FIELD OF INVENTION
The invention relates to a Resistance welding instrumentation system for measurement
of welding time, current and force developed during resistance welding of sheet metal.
More particularly the present invention relates to development of portable resistance
welding instrumentation system for welding of sheet material.
BACKGROUND OF THE INVENTION
In resistance spot welding generally many of the machines do not provide the value of
welding current and welding time and the force used to achieve a good weld.
In welding operation of steel sheets for example by spot welding which is a type of
resistance welding, stable welding is realizable by maintaining welding conditions such
as the shape of electrodes, welding pressure between both the electrodes, welding
current and weld time to respectively predetermined values.
Meanwhile, in the case when a spot welding is applied to galvanized sheet iron or high
tensile strength steel sheets, it is known that stable welding can not be achieved only
by maintaining the welding condition to the predetermined values.
Recently, in order to maintain the stable welding and to fabricate welded products
having high quality by the spot welding of the new material, a monitor for the
resistance welding is disclosed in a variety of patents.

For example, according to the Japanese Patent Sho 59-14812, a time-scale
variation of an inter electrode voltage (voltage across both electrodes) by which
optimum welding has achieved under a predetermined work condition such as a
material of the work to be welded and the thickness thereof is evaluated and
considered as a reference voltage variation. In actual spot welding operation, the
time-scale variation of the inter electrode voltage is measured and is compared
with the reference voltage variation. When the variation of the measured inter
electrode voltage is in coincidence with the reference voltage variation within a
predetermined allowable range, it is assumed that an adequate welding has been
achieved. Moreover, the welding condition such as the welding current is
controlled so that the inter electrode voltage matches the reference voltage
variation.
In the above-mentioned prior art, it is important to detect an accurate inter
electrode voltage.'The inter electrode voltage is regarded to be measured by
detecting an output voltage of two lead wires connected to both the electrodes.
However, large voltages are induced on the lead wires due to a magnetic field,
which is generated by a large current flowing through both the electrodes. Since
the induced voltages are generated by magnetic couplings on the lead wires, the
phase of the induced voltages are different from that of the voltage supplied
from the power source of the spot welder. The induced voltages vary by
magnetic coupling conditions of the lead wires, and produces an error in the
measured value of the inter electrode voltage. Consequently, the change of the
induced voltages results in occurrence of an error in determination of welded
result and the control of the spot welding apparatus.

An example of a conventional monitor of the spot welder having means for
eliminating the influece of the induced voltage is disclosed in the Japanese
Patent Sho 59-40550. According to the prior art, the inter electrode voltage is
detected at a time point at which a variation of the welding current becomes
zero (time point at peak of an alternate current waveform) and escapes from the
influence due to induction noise components. Consequently, the detected inter
electrode voltage is represented by sampled values of the inter electrode
voltage, which are sampled at a frequency as twice high as that of the welding
current. Since the sampled value is obtained only at the time point of the peak of
the alternating current of the welding current, the inter electrode voltage can not
be detected at an arbitrary time point. Therefore, the prior art is applicable only
to the alternating current resistance welder, and can not be applied to a
resistance welder having an inverter power source.
The prior art spot welding machines therefore do not have the facility to display
the welding parameters, since analog type of displays can not respond to the
extremely short durations of spot welding cycle times which are of the order of
few cycles of current.
SUMMARY OF THE INVENTION
Accordingly there is provided a resistance welding instrumentation system for
measurement and display of welding time, welding current, and force developed
during resistance welding of metal sheets, comprising a load cell (1) biasable to
output-signal representative of load information, the outputted signal transmitted
to an amplifier (4), the amplified load signal transmitted to an analog to digital
converter (7) for digitizing the load information into corresponding digital values

enabling further processing, a toroid coil (2) for sensing the welding current
whose output-being connected to an integrator (5) for measuring the current,
the output from the integrator (5) transmitted to the analog to digital converter
(7) for digitizing the analog current values, a trigger circuit (6) receiving the
output directly from the toroid coil (2), and activating a microcontroller (8) to
initiate current measurement, and the microcontroller (8) processing, computing
and displaying the digitized values of the welding time, welding current, and
force, the display being arranged through an LCD array (9) connected to the
microcontroller (8).
In resistance spot welding, welding current, welding time and welding force are
the three important welding parameters which are to be monitored for achieving
a quality weld. For measuring the welding current a toroid coil is employed,
which gives a signal proportional to the welding current. The welding time is
measured by measuring the duration of the current flow. The applied force is
measured by using a load cell when welding is not carried out.
The system is enabled to perform measurement of welding current, time' and
force, in which the welding current is measured using a toroid coil as a sensor to
sense the input current values. The welding time is measured from the duration
of the current flow in terms of number of cycles. The results are displayed on the
screen.The welding current and duration are measured during actual welding.
The force is measured when welding is not carried out. The welding current
waveform is shown in the display. The instrument is portable and handheld. It
has a chargeable battery. Once it is charged, the instrument can be employed for
4 hours.

According to the invention, the parameters are set as per the welding procedure
data sheet and the machine is working only as per the settings.
There are two ranges of current either 0 to 9.99kA and 0 to 99.9 kA.
The range for the welding force will be as much as 0 to 99Kg per spot welding.
The range for welding time is from 0 to 99cycles.
Further features of the invention, its nature and various advantages will be more
apparent from the accompanying drawing and the following detailed description
of the preferred embodiments.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
FIG. 1 Schematic diagram of a Portable Handheld Resistance Welding
Instrumentation system according to the invention.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
As shown in figure-1, a strain gauge based Load cell (1) is biased and the signal
output is connected to an amplifier (4). The amplified load signal is connected to
an Analog to Digital Converter-ADC (7). Welding current is sensed by a Toroid
Coil (2) and its output is connected to an integrator (5) through a range selector
switch (3) for measuring current in the range of 9.99 kilo ampere or 99.9 kilo
ampere. The output of the integrator (5) is connected to the ADC (7). This ADC

(7) digitizes the analog current/load information in to corresponding digital
values and is processed, computed and displayed by a micro controller (8).
The output of the toroid coild (2) is also connected to a trigger circuit (6) and
this signal activates the micro controller (8) to initiate current measurement. An
LCD array (9) is connected to the micro controller (8) for display of results. A
mini key pad (10) is used for setting mode and range of measurement. A set of
chargeable battery (11) powers up the instrument for using in shop floor without
need for aditional power connection. This battery can be charged through
external charger. The microcontroller (8) is programmed to take care of
functionality of the instrumentation system. The system is designed to be held by
hand. It is compact and portable. The system is required to be charged for 4
hours before operation so that the instrument can be used without power
connections for four hours.

WE CLAIM
1. A portable resistance welding instrument system for resistance welding of sheet
material, comprising:
- a strain gauge based load cell (1) connected to an amplifier (4) for measuring the
pressure applied between the clamped sheet metals
- a toroid coil (2) connected to an integrator (5) equipped with a range selector switch
(3) for measuring of true RMS input welding current
- an analog to digital converter, ADC(7) which receives amplified signals from strain
gauge based load cell (1) connected to the amplifier (4) and toroid coil (2) connected to
an integrator (5)
-trigger circuit (6) receiving the output directly from the toroid coil (2) and activating a
micro controller (8) to initiate measurement of current and pressure
- the microcontroller (8) for processing, computing and displaying the digitized values of
welding current, applied force and the welding current duration through a numeric LCD
display (9) connected to the micro-controller (8).
2. The system as claimed in claim 1, comprising a mini key pad (10) connected to the
micro controller (8) for setting mode and range of measurement.
3. The system as claimed in any of the preceding claims, comprising a set of
chargeable battery (11) acting as the power-source for the system.

4. A method of measurement in a system as claimed in claims 1 to 3, the welding
current, applied force and welding current duration during resistance welding of metal
sheets, comprising the step of:
- biasing a load-cell to output signal representative of load information,
- amplifying the output signals and transmitting the amplified signal for conversion from
analog to digital signal;
-transmitting the digitized signal for further processing, computation and display;
- sensing the welding current via a sensor, transmitting the sensed current to a
measurer for measuring the current,
- converting the measured current from analog to corresponding digital values;
- activating the microcontroller via a trigger circuit which is being fed directly from the
measurer to initiate measurement of current;
- feeding the digitized current values from the measurer to the micro controller; and
- displaying the measured values via an LCD display.

ABSTRACT

DEVELOPMENT OF PORTABLE RESISTANCE WELDING INSTRUMENTATION
SYSTEM FOR WELDING OF SHEET MATERIAL
A resistance welding instrumentation system for measurement and display of welding
time, welding current, and force developed during resistance welding of metal sheets,
comprising a load cell (1) bias able to output-signal representative of load information,
the outputted signal transmitted to an amplifier (4), the amplified load signal
transmitted to an analog to digital converter (7) for digitizing the load information into
corresponding digital values enabling further processing, a toroid coild (2) of sensing
the welding current whose output-being connected to an integrater (5) for measuring
the current, the output from the integrator (5) transmitted to the analog to digital
converter (7) for digitizing the analog current values, a trigger circuit (6) receiving the
output directly from the toroid coil (2), and activating a microcontroller (8) to initiate
current management; and the micro controller (8) processing, computing and displaying
the digitized values of the welding time, welding current, and force, the display being
arranged through an LCD array (9) connected to the microcontroller (8).