Magnetic gear tooth sensor with hall cell detector

• US 20030178989A1
• Filed: 03/25/2002
• Published: 09/25/2003
• Est. Priority Date: 03/25/2002
• Status: Active Grant

• Current Assignee: Allegro Microsystems Incorporated (Sanken Electric Company Limited)
• Original Assignee: Allegro Microsystems Incorporated (Sanken Electric Company Limited)
• Inventors: Vig, Ravi, Bilotti, Alberto, Bolsinger, Pablo Javier
• Granted Patent: US 6,690,155 B2
• Time in Patent Office: Days
• US Class Current: 324/207.2
• CPC Class Codes: G01D 5/147 influenced by the movement of a third element, the position of Hall device and the source of magnetic field being fixed in respect to each other
• CPC Class Codes: G01D 5/2448 Correction of gain, threshold, offset or phase control
• CPC Class Codes: G01D 5/24495 using previous values

Abstract

A sensor includes a Hall cell and a compensation circuit for subtracting a magnet background field from the Hall cell output to provide a signal corresponding to rotations of a toothed ferrous gear.

Claims

1. A sensor, comprising:

a Hall cell for providing an output signal having a first signal component corresponding to a background field from a magnet and a second signal component corresponding to rotations of a ferrous gear; and

a compensation circuit coupled to the Hall cell, the compensation circuit including a subtractor for subtracting a steady state compensation signal corresponding to the background field signal from the Hall cell output signal to output the second signal component.

2. The sensor according to claim 1, wherein the compensation circuit further includes a counter circuit providing the compensation signal by counting up until the compensation signal reaches a level corresponding to the background field.

3. The sensor according to claim 2, wherein the compensation circuit further includes a digital-to-analog circuit coupled to the counter for converting a value of the counter to an analog signal.

4. The sensor according to claim 3, wherein the compensation circuit further includes a comparator coupled to the counter via a logic circuit wherein the comparator has an output that changes an output state when the subtractor output reaches a predetermined threshold.

5. The sensor according to claim 4, wherein compensation circuit further includes a flip-flop circuit coupled to the comparator.

6. The sensor according to claim 4, wherein the compensation circuit further includes an amplifier coupled to the subtractor output.

7. The sensor according to claim 4, wherein the compensation circuit further includes a first differential transconductance amplifier coupled to the Hall cell.

8. The sensor according to claim 7, wherein the first transconductance amplifier includes the subtractor circuit.

9. The sensor according to claim 8, wherein the compensation circuit further includes a second transconductance amplifier coupled to the digital-to-analog circuit.

10. The sensor according to claim 1, wherein the sensor includes a single Hall cell.

11. A Hall sensor, comprising:

a magnet producing a background field;

a Hall cell proximate the magnet, the Hall cell providing an output responsive to the background field and a ferrous gear rotating in proximity to the Hall cell; and

a compensation circuit receiving the Hall cell output, the compensation circuit including

a subtractor having first and second inputs and an output, wherein the first input is coupled to the Hall cell output;

a comparator having first and second inputs and an output, wherein the first input receives a signal corresponding to a signal from the subtractor output;

a logic circuit having first and second inputs and an output, the first input receiving a clock signal and the second input receiving a signal from the subtractor ouput;

a counter receiving a signal from the logic circuit output; and

a digital-to-analog circuit converting a digital count value from the counter to an analog value provided to the second input of the subtractor.

12. The sensor according to claim 11, further including an amplifier for amplifying a signal from the output of the subtractor.

13. The sensor according to claim 11, further including a flip-flop circuit coupled to the comparator output.

14. A method of extracting a signal, comprising:

receiving a Hall cell output voltage, which includes a first component corresponding to a background field from a magnet and a second component corresponding to rotations of a ferrous gear proximate the Hall cell;

generating a steady state compensation voltage corresponding to the background field; and

subtracting the compensation voltage from the Hall cell output voltage.

15. The method according to claim 14, further including generating the compensation voltage using a counter.

16. The method according to claim 14, further including converting a digital value in the counter to an analog value.

17. The method according to claim 14, further including generating an increasing compensation voltage until reaching the steady state by comparing a difference of the compensation voltage and the Hall cell output voltage with a predetermined threshold voltage.

18. A method of manufacturing a Hall sensor, comprising:

securing a Hall cell to a magnet; and

fabricating a compensation circuit that is coupled to the Hall cell, the compensation circuit generating a steady state compensation signal corresponding to a background field of the magnet that is subtracted from an output of the Hall cell.