One of the primary motivations for developing the Mini Gimbal in-house was to allow users to swiftly interchange various payloads. Not only would this benefit customers, it would also help keep the manufacturing workflow streamlined, reduce the amount of different parts stocked in inventory, and allow us to lower the cost of the gimbal using manufacturing techniques better suited to higher volumes.

There would be a few challenges to overcome in this endeavor, the easiest being designing the mechanical mechanism itself. Before starting any design work I analyzed the scoped payloads for size, mass, and center of gravity; this in addition to constraints set by Astro allowed me to come up with the bounding parameters for payloads that we could integrate. 

Once those parameters were in place the mechanism design could commence. The concept was a tapered dovetail on the gimbal and corresponding socket on the payload mount; this mount provided a self-locating joint which could be consistently repeated for many cycles.

Finally, in order to ensure the gimbal and payload would be well balanced (crucial in gimbal design as any imbalance will cause unnecessary motor loading), the center of gravity (C.G.) of both the gimbal and payload needed to be known. To accurately determine the C.G. for each payload, I employed a method involving suspending the payload with a thin fishing line from multiple points, capturing photos of its equilibrium position, and overlaying these images to identify the intersection point of the lines. While the payloads' C.G. values were obtained experimentally, the gimbal's C.G. was calculated using precise weight and material specifications for each component in the CAD model, including screws, cables, PCBAs, and other manufactured parts. Leveraging this C.G. data, I was able to design payload mounts that align the payload's C.G. within 0.2mm of the gimbal's C.G., ensuring optimal balance.

You can picture a gimbal as a system made up of several springs in a series. This system’s objective is to detect a disturbance input at one end of the spring system as quickly as possible from the other end. The ideal scenario is that the entire gimbal is infinitely stiff so disturbances are instantly detected, allowing the control loop to respond to the disturbance knowing the exact state of the entire system. However, gimbals which are flown need to be light; every superfluous gram adds up and a penalty is paid in flight time. This trade off between stiffness and weight is a  key challenge when designing gimbals, especially ones which are airborne.

After receiving and testing the first prototype of Mini Gimbal, the motor,  arms, and payload dovetail design were highlighted as areas that did not meet the minimum resonant frequency target (stiffness metric). Motors are often one of the more flexible parts of a design since they need to be dynamic. 

However, through an investigation to find the root cause of the lack of stiffness I found some key design issues that needed to be changed in order for the motors to meet the stiffness requirements: 

• The springs preloading the motor stack were being compressed and plastically deformed resulting in less preload force than intended. The solution was to source a better spring and test them for adequate preloading and the ability to compress to nearly flat state without plastically deforming.
• The bearing spacing on the stator's shaft was undersized resulting in a poor ability to react to bending forces on the motor bell.  Solving this issue required a substantial design change to the stator mount; the max bearing spacing was also limited by other constraints in the gimbal and motor.
• The shaft's press fit into the motor bell was incorrectly sized causing it to break free after extended periods of use. This coupled with the deformed preload spring resulted in a motor that had more than 1mm of axial play once the shaft came free. I solved this by adjusting the fit between the shaft and bell as well as adding bearing loctite to the joint.

The issues with the gimbal arms and dovetail required more cross section to withstand the bending caused by the weight of the payload. After implementing these modifications and testing the second gimbal iteration, I confirmed the revised design met the expected performance standards and achieved the target gimbal stiffness.