JP4775693B2 - 2-wire transmitter - Google Patents

Description

  The present invention relates to a two-wire transmitter that can reliably perform burnout on the lower limit side in a two-wire transmitter that supplies power and transmits signals using two transmission lines.

  Since the two-wire transmitter transmits both the power source of the transmitter and the 4-20 mA current output signal through two transmission paths, the current output signal cannot be reduced below the current consumption of the transmitter. . In the transmitter, in order to indicate that a sensor or the like has failed, a lower limit side burnout is performed to reduce the output value to 0% or less. However, since the current output cannot be made lower than the lower limit of the transmitter in the two-wire transmitter, there is a problem that it is difficult to perform burnout on the lower limit side.

  Patent Document 1 describes a two-wire transmitter that solves this problem. The present invention will be described below with reference to FIG. In FIG. 7, a part of the portion not related to the gist of the invention is omitted.

  In FIG. 7, the output of the sensor 11 is input to the signal processing circuit 12 and converted into a pulse width signal. This pulse width signal drives the switch SW1. Since the switch SW1 switches between the reference power sources Vr1 and Vr2, the output is a pulse width signal in which the voltages on the high level side and the low level side are constant. This pulse width signal is input to the filter circuit 17 via the switch SW2, converted into a voltage signal, and input to the current output circuit 18. The current output circuit 18 controls the current flowing through itself so that the current flowing through the transmission line 19 is proportional to the output of the sensor 11.

  The monitoring circuit 13 monitors the operation of the signal processing circuit 12 and activates the bar-out signal BOS when an abnormality occurs. As a result, the switch SW2 is driven, and the voltage input to the filter circuit 17 is switched to the output of the reference power supply Vr3. As a result, burnout to the lower limit side is performed, but if the current consumption of the transmitter is large, the burnout signal cannot be output accurately.

  The voltage V1 supplied from the transmission line 19 is stabilized by the constant voltage circuits 15 and 16. The output voltage of the constant voltage circuit 15 is set so as to be larger than the output voltage of the constant voltage circuit 16. When the burnout signal BOS is not active, the FETs in the constant voltage circuit 16 are turned on by the inverters INV1 and INV2, and when the burnout signal BOS is active, the FETs in the constant voltage circuit 15 are turned on. The outputs of these constant voltage circuits 15 and 16 are input to the switching power supply 14. The switching power supply 14 supplies power to the sensor 11, the signal processing circuit 12, and the monitoring circuit 13.

  That is, the switching power supply 14 is supplied with power from the constant voltage circuit 16 when the burnout signal BOS is not active, and is supplied from the constant voltage circuit 15 when the burnout signal BOS is active. The output voltage of the constant voltage circuit 15 is set to be larger than the output voltage of the constant voltage circuit 16, and the efficiency of the switching power supply 14 does not change depending on the input voltage. Therefore, the current consumption of the transmitter when the burnout signal BOS is generated is reduced, and the lower limit burnout can be output accurately.

  Patent Document 2 discloses a two-wire transmitter that stops a microprocessor and performs a lower limit side burnout when a power supply voltage becomes a predetermined value or less, and can prevent output hunting. A type transmitter is described.

  Since a two-wire transmitter is usually connected with a receiving resistor in series, the power supply voltage supplied to the transmitter varies depending on the value of the output current. When the burnout is performed to the lower limit side because the power supply voltage has decreased, the voltage drop due to the receiving resistance is reduced and the power supply voltage is increased. Therefore, when the burnout is canceled and the output returns to the normal output, hunting that repeats the operation of decreasing the power supply voltage of the transmitter may occur again.

  In order to prevent this hunting, the idea described in Patent Document 2 is such that the difference between the power supply voltage for starting burnout and the power supply voltage for releasing is made larger than the fluctuation range of the power supply voltage due to the burnout operation. It is a thing. Thereby, output hunting can be prevented.

JP-A-6-60287 Japanese Utility Model Publication No. 6-25073

  However, such a two-wire transmitter has the following problems. The invention described in Patent Document 1 can surely generate the lower limit side burnout signal, but requires two constant voltage circuits and requires a switching power supply, so that the configuration is complicated. was there.

  Further, although the device described in Patent Document 2 can prevent hunting of the output current, restrictions are imposed on the power supply voltage for starting the burnout signal and the power supply voltage for releasing the burnout signal, so these voltages can be set freely. There was a problem that could not be done.

  It is conceivable that the current consumption increases due to erasure of the flash ROM built in the two-wire transmitter, the runaway of the microprocessor, or the increase of the leakage current, and the lower limit side burnout cannot be output normally. The invention described in Patent Document 2 has a problem that it cannot sufficiently cope with these problems.

  Accordingly, an object of the present invention is to provide a two-wire transmitter capable of reliably generating a lower limit side burnout signal with a simple configuration.

In order to achieve such a problem, the invention according to claim 1 of the present invention is:
Supplied with current from the load side through the two transmission lines, and to generate an alternating magnetic field, transmits an output current related to the electromotive force generated by the alternating magnetic field, the load side through the transmission line And a two-wire transmitter that performs a lower limit side burnout that causes the output current to fall below 0% when an abnormality occurs.
A microprocessor for signal processing;
A watchdog timer that detects abnormalities in this microprocessor;
An exciting coil for generating the alternating magnetic field;
Switch means for passing an alternating current through the exciting coil;
The output of the watchdog timer is input to generate a signal for controlling the switch means, and when the watchdog timer detects an abnormality of the microprocessor, the switch element is turned off and flows to the excitation coil. An excitation control circuit for generating a control signal for stopping the current;
Is provided. The lower limit burnout can be performed reliably.

The invention according to claim 2 is the invention according to claim 1,
A resistor for detecting a current flowing in the exciting coil is provided. The lower limit burnout can be performed reliably.

The invention according to claim 3 is the invention according to claim 2,
The excitation control circuit includes an error amplifier to which an output signal of the resistor is input, a pulse width converter that converts the output signal of the error amplifier into a pulse width signal, an excitation timing generation circuit controlled by the microprocessor, and the It is characterized by comprising a gate to which the output of the pulse width converter and the excitation timing generation circuit is input, and the output of the watchdog timer is input to the gate . The lower limit burnout can be performed reliably.

As is apparent from the above description, the present invention has the following effects.
According to the first, second, and third aspects of the present invention, there is provided a watchdog timer for detecting an abnormality of the microprocessor, and when the watchdog timer detects an abnormality of the microprocessor, an excessive current flows to the exciting coil. Disappear.

  Hereinafter, the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a two-wire transmitter according to the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 7, and description is abbreviate | omitted. In FIG. 1, reference numeral 21 denotes a microprocessor to which the output of the sensor 11 is input. The microprocessor 21 converts the input signal into a digital signal, performs a predetermined operation, then converts it into an analog signal and outputs it.

  SW3 is a switch that selects the output of the microprocessor 21 and the output of the reference power supply Vr4 and outputs them to the current output circuit. This switch SW3 is controlled by an output signal WDG of a watchdog timer right 24 described later. The reference power supply Vr4 is set to a voltage value that outputs a lower limit side burnout signal.

  A flash ROM 22 is accessed by the microprocessor 21 and stores programs and data used by the microprocessor 21. The flash ROM 22 can erase and rewrite data.

  A reset circuit 23 outputs a low level reset signal when the power is turned on. Reference numeral 24 denotes a watchdog timer, which is used to check whether the microprocessor 21 is operating normally. The watchdog timer 24 is a timer that is counted up by a predetermined clock (not shown), and outputs a low level signal WDG when the timer overflows. The watchdog timer 24 receives the output of the port P1 of the microprocessor 21.

  The microprocessor 21 resets at a predetermined interval using the port P1 so that the watchdog timer 24 does not overflow. If an abnormality occurs in the microprocessor 21, the watchdog timer 24 cannot be reset, and the signal WDG becomes low. This signal WDG controls the switch SW3. The switch SW3 selects the output of the microprocessor 21 when the signal WDG is at a high level, and selects the output of the reference power supply Vr4 when the signal WDG is at a low level.

  An AND gate 25 receives the output WDG of the reset circuit 23 and the watchdog timer 24 and outputs the output to the reset terminal RST of the microprocessor 21. Therefore, the microprocessor 21 is reset when either the reset circuit 23 or the output WDG of the watchdog timer 24 becomes low level.

  An AND gate 26 receives the output WDG of the watchdog timer 24 and the output of the port P2 of the microprocessor 21 and outputs the output to the chip select terminal CS of the flash ROM 22.

  As described above, when the microprocessor 21 is operating normally, the signal WDG is at a high level. When the microprocessor 21 accesses the flash ROM 22 to read / write / erase data, the port P2 is set to the high level and the flash ROM 22 is selected. When an abnormality occurs in the microprocessor 21 and the signal WDG becomes low level, the chip select terminal CS related to the level of the port P2 becomes low level, and the flash ROM 22 is not selected.

  A constant voltage circuit 27 supplies power to the microprocessor 21, the flash ROM 22, the sensor 11, and the like.

  Next, the operation of this embodiment will be described. As described above, in order to output the lower limit burnout signal, the current consumption of the transmitter must be reduced. However, a large current flows when data is written to or erased from the flash ROM 22. If the microprocessor 21 breaks down, the port P2 cannot be controlled, unintended writing or erasing may be performed, and the erasing operation may not be interrupted. If such unintended writing or erasing is performed, the current consumption of the transmitter increases, and the lower limit side burnout signal cannot be output.

  Therefore, by inputting the output signal WDG of the watchdog timer 24 and the output of the port P2 to the AND gate 26 and taking the logical product of them, the chip select terminal is forced when the watchdog timer 24 detects a failure of the microprocessor 21. Therefore, the flash ROM 22 is not selected. If the flash ROM 22 is not selected, its operation is stopped, so that unintended writing or erasing is not performed.

  Note that the configuration of the two-wire transmitter itself and the configuration for performing the lower limit side burnout are not limited to this embodiment, and can be modified as appropriate. In this embodiment, the flash ROM has been described. However, the present invention can be applied to a ROM (Read Only Memory), a RAM (Random Access Memory), and an EEPROM (Electric Erasable ROM) other than the flash ROM.

  An example of the configuration of the watchdog timer 24 is shown in FIG. In FIG. 2, reference numerals 31 to 35 denote D-type flip-flops. The data terminal D and the inverted output terminal _Q of the flip-flops 31 to 34 are connected, and the flip-flops 31 to 33 are further connected to the clock terminal CK of the next-stage flip-flop. The output Q of the previous stage is input to the clock terminal CK of the flip-flop 35 and the power supply voltage VDD is input to the data terminal D. The output Q is output to the outside via the inverter 36 as the signal WDG.

  A clear signal, that is, a port P1 output of the microprocessor 21, is input to the clear terminal CLR of the flip-flops 31 to 34, and a reset signal is input to the clear terminal CLR of the flip-flop 35. A clock is supplied from the oscillator 37 to the clock terminal CK of the flip-flop 31.

  In such a configuration, the flip-flops 31 to 35 constitute an asynchronous ripple counter and are counted up by the output clock of the oscillator 37. If the microprocessor 21 is normal, a clear signal is input before the flip-flop 35 is inverted, and the output WDG maintains a high level. When an abnormality occurs in the microprocessor 21 and the clear signal is not input, the flip-flop 35 is inverted and the signal WDG changes to a low level. Since the data terminal D of the flip-flop 35 is connected to the power supply VDD, this low level is maintained until a reset signal is input or the power is turned on again.

  FIG. 3 shows another embodiment of the present invention. In addition, the same code | symbol is attached | subjected to the same element as FIG. 1, and description is abbreviate | omitted. Further, since the configuration for performing the two-wire transmitter itself and the lower limit side burnout is the same as that in FIG. 1, it is omitted in this figure.

  Usually, a microprocessor is provided with a sleep terminal SLEEP that stops its operation and reduces power consumption. In this embodiment, the output WDG of the watchdog timer 24 is input to the sleep terminal SLEEP of the microprocessor 21. When the watchdog timer 24 detects an abnormality of the microprocessor 21, its output WDG becomes low level, the microprocessor 21 shifts to a sleep state and enters a standby state, and stops its operation. Since the current consumption is significantly reduced in the sleep state, the lower limit burnout signal can be reliably output.

  FIG. 4 shows still another embodiment. In addition, the same code | symbol is attached | subjected to the same element as FIG. 1, and description is abbreviate | omitted. Also in this figure, the configuration of the two-wire transmitter itself and the configuration for generating the lower limit side burnout are omitted as in FIG.

  In FIG. 4, 41 is a clock oscillator that generates a clock to be supplied to the microprocessor 21, and 42 is a switch that supplies the output clock of the clock oscillator 41 to the microprocessor 21 and stops the supply. The switch 42 is controlled by the output WDG of the watchdog timer 24.

  When the microprocessor 21 is normal, the signal WDG maintains a high level state. The switch 42 is turned on, and the output clock of the clock oscillator 41 is supplied to the microprocessor 21.

  When the watchdog timer 24 detects an abnormality of the microprocessor 21, the signal WDG becomes low level. The switch 42 is turned off and the clock is not supplied to the microprocessor 21. Normally, when the clock is not supplied, the microprocessor stops its operation, and the current consumption is greatly reduced. Therefore, the lower limit burnout signal can be output reliably.

  FIG. 5 shows still another embodiment. In this embodiment, when the current consumption increases due to the leakage current, a part of the circuit is disconnected. In FIG. 5, 51 is a power supply for supplying power to the two-wire transmitter, 52 is a starter circuit, 53 is a constant voltage circuit, R1 is a resistor for detecting an output current, and TR1 is a transistor for controlling the output current. The starter circuit 52, the constant voltage circuit 53, the resistor R1, and the transistor TR1 constitute the current output circuit 18 of FIG.

  A sensor circuit 57 converts a physical quantity into an electrical signal. A microprocessor 55 processes the physical quantity detected by the sensor circuit 57. The constant voltage circuit 54 supplies power to the microprocessor 55, the sensor circuit 57, and the like.

  R3 is a resistor connected between the constant voltage circuit 54 and the sensor circuit 57, and converts the consumption current of the sensor circuit 57 into a voltage signal. This voltage signal is input to one input terminal of the comparator 58. A constant voltage circuit composed of a series circuit of a resistor R2 and a Zener diode D1 is arranged between the output side of the starter circuit 52 and the common potential point. The constant voltage generated by this constant voltage circuit is the other voltage of the comparator 58. Input to the input terminal.

  The output of the comparator 58 is input to the clock terminal CK of the flip-flop 59. The data terminal D of the flip-flop 59 is connected to the power supply Vcc. The port output of the microprocessor 55 is connected to the clear terminal CLR of the flip-flop 59. A relay 56 is inserted between the resistor R3 and the sensor circuit 57, and is controlled by the output of the flip-flop 59.

  In such a configuration, when the consumption current of the sensor circuit 57 is small, the relay 56 is turned on and power is supplied to the sensor circuit 57. When the power consumption of the sensor circuit 57 increases due to leakage current or the like, the relay 56 is turned off, and the power supply to the sensor circuit 57 is stopped. Therefore, the sensor circuit 57 stops operating, the current consumption of the entire transmitter is reduced, and the lower limit burnout signal can be reliably output.

  FIG. 6 shows more embodiments. In this embodiment, the present invention is applied to a two-wire electromagnetic flow meter. In addition, description which is not necessary for the description is omitted. The same elements as those in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted. In FIG. 4, reference numeral 65 denotes an exciting coil, and a current flows alternately in the opposite direction by the switch elements 61 to 64. This current is detected by resistors R4 and R5. The switch elements 61 to 64 are controlled to be turned on and off by signals T1 to T4, respectively.

  The current values detected by the resistors R4 and R5 are amplified by the error amplifier 71 and converted into a pulse width signal by the pulse width converter 72. 73 is an oscillator, and 74 is an excitation timing generation circuit controlled by the microprocessor 21. The outputs of the pulse width converter 72, the transmitter 73, and the excitation timing signal generation circuit 74 are input to gates 75 to 78, and signals T1 to T4 are generated. The error amplifier 71, pulse width converter 72, transmitter 73, excitation timing generation circuit 74, and gates 75 to 78 constitute an excitation control circuit. Since this part is a known technique, detailed description is omitted.

  In such an excitation coil drive circuit of the electromagnetic flow meter, there is a problem that when the microprocessor 21 runs away, a large current flows through the excitation coil 65 and the lower limit side burnout signal cannot be output. For example, when all the switch elements 61 to 64 are turned on, current passing lines 61 → 63 → R4 and 62 → 64 → R5 are generated. Since the resistance values of the resistors R4 and R5 are small, a large current flows and the lower limit side burnout cannot be performed.

  Therefore, in this embodiment, the output of the watchdog timer 24 is inputted to the gates 75 to 78. These gates 75 to 78 are AND gates, and when the microprocessor 21 becomes abnormal, the output of the watchdog timer 24 becomes a low level, so all the signals T1 to T4 also become a low level. As a result, the switch elements 61 to 64 are turned off, and no excessive current flows.

It is a block diagram which shows one Example of this invention. It is a block diagram which shows the other Example of this invention. It is a block diagram which shows the other Example of this invention. It is a block diagram which shows the other Example of this invention. It is a block diagram which shows the other Example of this invention. It is a block diagram which shows the other Example of this invention. It is a block diagram of the conventional 2-wire type transmitter.

Explanation of symbols

11 Sensor 18 Current output circuit 21 Microprocessor 22 Flash ROM
23 reset circuit 24 watchdog timer 25, 26 AND gate 41 clock oscillator 42, SW3 switch 56 relay 57 sensor circuit 58 comparator 59 flip-flop 61-64 switch element 65 excitation coil 71 error amplifier 72 pulse width converter 73 transmitter 74 excitation Timing generation circuit 75 to 78 Gate R2 to R5 Resistor D1 Zener diode

Claims (3)

Supplied with current from the load side through the two transmission lines, and to generate an alternating magnetic field, transmits an output current related to the electromotive force generated by the alternating magnetic field, the load side through the transmission line And a two-wire transmitter that performs a lower limit side burnout that causes the output current to fall below 0% when an abnormality occurs.
A microprocessor for signal processing;
A watchdog timer that detects abnormalities in this microprocessor;
An exciting coil for generating the alternating magnetic field;
Switch means for passing an alternating current through the exciting coil;
The output of the watchdog timer is input to generate a signal for controlling the switch means, and when the watchdog timer detects an abnormality of the microprocessor, the switch element is turned off and flows to the excitation coil. An excitation control circuit for generating a control signal for stopping the current;
A two-wire transmitter characterized by comprising:   The two-wire transmitter according to claim 1, further comprising a resistor for detecting a current flowing through the exciting coil. The excitation control circuit includes an error amplifier to which an output signal of the resistor is input, a pulse width converter that converts the output signal of the error amplifier into a pulse width signal, an excitation timing generation circuit controlled by the microprocessor, and the It consists of a gate to which the output of the pulse width converter and excitation timing generation circuit is input,
3. The two-wire transmitter according to claim 2, wherein the output of the watchdog timer is input to the gate.

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