SNR improvement by selective modulation

Abstract

The accuracy of certain sensors is improved by improving their signal to noise ratio (SNR) in the presence of an interfering noise. Sensors were discovered which have a SNR which substantially changes when an operating parameter is selectively modulated to different magnitudes. In the simplest form, the sensor is operated where it is both stable and close to its best SNR. This is usually faster and less costly, but the noise is never completely eliminated. This invention has first been applied to Swain Meter® type clamp-on DC ammeters. Some results are good—the benefit in SNR is between 2 and 4. ® Swain Meter is a registered Trademark of the William H. Swain Co.

1 . A method and process for constructing and using a sensor with reduced error for measurement or control including means: 
 a core of low magnetic reluctance material, here called SQ,    a coupling sense winding on said core having a number of turns, here called N s ,    an inverter having an output current, here called i s , and an average said output current here called I s , and also constructed such that said inverter has an operating parameter which is the peak value in either direction of said current, here called I sm ,    a low input impedance means converting the said average value I s  of said inverter current to an output voltage here called V c ,    and said method includes:    positioning said core so that it is influenced by a conductor carrying a signal current I to be measured,    said position being within the effective range of a magnetic field noise, here called N, causing at least part of an error in the form of a change in zero offset of said output voltage V c , wherein the sensitivity of said V c  to said noise N is here called Ψ, and defined as the change in said V c  due to a unit change in said noise N divided by a gain g, i.e.,              Ψ   ≡       δ   ⁢           ⁢       V   c     /   δ     ⁢           ⁢   N     g       ,           where    said g is defined as the change in said output V c  due to a unit change in said signal current I; i.e.,              g   ≡       δ   ⁢           ⁢     V   c         δ   ⁢           ⁢   I         ,           and    said method also includes series connecting said N s , said inverter, and said low input impedance means converting;    and adjusting said means, including said N s  and said I sm , so that the change in said gain g is considerably less than the change in said noise sensitivity Ψ, as said noise sensitivity Ψ is reduced from a maximum to a value considerably less than said maximum, said reduced being accomplished by altering the value of said means, especially the number of turns on said winding N s  and the said peak inverter current I sm , said altering being preferably in the direction of a greater value of the product of said N s  and said I sm ,    and operating said sensor with said product of said N s  and said I sm  set so that said noise sensitivity Ψ is considerably reduced below said maximum,    thereby constructing and operating said sensor with said reduced error in zero offset due to said noise N.    
   
   
       2 . A Swain Meter type non-contact direct current ammeter with improved accuracy for measurement or control, which comprises: 
 a core, here called SQ, of low magnetic reluctance material,    a coupling sense winding, here called N s , on said core SQ,    an inverter with power supply, here called X, with output terminals with a current i s  flowing which has an average value I s , and also a peak value I sm  which is an operating parameter, all of said currents flowing in either direction in said output terminals,    a low input impedance means converting said average current I s  to an average output voltage V,    a current carrying conductor carrying a signal input current I, which is to be measured or controlled, positioned so that said current I influences said core SQ, and    said core SQ is within the effective range of an interfering magnetic field noise, here called N, and    said coupling sense winding N s  series connected with said output terminals of said inverter X and said low input impedance means converting,    said operating parameter I sm  set to a substantially greater magnitude than the magnitude corresponding to the minimum signal to noise ratio, here called SNR, so that thereby the said SNR is considerably increased over said minimum, so that said non-contact ammeter has considerably greater accuracy in the presence of said interfering magnetic field noise N.    
   
   
       3 . I claim a method for making a more accurate implement for at least one of measurement or control including the steps: 
 Construct a port for desired input signal I, which of necessity makes a port for undesired error producing interference N,    construct a port for said implement's output V c ,    acquire an Essential Characteristic type sensor having an output V responsive to said desired input signal I, and also    responsive to said undesired error producing interference N, and further having an operating parameter of magnitude Q;    show that said Essential Characteristic type sensor has a useful said Essential Characteristic evidenced by    a signal to noise ratio SNR of said sensor observed to change a lot when the said magnitude Q of said operating parameter is modulated over a practical range;    provide said implement equipped to:    support said sensor so as to:    considerably reduce said undesired interference N relative to said desired signal I at said output V c  by    holding said magnitude Q in a higher said SNR state and coupling said sensor output V to said implement output V c .    
   
   
       4 . I claim a more accurate sensor with implement for at least one of measurement or control, 
 including said sensor having a strong Essential Characteristic, and also an output V responsive to a physical quantity input I, the gain g given by              g   ≡       δ   ⁢           ⁢   V       δ   ⁢           ⁢   I         ,           and    said output V also responsive to an undesired error producing interference N,    the sensitivity Ψ being              Ψ   ≡       δ   ⁢           ⁢   V       δ   ⁢           ⁢   N         ,           and    said sensor also having an operating parameter of magnitude Q which modulates said Ψ, and to a lesser extent said g;    said sensor having been shown by at least one of calibration, proven manufacturing process, or other demonstration to have said strong said Essential Characteristic, i.e., the said sensitivity Ψ changes a lot more than said gain g when said magnitude Q is driven over a practical range of values; and    also including:    an error reduction form of said implement, fitted to support said sensor, and    fitted to drive said magnitude Q and hold it at a constant value, which is predetermined to cause said sensor to operate with said interference sensitivity Ψ a lot less than was heretofore customary, while said gain g is still good,    thereby making said sensor with said implement substantially more accurate than comparable transducers for said physical quantity I in the presence of said interference N.    
   
   
       5 . I claim a method for making a more accurate sensor with implement for at least one of measurement or control, made in steps: 
 obtain a said sensor having an output V responsive to a physical quantity input I, the gain g given by              g   ≡       δ   ⁢           ⁢   V       δ   ⁢           ⁢   I         ,           and    said output V is also responsive to an undesired error producing interference N, the sensitivity Ψ being              Ψ   ≡       δ   ⁢           ⁢   V       δ   ⁢           ⁢   N         ,           and    in addition, said sensor has an operating parameter of magnitude Q which modulates said Ψ, and to a lesser extent said gain g;    at least one of calibrate, or make by a proven process, or otherwise assure that said sensor has a strong Essential Characteristic evidenced by observing that said Sensitivity Ψ changes a lot more than said gain g when said magnitude Q is driven over a practical range of values; and:    provide an error reducing form of said implement, fitted to support said sensor, and    also fitted to drive said magnitude Q and hold it at a constant value, and by at least one of measurement or a proven process, set said magnitude Q at a value corresponding to a said sensitivity Ψ which is a lot less than heretofore while said gain g is still good, thus making said sensor with implement substantially more accurate than comparable transducers for said input I in the presence of said interference N.