Structural resonance in stabilized gimbal control

Structural resonance in stabilized gimbal control

Structural torsional resonances often limit the closed-loop bandwidth for large and mid-sized stabilized gimbals, but many more subtle structural effects can cause instability or impact performance.

IJK Controls engineers analyzed these more subtle structural effects to correct a poorly performing gyro stabilized gimbal. This high performance stabilized gimbal was resonating dangerously in only certain gimbal configurations. The servo control system was coupling with poorly understood structural modes.

Structural transfer functions were measured experimentally over a set of parameters, and an appendage mode was discovered in the gimbal structure. Appendage modes are a result of flexible elements that are not in between the actuator and the feedback sensor but are “hanging off” the structure. A simplified “lumped mass” model provided insight into the expected form of the transfer functions. The lumped mass model parameters were varied to fit the experimentally measured transfer function. The controller design was adjusted to take these dynamics into account.

Like many lightweight, high-performance multi-axis gimbals, this product exhibited surprising and difficult to diagnose coupling between degrees of freedom. The effects of cross products of inertia were bounded and found not to be the problem. Experimental modal analysis pointed to a “warping” of the gimbal support structure: Torque in one axis can actually induce motion in a different axis. For simple beams, the Cw constant characterizes a similar effect, but more complex 3-D structures can also “pitch” in response to a “roll” torque. A finite element model was built to validate a simple structural fix. The proposed change was implemented in the field at low cost and with minimal impact, producing the desired result while keeping the program on schedule and within its budget.

Insight from the effort was used to generate a set of design guidelines that were successfully applied across the product line.

keyword: servo-structure coupling, gimbal resonance, stabilized gimbal control, control of flexible structures, structural dynamics, structural transfer functions, cross products of inertia, gimbal design, finite element analysis