IES20020511A2 - A monitoring circuit for determining the state of a device, and a current emulating circuit for emulating current drawn by the device - Google Patents

Abstract

A monitoring circuit (1) for monitoring the state of a hall effect sensor (2) for determining the state of a seat belt buckle of a motor vehicle comprises a regulated voltage power supply (7) for applying a five-volt supply to the sensor (2) through a primary transistor (8) and a current sense resistor (10). A comparator (12) compares the voltage applied to the sensor (2) with a threshold voltage for detecting an over current condition in the sensor (2). A secondary transistor (14) is responsive to the output of the comparator (12) for switching off the primary transistor (8) in the event of an over current condition in the device (2). A microprocessor disables the secondary transistor (14) for a settling time period. The microprocessor (5) determines from the comparator (12) and the current sense resistor (10) an over current condition and the current being drawn by the sensor (2). The microprocessor (5) selects one of resistors (R1 to Rn) of a current emulating circuit (20) for developing a current through the current emulating circuit (20) which emulates the current drawn by the sensor (2), and a monitoring circuit (23) monitors the current. <Figures 1 & 2>

Description

“A monitoring circuit for determining the state of a device, and a current emulating circuit for emulating current drawn by the device The present invention relates to a monitoring circuit for determining the state of a 5 device having at least two states, and the invention also relates to a current emulating circuit for emulating current drawn by a device having at least two states.

Monitoring circuits for monitoring the state of a device by passing a current through the device or applying a voltage to the device are well known. A basic form of such 10 device is a bi-state switch, which in one state presents an open circuit to a monitoring circuit, and no current flows, and in the other state presents a closed circuit to the monitoring circuit and draws maximum current. Such switch devices when used in a circuit, while they can to some extent give an indication of the state of the device being monitored, they nonetheless suffer from a number of 15 disadvantages. In particular, since switch devices have only two states, one being open circuit and the other being closed circuit, the circuit in which the switch device is located may develop a fault and become either an open or closed circuit, and thus, when the circuit is being monitored the state read by the monitoring circuit could be spurious if the circuit being monitored had already developed a fault and had gone 20 either open or closed circuit.

To overcome this problem, more sophisticated devices are used, which sink currents of different values, or of values within different ranges depending on the state of the device. A typical one of such devices is a hail effect sensor. Such censors can be OPEN TO PUBLIC INSPECTION UNDER SECU, A 2b AND RULE 22 JNL NO..BM..............

I IE Ο 205 Π used to represent two or more states. When used with a 20mA current, such hall effect sensors are particularly suitable for representing two states, one state being represented when the current being sunk is approximately 6mA, or lies within a range of 4mA to 8mA, and a second state when the current is approximately 14mA and lies within a range of 12mA to 16mA. Currents outside these ranges in general are indicative of a fault in the circuit. For example, a zero current condition would indicate a fault in the circuit, and would be indicative of an open circuit, while a higher current could be indicative of a short circuit. The advantage of such hall effect sensors is that signals resulting from a fault in the circuit can be identified as io such, and distinguished from signals representing the state of the hall effect sensor.

Such hall effect sensors are commonly used in the motor industry for sensing the state of the buckle of a seat belt for indicating whether the seat belt buckle is fastened or otherwise. Typically, the hall effect sensor sinks a current of 4mA to 6mA when the buckle is in the unbuckled state, and sinks a current of approximately 12mA to 16mA when the buckle is in the buckled state.

Circuits for monitoring the current being sunk by such hall effect sensors when used in conjunction with a seat belt buckle are known, and in general, are incorporated within the electronic system and circuitry of the motor vehicle. In general, such circuitry in motor vehicles merely monitors the seat belt buckle for determining if the buckle is fastened or otherwise. While this is useful, it does not take account of whether a person is sitting in the seat corresponding to the seat belt or otherwise. It is desirable that a monitoring circuit for monitoring the state of a buckle of a seat belt should also permit monitoring of the seat to which the seat belt corresponds for determining if the seat is occupied or otherwise, and also permit monitoring of other parameters. However, in order to permit the monitoring of such other parameters, it is necessary to independently monitor the state of the seat belt buckle of the seat belt independently of the monitoring system operated under the control of the vehicle electronics. This presents serious problems, since any additional monitoring of the current drawn by the hall effect sensor has a consequential effect on the monitoring circuit of the vehicle electronics for monitoring the state of the seat belt buckle, and thus, the presence of the two monitoring systems would lead to spurious results, which would quite likely affect both monitoring systems.

There is therefore a need for a monitoring circuit for monitoring a hall effect sensor, or indeed, any other device having at least two states, and there is also a need for such a circuit which is useable in conjunction with other monitoring circuitry which also monitors the device.

The present invention is directed towards providing a monitoring circuit for determining the state of a device having at least two states. The invention is also directed towards providing a current emulating circuit for emulating a current drawn by a device which has at least two states.

According to the invention there is provided a monitoring circuit for determining the state of a device having at least two states, the device sinking currents of values within respective predetermined ranges corresponding to respective states of the IE Ο 2 05 1 1 device, the monitoring circuit comprising a regulated voltage power supply for supplying a current to the device, a comparing means for comparing the voltage applied to the device with a threshold voltage for detecting an over current condition, a primary switch means for switching the power supply to the device for facilitating monitoring of the state of the device, the primary switch means being responsive to the comparing means detecting an over current condition for isolating the device from the power supply, a timing means for timing a predetermined settling time delay, the primary switch means being non-responsive to the comparing means during the settling time delay for preventing operation of the primary switch means io for isolating the device from the power supply for the predetermined settling time delay for allowing settling of the circuit, and a current sensing means for sensing the current being sunk by the device.

Preferably, a control circuit is provided for periodically switching the primary switch means for periodically supplying current to the device for facilitating periodic monitoring of the device.

Advantageously, the control circuit reads the current sensing means for determining the state of the device.

In one embodiment of the invention the control circuit is responsive to the comparator for preventing reading of the current sensing means until the voltage applied to the device reaches the threshold voltage. ΙΕΟ 2 05 11 In another embodiment of the invention the primary switch means comprises a primary transistor, and preferably, the primary transistor is responsive to an enable signal from the control circuit for switching the power supply to the device.

In another embodiment of the invention the primary transistor is active low.

In a further embodiment of the invention a secondary switch means is provided for applying a disable signal to the primary switch means for disabling the primary switch means in response to the comparator determining that the voltage applied to io the device is below the threshold voltage after the settling time delay period.

Preferably, the secondary switch means comprises a secondary transistor.

In a further embodiment of the invention the current sensing means comprises a 15 current sensing resistor, and preferably, the current sensing resistor is located in series with the primary switch means, and ideally, is located downstream of the primary switch means.

In another embodiment of the invention the timing means for timing the settling time 20 delay period is provided in the control circuit. Preferably, the settling time delay period is sufficient for allowing the voltage applied to the device to cross over the threshold voltage assuming normal operation of the device.

In one embodiment of the invention the settling time delay period lies in the range of microseconds to 200 microseconds, in general, the settling time delay period lies in the range of 75 microseconds to 150 microseconds, and typically, the settling time delay period is 100 microseconds.

In one embodiment of the invention the monitoring circuit is adapted for monitoring the state of a sensor associated with a seat belt buckle of a seat belt in a motor vehicle.

In another embodiment of the invention the sensor being monitored by the io monitoring circuit is a hall effect sensor.

Additionally the invention provides a current emulating circuit for emulating a current drawn by a device having at least two states, the device sinking current of values within respective predetermined ranges corresponding to the respective states of the device, the emulating circuit comprising a means for receiving a regulated voltage power supply, at least two current sink impedance means for sinking current from the regulated voltage power supply of respective different values, a main switch means for selectively switching the impedance means for sinking respective currents from the regulated voltage power supply so that the currents sunk by the emulating circuit emulate those sunk by the device when connected to a regulated power supply similar to the regulated power supply to which the emulating circuit is connected for indicating the state of the device.

In one embodiment of the invention the respective impedance means are connected IE Ο 2 05 ι γ in parallel.

In another embodiment ofthe invention the main switch means comprises a plurality of switches, one switch being provided for each impedance means for selectively switching the corresponding impedance means for sinking current therethrough.

In another embodiment ofthe invention each impedance means comprises a resistor. io Ideally, the number of impedance means provided corresponds to the number of states which the device may take.

In one embodiment ofthe invention the emulating circuit is adapted for emulating currents sunk by a sensor associated with a seat belt buckle of a seat belt of a motor vehicle, and preferably, the emulating circuit emulates currents sunk by a hall effect sensor associated with a seat belt buckle of a seat belt of a motor vehicle.

Further the invention provides a circuit for monitoring the state of a device and for outputting a current indicative of the state of the device, the circuit comprising the monitoring circuit according to the invention for monitoring the device, and the emulating circuit for outputting one of a plurality of currents in response to the state of the device for indicating the state of the device, the main switch means of the emulating circuit being responsive to the control circuit of the monitoring circuit for selecting the impedance means to be switched for sinking current from the regulated IE Ο 2 05 1 Ϊ power supply feeding the emulating circuit.

In one embodiment of the invention the control circuit comprises a microprocessor.

In a further embodiment of the invention the circuit is adapted for monitoring a sensor associated with a seat belt buckle of a seat belt of a motor vehicle, and for emulating a current indicative of the state of the sensor associated with the seat belt buckle. io In one embodiment of the invention the sensor associated with the seat belt buckle is a hall effect sensor.

The invention will be more clearly understood from the following description of an embodiment thereof, which is given by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a block representation of a monitoring circuit according to the invention for monitoring the state of a device having at least two states, and the monitoring circuit forms part of a circuit for monitoring the state of the device and for emulating currents indicative of the state of the device being monitored, and Fig 2 is a block representation of the emulating circuit for emulating currents indicative of the state of the device being monitored by the monitoring circuit ΙΕΟ 2 05 1 1 of Fig. 1.

Referring to the drawings, there is illustrated a monitoring circuit according to the invention, indicated generally by the reference numeral 1 for monitoring the state of a device 2. An emulating circuit also according to the invention and indicated by the reference numeral 3 is provided for emulating currents outputted by the device 2 which are indicative of the state of the device. The emulating circuit 3 is operated under the control of the monitoring circuit 1 as will be described below. In this embodiment of the invention the device 2 is a sensor 2 for sensing the state of a io seat belt buckle of a seat belt of a motor vehicle. In other words, the sensor 2 senses whether the seat belt buckle is in the buckled state or the unbuckled state.

In this embodiment of the invention the sensor 2 is a hall effect sensor which when a 5-volt supply is applied to the sensor sinks one of two currents, namely, a 6mA current when the buckle is unbuckled, and a 14mA current when the buckle is buckled. These two currents may range within a predetermined range of 4mA to 8mA in the case of the belt being unbuckled, and may range in the range of 12mA to 16mA in the case of the buckle being buckled. Currents outside the ranges 4mA to 8mA and 12mA to 16mA are deemed to be fault currents, which in general, would indicate a fault in the circuitry, or in the hall effect sensor.

The monitoring circuit 1 comprises a control means, in this embodiment of the invention a microprocessor 5 which controls the operation of the monitoring circuit 1, and also the emulating circuit 3 as will be described below. A regulated voltage power supply 7 outputs a regulated voltage of approximately 5 volts, which can vary ί s between 4.9 volts and 5.1 volts. The 5-volt supply from the power supply 7 is applied to the sensor 2 through a primary switch means, which in this embodiment of the invention is provided by a primary transistor 8 which is active low. A current sensing means, namely, a current sense resistor 10 is provided in series with the primary transistor 8 so that the current drawn by the device 2 is drawn through the current sense resistor 10 for monitoring by the microprocessor 5 as will be described below. The primary transistor 8 is periodically enabled by an enable signal from an output pin of the microprocessor 5 for periodically applying the voltage from the power supply 7 to the device 2 for periodically monitoring the state thereof.

A comparing means comprising a comparator 12 which is powered by the power supply 7 compares the voltage applied to the device 2 with a threshold voltage of value of approximately 4.25 volts for detecting an over current condition in the device 2. An over current condition could be caused by a short-circuit in the device 2 and its related circuitry, and such a short-circuit would cause an excessive current to be sunk by the device 2 which in turn would cause the voltage applied to the device 2 to drop below 4.2 volts. The threshold voltage is derived from the regulated power supply 7 but is independent of the current drawn by the device 2. A secondary switch means provided by a secondary transistor 14 is responsive to the comparator 12 for outputting a logic high to the enable input of the primary transistor 8 for switching off the primary transistor 8 in the event of an over current condition developing in the device 2.

However, in order to allow a sufficient settling time for the device 2 after the voltage IE0 205 1 1 from the power supply 7 has been applied to the device 2, a timing means is provided for timing a settling time delay period after the primary transistor 8 has been enabled for applying the voltage from the power supply 7 to the device 2. In this embodiment of the invention the microprocessor 5 times the settling time delay period, which in this case is approximately 100 microseconds. The microprocessor 5 holds the secondary transistor 14 disabled until the settling time delay period has been timed out. On the settling time delay period having been timed by the microprocessor 5, an enable signal is applied to an enable input of the primary transistor 8 from an enable output pin of the microprocessor 5 for enabling the io secondary transistor 14. Therefore at any time after the settling time delay period should the output from the comparator 12 indicate an over current condition in the device 2, the secondary transistor 14 outputs a logic high for switching the primary transistor 8 for in turn isolating the device 2 from the power supply 7.

The voltage across the current sense resistor 10 is applied to analogue to digital input pins of the microprocessor 5, and the microprocessor 5 determines the state of the device 2 from the voltage across the current sense resistor 10. By reading the voltage developed across the current sense resistor 10, an accurate determination of the current being drawn by the device 2 is made by the microprocessor 5.

Additionally, the microprocessor 5 reads the output of the comparator 12 for determining when the voltage applied to the device 2 has crossed the threshold . voltage. On the microprocessor 5 determining that the output from the comparator 12 is indicative of the voltage applied to the device 2 crossing over the threshold voltage, the microprocessor 5 then commences to read the voltage across the ¢0205 f f current sense resistor 10 for determining the current being drawn by the device 2. It is only when the voltage applied to the device 2 has crossed the threshold voltage that an accurate reading of the state of the device 2 can be made.

Before proceeding to describe the emulation circuit 3, the operation of the monitoring circuit 1 will first be described. The monitoring circuit 1 at periodic intervals monitors the state of the device 2 for determining the state of the buckle of the seat belt. The length of the intervals between each monitoring of the device 2 may be any desired interval, but typically, will be in the order of 100 milliseconds. A monitoring cycle commences with the microprocessor 5 outputting an active low on the output pin of the microprocessor 5 to an enable input of the primary transistor 8 for switching the primary transistor 8 for applying the regulated voltage of the power supply 7 to the device 2. Current from the power supply 7 passes through the current sense resistor 10 to the device 2. Immediately on the output pin of the microprocessor 5 going low the microprocessor 5 commences to time the settling time delay period, and while the microprocessor 5 is timing the settling time delay period the output from the enable pin of the microprocessors holds the secondary transistor 14 off. The comparator 12 compares the voltage applied to the device 2 with the threshold voltage and outputs a signal indicative of the difference between the respective voltages. On the microprocessor 5 determining from the output of the comparator 12 that the voltage applied to the device 2 has crossed over the threshold voltage, the microprocessor 5 commences to read the voltage developed across the current sense resistor 10 for determining the state of the device 2 and in turn the state of the seat belt buckle. At the end of the settling time delay period the secondary transistor ΙΕο 205 j j is enabled by an enable output from the enable pin of the microprocessor 5. If at the end of the settling time period or at any time thereafter, the output from the comparator 12 is indicating that the voltage applied to the device 2 has not yet crossed the threshold voltage, the secondary transistor 14 outputs a logic high to the enable pin of the primary transistor 8 for disabling the primary transistor 8. In general, it is envisaged that each monitoring cycle will last approximately 100 microseconds, which is sufficient time to allow the voltage developed across the current sense resistor 10 to give an accurate indication of the current being drawn by the device 2, and in turn the state of the device 2 and in turn the state of the seat belt buckle. The next monitoring cycle commences after the appropriate interval has been timed out by the microprocessor 5.

Turning now to the emulating circuit 3, the emulating circuit 3 is adapted to be powered by a regulated voltage power supply 20 which preferably, is of similar value to the regulated voltage of the power supply 7 of the monitoring circuit 1. The voltage from the power supply 20 is applied to the emulating circuit 3 through a series resistor Rrs. The emulating circuit 3 comprises a plurality of impedance means, namely, resistors R1 to Rn which are connected in parallel for sinking current from the power supply 20 to ground. One resistor R1 to Rn is provided for each value of current to be emulated, and the values of the resistors R1 to Rn are selected for providing the appropriate respective values of emulated current. In this embodiment of the invention since two currents indicative of the two states of the device 2 are required to be emulated and two currents indicative of two fault states of the device 2, namely, under and over current states only four resistors R1 to R4 IE 0 2 05 ίι are required. A main switch means comprising a plurality of transistor switches T1 to Tn are provided for selectively switching the appropriate one of the resistors R1 to Rn for connecting the appropriate resistor R1 to Rn to ground for sinking the current of the value to be emulated. The transistor switches T1 to Tn are controlled by corresponding digital outputs Select 11 to Select In from the microprocessor 5.

A current sensing means, namely, a current sense resistor 22 is provided in series with the resistors R1 to Rn for permitting a reading of the value of the current emulated by the emulating circuit 3 to be made. A current monitoring circuit 23 io monitors the voltage developed across the current sense resistor 22 for determining the current being sunk by the emulating circuit 3.

In this embodiment of the invention the resistor R1 is a low value resistor for emulating a current corresponding to an over current state of the device 2. The resistor R4 is a high value resistor corresponding to an open circuit state occurring in the device 2. The open circuit state is determined by the microprocessor 5 from the voltage developed across the current sense resistor 10. The resistors R2 and R3 of the emulating circuit 3 are of value to provide emulated currents corresponding to the currents sunk by the device 2 which correspond to the respective unbuckled and buckled states of the seat belt buckle.

In use, on the monitoring circuit 1 determining an over current condition in the device 2 the microprocessor 5 outputs logic zeros on the select outputs Select I2, Select I3 and Select I4 and outputs a logic high on the output Select 11 for switching the resistor R1 to ground for sinking a current corresponding to the over current drawn by the device 2. On an open circuit condition in the device 2 being determined by the microprocessor 5 the select outputs Select 11, Select I2 and Select I3 are held low while the output Select I4 outputs a logic high for switching the transistor switch T4 for connecting the resistor R4 to ground for sinking a current corresponding to an open circuit state in the device 2. On the microprocessor 5 determining that the seat belt buckle is unbuckled, the microprocessor 5 outputs logic lows on the select outputs Select 11, Select I3 and Select I4 and puts a logic high on the Select I2 output for switching the transistor switch T2 for in turn switching the resistor R2 to io ground for sinking a 6mA current to ground for emulating the 6mA current drawn by the device 2 when the buckle is in the unbuckled state. When the microprocessor 5 determines that the current sunk by the device 2 corresponds with the buckle being in the buckled state, the outputs Select 11, Select I2 and Select I4 are held low, while the output Select I3 of the microprocessor 5 applies a logic high to the transistor switch T3 for in turn switching the resistor R3 to ground for sinking in a current of approximately 14mA, to emulate the current sunk by the device 2 when the buckle is in the buckled state.

Accordingly, the combined monitoring circuit 1 and the current emulating circuit 3 can be interposed in the electronic circuitry of a motor vehicle between the seat belt buckle sensor and the monitoring electronics of the motor vehicle without interfering with the motor vehicle electronics, and furthermore, independent monitoring of the seat belt buckle can be carried out while still providing corresponding currents to the vehicle electronics for monitoring by the vehicle electronics. ΙΕο 2 05 1 1 While the monitoring circuit has been described for determining only two states of the device 2, it will be readily apparent to those skilled in the art that the monitoring circuit could be adapted for determining many more than two states of the device. It will also, of course, be appreciated that the device 2 may be any other type of device besides a hall effect sensor.

While the emulating circuit has been described as comprising four resistors R1 to R4 for emulating four currents, any other number of resistors R1 to Rn may be provided io for emulating more than four currents.

It will of course be appreciated that impedance means besides resistors R1 to R4 may be used for emulating the currents in the current emulating circuit.

It will also be appreciated that the emulating current circuit may be provided without the current sense resistor 22. Any other suitable means may be provided for sensing the current being emulated by the current emulating circuit. It will also of course be appreciated that the current sense resistor, or any other current sensing means instead of being provided in the emulating current circuit, may be provided adjacent the power supply 20 powering the current emulating circuit.

It will also be appreciated that any other suitable main switch means besides that described for selectively switching the resistors R1 to Rn to ground for emulating the respective currents may be used.

While the monitoring circuit has been described as being provided in conjunction with the current emulating circuit, it will be readily apparent to those skilled in the art that the monitoring circuit may be provided on its own. Additionally, while the emulating circuit has been described as being provided in combination with the monitoring circuit, the emulating circuit may be provided independently of the monitoring circuit, and indeed, may be provided with any other suitable monitoring circuit, or indeed, any other suitable circuitry for selecting the appropriate resistors R1 to Rn for providing the desired emulated current.

The invention is not limited to the embodiments hereinbefore described, which may be varied in construction and detail.

F.F. GORMAN & CO.

IEO 2 05 1 1

Claims (5)

1. A monitoring circuit for determining the state of a device having at least two states, the device sinking currents of values within respective predetermined ranges corresponding to respective states of the device, the monitoring circuit comprising a 5 regulated voltage power supply for supplying a current to the device, a comparing means for comparing the voltage applied to the device with a threshold voltage for detecting an over current condition, a primary switch means for switching the power supply to the device for facilitating monitoring of the state of the device, the primary switch means being responsive to the comparing means detecting an over current 10 condition for isolating the device from the power supply, a timing means for timing a predetermined settling time delay, the primary switch means being non-responsive to the comparing means during the settling time delay for preventing operation of the primary switch means for isolating the device from the power supply during the predetermined settling time delay for allowing settling of the circuit, and a current 15 sensing means for sensing the current being sunk by the device.

2. A monitoring circuit as claimed in Claim 1 in which a control circuit is provided for reading the current sensing means for determining the state of the device, and for outputting signals indicative of the state of the device.

3. A monitoring circuit as claimed in Claim 2 in which a current emulating circuit is provided for emulating the currents drawn by the device corresponding to the respective states of the device, the emulating circuit comprising a means for receiving a regulated voltage power supply, at least two current sink impedance ΙΕΟ 205 1 Τ means for sinking current from the regulated voltage power supply of respective different values which emulate the currents drawn by the device in its respective different states, a main switch means responsive to the signals outputted by the control means for selectively switching the impedance means for sinking a current 5 from the regulated voltage power supply which emulates the current drawn by the device.

4. A monitoring circuit as claimed in any preceding claim in which the monitoring circuit is adapted for monitoring the state of a sensor of a device associated with a 10 seat belt buckle of a seat belt in a motor vehicle for determining the state of the seat belt buckle.

5. A monitoring circuit for determining the state of a device having at least two states, the monitoring circuit being substantially as described herein with reference 15 to and as illustrated in the accompanying drawings. F.F. GORMAN & CO. JS Ο 2 S3 5 Η hie-, i- ίΕ Ο IΟ 5 π •S-eZixA XI JkxZee/'J.n