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Positioning A gimbal application like the one shown in (Figure 4) performs pan & tilt operations in most application scenarios. Some engineers may consider stepper motors for positioning motion patterns. Indeed, stepper motors are designed for positioning and have some intrinsic advantages. For example, a stepper motor typically has 200 steps per revolution (360 degrees) and can hold its rotor at any step by design, and stepper motor control is simple and open-loop. However there are also disadvantages inherent to this type of motor which make them less suitable for positioning in BLDC gimbal motor applications, particularly when used for a drone carrying a camera. 1. Stepper motors are typically slow and thus cannot move a camera with enough speed to compensate for drone movement. 2 A stepper motor's open-loop control cannot achieve the level of power efcacy that a BLDC motor can reach. 3. Stepper motors are generally more expensive than BLDC motors. In gimbal axis motion control, a motor is commanded to move from position A (e.g. 25 degrees) to position B (e.g. 67 degrees) in a forward or backward direction. When at position A, the motor is stationary. It then accelerates at a specied ratio, reaches the full specied speed, and continues traveling. As it approaches position B (destination), it decelerates at a specied ratio before nally stopping at position B. The motor current must be circulated in a particular manner while the motor stops at position B. Below, in (Figure 5), is the trapezoidal speed prole that the motor will follow. Regarding Gimbal axis position control, it can be benecial to abstract/encapsulate its motion pattern into a set of atomic motion commands, which are needed in host-gimbal communication. Below is a basic example to illustrate this concept. • positioning commands: GoTo_ABS, GoTo_DIR, GoHome • motion commands Spin • Stop command: Stop, hardstop Parameter definition: • ABS_POS Absolute position encoder degree • INC_POS Incremental position encoder degree Positioning command: • GoHome: Brings the motor into HOME position (absolute encoder counter is zero) from existing position (minimum path) • GoTo_ABS(ABS_POS): Bring the motor into ABS_POS position (minimum path) • GoTo_DIR(DIR, ABS_POS): Bring the motor into ABS_POS position at specied direction (forward or reverse) • GoTo(INC_POS): Bring the motor into INC_POS position (could be multiple rounds (e.g. n • 360 degree + m degrees) Motion command: • Spin(DIR,eHz): spin the motor in a given direction at target electrical speed (Hz or inc_pos/s) Figure 5. Trapezoidal speed prole in gimbal axis motion control. (Source: STMicroelectronics) 10 Figure 4. Example of a drone-mounted gimbal application for camera pan & tilt. (Source: STMicroelectronics)

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