JP5389614B2 - Process control device - Google Patents

Description

  The present invention relates to a process control apparatus compliant with a functional safety standard (IEC 61508) established by IEC (International Electrotechnical Commission), and more particularly to a technique for making process control highly reliable.

  Conventionally, in a process control device that requires high reliability, as shown in Non-Patent Document 1, a process control processing device and a process input / output device are combined into a multiplexed configuration, or a plurality of process input / output devices are provided. It was configured to be shared by all the process control processing units.

  Patent Documents 1, 2, 3, and 4 disclose techniques for multiplexing analog signal output devices (circuits) in order to make an analog signal output to a process highly reliable. In these known prior arts, two or more analog signal output devices (circuits) consisting of an active system (normal system) and a standby system are connected in parallel to a load controlled by an analog current signal, and each analog By controlling ON / OFF the output current supplied to the load from the signal output device (circuit), the output source is set so that the output current from one of the normal standby systems becomes valid when a failure occurs in the active system. Was switching. Furthermore, a technique is disclosed in which output currents from a plurality of analog signal output devices (circuits) are added and supplied to a load so that a certain level of output is maintained even when a failure occurs in some systems. Yes.

JP-A-5-199570 Japanese Patent Laid-Open No. 7-20903 JP 2000-22001 A JP 2003-29858 A

Hitachi Review Vol. 68-6 P18 Figure 5 (1986.6)

  However, the method of switching the output source from the active system to the standby system among the above-described prior arts, the output current once becomes 0 between the occurrence of a failure in the active system and the switching of the output source to the standby system. Then, since it suddenly recovered to the original level, there was a disadvantage that it was difficult to apply to a device having high responsiveness. Also, the method of adding all output currents from a plurality of output devices and outputting them has the disadvantage that the output after a failure cannot be recovered to the original level.

  The present invention has been made to eliminate such inconveniences, and is a process control capable of reducing fluctuations in output current when a failure occurs in a part of the output system and recovering it to the original level in a short time. It is an object to provide an apparatus.

  In order to solve the above problems, the process control apparatus of the present invention includes a multiplexed analog signal output circuit that outputs a current signal for controlling a controlled object, and is input to each analog signal output circuit. By comparing the digital output value with the result of analog conversion, the failure of the analog signal output circuit is detected, the failed analog signal output circuit is disconnected, and the current to be output to the controlled object A digital output value is evenly distributed and input to each analog signal output circuit so that a signal is generated by synthesizing current signals output uniformly from one or more analog signal output circuits that are not faulty. It is characterized by doing.

  ADVANTAGE OF THE INVENTION According to this invention, the process control apparatus which can reduce the fluctuation | variation of the output current when a failure generate | occur | produces in a part of output system, and can recover to the original level in a short time can be provided.

It is the conceptual diagram which showed the operating principle of the process control apparatus of this invention. It is the block diagram which showed the detailed structural example of the analog signal output system. It is a flowchart of an analog signal output processing program. It is the graph showing the change of the output current accompanying the failure of an analog signal output system. It is the block diagram which showed the detailed structural example of the analog signal input system. It is the block diagram which showed the structural example of the duplex process control apparatus. It is explanatory drawing which showed the operation example of the duplex process control apparatus. It is a flowchart of a fault diagnosis processing program for the active system. It is a flowchart of a failure diagnosis processing program for a standby system. It is explanatory drawing regarding the detection of the failure of an analog signal input system.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings as appropriate.

  FIG. 1 is a conceptual diagram showing the operation principle of the process control apparatus of the present invention. As shown in FIG. 1, the process control device 1 is based on, for example, an analog signal input circuit 3 for converting an analog process signal transmitted from a sensor 2 into a digital value and inputting it, and an input process signal. A digital controller 4 as a process control arithmetic unit for executing a predetermined process control operation and calculating an output value to the process, and according to an instruction from the digital controller 4, the digital output value is converted into an output current to be controlled. Multiplexed analog signal output circuits 5a, 5b and 5c for supplying a load 7 for controlling the target valve 8 are provided. In the following, an example in which the analog signal output circuit is tripled or duplexed will be mainly described, but the number of multiplexed analog signal output circuits is not limited to this. In FIG. 1, only one process signal is input / output. However, generally, a plurality of process signals are input and a plurality of process signals are output. In addition to analog signals, the process signals that are input and output include digital signals that are output to input contact signals and to control ON / OFF of relays and switches. Illustration of signals is omitted.

  FIG. 1 (a) shows an operation when all three output systems are normal in the process control apparatus 1 including three output systems configured by the tripled analog signal output circuits 5a, 5b, and 5c. It is shown. As shown in FIG. 1A, when all the output systems are normal, the digital controller 4 sends to each analog signal output circuit 5a, 5b, 5c an output current corresponding to the calculated final output value to the load 7. The output values are distributed evenly so that 1/3 of each value is output. Then, 1/3 currents converted into current signals by the analog signal output circuits 5a, 5b, and 5c are synthesized at the junction 6, and an output current corresponding to the final output value before distribution is applied to the load 7. Supplied.

  FIG. 1B shows the operation when one output system constituted by the analog signal output circuit 5a fails in the process control apparatus 1 in the state shown in FIG. The analog signal output circuit 5a in which the failure is detected is disconnected from the apparatus and the output is taken over by the remaining two normal output systems, as indicated by the “x” mark and the broken-line arrow in FIG. Become. In this case, the digital controller 4 outputs to each of the normal two analog signal output circuits 5b and 5c a value that is ½ each of the output current corresponding to the calculated final output value to the load 7. Then, evenly distributed output values are input, and ½ current obtained by converting them into current signals is synthesized at the junction 6 and supplied to the load 7.

  FIG. 1C shows the process control apparatus 1 in the state shown in FIG. 1B, in which another output system constituted by the analog signal output circuit 5c fails and two output systems fail. It shows the operation when The analog signal output circuits 5a and 5c in which a failure is detected are disconnected from the apparatus and output by an output system constituted by the analog signal output circuit 5b, as indicated by the “x” mark and the broken-line arrow in FIG. Will be taken over. In this case, the final output value from the digital controller 4 to the load 7 is input as it is to the analog signal output circuit 5b, and the result of converting it into a current signal is supplied to the load 7 alone.

  FIG. 2 is a block diagram showing an example of a detailed configuration of an analog signal output system constituted by analog signal output circuits. As shown in FIG. 2, the analog signal output circuit 5 includes an output signal control circuit 52 that controls an output signal in accordance with an instruction from the digital controller 4 transmitted via the input / output bus 9, and an output signal control circuit 52. The D / A (Digital / Analog) converter 53 that converts the digital output value instructed by the digital controller 4 relayed by the digital controller 4 into an analog DC current value and outputs the analog DC current value is output from the D / A converter 53. An A / D (Analog / Digital) converter 54 that converts an analog DC current value into a digital value and outputs the digital value, and a digital value output from the A / D converter 54 via the input / output bus 9 A readback signal input circuit 55 for reading the power, an insulated power supply 51 for supplying power to each circuit and an external load, and a switch circuit 57 for disconnecting the circuit , A switching control circuit 56 for controlling the switch circuit 57 in accordance with an instruction from the digital controller 4, a butt diode 58 for preventing a backflow of current from the outside, and an analog DC current value output from the D / A converter 53. And an output transistor 59 for supplying an output current to the load 7.

  A plurality of such analog signal output circuits 5 can be connected to the input / output bus 9, the output terminals of the butt diodes 58 of all the analog signal output circuits 5 connected to the input / output bus 9 and the collector of the output transistor 59. By connecting the terminals to each other, a current combining unit corresponding to the junction 6 in FIG. 1 is formed, and the load 7 is connected between them (between 6a and 6b). The contact of the switch circuit 57 is ON in the normal state in which the analog signal output circuit 5 is operating normally. As a result, a current signal as a result of adding up all the output currents of the output transistors 59 is obtained. , Supplied to the load 7.

  Further, the digital controller 4 compares the output value designated by itself to each analog signal output circuit 5 with the output value from the A / D converter 54 read from the readback signal input circuit 55, so that the analog controller 4 When the failure of the signal output circuit 5 is diagnosed and the failure is detected, the switching control circuit 56 is instructed to disconnect the output system, and the switching control circuit 56 turns off the contact of the switch circuit 57 for insulation. By disconnecting between the power source 51 and the butt diode 58 and between the D / A converter 53 and the output transistor 59, the output system is disconnected. The output system disconnected in this way is replaced with a spare circuit or repaired, and then reconnected again in accordance with an instruction from the digital controller 4.

  FIG. 3 is a flowchart of an analog signal output processing program executed by the digital controller 4 and embodying the above-described operation. Details of the analog signal output processing will be described below with reference to the flowchart of FIG.

  When the program is started, first, in step S1, the number of mounted analog signal output circuits 5 is stored in a variable n. In the next step S2, an analog output value X to be output to the controlled object is calculated, and in step S3, each mounted analog signal output circuit 5 is instructed to output X / n, and step S4. Then, the output value obtained as a result of the D / A conversion is read back (see FIG. 2) and compared with the designated output value to detect the presence / absence of a failure in each analog signal output circuit 5.

  In step S5, it is determined whether or not a failure has been detected in any of the analog signal output circuits 5. If no failure has been detected ("No" in step S5), the process returns to step S2 to Repeat the process.

  If a failure is detected ("Yes" in step S5), the analog signal output circuit 5 that detected the failure is disconnected in step S6, and then the value of the variable n is set to 1 in step S7. In step S8, it is determined whether or not the value of the variable n is 1 or more. As a result, if the value of the variable n is 1 or more (“Yes” in step S8), the process returns to step S3 to calculate the value to be output to each analog signal output circuit 5 using the updated value of the variable n. Then repeat the above process. If the value of the variable n is not 1 or more (“No” in step S8), the process is terminated after the device is switched to the manual operation mode in step S9. Note that the output current supplied from the backup circuit (not shown) after switching the device to the manual operation mode is maintained at the current value or programmed in advance so that the fail-safe operation is performed according to the characteristics of the controlled object. It is preferable to set the value specified by a tool or the like.

  Through the analog signal output processing described above, the digital controller 4 disconnects the output system in which the failure is detected, and distributes the output value evenly to the remaining normal output system to continue control.

  Next, with reference to FIG. 4, the transition of the change in the output current accompanying the failure of the analog signal output system will be described. FIG. 4 is a graph showing a change in the output current due to a failure of the analog signal output system. FIG. 4A shows a case where the output system is switched from the active system to the standby system according to the prior art. FIG. 4C shows a case where the output of the failed output system is supplemented by another normal output system according to the present invention. FIG. 4 (b) shows a case where the number of output systems is reduced from 3 systems to 2 systems, and FIG. 4 (c) shows a case where the number of output systems is reduced from 2 systems to 1 system. ing.

  In the prior art, when a failure of the power supply system occurs in the output system of the active system and the output system is switched from the active system to the standby system by a switch circuit or the like, as shown in the graph of FIG. The current supply from the standby system is started after the output current output to the controlled object once decreases to 0 between the failure and the start of the switching operation to the standby system. A settling time t1 depending on the response characteristics of the circuit is required until the output current reaches the original 100% level.

  On the other hand, according to the present invention, for example, when a failure occurs in one of the three systems and the operation is switched from the three systems to the two systems, as shown in FIG. Even if the output current from the faulty system becomes 0, the output current is continuously supplied from the remaining two systems, so the temporary decrease in the output current output to the controlled object Can be reduced to 1/3. Further, the settling time t2 for recovering the output current to the original level of 100% can be shorter than t1 by the amount of decrease in the output current.

  Further, according to the present invention, even when a failure occurs in the second system in the operation state with two systems and the operation is continued only with the remaining one system, FIG. As shown in FIG. 4, since the temporary decrease in the output current output to the controlled object at the time of the failure is ½ that of the prior art, the settling time t3 required for the output recovery is longer than t2. It can be shorter than t1.

  As is clear from the above, according to the present invention, the larger the number of multiplexed analog signal output systems (the number of multiplexed signals), the more temporary the output current when a failure occurs in one system. The time required for output recovery can be shortened while reducing the decrease. However, if the number of multiplexing is large, the apparatus becomes correspondingly large and the cost also increases. Therefore, it is preferable to select an appropriate number of multiplexing in consideration of the characteristics of the controlled object and the required level of safety.

  In the above description, for the sake of easy understanding, the digital output value from the digital controller 4 has been described as being equally distributed to each analog signal output circuit 5, but in practice, it is a standard in the field of process control. The direct current signal is defined so that 0 to 100% of the signal value corresponds to a current of 4 mA to 20 mA. Therefore, it is necessary to correct the bias value corresponding to 4 mA. For example, when a signal value of 100% is distributed and output by 1/3, approximately 27.1%, which is a signal value corresponding to 20/3 mA obtained by dividing 20 mA converted into a current value by 3, is obtained for each analog. The signal is input to the signal output circuit 5. Further, for example, if an output value of 0% (equivalent to 4 mA) is distributed and output by 1/2, -12.5% corresponding to 2 mA is input. Therefore, the analog signal output circuit 5 is capable of outputting a current from a minimum of 0 mA (corresponding to −25% input) to a maximum of 20 mA (corresponding to 100% input) over a wider input range than usual. Therefore, it is necessary to provide linear conversion characteristics.

  Subsequently, in order to make the process control apparatus according to the present invention more reliable, a method for multiplexing an analog signal input system and a digital controller will be described.

  FIG. 5 is a block diagram showing a detailed configuration example of the analog signal input system. As shown in FIG. 5, the analog signal input circuit 3 includes a switch circuit 31 for separating the sensor 2 that detects the state of the controlled object from the apparatus, and an analog signal transmitted from the sensor 2 via the switch circuit 31. A / D converter 32 for converting the digital value into a digital value and a sensor signal input circuit 33 for the digital controller 4 to read the digital value output from the A / D converter 32 via the input / output bus 9; And a switching control circuit 34 that controls ON / OFF of the contacts of the switch circuit 31 in accordance with an instruction from the digital controller 4. The contact of the switch circuit 31 is ON in a normal state where the analog signal input circuit 3 is operating normally. As a result, the digital controller 4 is converted into a digital value by the A / D converter 32. An analog signal from the sensor 2 can be read from the sensor signal input circuit 33 via the input / output bus 9.

  FIG. 6 shows a configuration example of a duplex process control apparatus in which all four of the sensor, the analog signal input system, the digital controller, and the analog signal output system are duplexed as an example of the process control apparatus multiplexing method. FIG. As shown in FIG. 6, the duplex process control device 1A includes two digital controllers 4a and 4b that are communicably connected to each other via a communication cable 10 and two analog signals that receive analog signals from different sensors 2a and 2b, respectively. The input circuits 3 a and 3 b and the two analog signal output circuits 5 a and 5 b are connected via an input / output bus 9.

  With such a configuration, each of the digital controllers 4a and 4b inputs and outputs to both the analog signal input circuits 3a and 3b and the analog signal output circuits 5a and 5b via the input / output bus 9. The redundancy control mode in the duplex process control device 1A can be arbitrarily changed by a redundancy control program executed by the digital controllers 4a and 4b.

  For example, in a normal operating state in which all input / output and control systems are normal, the digital controller 4a is operated as the active system and the digital controller 4b is operated as the standby system, and the analog signal input circuit 3a is operated by the active digital controller 4a. The output value calculated based on the sensor signal input from the sensor 2a via the output can be equally distributed to the analog signal output circuits 5a and 5b for output. Alternatively, the two output values separately calculated by the digital controllers 4a and 4b based on the signal from the sensor 2a and the signal from the sensor 2b, respectively, are halved, and the analog signal output circuit 5a You may make it output in parallel with 5b. Further, the digital controllers 4a and 4b may receive both the signal from the sensor 2a and the signal from the sensor 2b, and execute the process control calculation using the average value thereof.

  Next, the operation when a failure occurs in the duplex process control device will be described with reference to FIG. FIG. 7 is an explanatory diagram showing an operation example of the duplex process control device. Note that the duplex process control device 1B shown in FIG. 7 includes a single sensor 2 and an analog signal branch circuit 11 instead of the duplex sensors 2a and 2b in FIG. 6, and an analog signal transmitted from one sensor. The signal is branched into analog signal input circuits 3a and 3b and input in parallel.

  FIG. 7A shows an operation example of <normal operation> in which all input / output and control systems are normal. In the example of FIG. 7A, the digital controller 4a operating as the active system executes a process control calculation based on the sensor signal input from the active analog signal input circuit 3a, and outputs an output value to the load 7. The analog signal output circuits 5a and 5b are divided and output by 1/2 each. The standby digital controller 4b executes the same process control calculation based on the sensor signal input from the standby analog signal input circuit 3b to obtain an output value to the load 7, but the analog signal output circuit 5a. , 5b is not output.

  In FIG. 7B, in the state of FIG. 7A, an abnormality is detected in the analog signal input system from the sensor 2 to the active digital controller 4a, and the standby analog signal input circuit 3b is switched to the active system. In other words, an operation example after <switching the working analog signal input circuit> is shown. An input abnormality from the analog signal input circuit 3a, such as an input signal being interrupted or an illegal value being detected, is detected by the active digital controller 4a, and in accordance with an instruction from the digital controller 4a, the analog signal input circuit 3a Disconnection and switching of the analog signal input circuit 3b from the standby system to the active system are performed. Thereafter, the signal from the sensor 2 is input to the active digital controller 4a via the analog signal input circuit 3b and processed. Will continue.

  FIG. 7 (c) shows the state after the abnormality is detected by the self-diagnosis of the active digital controller 4a in the state of FIG. 7 (b) and the digital controller 4b is switched from the standby system to the active system. An example of operation after switching the system digital controller is shown. The abnormality of the active digital controller 4a detected by the self-diagnosis is transmitted to the standby digital controller 4b via the communication cable 10 (see FIG. 6), and the digital controller 4b immediately operates from the standby system to the active system. After the mode is changed, the processing is continued by the digital controller 4b. As shown in FIG. 7A, when the active digital controller 4a and 4b of the active system and the standby system are switched from a state in which the input system is different, a new active system is used. The digital controller 4b to be used continues to use the input from the input system (the input system on the analog signal input circuit 3a side in the example of FIG. 7A) that has been used for the process control calculation so far. It is preferable to reduce the fluctuation of the output signal due to the switching of the active digital controller by switching the input source.

  FIG. 8 is a flowchart of a failure diagnosis processing program executed at a predetermined cycle (for example, every 5 seconds) by the active digital controller. Hereinafter, the fault diagnosis process for the active system will be described in detail with reference to this flowchart.

  First, in step S11, diagnosis of an active analog signal input circuit is executed. In this diagnosis, the presence / absence of a failure is diagnosed by checking the interruption of the input signal from the working analog signal input circuit, comparison with predetermined upper and lower limit values, sudden change, and the like.

  In the next step S12, it is determined whether or not a failure has been detected by this diagnosis. If no failure has been detected ("No" in step S12), the process proceeds to step S15. If a failure is detected (“Yes” in step S12), it is determined in step S13 whether or not the standby analog signal input circuit is operating. If it is operating (in step S13). In step S14, the faulty active system analog signal input circuit is disconnected, the standby system is changed to the active system, and the process proceeds to step S15. Specifically, the broken analog signal input circuit is disconnected by switching the contact of the switch circuit 31 from ON to OFF via the switching control circuit 34 shown in FIG. The standby system analog signal input circuit is changed to the active system by changing the input source in the redundancy control program executed by the active digital controller.

  If the standby analog signal input circuit is not in operation (“No” in step S13), the apparatus is switched to the manual operation mode in step S19, and then the operation is stopped in step S20 to complete the process. To do. This is because it is no longer possible to capture normal input from the sensor.

  In step S15, a self-diagnosis is executed. In this self-diagnosis, a self-diagnosis program for detecting a hardware or software failure is executed. In the subsequent step S16, it is determined whether or not a failure has been detected by self-diagnosis. If no failure has been detected ("No" in step S16), the process ends. If a failure is detected (“Yes” in step S16), it is determined in step S17 whether or not the standby digital controller is operating. If it is operating (“Yes in step S17”). In step S18, after communicating with the standby digital controller via the communication cable 10 (see FIG. 6), the standby digital controller is changed to the active system, and in step S20, its own operation is performed. Stop and end processing.

  If the standby digital controller is not operating (“No” in step S17), the apparatus is switched to the manual operation mode in step S19, and then the operation is stopped in step S20 and the process is terminated. This is because it is no longer possible to continue normal control. Note that the output current after switching the device to the manual operation mode is either maintained at the current value or set to a value specified in advance by a programming tool or the like so that a fail-safe operation is performed according to the characteristics of the controlled object. It is preferable to do so.

  FIG. 9 is a flowchart of a failure diagnosis processing program executed by the standby digital controller at a predetermined cycle (for example, every 5 seconds). Hereinafter, the failure diagnosis process for the standby system will be described in detail with reference to this flowchart.

  First, in step S21, the standby analog signal input circuit is diagnosed. In this diagnosis, the presence / absence of a failure is diagnosed by checking the interruption of the input signal from the analog signal input circuit of the standby system, comparison with predetermined upper and lower limit values, sudden change, and the like.

  In the next step S22, it is determined whether or not a failure has been detected by this diagnosis. If no failure has been detected ("No" in step S22), the process proceeds to step S24. If a failure is detected (“Yes” in step S22), the failed standby analog signal input circuit is disconnected in step S23, and the active digital controller is connected via the communication cable 10 (see FIG. 6). After notifying the failure of the circuit, the process proceeds to step S24. Specifically, the broken analog signal input circuit is disconnected by switching the contact of the switch circuit 31 from ON to OFF via the switching control circuit 34 shown in FIG.

  In step S24, a self-diagnosis is executed. In this self-diagnosis, a self-diagnosis program for detecting a hardware or software failure is executed. In subsequent step S25, it is determined whether or not a failure has been detected by self-diagnosis. If no failure has been detected ("No" in step S25), the process is terminated. When a failure is detected (“Yes” in step S25), in step S26, the failure of the own system is notified to the active digital controller via the communication cable 10 (see FIG. 6), and then in step S27. Then, the operation is stopped and the process is terminated.

  Next, a method for detecting a failure in the analog signal input system will be described with reference to FIG. FIG. 10 is an explanatory diagram regarding the detection of a failure in the analog signal input system. As shown in FIG. 10, in a DC current signal that is typically used to represent an analog signal in the process control field, a range of 0 to 100% of the signal value is associated with a current value in the range of 4 to 20 mA. . Therefore, it is possible to detect the occurrence of an abnormality in the input system by determining whether or not the value of the input current is within a normal value range of 4 to 20 mA. Therefore, in the A / D converter 32 (see FIG. 5) of the analog signal input circuit 3, the input current from the sensor 2 is received in a range wider than the normal value range, for example, 0 to 22.8 mA, and is −25. By performing digital conversion to a signal value of ˜117.5%, the digital controller 4 determines that a failure has occurred in the input system if the value after digital conversion is outside the range of 0 to 100%. be able to.

  Alternatively, the analog signal input circuit 3 is provided with a circuit for determining whether or not the value of the input current is within a normal value range of 4 to 20 mA so that the analog signal input circuit 3 self-diagnose the failure. Also good.

  As described above, even if a failure occurs in one analog signal input system or digital controller by duplicating the analog signal input system or digital controller, the other analog signal input system or digital controller can be connected. Since it becomes possible to continue control using it, the operating rate of a process control apparatus can be improved. In addition, although the example of the duplex process control apparatus was demonstrated above, it is not restricted to this, Even if it is an apparatus with much larger multiplexing number, the improvement of a further operation rate can be implement | achieved similarly.

DESCRIPTION OF SYMBOLS 1 Process control apparatus 1A, 1B Duplex process control apparatus 2, 2a, 2b Sensor 3, 3a, 3b Analog signal input circuit 31 Switch circuit 32 A / D converter 33 Sensor signal input circuit 34 Switching control circuit 4, 4a, 4b Digital Controller (Process control operation unit)
5, 5a, 5b, 5c Analog signal output circuit 51 Isolated power supply 52 Output signal control circuit 53 D / A converter 54 A / D converter 55 Readback signal input circuit 56 Switching control circuit 57 Switch circuit 58 Butting diode 59 Output transistor 6 Junction point 7 Load 8 Valve (to be controlled)
9 I / O bus 10 Communication cable 11 Analog signal branch circuit

Claims (3)

A pulp process control device to output the current signal for controlling the controlled object,
One is an active system and the other is a standby system, and a redundant process control calculation unit that calculates an output value to be output to the controlled object;
A superimposed analog signal that outputs a current signal for controlling the controlled object corresponding to the output value of the process control arithmetic unit, and that is added to at least two outputs and multiplexed to the controlled object An output circuit;
One of the active system and the other operate as a standby system, and includes a redundant analog signal input circuit for inputting an analog signal used for process control calculation,
An active process control calculation unit that calculates an output value to be output to the controlled object,
A failure of the analog signal output circuit is detected by comparing the digital output value instructed to the analog signal output circuit with the result of analog conversion, and the analog signal output circuit detecting the failure is disconnected. With
It said current signal corresponding to the output value to be output to the controlled object, evenly SL before you have not been distributed failure as output analog signal output circuit or, et al., Each of the analog signal does not fail Instruct the output circuit to output an equal digital output value ,
When the active process control arithmetic unit becomes abnormal, the control is continued by switching one of the standby process control arithmetic units to the active system,
The active and standby analog signal inputs are characterized in that the analog signal input circuit continues the active and standby states when the standby process control operation unit is switched to the active system. Process control device. The current signal for controlling the controlled object is synthesized by connecting the output terminals of the butt diodes provided in the analog signal output circuit and the output terminals of the output transistors in parallel, respectively. Item 2. The process control device according to Item 1. The standby process control operation unit diagnoses the standby analog signal input circuit at a predetermined cycle, and disconnects the standby analog signal input circuit when a failure is detected. Notify the failure via the communication cable,
Furthermore, the standby process control operation unit performs a self-diagnosis, and upon failure detection, notifies the active process control operation unit of the failure,
The active process control operation unit diagnoses the active analog signal input circuit at a predetermined cycle, and determines whether the standby analog signal input circuit is operating when a failure is detected. If so, disconnect the failed active analog signal input circuit, change the standby analog signal input circuit to active,
In addition, the active process control calculation unit performs self-diagnosis and determines whether or not the standby process control calculation unit is operating when a failure is detected. The process control device according to claim 1 or 2, wherein the section is changed to a current one.

Images