Rate Table - Precision Velocity Control
Cranfield Precision lead the development and manufacture of high and ultra high precision rotary axis motion systems for the calibration, test, and evaluation of sensors for angular position, rate, and acceleration.
Cranfield Precision has been designing and manufacturing the range of motion systems since 1970, supported in this pursuit by both the UK Ministry of Defence and the UK Department of Trade and lndustry.
Although the design requirement for single axis table is deceptively simple: to provide a flat surface capable of rotation at any preset angular speed between 0.0001 degrees per second and 1000 degrees per second, with an accuracy of 0.1 percent of set speed, the reality is that these systems are extremely costly to develop. The development costs for the single axis systems alone were (in the 1970s) over £500,000. Cranfield Precisionís investment in rate table technology over 40 years means that we can supply one, two and three axis systems to extremely high performance specifications.
Drive units contain a non-magnetic stainless steel spindle of high stiffness, supported on precision angular contact bearings spring preloaded ABEC9 angular contact bearings, powered by direct drive rare earth magnet motors, with analogue rate measured by direct drive dc tachogenerators providing velocity feedback to 0.1 percent ripple and linearity
High accuracy and low ripple rate are ensured by magnetic shielding between motor and tachogenerator, earthed non-magnetic spindle, and optimised tachogenerator commutation. Positioning, and position-tracking functions such as precision digital rate, are provided by adding on-axis position encoders.
Electrical power ways and signal ways are carried through the spindle by low noise, multipIe brushed, gold on gold slip rings.
The platen may be extended into an environmental chamber by fitting a heated and thermally insulating tube [of high stiffness and stability) between the drive spindle and the platen.
In two or three axis systems the drive units are used in pairs, one each side of the axis structure to provide for maximum stiffness and bandwidth, and with one of the units containing limited axial floatation bearings to avoid over-constraint.
Direct drive axes offer the maximum versatility for motion generation. There are occasions however when non-direct drives can offer cost or technical advantages.
Friction drives are now widely accepted as the ideal motion generators of precision movement for telescopes. These same drives can be used to supply very high torques for systems requiring high acceleration and high frequency response, usually over limited angles and at high accuracy.
Cranfield Precision pioneered the use of friction drives and has supplied friction drives to manufacturers of a wide range of machines.
Another form of off-axis drive, the spiroid gear drive, has very limited dynamic response capability, but does offer high torque/cost ratios for position only axes.
Air Bearing Systems
Tables requiring the ultimate in precision are supplied with unique hemispherical air bearings developed by Cranfield Precision to meet the exceptional accuracy and reliability criteria demanded by defence programmes in Europe and the USA.
These unique air bearings use a porous graphite lined stator and double hemispherical self aligning rotor. The design is well proven for stability of high accuracy over long periods and for rugged operation.