JP2013145509A - Transformer-isolated analog input device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transformer-isolated analog input device that compensates a sag associated with parasitic impedance of a signal transformer and suppresses deterioration in accuracy of an input signal.SOLUTION: The transformer-isolated analog input device comprises a waveform shaping circuit 9 arranged on the second side of a signal transformer 7. The waveform shaping circuit 9 includes: an inductor 9series connected to a signal line; and a snubber circuit 9shunt connected to a subsequent stage of the inductor 9and composed of a resistor 9and a capacitor 9. The inductor 9of the waveform shaping circuit 9 generates counter electromotive force depending on a change of current flowing through the inductor 9. The current flowing through the inductor 9and the current change depend on the snubber circuit 9arranged on the subsequent stage. Thus, counter electromotive force generated across the inductor 9can compensate a sag associated with parasitic impedance of the signal transformer 7 by setting appropriate element values of the inductor 9and the snubber circuit 9.

Description

  The present invention relates to a transformer-isolated analog input device that collects an analog signal output from a sensor or the like via an isolation transformer in the field of FA (Factory Automation).

  An analog input device frequently used in the FA field has a function of inputting an analog signal output from a temperature sensor or the like to an AD converter with high accuracy. In general, an analog input device requires insulation in order to avoid a failure caused by a ground loop. Photocoupler insulation and transformer insulation are known as methods for this insulation.

Photocoupler insulation has an advantage that a circuit can be easily configured, but an AD converter and an amplifying element are required on the insulation side, which increases the circuit scale.
On the other hand, the transformer insulation is advantageous in terms of miniaturization and high density of the circuit because the insulation side can be constituted only by simple passive elements.

  For example, Patent Document 1 and Patent Document 2 below describe an analog input device using a transformer element as an insulating means. FIG. 6 shows a configuration diagram of the system described in the preceding case, and FIG. 7 shows a waveform sequence.

The analog signal output from the sensor 3 is transmitted to the AD converter 12 via the switch 6 and the signal transformer 7 of the transformer isolated analog input device 101.
The analog signal from the sensor 3 may be a very low frequency signal (substantially DC), and insulation transmission through the signal transformer 7 that is an AC element cannot be performed (or the transformer becomes extremely large).

Therefore, the control circuit 113 includes a sequence unit 113 ₁ and generates a control pulse W2 at the timing of inputting an analog signal. The control pulse W2 is insulated and transmitted through the control transformer 8, and then the switch 6 is turned on to perform a switching process on the sensor signal.
In this way, by performing on / off control of the switch 6 and making the input signal W1 from the sensor 3 have a transient pulse waveform shape, isolated transmission through the signal transformer 7 becomes possible.

  The analog signal output from the signal transformer 7 is input to the AD converter 12 via the subsequent amplifier 10 and the sample / hold circuit 11. A series of circuits for inputting these analog signals needs to be configured so that an input error can satisfy a very high-precision specification, for example, ± 0.1%.

  In the waveform sequence shown in FIG. 7, the input signal W1 observed at the output section of the signal transformer 7 has a shape of a transient pulse by switching processing. FIG. 7 shows the cases of 10 V, 5 V, and 2 V as three examples of sensor voltage values, and the amplitude of the transient pulse corresponds to each sensor voltage value.

Japanese Patent Laid-Open No. 03-044507 Japanese Patent Publication No. 60-10449

The conventional transformer-insulated analog input device has the following problems.
With the switching process of the switch 6, there is a problem that a sag (amplitude deterioration) occurs in the amplitude level of the transient pulse waveform W1.

The state of the conventional sag is shown in the input error waveform of FIG. When the difference (W1−Vs) between W1 and Vs is enlarged and displayed as an input error, a sag (amplitude deterioration) occurs according to the sensor voltage Vs.
In general, a signal transformer has a parasitic impedance typified by a winding resistance, and a voltage drop caused by the parasitic component causes a sag.
The sag due to the parasitic impedance of the signal transformer becomes a cause of deterioration of the accuracy of the input signal, which is a big problem for the input accuracy specification of ± 0.1%.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a transformer-isolated analog input device that compensates for sag due to parasitic impedance of a signal transformer and suppresses deterioration in accuracy of an input signal. And

  The transformer-isolated analog input device of the present invention includes a waveform shaping circuit arranged on the secondary side of the signal transformer, and the waveform shaping circuit is connected in series with the signal line and shunt-connected to the subsequent stage of the inductor, and has a resistance. And a snubber circuit composed of a capacitor.

According to the present invention, the inductor of the waveform shaping circuit generates a back electromotive force according to a change in current flowing through the inductor. Here, the current flowing through the inductor and the current change depend on the snubber circuit arranged in the subsequent stage. Therefore, if the element values of the inductor and the snubber circuit are appropriately set, the sag accompanying the parasitic impedance of the signal transformer can be compensated by the back electromotive force generated at both ends of the inductor.
Thus, there is an effect that the sag accompanying the parasitic impedance of the signal transformer can be compensated and the accuracy deterioration of the input signal can be suppressed.

It is a block diagram which shows the transformer insulation analog input device by Embodiment 1 of this invention. It is a circuit diagram which shows the principal part equivalent circuit of the transformer insulation analog input device by Embodiment 1 of this invention. It is a wave form diagram which shows the current waveform which flows into each part of the transformer insulation analog input device by Embodiment 1 of this invention. It is a wave form diagram which shows the waveform of each part of the transformer insulation analog input device by Embodiment 1 of this invention. It is a circuit diagram which shows the principal part equivalent circuit of the transformer insulation analog input device by Embodiment 2 of this invention. It is a block diagram which shows the conventional transformer insulation analog input device. It is a wave form diagram which shows the waveform of each part of the conventional transformer insulation analog input device.

Embodiment 1 FIG.
FIG. 1 is a block diagram showing a transformer-insulated analog input device according to Embodiment 1 of the present invention.
In the figure, a transformer-insulated analog input device 1 includes input terminals 2 ₁ and 2 _2, and an analog signal from a sensor 3 is input via a cable 4.

In the transformer-isolated analog input device 1, the capacitor 5 removes high-frequency noise from the analog signal.
The switch 6 is disposed on the primary side of the signal transformer 7 and is driven according to a control pulse W2 input via the control transformer 8.

The waveform shaping circuit 9 is disposed on the secondary side of the signal transformer 7 and is a snubber circuit including an inductor 9 ₁ connected in series to the signal line and a shunt connected to a subsequent stage of the inductor 9 ₁ , and including a resistor 9 ₂₁ and a capacitor 9 _22. 9 ₂ .
The amplifier 10 amplifies the analog signal.

The sample / hold circuit 11 includes a switch 11 ₁ , a resistor 11 ₂ that removes high-frequency noise from an analog signal, and a capacitor 11 _3, and the switch 11 ₁ is driven according to an input control pulse W 3.
The AD converter 12 converts an analog signal into a digital signal.

The control circuit 13 inputs the digital signal from the AD converter 12.
Further, the sequence means 13 ₁ of the control circuit 13 generates a control pulse W 2 to the switch 6 through the control transformer 8 and generates a control pulse W 3 to the switch 11 ₁ of the sample / hold circuit 11.

Next, the operation will be described.
In the transformer-isolated analog input device 1, for example, a monitoring device such as a sensor 3 is connected via a cable 4. Here, when inputting an analog signal from the sensor 3, isolated transmission through the signal transformer 7 is necessary for the purpose of avoiding a failure associated with the ground loop.

At this time, since the analog signal from the sensor 3 may have a very low frequency (substantially DC) in some cases, the switch 6 is used to perform the isolated transmission via the signal transformer 7 that is an AC element. Switching processing is required.
FIG. 4 shows a waveform sequence when the signal transformer 7 is transmitted as a transient pulse waveform W1 by switching the analog signal Vs from the sensor 3.

The sag (amplitude deterioration) that has been a problem with conventional devices is mainly caused by parasitic impedance such as winding resistance of the signal transformer 7.
An equivalent circuit of the signal transformer 7 is shown in FIG. The signal transformer 7 is equivalently configured by a first parasitic impedance 7 ₁ that is a parasitic component of the primary winding, a second parasitic impedance 7 ₂ that is a parasitic component of the secondary winding, and a mutual inductor 7 _3. It was supposed to be.
A voltage drop occurs in each of the parasitic impedances 7 ₁ and 7 ₂ of the signal transformer 7 due to the flowing current, and this causes a sag.

In the first embodiment, the problem of the sag is solved by arranging the waveform shaping circuit 9 in the subsequent stage of the signal transformer 7 with respect to the conventional analog input device.
As shown in FIGS. 1 and 2, the waveform compensation circuit 9 includes an inductor 9 ₁ connected in series to the signal line and a snubber circuit 9 ₂ shunt connected to the subsequent stage. Here, the snubber circuit 9 ₂ comprises a serial connection of a resistor 9 ₂₁ and the capacitor 9 _22.

The current flowing through the mutual inductor 7 ₃ is I ₁ , the current flowing through the secondary side of the signal transformer 7 is I ₂ , and the respective current waveforms are shown in FIG.
The exciting current I ₁ flowing through the mutual inductor 7 ₃ increases linearly at a rate of change proportional to the reciprocal of the inductance value L ₁ . On the other hand, the current I ₂ flowing through the secondary side of the signal transformer 7 is a time characteristic to decrease a curve corresponding to the time constant of the snubber circuit 9 _2.

Since the current I ₂ corresponding to the time constant of the snubber circuit 9 ₂ flows through the inductor 9 ₁ constituting the waveform shaping circuit 9, a back electromotive force proportional to the time change of I ₂ is generated. With current I ₂ is the waveform time, since there is a tendency to decrease, back electromotive force generated in the inductor 9 ₁ is a direction to boost the signal waveform. Thus, the counter electromotive force of the inductor 9 ₁ compensates the amplitude deterioration due sag of the signal transformer 7, thereby improving the input error.

  FIG. 4 shows waveforms at various parts in the first embodiment. In FIG. 4, the case of 10V, 5V, and 2V is shown as three examples of the sensor voltage Vs. An input signal W1 obtained as an output of the signal transformer 7 has a shape of a transient pulse, and its amplitude corresponds to each sensor voltage value.

The manner in which sag is improved by the counter electromotive force of the inductor 9 _1, shown in the waveform of the input error of FIG. When the difference (W1−Vs) between W1 and Vs is enlarged and displayed as an input error, it can be seen that the amplitude boost and sag accompanying back electromotive force are balanced and the error is compensated.
When inputting a signal to the AD converter 12, the sample / hold circuit 11 controls the control pulse by the sequence means 13 ₁ of the control circuit 13 so that the voltage is monitored (sampled) at the timing when the error is balanced. W3 is generated.

As described above, according to the first embodiment, the inductor 9 ₁ of the waveform shaping circuit 9 generates a back electromotive force in accordance with a change in the current flowing through the inductor 9 ₁ . Here, the current flowing through the inductor 9 ₁ and the current change depend on the snubber circuit 9 ₂ disposed in the subsequent stage. Therefore, if the element values of the inductor 9 ₁ and the snubber circuit 9 ₂ are set appropriately, the sag accompanying the parasitic impedance of the signal transformer 7 can be compensated by the back electromotive force generated at both ends of the inductor 9 ₁ .
In this way, it is possible to compensate for the sag associated with the parasitic impedance of the signal transformer 7 and to suppress deterioration in accuracy of the input signal.

Further, the control circuit 13, the time in which the amplitude boosting was balanced by the signal transformer 7 according to sag and the inductor 9 ₁ of the counter electromotive force of the waveform shaping circuit 9, at the sample / hold circuit 11, the subsequent waveform shaping circuit 9 Since the control is performed so as to monitor the voltage in the signal line, the voltage can be read at the timing of compensating the sag, and the input accuracy of the analog signal can be greatly improved.

Embodiment 2. FIG.
FIG. 5 is a circuit diagram showing an equivalent circuit of a main part of a transformer-insulated analog input device according to Embodiment 2 of the present invention.
In the figure, the leakage inductance 7 ₄ is a inductor that does not contribute to the mutual inductance 7 ₃ signal transformer 7.
Embodiment 2 In the first embodiment, remove the inductor 9 ₁ constituted the waveform shaping circuit 9, instead, by using the leakage inductance 7 ₄ signal transformer 7, to compensate for sag It is a configuration.
Other configurations are the same as those described in the first embodiment.

Next, the operation will be described.
Leakage inductance 7 ₄ is originally a component of parasitic inductance can not contribute as a mutual inductor 7 _3, a back electromotive force generated in the leakage inductance 7 ₄ is a parasitic component actively utilizing, guarantee the amplitude deterioration by sag To do.

As described above, according to the second embodiment, the leakage inductor 7 ₄ that does not contribute to the mutual inductor 7 ₃ of the signal transformer 7 is used as the inductor 9 ₁ of the waveform shaping circuit 9. In addition to the effect, it is not necessary to mount the inductor element for generating the counter electromotive force as a board component, and the component cost and the component area can be improved.

  In the present invention, within the scope of the invention, any combination of the embodiments, or any modification of any component in each embodiment, or omission of any component in each embodiment is possible. .

₁ , 101 transformer isolated analog input device, 2 ₁ , 2 ₂ input terminal, 3 sensor, 4 cable, 5, 9 ₂₂ , 11 ₃ capacitor, 6, 11 ₁ switch, 7 signal transformer, 7 ₁ first parasitic impedance, 7 _2nd parasitic impedance, 7 ₃ mutual inductor, 7 ₄ leakage inductor, 8 control transformer, 9 waveform shaping circuit, 9 ₁ inductor, 9 ₂ snubber circuit, 9 ₂₁ , 11 ₂ resistance, 10 amplifier, 11 sample / hold Circuit, 12 AD converter, 13, 113 control circuit, 13 ₁ , 113 ₁ sequence means.

Claims (3)

In a transformer-isolated analog input device for driving an input signal from the primary side of the signal transformer to the secondary side by driving a switch disposed on the primary side of the signal transformer.
A waveform shaping circuit disposed on the secondary side of the signal transformer;
The waveform shaping circuit is
An inductor connected in series to the signal line;
A transformer-isolated analog input device comprising a snubber circuit comprising a resistor and a capacitor shunt-connected to the subsequent stage of the inductor.   And a control circuit that controls to monitor a voltage in a signal line subsequent to the waveform shaping circuit at a time when the sag by the signal transformer and the amplitude boost by the back electromotive force of the inductor of the waveform shaping circuit are balanced. The transformer-insulated analog input device according to claim 1. The inductor of the waveform shaping circuit is
3. The transformer isolated analog input device according to claim 1, wherein the transformer isolated analog input device is a leakage inductor that does not contribute to a mutual inductor of the signal transformer.

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