Patent Description:
Industrial controllers are specialized computer systems used for the control of industrial processes or machinery, for example, in a factory environment. Generally, an industrial controller executes a stored control program that reads inputs from a variety of sensors associated with the controlled process and machine and, sensing the conditions of the process or machine and based on those inputs and a stored control program, calculates a set of outputs used to control actuators controlling the process or machine.

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 than conventional computers. The processors and operating systems are optimized for real-time control and are programmed with languages designed to permit rapid development of control programs tailored to a constantly varying set of machine control or process control applications.

Generally, the controllers have a highly modular architecture, for example, that allows different numbers and types of input and output modules to be used to connect the controller to the process or machinery to be controlled. As part of their enhanced modularity, industrial controllers may employ input and output modules or various other modules and devices dedicated to a particular type of electrical signal and function, for example, detecting AC or DC input signals or controlling AC or DC output signals. Each of these modules may have a connector system allowing them to be installed in different combinations in a housing or rack along with other selected modules or devices to match the demands of the particular application. Multiple or individual modules or devices may be located at convenient control points near the controlled process or machine to communicate with a central industrial controller via the control network.

Output modules provide an interface between a program executing on the industrial controller and the industrial equipment or devices which the program is intended to control. Instructions in the program may determine, for example, when an actuator is to turn on or off, when a relay is to open or close, or when a motor is to start or stop. The digital signal within the industrial controller is provided to the output module. The output module receives power for the output terminals from an external power source and selectively connects the power to individual output terminals according to the digital signals received from the control program in order to control operation of the external device.

As is known to those skilled in the art, the signal from the industrial controller is typically provided as a digital signal. The digital signal may be a logical zero and be at a ground, or common potential, for example, of zero volts (<NUM> VDC) or the digital signal may be a logical one and be at a high reference voltage, such as <NUM> VDC or <NUM> VDC. The logical voltage reference, however, is commonly a different voltage than the desired output voltage at the terminal. The desired output voltage may be +/- <NUM> VDC, +/- <NUM> VDC, +/- <NUM> VDC, or the like. Further, the total current requirements for all of the controlled devices connected to one output module typically exceeds the power capacity of the power supply provided for the logical circuit components. Thus, a separate power supply is often provided with one terminal of the output module configured to receive power from the power supply at the desired output voltage, where the power supply has a sufficient power rating to supply current to each controlled device at the desired output voltage. This power is selectively connected to individual output terminals by a switch where the switch for each output terminal is controlled responsive to the digital signal received for the corresponding output terminal.

Providing an external power supply to the output module to drive each of the external devices connected to that output module is not without certain drawbacks. A parallel circuit is established internally to the output module between the input terminal connected to the external power supply and each of the output terminals. Although the switch may selectively connect each output terminal to the power supply, when multiple switches are enabled and driving multiple output terminals in tandem, a conductive path is established between each of the driven output terminals. Under normal operation, this parallel operation is acceptable. If, however, one terminal experiences a high current due, for example, to an inrush current when an inductive load closes or to a short circuit condition, each of the output terminals being driven in tandem are connected to the circuit in which the high current is being conducted. A potential exists, for damage to other devices being controlled in tandem with the output terminal on which the high current is being conducted.

Thus, it would be desirable to provide an output module having multiple output terminals with electrical isolation between each of the terminals. <CIT> discloses an isolation module including a first input connector and a first output connector. The input connector is configured to receive an input power signal into the isolation module, and the output connector is configured to provide a voltage-clamped output power signal from the isolation module. The input power signal passes through the input connector <NUM> and is received at a fuse <NUM>. The fuse represents a structure configured to break in order to prevent excessive current from flowing further into the isolation module. An in-rush control circuit is configured to receive the input power signal through the fuse and to limit the current passing through the control circuit, such as when the isolation module is initially powered-on. A voltage limiting circuit is coupled to the output of the in-rush control circuit. <CIT> discloses Power Protection and Current Limiter Circuitry included in an upstream terminal. The circuitry provides a stable source of power to one or more downstream distribution terminals. Power Protection and Current Limiter Circuitry provides up to <NUM> VA from power supply <NUM> at a -<NUM> volts between the center tap of the isolation transformer and the center tap of isolation transformer connected to the return of the power supply. Thus, the voltage output of the -<NUM> volt power is carried to downstream distribution terminal by both conductors of a transmission line pair connected to the isolation transformer. <CIT> discloses protection circuitry which receives serial data along lines from a digital logic section for transmission on a serial link and receives data from the serial link to be transmitted to the digital logic section. The circuitry also receives a source of intrinsically safe power from the isolator. The data is next received by power control circuitry, which places limits on the voltage and current of the data and also boosts the band limited data in power appropriate for the communication speeds. This circuitry may include conventional amplification electronics receiving intrinsically safe power accompanied with current and voltage limiting circuits, i.e. shunting Zener diodes. The power control circuitry <NUM> is followed by a galvanic isolator which is a saturable core transformer providing further current limiting and isolation by virtue of the insulated transformer gap.

It is the object of the present invention to provide an improved current limiting system.

This object is solved by the subject matter of independent claim <NUM>.

The subject matter disclosed herein describes an output module for an industrial controller having multiple output terminals with electrical isolation between each of the terminals. The output module receives control signals via an internal bus, where the control signals indicate a desired output state for each of the output terminals. An external power supply is connected to the output module to supply power for each of the output terminals. Switching circuitry within the output module selectively connects the external power supply to the output terminals responsive to the control signals.

Electrical isolation is provided between each of the output terminals in order to prevent a spike in current at one terminal from adversely impacting devices connected at any of the other terminals. According to one aspect of the invention, the electrical isolation is provided by a transformer connected between the external power supply and each output terminal. A control circuit for each output terminal receives power from the isolated side of the transformer and controls a switch connected between the isolated side of the transformer and the output terminal to selectively connect the output of the transformer to the output terminal.

If a fault condition, switching condition, or other event occurs at the output terminal resulting in a current spike being drawn from the output terminal, the spike may cause the transformer to enter a shut-down mode in which it is unable to maintain the power supplied to the output terminal. In this shut-down mode, the voltage supplied from the transformer may drop such that it causes the control circuit to shut down. Shut down of the control circuit may result in an unintended disabling of the output terminal, and unintended shut down of the control circuitry may create an unsafe operating condition in a safety module or reduce availability of a fault tolerant module. Additionally, when the control circuit shuts down, the connection between the transformer and the output terminal may be broken, removing the current draw from the transformer. As the transformer exits the shut-down mode and the voltage level returns to a normal operating level, the control circuit is re-enabled and the terminal may be reconnected to the transformer. Reconnection of the output terminal to the transformer may re-establish a connection under the fault condition or cause another outrush spike of current to a load device at the terminal. Repeating this sequence may also result in high-frequency oscillations of current at the output terminal.

A current limiting circuit is provided between the isolated side of the transformer and the output terminal to prevent the voltage level output from the transformer from dropping enough to disable the control circuit. As a result, the control circuit remains on during periods of high current draw at an output terminal and may maintain the connection between the transformer and the output terminal, if desired. The control circuit may further be used to monitor the current flow output at each terminal and to generate a signal indicating a short circuit condition for reporting back to the industrial controller in which the output module is mounted.

According to one embodiment of the invention, an output circuit for an output module used in an industrial controller is disclosed. The output module includes multiple output terminals, and the output circuit includes a system side control circuit operative to receive multiple digital signals from the industrial controller, where each of the digital signals corresponds to a desired output signal at one of the terminals. For each of the output terminals, the output circuit further includes an isolation circuit, a terminal control circuit, and a current limit circuit. The isolation circuit includes an input configured to receive power from a power source and an output configured to provide power electrically isolated from the power source to the corresponding output terminal. The terminal control circuit is operative to receive power from the output of the isolation circuit, receive the digital signal for the corresponding output terminal, and selectively connect the output of the isolation circuit to the corresponding output terminal. The current limit circuit is operative to limit a current conducted between the output of the isolation circuit and the corresponding terminal to a predefined threshold.

According to another embodiment of the invention, an output circuit for an output module used in an industrial controller is disclosed. The output module includes multiple output terminals, and the output circuit includes multiple isolation circuits. Each isolation circuit corresponds to one of the output terminals and each isolation circuit is configured to receive power from a power source at an input to the isolation circuit and to provide electrically isolated power from an output of the isolation circuit to the corresponding output terminal. The output circuit also includes multiple terminal control circuits and multiple current limit circuits. Each terminal control circuit corresponds to one of the output terminals and is operative to selectively connect the electrically isolated power from the corresponding isolation circuit to the corresponding output terminal. Each current limit circuit corresponds to one of the output terminals and is operative to limit a current conducted between the output of the isolation circuit and the corresponding terminal to a predefined threshold.

According to still another embodiment of the invention, a method of limiting current in an output module used in an industrial controller is disclosed, where the output module includes multiple output terminals. A digital input signal is received for each of the output terminals. Each digital input signal defines a desired output state of the corresponding output terminal. Power is received from a power source at an input for multiple electrical isolation devices, where each electrical isolation device corresponds to one of the output terminals. Power is selectively supplied from an output of each of the electrical isolation devices to the corresponding output terminal. A switching device in a terminal control circuit receives the electrically isolated power from the output of the electrical isolation device and selectively supplies the power to the corresponding output terminal responsive to the corresponding digital input signal. A current present at each of the plurality of output terminals is monitored, and the current conducted between the output of the isolation circuit and the corresponding terminal is limited to a predefined threshold with a current limit circuit.

These and other advantages and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation.

Turning initially to <FIG>, an output circuit <NUM> for an output module used in an industrial controller includes multiple sub-circuits. According to the illustrated embodiment, the output circuit <NUM> includes a system control circuit <NUM> and a reverse protection circuit <NUM>. For each output terminal <NUM>, the output circuit <NUM> further includes an electrical isolation circuit <NUM>, a terminal control circuit <NUM>, a current limit circuit <NUM>, and a switching circuit <NUM>. The output circuit <NUM> is divided into the sub-circuits for ease of illustration and discussion, and the illustrated embodiment is not intended to be limiting. The illustrated embodiment is one embodiment of the invention and it is contemplated that various elements discussed with respect to one sub-circuit may be incorporated into another sub-circuit to achieve an identical function without deviating from the scope of the invention.

The illustrated system control circuit <NUM> provides an interface between the industrial controller and the output module. The system control circuit <NUM> may include, for example, a backplane connector operative to connect to a backplane such that the system control circuit <NUM> may receive digital signals <NUM> from a processor module or from other modules within the industrial controller. The processor module (not shown) may execute a control program to generate desired operation of devices controlled by the industrial controller. The control program generates digital signals <NUM> which are transmitted to the output module via the backplane and received by the system control circuit <NUM>.

The digital signals <NUM> may be passed to the output module in various forms. Discrete signals may be passed over dedicated channels on a data bus. Optionally, the data signals <NUM> may be included in a data packet and transmitted via the data packet over the backplane to the output module. The system control circuit <NUM> may include, for example, buffers to receive the data packets and a processor executing instructions to receive the data packet and extract the data signals. Optionally, the processor may perform some further processing on the data signals <NUM> prior to using the data signals to enable/disable individual output terminals <NUM> on the output module.

A data bus <NUM> is provided as an output from the system control circuit <NUM> to each of the output terminals <NUM>. Appropriate interface circuitry may be provided within a terminal control circuit <NUM> for each output terminal to extract control signals <NUM> from the bus <NUM>. Optionally, the bus <NUM> may consist of multiple individual traces on which each control signal <NUM> is separately conducted from the system control circuit <NUM> to the corresponding terminal control circuit <NUM> for each output terminal. For ease of illustration, two output terminals 24A, 24B, with associated control circuitry for each output terminal, are illustrated in <FIG>. It is contemplated that the output module may include a single output terminal <NUM> or may include still additional output terminals beyond the two illustrated terminals <NUM>. Output modules may be configured, for example, with eight (<NUM>) or sixteen (<NUM>) output terminals <NUM>, where the control circuitry for one output terminal is duplicated for each output terminal in the module.

With reference also to <FIG>, the terminal control circuit <NUM> for each output terminal <NUM> may include a microprocessor <NUM>. The microprocessor <NUM> is powered by a control voltage <NUM> received as an output from the isolation circuit <NUM>. The microprocessor <NUM> receives the control signal <NUM> as an input to indicate when the corresponding output terminal <NUM> is to be energized. A pair of outputs <NUM>, <NUM> are provided to the current limit circuit <NUM>, where a first output <NUM> indicates that the output terminal <NUM> is to be energized and a second output <NUM> indicates that the output terminal is to operate in a current limiting state. According to one embodiment of the invention, the control signal <NUM> and the control voltage <NUM> are the same signal. The system control circuit <NUM> may output the control signal <NUM> as an enabling voltage to the microprocessor <NUM>. The enabling voltage may both energize the microprocessor and indicate that the output terminal <NUM> is to be energized. When used as an enabling voltage and as a control signal <NUM>, the control voltage <NUM> may also be provided to the current limit circuit <NUM> as the first output signal <NUM>, indicating that the output terminal is to be energized.

A current sensor <NUM> is provided to generate a current feedback signal <NUM> corresponding to the current being conducted at the output terminal <NUM>. As illustrated in <FIG>, the current sensor <NUM> is incorporated in the terminal control circuit <NUM>. As illustrated in <FIG>, the current sensor (although not shown in <FIG>) is incorporated in the switch circuit <NUM> and provides the current feedback signal <NUM> to the terminal control circuit <NUM>. The current sensor <NUM> may be, for example, a current sensing resistor connected in series with the output terminal <NUM>. A voltage drop across the current sensing resistor may be measured and provided as the current feedback signal <NUM> which is input to the microprocessor <NUM>.

It is contemplated that the microprocessor <NUM> may be configured to receive the control signal <NUM> and the current feedback signal <NUM> as input signals, to execute stored instructions, and to generate the output signal(s) <NUM>, <NUM> responsive to the input signals. Optionally, the microprocessor may be a dedicated integrated circuit (IC) performing a single function, such as a comparison function, where the current feedback signal <NUM> is compared to a threshold and the second output signal <NUM> is generated when the current conducted at the output terminal <NUM> exceeds the threshold. According to still another embodiment of the invention, the microprocessor <NUM> may be a field programmable gate array (FPGA) or an application specific integrated circuit (ASIC) configured to generate the output signal(s) <NUM>, <NUM> responsive to the measured current feedback signal <NUM> and control signal <NUM>. A single IC may be provided for each output terminal <NUM> or one IC may incorporate multiple control circuits <NUM> to control operation of multiple output terminals <NUM>.

With reference also to <FIG>, the current limit circuit <NUM> is used, in combination with the control circuit <NUM> to control operation of the switching circuit <NUM>. The switching circuit <NUM> includes a field-effect transistor (FET) <NUM>, such as a metal-oxide-semiconductor field-effect transistor (MOSFET). The FET <NUM> is controlled in one of three operating states to determine the output of the corresponding output terminal <NUM>. In a first operating state, the FET <NUM> is disabled, preventing current conduction through the device and turning off the output terminal <NUM>. In a second operating state, the FET <NUM> is enabled, or operating, in a saturation mode, allowing "full" conduction through the FET and turning on the output terminal <NUM>. The first two operating states are considered the "normal" operating states for each output circuit. A third operating state is also provided for a "current limiting" operating state. The FET <NUM> is operated in a linear mode, where the voltage supplied to the gate is varied such that the FET operates as a variable resistor to vary the amount of current conducted through the FET.

In operation, the current limit circuit <NUM> outputs a switching signal <NUM> to the switching circuit <NUM> to select the desired operating mode of the FET <NUM> and, thereby limit the current output from the output terminal <NUM>. When the control signal <NUM> indicates the corresponding output terminal <NUM> is to be turned off, the control circuit <NUM> and the current limit circuit <NUM> operate in tandem to disable the FET <NUM> and operate in the first operating mode. When the control signal <NUM> indicates the corresponding output terminal <NUM> is to be turned on, the control circuit <NUM> and the current limit circuit <NUM> operate in tandem to supply a first voltage to the gate terminal of the FET, causing the FET to operate in the saturation mode. If the control circuit <NUM> detects a signal from the current sensor <NUM> indicating the current at the output terminal <NUM> exceeds a predefined threshold, the control circuit <NUM> and current limit circuit <NUM> operate in tandem to supply a second voltage to the gate terminal of the FET, causing the FET to operate in a linear mode, restricting the amount of current output from the terminal.

Power to drive external devices connected to each output terminal <NUM> is supplied to the output module by an external power source. The external power source may be a power supply providing, for example, <NUM> VDC, <NUM> VDC, <NUM> VDC or any other suitable voltage level which is connected to a first terminal <NUM> of the output module. This first terminal <NUM> is also referred to herein as a power supply terminal. The power source is configured to have a sufficient power rating to supply current at the desired output voltage for each of the output terminals <NUM> in the output module to which it is connected. A reverse protection circuit <NUM> may be provided to prevent damage to the output module if the external power source is erroneously connected to the output module with a reverse polarity.

Although the power for each output terminal <NUM> is provided from a single power source, the output module includes electrical isolation provided between output terminals to prevent a fault condition at one terminal from damaging devices connected at other terminals. An electrical isolation device <NUM> is provided for each output terminal between the power supply terminal <NUM> and the circuitry to control operation of the corresponding output terminal <NUM>. According to the embodiment in <FIG>, a first output terminal 24A includes a first isolation transformer 26A, and a second output terminal 24B includes a second isolation transformer 26B. Each isolation transformer <NUM> includes a primary winding electrically connected to the power supply terminal <NUM> and a secondary winding at which a control voltage <NUM>, which is electrically isolated from the power supply voltage, is provided. A DC-to-DC power converter <NUM> receives the control voltage <NUM> as an input and outputs one or more DC voltages, such as <NUM> VDC, <NUM> VDC, or the like, suitable to supply power to ICs, processors, or other electrical devices within the control circuit <NUM> for the corresponding output terminal <NUM>.

With the DC voltage from the output of the DC-to-DC power converter <NUM> enabling each control circuit 30A, 30B, the control circuit monitors the digital signal 17A, 17B received from the system control circuit <NUM> to set the respective output terminal 24A, 24B to the desired state as indicated by the digital signal. When the digital signal <NUM> is off, or set to a logical zero, both the first output <NUM> and the second output <NUM> from the control circuit <NUM> are off. As illustrated in <FIG>, the first output <NUM> from the control circuit is used to enable the first transistor <NUM> in the current limit circuit, and the second output <NUM> from the control circuit is used to enable the second transistor <NUM> in the current limit circuit <NUM>. Thus, when both the first and second outputs are off, the first transistor <NUM> and the second transistor <NUM> in the current limit circuit are similarly disabled. The output <NUM> of the current limit circuit is similarly off, which disables the FET <NUM> in the switching circuit <NUM>. When the digital signal <NUM> is on, or set to a logical one, the first output <NUM> from the control circuit is similarly on, or set to a logical one, which enables the first transistor <NUM> in the current limit circuit. The output <NUM> of the current limit circuit is set to a first voltage, which causes the FET <NUM> to enter the saturation mode. As previously indicated, the first output <NUM> may also be tied directly to the digital signal <NUM> such that the digital signal <NUM> is used both to enable the control circuit <NUM> and to enable the first transistor <NUM> in the current limit circuit <NUM>, thereby enabling the FET.

The control circuit <NUM> monitors the current feedback signal <NUM> from the current sensor <NUM> to control operation of the second transistor <NUM> in the current limit circuit <NUM>. According to one embodiment of the invention, the current feedback signal <NUM> is converted to a digital signal, either via the current sensor <NUM> or via an analog-to-digital (A/D) converter connected in series between the current sensor <NUM> and the processor <NUM>. The processor <NUM> receives the digital current feedback signal and compares the value to a preset threshold which may be stored in memory either within or in communication with the processor <NUM>. According to another embodiment of the invention, an external reference value may be provided to the processor <NUM> either digitally or via an analog signal which may be converted to a digital signal prior to comparison with the current feedback signal <NUM>. According to still another embodiment of the invention, the processor <NUM> may be a dedicated comparator circuit which receives the current feedback signal <NUM> and a reference signal as analog signals and compares the two signals. In any of these embodiments, the reference signal or stored value define a threshold for a maximum current value desired at the output terminal <NUM>. When the current present at the output terminal <NUM> (as indicated by the current feedback signal <NUM>) is less than the threshold, the second output of the control circuit <NUM>, as set by the processor <NUM> or comparator, is off or set to a logical zero. When the current present at the output terminal <NUM> is greater than the threshold, the second output <NUM> of the control circuit is on or set to a logical one by the processor <NUM> or comparator. The second output <NUM> from the control circuit <NUM> is, in turn, provided to the current limit circuit <NUM>.

The current limit circuit <NUM> uses the second output <NUM> to keep the current present at the output terminal <NUM> below a desired maximum value. A graphical representation of the operation of the current limit circuit <NUM> at one output terminal <NUM> is shown in <FIG>. At time t<NUM>, the control signal <NUM> for the output terminal is set. The first output signal <NUM> is set either by the processor <NUM> or directly via the control signal <NUM> to enable the first transistor <NUM> in the current limit circuit <NUM>. With the first transistor <NUM> enabled and the second transistor <NUM> disabled the FET <NUM> is enabled to operate in saturation mode. Under normal operation, illustrated between times t<NUM> and t<NUM>, an input voltage <NUM> is drawn from the isolation transformer <NUM> and is present at a first voltage level, V<NUM>. The output terminal <NUM> has an output current <NUM> and an output voltage <NUM> present at a first current level, I<NUM>, and an output voltage level, V<NUM>, respectively. At time t<NUM>, the current <NUM> at the output terminal <NUM> begins to increase. The increase may be caused by a change in load, short circuit, or other operating or fault condition. As the current <NUM> increases, the input voltage <NUM> drawn from the isolation transformer begins to increase as well. At time, t<NUM>, the output current reaches the maximum threshold. The control circuit <NUM> sets the second output <NUM> and the current limit <NUM> enables the second transistor <NUM>. Enabling the second transistor <NUM> causes the FET <NUM> to operate in a linear mode. The output current <NUM> remains constant, but the output voltage <NUM> is reduced to prevent further increase in the output current. Operating the FET <NUM> in this manner to limit the output current <NUM> also prevents the input voltage <NUM> drawn from the isolation transformer <NUM> from increasing further. As a result, the isolation transformer <NUM> remains active and is prevented from entering a shut down mode. As a further result, the control voltage <NUM> supplied to the DC-to-DC converter <NUM> also remains present, allowing the control circuit <NUM> to continue operation and to maintain control over the output terminal <NUM>.

Claim 1:
An output circuit for an output module used in an industrial controller, wherein the output module includes a plurality of output terminals (<NUM>), the output circuit comprising:
a plurality of isolation circuits (<NUM>), wherein each isolation circuit corresponds to one of the plurality of output terminals and each isolation circuit is configured to receive power from a power source at an input (<NUM>) to the isolation circuit and to provide electrically isolated power from an output of the isolation circuit to the corresponding output terminal;
a plurality of terminal control circuits (<NUM>), wherein each terminal control circuit corresponds to one of the plurality of output terminals and each terminal control circuit is operative to selectively connect the electrically isolated power from the corresponding isolation circuit to the corresponding output terminal, wherein each of the plurality of terminal control circuits further comprises a transistor operatively connected between the
output of the corresponding isolation circuit and corresponding output terminal configured to
selectively connect the output of the corresponding isolation circuit to the corresponding output terminal; and
a plurality of current limit circuits (<NUM>; <NUM>), wherein each current limit circuit corresponds to one of the plurality of output terminals and is operative to limit a current conducted between the output of the isolation circuit and the corresponding output terminal to a predefined threshold.