Active rfid asset tracking tag with current-sensing cable clamp

Abstract

Configurations are provided for measuring electrical current flow through an AC power cable to a host device. An enclosure is provided that includes various components, such as components of an active RFID tag. A cable clamp is provided that is configured to attach the enclosure to the AC power cable. The cable clamp includes magnetic sensors that are responsive to electrical current flowing through the AC cable to generate signals indicative of the current flowing through the AC cable.

1 . A method comprising:
 affixing a single magnetic field sensor to an outer insulator of an electrical power cable, wherein the outer insulator of the electrical power cable is wrapped around two or more insulated inner conductors carrying electrical current in equal but opposite directions in the electrical power cable to a host device;   physically constraining the magnetic field sensor to the outer insulator of the electrical power cable so that it does not change position for a period of time;   obtaining a first plurality of magnetic field strength measurements from the magnetic field sensor while it is physically constrained to the outer insulator of the electrical power cable;   calculating a root mean squared (RMS) of the first plurality of magnetic field strength measurements;   deriving a measure of the electrical current flowing through the electrical power cable based on the RMS of the first plurality of magnetic field strength measurements; and   determining a usage state of the host device based on the measure of electrical current.   
     
     
         2 . The method of  claim 1 , wherein deriving comprises scaling the RMS of the first plurality of magnetic field strength measurements by a constant. 
     
     
         3 . The method of  claim 2 , further comprising computing the constant during a calibration step after the magnetic field sensor is physically constrained to the outer insulator of the electrical power cable by:
 obtaining a second plurality of magnetic field strength measurements from the magnetic field sensor when a known RMS current level is flowing through the electrical power cable;   calculating a RMS of the second plurality of magnetic field strength measurements; and   dividing the RMS of the second plurality of magnetic field strength measurements by the known RMS current level to derive the constant.   
     
     
         4 . The method of  claim 2 , further comprising deriving the constant by measuring a maximum RMS magnetic field strength from the magnetic field sensor over some time interval. 
     
     
         5 . An apparatus comprising:
 a single magnetic field sensor configured to be affixed to an outer insulator of an electrical power cable, wherein the outer insulator of the electrical power cable is wrapped around two or more insulated inner conductors carrying electrical current in equal but opposite directions in the electrical power cable to a host device;   an analog-to-digital converter coupled to an analog output of the single magnetic field sensor and configured to convert the output to a digital output; and   a processor coupled to the analog-to-digital converter and configured to:
 receive the digital output of the analog-to-digital converter for a first plurality of magnetic field strength measurements from the magnetic field sensor while it is physically constrained to the outer insulator of the electrical power cable; 
 compute a root mean squared (RMS) of the first plurality of magnetic field strength measurements; 
   derive a measure of the electrical current flowing through the electrical power cable based on the RMS of the first plurality of magnetic field strength measurements; and   determine a usage state of the host device based on the measure of electrical current.   
     
     
         6 . The apparatus of  claim 5 , wherein the processor is configured to derive the measurement of electrical current by scaling the RMS of the first plurality of magnetic field strength measurements by a constant. 
     
     
         7 . The apparatus of  claim 6 , wherein the processor is configured to compute the constant during a calibration step after the magnetic field sensor is physically constrained to the outer insulator of the electrical power cable by:
 obtaining a second plurality of magnetic field strength measurements from the magnetic field sensor when a known RMS current level is flowing through the electrical power cable;   calculating a RMS of the second plurality of magnetic field strength measurements; and   dividing the RMS of the second plurality of magnetic field strength measurements by the known RMS current level to derive the constant.   
     
     
         8 . The apparatus of  claim 6 , wherein the processor is configured to derive the constant by measuring a maximum RMS magnetic field strength from the magnetic field sensor over some time interval. 
     
     
         9 . (canceled) 
     
     
         10 . (canceled) 
     
     
         11 . (canceled) 
     
     
         12 . A system comprising:
 a host device that draws electrical power from an electrical power cable;   a single magnetic field sensor configured to be affixed to an outer insulator of an electrical power cable, wherein the outer insulator of the electrical power cable is wrapped around two or more insulated inner conductors carrying electrical current in equal but opposite directions in the electrical power cable to a host device;   an analog-to-digital converter coupled to an analog output of the single magnetic field sensor and configured to convert the output to a digital output; and   a processor coupled to the analog-to-digital converter and configured to:
 receive the digital output of the analog-to-digital converter for a first plurality of magnetic field strength measurements from the magnetic field sensor while it is physically constrained to the outer insulator of the electrical power cable; 
 compute a root mean squared (RMS) of the first plurality of magnetic field strength measurements; 
 derive a measure of the electrical current flowing through the electrical power cable based on the RMS of the first plurality of magnetic field strength measurements; and 
 determine a usage state of the host device based on the measure of electrical current. 
   
     
     
         13 . The system of  claim 12 , wherein the processor is configured to derive the measurement of electrical current by scaling the RMS of the first plurality of magnetic field strength measurements by a constant. 
     
     
         14 . The system of  claim 13 , wherein the processor is configured to compute the constant during a calibration step after the magnetic field sensor is physically constrained to the outer insulator of the electrical power cable by:
 obtaining a second plurality of magnetic field strength measurements from the magnetic field sensor when a known RMS current level is flowing through the electrical power cable;   calculating a RMS of the second plurality of magnetic field strength measurements; and   dividing the RMS of the second plurality of magnetic field strength measurements by the known RMS current level to derive the constant.   
     
     
         15 . The system of  claim 13 , wherein the processor is configured to derive the constant by measuring a maximum RMS magnetic field strength from the magnetic field sensor over some time interval. 
     
     
         16 . The method of  claim 1 , wherein the electrical power cable has a circular cross-section. 
     
     
         17 . The method of  claim 1 , wherein the single magnetic field sensor is a single inductive coil. 
     
     
         18 . The method of  claim 1 , wherein the calculating, deriving and determining are performed by a computing device. 
     
     
         19 . The method of  claim 18 , wherein the computing device is contained within an RFID tag, wherein the RFID tag is connected by a cable to a package that includes the single magnetic sensor. 
     
     
         20 . The apparatus of  claim 6 , wherein the electrical power cable has a circular cross-section. 
     
     
         21 . The apparatus of  claim 6 , wherein the single magnetic field sensor is a single inductive coil. 
     
     
         22 . The apparatus of  claim 6 , wherein the analog-to-digital converter and the processor are contained within an RFID tag, wherein the RFID tag is connected by a cable to a package that includes the single magnetic sensor. 
     
     
         23 . The apparatus of  claim 6 , wherein the electrical power cable has a circular cross-section.