Three Axis Rate Table
The picture to the right shows one of Cranfield Precision’s three axis motion simulation system (type 1286), supplied to a European MoD. The motion simulator is a high precision, high dynamic performance system for the test and evaluation of guidance systems.
The system will position a payload of 50 Kg mass and 100 litres volume at accuracies of 1.5 arc seconds with payload rates of up to 1000°/s, 750°/s, and 1600°/s and accelerations of 30 rad/s2, 5 rad/s2, and 3 rad/s2 for the inner, middle and outer axes respectively
The system maintains axis wobble of less than 3 arc seconds and orthogonality errors of less than 2 arc seconds, whilst preserving stability and a positioning accuracy of 1 arc second.
The opposing requirements of high accuracy and high dynamic performance resulted in severe design problems, as illustrated by the sets of conflicting criteria below.
- High positional accuracy, low orthogonality errors, and low axis wobbles require very stiff and stable axis structures. Such structures tend to high inertias.
- High acceleration requires low axis inertias, and high torque motors. High- torque motors have large mass, resulting in high inertia for any axis carrying the motors.
- The motors to drive the inner axis add significant inertia to the middle axis, thereby increasing the driving torque required to accelerate the middle axis with a ripple-on effect to the outer axis
- Because of` the comparatively high inertias, maximum rate on all axes produces severe gyroscopic torque, which in turn increases the size of the motors with consequent increase in inertia.
As shown in the photograph, the system comprises three stacked rotary axes with the outermost axis vertical, each capable of continuous rotation for position and rate based functions. The dynamic elements of the system are supported on a massive 3 column support structure of polymer concrete, for high stability, stiffness, inertia and damping.
The rotating axis structures are designed for maximum strength to inertia ratio using composite materials.
Orthogonality errors were minimised by a combination of precision machining and design for mechanical error correction following build and initial calibration (see two axis systems).