Note: Descriptions are shown in the official language in which they were submitted.
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The invention concerns an electrostatic enameling
system with a revolving spray bell supported by a spray head,
a spray bell drive, an enamel line through which the enamel
can be fed, facultatively separate lines for feediny
compressed air and solvent, and with a voltage supply serving
to apply voltage across the spray bell.
Revolving spray disks or spray bells supported by a
spray head which carries high voltage have been used for a
long time in electrostatic enameling. For enameling, the
enamel to be sprayed is fed to the spray disks or spray bells
continuously. The spray~dlsks or spray bells were mostly
driven by electromotors at ground potential, but also by
slow-running electromotors on high~voltage potential. In
addition, pneumatic motors were employed, which were mostly
at high voltage potential.
While previously speeds~of ~rotation of 1,000 to
3,000 RPM were customary, rotational speeds from 10,000 to
30,000 RPM have been used recently,~ especially due to newer
enamel materials. To achieve such rotational speeds with
electromotors on ground~ potential, gearing and/or V-belt
drives connected to~high voltage potential were used at the
proper ratio. While pneumatic motors were usable on the high
voltage side, lubrication problems arose in conjunction with
these motors and~the useful life of their ball bearlngs.
Newly developed enamel materials, such as water
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enamel or so-called solventless enamel, require still greater
speeds of rotation of approximately 40,000 to 70,000 RPM for
which so-called turbodrives, con~ected to high voltage, are
used. The turbodrives consist usually of a compressed air
nozzle and an impeller wheel at which the compressed air from
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the nozzle is directed. ~he spray bells used at such high
speeds of rotation have usually a relatively small diameter,
about between 20 and 80 mm. Depending on diameter, the
operational peripheral speeds of these spray bells range from
100 to 250 m/s. One problem of this drive type is
constituted by the adjustment of the individually most
favorable speed of rotation and by keeping it constant. In
idling, with no enamel being fed, the speed of rotation is
very high while under load, when enamel is being fed for
spraying, it drops considerably. As a result, bearing wear
is high in idling. When no spray bell is mounted, even
higher and dangerous speeds of rotation are reached. In
addition, an excessive speed of rotation produces a spray
which is too dry and has an adverse effect on the finish. At
a low speed of rotationj a high degree of atomization is not
achieved, and thus no optimum enameling results either.
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~ Based on these problems, the invention attempts,
among other things, to advance an electrostatic enamel
application system of the initially named type in a fashion
such that an exact control of the speed of rotation of the
turbodrive for the spray disk or spray bell (herein always
referred to as spray bell for convenience) is accomplished
without incurring problems in connection with the high
voltage.
These problems are solved in the invention by
providing a rotational speed sensor for sensing the speed of
rotation of the spray bell, which sensor features an
electrically insulating signal-transmission device which
extends a sufficient insulation distance between a point
connected to high voltage potential and a point connected to
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ground potential, in that the output signal of the rotational
speed sensor is transmitted via signal-processing and
signal-forming stages to a comparator to which a set value is
fed by a set value emitter, and in that the output signal of
the comparator is passed by way of further signal-processing
stages to a control component. --
A desired speed of rotation can accurately beachieved and kept constant at any operating conditions with
the above characteristics. The constancy of the desired
speed of rotation can be achieved also in the case of changes
of the amount of enamel supplied or of the temperature or
other parameters which usually have an influence upon the
speed of rotation. With a varying size of the enameling
surfaces, e.g., enamel quantity changes are preprogrammed in
electrostatic enameling systems used in automotive facilities,
which quantity changes require likewlse corresponding
rotational speed changes for optimization which, according to
the invention, can be preprogrammed as well.
Experiments have shown that any enamel material,
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even enamel materials of the same type but with different
shades can be optimally applied only at a specific speed of
rotation which varies from the optimum RP~ for other enamel
materials and other shades. The invention makes it possible
to adjust a specific speed of rotation which is governed by
the enameling material and the selected shade which, in the
case of multicolor systems, can be handled automatically,
e.g., with every paint change according to previous
programming.
When the rotational speed is measured on a part
connected to high voltage, for instance either the spray bell
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or a spray bell drive connected to high voltage, a problem arises: that is,
transmitting khe signal obtained through rot;ational speed measuring, for
further processing by the Gontrol circuitry, from the high voltage side to
a point which is approximately at ground potential. For that purpose, the `
invention provides a rotational speed sensor which features an electrically
insulating signal transmission device with a signal transmission path which,
with a sufficient insulating length, extends between a point connected to
high voltage potential and a point connected to ground potential.
According to one broad aspect of the present inve~tion, there
is provided a speed control system for a fluid motor having a motor fluid
input, the system comprising a rotational speed sensor which derives an
output signal indicative of the rotational speed of the motor, means for
converting the output signal to a fluid signal, means for coupling the
converting means to the motor input and means for braking the motor, the
converter and braking means controlling motor speed.
According to another broad aspect of the present invention, there ;
is provided a feedback system for controlling the speed of a fluid motor
having a motor fluid input, the system including an optical-signal trans-
mitter, an optical-signal receiver, means`providing an optically sensitive
marker, means for coupling the marker to the motor in driven engagement, the
marker selectively coupling the optical-transmitter signal to the optical-
signal receiver to provide an optical output signal indicative of motor
rotation, means for converting the optical output signal to a fluid signal,
means for coupling the converter to the motor input, and means or braking
the motor, the converter and braking means controlling motor speed.
According to a further broad aspect of the present invention, there
is provided a method for controlling rotation rate of a fluid motor,
comprising the steps of establishing a desired rotation rate, monitoring the
actual rotation rate, comparing the actual rotation rate to the desired
rotation rate, generating a driving fluid error signal in response to such
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comparison, and supplying said error signal to the motor to correct the
~ctual rotation rate toward the desired rotation rate, the motor being a
motor for rotating a rotary atomi~ing device of a coating material atomizing
and dispensing system including means for feeding a selected coating material
from a selected one of a plurality of sources for such coating materials to
the atomizing device, supplying highrmagnitude electrical potential to the
rotary atomizing device, the step of monitoring the actual rotatlon rate
comprising the step of generating a light signal, coupling the light signal
through a fiber optical conductor to a member which rotates with the rotary
atomizing device, detecting reflections of the light signal from the member
through a fiber optical conductor to maintain the electrically insulated
integrity of the atomizing device from the return of the source of high-
magnitude electrical potential while permitting the feedback of motor speed-
related signals for motor and atomizer speed control, and processing the
reflected signal coupled through the fiber optical conduGtor to an optical
signal receiver, and processing the refIected light signal received by the
optical signal receiver to obtain the actual rotation rate.
According to a particularly favorable embodiment, the speed sensor
comprises the following components: an optical marking located in the high
voltage area and revolving at a speed of rotation which is proportional to
the speed of rotation of the spray bell, a light source, a photoelectric
receiver connected to approximately ground potential and capable of emitting ~;
an electrical signal corresponding to the incident light, and a light-
transmission segment with a first light-transmission section between the
light source and the marking, and a second light-transmission section between
the marking and the photoelectric receiver, with each light-transmission
section possessing a sufficient insulating length between any points
connected to high voltage and approximately ground potential. This rotation-
al speed sensor design avoids conductive signal transmission devices, such as
electrical lines and cables. By using, for the transmission of the signal
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proportional to the speed of rotation, a light which is modulated in accord- :
ance with the rotational speed of the spray bell, insulation problems are
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precluded. The marking must be provided on a component which
revolves together with the spray bell, for instance on the
turbine wheel of a turbodrive or on some other wheel. The -
wheel may be provided with a single marking or with a number
of markings. When using only a single marking, signals
corresponding to the speed of rotation are obtained. When
using several markings in the peripheral direction on the
wheel, a signal is obtained which equals an appropriate
multiple of the rotational speed of the spray bell. The
length of the light transmission sections must be selected at
least large enough to ensure that, contingent on the high
voltage applied, no arc-over to components at ground
potential can occur, for instance to the light source and the
photoelectric receiver.
The optical marking may favorably be ~ashionedj
e~g., as a mirror wheel with a plurality of mirror segment
surfaces. Owing to the good reflection of the mirror segment
sarfaces, a weaker llght source can be employed.
~ In order to eliminate outside Iight lnfluences and,
on the other hand, to make do with a relatively weak light
soarce, a glass fiber OptlC transmitter (glass fiber light
conductor~ is favorably provided as a light-transmission
section between the light source and the marking and as a
light-transmission section between the marking and the
photoelectric component.
According to another suitable embodiment, the
rotational speed sensor comprises the following components:
an electric signal generator coupled with the spray bell, for
instance a revolving tachometer generator which emits
intermittent signals at a frequency which is proportional to
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the speed of rotation of the spray bell, a light source
capable of emitting at a high frequency intermittent light
signals with the signals being supplied to the light source
by the signal generator, facultatively by way of a
pulse-forming circuit, and a light-transmission section,
favorably with optical fibers and su~ficient insulation gap
which passes the light signals from the light source to a
photoelectric receiver which approximately carries ground
potential. This embodiment has the advantage that the light
transmission path from the light source via the optical
fibers and to the photoelectric receiver can be completely
encapsulated so that the path is practically not exposed to
any outside influences, and specifically not to contamination
and any deterioration of transmission properties caused
thereby. Generally, it can be pointed out that by
transmitting digital signals the transmission path of the
light signals is at a rate less sensitlve than when
transmltting analog light signals.
The light source is suitably a light-emitting
diode, especially for reason of utility life and heat
generatlon. According to a preferred embodiment, the spray
head is attached to the end of an insulated support tube
whose other end is fastened to a point of the enameling
system which is connected to ground potential, with the
optical fiber light conductors being run through the interior
of the insulating tube. This makes for a particularly
protected arrangement of the optical fibers. Besides, they
are not subjected to bending stress when the spray head is
moved to and fro. This design permits a practically complete
elimination of oatside light influences. A still more rugged
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and operationally safe design of the rotational speed sensor
according to the invention is obtained by molding the light
source, the photoelectric receiver, their electrical leads,
and the optical fiber parts terminating at them, in
pressure-resistant fashion in a housing which favorably is
fastened to the insulating tube.
According to another suitable embodiment, a
compressed air generator driven by the spray bell is coupled
with it as a rotational speed sensor, which compressed air
generator is capable of producing in a compressed air line
constructed from insulating material a static air pressure
which is a function of the speed of rotatian of the bell,
with a transducer, preferably a pressure-electric transducer,
being provided on the other end of the pressure line in order
to generate an output signal proportional to the speed of
rotation of the spray bell. Again, the compressed air
generator may be a turbine coup~ed with the shaft of the
spray bell, but acting as ~a pressure generator. A rotational
speed generator of such design permits extreme simplicity and
operational dependability.
According to a modified embodiment, an ultrasonic
generator coupLed with the spray bell may suitabIy be provided
as a rotational speed sensor, generating ultrasonic pulses of
a frequency which is proportional to the speed of rotation of
the spray bell, with the ultrasonic pulses being transmitted
via an insulating transmission line with sufficient
insulation length to a signal transformer which lies at
approximately ground potential and emits an electrical output
signal. This embodiment may as well be designed extremely
simple, rugged, and resistant to outside interference.
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A pneumatic drive is suitably provided for the
spray bell while the comparator is coupled, facultatively
through the intermediary of an amplifier and an adaptor
circuit, by an electric-pneumatic transducer whose output
signal is transmitted to a pressure amplifier for the drive
air of the pneumatic drive of the spray bell. According to a
preferred embodiment, the signal-processing and forming
stages arranged after the rotational speed sensor comprise a
flip-flop ampl~ifier and, following the latter, a
pulse-voltage~transducer, while with a suitably modified
embodiment, the signal-processing and forming stages arranged
after the rotational speed sensor ~eature a flip-flop
amplifier, a frequency meter, and a digital-analog converter.
Thus, the actual signal processing is han~dled electrically.
For insulation reasons and because of the high speed of
rotation, however, a pneumatic~system is preferred for the
spray bell drive. Favorably provided is an enameling
control where, in the case of changing from one color to
another, a new set value is stored in the comparator and,
if the actual speed of rotation is greater than the new
set value, a switching amplifier will cause a valve to open
and feed braking air to the pneumatic drive of the spray
bell. If, upon changing from one enamel color to another, a
rotational speed lower than the one used before is selected,
a long time will pass upon shutoff of the compressed air used
for the drive until the rotational speed of the spray bell
drops to the new value. Therefore, the braking process is
suitably accelerated in the simplest fashion by an additional
nozzle which acts on the turbine wheel in a braking
direction.
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Emb~diments of the invention will be explained hereafter with
reference to the attached drawings, in which contextexpress re~erence is
made to the drawings withi~regard to the teaching of the invention, due to the
simplicity and clarity of the drawings.
Figure loshows a schematic of a first embodiment according to the
invention;
Figure 2 shows a~lschematic of a modified part of Figure l; and
Figure 3 shows a rotational speed curve of two electrostatic -
enamel~ng systems with a revolving spray beIl, one operating controlled and
one operating uncontrolled.
To begin with~ reference is made to Figure 1. A spray head 1
supports a spray bell 2 and is mounted on a support tube la constructed
from insulating material. One end oflthe insulating tube la supports the
spray head l~ and the other end of the insulating tube la is fastened to an
enameling system point which is connected to ground potential. The enamel to
be atomized is fed to the spray bell 2 via an enamel line (not shown). Other
llnes~referred to below) make it possible to feed to the spray bell com-
pressed air for improving the spray pattern and a solvent for cleaning the
,
spray bell in the case of color changes. The spray head is kept at high
voltage potential (100 to 150 KV direct voltage) by a high voltage supply
not illustrated in detail.
~ The spray bell 1 is driven by a turbodrive which, while not il-
lustrated in detaill~ is connected with the spray bell 2 in non rotating
fashion and features an impeller whbel 3~ and a compressed air nozzle ~not
shown). Compressed air is fed via a compressed air line ~ to the compressed
air nozzle.
Arranged on the back of the impeller wheel 3 is a mirror wheel 5
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with n segment faces, with the mirror wheel 5 revolving past a reflection
head 6. Light from a light-emltting diode 8~en~ergi~ed ~ya voltage source 8
is transmitted via an optical fiber conductor 7 and the reflection head 6 to
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the mirror wheel 5. The bright segments of the mirror wheel 5 reflect the
light back to the reflection head 6 and pass it via another fiber optics
conductor 9 to a photoelectric receiver 10, for instance a photoelectric
resistor. A potential separation is accomplished through the two light con-
d~ctors 7, 9 between the spray head connected to high voltage and the emitter-
receiver electronics connected to ground potential.
In order to keep the light conductors 7 and 9 as short as pos-
sible and provide mechanical protection for them, they are run through the
interior of the insulating support tube la. The light conductors Z, 9, as
well as the light-emitting diode 8a and the photoelectric receiver 10 and
their leads, are molded in a pressure-resistant manner in a PTB-tested~
housing. This housing is mounted at the rear end of the insulating support
tube la.
Another possibility would be to arrange the light-emitting diade `
and ~he photoelectric receiver outside the hazardous area (indicated by the
broken double lines in Figure 1) and the use of longer llght conductors.
However, these light conductors must be run from the turbodrive to the wall
of the enameling booth. In the case of a~vertically moving spray head, they
~ must be capable of withstanding flexing. In addition, with such an arrange-
- ment, an increased light loss may have to be tolerated. This loss can be
compensated for by increased transmitting power of the light-emitting
diode andlor by more sensitive photoelectric receivers.
The light pulses ~rotational speed multiplied by number of mirror~
wheel segments) received by the photoelectric receiver 10 are amplified by a
flip-flop amplifier 11 (Schmitt trigger) and so processed that they can be
passed to a frequency-to-voltage converter 12. Ihe output of this converter
12 is a voltage proportional to the speed of rotation and permits indication,
for instance by means o~ a digital voltmeter 13. Alternatively, a frequency -~
meter 14 may as well be connected before the transducer 12, as is indicated
by broken lines in Figure 1.
The voltage signal proportional to the speed of rotation is
passed as an actual value to the comparator 15 which, contingent on the set
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value of a manually adjustable set value potentiometer 16 and/or of the set
value output signal of a computer 17, transmits a control voltage to an am-
plifier 18. Adjusted on this amplifier 18 are the zero point and the limit
for a voltage-pressure ~V/P) transducer 19. The output signal of 0.2 to 1
bar of transducer 19 is amplified by a pressure amplifier 20 to a value of
approximately 1 to 6 bars. The air flow to the turbodrive can be shut off
completely by means of a solenoid valve 21.
In order to enable a quick rotational speed drop when changing
an enamel which is applied with a spray head 1, the turbine is equipped with
an additional braking air connection which is supplied by a braking air
line 22. The air line 22 is connected to an appropriate braking nozzle (not
shown) which is disposed to direct air on to the turbine wheel in a hraking
direction.
A control command "rotational speed reduction" is given by the
overriding enameling control to the electronic control circuitry of the tur-
bine. This control command stores by way of the computer 17 a new set value ;
in the comparator 15. As long as the actual value exceeds this new set val-
ue9 the braking valve 24 is opened by the switching amplifier 23 and the tur-
bodrive slowed down briefly by the braking air supplied by way of the braking
air line 22 to the braking nozzle. Instead of using a separate braking line
22 and braking nozzle a similar effect could be achieved by venting the drive
line to atmosphere to stop drive air from passing to the turbodrive.
In the operational shutoff of the turbodrive by way of the solenoid
valve 21, the braking valve 24 is inactive; the turbine comes gradually to a
standstill.
A modification of Figure 1 is illustrated in Figure 2. Instead
of the frequency-to-voltage converter 12 and the digital voltmeter 13 be-
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tween flip-flop amplifier 11 and comparator 15, a frequency meter 25 is .
coupled to the output of the frequency-to-voltage transducer 1~ and the digi-
tal value of the frequency meter 25 is trcmsmitted to a digital-to-analog
converter 26 which, for instance~ transforms a BCD-coded number ~` an ana-
log voltage and feeds it as actual value to the comparator 15.
Figure 3 shows clearly that, when the turbodrive is adjusted
to a specific idling speed and a speed regulation is not provided, the RPM
will drop with the amount of enamel fed to the spray bell per unit of time.
On the other hand, with the speed control system .according to
10the invention it is possible to keep a rotational speed adjusted in idling
condition constant,`even when the spray bell is supplied with increasing
amounts of enamel.
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