GB2068152A - Automatic control of fluid motor speed - Google Patents

Abstract

A feed-back system, for controlling the speed of a fluid motor (1) includes an optical-signal transmitter (8a), an optical-signal receiver (10), an optically sensitive marker (5) provided on the motor and operable through a reflection head (6) to couple the transmitter to the receiver intermittently as the motor rotates and provide an output signal indicative of motor rotation. The output signal is converted to a voltage in converter 12 and compared at 15 with a desired value set manually at 6 or from a computer at 17. The output of comparator 15 is used to control the supply of motive fluid through valve 21 and braking fluid through valve 24. In a modification the comparator 15 may be wholly digital. <IMAGE>

Description

SPECIFICATION Motor speed control This invention relates to a feedback system for controlling the speed of a fluid motor and is particularly applicable to an electrostatic enameling system of the kind having a revolving spray bell supported by a spray head, a spray bell drive, an enamel line through which the enamel can be fed, separate lines for feeding compressed air and solvent, and a voltage supply serving to apply voltage across the spray bell.

Revolving spray discs 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 discs or spray bells continuously. The spray discs or spray bells were mostly driven by electromotors at ground potential, but also by slowrunning 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 bearings.

Newly developed enamel materials, such as water 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, connected to high voltage, are used. The turbodrives consist usually of a compressed air nozzle and an impeller wheel at which the compressed air from the nozzle is directed. The 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 favourable 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 rotation, a high degree of atomisation is not achieved and thus no optimum enameling results either.

It is the principal object of the present invention to accomplish exact control of the speed of rotation of a fluid motor capable of use as the turbodrive for the spray disc or spray bell (hereinafter referred to as spray bell for convenience) of an electrostatic enamel application system of the kind referred to without incurring problems in connection with the high voltage.

To this end, according to the 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 transmitter, 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 for braking the motor, the converter and braking means controlling motor speed.

When the invention is applied to an electrostatic enamel application system of the kind referred to, a desired speed of rotation can accurately be achieved and kept constant at any operating conditions. 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 likewise corresponding rotational speed changes for optimisation which, according to the invention, can be preprogrammed as well.

Experiments have shown that any enamel material, even enamel materials of the same type but with different shades can be opti mally applied only at a specific speed of rotation which varies from the optimum RPM 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 materials and the selected shade which, in the case of multicolour 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 or a spray bell drive connected to high voltage, a problem arises: that is, transmitting the signal obtained through rotational speed measuring, for fur ther processing by the control circuitry, from the high voltage side to a point which is approximately at ground potential. For that purpose, a rotational speed sensor is preferably provided 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 a particularly favourable 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 rotational speed sensor design avoids conductive signal transmission devices, such as electrical lines and cables. By using, for the transmission of the signal proportional to the speed of rotation, a light which is modulated in accordance with the rotational speed of the spray bell, insulation problems are 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 fashioned, e.g., as a mirror wheel with a plurality of mirror segment surfaces. Owing to the good reflection of the mirror segment surfaces, a weaker light source can be employed.

In order to eliminate outside light influences and, on the other hand, to make do with a relatively weak light source, a glass fiber optic 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 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 lighttransmission section, favorably with optical fibers and sufficient 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 sensitive than when transmitting analog light signals.

The light source is suitably a light-emitting diode, especially for reason of utility life and heat generation. 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 outside light influences. A still more rugged and operationally safe design of the rotational speed sensor is obtained by moulding the light source, the photoelectric receiver, their electrical leads, and the optical fibre parts terminating at them, in pressureresistant fashion in a housing which favourable is fastened to the insulating tube.

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 embodi- ment, the signal-processing and forming stages arranged after the rotational speed sensor comprise a flip-flop amplifier 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 feature a flip-flop amplifier, a frequency meter, and a digital-analog converter. Thus, the actual signal processing is handled 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 beil. 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.

Embodiments of the invention will be explained hereafter with reference to the attached drawings, in which context express reference is made to the drawings with regard to the teaching of the invention, due to the simplicity and clarity of the drawings.

Figure 1 shows a schematic of a first embodiment according to the invention; Figure 2 shows a schematic of a modified part of Fig. 1; and Figure 3shows a rotational speed curve of two electrostatic enameling systems with a revolving spray bell, one operating controlled and one operating uncontrolled.

To begin with, reference is made to Fig. 1.

A spray head 1 supports a spray bell 2 and is mounted on a support tube 1 a constructed from insulating material. The one end of the insulating tube 1 a supports the spray head 1; 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. Other lines make it possible to feed to the spray bell compressed 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 illustrated in detail, is connected with the spray bell 2 in non-rotating fashion and features an impeller wheel 3, and a compressed air nozzle. Compressed air is fed via a compressed air line 4 to the compressed air nozzle.

Arranged on the back of the impeller wheel 3 is a mirror wheel 5 with n segment faces, with the ri'irrnr wheel 5 revolving past a reflection head 6. Light from a light-emitting diode 8a is transmitted via an optical fiber conductor 7 and the reflection head 6 to the mirror wheel 5. The bright segments of the mirror wheel 5 reflect the light to the reflection head 6 and pass it via another fiber optics conductor 9 to the photoelectric receiver 10, for instance a photoelectric resistor.

A potential separation is accomplished through the two light conductors 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 possible and provide mechanical protection for them, they are run through the interior of the insulating support tube la. The light conductors 7, 9, as well as the lightemitting diode 8a and the photoelectric receiver 10 and their leads, are molded in a pressure-resistant manner in a PTB-tested housing. Said housing is mounted at the rear end of the insulating support tube 1 a.

Another possibility would be the arrangement of the light-emitting diode and the photoelectric receiver outside the hazardous area (indicated by the broken double lines in Fig.

1) and the use of longer light 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 arrangement, an increased light loss may have to be tolerated. This loss can be compensated for by increased transmitting power of the light-emitting diode and/or 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 transducer 1 2. The output of this transducer 1 2 is a voltage proportional to the speed of rotation and permits indication, for instance by means of a digital voltmeter 1 3. Alternatively, a frequency meter 1 4 may as well be connected before the transducer 12, as is indicated by broken lines in Fig. 1.

The voltage signal proportional to the speed of rotation is passed as an actual value to the comparator 1 5 which, contingent on the set value of a manually adjustable set value potentiometer 1 6 and/or of the set value output signal of a computer 17, transmits a control voltage to an amplifier 1 8. Adjusted on this amplifier 18 are the zero point and the limit for the voltage-pressure (V/P) transducer 19.

The output signal of 0.2 to 1 bar of transducer 1 9 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.

A control command "rotational speed reduction" is given by the overriding enameling control, which may be of the type described in U.S. Serial No. 35,105, filed May 1, 1979, and assigned to the assignee of the present invention, to the eiectronic control circuitry of the turbine. This control command stores by way of the computer 1 7 a new set value in the comparator 1 5. As long as the actual value exceeds this new set value, the braking valve 24 is opened by the switching amplifier 23 and the turbodrive slowed down briefly by the braking air supplied by way of the braking air line 22.

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 Fig. 1 is illustrated in Fig.

2. Instead of the frequency-to-voltage transducer 1 2 and the digital voltmeter 1 3 between flip-flop amplifier 11 and comparator 15, a frequency meter 25 is coupled to the output of the frequency-to-voltage transducer 1 2 and the digital value of the frequency meter 25 is transmitted to a digital-to-analog converter 26 which, for instance, transforms a BCD-coded number to an analog voltage and feeds it as actual value to the comparator 1 5.

Fig. 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.

With the inventional speed control according to Fig. 3, 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.

Claims (8)

1. A feedback system for controlling the speed of a fluid motor having a motor fluid input, the system including an optical-signal transmitter, 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 opticalsignal 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 for braking the motor, the converter and braking means controlling motor speed.

2. The system of Claim 1, wherein the braking means includes means for comparing motor speed to desired motor speed, and valve means responsive to the comparing means for controlling braking fluid flow in the motor.

3. The system of Claim 2, wherein the valve means includes means for venting fluid from the motor when motor speed is excessive.

4. The system of Claim 2, wherein the valve means includes means for restricting the flow of fluid from the motor fluid input into the motor.

5. The system of Claim 2, wherein the valve means includes a nozzle for directing braking fluid against motor components to effect braking of the motor.

6. An electrostatic enameling system comprising a fluid motor-driven spray bell and means according to any preceding claim for controlling the speed of rotation of said spray bell.

7. A feed-back system for controlling the speed of a fluid motor, substantially as hereinbefore described with reference to the accompanying drawings.

8. An electrostatic enameling system according to Claim 6, substantially as hereinbefore described with reference to the accompanying drawings.