Anchored in Inertial Space: Interactive Apollo Inertial Measurement Unit (IMU)

The Apollo Project’s Inertial Measurement Unit (IMU) was, in my opinion, a mechatronical wonder with an astonishing property: it can measure absolute orientation!

Architecturally speaking, it consisted of an outer shell with three nested gimbals inside. In the abstracted visualization below, the outer shell is in gray, the outer gimbal is in red, the middle gimbal is in green, and inner gimbal/platform is in blue. The gimbals were suspened on three orthogonal axes and electromechanically stabilized with the output signals of three orthogonal gyroscopes (mounted on the inner, blue, platform) as basis. Resolvers digitized the angles of the gimbals, and corresponding pulses were fed into the Apollo Guidance Computer (AGC) to calculate the absolute orientation of the vessel.

In the interactive numerical simulation below, use drag-and-drop to rotate the IMU as a whole and witness how the “stable member” (blue) remains fixed in inertial space; in other words how its absolute orientation does not change (with the exception of slight drift that increases around a state called “gimbal lock”).