deflection currents for a camera lacking the image stabilization apparatus according to the invention. In order to effect the required compensation described above, converted signals I1x and I1y must be sent through the coils 4 and 5, respectively, the values of these signals being as follows: I1x = Ix cos γ + Iy sin γ I1y = Iy cos γ - Ix sin γ The signals Ix and Iy are generated by the camera, and the signals sin γ and cos γ can be generated by a sine and cosine converter associated with a rate gyroscope integrally connected with the camera and placed in such a manner that it provides a measure of the angular velocity of the camera about the axis of its objective. The entire apparatus thus functions as a vibration or oscillation damper for an elastically suspended camera. In Fig. 3, which shows a block diagram of the apparatus, a rate gyroscope 10 has been shown, which is integrally connected to the camera of Fig. 1 and is located in such a manner that it provides a measure of the angular velocity of the camera about the axis OZ of its objective. The rate gyroscope 10 is connected at the output to an integrator 11 provided with a slow return to zero, so as to convert the angular velocity into an angle for rapid movements. This integrator is connected to a converter 12 of sine and cosine functions, which in turn is connected to a linear function converter circuit 13. At its output, the converter 12 thus furnishes two values, sin γ and cos γ, which are proportional to the sine and cosine of the angles of inclination of the camera with respect to its prior position, for rapid angular variations, and furnishes an increasingly reduced fraction of these functions when the angular velocity decreases. The sine and cosine converters mentioned above can be replaced by converters operating upon the angle measured by the gyroscopic device to obtain polynomials or power series (Taylor) thereof respectively corresponding to these functions, which may use, in a first approximation, the first-order (linear) member only. If more precision is desired, higher-order polynomials may be used. The linear function circuit 13 is connected to the output of the circuit of the camera that furnishes the signals Ix and Iy, this circuit being shown at 14. The linear function circuit 13 comprises a multiplier and adder circuit suitable for performing multiplications and additions of signals, so that the above-mentioned converted signals I1x and I1y can be obtained. Multiplications and additions in this manner of signals generated on the one hand by the camera, relating to Ix and Iy, and on the other hand by the rate gyroscope, the integrator and the converter relating to γ, are very easily performed in real time by analog or numerical circuits of a known type. It has been assumed above that the camera is angularly displaced about the optical axis OZ of its objective. Let it now be supposed that the camera is displaced about an axis perpendicular to the optical axis of the camera, for example about the axis x of Fig. 2, which is in the horizontal plane. Any rotational movement α about the axis x is translated into a displacement of the optical image 7 in the direction y, which is proportional to the angle α (small α). In order to prevent such a rapid displacement of the optical image 7 from being translated into a corresponding displacement of the electronic image at the receiver, a direct current proportional to α must be added to the vertical scanning current Iy. Thus the cathode ray corresponding to a point is made to coincide with this point independently of the aforementioned rotational movement. The converted signal thus assumes the following value: Iiy = Iy + K1α An identical induction for a rate gyroscope, with which, after integration, angular varations β about the y axis of Fig. 2 can be obtained, demonstrates that the converted signal is then as follows: Iix = Ix + K2β The use of such a converted signal is not generally necessary, because the shocks to which the cameras are subjected as they move are seldom diminished by movements about the y axis that displace the image in the x direction proportional to the angle β. Fig. 4 shows the entire stabilization apparatus for arbitrary angular variations of the camera making use of the aforementioned provisions. Any angular variation of the camera is defined in polar coordinates by the angles α, β and γ above, so that the apparatus thus includes three rate gyroscopes 10a, 10b, 10c similar to the rate gyroscope 10 of Fig. 2, which are provided in order to furnish information corresponding to the angular variations γ, α and β, respectively. The coefficients K1 and K2 multiplying the angles by which the camera pivots about a horizontal and vertical axis, respectively, both of which are perpendicular to the optical axis OZ, depend on the distance for which the objective focuses the image of the subject onto the photosensitive surface of the target plate, because this determines the displacement of the optical image in the target plane. If a zoom lens, which is capable of varying the focal length of the objective, is used, the coefficients K1 and K2 are made variable as a function thereof. In order to obtain this, as shown in Fig. , the zoom lens, which is controlled by rotation about the optical axis, is mechanically coupled (by gears or a belt and pulleys) to the wiper shaft of a potentiometer whose resistive path is connected between the terminals of a DC power supply, in order to deliver a voltage that is variable with the adjustment of the zoom lens. If the variations in the focusing of the zoom lens are nonlinear with the angle of rotation of its control device, the resistor of the potentiometer can be made to follow this nonlinear function. It is also possible to connect two such potentiometers, connected in the manner of rheostats, into the feedback path of two operational amplifiers, which respectively amplify the signals given by the two further gyroscopic devices (cascaded rate gyroscopes and integrators), in order to vary the respective gains of these amplifiers. The various rate gyroscopes 10a, 10b, 10c are connected to integrators 11a, 11b, 11c, which in turn are connected to converters 12a, 12b, 12c. The converter 12a is a sine and cosine function converter, while the converters 12b and 12c respectively furnish a direct current K1α and K2β proportional to the angular variation detected by the corresponding gyroscope. A linear function circuit 13′, which receives the signals Ix and Iy from a circuit 14′ that is identical to the circuit 14 of Fig. 3, is connected to the output of the converters 12a, 12b and 12c. The linear function circuit 13′ thus furnishes at its output signals that are the linear functions of Ix, Iy, sin γ, cos γ, α and β as defined above. These signals are applied to the deflection coils 4 and 5 of Fig. 1 and can be written as follows: I2x = Ix cos γ + Iy sin γ + K2β I2y = Iy cos γ - Ix sin γ + K1α In the case where the deflection via a coil is replaced with a static deflection, the same induction must naturally be applied, in order to proceed to the linear operations described above on the voltages applied to the deflection plates. It should be noted that in the entire foregoing description, the dimensions of the image 7 furnished by the objective 1 and the target plate 2 of the camera must be such that the above values of the converted signals remain within the boundaries of the target plate. Moreover, a switch located on the camera makes it possible to prevent functioning of the stabilization apparatus except when this is deemed useful. If the camera uses a charge-coupled device solid-state picture sensor, shown in Fig. 5, the scanning is carried out in a matrix fashion, using a digital computer or microprocessors. The linear vertical and horizontal deflection of the scanned image field or of the starting point of the scan are relatively simple operations on the addresses of the read-out points. However, the rotation of the image about the optical axis of the camera is a more complex operation, because in order to obtain a line, several rows of the CCD elements have to be read consecutively. As a matter of fact, depending on the value of the angle of rotation (γ) a certain number of successive elements of the nth row will have to be read, then the same number of elements of the (n + 1)th and so on, until the end of each line. Thereafter, the following (n + 1)th or (n + 2)th row (if the scan is interlaced) is read in the same manner as before, by passing from one row to the next. If the rotation is in the opposite direction, the elements of the nth row are followed by the (n - 1)th and then by the (n - 2)th, and so on. An adequate algorithm can be stored in a read-only memory associated with a micro-processor, to carry out the desired shift in the address of the read-out. The shift functions will correspond substantially to the sine and cosine of the angle of rotation. The word "gyromètre" actually means a "rate gyroscope" or "rate gyro" which generates a signal proportional to the angular velocity of the rotation of the camera about its optical axis (a rate gyro being a gyroscope whose gimbal is restrained into a fixed position relatively to the object on which it is mounted by means of drawsprings and whose motion is damped). The signal is integrated in an integrator having a slow discharge rate (return to zero) to give an output signal proportional to the angle of rotation. Any other type of instrument (transducer) giving a voltage proportional to the angular displacement of the camera may be used. In an experimental model of the invention a gyroscope for model airplanes was successfully applied.

Claims: An image stabilization apparatus for a portable video camera comprising lens means for projecting an image onto a plane XY perpendicular to their optical axis Z, said image being analysed along axes X and Y by means of photosensitive means receiving scanning signals X and Y, said apparatus further comprising electrical means supplying said scanning signals X and Y under the control of at least one measuring arrangement including gyrometric means for sensing the angular velocity of the camera body about one of said axes X, Y or Z, and following said gyrometric means, integrator means having a return to zero response and providing, for rapid angular variations, an integrated angular velocity representing the angles of inclination of said camera with respect to its prior position so as to control said scanning signals X and Y, characterized in that the slope of said return to zero response of said integrator means is chosen such that said integrated angular velocity furnishes an increasingly reduced fraction of said angles of inclination when said angular velocity decreases. An image stabilization apparatus as claimed in claim 1, characterized in that for angular variations y about the optical axis (OZ) of the camera, and for vertical deflection currents Ix and horizontal deflection currents Iy of the camera in the absence of stabilization, the deflection currents are converted by an angular measurement device integrally connected with the camera and take the following form: I1x = Ix cos γ + Iy sin γ I1y = Iy cos γ - Ix sin γ where I1x is the converted horizontal deflection current, and I1y is the converted vertical deflection current, which are applied respectively to the corresponding horizontal and vertical deflection coils (4 and 5, respectively) of the camera. An image stabilization apparatus as claimed in claim 1, characterized in that for angular variations α about a horizontal axis (x) perependicular to the axis (OZ) of the objective and for a vertical deflection current (Iy) of the camera in the absence of stabilization, the vertical deflection current applied to the vertical deflection coil (5) is converted by an angular measurement device integrally connected to the camera and takes the following form: Iiy = Iy + K1α. An image stabilization apparatus as claimed in claim 1, characterized in that for angular variations β about a vertical axis (y) perpendicular to the axis (OZ) of the objective and for a horizontal deflection current Ix of the camera in the absence of stabilization, the horizontal deflection current applied to the horizontal deflection coil (4) is converted by an angular measurement device integrally connected to the camera and takes the following form: Iix = Ix + K2β. An image stabilization apparatus for a portable video camera in accordance with all of the claims 1-3, characterized in that for an arbitrary angular variation of the camera, the vertical and horizontal deflection currents applied to the horizontal and vertical deflection coils (4 and 5, respectively) via an angular measurement device integrally connected with the camera are the linear functions of Ix, Iy, sin γ, cos γ, α and β obtained via an integrator receiving the signals and can be written as follows: I2x = Ix cos γ + Iy sin γ + K2β I2y = Iy cos γ - Ix sin γ + K1α. An image stabilization apparatus for a portable video camera dependent on one of the claims 1-4, characterized in that the angular measurement device integrally connected with the camera includes at least one rate gyroscope (10) connected to an integrator (11) connected to an analog or numerical converter (12), the output of which is connected via a linear function circuit (13) to the coils (4 and 5) or to the horizontal and vertical deflection plates of the camera.

Inventors: FOURNIER ANTOINE, FOURNIER, ANTOINE
Applicants: FOURNIER ANTOINE, FOURNIER, ANTOINE
Application year: 1988
Main classification: H04N5
IPC codes: H04N5