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

69 view being the most demanding. Bigger systems have had very good stabilisation performance for many years, but lately smaller ones have been improving in this respect thanks to the miniaturisation of inertial sensors and high-performance torque motors. Fundamentally, there are only three axes of motion representing the three spatial dimensions x , y and z , but any object, such as a camera, can both rotate about those axes and move along them in a linear fashion (sometimes called translation). The rotating motions are known as yaw, pitch and roll when talking about vehicles or pan, tilt and roll when discussing cameras, while their three translational counterparts are termed heave (up and down), surge (back and forth) and sway (side to side). Gimbals only permit rotational motion – they can be likened to axles – but they can be nested inside each other, which is why some stabilised payloads have more than three gimbals. For example, a camera might be mounted on a platform with pan, tilt and roll gimbals, all of which are mounted on another, larger pan gimbal about which the whole turret is steered. Likewise, a turret in an application that requires it to tilt through large angles is likely to have an extra gimbal on the tilt axis, bringing the total number to five. Another reason for using more than three gimbals is to overcome a phenomenon known as gimbal lock. This is the effective loss of one axis of rotation in a three-gimbal system when the other two are driven into parallel orientations. Which axis is locked in this way depends on the order in which the axes rotate. The three are sometimes referred to as the parent (outer), middle (intermediate) and child (innermost) axes. When the parent axis rotates, so do the other two. When the middle axis rotates so does the child axis but the parent axis is unaffected; only the child axis can rotate without affecting either of the others. However, when the middle axis lines up with the parent axis, the child axis locks so no rotation about it is possible. In conventional terminology, the parent axis is the third on the list, so in a pan/ tilt/roll set-up the roll axis would be the parent. Here, the system is free to point the camera’s line of sight in any direction in space with the camera at any roll angle, but when the tilt angle reaches ± 90° the pan and roll angles become parallel and the pan axis locks and the system cannot be fully stabilised because only the tilt and roll axes can move. Three-gimbal systems can be designed with any of the axes in the parent, middle and child positions, and there are six possible combinations. System designers choose the configuration in which gimbal lock is least likely to occur on the most important axis for the application. For example, a system like the one above but with a pan/roll/tilt gimbal order, gimbal lock would happen at a roll angle of 90°, which the system is much less likely to approach. Furthermore, the gimbals are just components of the stabilisation system that also contains servo motors, inertial measurement units (IMUs) and electronic feedback loops that sense motion about the stabilised axes and send commands to the servo motors to apply corrections. As well as the tiny movements used to stabilise the sensors, the turrets also use servo motors to make larger movements to point them in azimuth and elevation. The turrets are also environmental enclosures that protect the sensors and electronic and electrical components inside them from extremes of temperature, shock and vibration, dust and water ingress and electromagnetic interference. What is more, they must do all that while remaining within the cost and Gimbals | Focus Unmanned Systems Technology | August/September 2017 This five-axis system with three gimbals inside the ball provides fine stabilisation for the x, y and z axes (Courtesy of Dynamic Perspective)