BACKGROUND OF THE INVENTION

This invention relates to two-terminal Hall sensors and their use in magnetic field monitoring systems, and more particularly pertains to a monitoring systems with fault detection and diagnostic capabilities wherein each of a plurality of remote Hall sensors are connected by two conductors to a sensors-addressing and control module.

A two terminal Hall sensor is described in the U.S. Pat. No. 4,374,333 to Grant Avery, issued Feb. 15, 1983 and assigned to the same assignee as is the present invention. This integrated circuit Hall sensor includes an on-board voltage regulator and the output stage is a current source providing an output current of predetermined value when the ambient magnetic field exceeds a prescribed value and no output current otherwise.

Such two terminal Hall sensors are employed in a vehicle monitoring system that is described in the U.S. Pat. No. 4,791,311 to Ravi Vig, issued Dec. 13, 1988 and assigned to the same assignee as is the present invention. These remote multiplexible two-terminal Hall sensors are all connected in parallel along a single pair of conductors and to a control module that addresses each sensor in sequence by multiplexing signals; the wiring is greatly simplified whereas the sensors must be multiplex-addressable.

The system of the present invention is intended for building security systems and vehicle monitoring tasks wherein simplicity and reliability are foremost considerations, and distinguishes from the multiplexing system in U.S. Pat. No. 791,311 by employing a plurality of simple switching two-terminal Hall sensors, e.g. as described in U.S. Pat. No. 4,374,333 each Hall sensor being connected to the control module by its own wire(s).

It is an object of this invention to provide such a vehicle monitoring system that uses a simple control module for switching from one Hall sensor to the next and for detecting and decoding in each Hall sensor the output current to determine whether a fault exists between the control module and the sensor and if not whether the magnetic field strength exceeds a certain value there.

SUMMARY OF THE INVENTION

In a first embodiment of this invention, a magnetic-field monitor includes a Hall sensor of the kind having a voltage reference terminal and a signal output terminal for being energized by applying a DC voltage across the output terminal and the reference terminal. The two terminal Hall sensor is adapted for generating through the output terminal an output current of a low amplitude Ie when the ambient magnetic field strength at the sensor lies within a first predetermined range M1 and generates a relatively high output signal current (Ie+Im) when the ambient magnetic field strength lies within a second predetermined range M2.

A Hall-sensor decoder circuit includes a signal input terminal that is connected by one conductor to the Hall-sensor output terminal and a ground terminal connected to the Hall-sensor reference terminal. The decoder circuit also has a supply voltage terminal to which a DC energizing voltage source may be connected, and a current mirror circuit having an input branch connected between the voltage supply terminal and the signal input terminal. A resistive voltage divider circuit is connected in the output branch of the current mirror circuit and to the decoder ground terminal. The decoder circuit further includes three voltage comparators, each comparator having a different threshold voltage and the inputs of the three comparators are connected respectively to three points in the voltage divider circuit.

The three composite binary output signals from the decoder indicate whether a short circuit fault or an open circuit fault condition exists in the conductors connecting the Hall element to the decoder, and when there is no fault further indicates whether the ambient magnetic field at the remote Hall sensor is high or low, e.g. whether a magnet is close by or remotely distant from the sensor.

In a second embodiment of this invention, a magnetic-field monitor system includes a plurality of Hall sensors of the kind employed in the first embodiment. A clocked sequential switch sequentially connects the output of each Hall sensor in turn to a signal Hall-sensor decoder of the kind used in the first embodiment.

In a third embodiment, another magnetic-field monitor system includes a plurality of Hall sensors of the same kind as in the first embodiment, but also includes a plurality of Hall-sensor decoders of the kind used in the first embodiment. Each sensor is connected directly to a corresponding one of the decoders via a separate pair of conductors, respectively. The outputs of all the decoders are connected to a logic circuit that produces, when there are no circuit fault conditions, binary signals indicating at every instant the magnetic field status at every Hall sensor and providing a fault signal whenever a fault occurs. This real time magnetic-field monitoring system is especially suitable for use in an automatic braking system (ABS) of a vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit diagram of two-terminal integrated-circuit Hall sensor of the prior art.

FIG. 2 shows a circuit diagram of a Hall-sensor signal decoder of the present invention.

FIG. 3 shows a graph depicting total sensor current, Is, in the decoder of FIG. 2 under different field and fault conditions, and depicting decoder-comparator threshold values in a monitoring system of this invention.

FIG. 4 shows a circuit diagram of another Hall-sensor signal decoder of this invention.

FIG. 5 shows a block diagram of a monitoring system of this invention suitable for use in a building-security system.

FIG. 6 shows a block diagram of another monitoring system of this invention that is suitable for use in an anti-skid vehicle-braking system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The two-terminal integrated circuit Hall sensor 10 of FIG. 1 includes a Hall element 12, a Hall-voltage amplifier 14, a Schmitt trigger circuit 16 and a switchable current source 18 all of which are connected in tandem. When a positive DC exciting voltage is connected, via conductor 45 and transistors 28 and 32 of the decoder circuit 26 of FIG. 2 to the terminal 20, with respect to the ground or reference terminal 22, the current flowing through terminal 20 and lead conductor 21 consists of the exciting current, Ie, plus any current that may be generated by the current source 18. The current source 18 is caused to produce an output current Im of predetermined amplitude only when the ambient magnetic field at the sensor exceeds a certain magnitude. Thus the total current, Is, in terminal 20 and lead conductor 21 is composed only of the exciting current Ie when the magnetic field is less than the certain magnitude, and otherwise is composed of the total current Ie + Im when the magnetic field is greater than the certain magnitude. Thus to know the amplitude of the total current Is, is to know the range of ambient magnetic field strength. And, the same current Is flows in both terminals 20 and 22 as well as in both lead wires 21 and 23 so that detection can be effected at any point in this series circuit.

The Hall-sensor signal decoder 26 of FIG. 2 is composed of a current mirror circuit including transistors 28 and 29, a saturating high-current protection circuit including the transistor 32 and voltage source 34, a voltage divider circuit including resistors 36, 37 and 38, and three comparators 41, 42 and 43. Decoder circuit 26 is to be connected to the output of a Hall sensor such as the sensor 10 of FIG. 1. When the reference conductor 23 of chip 10 is connected to ground, as is the conductor lead 49 of decoder circuit 26, and sensor lead 21 is connected to decoder lead 45, then a current Id, which is proportional to the Hall sensor current Is, flows through the divider resistors 36, 37 and 38.

To the reference inputs of the comparators, there are respectively connected the three reference voltage sources 46, 47 and 48 which establish the three threshold voltages Vref1, Vref2 and Vref3. The comparators 41, 42 and 43 produce at their respective outputs 51, 52 and 53 a binary zero signal when their respective signal inputs (appearing at the top of resistors 36, 37 and 38) are less than the corresponding threshold voltages Vref1, Vref2 and Vref3. On the other hand, when the input voltage to any one of the comparators is greater than the corresponding threshold reference voltage, that comparator produces at its output a binary one signal.

Now with reference to FIG. 3 and the Truth Table below, the decoder circuit 26 generates three binary output signals at the decoder outputs 51, 52 and 53 reflecting the status of the connected Hall sensor 10 and its wiring to the decoder input.

For example, Vref1 is established at a level I₁ below that which corresponds to the Hall-sensor exciting current, Ie, so that if an open circuit has developed in conductors 21 and 23 between the sensor 10 and the decoder 26, the exciting current drops or becomes zero and the output 51 of comparator 41 becomes a binary zero (L).

Similarly, Vref3 is established at a level above that which corresponds to a maximum total Hall-sensor output current, Ie + Im, under the conditions of no faults and existence of an ambient magnetic field at the connected sensor 10. Thus when an open circuit or high resistance develops in the connection between the sensor 10 and the decoder 26, then the output 53 of comparator 43 becomes a binary one (H) whereas without the open circuit fault it would have been a binary zero (L).

The threshold voltage Vref2 is established about in the center of the range of voltages appearing at the signal input of comparator 42 corresponding to the ambient magnetic field at sensor 10 so that the Schmitt trigger circuit is operating at about the center of its hysteresis loop, and under the condition of no-faults in the connection between the sensor 10 and the decoder 26.

Referring to FIG. 3, with the assumption that the base-emitter areas of current mirror transistors 28 and 29 are the same, the comparator reference voltages are each related to corresponding Is threshold current as follows:

I1 =Vref1/R36 

I2 =Vref2/(R36 +R37)

I3 =Vref3/(R36 +R37 +R38)

Truth Table I summarizes the operation of decoder

______________________________________TRUTH TABLE I                              Decoder 26Typical                            BinaryCurrents                 Magnetic  OutputIp      Is         Fault     Field   51  52  53______________________________________ <3 ma  Is < I1              OPEN      --      L       L 3-10 ma   I1 < Is < I2              --        LOW     H   L   L10-20 ma   I2 < Is < I3              --        HIGH    H   H   L>20 ma  Is > I3              SHORT     --      H       H______________________________________

An alternate decoder circuit 60 having similar features is shown in FIG. 4 whereby a single three-threshold comparator 62 provides a magnetic-field status binary signal at output conductor 64, and a fault condition binary signal at output terminal 66 indicates any fault, e.g. goes high for either a short or an open fault condition. Under fault conditions, either short or open, it is desirable to use the binary fault indicator signal to shut down the monitoring system to avoid producing an erroneous magnetic-field indication, and further to turn on a fault alarm.

Referring now to FIG. 5, a monitoring system employs four essentially identical Hall sensors 10a, 10b, 10c and 10d that are the same or equivalent to the Hall sensor 10 of FIG. 1. Each of the four Hall sensors (e.g. 10a) has its primary terminal (e.g. 21a) connected to a microprocessor control module 68. The control module 68 includes a sequential address switch 70 that sequentially connects each of the four Hall sensors for a fixed period of time in turn, via conductor 45 to the input of the decoder 26. During that each period, the connected Hall sensor is excited by application of Vcc (or scaled down portions thereof) and the magnetic field status and the fault status of the connected Hall sensor is indicated by the binary signals produced at outputs 51, 52 and 53 of decoder 26. The switch 70, and therefore the time periods during which each Hall sensor is excited, are under control of a clock and logic circuit 72. Circuit 72 may also include memory (RAM and/or ROM) for remembering the fault status and the identity of its associated Hall sensor. A binary magnetic-field (high or low) status signal is produced at terminal 74 and a binary fault status signal is generated at terminal 76. The Hall sensor identification signal is provided in a composite two-digit binary form at terminals 78 and 80. All three of these output signals may be said to be contained in "the signal" produced at the composite four-digit binary output composed of the four terminals 74, 76, 78 and 80.

Referring now to FIG. 6, another monitoring system is shown also employing the four essential identical Hall sensors 10a, 10b, 10c and 10d, each of the four Hall sensors having its primary terminal (e.g. 21a) connected to a corresponding one of four decoders 26a, 26b, 26c and 26d, respectively. A microprocessor control module 88 includes clocked address switches 90a and 90b that sequentially connect the pair of faults output signals from each of the four Hall decoders, e.g. via a pair of conductors 51d and 53d, to the logic circuit 91, and in subsequent periods respectively via the pairs of fault-signal conductors 51a/53a, 51b/53b and 51c/53c. The decoders 26a through 26d are identical to the decoder 26 of FIG. 2. Unlike for the monitoring system of FIG. 5, the four sensors 10a through 10d are continuously energized via the four decoders 26a through 26d respectively. Thus each continuously produces a binary magnetic-field indicating signal at the field status outputs 92, 93, 94 and 95 of logic circuit 91 and control module 88, unless a fault condition is present. A fault, either a short or an open, is indicated by a binary fault signal at output terminal 97 of the control module 88 and the sensor with which the fault is associated is indicated by the binary signal appearing at the output conductors pair 98 and 99.

Monitoring systems of this invention are especially well suited for use in a vehicle automatic braking system (ABS) wherein the relative rotational positions of each of four wheels could be detected and monitored and controlled (especially for skid sensing and control) by mounting a multiple-tooth ferrous gear to each wheel, assembling magnets to the Hall sensors and mounting the Hall sensor/magnet assemblies near the rotating teeth of the gears to sense each tooth passing by a Hall sensor. The integrated circuit Hall sensors employed in the vehicle monitoring system of this invention may be of the dual Hall type that work especially well for gear tooth detection. Descriptions of such dual Hall sensors are provided in U.S. Pat. No. 4,518,918 issued May 21, 1985 and U.S. Pat. No. 5,045,920 issued Sept. 3, 1991, both being assigned to the same assignee as is the present invention.