Unmanned Systems Technology 015 | Martin UAV V-Bat | William Sachiti | Sonar Systems | USVs | Desert Aircraft DA150 EFI | SeaCat AUV/ROV | Gimbals

72 detector, which is a measure of the distance between its individual elements. Using that angular movement limit as a starting point, engineers design the system and specify the required accuracies of all the individual components and the number of axes to which they are to be applied. In current practice, if the required stabilisation level is as demanding as 15-20 microradians then four gimbals would typically be used, while for systems with less stringent needs three are usually enough. The number can also be reduced if the performance requirement allows, with two-axis stabilisation considered enough for fields of view down to 2°. However, four-axis stabilisation is useful in applications that require zooming to fields of view smaller than 2°. Among today’s high-end payload manufacturers, of which there are several, some use two-gimbal designs while others use four-gimbal designs, an engineer notes. From the image quality at comparable fields of view, there are no obvious differences in performance, he says. The specific application plays an important role. For quadcopter payloads designed with cinematography in mind, it is important to have a three-axis gimbal, the engineer says, adding that although the large, high-resolution cameras often operate at wide fields of view, the extra stability enhances the aesthetics of the image. Seeking balance For ISR applications in miniature UAVs, the engineer adds, it is necessary to fit the camera with the narrowest field of view in the smallest turret diameter possible. The goal is to maximise the detectability range for a human target in both day and night conditions. In this application, a two-axis direct- driven gimbal can provide the required stability while keeping the size and weight of the turret to a minimum. Ideally, a sensor would be perfectly balanced on its gimbals, with each axis minimised to reduce drag, the engineer adds, it is also vital to ensure that each sensor has an unobstructed view out of the turret throughout its entire zoom range. For example, it is not a good design if, on a wide field-of-view setting, the sensor starts to show the edges of the aperture or the internal structure of the turret. Gimbals also include bearings that would be frictionless in an ideal world, but in the real world are not. The friction they generate allows coupling forces to be transmitted from axis to axis and, along with forces generated by any cables that cross the axes, tend to make the camera follow the motion of the platform from which the gimbals are intended to isolate it. As gimbals permit rotational motion, gyroscopic forces also have to be managed. The principal unwanted gyroscopic force is precession, which can affect anything that is rotating when it is subjected to a second rotational force about an axis at a right angle to the first, with the resulting motion taking place about a third axis at a right angle to both of the others. A design that balances the sensors August/September 2017 | Unmanned Systems Technology Focus | Gimbals of rotation passing through its centre of gravity. In the real world, however, this is rarely possible and minor imbalances can generate coupling forces in which motion about or along one axis can cause disturbances on others. A very simple way to illustrate this is to hold a pencil between forefinger and thumb halfway along, and move it up and down; as it is balanced it will stay level. Try the same thing while holding the pencil slightly off centre though and it will rock like a see-saw as well. Balance is always something that must be kept in mind throughout the entire design process, the engineer says. With cameras, which must be mounted on the stabilised optical platform supported by the gimbals within the turret, their centres of gravity are often shifted towards the lens by the weight of the optics; however, the design objective is to minimise the amount of ballast used to balance it out, otherwise the turret can become too heavy. Most of the lens needs to be located as close to the outside window as possible to keep the diameter of the window small and minimise the diameter of the turret. While the diameter of the turret must be The Fox thermal camera has a long-range continuous optical IR zoom lens and a cooled mid-wave IR focal plane detector array made from indium antimonide (Courtesy of Controp)