CCD Sensor: DC Motor Applications
Geo-scanner makes extreme demands on synchronous motors
There can be no doubt that the successful advance of digital photography will continue well into the future. But cameras with films are still required for special fields of application in photography, for example making aerial photographs for surveying purposes. These special-purpose devices, frequently weighing two hundred pounds, are far too valuable to be replaced by digital technology. Today the films are digitized with the help of high-quality scanners. Such applications call for accuracy and resolution levels in the range of micrometers. At the same time, it is necessary to accelerate large volumes – some of the rolls have a length of up to 150 m and weigh around 5 kg. This not only makes enormous demands on the optical systems, but the actuators deployed within this area also need to come up with a spectacular performance.
The individual image sections are taken from various directions with a generous overlap in order to produce three-dimensionality from two-dimensional photographs. The series of images are transformed to a common coordinate system for processing. A special program then computes the 3-D coordinates of the specified elevation points. The individual photographs have to be scanned with maximum geometrical and radiometric accuracy in order to produce correct images as part of the superimposition process. Therefore, high-performance scanners are essential.
PhotoScan by INTERGRAPH was developed specifically for scanning photogrammetric images.
It achieves resolutions up to 7 µm. The images are each scanned with the required resolution, meaning that losses in quality owing to resampling are a thing of the past. The unit is suitable for scanning standard 236 x 236 mm² films through roll films with a length of 150 m. The negative carrier is stationary. The mobile, 40 mm-wide CCD sensor scans the image line by line. This process achieves a geometrical accuracy of better than 2 µm RMS per axis.
Actuators for constant speed
Klaus Neumann, Project Manager at INTERGRAPH, is aware that far more than just a high-resolution CCD sensor is required: "A high radiometric accuracy of the sensor is of no particular benefit if the actuator is not able to provide at least the same mechanical resolution. Therefore, one of the key requirements was that the actuator had to provide a resolution accurate to the micrometer". The high-performance scanner is equipped with several actuators developed by FAULHABER, the micromotor specialist. As Neumann continues: "We have been working together with FAULHABER for many years now and have nothing but praise for their actuator technology. Besides this, we have always received good support when developing new products".
An actuator moves the CCD sensor at constant speed in x direction. As Neumann emphasizes: "Even a small jolt is noticeable with these resolutions. So it is important for the actuator to operate at constant speed. Otherwise the image is subject to signal noise. We use a copper graphite-commutated DC micromotor. It features extreme synchronism even at low speeds and operates very precisely." The bell-type armature motor displays no signs of cogging thanks to an ironless rotor coil, thus ensuring high speed constancy without “torque ripple”. This allows the rotational speed and the position to be controlled precisely using the tachogenerator mounted on the motor, together with a pulse generator. The analog tachogenerator supplies information on velocity extremely quickly, while the pulse generator supplies precise position information.
When the sensor has scanned one line, the actuator moves it back in the x direction. At the same time, another actuator moves the sensor in the y direction precisely through its width. The next scanned line continues seamlessly from the previous one. However, owing to the design, the x axis and the y axis of the scanner are never exactly at right-angles. Even minimal angle offset may have devastating consequences with a required resolution of 1 µm. For this reason, the second actuator not only has to position the sensor next to the last scanned line, but also position it accurately to the micrometer. During traverse in x direction, the actuator must also compensate for the positional error of the axis so that the scanned line follows the previous one precisely. The perfect solution: a DC bell-type armature motor with an attached pulse generator or encoder, which, as the active element of a control loop with incorporated electronic circuitry, is able to respond in an extremely short time very precisely and without overshoot.
Small, compact and powerful
The photo-scanner achieves very high scanning speeds. Seven minutes to scan a standard 236 x 236 mm² film with a pixel size of 14 µm is an exceptional performance. But when scanning an entire film with up to 550 images, it is important that the process can be performed independently, for instance overnight. The meter-long films are fed automatically via a roll-film attachment. Once again, DC bell-type armature motors play a pivotal role. In this case, it is important for the actuators to provide high torque levels, while at the same time being compact and energy-efficient. Absolute accuracy is not necessarily required in such cases. The film simply has to be positioned accurately to a few millimeters. The scanner itself determines its precise start point by conducting a comparison with a frame mask present on the film. The interplay of high-resolution optics and high-performance, compact actuator technology delivers picture-perfect results.