Abstract:
An I/O circuit for use with an industrial controller provides a zero-crossing detector circuit with low power dissipation through the use of a zero-crossing circuit that activates a light emitting diode of a photo coupler only for a very brief period of time at the zero-crossing (as opposed to at all times other than the zero-crossing). The circuit is coupled with a power supply circuit that uses a reactive element for voltage dropping as opposed to a resistive voltage drop element further reducing power consumption possible with the low power consumption of the photo coupler.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to I/O modules for industrial control systems and in particular to a low heat dissipation I/O module that may flexibly accommodate the digital control of AC power with zero-crossing detection. 
     Industrial controllers are specialized computer systems used for the control of industrial processes or machinery, for example, in a factory environment. Industrial controllers differ from conventional computers in a number of ways. Physically, they are constructed to be substantially more robust against shock and damage and to better resist external contaminants and extreme environmental conditions. The processors and operating systems are optimized for real-time control and execute languages allowing ready customization of programs to comport with a variety of different controller applications. Typically, the controllers have a highly modular architecture that allows different numbers and types of input and output modules to be used to connect the controllers to the process or machinery to be controlled. This modularity is facilitated through the use of special “control networks” suitable for highly reliable and available real-time communication. Such control networks (for example, Ethernet IP) differ from standard communication networks (e.g. Ethernet) by guaranteeing maximum communication delays, for example, as obtained by pre-scheduling the bandwidth of the network, and/or providing redundant communication capabilities to high-availability. 
     As part of their enhanced modularity, industrial controllers may employ I/O modules dedicated to a particular type electrical signal and function, for example, detecting input AC or DC signals or controlling output AC or DC signals. Each of these I/O modules may have a connector system allowing them to be installed in different combinations in a housing or unit along with other selected I/O modules to match the demands of the particular application. Multiple units may be located at convenient control points near the controlled process or machine to communicate with a central industrial controller via a special control network such as Ethernet IP. 
     Normally different I/O modules are required for the control of AC and DC signals reflecting differences in the control devices used for the signals (e.g. triacs versus transistors). Different I/O DC modules may be required depending on whether the DC signal is “sourced” from the DC output module or “sinked” into the DC output module. 
     The circuitry of the I/O module communicating with the industrial controller (controller-side) is typically optically isolated from the circuitry communicating with the controlled process or machine (field-side) in order to prevent faults in the field-side from damaging the industrial control system or other I/O modules. For output circuits (such as AC output or DC output circuits) it can be necessary to provide a source of low voltage DC for operation of the field-side circuitry independent of the power available on the controller-side. Often this is done through the use of a voltage dropping resistance converting the high voltages controlled by the I/O module into a lower voltage suitable for powering the field-side circuitry. Such voltage dropping resistors are often physically large to handle the necessary power dissipation and must be spaced apart to aid in dissipating the incident heat. 
     The control of AC voltages is often provided by a triac or thyristor that may be switched at a zero-crossing of the AC signal (to reduce electrical switching noise and power dissipation) by a conventional trigger circuit relying on a phase shifted version of the AC signal being controlled. 
     SUMMARY OF THE INVENTION 
     The present invention provides a zero-crossing detector for use with an I/O module providing phase control of AC signals, the zero-crossing detector having substantially reduced power dissipation. A transistor circuit provides for activation of an optical isolator for only a brief period of time near the zero-crossing of the AC waveform, substantially reducing the power dissipated by this relatively high current element. Reduced current demand allows an impedance (rather than resistance) limited power source avoiding the power dissipation attendant to voltage dropping resistors normally used to moderate the voltage of the AC waveform for use by the transistor circuitry. Reduced power dissipation allows significantly smaller components with closer spacing for more compact I/O module. 
     More specifically, one embodiment of the present invention provides an I/O module circuit for use in an industrial control system providing a housing with terminals supported by the housing for receipt of electrical power to be controlled by the I/O module circuit. A circuit contained in the housing provides a transistor switch element having an input connected to a terminal for receiving an AC waveform therefrom and having an output providing a zero-crossing signal indicating a voltage of the AC waveform within a first and second predetermined voltage threshold near zero voltage of the AC waveform. An optical isolator having a light emitting diode is connected to the transistor switch element to switch on and off according to the zero-crossing signal and having a photosensitive solid-state switch receiving light from the LED to provide an optically isolated zero-crossing signal wherein the zero-crossing signal from the transistor switch element provides conduction of the LED only during times when the AC waveform is between the first and second voltage threshold. 
     It is thus a feature of at least one embodiment of the invention to provide the conduction of the LED for a small fraction of the AC cycle thereby substantially conserving electrical power. 
     The transistor switch element may provide a duty cycle of less than five percent for AC waveforms in the range of 120 volts AC to 240 volts AC. 
     It is thus a feature of at least one embodiment of the invention to provide a more compact I/O circuit by limiting the need for heat dissipation over a range of expected voltages. 
     The transistor switch element may include a first transistor that conducts when the AC waveform is above a first threshold and a second transistor that conducts when the AC waveform is below the second threshold and a third transistor communicating with the first and second transistors to conduct only when neither the first or second transistor is conducting. 
     It is thus a feature of at least one embodiment of the invention to provide a relatively simple circuit for defining a narrow range near zero voltage for AC zero voltage detection such as normally might require more complex voltage comparator circuits. 
     The first transistor may be an NPN transistor having a base connected to the AC waveform and the second transistor may be a PNP transistor having a base connected to the AC waveform and wherein the NPN transistor and PNP transistor are connected in parallel to each other and in series with a resistance to a voltage provided by the power circuit and wherein the junction between the resistance and the parallel connected NPN and PNP transistors is connected to the base of the third transistor which controls current through the LED of the optical isolator. 
     It is thus a feature of at least one embodiment of the invention to make use of the intrinsic turn on characteristics of bipolar transistors to provide a low voltage detection threshold. It is a further feature of at least one embodiment of the invention to make use of NPN and PNP transistors to provide for the necessary two different thresholds defining a range near zero voltage. 
     The transistor switch element may be connected directly to the AC waveform through a resistance without rectification. 
     It is thus a feature of at least one embodiment of the invention to eliminate the need for a rectifier bridge or the like to define multiple voltage thresholds, for example, by a full wave rectifier followed with a single threshold detector. 
     The I/O module circuit may further include a microprocessor for receiving the optically isolated zero-crossing signal and a switch element receiving a control signal from the microprocessor to control a switching of the AC waveform across the terminals. 
     It is thus a feature of at least one embodiment of the invention to make use of the microprocessor circuitry normally found in an I/O module to replace the phase delay circuitry often used for triac control. 
     The microprocessor may provide a switch signal at a predetermined phase delay with respect to isolated zero-crossing signal according to a phase control signal received by the microprocessor 
     It is thus a feature of at least one embodiment of the invention to provide for flexible AC control not limited to zero-crossing control. 
     The I/O module circuit may further include a power circuit receiving the AC waveform to rectify the AC waveform to provide power to the transistor switch element. 
     It is thus a feature of at least one embodiment of the invention to provide power to the necessary zero-crossing detection circuitry from the AC waveform being controlled. 
     The power receiving circuit may include a series connected nonresistance impedance limiting current to the power circuit. 
     It is thus a feature of at least one embodiment of the invention to eliminate power dissipation inherent in resistive voltage dropping systems. 
     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 perspective view of a remote I/O unit of a type suitable for use with the present invention showing a housing having multiple I/O modules contained therein for communication with an industrial process via terminals and a remote industrial controller via a network cable; 
         FIG. 2  is a block diagram showing the I/O unit of  FIG. 1  in the context of the industrial control system incorporating multiple remote I/O units and an industrial controller; 
         FIG. 3  is a block diagram of the remote I/O unit showing connectors for attaching I/O modules installed in the I/O unit to a common bus communicating with the network cable and electrical power, and showing one I/O module providing a power control circuit and a zero-crossing circuit controlled by a micro-controller; 
         FIG. 4  is a detailed schematic of the zero-crossing circuit of  FIG. 3  such as may receive an AC waveform from the output terminals to provide an isolated signal through an optical isolator; 
         FIG. 5  is aligned graphs showing the AC waveform and the switching of the LED of the optical isolator for a short period of time only near the zero-crossing of the AC waveform; 
         FIG. 6  is a plot of the state of switching of the LED of the optical isolator versus AC waveform voltage for the circuit of  FIG. 4 ; and 
         FIG. 7  is aligned graphs showing a change in the width of the pulse of  FIG. 6  with different amplitudes of AC waveforms allowing indirect amplitude monitoring. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIGS. 1 and 2 , an I/O block  10  may provide for a unit housing  12  having multiple openings  14  for orienting and supporting corresponding replaceable or permanently installed I/O modules  16 . The I/O modules  16  generally provide an electrical interface between an industrial controller  17  and an industrial process or machine  18 , the latter having actuators presenting loads  20  as powered by external field-side power sources  22  which may be AC sources (as shown) or DC sources as will be described. Each of the I/O modules  16  may have exposed screw terminals  24  for connecting circuitry of the I/O module  16  via electrical conductors  26  to the load  20  and field-side power source  22  of the industrial process or machine  18 . 
     The housing  12  of the I/O block  10  may be supported, for example, on a DIN rail  29  of a type known in the art or may provide for another mounting arrangement allowing it to be located close to the desired controlled loads  20 . The I/O block  10  may connect via a control network  28 , for example ControlNet or Ethernet IP, to industrial controller  17  for communication of control data between the I/O modules  16  of the I/O block  10  and the industrial controller  17 . 
     Generally, the industrial controller  17  will include at least one processor  30  executing a stored control program  32  contained in a communicating memory  34  to provide for communication with the control network  28  to exchange signals with the I/O block  10  for control of the industrial process or machine  18  according to a stored control program  32 . The stored control program  32  reads signals indicating the state of the industrial process or machine  18  (from sensors or the like) and writes outputs to the I/O block  10  to control the loads  20  according to those sensed inputs and the logic of the control program  32 . The industrial controller  17  may include a connection to a user interface terminal  35  (for example including a display, computer, keyboard, and mouse or the like) allowing programming and other control of the industrial controller  17  and providing for output of information to an operator. Industrial controllers  17  suitable for use with the present invention include, for example, programmable controllers commercially available from Rockwell Automation, Inc. 
     Generally, the I/O modules  16  of each I/O block  10  include an electrical isolation barrier  36  between the electrical power and circuitry of the industrial process or machine  18  and directly connected portions of the I/O module  16 , and the circuitry of the industrial control system including the industrial controller  17 , the network  28 , the I/O blocks  10  and remaining portions of the I/O modules  16 . Electrical isolation, as understood herein, refers to the portions of an electrical circuit and do not communicate by a DC electric path within the I/O block  10 . Circuitry on the side of the electrical isolation barrier  36  toward the industrial process or machine  18  will be termed “field-side” circuitry and circuitry on the side of the electrical isolation towards the industrial controller  17  will be termed “controller-side” circuitry. Electrical isolation, as is understood in the art, may be accomplished by an optical isolator or transformer that prevents DC conduction through the electrical isolation barrier  36  to prevent damage to the controller-side circuitry in the event of fault on the field-side circuitry. This isolation prevents simple sharing of electrical power between the circuits on the controller-side in the field-side by conductors spanning the isolation barrier  36 . 
     Referring now to  FIG. 3 , each I/O block  10  may provide a backplane  40  providing a set of electrically releasable or permanently connected electrical connectors  42  communicating with a backplane bus  44  providing parallel or serial digital communication and electrical power. Electrical power may be provided by a power supply  43 . The digital communication on the backplane bus  44  may be moderated by a unit controller  46  including network circuit  45  communicating with the industrial control network  28 . The unit controller  46  and network circuit  45  may be, for example, one or more electronic processors executing a stored program stored in memory or the like. 
     The electrical connectors  42  forming the backplane bus  44  may connect with corresponding electrical connectors  47  positioned at the rear of each I/O module  16  allowing electrical connection between controller-side circuitry of the I/O block  10  and the controller-side circuitry of the I/O module  16  for directly communicating data and power therebetween when an I/O module  16  is inserted into the I/O block  10 . Within each I/O module  16 , a module controller  48 , for example a microcontroller, may execute a stored program  50  to coordinate functions of the I/O module  16  by communicating with multiple output circuits  52  and corresponding multiple zero-crossing detector circuits  54 . Each of these output circuits  52  and zero-crossing detector circuits  54  may provide internally for an electrical isolation barrier  36  between controller-side and field-side circuitry as will be described. Each of these output circuits  52  and zero-crossing detector circuits  54  also communicate with a shared set of terminals  24  (only three shown for clarity) that may, for example, be screw type terminals releasably receiving electrical conductors  26 . 
     As will be understood in the art, the zero-crossing detector circuit  54  provides a detection of zero-crossing of the waveform applied to the terminals  24  connected across an AC power line (the outer two terminals  24  as depicted) to produce a zero-crossing signal  55  transmitted to the module controller  48  and indicating the timing of the zero-crossing of AC waveform on the terminals  24 . 
     The module controller  48  receiving this zero-crossing signal  55  provides a switch state signal  56  for each output circuit  52  indicating a desired control of electrical power at an output terminal  24  (the center terminal  24  as depicted) based on the switch state signal  56 . In one mode of operation, conduction between output terminal  24  and one other terminal  24  is changed by output circuit  52  via the switch state signal  56  either by opening or closing the circuit between output terminals  24  and a line terminal  24  only at the zero-crossings of the AC waveform to: (1) reduce power dissipation in the internal switching elements used to control power conducted through the output terminals  24 , (2) accommodate the architecture of the switching element characteristics (for example when a triac is used), and/or (3) reduce the generation of electromagnetic interference. 
     Alternatively, in a different mode of operation, the module controller  48  may receive a switch command signal, for example, from the backplane  40  as communicated from the controller  17 , commanding a switching of conduction between terminals  24 ′ and  24  at times other than zero-crossings of the AC waveform, for example, to provide for duty cycle control of the AC waveform. This latter mode of operation may control, for example, average current for a resistive heating element more precisely or may be used to deal with reactive loads. This phase offset control of the switching with respect to the zero-crossing signal may be implemented by appropriate instructions in the stored program  50 . 
     Referring now to  FIG. 4 , the zero-crossing detector circuit  54  may receive an AC waveform  60  across the terminals  24 , for example a ground reference  62  at one terminal  24 ′ and the AC waveform at terminal  24 . The AC waveform will, in one embodiment, be at approximately 60 Hz and have a voltage between 120 and 240 volts AC. 
     The terminal  24  receiving the AC waveform may connect to an impedance element  64  comprising a series connected inductor  66  and capacitor  68  which in turn connects to the cathode of the zener diode  70  having its opposite terminal connected to ground  62 . 
     The zener diode  70  may have a relatively low zero zener voltage (e.g. five volts) providing a voltage drop across the impedance element  64  of substantially the entire voltage of the AC waveform  60  when the zener diode  70  is conducting. For this reason, impedance element  64  must be sized to provide this necessary voltage drop and to limit current flow through the zener diode  72  and the acceptable value for that component, typically a few hundred milliamps or less. It will be understood that a reactive impedance element  64  may block current flow without dissipating substantial energy in contrast to a blocking resistance so that substantially no heat is generated in this current limiting process. 
     The junction between the impedance element  64  and the zener diode  70 , being at the low zener regulated voltage, may connect to the anode of a rectifying diode  72  whose cathode connects to a filter capacitor  78 , the latter connected between the cathode and ground. The combination of the rectifier  72  and the filter capacitor  78  provide at their junction a source of regulated DC power to resistor  74  and resistor  76 , both connected to that junction. 
     Resistor  74  passes to the anode of a light emitting diode (LED)  80  being part of an optical isolator  83 . The cathode of the LED  80  connects in turn with the collector of an NPN switching transistor  82  whose emitter is connected to ground  62 . When the switching transistor  82  is turned on, the LED  80  is illuminated providing optical signal  84  to phototransistor  86  of the optical isolator  83  finding the isolation barrier  36 . The phototransistor  86  may be output as the zero-crossing signal  55 , as described above, leading to module controller  48  shown in  FIG. 3 . 
     Resistor  76 , described above and also connected to the source of regulated DC power at the junction of rectifier  72  and filter capacitor  78 , provides regulated DC power to the emitter of a first PNP transistor  90  and a collector of a second NPN transistor  92 . The collector of PNP transistor  90  and the emitter of NPN transistor  92  are both connected to ground and the bases of the transistors  90  and  92  are joined to a high impedance resistance  94 , the other end of which is attached to the terminal  24  receiving the AC waveform  60 . This resistance  94  provides a trigger signal that is in phase with the AC waveform  60 , avoiding the phase shift of the impedance element  64 . 
     The emitter of PNP transistor  90  and collector of NPN transistor  92  also join with the base of switching transistor  82  to provide a base voltage  91  such that switching transistor  82  will turn on (conduct current between its collector and emitter) only when both transistors  90  and  92  are turned off (block current between collector and emitter) and will conversely be turned off when either of transistors  90  and  92  are turned on. It will be further understood that when the PNP transistor  90  and NPN transistor  92  are turned off, the emitter of PNP transistor  90  and collector of NPN transistor  92  will be retained at substantially 0.7 volts being the base to emitter forward bias voltage of transistor  82  when both of transistors  90  and  92  are off. 
     Referring now also to  FIGS. 5 and 6 , when the AC waveform  60  connected between terminals  24  and  24 ′ is at a voltage below zero volts, PNP transistor  90  will be turned on pulling the base voltage  91  of switching transistor  82  to a low-voltage turning the switching transistor  82  off and stopping conduction of current through the LED  80 . 
     When the AC waveform  60  is between approximately zero and 0.7 volts, PNP transistor  90  will turn off and NPN transistor  92  will likewise be turned off again for lack of sufficient forward biasing of its emitter junction. At this time, the base voltage  91  of transistor  82  rises sufficiently to turn on switching transistor  82  and LED  80 . 
     When the AC waveform  60  rises above approximately 0.7 volts, PNP transistor  90  has its emitter back-biased and turns off but the emitter of NPN transistor  92  is forward biased to turn on, again pulling base voltage  91  close to ground by the saturation voltage of NPN transistor  92  and turning off switching transistor  82 . 
     Accordingly it will be understood that the switching transistor  82  and the LED  80  are off for most of the cycle of the AC waveform substantially conserving power. Typically the duty cycle of the base voltage  91  and hence of the LED  80  will be less than 10 percent and more typically much less than one percent of the period of the AC waveform  60 . This reduced duty cycle provides less current drain of the power source provided by the impedance element  64 , rectifier  72 , and filter capacitor  78  allowing these components to be comparably smaller and permitting a low current zener diode  70  to be used. 
     Referring now to  FIG. 7 , it will be understood that the width of the pulse of waveform  91  and hence of the output zero-crossing signal  55  can be used to determine a coarse voltage level of the AC waveform  60  within AC waveform. For example, when the AC waveform  60  is 240 volts, the zero-crossing will be steeper resulting in a relatively narrow pulse  91  measured in absolute time. Conversely, when the AC waveform  60 ′ is a relatively lower voltage, for example 120 volts, the zero-crossing will be slower extending the pulse of base voltage  90 ′. In this way, operation of the I/O module at 120 or 240 volts may be determined and other simple voltage measurements may be made. 
     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. 
     References to “a microprocessor” and “a processor” or “the microprocessor” and “the processor,” can be understood to include one or more microprocessors that can communicate in a stand-alone and/or a distributed environment(s), and can thus be configured to communicate via wired or wireless communications with other processors, where such one or more processor can be configured to operate on one or more processor-controlled devices that can be similar or different devices. Furthermore, references to memory, unless otherwise specified, can include one or more processor-readable and accessible memory elements and/or components that can be internal to the processor-controlled device, external to the processor-controlled device, and can be accessed via a wired or wireless network. 
     Co-pending patent applications entitled Input Circuit For Industrial Control With Low Heat Dissipation Ser. No. 13/474,351 and Output Module For Industrial Control With Sink And Source Capability And Low Heat Dissipation Ser. No. 13/473,711 filed on even date herewith and assigned to the same assignee as the present invention are hereby incorporated by reference in their entirety. 
     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.