Abstract:
A three-wire sensor interface for field wiring of conventional on/off type control sensors that delivers power to the sensor, provides a normally open control output from the sensor and, concurrently, provides a normally closed control output from the sensor. The sensor interface includes a plurality of external terminals including a normally open external terminal for electrically connecting to zero, one, or more normally open load devices to control the operation of the normally open load device, and a normally closed external terminal for electrically connecting to zero, one, or more normally closed load devices to control the operation of the normally closed load device. A plurality of interface terminals are preferably electrically coupled between the sensor interface and the sensor and include a normally open control terminal and a normally closed control terminal. The sensor interface further includes an interface circuit adapted to receive the control signals from the sensor subsystem through the normally open and normally closed control terminals and to couple these control signals to the appropriate normally open or normally closed external terminals for controlling the external load devices.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to interface devices and methods used to provide power to sensors and to provide normally open and/or normally closed outputs from the sensors to drive external loads. 
     BACKGROUND OF THE INVENTION 
     Conventional sensor interface units provide a means for supplying power to a control sensor and for translating the sensor&#39;s control outputs to drive external loads. These conventional interface units are typically used with various types of on/off control sensors, proximity detectors, level sensors, and any other devices that take or provide measurements that can be used to control devices or translated into control outputs, such as normally open and/or normally closed control outputs. The interface units are typically configured to control various types of loads such as, for example, starter relays connected to motors, solenoid actuators and programmable logic controllers (PLCs). 
     One conventional sensor interface unit is a standard two-wire interface, as shown in FIGS. 1 a  and  1   b , that operates in either a normally open or a normally closed configuration (one or the other, but not both). In such a conventional two-wire interface, power for the sensor is provided from the current through the load, which requires a minimum leakage current through the load when the control output is in an inactive or off state. These two-wire interfaces may be designed to operate on AC power, DC power, or configurable to use either AC or DC power. Prototypical interface units of this type are model Bi25-G47SR-FX3X2, available from Turck USA and model NJ2-12GM50-WS, available through Pepperl+Fuchs. 
     A conventional variation on the standard two-wire interface unit is a field programmable unit that provides the ability to set the active state, i.e., either normally open or normally closed, via a switching mechanism in the field. For example, this may be provided by including a user interface on the sensor or by using additional wiring such as two terminals that are shorted to configure as a normally closed interface and left open to configure as a normally open interface. 
     Another type of conventional sensor interface unit is a three-wire unit in which power to the sensor is provided separately from the control outputs from the sensor, wherein the power and one of the control functions share a common wire. This type of configuration is illustrated in FIGS. 1 c  through  1   f . FIGS. 1 c  and  1   d  show a three-wire normally open (FIG. 1 c ) and normally closed (FIG. 1 d ) source interface connected to a load device  110 . The source interface  112  is powered through terminals  103  and  105 . Switched power (in a source configuration) is available to the load between terminals  103  and  104 . For the three-wire normally open source interface  112  shown in FIG. 1 c , the load is switched on when the source interface  112  is active and switched off when the source interface  112  is inactive. For the three-wire normally closed source interface  112  shown in FIG. 1 d , the load is switched on when the source interface  112  is inactive and switched off when the source interface  112  is active. FIGS. 1 e  and  1   f  illustrate a prior art three wire normally open (FIG. 1 e ) and normally closed (FIG. 1 f ) sink interface  114 . The sink interface  114  is powered through terminals  106  and  108 . Switched power (in a sink configuration) is available to the load between terminals  106  and  107 . For the three-wire normally open sink interface  114  shown in FIG. 1 e , the load is switched on when the sink interface  114  is active and switched off when the sink interface  114  is inactive. For the three-wire normally closed sink interface  114  shown in FIG. 1 f , the load is switched on when the sink interface  114  is inactive and switched off when the sink interface  114  is active. 
     Yet another conventional interface unit includes four wires that is capable of simultaneously providing both normally open and normally closed control outputs. Two of the four wires are used to provide supply power to the sensor, one of the wires provides the normally open control output, and the last wire provides the normally closed control output. The additional fourth wire can be a significant disadvantage in small sensors, where space (especially wiring space) is at a premium. 
     While these above-mentioned sensor interface units are acceptable in certain applications, a need still remains for a sensor interface capable of simultaneously providing a normally open and a normally closed control output using only three wires. A preferred sensor interface would include a maximum of three external terminals for connection to three wires or conductors including a power terminal and terminals for connecting the sensor interface to load devices, such as a normally open and a normally closed load device. The sensor interface would preferably be adapted to control the normally open and the normally closed load devices concurrently. 
     SUMMARY OF THE INVENTION 
     The current invention provides a means for field wiring of conventional on/off type control sensors using a three-wire sensor interface that (i) delivers power to the sensor; (ii) provides a normally open control output from the sensor; and, concurrently, (iii) provides a normally closed control output from the sensor. 
     In one embodiment, the three-wire sensor interface is capable of being coupled to a sensor subsystem for conducting signals to and from the sensor subsystem. Preferably, the sensor subsystem provides normally open and normally closed control signals and the sensor interface provides interfaces for communicating these signals to control normally open and/or normally closed load devices. The preferred interface unit includes a plurality of external terminals including a normally open external terminal that may be electrically connected to one or more normally open load devices to control the operation of the normally open load devices, and a normally closed external terminal that may be electrically connected to one or more normally closed load devices to control the operation of the normally closed load devices. The interface unit also preferably includes an external power terminal for connection to a source of electrical power for supplying power to the sensor interface and, in one embodiment, to the sensor subsystem. A plurality of interface terminals are preferably electrically coupled between the interface unit and the sensor subsystem and include one or more control terminals, e.g., a normally open control terminal and a normally closed control terminal. The interface unit also includes an interface circuit adapted to receive the normally open and normally closed control signals from the sensor subsystem through the normally open and normally closed control terminals and to couple these control signals to the appropriate normally open or normally closed external terminals for controlling the external load devices. 
     In another embodiment, the interface circuit includes a normally open control path that receives the normally open control signal from the sensor subsystem and couples the normally open control signal to the normally open external terminal, and a normally closed control path that receives the normally closed control signal from the sensor subsystem and couples this signal to the normally closed external terminal. The interface circuit also receives supply power through the external power terminal from an external power supply. In one embodiment in which both types of load devices (normally open and normally closed) are connected, the circuit couples the supply power to the interface power terminal through the normally open control path when the normally open control signal is inactive or, alternatively, couples the supply power to the interface power terminal through the normally closed control path when the normally closed control signal is inactive. When used in a configuration in which only one type of load device is present, the circuit couples the supply power to the interface power terminal through either the normally open or normally closed control path depending on which type of device is connected to the sensor interface. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings wherein: 
     FIG. 1 includes diagrammatic representations of prior art sensor interface designs; 
     FIG. 2 is a block diagram illustrating a preferred sensor interface connected to a sensor subsystem and normally open and normally closed load devices; 
     FIG. 3 is a circuit level diagram of a preferred sensor interface circuit for coupling the control outputs of the sensor subsystem to the load devices; and 
     FIG. 4 is a circuit level diagram of a preferred voltage regulator for use in the present invention. 
    
    
     These drawings are provided for illustrative purposes only and should not be used to unduly limit the scope of the present invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 illustrates one embodiment of the three-wire sensor interface  10  configured in accordance with the present invention. The three-wire sensor interface  10  is preferably coupled to three external terminals (shown as ref. numerals  12 ,  14 , and  16 ), and four interface terminals (shown as ref. numerals  26 ,  28 ,  30 , and  32 ) connected to a sensor subsystem  20 . One of the three external terminals coupled to the sensor interface  10 , for example external power terminal  12 , provides a power input for energizing the circuitry contained within the sensor interface  10  and the attached sensor subsystem  20 . Preferably, the sensor interface  10  may operate on alternating current or either polarity direct current. Preferably, the supply power (not shown) can vary over a large range of values, for example, between 6 and 340 volts for a preferred proximity detector used as a component of the sensor subsystem  20 . One preferred voltage regulator designed to operate with this wide range of input voltages is described below. The two remaining terminals,  14  and  16 , provide a normally open control output (for example, at a normally open external terminal  14 ) and a normally closed control output (for example, at a normally closed external terminal  16 ). In operation, the normally open control output at terminal  14  is coupled to zero, one or more normally open load devices  22  and the normally closed control output at terminal  16  is coupled to zero, one or more normally closed load devices  24 . The load devices may be, for example, starter relays connected to motors, solenoid actuators and programmable logic controllers (PLCs). 
     The sensor subsystem  20  may preferably include any conventional type of on/off control sensor, such as a movement sensor, a proximity detector, a Hall-effect switch, an ultrasonic level sensor, or any similar sensor operable to control a plurality of loads in accordance with predetermined criteria. A preferred proximity detector is disclosed in an application entitled “Proximity Detector for Hard-to-Detect Materials,” filed on Jun. 23, 1999, and assigned to the assignee of the present invention, the contents of which are incorporated herein in its entirety for all purposes. The sensor subsystem  20  and the sensor interface  10  may be packaged together in a common circuit board assembly. 
     Power is coupled to the sensor subsystem  20  from the sensor interface  10  through power conductor  26  and a ground connection is attached to the sensor subsystem  20  through ground connector  28 . The sensor subsystem  20  performs appropriate sensing operations, such as, for example, detecting the presence of certain materials or detecting the level of material or fluid in a holding container, and outputs one or more control signals, e.g., a normally open control output signal  30  and a normally closed control output signal  32 —to the sensor interface  10 . Importantly, the sensor subsystem  20  must be designed such that only one of the control signals, either the normally open control output signal  30  or the normally closed control output signal  32 , is active at any given point in time. 
     In an alternative embodiment, only one control output signal (either normally open or normally closed) is provided by the sensor subsystem  20  and the opposite control output signal is derived by the logical negation of the available control output signal. For example, if the available control output signal is the normally open control output signal, the normally closed control output signal is derived from the normally open control output signal by negating the normally open control output signal (i.e., if normally open control output signal is active, normally closed control output signal is inactive). In such an embodiment, the sensor interface  10  may only require one interface terminal to connect to the only one control output signal from the sensor subsystem  20 . 
     FIG. 3 illustrates an embodiment of the circuitry contained within the sensor interface  10 . Supply power is coupled to the circuitry via external power terminal  12  and the current flows through fuse  40 . The fuse  40  is preferably 125% of rated current. While the preferred sensor interface  10  may be used in various configurations, for ease of explanation, initially disclosed is a configuration wherein the sensor interface is adapted to control only one type of load device—either a normally open or a normally closed load device. Thus, the three-wire sensor interface  10  may be configured in a manner similar to conventional two-wire interface units and provide either a normally open or a normally closed control output, but not both. For example, when only one load device is present, for example, the normally closed load device  24 , the normally closed control path extends from external power terminal  12  to external normally closed terminal  16  and includes fuse  40 , the diode bridge formed by four unidirectional diodes  41 ,  42 ,  43 , and  44 , and a transistor  50 . Preferably, the diodes may be 1N4004 having a peak inverse voltage above the worst case voltage anticipated (340V), for example at 400V. Preferably, the transistor  50  may be an enhancement mode n-channel MOSFET (e.g., MTP10N40E Mot., or an MTP2N60E Mot.). 
     Assuming an alternating current power supply is used, at which power terminal  12  is at a higher voltage than normally closed terminal  16  during the positive half-cycle, the current flows along the path formed by power terminal  12 , fuse  40 , diode  44 , transistor  50 , diode  41 , and on to the normally closed terminal  16 . For the negative half cycle, during which the power terminal  12  is at a lower voltage that the normally closed terminal  16 , the current flows along the path formed by normally closed terminal  16 , diode  43 , transistor  50 , diode  42 , fuse  40  and power terminal  12 . If the sensor interface  10  is coupled to a direct current power supply, the current flow path will follow one of the two above-described flow paths. For example, when using a positive voltage supply, the current flow path will follow the positive half-cycle flow. During both half cycles for an alternating current power supply (and for a direct current power supply) the current path through the transistor  50  is always from its drain terminal to its source terminal. Power is supplied to the sensor subsystem  20  via diode  54  and power conductor  26  by regulating the gate voltage on the transistor  50  in the normally closed path to ensure that the voltage differential between power conductor  26  and the ground conductor  28  is greater than the minimum voltage required to power the sensor subsystem  20 . When the sensor subsystem  20  commands the normally closed control output signal  32  active (commanding the normally closed terminal  16  to open), the sensor interface  10  and the sensor subsystem  20  are powered by leakage current through the normally closed load device, i.e., current flowing through the load device that is insufficient to turn the device on. Thus, the transistor  50  in the normally closed path is off and the current flows from the power terminal  12  through the fuse  40 , through the diode bridge ( 41 - 44 ), through diode  54 , through the power conductor  26 , the sensor subsystem  20 , the ground conductor  28  and through diode  48 . In this case, the voltage difference between the power conductor  26  and the ground conductor  28  is equal to the voltage used to supply the normally closed load  24  (when the load is powered). Thus, for a 240 VAC normally closed load  24 , the peak voltage is approximately 340 volts. 
     In the event that only a normally open load device  22  is connected to the sensor interface  10  (i.e., the normally closed load device  24  is not present), the current flow paths may be similarly described. When the sensor subsystem  20  commands the normally open control output signal  30  active, the normally open flow path is between power terminal  12  and the normally open terminal  14  and includes the fuse  40 , a diode bridge formed by four diodes  53 ,  45 ,  46 , and  47 , and transistor  52 . Assuming an alternating current power source is coupled to the power terminal  12 , during the positive half-cycle the current flows from the power terminal  12 , through the fuse  40 , through the diode  53 , through the transistor  52 , through the diode  47  and out through the normally open terminal  14 . During the negative half-cycle, the current flows from normally open terminal  14 , through the diode  46 , through the transistor  52 , through diode  45 , through the fuse  40  and out through the power terminal  12 . Again, if the sensor interface  10  is coupled to a direct current power supply, the current flow path will follow one of the two above-described flow paths. During both half cycles for an alternating current power supply (and for a direct current power supply) the current path through the transistor  52  is always from its drain terminal to its source terminal. Alternatively, the sensor subsystem  20  may command the normally open control output signal  30  to be inactive. In this configuration (i.e., the normally open load device  22  present, with the normally closed load device  24  not present), the sensor interface  10  and the sensor subsystem  20  are powered by leakage current through the normally open load device  22 . Thus, the transistor  52  is off and the current flows from the power terminal  12  through the fuse  40 , through the diode bridge ( 53 ,  45 - 47 ), through diode  55  through the conductor  26 , the sensor subsystem  20 , the ground conductor  28  and through diode  49 . In this case, the voltage difference between the power conductor  26  and the ground conductor  28  is equal to the voltage used to supply the normally open load device  22  (when the load is powered). Thus, for a 240 VAC normally open load  22 , the peak voltage is approximately 340 volts. When the sensor subsystem  20  commands the normally open control output signal  30  active (commanding the normally open terminal  14  to close), power is supplied to the sensor subsystem  20  via diode  55  and power conductor  26  by regulating the gate voltage on the transistor  52  to ensure that the voltage differential between conductor  26  and the ground conductor  28  is greater than the minimum voltage required to power the sensor subsystem  20 . 
     While the above description of FIG. 3 assumes that only one load is present (either the normally open load device  22  or the normally closed load device  24 ), the present invention may be operated with both types of device loads present. In the default state, in which the normally open control signal  30  from the sensor subsystem  20  is commanded open, and the normally closed control signal  32  from the sensor subsystem  20  is commanded closed, the transistor  50  in the normally closed flow path is on and the transistor  52  is off. In this case, the voltage at the source of the transistor  52  is at a lower potential than the voltage at the source of the transistor  50 . The current flows from the power terminal  12 , through the fuse  40 , into the diode bridge though diodes  53  and  45 - 47 , through diode  55 , through the power conductor  26  into the sensor subsystem  20 , out though the ground conductor  28 , through the diode  49  and through the normally open terminal  14 . In this configuration, the diode  48  in the normally closed loop is preferably reversed biased, thus reducing the possibility that the current which flows through the transistor  50  in the normally closed path will inadvertently power the normally open load device  22 . 
     As the sensor subsystem  20  commands the system into an energized state, by, for example, commanding the normally closed signal  32  to open and the normally open control signal  30  to closed, the transistor  52  is on and the transistor  50  is off. In this case, the voltage at the source of the transistor  50  is at a lower potential than the voltage at the source of the transistor  52 . The current flows from the power terminal  12 , through the fuse  40 , into the diode bridge though diodes  41 - 44 , through diode  54 , through the power conductor  26  into the sensor assembly  20 , out though the ground conductor  28 , through the diode  48  and through the normally closed terminal  16 . In this configuration, the diode  49  in the normally open loop is preferably reversed biased, thus reducing the possibility that the current which flows through the transistor  52  will inadvertently power the normally closed load device  24 . 
     In each of these configurations (i.e., when the sensor subsystem  20  is either commanding the normally open control signal  30  or the normally closed control signal  32  to active), the power supplied to the sensor subsystem  20  through the power conductor  26  is supplied by the leakage current associated with the load that is turned off. 
     FIG. 4 illustrates an embodiment of a closed loop high voltage linear regulator circuit  60  preferably included within the sensor subsystem  20  and designed to handle a wide range of input voltages for generating a regulated supply voltage. This design is described in application note AN-D17, “High Voltage Off-Line Linear Regulator,” by Jimes Lei, Supertex, Inc., the contents of which are incorporated herein in its entirety for all purposes. The regulator circuit  60  preferably includes an integrated circuit linear regulator  64 , such as, for example, Ricoh part no. RH5RA50AA, which is a 5.0V regulator. The regulator circuit  60  also preferably includes a transistor  62 , such as an N-channel depletion mode device with a threshold voltage less than the dropout voltage associated with the integrated circuit regulator  64  and a breakdown voltage greater than the maximum value of the voltage present at power terminal  12 . Preferably, the transistor  62  is connected in a source-follower configuration, wherein its gate is connected to the output, V cc , and to ground through capacitor C 3 , its source terminal is connected to the input of the integrated circuit regulator  64  and to ground potential through capacitor C 2 , and its drain terminal is connected to V+ (power conductor  26 ). Preferably, all of the capacitors, C 1 , C 2 , and C 3 , are bypass capacitors. Power from an external power source is applied to V+ and as V+ increases, V cc  will also increase until V cc  reaches its regulated voltage as controlled by the linear regulator  64 , for example 5.0 V. 
     Although the present invention has been described in considerable detail with reference to certain presently preferred embodiments thereof, other embodiments are possible without departing from the spirit and scope of the present invention. Therefore the appended claims should not be limited to the description of the preferred versions contained herein.