Temperature compensated permanent magnet/moving coil accelerometer

A temperature compensated accelerometer of the type having a permanent magnet, moving coil rebalance system is disclosed. A cylindrical permanent magnet and surrounding sleeve produce radial magnetic fields in annular regions in which are located rebalance coils on a suspended bobbin. A noninductive temperature sensing winding on the magnet is connected in a bridge input circuit to an amplifier which produces a current in an auxillary winding for augmenting the fields produced by the permanent magnet so as to maintain constant magnetic field strength in the annular regions.

BACKGROUND OF THE INVENTION 
The present invention relates generally to acceleration responsive devices, 
and more specifically to an accelerometer employing a permanent 
magnet/moving coil rebalance system and unique temperature compensation 
means for minimizing variations in accelerometer response due to changes 
in temperature. 
Acceleration responsive devices perform essential sensing functions in a 
wide variety of systems. As performance requirements and available 
technology have advanced, the demand has increased for sensors 
characterized by much improved sensitivity, stability, accuracy, linearity 
of response, reliability and ruggedness, in addition to fast reaction time 
and low cost. Implicit in the stability, sensitivity and response 
linearity requirements is a requirement that precision be maintained over 
a wide temperature range. The present state of the art is such that it has 
been difficult to achieve improvements in all of the foregoing 
characteristics simultaneously, or, in some instances, even to achieve 
improvement in one characteristic without adversely affecting another. 
Nevertheless, requirements exist, particularly in aircraft navigation and 
missile guidance systems, for a single acceleration sensor with superior 
performance in all of the noted areas. 
One of the functions which must be provided for in a nonintegrating 
accelerometer is that of returning the acceleration sensing mass to a 
predetermined rest position after it has been displaced by an 
acceleration. A variety of system variations for performing this rebalance 
function have been devised and are presently employed. One such system 
variation described as a permanent magnet/moving coil rebalance system has 
shown considerable promise in simultaneously meeting the performance and 
low cost requirements. However, an inherent characteristic of permanent 
magnet/moving coil systems is that any variation in magnetic flux density 
in the region surrounding the moving coil results in a variation in the 
magnitude of response resulting from a given input acceleration, or 
variation in scale factor. One source of variation in magnetic flux 
density stems from the temperature dependent permeabilities of known 
magnetic materials. Although the most stable magnetic materials have 
comparatively small temperature dependencies, no such material has been 
found which can meet the requirements for the most demanding accelerometer 
applications. 
One known technique for improving accelerometer performance under varying 
temperature conditions is to provide a stable temperature environment for 
the accelerometer by means of a heater or other temperature control 
device. However, suitable forms of temperature control are complex, costly 
and result in slow accelerometer reaction time. Thus, the accelerometer 
designs requiring temperature control are not suitable for the most 
demanding aircraft navigation and missile guidance systems, and further do 
not meet the low cost objective dictated for many applications. 
The applicant has avoided the above-described problems by providing a 
unique system for augmenting the magnetic field(s) produced in permanent 
magnet/moving coil accelerometer rebalance apparatus. The compensation 
system is simple, reliable and low in cost, and does not detract from the 
fast reaction time potential of permanent magnet/moving coil 
accelerometers. Accelerometers employing the applicant's temperature 
compensation system have been found capable of performance and reliability 
heretofore unavailable from simple low cost apparatus. 
SUMMARY OF THE INVENTION 
The invention herein set forth is an accelerometer including permanent 
magnet means for producing a magnetic field in which is located coil means 
forming at least a portion of a mass suspended for movement relative to 
the magnet means, a servo system for supplying current to the coil means 
to maintain the mass in a predetermined position relative to the magnet 
means and pickoff means for supplying a signal indicative of displacement 
of the mass from the predetermined position. Temperature compensation is 
provided by temperature sensing means for producing a temperature signal 
indicative of the temperature of said permanent magnet means, an auxillary 
winding associated with said permanent magnet means for augmenting the 
magnetic field produced thereby and amplifier means for supplying a 
temperature compensation current to the auxillary winding in response to 
the temperature signal. 
The permanent magnet means may comprise a permanent magnet aligned with an 
input axis and a magnetically permeable sleeve cooperating with the 
permanent magnet to define first and second annular gaps containing 
substantially radial inwardly and outwardly directed magnetic fields 
respectively. The mass may comprise a tubular bobbin concentrically 
suspended between the permanent magnet and the sleeve, and carrying first 
and second circumferential coils wound in opposite directions, connected 
in series and centered in the first and second annular gaps respectively. 
The temperature sensing means and auxillary winding may comprise windings 
on the permanent magnet. 
Accordingly, the primary object of this invention is to provide an 
accelerometer capable of superior performance. 
A further object of this invention is to provide an accelerometer 
characterized by improved linearity of response and a constant scale 
factor. 
A further object is to provide a unique temperature compensation system 
which does not detract from the superior performance characteristics of 
accelerometers employing permanent magnet/moving coil rebalance apparatus. 
A further object is to provide a permanent magnet type of accelerometer in 
which the magnetic field is augmented so as to minimize the effects of 
temperature changes. 
Yet a further object is to provide a temperature compensated linear 
accelerometer employing a permanent magnet/moving coil rebalance system. 
Additional objects of the invention may be ascertained from a study of the 
disclosure, drawings and appended claims.