Figure 1a: Image stabilization algorithms and technologies need repeatable test conditions to be comparable and allow for faster progress. Hexapods are used to simulate unwanted camera vibration and motion in X, Y, Z, pitch, yaw, and roll during image recording.

Hexapod Motion Simulators Are Improving Image Stabilization for the Industry

June 23, 2021
Taking sharp pictures despite fast-changing lighting conditions, taking snapshots without blurring or identifying dangerous situations in surveillance systems—all of this is possible today with the help of modern cameras.

End users, manufacturers of smartphones and cameras, companies in the automobile and aerospace industries, companies in the security or automation technology sectors—they all place high demands on the quality of still and video images. As a consequence, cameras are being evaluated in comprehensive tests on the basis of specific image attributes. These attributes include resolution, contrast, color, texture, zoom, autofocus, exposure, and image stabilization. For each of these attributes, thousands of images are taken and evaluated to obtain statistically significant results. In order for the image quality to be comparable, cameras and camera components must always be tested under the same conditions and using the same methods.

Reliable Analytical Methods

DXOMARK (headquartered in Boulogne, France) specializes in complete laboratory solutions for the analysis, comparison, and optimization of image quality. The Analyzer systems (Figure 2) consist of hardware, software, and extensive testing protocols, bundled within serval modules that ensure repeatable, operator-independent results.

Depending on the task at hand or on the image quality attributes being evaluated, different modules can be used and combined. A special capability of Analyzer is the visual noise measurement. It produces visual noise metrics, which directly correlate with visual perception. The video analysis encompasses exposure, white balance, definition, and texture for changing lighting conditions. Automated lighting scenarios can also be programmed for user-specific test requirements. The latest version of Analyzer includes the Selfie module to enable accurate and repeatable smartphone front-camera testing.

The measurement methods on which the DXOMARK evaluation is based, have been developed in collaboration with several companies in the imaging industry that collaborate in international standardization working groups such as IEEE/CPIQ and ISO TC42-WG18.

Testing Image Stabilization

The image stabilization test assesses how well the optical and electronic image stabilization systems built into cameras work. These systems are designed to compensate for movements and, therefore, avoid blurred images. In order to guarantee stable image capturing, sensors measure linear and angular motion accelerations, for example, the photographer's shaky hands holding the camera or the vibrations of vehicles and aircraft so that the image stabilization system can automatically offset these influences and avoid blurred images. The effectiveness of image stabilization systems, whether optical or electronic, therefore, follows the CIPA protocol. CIPA (Camera and Imaging Product Association) is an alliance of Japanese camera manufacturers that has developed standards for defining the test conditions for motion simulation.

Motion Simulation with High Repeatability

For all image stabilization test methods, it is important that the shaking camera or the vibrations in vehicles and aircraft are reproducible.

"We must ensure that the simulated frequencies and movements, for example, around the rotation axes θX, θY, θZ (pitch, yaw, roll), are identical for each test," emphasized Nicolas Touchard, Vice President of Marketing at DXOMARK. "We are currently using hexapods in the latest version of Analyzer because they allow precisely reproducible motions up to frequencies of 30 Hz, increasing the number of potential test scenarios. We have to ensure that the simulated motion remains unchanged for all tests. Hexapods are very well suited to perform this task."

Camera and smartphone tests typically require up to 12 Hz, however, the image stabilization function of driver assistance systems offset much higher frequencies. Thanks to their parallel-kinematic design, hexapods are virtually predestined for several reasons for the precise simulation of motions and vibrations. Their advantages compared with serial (stacked) systems, are, that they have much better path accuracy, repeatability, and flatness. In addition, the moved mass is lower, enabling better dynamic performance, which is the same for all motion axes. Also, cable management is no longer an issue, because cables are not moved, and the system has a much more compact design.

Suitable Specifications and Qualified Support

The hexapods used in the latest Analyzer version are from the German company PI (Physik Instrumente). The H-840 hexapod, for example, is designed for the testing of image stabilization systems and certified to CIPA Standard DC-011-2015.  This standard defines rotational axes, test frequencies, and vibration amplitudes that are necessary for certification.

Since mid-2019, another hexapod is being used by the Analyzer (Figure 4).  The H-860 hexapod is designed for the testing of image stabilization systems. It offers stimulation frequencies up to 30 Hz and travels predefined trajectories (sinusoids) and freely definable paths with a high degree of trajectory control. Due to the friction-free voice coil drives and the lightweight design consisting of extremely stiff carbon fiber parts with slow-moving masses, it is possible to achieve fast and smooth motion as well as high acceleration.

The hexapod used for testing is fixed to a base plate. The brackets for the test items are installed on the base plate. They keep the test device safely in place on the hexapod during vibration simulation.

"We have not only decided in favor of these hexapods because they have the perfect specifications for vibration simulation but also because PI provides support, for example, for the adaptation of the software drivers," Touchard adds.

Easy Control and a Freely Definable Pivot Point

The high-performance C-887 digital controller (Figure 5) takes care of the control for the hexapods, and the software enables easy commanding. Positions are specified in Cartesian coordinates and transformations for the six individual drives that take place in the controller. The freely definable rotation or pivot point is an essential feature of the hexapod. This means that the motion of the hexapod platform can be specifically adapted to the position of the image stabilization component in the camera so that, for example, the image sensor is located in the middle of all six degrees of freedom.

By now, hexapods have proven their worth for image stabilization testing. The six-axis parallel-kinematics also provide advantages in other applications and are at the forefront of the race for many motion simulations, like in mechanical engineering and drive technology, tests for multi-dimensional position sensors, as well as the calibration of gyroscopic compasses pursuant to DIN ISO 22090-1 for shipping and marine technology.