Simulating camera shake with high precision
03 August 2016
Obtaining razor-sharp images without using a tripod and at the same time, compensate for natural and artificial motion is currently one of the decisive factors for both cameras and smartphones.
In cameras, sensors detect the shaking motion of the photographer or the vibrations of a vehicle or airplane so that the image stabilisation system can compensate automatically.
For this purpose, optical compensatory elements in the lens are shifted or the image sensor is moved. However, the corresponding algorithms and the mechanics must be optimised and tested. Parallel-kinematic Hexapods have proven to be effective for the required highly dynamic simulation of defined precision motion.
More than two axes
An effective test procedure is of particular interest for both manufactures and users. This is where Image Engineering plays a key role. Their core expertise is measuring the digital image quality as well as developing corresponding test procedures and comprehensive test systems for cameras.
Evaluating the image quality and testing the image stabilisation place high technical demands on the test procedure and its components. Above all, the objective is precision simulation of the trembling, shaking human hand.
CIPA (Camera and Imaging Product Association), an alliance of Japanese camera manufacturers similar to an ISO committee, has developed the DC-011-2015 technical standard for defining the test conditions for motion simulation. It defines rotations around the Z and Y axes as well as the test frequencies and oscillation amplitudes that are necessary for certification. This standard will shortly be adopted by an international ISO standard. However, the differences in motion behaviour when using classical digital cameras and mobile devices require the test procedure to be adapted to the individual circumstances of the smartphone user.
Dietmar Wüller, Managing Director of Image Engineering, said camera manufacturers do not just correct motion on two axes as required by CIPA, but are increasingly calling for image stabilisation on up to five axes. Depending on the weight of the camera, shaking is different, and simulating two axes of motion does not provide significant test results anymore. The demands on smartphones are even higher. As a camera, they don't fit well into the hand and when taking photos, they are often held at arm's length or with the finger tips and in addition, the display is pressed to trigger the shutter, which tends to push it back.
Parallel kinematics simulates shaking
However, in the case of multi-axis test systems, the conventional, serial or stacked systems quickly reach their limits because of the accumulation of guiding errors. Parallel-kinematic systems are the best choice. Their advantages include increased path accuracy, repeatability and flatness of travel as well as a lower moved mass that enables the same higher dynamic performance on all motion axes. Also, they don't need complex cable management and the design is much more compact.
Wüller reports that with the Hexapods from PI (Physik Instrumente) can reproduce exactly defined "camera shake" and can be controlled according to individual specifications and test procedures.
The frequency of human shaking goes up to 12Hz, which means that the tests must be able to cover a range from 0 to 12Hz. The bulk of the frequencies required is in the range between 4 and 8Hz. Hexapods are predestined for this purpose.
Different weight classes and travel ranges
As test systems, the image stabilisation engineers opted for two different Hexapods, depending on the weight of the test items and the travel range required:
The H-811 mini Hexapod is used in test systems for smartphones. The universal H-840 is used for heavier cameras or larger travel ranges. Both of them are perfectly suited for testing image stabilising systems and are already certified by CIPA.
The mini Hexapod can simulate vibration, e. g., rotary motion, with dynamics of 20Hz and a deflection of 0.1°. For this purpose, the parallel-kinematic system performs a repeatable and defined test trajectory. It provides travel ranges up to 34mm on the XY plane and up to 13mm in the Z direction. The tilt angles are 20° around the X and Y axes and up to 42° around the vertical. The H-840 carries higher loads, provides travel ranges up to 100mm and rotational angles up to 60°.
Easy control and a freely definable pivot point
The high-performance C-887 digital controller takes care of the control for both Hexapods that, thanks to the user-friendly software, enables easy commanding. Positions are specified in Cartesian coordinates and transformations for the six individual drives takes place in the controller.
The freely definable rotation or pivot point is an essential feature of the Hexapod. This makes it possible to match the motion of the Hexapod platform directly to the respective camera and integrate it into the overall process.
Thanks to the high sampling rate of the controller, it is easy to create and transfer customer-specific test procedures with the software provided by Image Engineering.
What is tested?
The Hexapod is fixed to a base plate for the tests. The camera holder and a so-called digitus, which is a mechanical finger for triggering the camera, is mounted onto its platform. An image of the full format test chart is acquired. Images of the test chart are acquired while the Hexapod shakes the camera according to the test trajectories. The motion blur is then analysed from this. The process is designed to test the behaviour of the width of several diagonal edges at different exposure times with the motion control switched on and off. The image stabilisation should be able to reduce shifts in the image.
Image stabilisation is not just designed for photography. Other examples come from the automobile industry, where such tests are useful for example, for automatic recognition of traffic signs, where reliable function is necessary under strong vibration or adverse lighting conditions.
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