Abstract:
A high-speed input circuit for industrial control provides an optoisolator protected by a series current regulator and shunting voltage regulator. The combined effect of the regulators is to allow the input circuit to work over a wide range of voltages while providing extremely fast response time.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     CROSS REFERENCE TO RELATED APPLICATION 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an industrial control systems and in particular to an I/O module for industrial control systems, the I/O module accommodating large voltage ranges and sensitive to high-speed switching. 
     Industrial control systems were originally developed to provide control logic for equipment in factories operating to manufacture products on an assembly line or in batch processes. Early industrial control systems replaced racks of relays with specialized circuitry that mimicked these relays but used a programming language. 
     Current industrial control systems continued this evolution, using modern electronic computer processors and circuits, but may be distinguished from standard computers by architectures that address the needs for high reliability, safety, and resistance to environmental damage and normal contamination. This latter concern generally means that industrial control system components are designed without cooling fans or ports that communicate with the outside air, placing a premium on low heat dissipation. 
     Industrial control systems must function in a highly varied set of manufacturing applications. For this reason, industrial control systems normally employ a modular construction form in which one or more modular controllers are teamed with local or remote I/O modules and other control devices that provide the actual interface between the controller and the hardware of sensors and actuators driving the industrial process. 
     One type of input I/O module operates as an interface to receive an input signal from a sensor or the like, the sensor providing an output voltage in a first voltage range indicating a low state and an output voltage in a second voltage range indicating a high state, Generally, a low state may be represented by an input voltage between zero and 10 volts and a high state may be represented by an input voltage from 10 to 32 volts. 
     It is important to prevent high electrical voltages originating in the controlled equipment or process from damaging the industrial control system. For this reason, I/O modules normally include an isolator. One possible isolator is an optical isolator (optoisolator/optocoupler) which provides an electrically actuated light source (an LED) insulated and isolated from a photodetector (for example, a photo diode) the latter that communicates with the remainder of the industrial control system. The optical isolator provides for good common mode rejection (that is, canceling noise that is present on both the ground and input side of the I/O module) and protects the industrial control system from extremely large voltage inputs which may accidentally be applied to the inputs of the I/O module. 
     Different sensors attached to an I/O module may have different output voltage ranges with high state voltages ranging from as low as 5 volts to 30 volts or more. One way of accommodating this voltage range without exceeding the current capacity of the optical isolator is to use a series limiting resistor. Such an approach may require different limiting resistors for different input voltage ranges and thus the the fabrication of multiple I/O modules for different applications. 
     An alternative approach is disclosed in U.S. Pat. No. 6,043,703 to Bavol, assigned to the assignee of the present invention and hereby incorporated by reference, In this invention, a transistor-based, series current limiter  28  is placed between one input of the I/O module and the light source of the optocoupler to provide to limit the current to the optocouplers over the wide input voltage range. This active regulation of series current allows a single I/O module to work over a wide input voltage range. 
     In these prior art designs, resistance to electrical noise is provided by a noise limiting capacitor and shunt resistor placed in parallel across the optocoupler&#39;s input. The capacitor serves to bypass high-frequency noise signals, and a resistor and a capacitor serve to provide a load that tends to diminish low-power high-voltage spikes. 
     Modern control environments may require the processing of electrical signals from a sensor connected having extremely high switching rates, for example, operating at frequencies of up to four megahertz and requiring a pulse width measurement having accuracies within plus or minus 30 nanoseconds. It is difficult to obtain high frequency response over a large voltage range of input voltages while preserving immunity against electrical noise. 
     SUMMARY OF THE INVENTION 
     The present invention provides an I/O input module that works over a wide range of input voltages with good noise immunity and yet provides sensitivity to extremely short pulse widths. This is possible through the use of a two-step limiting approach which employs both a transistor-based current limiter in series with the optical isolator input and a transistor-based active voltage clamp bridging the input of the optical isolator. 
     Specifically, the invention provides an input circuit for an industrial control system having input terminals for receiving a sensed voltage and providing a current path between the input terminals. An optical isolator has an input LED connected in series along the current path between the input terminals so that current flow between the input terminals in a first direction illuminates the LED and an active current limiter is placed in series along the current path with the LED to limit current flow to the LED. An active current shunt is placed in parallel across the LED to limit current flow to the LED. 
     It is thus a feature of at least one embodiment of the invention to provide an input circuit that may respond to voltage pulses having a wide range of peak values while limiting the current through the LED of the optical isolator to prevent degradation of its switching speed. 
     The active shunt may provide at least one transistor operating to shunt current of the current path around the LED according to an input signal. 
     It is thus a feature of at least one embodiment of the invention to employ an active shunting that can provide high-speed shunting when high-voltage pulses are applied to the input terminals. 
     The input signal is a function of current passing through the LED. 
     It is thus a feature of at least one embodiment of the invention to provide a shunting that is responsive to current flow through the LED so that the LED is guaranteed to be properly forward-biased before voltage limiting. 
     The transistor may be biased to an off state when there is no current flow in the current path. 
     It is thus a feature of at least one embodiment of the invention to reduce unnecessary power dissipation in the input circuit allowing high circuit density and yet permitting the I/O module to be sealed against the environment without fans. 
     The transistor may limit the voltage across the LED to a constant predetermined value for a range of voltages across the input terminal. 
     It is thus a feature of at least one embodiment of the invention to ensure fast recovery time of the photo diode at a variety of operating inputs. 
     The active current limiter may also provide at least one transistor operating to control current along the current path to the LED according to an input signal received by at least one transistor. 
     It is thus a feature of at least one embodiment of the invention to provide active current limiting that permits the optical isolator to operate within a standard current range despite wide variations in input voltages. 
     The input signal may be a measure of current along the current path. 
     It is thus a feature of at least one embodiment of the invention to provide current-sensitive current limiting to ensure proper biasing of the LED of the optical isolator. 
     One or both of the transistors may be a depletion mode field effect transistor biased in the conduction when there is no current flow along the current path. 
     It is thus a feature of at least one embodiment of the invention to provide a normally on current path providing rapid response to measurement of leading edges of input pulses. 
     The active limiter may provide at least two series-connected transistors operating to control current along the current path. 
     It is thus a feature of at least one embodiment of the invention to permit reduced cost components that may divide the input voltages and dissipated power among separate packages without exceeding their specifications. 
     The LED may not be shunted by a capacitor greater than one picofarad connected between the first current limiter and the LED. 
     It is thus a feature of at least one embodiment of the invention to eliminate the need for a capacitor for noise rejection relying instead on a high-speed active shunting circuit thereby also improving the response rate of the optical isolator. 
     The input circuit may further include a diode shunting the current path and oriented to be back-biased when there is a flow of current along the current path activating the LED. 
     It is thus a feature of at least one embodiment of the invention to provide protection of the optical isolator and current limiting circuitry against reverse voltages applied to the terminals. 
     The input circuit may further include a surge limiter shunting the input terminals. 
     It is thus a feature of at least one embodiment of the invention to protect the I/O circuitry from over voltages of the correct polarity. 
     The input circuit may include a parallel connected diode and resistor in series along the current path to conduct with current flow in a direction of current flow along the current path activating the LED. 
     It is thus a feature of at least one embodiment of the invention to permit limited reverse current flow. 
     The optical isolator may provide a photodetector providing a signal to a Schmitt trigger circuit driving an output of the optical isolator. 
     It is thus a feature of at least one embodiment of the invention to limit noise sensitivity through the use of a Schmitt trigger circuit that also avoids the need for a shunting capacitor on the input of the optical isolator. 
     These particular objects 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 DRAWINGS 
         FIG. 1  is a simplified representation of an example of an industrial control system having a modular construction with an I/O module that may be installed in a housing or the like, the I/O module providing exposed terminals for communicating with an industrial control process and a rear connector for communicating with a common industrial control backplane; 
         FIG. 2  is a functional block diagram of the circuit of the I/O module of  FIG. 1 ; and 
         FIG. 3  is a detailed schematic diagram of the circuit of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , an industrial control system  10  may provide for a housing  12  or similar support that assembles together multiple modules  14  to communicate together on a common industrial control backplane  15  when so assembled. Modules  14  may generally include an industrial controller  14   a  executing a control program, a communication module  14   b , for example, communicating on an industrial control network (such as those using Common Industrial Protocols (CIP) such as EtherNet/IP, DeviceNet, and ControlNet), a power supply  14   c , and one or more I/O modules  14   e  to be discussed herein. 
     One I/O module  14  may be an input module  16  having a set of electrical terminals  18  on a front face of a housing  19 , the terminals  18  adapted to be connected to conductors  20  that may communicate with various sensors  22  and  24  of an industrial process  26 . A rear face of the housing  19  may provide an electrical connector  28  that may communicate with a corresponding connector on the backplane  15 . 
     It will be appreciated that backplane  15  may be alternatively contained partially within each module  14  and formed by interconnections via connector pairs (not shown) on each of the modules  14  connecting with adjacent modules when the modules  14  are assembled together. 
     Referring now also to  FIG. 2 , the input module  16  may include an input circuit  30  held within the housing  19  and providing electrical isolation to protect the industrial control system  10  from external high voltages. The input circuit  30  may receive, across a first and second terminal  18   a  and  18   b , one or more pulses  32  at input terminal  18   a  from an associated sensor and being either positive- or negative-going with respect to a ground reference at terminal  18   b . Desirably, the input circuit  30  may respond to a positive-going pulse from 4 to 32 volts occurring at frequencies as high as four megahertz and providing for a pulse-width detection accurate to plus or minus 30 nanoseconds. 
     A positive-going pulse  32  generates a current along a current path  39  received by an input of an optical isolator  34  which provides outputs  36  which may be communicated through a backplane interface to the backplane  15 . 
     Terminals  18   a  and  18   b  of the input circuit  30  are shunted by a surge protector  38  that limits the voltage difference between terminals  18   a  and  18   b . As shown in  FIG. 3 , the surge protector  38  is placed to bypass the current path  39  and may be a transient-voltage-suppression (TVS) diode  40  in parallel with a capacitor  41 . The TVS diode  40  will be rated at a voltage higher than the highest expected operating voltage of the input circuit  30 . 
     Following the TVS diode  40  may be a reverse current limiter  42  placed along the current path  39  and connected to the junction between terminal  18   a  and surge protector  38 . The reverse current limiter  42  may limit current passing along current path  39  in a reverse direction from terminal  18   b  to terminal  18   a , for example, when the pulse  32  is negative-going. This reverse current limiter  42  may comprise a diode  44  back-biased by reverse current along current path  39  and shunted by a resistor  46 . Forward current along current path  39  from terminal  18   a  and  18   b  passes through the diode  44  with a minor forward diode voltage drop while reverse current is blocked by the diode  44  and must pass through resistor  46  which limits this current. 
     Following the surge protector  38  and reverse current limiter  42  is a reverse voltage limiter  50  which shunts the current path  39  to allow the flow of current from terminal  18   b  through the reverse voltage limiter  50  backward through the reverse current limiter  42  to the terminal  18   a  thereby bypassing the remainder of the input circuit  30 . The reverse voltage limiter  50  may comprise two series-connected diodes oriented to be forward biased by current passing backward along the current path  39  from terminal  18   b  to terminal  18   a  and limiting the voltage difference across the reverse voltage limiter  50  (and thus the voltage applied to the remainder of the input circuit  30 ) to approximately two diode drops or approximately 1.4 volts. 
     Along the current path  39  from terminal  18   a , past the reverse voltage limiter  50 , is a forward current limiter  56  which is in series with the input of the optical isolator  34  along the current path  39 . The forward current limiter  56  thus operates to limit current to the input of the optical isolator  34 . The forward current limiter  56  may comprise two series-connected N-channel depletion JFET devices  58  and  60  where current passing along current path  39  passes into the drain and out of the source of device  58  and then into the drain and out of the source of device  60 . Current exiting the source of device  60  passes through a resistor  62  and then to the anode of a light emitting diode  64  providing input of the optical isolator  34 . A junction between the resistor  62  and the input of the optical isolator  34  is connected in parallel to the gates of devices  58  and  60 . This biasing by the voltage drop across resistor  62  causes devices  58  and  60  to be normally on with low current flows and then to be progressively turned off (to higher resistance values) as current flow increases. A shunting resistor  65  may be placed across the drain and source of device  58  to reduce heat dissipation from device  58 . The forward current limiter  56  allows the input circuit  30  to operate over a wide variety of different voltages at the terminals  18  without overloading the current capabilities of the input of the optical isolator  34 . 
     A second mechanism limiting the application of electrical power to the light emitting diode  64  of the optical isolator  34  is a shunting bipolar NPN transistor  66  which provides a shunt current limiter  67  creating a bypass path  68  around the optical isolator  34 . In one example, the emitter of transistor  66  communicates with the ground reference of terminal  18   b  and the collector of transistor  66  is connected through diode  70  to the output of the forward current limiter  56 . Diode  70  is oriented to allow current flow through the transistor  66  from collector to emitter. 
     The base of transistor  66  is connected to a voltage divider comprised of a resistor  72  passing from the output of forward current limiter  56  (and one input of the optical isolator  34 ) to the base of transistor  66 , and of resistor  74  passing from the base of transistor  66  to the ground reference of terminal  18   b . The light emitting diode  64  of the optical isolator  34  is connected across the ends of resistor  72  in a direction to conduct electricity passing forward along the current path  39 . A capacitor  76  of less than 1000 picofarads is connected across the ends of resistor  74  in order to speed up the response of the optical isolator. 
     It will be appreciated that the circuit of the shunt current limiter  67  limits the current through the light emitting diode  64  to less than that which would occur at a voltage equal to the sum of saturation voltage of transistor  66  and the forward diode drop of diode  70 . By clamping this voltage, the response time of light emitting diode  64  in turning off is greatly reduced for example, by limiting the charging of parasitic capacitances and the like. 
     Light emitting diode  64  transmits light to a photodetector  80  within the optical isolator  34  which communicates with Schmitt trigger circuit  82 . The Schmitt trigger circuit  82  provides hysteresis in the switching of the output  36  of the optical isolator  34 . An optical isolator  34  providing these features is commercially available from Toshiba of Japan under the trade designation TLP2361 and provides a high output, gallium aluminum arsenide light emitting diode coupled with a high gain high-speed photodetector. 
     In one embodiment, a stability capacitor  84  may be placed across the outputs  36  of the optical isolator  34 . 
     Certain terminology is used herein for purposes of reference only, and thus is not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “bottom” and “side”, describe the orientation of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import. Similarly, the terms “first”, “second” and other such numerical terms referring to structures do not imply a sequence or order unless clearly indicated by the context. 
     When introducing elements or features of the present disclosure and the exemplary embodiments, the articles “a”, “an”, “the” and “said” are intended to mean that there are one or more of such elements or features. The terms “comprising”, “including” and “having” are intended to be inclusive and mean that there may be additional elements or features other than those specifically noted. It is further to be understood that the method steps, processes, and operations described herein are not to be construed as necessarily requiring their performance in the particular order discussed or illustrated, unless specifically identified as an order of performance. It is also to be understood that additional or alternative steps may be employed. 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein and the claims should be understood to 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. All of the publications described herein, including patents and non-patent publications, are hereby incorporated herein by reference in their entireties.