Abstract:
An apparatus for sensing electrical current flowing in a conductor includes a toroidal core formed of a ferric material and having a slot defining an air gap, a Hall Effect sensor integrated circuit positioned in the air gap for sensing electrical current flowing in a conductor received in a central aperture of the core, and a housing enclosing the core and the integrated circuit. The integrated circuit can be programmed to set functions such as output offset, gain, temperature compensation and a current sensing range.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to an apparatus for calibrating electric current sensors. 
     BACKGROUND OF THE INVENTION 
     Prior art analog based current sensors are designed to operate in a fixed range of current magnitudes. The lack of programmability means that multiple variations of current sensors need to be manufactured to accommodate applications requiring different current sensing ranges. Further, the prior art current sensors set the device gains/offsets by scribing resistive elements before final assembly or adjusting potentiometers. Both of these methods have drawbacks. 
     SUMMARY OF THE INVENTION 
     The present invention concerns an apparatus for calibrating sensors of electric current to ease problems brought on by part and process variations which affect sensor performance. The apparatus according to the present invention affords these sensors the flexibility to be calibrated to measure current levels in different working ranges appropriate for given applications. The innovative apparatus incorporates into the design of the sensor a Hall-effect magnetic field transducer element that enables the current measurement response function of the sensor to be programmed via the electrical interface connection of the sensor device. The primary parameters to be set via the connector interface are the transducer gain and offset. 
     The apparatus for sensing electrical current flowing in a conductor includes a toroidal core formed of a ferric material and having a slot extending through the core defining an air gap, the core having a central aperture, and an electrical current carrying conductor received in said central aperture, said core concentrating in said air gap magnetic flux generated by electrical current flowing in said conductor. A power supply is connected to the conductor for generating a known electrical current in the conductor and a Hall Effect sensor integrated circuit is positioned in the air gap and has a power supply pin and an output pin. A programming means is connected to the power supply pin and the output pin, whereby the integrated circuit responds to an application of a lower voltage first signal applied at the power supply pin by the programming means by generating at the output pin an output signal representing an amount of the known current flowing in the conductor and the integrated circuit responds to a higher voltage second signal applied at the power supply pin by accepting a programming signal applied at the output pin and calibrating the output signal in response to the programming signal. 
     The benefits of incorporating a sensing element programmable through the connector include: a) the ability to eliminate part-to-part variability and process variation of the end-of-line packaged units; and b) programmability allows the setting of both gain and offset after complete assembly thus permitting a single hardware implementation to meet the working electrical current ranges for different applications. The programmability enables the manufacturer to reduce the number of versions of parts needed to accommodate customer applications, thereby reducing costs. 
     Incorporation of the means to program the electric current sensor through the external electrical connector after final assembly bypasses the problems associated with the prior art current sensors in an economical and easily manufacturable way. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
     FIG. 1 is a schematic view of a current sensing apparatus in accordance with the present invention; 
     FIG. 2 is a schematic view of a current sensor pickup used in the apparatus shown in FIG. 1; and 
     FIG. 3 is a circuit schematic of the programmable integrated circuit shown in FIG.  3 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     There is shown in FIG. 1 an electrical current flow path  10  for which it is desired to sense the parameters of the current flowing along the path. The path  10  is representative of any circuit configuration such as an electronic module to be tested for proper operation after assembly or a winding of an electric motor to be monitored for current flow. Opposite ends of the path  10  are releasably connected to a pair of switch terminals  12  and  14  of a double pole double throw (DPDT) switch  16 . The first terminal  12  is associated with a first pole of the switch  16  and can be switched between a first contact  18  and a second contact  20 . The second terminal  14  is associated with a second pole of the switch  16  and can be switched between a third contact  22  and a fourth contact  24 . As shown, the poles are switched simultaneously so that, for example, when the terminal  12  is connected to the contact  18 , the terminal  14  is connected to the contact  22 . 
     The DPDT switch  16  is included in a test and calibration circuit  26  having an ammeter  28  and a current source  30  connected in series. The second contact  20  and the third contact  22  are connected to one terminal of the ammeter  28  having another terminal connected to a terminal of the current source  30 . Another terminal of the current source  30  is connected to the first terminal  18  and the fourth terminal  24 . The current source  30  can generate AC or DC current as required for testing and calibrating the current sensor described below. In the position of the switch  16  shown in FIG. 1, the first terminal  12  is connected to the current source  30  and the second terminal  14  is connected to the ammeter. Switching the switch  16  reverses the connections of the power supply  26  to the current flow path  10 . Thus, the polarity of DC current supplied to the current path  10  can be reversed. 
     Current flowing in the path  10  is sensed by an electric current sensor  32  according to the present invention. The sensor  32  includes an annular housing  34  having a terminal block  36  mounted on an exterior surface thereof. The housing  34  is formed of a suitable material, such as a molded plastic material, and encloses an inductive pickup  38  shown in FIG.  2 . The pickup  38  includes a slotted (ferric or soft magnetic) ferrite core  40  extending through a pickup winding  42 . Positioned in a slot  44  of the core  40  is a programmable linear Hall Effect sensor integrated circuit  46 , for example, a MLX90237 chip manufactured by Melexis and available in the United States from Dominion Group of Fishers, IN. This IC utilizes a single chip and is digitally programmable through its electrical leads that are connected to the terminal block  36 . 
     A schematic circuit diagram of the connections of the IC  46  is shown in FIG. 3. A supply voltage pin I 1  is connected to a positive potential terminal of a supply voltage source  48 . A variable voltage supply is used to control the operation of the IC  46  as explained below. A pin I 2  is a test pin for readback diagnostic use only. A pin I 3  is connected to the circuit ground potential. An output pin I 4  is used for a sensed current output signal of the IC and can be changed to an input. 
     The electric current sensor  38  is an inductive pickup (non-invasive) device. A wire (current path  10  in FIG.  1  and wire  42  in FIG. 2) carrying the current to be measured is passed through the center of the C-shaped steel toroid  40 . Inserted into the flux gap  44  of the split toroid  40  is the digitally programmable Hall Effect IC  46 . Following the completed assembly of the current sensor  32  (FIG.  1 ), with the pins of the IC  46  accessible at the terminal block  36 , the output offset, gain and temperature compensation of the IC  46  can be adjusted and set. The pin I 1  is connected to a terminal T 1 , the pins I 2  and I 3  are connected to a pair of terminals T 2  and T 4 , and the pin I 4  is connected to the terminal T 3 . 
     A programming technique allows the normally analog signal measurement output pin I 4  connected to the terminal T 3  to be utilized as a digital serial data input to the IC  46 . For example, when the supply voltage at the terminal T 1  is in the range of 4.5 V to 5.5 V, the output at the terminal T 3  behaves normally. When the supply voltage is raised to 13 V, the pin I 4  connected to the terminal T 3  functions as an input allowing a 31-bit word programming signal to be clocked in. All data is loaded through a single line in a load sequence with no dedicated clock signal. The clock and data are integrated into one programming signal that is initiated with the beginning of the load sequence and clocked with the positive edge of each bit. No extra auxiliary programming interface pins are required. Also avoided is the expense of potentiometer adjustment or resistor laser trimming. Part-to-part variability is improved and cost is reduced. 
     There is shown in FIG. 1 a programmer  50  manufactured by Melexis as a model PTC-01 for programming the IC  46 . Programming ports of the programmer  50  are connected to the terminals T 1  through T 3  on the terminal block  36 . An RS232 serial interface port is connected with a processor  52  of a personal computer  54 . The computer  54  runs standard windows based Melexis software for programming the IC  46 . The software and the programmer  50  permit the user to load magnetic response function settings, take measurements, and calibrate the sensor  32 . Using the test and calibration circuit  26 , the sensor  32  can be programmed to operate with a wide variety of current flow paths  10 . For example, a separate one of the sensors  32  can be associated with each current flow path of a polyphase electric motor to generate feedback signals to a controller. Each of the sensors  32  can be calibrated to the associated current flow path after installation. The sensor  32  can be used with different size electric motors by programming the current range to be sensed. 
     The slotted core  40  is generally circular in cross section and is held in place in the plastic housing  34  by an integrally molded tab (not shown) extending into the slot  44 . The tab is smaller than the slot  44  yet larger than the width of the Hall IC  46 . This serves to align the core  40  as it constrains rotation of the core and enables the use of the circular cross section core instead of the more traditional rectangular core. A circular cross section core is less costly to manufacture than a rectangular core. Also, there is a more uniform field density in a circular toroidal core, an attribute that improves performance and reduces the amount of material needed for the core. 
     In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.