Note: Descriptions are shown in the official language in which they were submitted.
3~
The present invention relates to a process ~or connect-
ing pipe components made of weldable plastic, by -~elding their
ends by means of elec-trical resistance heating, and a device for
carrying out the process.
When piping made of weldable plastic is being assembled
the process and the device with which the piping components are
connected to each other by means of electrical resistance heating
play a very considerable role. The expression "piping components"
means pipe, fittings and valves that are combined to form pipe
systems and are to be connected to each other. The completion of
the connections for these piping components is a costly sub-
operation, and in addition determines the quality of the piping
and for this reason has -to be very carefully completed.
Either sleeve joints, that constitute a part of the
pipe fittings or welding couplings that are separate from the
pipe fittings and are installed on the ends of two fittings and
connected to them, are used to connect the pipe components. A
coil of resistance heating wire is installed in the overlapping
areas of these connectors and this coil is heated electrically ~ ~
to make the connection, so that in the area around the coil the -
material of the overlapping ends of the piping components melts,
thereby welding these components together. The thermal energy ~ -
that is passed through the coil is quantitatively regulated in
such a manner that an optimum connection results, i.e., neither
insufficient nor excessive heating is applied to the components
that are to be connected. A device in which the adjustment of
the thermal energy is made either manually or automatically is
used for regulating the thermal energy according to the pipe com-
ponents that are to be connected.
When welding the aforesaid connections by using such a
device it is impossible to avoid the situation that, since the
connections are to be completed on a construction site, some of
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the connections will npt be of sufficient quality. In some cases
this IS because the amount oE thermal energy that is applied in
making the conne~tion is too small with regaxd to the type of
connection and the prevailing environmental ~actors, e,g., the
~mbten-t tempe~ature or a fault~r connection~ ;eO~ example, loss of
contact at a plu~ tn connector, in most instances this leads to
the condition that although the connection is capable of withstand-
ing mechanical loads it is not tight,
Thus, the present invention arranges a process of the
type described above, in such a manner that the laryest number
of influencing factors are taken into consideration, at least
approximately.
According to -the invention there is provided in a pro~
cess for joining piping components made of weldable plastic which
comprises welding their ends by applying heat from an electrical
resistance heater having a welding voltage applied thereto, the
improvement in which the welding voltage is monitored, squared
and converted to a pulse train having a repetition frequency
corresponding to the square of the welding voltage , the pulses
are counted by a counter, and the welding process is terminated
once a preselected pulse count is reached.
The present invention also provides a control device
for use with welding apparatus for joining pipe components made of
weldable plastic by welding their ends by applying heat from a
resistance heater having a welding voltage applied thereto, said
device comprising means for measuring the welding voltage, means
for driving the square of said welding voltage, means for producing
a train of pulses having a repetition frequency corresponding to
said square o~ the welding voltage, a counter for counting said
pulses, and means for terminat~n~ the welding when the pulse count
in said counte~ reaches a preselected ~alue representative of the
total welding energ~
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With the aid of this dev~ce the welding energy that is
usedcanbe kept Ve~ close to the'n`omi,n~l value o~ the welding
energy and~ in ~dd~t~on, certain environmental factors, e.g., amb-
ient temperature and coupllng tenl~e~àture can be taken into consi-
deratlon.
~ n additional safety feature with regard to the production
of a welded joint is provided by a device that is suitable for the
process accordin~ to the invention and has an electrical circuit
that has a switch stage that can be switched off after the welding
energy has been supplied, in, so far as the switch stage incorporates
interrupters if there is a failure of the power circuit.
The present invention will be further illustrated in
the accompanying drawing which is a schematic representation of
a device for connecting piping components made of weldable plastic
. :
in accordance with one embodiment of the present invention and is
presented in the form of a block diagram.
In the device illustrated in the drawing, a power is sup-
plied through a power connector 1. The power circuit is controlled
by means of a switch stage 7 . A welding collar 3 is connected to
the output of the switch stage through a schematically represented
plug 4. A power monitor 6 is incorporated at the output of the
switch stage 7. This device monitors the flow of current and
maintains the switch stage, which incorporates, for example, a
relay, in the operating position.
A po~er pack 2 is connected to the power circuit, and
this power pack provides a low voltage supply, e.g. i2 volts, that
powers the device. The power pack 2 also feeds the line voltage U
to a transducer ~ where it is measured and converted to a s~uared
product of the supplv ~oltage U2, The output of the transducer 8
is connected to the in~ut of a pulse generator 9, that generates
a pulse frequency that correspond$ to the square of the voltage.
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This squared value is forMed from the charging curve of a con-
denser that is supplied directly or indirectly through a resis-
tance from the voltage -that is applied to -the welding co]lar.
If this colldenser is connected to a DC source -the condenser
voltage can only increase as high as the level of the supply
voltage. Its -time constant T, i.e., the time in which 63% of
the maximum possible condenser voltage is reached, is determined
from the equation T = C . R, wherein C is the capacity of the
condenser and ~ is its ohmic resistance. As is known, the con-
denser voltage does not increase linearly, but according to the
equation
UC = U (1 - e-t/T~
wherein Uc is the condenser voltage, U is the charging voltage
and t is the charging time. If, when a specific charging voltage
is reached, the condenser is discharged by means of a switch
stage which incorporates, for example, a comparator, a new charge
cycle will begin and will result in a frequency that is controlled
by the charge voltage or the supply voltage.
The formation of the square function of the supply
voltage in the range, for example, from 180 - 260 V can be
achieved by the fact that the switch point of the discharge,
e.g., to a value of 0.39 of the supply voltage is set at 220 V.
If the voltage deviates from this value, there will be an over-
compensation of the squared supply voltage. At low supply vol-
tages, this over-compensation is necessary to counteract the
large thermal loss in plastics, that occurs as a result of the
lengthy welding time. Thus the welding time is not only increased
correspondingly to the low supply voltage, but additionally as
well. It is thereby possible to achieve a constant weld quality,
even if, for example, at 180 V the welding time increases by a
factor of 1.5. In a similar manner, suitable selection of the
partial value of the charge voltage or line voltage provides for
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over-compcnsation at vo]tages that are in e~cess of the selccted
value o~ the charge or line voltage.
A counter 10 is incorporated at the output of the p-~lse
ellerator, and the pulscs are suMmed in this countcr. The ca~a-
city of this counter amoullts, for exall~p:le l:o 28 _ 224 plllses.
In accordallce wi.th tlle`dependellcy o~ the pulse ~rc~uency of the
~ulse g~llerat:or 9 on the scluare value oE the voltage, a total
nurnber of pulses surnmed in the counter 10 will indica~e a speci-
fic thermal energy~ ~
It is important that during welcliny of the welcling
collar 3 the number of pulses in the coun~er 10 corresponds to
the desil-ed weldiny energy. This is ensured by means of a fun~c-
tion monitor 5. In the event that the ~ulse coullter 10 has not
yet reachcd itS prese]ected pulse COUllt, e.g., as a result of
a break in the power circu;t between the device and t}le ~elding
collar during the welcling process, the function moni~or generatcs
a warnillg siynal~ e.g., a red ~arnillg liqht will flash. ~s soon
as the Warllinc3 sic~nal staL-tS~ this indicates that the co~npleted
~re]ding of the welding collar 3 is to be checked. At tlle same
tilne as the t.7arning signal is displayed, -the line voltage to the
teldillg col]ar is cut oEf by the s~itch unit 7. Switching on
the potver cirucit by switching on the switch stage 7 is controll-
ed by a pGWer mon1tor 11. The power~mon;tor 11 uses a low volta~e `
-to detorr,line whe~her a wel(~ g collar 3 or an equivalent resis-
tarlce is connected to the po~er Cil-Cuit outpilt 4. It is expeclient
that the pcwcr moni~or system l~e rlesi(3ned in such a mal~ner that
the pot.er circuit call only be switched on whell the user resis-
tance is smal1er than a certain value, e.g., 3 k-ohms.
The device cllsu~es m.lx;mll3ll o~eratillg safety. The out-
~0 uut Eor L:he p~wcr circuit of Llle dcvicc rell)c~ s locked until a
u:^;er resistallce having a res;sl:allce th-l~ is lower than a thrc~llold
lcsistLIllce ;s coJ-Inectcd. It~is only tllcll l.]~.lt thc ~ower mclli lor
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38~3
11 is unlocked and the switch stage can be switched on by means
of a push-buttom that is not illustrated.
In addl-tion, environmental Eactors will also be elim-
inated, at least in part. Since thermal energy hased on voltage
is determined according to the Eormula u2 . t/R, this means that
ak low ambient temperatures the welding performance is greater,
always assuming that the electrical resistance heating wire is a
cold conductor. In like manner a form factor that takes into
account the shape of the welding collar can also be considered
for the dependency of the voltage or its squared value on the
pulse frequency. Because of the high counter capacity it is
possible to keep very closely to the welding time, that can be
in the order of 70 - 80 seconds, and still stay very closely to
times of at least 20 minutes duration.
It is important that because of this layout of the -
device, its construction is very simple and at the same time, it ~`
is possible to achieve a weight saving of up to 80~ compared to
familiar welding devices.
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