JPS5914853Y2 - Computing device for electromagnetic flowmeter - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to an arithmetic device for an electromagnetic flowmeter.

In general, in an electromagnetic flowmeter, the electromotive force obtained from the electromagnetic flowmeter transmitter is proportional to the flow velocity of the fluid and the magnetic flux density of the magnetic field, so when the excitation current of the transmitter changes, the magnetic flux density changes and the electromotive force is proportional to the flow velocity of the fluid and the magnetic flux density of the magnetic field. Electricity is affected.

The present invention uses an integrator in which the flow rate signal voltage related to the electromotive force from the electromagnetic flowmeter transmitter and the excitation signal voltage proportional to the excitation current of the transmitter are applied with opposite polarity to each other to eliminate the influence of magnetic flux density. , a comparator with hysteresis to which the output of this integrator is added, and a switch that is driven by the output of this comparator and turns on and off the excitation signal voltage, and a pulse generated at the output of this self-excited oscillation loop. The duty ratio of the width signal is related only to the flow velocity of the fluid, and the pulse width signal is further transmitted through an insulating circuit and then smoothed to obtain an output signal voltage proportional to the duty ratio of the pulse width signal, resulting in a simple configuration. This has resulted in a high-precision, input-output isolation type electromagnetic flowmeter calculation device that can effectively eliminate the effects of magnetic flux density.

FIG. 1 is a connection diagram showing one embodiment of the device of the present invention.

In the figure, MFT is an electromagnetic flowmeter oscillator, A is an amplifier that amplifies the electromotive force e obtained from MFT, SDl, SD2
are synchronous rectifier circuits, SDl converts the output of amplifier A into a DC flow signal voltage E1 and applies it to terminal 1, and SD2 converts a voltage e proportional to the excitation current obtained from the transformer T into a DC excitation signal voltage. This converts the signal into 2 and applies it to terminal 2.

COM rectifies the voltage e proportional to the excitation current and generates S
Dl.

A circuit that drives SD2, 3 is a terminal to which a constant DC voltage E3 is applied, and 4 is an output signal voltage E.

This is the output terminal where .

5 is a switch for turning on and off the input signal E2, and is composed of a field effect transistor FET1.

Reference numeral 6 denotes an integrator, which is composed of an operational amplifier OP1 and a capacitor C1 connected to its feedback circuit.

The integrator 6 has a flow rate signal voltage E1 applied to its input terminal via a resistor R1, and an excitation signal voltage E2 applied to its input terminal via a resistor R2.
, R3 and the switch 5, the output E1 increases when the switch 5 is on and decreases when the switch 5 is off.

A comparator 7 is configured to provide positive feedback to the operational amplifier OP2 through resistors R4 and R6, and to have a hysteresis width of 2h.

The output E of the integrator 6 is applied to the input terminal of the comparator 7 via a resistor R4, and the output terminal has an on/off switch that turns off the switch 5 when El>h and turns it on when El<-. produces an output Eo.

8 is an isolation circuit, consisting of a transistor TR and a fort coupler P.
C and an operational amplifier OP3.

The transistor TR is turned on and off by the output of the comparator 7, and causes the light emitting element P1 of the fort coupler PC connected to its collector to blink.

The light-receiving element P2 of Fort Coupler PC receives the blinking of the light-emitting element P1, changes the potential of the input terminal (-) of the operational amplifier OP3, and outputs a direct current signal to the output terminal of the operational amplifier OP3 in synchronization with the on/off output of the comparator 7. Generates an isolated on/off signal.

9 is a switch for turning on and off a constant DC voltage, which is composed of a field effect transistor FET2, and an operational amplifier OP3.
is driven by the on-off output of

Reference numeral 10 denotes a smoothing circuit, which is composed of an operational amplifier OP4, a parallel circuit of a resistor R15 connected to its feedback circuit, and a capacitor C2.

The smoothing circuit 10 smoothes a pulse signal obtained by turning on and off a constant DC voltage applied to its input terminal using a switch 9, and outputs a signal voltage E.

The signal is sent to the output terminal 4 as a signal.

The operation of the device of the present invention constructed as described above will be explained next with reference to FIG.

If the switch 5 is now turned on by the output of the comparator 7, the integrator 6 integrates only the flow rate signal voltage E1, and the output E1 increases linearly.

When the integrator output E1 becomes E,>h at point a in FIG. 2, the output of the comparator 7 is inverted and the switch 5 is turned off.

As a result, the integrator 6 integrates the difference between the signal voltages E1 and R2, and its output E decreases linearly.

As a result, when the integrator output E1 becomes less than El<- at point b in FIG. 2, the output of the comparator 7 turns on the switch 5 and the integrator output E1 increases again.

In this way, the loop consisting of the switch 5, the integrator 6, and the comparator 7 repeatedly turns on and off with a period T and undergoes self-excited oscillation.

The period T of the self-excited oscillation is mainly determined by the hysteresis width of the comparator 7, the time constant of the integrator 6, and the input signal level, and the oscillation operation is stable.

In this example, the oscillation frequency of the self-excited vibration loop is several kHz.
has been adjusted to.

In this self-excited oscillation loop, the hysteresis width 2h is selected to be sufficiently small and it operates so that the average value of the integrator output E1 becomes zero, so if the off time of the on/off output is t, the following relationship is obtained. .

Therefore, the pulse width signal output by the self-oscillating loop,
The output voltage of t is as follows, which is the ratio of the input signal voltages E1 and R2.

As a result, the component proportional to the magnetic flux density included in the flow rate signal voltage E1 is removed, and the pulse width signal
The current value corresponds only to the flow velocity of the fluid.

The pulse width output of this self-oscillating loop is applied to a switch 9 via an isolation circuit 8.

Therefore, the constant DC voltage E3 has a pulse width of t and an amplitude of R3.
is converted into a pulse voltage of

This pulse voltage is smoothed by a smoothing circuit 10, and its average value voltage E is obtained.

is as follows.

From equations (2) and (3), becomes.

That is, the output voltage E. corresponds only to the fluid flow velocity.

Furthermore, it is not affected by changes in the repetition period T of the pulse width signal.

Note that the flow rate signal voltage E1 is shown in the case of only negative polarity, but when it has both positive and negative polarity such as positive and negative flow, or when it has only positive polarity, the excitation signal E2 is used as shown by the dotted line in the figure.
is connected to the integrator 6 via the inverter IV1 and the resistor R2'.
All you have to do is add it to the input.

In this way, the output of the self-excited oscillation loop becomes the duty cycle of the on and off outputs.For example, if the value of the resistance R2' is chosen so that 2 becomes 0.5, the flow rate of both positive and negative polarities becomes If the value of resistor R2' is selected so that an output of 0 to 100% can be obtained with respect to the signal voltage E1, and R2 is 1, an output of 0 to 100% can be obtained with respect to the positive flow rate signal voltage E1. possible, and resistance R
The output can be arbitrarily shifted depending on the value of 2'.

Furthermore, even if the flow rate signal voltage E0 is of negative polarity, if the excitation signal voltage E2 is applied to the input of the integrator 6 via the resistor R2' except for the inverter I1, the duty ratio 〒 becomes, The output can be shifted arbitrarily depending on the value of resistor R2'.

In this way, in the present invention, the calculation for removing the influence of magnetic flux density is performed by configuring a self-excited oscillation loop with the integrator 6, comparator 7, and switch 5, and the pulse width signal generated at the output of the self-excited oscillation loop is After the duty ratio of E is related only to the fluid flow velocity, this pulse width signal is transmitted through an insulating circuit 8 and then smoothed to obtain an output signal voltage E proportional to the duty ratio of the pulse width output.

Therefore, it is possible to easily achieve high accuracy without being affected by the performance of the integrator 6, comparator 7, etc., and to realize a calculation device for an electromagnetic flowmeter that is resistant to noise with a simple circuit configuration.

Further, in the present invention, since the insulation circuit 8 is provided between the self-excited vibration loop and the switch 9, input/output insulation can be achieved.

Moreover, if the insulating circuit 8 is configured with the transistor TR, fort coupler PC, and operational amplifier OP3 as shown in the figure,
The output of the self-excited vibration loop can be accurately transmitted to the switch 9 without being affected by the non-linearity of the input/output characteristics of the Fort coupler PC.

Furthermore, in the present invention, the integrator 6, the comparator 7, the smoothing circuit 10, etc. can be constructed mainly from IC operational amplifiers, so that the device can be made smaller.

Note that electromagnetic flowmeters transmit some output even when the flow rate is zero, depending on the usage conditions (for example, how dirty the electrodes are, the status of the fluid in the transmitter, etc.).

Moreover, as the situation changes, this error output also changes.

In particular, if the state of zero flow rate continues for a long time, the integration error becomes large.

Therefore, if the output is below the normal value, these errors will not be integrated if the output is forcibly set to zero.

Therefore, in the embodiment shown in FIG. 3, a zero cut circuit 11 consisting of a gate circuit G and an operational amplifier OP5 is inserted between the self-oscillating loop and the isolation circuit 8.

That is, the operational amplifier OP5 compares the flow rate signal voltage E1 with the set value E2, and when El<E2, G is closed to prevent the output of the self-excited vibration loop from being transmitted to the isolation circuit 8, and the output signal E is .

to zero. By configuring the pulse width output of the self-excited oscillation loop to be taken out via the gate circuit G in this manner, zero cut can be easily performed.

The operational amplifier OP5 is provided with positive feedback to have hysteresis to stabilize the comparison operation.

In the case of a positive and reverse flow rate, since the flow rate signal voltage E1 has both positive and negative polarities, a bias is applied and the output E is, for example, 50% when the flow rate is zero.

Therefore, as shown in Fig. 4, when El<E7, the field effect transistor FET3 is turned off by the output of the operational amplifier OP5, and the flow rate signal voltage E1 is forced to zero. good.

At this time, the pulse width output of the self-excited oscillation loop is as shown in Figure 4.
Alternatively, the gate circuit G may be omitted and added directly to the insulating circuit 8 as shown in FIG. 1.

In the embodiments shown in FIGS. 3 and 4, the integral output E increases when the switch 5 is off, and decreases when the switch 5 is on.

Further, although field effect transistors are illustrated as the switches 5 and 9, various configurations may be adopted as necessary.

[Brief explanation of drawings]

FIG. 1 is a connection diagram showing one embodiment of the device of the present invention, FIG. 2 is an explanatory diagram of its operation, and FIGS. 3 and 4 are connection diagrams of other embodiments of the device of the present invention. El, E2. E3... Input signal voltage, Eo...
...Output signal voltage, 5,9...Switch,
6...Integrator, 7...Comparator, 8...
...Insulation circuit, 10...Smoothing circuit, 11.
...Zero cut circuit, MET...Magnetic flowmeter transmitter, T...Transformer, SDl, SD2.
...Synchronous rectifier circuit, COM...SDl,
SD2 drive circuit.

Claims (1)

[Scope of utility model registration request] an integrator to which a flow signal voltage related to the electromotive force from the electromagnetic flowmeter transmitter is applied; and an excitation signal voltage related to the excitation current of the electromagnetic flowmeter transmitter is applied to the integrator via a first switch to control the flow rate. a comparator having hysteresis to which the output of the integrator is added; and the output of the comparator drives the first switch, and means for outputting a pulse width signal by forming a self-excited oscillation loop with a switch; a second switch that is driven by the pulse width signal applied via an insulating circuit and turns on and off a constant DC voltage; An arithmetic device for an electromagnetic flowmeter, comprising means for smoothing the voltage turned on and off by the switch No. 2 and obtaining an output signal voltage.