Biaxial Zero-Force Backlash Region Assembly
The TALOS mechanical design process begins by analyzing the high-level system requirements. Once a design concept is approved by the client, our mechanical design team will generate a model using SolidWorks. TALOS uses Commercial Off The Shelf (COTS) hardware when possible to reduce risk, lead time, and cost. However, if custom parts are required, TALOS has extensive design experience and a supply chain of quick turn machinists.
For this project, our client contracted TALOS to design a custom Automated Test Equipment (ATE) that would be used for validation testing of a commercial airplane’s flight control yoke. The primary mechanical design requirement specified that the ATE must be able to actuate the yoke roll (moving the wheel interface clockwise and counterclockwise) and pitch (pushing or pulling the yoke by applying a force orthogonal to the wheel interface plane) while taking force and position measurements throughout the actuation. Additionally, our client requested the ATE can transition between pitch and roll tests without operator setup. To ensure the Unit Under Test (UUT) was not being preloaded by the ATE prior to actuation, a zero-force backlash region for both pitch and roll was requested.
The above diagram illustrates the design implemented to satisfy the primary mechanical requirements. Once the UUT is loaded into the test fixture, the ATE is attached to the UUT at the wheel interface. A biaxial load cell was strategically placed as close to the wheel interface to reduce the risk of system-induced force reading error. When the UUT pitch is actuated, the load cell measures tension and compression axially. When the UUT roll is actuated, the load cell measures torque. The biaxial backlash assembly, which is shown in greater detail below, is the custom hardware which allows both the UUT pitch and roll to be actuated without operator setup. The biaxial blacklash assembly passes through the absolute rotary ring encoder and is coupled to the stepper motor assembly. The entire assembly above is mounted to the pitch actuator arm (shown in below images) via the four linear bearings which allow the assembly to move linearly in the zero-force pitch backlash region.
The biaxial zero-force backlash region requirement was achieved by designing two custom parts from 304 stainless steel to bear the load of pitch and roll actuation shown on the right. Smooth rotation between the bearing housing and drive shaft is possible via the thrust bearings inside the bearing housing. Thrust bearings were selected to handle the axial force exerted through the assembly, and the clamping force from the distorted-thread locknut used to fasten the drive shaft and bearing housing together. The end of the drive shaft is coupled to the stepper motor assembly and supported by another bearing in the linear motor carriage, and the end of the bearing housing is fastened directly to the biaxial load cell. When assembled, the four cylindrical bosses on the near face of the bearing housing fit into the four slots in the drive shaft. The roll actuation is in the zero-force backlash region when the cylindrical bosses are not contacting the slot ends.
To produce roll actuation, the stepper motor assembly will rotate the drive shaft clockwise or counterclockwise until the edges of the slots in the drive shaft contact the bearing housing bosses. This will actuate the roll by producing a torque which will be read by the biaxial load cell. The pitch zero-force backlash region is achieved by implementing hard-stops in the pitch actuator arm for the larger diameter sections for the drive shaft and bearing housing to contact as the linear motor carriage slides during actuation (shown in images below).
Once our client approved of the design concept, TALOS finalized the mechanical design and provided rendered graphics, motion studies, and validation calculation for final approval. TALOS works closely with our suppliers to ensure custom parts are manufactured to meet specifications. For this project, our client requested the stainless steel components to have a passivized finish, and all aluminum components to have a hard black-anodized finish to match the 8020 frame.