Abstract:
A current-loop output circuit for an industrial controller provides for low power dissipation and reduced part count by driving current loads of different resistances directly from a switched voltage source. Proper filtering and design of a feedback loop allows the necessary transient response times to be obtained.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 11/680,228, filed on Feb. 28, 2007 now U.S. Pat. No. 7,804,287. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to industrial controllers and, in particular, to current-loop circuits used to connect industrial controllers to controlled equipment and processes. 
     Two wire, current-loop circuits are used to provide electrical signals to a variety of industrial devices, for example, valve actuators or meters. Such circuits typically produce a defined current output, generally having an on state from 4-20 mA, according to a received control signal. By controlling output current instead of output voltage, changes in resistance caused by different lengths of wire connecting the current-loop circuit to the load and variations in load resistance, are better accommodated. 
     A typical current-loop circuit may use a current mirror circuit providing a controllable current source that generates the 4-20 mA signal based on the control signal. The current source is supplied with power from a voltage supply, commonly referred to as the compliance voltage, having a voltage value sufficient to drive the peak current required across the range of expected loads. For example, the loads may range in resistance from approximately 750 ohms for a solenoid valve to approximately 0 ohms for a panel meter. To provide adequate range of currents for these different loads, typical loop driving circuits are provided with compliance voltage sources of at least 24 volts. 
     While the original current-loop circuits operated in a binary mode, current-loops are also used to provide for “analog” control of current providing any current within a predetermined range of currents. 
     The power dissipated by a current source used in a current-loop driver will depend on the excess compliance voltage beyond the voltage needed to provide the desired current output. Thus, while it is desirable to have a high compliance voltage to provide high current outputs to high resistance loads, such high compliance voltages can produce high power dissipation in the current drive circuits when higher currents are output or lowered resistance loads are used. 
     One solution to this dilemma is to provide the current source, typically a transistor that is powered by a multi-mode power supply providing one or more different compliance voltages or a continuous range of compliance voltages. The transistor provides rapid current control and the power supply is switched between voltages at a slower rate depending on the amount of excess compliance voltage for the given current that is required. U.S. patent application 2006/0066379, filed Mar. 30, 2006, assigned to the assignee of the present invention and hereby incorporated by reference, describes such a system. In this system, the power supply is a boost converter, operating with very low power dissipation, to provide a range of compliance voltages to a field-effect transistor (FET) that provides the current control. The particular compliance voltage level to be used is determined by comparing the voltage dropped across the FET against the voltage drop across the load so that the compliance voltage may be tailored to the particular resistance of the load and the desired current level. 
     SUMMARY OF THE INVENTION 
     The present inventors have determined that a synchronous “buck converter”, when used as power supply, can provide compliance voltages that can be changed fast enough, even after the necessary filtering, for direct current control for an I/O current-loop circuit, eliminating the need for a current control transistor and the heat dissipation of this element. The reduced heat dissipation allows high-density I/O modules. 
     Specifically then, the present invention provides a current-loop input/output module for an industrial control including at least one output circuit with load terminals connectable to a load and a control input receiving a control value from the industrial control indicating a desired current to the load. A current sensor provides a current value measuring current received by the load through the load terminals and a feedback circuit receives the control value and the current value to provide an error output. A switching circuit having at least one solid state switch receives the error output and a one supply voltage, and based on the control input periodically connects the supply voltage through an inductor to a load terminal to control the current to the load according to the control value. When the supply voltage is not connected to the inductor a diode or second solid state switch continues the current to the load terminal. All the solid state switches of the switching device operate exclusively in a switching mode being driven to either fully on or fully off states, and current from the switching circuit connects to the load terminal without passing through any additional solid state switching elements not operated in a switching mode. 
     It is thus one aspect of at least one embodiment of the invention to provide a current-loop circuit with low power dissipation and low parts count. Direct feedback control of the compliance voltage eliminates the needs for a power dissipating current mirror operating in a non-switching mode. 
     The switching circuit may include a low pass filter attenuating current flow at a frequency of switching of the switching circuit. 
     It is another aspect of at least one embodiment of the invention to provide for a compliance voltage with low ripple without significantly increasing the power dissipation. 
     The low pass filter may include multiple stages and the feedback circuit may further receive a signal from before at least one of the multiple stages to provide the error output. 
     It is another aspect of at least one embodiment of the invention to provide for a sophisticated feedback control of the switching circuit that provides rapid transient response from a DC power supply. 
     The current sensor may be positioned after the filter. 
     It is another aspect of at least one embodiment of the invention to provide a stable current feedback signal suitable for feedback control. 
     The switching circuit may be a buck converter controlling a duty cycle of switching of a supply voltage to the load. 
     It is thus another aspect of at least one embodiment of the invention to use a switching circuit that can provide for rapid changes in current. The buck converter provides reduced high frequency output that allows the design of a filter that is consistent with a need for high transient response. 
     The switching circuit may include a first solid-state switching device connected from the supply voltage through an inductor to the load terminal and a second solid state switching device connected from a ground through the same inductor to the load terminal, the switching devices being activated alternately. 
     It is thus another aspect of at least one embodiment of the invention to provide a switching circuit that actively pulls the output either high or low for rapid transient response. This is accomplished by the solid state switching device connected to ground pulling current from the load terminal and transferring that current to the supply voltage when the first solid state switch is subsequently turned on. 
     The invention may further provide a housing holding multiple output circuits. 
     It is another aspect of at least one embodiment of the invention to provide a circuit having both reduced part count and heat dissipation to allow multiple circuits to be packaged in an extremely compact I/O module or together on an integrated circuit. 
     These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a block diagram of an industrial control system suitable for use with the present invention showing an I/O module having multiple current-loop circuits; 
         FIG. 2  is a fragmentary schematic representation of a prior art current-loop circuit showing a non-switching mode transistor operating as a current source to drive a load; 
         FIG. 3  is a figure similar to that of  FIG. 2  showing a prior art improvement over the current-loop of  FIG. 2  in which a compliance voltage feeding the non-switching mode transistor is adjusted to reduce power dissipation in the transistor; 
         FIG. 4  is a figure similar to that of  FIGS. 2 and 3  showing a current-loop circuit of the present invention in which a switching mode compliance voltage source directly drives the load; 
         FIG. 5  is a detailed block diagram of the circuit of  FIG. 4  showing a synchronous buck converter such as forms the switching mode compliance voltage source of  FIG. 4  and configured to provide high transient response; 
         FIG. 6  is a schematic diagram of the converter of  FIG. 5 ; 
         FIG. 7  is a set of plots of voltage versus time for different points in a schematic of  FIG. 6 ; and 
         FIG. 8  is a block diagram of the principle components of the invention as may be incorporated into a single integrated circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , an industrial control system  10  may include a controller  12 , for example, executing a stored program for the control of an industrial process  14  or the like. 
     The controller  12  may provide for local I/O modules (not shown) or may provide a network connection  16  to a remote I/O module  18 . The I/O modules  18  may include a power supply  20  and one or more current-loop circuits  22 . Each current-loop circuit  22  may provide an electrical connection  24  to a load  26  such as actuators or sensors connected to the industrial process  14 . 
     Referring now to  FIG. 2 , in a prior art I/O module  18 , logic circuitry  30  provides a command signal  32 , indicating a desired current to be output over connection  24  to load  26 . The command signal  32  is received by a linear current source  34 , which may, for example, be an FET receiving a compliance voltage  36  from the power supply  20  (shown in  FIG. 1 ) and operating in a non-switching mode to provide a desired current level (e.g. 20 milliamps) when the command signal  32  is high. For an arbitrary low load resistance for load  26  and a high compliance voltage  36  substantial power dissipation occurs in linear current source  34 . 
     Referring now to  FIG. 3 , and as taught in U.S. patent 2006/0066379 entitled: “Current-Loop Drive Module With Dynamic Compliance Voltage”, filed on Sep. 30, 2004 and hereby incorporated by reference, the circuit of  FIG. 2  may be improved by the introduction of an adjustable compliance voltage converter  38  between the compliance voltage  36  and the controllable linear current source  34 . In this embodiment the amount of power dissipated in the linear current source  34  is monitored so that for loads  26  with low resistance, the voltage output of the compliance voltage module  38  is reduced thereby reducing the power dissipation in the linear current source  34 . The compliance voltage converter  38 , which varies the voltage provided to the linear current source  34 , uses a “boost converter” whose power dissipation is low and largely independent of the amount of voltage output by the compliance voltage converter  38 . 
     Referring now to  FIG. 4 , the present invention provides an improvement over the circuit of  FIGS. 2 and 3  by eliminating the controllable linear current source  34  and providing an adjustable voltage directly from a compliance voltage module  40  to the load  26 , adjusting that voltage rapidly to provide the desired current flow through the load  26 . This eliminates the need for the linear current source  34  and eliminates the heat dissipated in linear current source  34 . Critical to this ability is the recognition that a simple circuit could be used to implement compliance voltage module  40  that would also provide rapid transient response comparable to the linear current source  34 . 
     Referring now to  FIG. 5 , the compliance voltage module  40  may comprise four principal components. The first is a synchronous buck converter  42  receiving the compliance voltage  36  and producing a switched output  44  having an average value suitable for producing a desired current flow through connection  24 . Because converter  42  operates in a switched mode, either connecting compliance voltage  36  directly to switched output  44  or connecting switched output  44  to ground, the solid state switching devices of the converter  42  provide extremely low power dissipation. The switched output  44 , having a desired average voltage, is received by a multistage filter  46  having series connected low-pass networks of a type well known in the art, using reactive components (e.g. capacitors and inductors) having essentially no power dissipation and low resistance resistors providing minimal power dissipation. Thus the multistage filter  46  operates as a low pass filter, blocking frequency components at and around the switching frequency of the converter  42  which are generally many octaves above the desired transient response of the signal on connection  24  to the load  26 . 
     The multistage filter  46  provides an output voltage  48  that is connected to the load  26  to provide a desired current flow throughout a range of possible load resistances as will be described. Precise adjustment of the voltage  48 , to obtain the desired current flow through the connection  24 , is obtained by means of a feedback mechanism that uses a current signal measured by a current sensing resistor  50 . The current sensing resistor  50  is in series with the current that has passed through the load  26  and is returned on a return connection  24 ′ on the way to ground. 
     The current signal  52  from the current sensing resistor  50  is provided to a feedback error block  54  which receives the command signal  32  from the logic circuitry  30  and determines whether the voltage  48  is too high or too low to produce the desired current as determined from the current signal  52 . 
     The current feedback from the current sensing resistor  50  has some phase lag as a result of the action of the multistage filter  46  and this phase lag may impair the transient response of the system. Accordingly an anticipating signal  56  from an early stage in the multistage filter  46  is also used by the feedback error block  54  to provide improved transient response. 
     An error signal  57 , output from the feedback error block  54 , indicates whether voltage  48  is too high or too low, and is provided to the converter  42  to adjust the switched output  44  completing the feedback loop. 
     Referring now to  FIGS. 6 and 7 , the synchronous buck converter  42  may provide for a first and second solid-state switch  60  and  62 , for example, being field effect switches, with solid-state switch  60  receiving the compliance voltage  36  and connecting to a junction point being the switched output  44  of the synchronous buck converter  42  and the switch  62  connecting from the switched output  44  to ground. Fly-back diodes  64  may be connected in parallel with each of the switches  60  and  62  as understood in the art. 
     Each of switches  60  and  62  may be operated alternately by a “Q” output  65  and “Q-not” output  66  of a flip-flop  68 . The flip-flop  68  thus ensures that only one of switches  60  and  62  will be activated at a time preventing a possible short circuit from compliance voltage  36  to ground. 
     The flip-flop  68  is “set” by the output of a comparator  70  which receives the error signal  57  described above and compares it to a ramp wave  72  produced by ramp generator  74 . Referring now to  FIG. 7  when the ramp wave is greater than the error signal  57 , for example, at the time  76 , the output of a comparator  70  will rise, setting the flip-flop  68  and causing its Q output  65  to rise and it&#39;s Q-not output to fall. Correspondingly this causes switch  60  to turn on and switch  62  to turn off. 
     Ramp generator  74  also produces a reset pulse  80  when the ramp resets which also resets the flip-flop  68  causing the states of Q and Q-not outputs to reverse, that is, the Q output  65  to fall, and the Q-not output to rise. It will be understood that the higher the error signal  57 , indicating that insufficient current is flowing through the load  26 , the longer the duty cycle of the Q output  65  and thus the more time that switch  60  is closed increasing the average voltage of the switched output  44 . 
     The switched output  44  of the converter  42  is a square wave and is received by multistage filter  46  described above and consisting of a first stage being a series inductor  75  shunted by capacitor  77  to ground. The anticipating signal  56  to be described below is taken after this first stage at the junction of the inductor  75  and capacitor  77 . The next two stages consist of series resistors  78  and  79  shunted respectively by capacitors  82  and  84  with the first series resistor connected to the junction of the inductor  75  and capacitor  77  and the second series resistor connected to the junction of the resistor  78  and capacitor  82 . The junction of the resistor  79  and capacitor  84  forms the output voltage  48 . In the preferred embodiment, resistor  79  is replaced with a short and capacitor  84  is omitted. 
     The output voltage  48  from the filter is received by the load  26  and passes through the current sensing resistor  50  which is a precision low ohmage resistor  83 . The voltage across this resistor  83  forms a current signal  52  and is received by an integrator formed of an operational amplifier  85  having a noninverting input receiving command signal  32 , and an inverting input receiving the sum of the current signal  52  and anticipating signal  56  each through a gain setting resistance and the latter through a high pass filter selected for the appropriate transient response. The inverting input of the operational amplifier  85  is shunted by a capacitive network producing an integrated output providing the error signal  57 . 
     Referring now to  FIG. 8 , each of the elements of the buck converter  42  and the feedback error block  54  may be placed on a single integrated circuit  90  for multiple current-loop circuits  22 . By eliminating devices operating in the non-switched region, all the switch elements may be on the integrated circuit  90  significantly improving the manufacturability of the current-loop circuits  22 . For purposes of isolation, different compliance voltage module  40  may be on different integrated circuits  90 . 
     While the present invention has been described with respect to a digital command signal  32  it will be understood that the identical circuit may be used to provide for analog current-loop control as well simply by varying the command signal  32  among different ranges of voltage rather than simply between two voltages as may be provided by a digital to analog converter communicating with the logic circuitry  30 . 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein. For this reason, the invention may include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.