PIGA Accelerometer - Principle of Operation

Principle of Operation

The sensing element of a PIGA is a pendulous mass free to pivot by being mounted on a bearing. A spinning gyroscope is attached such that it would restrain the pendulum against "falling" in the direction of acceleration. The pendulous mass and its attached gyroscope are themselves mounted on a pedestal that can be rotated by an electric torque motor. The rotational axis of this pedestal is mutually orthogonal to the spin axis of the gyroscope as well as the axis that the pendulum is free to move in. The axis of rotation of this pedestal is also in the direction of the measured acceleration.

The position of the pendulum is sensed by precision electrical contacts or by optical or electromagnetic means. Should acceleration displace the pendulum arm from its null position the sensing mechanism will operate the torque motor and rotate the pedestal such that the property of gyroscopic precession restores the pendulum to its null position. The rate of rotation of the pedestal gives the acceleration while the total number of rotations of the shaft gives the speed, hence the term "integrating" in the PIGA acronym. A further level of integration of shaft rotations by either electronic means or by mechanical means, such as a ball and disk integrator, can record the displacement or distance traveled, this latter mechanical method being used by early guidance systems prior to the availability of suitable digital computers.

In most implementations of the PIGA the gyroscope itself is cantilevered on the end of the pendulum arm to act as the pendulous mass itself. Up to three such instruments may be required for each dimension of an INS with the three accelerometers mounted orthogonally generally on a platform stabilized gyroscopically within a system of gimbals.

A critical requirement for accuracy is low static friction (stiction) in the bearings of the pendulum; this is achieved by various means ranging from double ball bearing with a superimposed oscillatory motion to dither the bearing above its threshold or through the use of gaseous or fluid bearings or by the alternative method of floating the gyroscope in a fluid and restraining the residual mass by jewel bearings or electromagnetic means. Although this later method still has the viscous friction of the fluid this is linear and has no threshold and has the advantage of having minimal static friction. Another aspect is the accurate control of the gyroscope's rotational rate.

Missiles using PIGAs were the Polaris, Titan, Redstone, Jupiter, Saturn Series and MX Peacekeeper.