3320. Static Balancing a Device with Two or More Degrees of Freedom – (The Key to Obtaining High Performance on Gimbled Missile

Paper

Abstract

When a police officer in a helicopter is trying to use a telescope to read a license plate on a car, it is vital that the image not jitter; otherwise it will be too blurred to read. There is a constant vibration on a helicopter, plus there are g forces introduced by the motion of the helicopter. Normally, these external forces would cause the telescope image to jump around. However, if the telescope is balanced so its CG is exactly at the center of rotation of the gimbaled mount, then the image miraculously settles down.
The same dramatic improvement in performance occurs when a gimbaled seeker in a missile or smart weapon is balanced. In addition to affecting targeting accuracy, the degree of balance is the driving force that determines the torque requirement of the positioning motors in the seeker. If the seeker can be balanced with high precision, then the motors can be made much smaller, allowing the entire seeker to decrease in size. So this is a case where center of gravity control drives the limits of performance and also sets the requirements for basic elements in the design.
In addition to unbalance due to CG offset, unbalance due to product of inertia is a critical factor in the performance of a seeker. Rapid changes in direction introduce disturbing forces if the principal axes of the seeker are not coincident with the rotational axes. This paper outlines the current state of the art in gimbal balance. Some questions answered in this paper are:
1. How do you balance a gimbaled device (such as a missile seeker)?
2. Where should you locate the ballast weights on the gimbaled device for optimum reduction of unbalance?
3. How do you calculate the position and mass of the ballast weights, given that the seeker has at least two degrees of freedom, and sometimes has as many as five?
4. How do you fixture the seeker in the gimbal balance instrument?
5. How do you minimize practical limitations to minimum unbalance such as wire bundle movement and uncertainty in protective cover location?
6. What considerations should be given to minimizing product of inertia?
The gimbal balance machine described in this paper is one of the most accurate mechanical measuring devices in the world. The full-scale moment of each axis is 10,000 g-cm. The readability of the moment measurement is 0.03 g-cm, and the combined error due to drift, moment uncertainty, and linearity when measuring the final unbalance of a gimbal after ballast correction is 0.1 g-cm. This is a 0.001% error expressed in terms of full-scale moment! In other words, the gimbal balance machine is capable of reliably detecting unbalance as small as 1 part in 100,000. It is often hard for engineers to fully appreciate the incredible accuracy of this balancing machine. What makes this even more remarkable is that we now are able to achieve this with a single range of readout. It is no longer necessary to switch ranges as the unbalance is reduced. This extraordinary performance has driven the industry. Missile seekers can now be made smaller and lighter because the higher degree of balance has reduced the torque requirements on the positioning motors.