RU1787090C - Device for measuring and checking effective value of welding current - Google Patents

Abstract

A device for measuring and monitoring the effective value of the welding current relates to electrical equipment for spot welding with alternating current of industrial frequency and can be used to automatically control the welding mode. SUMMARY OF THE INVENTION: the derivative of the welding current is measured, the measured signal is integrated, and the effective value of the welding current is determined. The device comprises a current sensor made in the form of a Rogowski bar, an integrator and a scaling amplifier, an analog-to-digital converter, a microcomputer, an address register, a storage device, an input-output expander, a current limiter, and a digital indicator. 2 ill.

Description

The invention relates to electrical equipment for spot welding and is intended for measuring the effective value of the welding current and monitoring the welding mode.

Devices for measuring the effective value of the welding current are known. The device comprises a current sensor (according to Rogowski) with an integrator and a functional converter performing the operations of squaring, integrating and extracting the square root. The essence of the method is as follows. From the Rogowski output, a voltage proportional to the derivative of the welding current is supplied to the input of the first integrator, from which a value is taken that is proportional to the instantaneous value of the welding current. Then, using analog functional converters, squaring, integration, extraction

square root and storing the effective current value. Such a method and apparatus for its implementation have sufficiently large errors due to the use of analog functional converters.

The closest in technical solution to the invention is a device for measuring the effective value of the welding current, as well as a method of measuring the effective value of the welding current. The device comprises a current sensor, a current amplitude meter, a unit for measuring the angle of combustion of the valves of the power thyristor contactor, the inputs of which are connected respectively to the output of the current sensor, to the output of the phase shifter and to the output circuits of the thyristor contactor, and the outputs are connected to the indicator device through a computing unit. A voltage proportional to the instantaneous value of the welding current is obtained at the output of the current sensor.

(L

WITH

j with VJ o o o

with

The amplitude of the current is measured and stored. At the same time, p ./ measures the angle of the valves and the angle of combustion. Signals proportional to these values are supplied to a computing unit, which, according to a predetermined algorithm, calculates the effective value of the welding current. The disadvantage of the prototype is its low accuracy, since it includes analog units for measuring three values: current amplitude, turn-on angle and thyristor burning.

The purpose of the invention is to increase the accuracy and expand the functionality of the device.

The goal is achieved in that a table of the effective values of the welding current dt, each of which corresponds to a value, is written to the memory of a single-chip microcomputer in advance for the entire operating range

Qm

j 1

"(0.

0)

where k is the number of time-division intervals from 0 to T / 2, T is the voltage period of the supply network; i (t) is the welding current; m (.,. n) is the number of the table element, and the number of table elements n is selected from the condition that the difference between the corresponding values of dt and 1dt + 1 does not exceed the required measurement error, then determine the time to start the calculation from the condition that the sum of p measured subordinate instantaneous current values, where p 5 2, exceeds the predetermined value set, taking into account the noise immunity, equal to (0.1-0.5) 1d.min, measure the instantaneous value of the welding current Izm at regular intervals and calculate k

S 2 meas.p 1 (2) p - 1

and the current value of the welding current is taken to be an element from table 1dt, which corresponds to Qm equal to S or close to S.

To calculate the effective value of the welding current, an array of dm values is formed, which correspond to Qm values located in steps that provide the required measurement accuracy and which is different for different parts of the array. In this case, the search for the sum Qm closest to the value S is performed, starting with the upper QI (or lower Qn value from the set of Qm values with the set step), compare the values S and QI - i STEP (or Qn + if- хШГ) and at the next search step when

the difference between these values changes the sign to the opposite, an element from the array 1dt (... n) with the number i is taken as the effective value of the welding current, i.e.

| D |.

In the proposed device, the driver of the actual value contains a microcomputer, an address register, read-only memory, and the register

0 addresses are connected to the address / data bus, its output is connected to the address bus of the storage device. An I / O expander and a current limiter have been introduced, the switch being connected to

5, the first port is an I / O expander, and the indicator is with the second.

In the proposed method, the required measurement accuracy is ensured by the selection of a table (i.e., an array of 1 dt values) of the required volume, moreover, the array elements are directly codes for a seven-segment indicator. Thus, in the table, each cell (byte) corresponds to one dt value from the array,

5 covering the entire working range, and there is no need to store in memory an array of large Qm numbers used to find q. In addition to the 1Dt array, the first (maximum) value of the sum Qi and a set of search steps are selected in such a way that to optimally cover the entire working range. The developed device provides a given error

5 measurements of not more than + 2% when using a table of 256 bytes for each of the ranges 0.3-10 kA and 10-100 kA (one table for both ranges). If a table of 512 bytes is used, a reduced error of not more than ± 1% is provided. In addition, the accuracy of measurements is improved by using a program start, while eliminating the possibility of skipping the beginning of the welding

5 pulses, since the prehistory of the current pulse is stored in the microcomputer and the first measurements are not lost, but are taken into account when calculating Qm and, therefore, when finding q.

0 Simplification of the device circuit is achieved by the fact that with the program start-up method there is no block for generating a measurement start signal, since there is no need to measure the switching angle and the burning of 5 thyristors or generate a synchronizing pulse.

The expansion of functionality is provided by the introduced I / O expander, with the help of which the upper and lower settings, indication, issuance of additional information on the output of the effective value beyond the tolerance field, and signal output to the rejection device are implemented.

In FIG. 1 shows the principle of soft start of a device for measuring and monitoring welding current; in FIG. 2 is a block diagram of a device for measuring and monitoring the effective value of the welding current.

The essence of the method is that immediately after turning on the power, the signal value at the output of the analog part (integrator and scaling amplifier) is measured. These measurements are performed with a fixed frequency m, where m is the time interval between consecutive measurements of instantaneous values of the welding current (Fig. 1).

The time t0 corresponds to the moment the device is turned on and the moment of the first measurement of the welding current. After each such test measurement, the sum p of the last values of the measured signal is calculated, which is compared with the value corresponding to the predetermined threshold value P. We will use it to illustrate the method. If j is the last current measurement, then the condition is checked

J ij P. (3)

J-2

In the case when this sum exceeds P, the last three test measurements are considered the first three measurements of the instantaneous value of the welding current, the squares of which are added to the desired sum to calculate the effective value.

Finding the effective value of the welding current is based on the use of a table. The volume of the table is selected based on the required measurement accuracy and operating range.

The effective current value is calculated by the formula

1eff / I i2 dt

(4)

where T is the period corresponding to the frequency of the supply network.

For k intentions at intervals of r over a period, the integral is converted to the sum

 DI i.e. i j i

(5)

Moreover, the accuracy of calculating the eff is determined by the number of measurements. Here, the current is measured in arbitrary units (i1) proportional to the true value of the current (i) in kiloamperes, i.e.

where f is the conversion factor.

After simple transformations, you can get

 -----.

| .Ff4g | ch1) g

fifteen

1 j

(6)

20If you designate the coefficient before the radical

then

Wj

. lg

 2) (

(7)

(8)

i.e., for each possible value of sum 2

:.12

we s i: o /)

j 1

can be calculated accordingly

0

0

5

The corresponding value is 1eff in kiloamperes. These eff values are elements of a table. Elements of this table are arranged unevenly with a varying distance of one relative to the other in order to ensure a given measurement accuracy and coverage of a given operating range.

Finding bff is performed as follows. The maximum value of the measured eff (upper value of the measuring range) corresponds to the sum of 5Max. The amount of So STEP corresponds to 1eff1. Any other value of S corresponds to the value of current Lffk. In this case, the condition

ff.1 1eff.1 + 1s

effective top

where d is the permissible reduced measurement error;

S is the amount obtained as a result of the program after the passage of the welding pulse.

Thus, the elements of the table are directly the values of Lff in

kiloamperes previously calculated in accordance with the expression (8) and with the expression for the welding current with a full-phase signal

i lasinwt,

where la is the amplitude value of the pulse of the welding current.

Those. the coefficient F and the radical are taken into account in the table, which greatly simplifies the device software. By selecting the appropriate volume of the table, the required measurement accuracy can be ensured.

The device comprises a current sensor 1, made in the form of a Rogowski strip 1, the output of which is connected to the input of an analog block 2, consisting of an integrator and a scaling amplifier, the output of which is connected to the input of an analog-to-digital converter 3 connected to the microcomputer port 4. Bus address / the microcomputer data is connected to the address register 5 and the address port of the storage device 6. The I / O expander 7 is connected to the microcomputer port, the current limiter 8 with the first port of the I / O expander, indicator 9 - with the second port m kroEVM.

The device operates as follows.

After the power is turned on, the signal from the integrator and the scaling amplifier is measured. The measurement is carried out at a fixed frequency under the control of a microcomputer.

In case the condition for starting the device, i.e. upon receipt of a welding pulse, the microcomputer 4 accumulates data for the current period or half-period of the mains voltage and again switches to standby mode for the next welding pulse. In this case, the processing of the obtained data is carried out: calculation of the current value of the welding current according to the table, its comparison with the settings and output to the digital indicator 9. You can view and change at any time

settings using the setpoint 8 current limits.

The proposed method for calculating the effective value of the welding current and the method of starting the device can increase

current measurement accuracy.

The proposed device allows you to measure the effective value of the welding current with the required accuracy, compare it with pre-set settings, promptly set from the controls, which expands its functionality without significantly complicating the circuit.

Claims (1)

The claims A device for measuring and monitoring the effective value of the welding current, comprising a current sensor connected to an integrator input with a scaling amplifier, an analog-to-digital converter, an effective current value generator, and a digital indicator, characterized in that, for the purpose of increasing accuracy and expanding functionality, the generator acting the value contains a microcomputer, an address register, and read-only memory, the address / data bus of the microcomputer being connected to an address register and an address port of the storage device, the output of the address register is connected to the address bus of the storage device, moreover, an input / output expander connected to the microcomputer and a current limiter are additionally inserted into the device, and the limiter is connected to the first port of the input / output expander, and the indicator is connected to the second, while the sensor The current is made in the form of a Rogowski band.