Abstract:
Fieldbus is a digital, two-way communication link for remotely located sensors, controllers and actuators used in industrial applications. The fieldbus simultaneously carries a digital signal for communication with devices attached to the bus and DC power to operate those devices. Unless isolated from the fieldbus, the constant voltage DC power supply would prevent the propagation of the varying voltage of the digital communication signals on the fieldbus. An adaptive current source is provided between the output the constant voltage power supply and the fieldbus wiring to isolate the power supply. The adaptive current source can also be used to isolate the input of DC/DC voltage converters providing power to devices attached to the fieldbus.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a power conditioner for a DC power supply for a fieldbus network and, more particularly, to an adaptive current source to isolate a DC power supply from a communication bus simultaneously carrying digital communication signals and DC power to attached devices. 
     In a typical industrial plant application, sensors measure position, motion, pressure, temperature, flow, and other parameters related to the operation of process machinery and activities. Actuators, such as valves and motor controllers, control the operation of the machinery and process activities. The sensors and actuators are remotely located from the human and computerized controllers which gather information from the sensors and direct operation of the actuators. A communication network links the controllers with the sensors and actuators located in the field. 
     Heretofore, communication between controllers and remote sensors and actuators in industrial applications has been by means of analog signaling. The prevailing standard for analog networking of field devices and the control room in industrial applications has been the Instrument Society of America standard, ISA S50.1. This ISA standard provides for a two wire connection between the controller and each field device. One wire of the system carries the analog signal between the remote device and the controller. The analog signal may be converted to a digital signal useful to a computerized controller. The second wire of the circuit supplies DC power for operation of the remote sensor or actuator. 
     Communication utilizing digital signaling reduces the susceptibility of the communication system to noise and provides a capability for conveying a wide range of information over the communication network. Digital communication also permits several different devices to communicate over a single pair of wires. Remote devices used in connection with a digital communication system typically incorporate local “intelligence.” This permits sensors and actuators to perform diagnostic, control, and maintenance functions locally. Further, the local intelligence permits the devices to communicate directly with each other and perform some functions without the necessity of involving a central control facility, thus promoting the development of distributed control systems. 
     Fieldbus is a generic term used to describe a digital, bidirectional, multidrop, serial communication network for connecting isolated field devices, such as controllers, actuators and sensors, in industrial applications. One such fieldbus is defined by the Instrument Society of America standard, ISA SP50.02. This system utilizes a two wire bus to provide simultaneous digital communication and DC power to remotely located devices. 
     While fieldbus installations are as varied as the industrial applications with which they are used, an exemplary fieldbus installation is illustrated in FIG. 1. A twisted pair cable, referred to as the home run  2 , connects a digital control system  4  and a DC power supply  6  with a number of devices  8  (actuators, sensors and local controllers) in the field. The digital control system  4  and the DC power supply  6  may be located in a control room  10 . On the other hand, the power supply  6  could be located in the field or at a marshaling panel. The development of the digital fieldbus may also mean that controllers are located in the field. Several devices  8  can be connected to the home run  2  by spur cables  14  at a terminal referred to as a chicken foot  12  which incorporates signal termination for the home run. In addition, devices can be connected along the home run cable  2  with spur cables  14  that are connected to the home run by spur connectors  13 . 
     Termination is required at each end of the home run  2 . To send a digital signal over the network, a connected device varies the amount of current it draws from the bus. The terminators  16  comprise a resistor  18  of approximately the characteristic impedance of the wires of the bus and a capacitor  20  connected in series across the pair of wires of the home run cable  2 . When the transmitting device varies the current drawn from the bus, the voltage drop across the terminating resistor  18  varies producing the varying voltage of the digital signal. The capacitor  20  of the terminator  16  prevents dissipation of the DC power through the terminator resistor  18  while permitting transmission of the high frequency digital signal on the bus. In addition, the terminators  16  serve to prevent signals from reflecting from the ends of the home run wires  2 . 
     A power conditioner  22  is necessary to isolate the DC power supply  6  from the bus. The constant voltage power supply  6  will attempt to maintain a constant output voltage. This would prevent the propagation of the varying voltage of the digital signal if the power supply  6  was connected directly to the bus. Typically, the power conditioner  22  comprises an inductor and a resistor in series with the output of the power supply  6 . The inductor allows a DC voltage to be conducted to the bus, but blocks the high frequency digital communication signals from the output of the power supply  6 . The combination of an inductor in the power conditioner  22  and the capacitors  20  in the terminators  16  may make the network “ring” or oscillate disrupting signals on the network. A resistor in series with the inductor of the power conditioner  22  is used to critically damp the circuit to prevent oscillation. While a power conditioner  22  based on an inductor and resistor is effective in isolating the DC power supply, if a reasonable amount of current is to be supplied to the network the inductor must be physically large to avoid saturation of the magnetic material from which it is constructed and, as a consequence, will be costly. In addition, the DC current for the network flows through the series resistor of the power conditioner  22  dissipating power which would otherwise be available to the devices connected to the network. 
     Further, it may be desirable to provide multiple power supplies, particularly for large networks with long wiring runs. The resistance of the long wiring runs may dissipate sufficient power to make powering the network from one power source impractical. The tendency of the inductor isolated circuit to ring and the difficulty of designing a circuit with muWiple inductor-based isolators makes multiple power sources extremely difficult to incorporate in the fieldbus network. 
     What is desired, therefore, is a compact and efficient apparatus, adaptable to the power needs of a wide variety of installations, for isolating the output a DC power supply from voltage fluctuations on a bus simultaneously carrying digital communication signals and DC power. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the aforementioned drawbacks of the prior art by providing an apparatus for isolating the output of a constant voltage power source from a voltage variation in an electrical circuit connected to the output comprising a current source interposed between the output of the power source and the electrical circuit. The current source provides a variable resistance conductive path for current flow from the output of the power source to the wiring of a fieldbus circuit. The current source can provide a fixed current output or can be constructed to provide current output-that is adaptable to relatively slow changes in the current requirements of the fieldbus circuit while isolating the output of the power supply from the relatively high frequency digital signals on the bus. 
     The present invention also provides an apparatus for isolating the input of a direct current voltage converter from voltage variation in the electrical circuit supplying power to the input of the voltage converter comprising a current source interposed between the input of the voltage converter and the electrical circuit. 
     The adaptive current source has the circuit appearance of a large impedance and permits DC current to pass through the current source while isolating the low impedance constant voltage power supply or, as the case may be, the DC/DC voltage converter from the relatively high frequency digital communication signals on the bus. The current source is more compact and efficient than inductor-based power supply isolators and avoids signal distortion due to ringing in the circuit. 
     The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic drawing of a fieldbus circuit. 
     FIG. 2 is a schematic drawing of a constant current source according to the present invention. 
     FIG. 3 is a schematic drawing of an adaptive constant current source according to the present invention. 
     FIG. 4 is a schematic drawing of an adaptive current source according to the present invention employing MOSFET transistors and including over current protection. 
     FIG. 5 is a schematic drawing of an adaptive current source according to the present invention employing N-channel MOSFET transistors, npn-bipolar transistors, and including over current protection. 
     FIG. 6 is a block diagram illustrating use of an adaptive constant current source for isolation of the input a DC/DC converter supplying power to a device attached to a fieldbus. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a typical fieldbus installation comprises a number of remotely located attached devices  8  (sensors, distributed controllers, and actuators) connected to a digital control system  4 ; often, but not necessarily, located in a control room  10 ; and a DC power supply  6  by a bus comprising a twisted pair cable referred to as the home run  2 . Several devices  8  can be connected to the home run  2  by spur cables  14  at a connection referred to as a “chicken foot”  12 . In addition, devices  8  can be attached along the home run cable  2  with spur cables  14  that are connected to the home run by a spur connector  13 . For fieldbus installations with long wiring runs, an additional power supply  15  may be desirable. The resistance of a long wiring run may absorb sufficient power from the fieldbus to make powering remote devices from one point in the network impractical. 
     Termination is required at each end of the home run  2 . To send a digital signal over the network, an attached device  8  varies the amount of current it draws from the bus. The terminators  16  comprise a resistor  18  of approximately the characteristic impedance of the wires of the bus and a capacitor  20  connected in series across the pair of wires of the home run cable  2 . When the transmitting device  8  varies the current drawn from the bus, the voltage drop across the terminating resistor  18  changes producing the varying voltage of the digital signal. The capacitor  20  of the terminator prevents the flow of DC current through the terminator resistor  18  avoiding dissipation of power in the resistor while permitting the high frequency digital signal to be transmitted on the bus. In addition, the terminators serve to prevent signals from reflecting from the ends of the wires of the home run  2 . 
     A power conditioner  22  is necessary to isolate the DC power supply  6  or  15  from the bus. The DC power supply  6  or  15  will attempt to maintain a constant output voltage and if connected directly to the bus would prevent the propagation of the varying voltage of the digital signal. To permit the digital signals and the DC voltage to exist simultaneously on the same wires, the present invention utilizes a power conditioner  22  comprising a current source connected between the output of the power supply  6  or  15  and the bus. The current source permits DC current to flow from the power supply  6  or  15  onto the bus  2  but blocks the passage of the high frequency digital signals from the bus to the output of the power supply. While the current source provides relatively constant current isolating the output of the power supply from the high frequency voltage variations of the digital communication signals, it has the capability of adjusting the output to adapt to longer term changes in the current requirements of the devices attached to the bus. The current source has the appearance of a large impedance in the bus circuit, similar to that of a large inductor, but is physically smaller, less costly and reduces the likelihood of circuit ringing, particularly when multiple power supplies are attached to the bus circuit. 
     A first embodiment of an isolating constant current source according the present invention is illustrated in FIG.  2 . In this first embodiment of the current source  30 , current from the power supply  6  flows through a first resistor (R1)  32  and a first pnp, bipolar transistor (Q1)  34  to the home run wiring of the fieldbus  2  when the first transistor (Q1)  34  is turned “ON.” The first transistor (Q1)  34  is turned “ON” or conducts when current is drawn from its base through a second resistor (R2)  36  which is connected between the base of the first transistor (Q1)  34  and the negative side of the fieldbus circuit  2 . 
     When current flows through the first resistor (R1)  32  a voltage drop develops across the resistor equivalent to the emitter-base junction voltage of a second transistor (Q2)  38 . When the emitter-base junction voltage reaches or exceeds the threshold forward bias voltage (v eb ) of the second transistor (Q2)  38 , the second transistor  38  is turned “ON.” When the second transistor (Q2)  38  conducts, current flows from its collector and through the second resistor (R2)  36 . As a result, the voltage drop through the second resistor (R2)  36  increases and the current flowing from the base of the first transistor (Q1)  34  decreases. This causes the first transistor (Q1)  34  to tend toward an “OFF” or non-conducting state increasing the resistance from its emitter to its collector, reducing current flow to the bus  2 , and reducing the voltage drop across the first resistor (R1)  32 . Equilibrium is obtained when the current flowing through the first resistor (R1)  32  produces a voltage drop equal to the threshold forward bias voltage (v eb ) of the second transistor (Q2)  38 . As a result, the value of the first resistor (R1)  32  can be selected to permit a specific current flow through the current source  30  of the power conditioner. If the voltage in the wiring of the fieldbus  2  varies when the current source is in the current limiting mode, the current output of the current source  30  remains constant. Thus, the current source  30  has the appearance of large impedance isolating the output of the power supply  6  from the varying voltages of digital signals on the bus  2 . However, the constant current resulting from use of a fixed first resistor (R1)  32  is of limited practicality because the current requirements of the devices attached to the network are seldom known without analysis of each specific network installation. 
     Referring to FIG. 3, a second embodiment of the current source incorporates the capability of adapting to longer term changes in the current requirements of the bus. In this second embodiment the fixed first resistor (R1) of the first embodiment is replaced by a resistance which varies in response to relatively long term changes in the current requirements of the fieldbus but is unresponsive to the short term changes resulting from the high frequency digital signal. In the second embodiment, a P-channel, metal oxide field effect (MOSFET) third transistor (Q3)  42  replaces the first resistor (R1) of the first embodiment (see FIG.  2 ). A first transistor (Q1)  44  and the third transistor (Q3)  42  provide a conducting path with selective resistance for current flow from the output of the power supply  6  to the bus wiring  2 . As in the first embodiment, the first transistor (Q1)  44  conducts when the threshold forward bias voltage (v eb ) is applied between the emitter and base. The bias voltage of the first transistor (Q1)  44  is controlled by the voltage drop across the second resistor (R2)  46  which is a function of the current conducted by the second transistor (Q2)  48 . In this second embodiment of the invention, the operation of the second transistor (Q2)  48  is a function of the source to drain resistance of the third transistor (Q3)  42 . 
     The source to drain resistance of the third transistor (Q3)  42  is determined by its source to gate voltage. When the voltage in the fieldbus wiring  2  is reduced, the gate voltage of the third transistor (Q3)  42  is drawn low through the resistor (R3)  46  and the source to drain resistance of the third transistor (Q3)  42  is reduced. As the source to drain resistance of the third transistor (Q3)  42  decreases, the emitter-base voltage of the second transistor (Q2)  42  decreases causing the second transistor (Q2)  48  to tend toward reduced conduction. This causes the first transistor (Q1)  44  to turn “ON” and conduct increasing amounts of current from the power supply to the fieldbus wiring  2 . As current flow increases in the fieldbus wiring  2 , the voltage drop across the resistor (R3)  46  decreases reducing the source to gate voltage of the third transistor (Q3)  42  and increasing its source to drain resistance. This tends to turn the second transistor (Q2)  48  “ON” which, in turn, tends to turn “OFF” the first transistor (Q1)  44  reducing the output of the current source  40 . An equilibrium condition is attained when the source to gate threshold voltage of the third transistor (Q3)  42  is such that the source to drain voltage drop of the third transistor (Q3)  42  is equal to the threshold forward bias voltage (v eb ) of the second transistor (Q2)  48 . 
     A capacitor (C1)  50  maintains a relatively constant source to gate voltage for transistor (Q3)  42  when the voltage of the fieldbus fluctuates during digital signaling. As a result, the source to drain resistance of the third transistor (Q3)  42 , and therefore, the output of the current source remains relatively constant. If, on the other hand, the rate of change in voltage on the fieldbus wires  2  is relatively slow, as when an additional device is added to the network, the capacitor (C1)  50  will discharge through the resistor (R3)  46 . As a result, the voltage at the gate of the third transistor (Q3)  42  will drop and the output of the current source will adjust to the increased current requirements of the fieldbus. The values of the third resistor (R3)  46  and the capacitor (C1)  50  are selected such that the output of the current source  40  remains constant at a threshold frequency somewhat below the frequency of the communication signal specified for the fieldbus. The adaptive current source has the appearance of a large impedance in the circuit preventing the high frequency communication signals from reaching the output of the power supply. 
     A third embodiment of the current source is illustrated in FIG.  4 . In this third embodiment, the pnp, bipolar transistor (Q1) of the first and second embodiments is replaced with a field effect transistor (Q4)  62 . The field effect transistor requires much less gate current than a bipolar transistor. As a result, the resistance of the bias resistor (R2)  64  can be increased reducing the amount of power dissipated in sinking the current of the second transistor (Q2)  66 . 
     The third embodiment of the invention also incorporates over current protection in the event that the field bus wiring should be shorted. Over current protection is provided by adding a fixed, over current protection resistor (R4)  68  between the drain of the third transistor (Q3)  70  and the base of the second transistor (Q2)  66 . In the event of a short in the fieldbus wiring  2 , the source to drain resistance of the third transistor (Q3)  70  will drop very low. However, the maximum emitter-base voltage of the second transistor (Q2)  66  will limited by the resistance of the over current protection resistor (R4)  68 . As a result, the first transistor (Q4)  62  will not be turned fully “ON” and the current flow through the adaptive current source will be limited. 
     While all embodiments of the current source can be constructed with pnp bipolar transistors and P-channel field effect transistors as illustrated in FIGS. 2-4, the current source can also be constructed using npn, bipolar transistors and N-channel field effect transistors. An adaptive, constant current source  80  including over current protection, similar to the current source illustrated in FIG. 4 but constructed with NPN, bipolar and N-channel field effect transistors, is illustrated in FIG.  5 . This current source is connected to the negative side of the fieldbus wiring  2  and the power supply  6 . 
     Referring to FIG. 6, the adaptive current source  90  of the present invention can also be used as a power conditioner to isolate the input to devices attached to the fieldbus. The ISA standard provides for supplying a DC power voltage between 9 and 32 volts to the fieldbus. However, many attached devices require a lower internal voltage. A DC/DC converter  92  can be used to change the voltage supplied by the bus wiring  96  to the voltage required internally by the attached device  94 . However, the converter  92  has a low impedance and, like the power supply, would absorb the digital signals from the bus if not isolated. The adaptive constant current source  90  can be used to provide high impedance to isolate the input of a DC/DC converter  92  from the varying voltage of the digital signals on the bus wiring  96 . 
     The terms and expressions that have been employed in the foregoing specification are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow.