Method and Circuit for Operating a Spray Gun Having a Vibrating Armature Drive

The invention relates to a spray gun having a vibrating armature drive and to circuits for energizing the vibrating armature drive.

Spray guns with vibrating armature drives typically have a piston which is movable in a pump cylinder, is urged toward one position by a tension spring to suck in liquid and can be shifted against the force of the tension spring by an electromagnet to convey the sucked-in liquid to a spray head of the spray gun. The pump piston, the armature of the electromagnet and the tension spring together form a system capable of vibrating in response to energy supplied continuously thereto from an a.c. voltage network during spraying of the liquid.

In these known spray guns, it is possible to adjust the spray output, namely, the amount of liquid which is sprayed per unit of time, by limiting the suction stroke of the pump piston with an adjustable mechanical stop. In this manner, however, the spray output can only be reduced to approximately 300 g/min., since a further reduction of the piston suction stroke will cause the liquid to be no longer satisfactorily sprayed.

The purpose of the present invention is to provide a method and a circuit for operating a spray gun having a vibrating armature drive which assures good spraying of the liquid even in the case of a small spray output.

The foregoing and other purposes of the invention are met by providing a method and circuit for controlling the vibrating armature drive of a spray gun of the foregoing type in which an a.c. energizing signal having abruptly rising leading edges and a variable pulse width is supplied to the vibrating armature drive.

Up to now, the adjustment of the spray output in vibrating armature driven spray guns was effected by adjusting the end of the suction stroke, the end of the conveying stroke being chosen independently of the spray output and at most being adjusted according to the viscosity of the liquid to be sprayed, and the pump piston at the same time effecting a forced operation of a check valve on the conveying side thereof (compare Swiss Pat. No 360 625). The present invention works with a fixed and predetermined end point for the suction stroke and, through variation of the total effective electrical power fed to the vibrating armature drive, varies the end point of the piston conveying stroke. With this, it is possible to supply with the pump piston a very small amount of liquid to the spray head with each stroke by stopping the pump piston shortly after the point in time at which it passes a suction opening provided in a sidewall of the pump cylinder. One obtains at the spray head the high pressure which is necessary for a fine spraying of the liquid because the vibrating armature drive is energized by successive pulses having abruptly rising leading edges which effect abrupt acceleration of the piston.

The present invention has further important advantages, compared with the state of the art, including a considerable reduction in the noise produced during operation and a better stability of the spray output once it has been adjusted. In the known spray guns, a hard hitting of the armature connected to the piston occurs against a housing-fixed stop at the end of the suction stroke, which results in an unpleasant rattling noise. Also, this hard hitting can alter the positional adjustment of the stop itself, namely, in a direction effecting a greater spray output, which is very undesirable, especially during the application of fine lacquers.

In one further development of the invention, a particularly abrupt stopping of the pump piston is obtained. With this, it is possible to adjust the spray output to a particularly small value but still assure that during the quick movement of the pump piston between the abrupt acceleration and the abrupt stopping the necessary high pressure in the spray head of the spray gun is built up.

A further development can be carried out particularly simply by using easily available supply voltage sources. For this, in particular, the common a.c. voltage network and d.c. voltage sources can be used.

The invention will be discussed in greater detail hereinafter in connection with exemplary embodiments with reference to the enclosed drawing, in which:

FIG. 1 is a schematic illustration of a spray gun with a vibrating armature drive and an operating circuit therefor;

FIG. 2 illustrates a modified embodiment of the operating circuit of FIG. 1; and

FIG. 3 is a circuit diagram illustrating an operating circuit for a spray gun having a vibrating armature drive.

FIG. 1 schematically illustrates a spray gun 10 having a vibrating armature drive which is not shown in detail and which includes a pump piston which can be reciprocated in a horizontal direction within a pump cylinder. A suction opening is provided in the cylinder wall of the pump cylinder and is connected by a suction pipe to a storage reservoir 12 for the liquid to be sprayed. The pump which is formed by the pump piston and the pump cylinder does not have a separate suction valve on the suction side, but rather operates on the suction side with aperture control effected by the piston itself. A check valve is provided in the pump cylinder on the outlet side thereof, through which valve the conveyed liquid is fed to a spray head 14. These details of the pump design and of the vibrating armature drive are not illustrated in the drawings because they are known to the man skilled in the art.

An operating circuit for the spray gun 12 is disclosed in the form of a block diagram in FIG. 1 and includes a supply voltage source 16, a controllable switch 18 and a signal generator 20. The supply voltage source 16 is preferably a d.c. supply, for example, a battery or an a.c. power supply and a rectifier. The signal generator 20 has an adjustable pulse width and preferably has in addition an adjustable frequency. The controllable switch 18 is preferably a semiconductor switch, in particular a power transistor or a thyristor. These electronic building elements are actually known and can be obtained in commerce in various forms and need not be described in any greater detail here.

The arrangement which is shown in FIG. 1 operates as follows. The signal generator 20 produces a square wave signal with abruptly rising leading edges and abruptly falling trailing edges. This control signal is made available with little output power and serves to modulate by means of the switch 18 the supply voltage applied to the input side of the switch 18. Thus, one obtains at the output of the switch 18 an energizing signal for the spray gun having the same frequency and the same keying ratio (period/pulse width) as the control signal, but having a higher output power. With this, a magnetic field is periodically and abruptly built up and reduced in the field coil of the vibrating armature drive of the spray gun, causing the pump piston to be repeatedly and abruptly accelerated and stopped. The amplitude of the pump conveying stroke can be reduced by increasing the keying ratio of the control signal, for example, by reducing the pulse width of the control signal, since the total energy per unit time which is fed to the vibrating armature drive is reduced accordingly but very quick build-up and reduction of the magnetic field is still achieved. For an optimum adjustment of the control signal to the dynamic characteristics of the vibrating armature drive and the pump at small conveying stroke amplitudes, it is possible to change the frequency of the control signal slightly when changing the keying ratio. This change in frequency can in practice be effected simultaneously with the change in the keying ratio, for example, by mechanically coupling two potentiometers of the square wave signal generator which respectively control the pulse width and the period.

FIG. 2 illustrates an operating circuit very similar to the one in FIG. 1, the only significant difference being that a power amplifier 22 is provided in place of the supply voltage source and controllable switch and amplifies the control signal from the signal generator 20 to produce the energizing signal fed to the spray gun 10. Operation of this operating circuit produces the same result as operation of the operating circuit of FIG. 1.

FIG. 3 illustrates details of a particular operating circuit for the embodiment of FIG. 1, wherein the supply voltage source is shown as power terminals 24,26 connectable to an a.c. power network.

A thyristor 28 is provided as the controllable switch and is switched in series with the coil 30 of the vibrating armature drive and a main switch 32 which can be operated manually. The gate terminal of the thyristor 28 is connected through a bi-directional diode 34 to a phase-shifting voltage divider circuit which is connected in series across the thyristor 28 and consists of an adjustable resistor 36 and a capacitor 38, the latter being bridged by yet a further series connection of a resistor 40 and a capacitor 42. The network junction between the resistor 40 and the capacitor 42 represents the output of the phase-shifting voltage divider circuit and is connected to the bi-directional diode 34.

By adjusting the resistor 36 it is possible to adjust the phase angle at which the thyristor 28 fully modulates. When full modulation occurs, voltage is applied to the coil 30 with a quickly rising edge so that quick acceleration of the pump piston occurs. By increasing the phase angle at which the thyristor 28 fully modulates, the keying ratio can be increased and the amplitude of movement of the pump piston can be reduced. In practice, phase angles of between 15.degree. and 170.degree. can be effected, and consequently the stroke of the pump piston can be varied within very wide limits.

The operating circuit which is shown in FIG. 3 can, if desired, be easily incorporated into the housing of the spray gun with the vibrating armature drive.

If one uses a triac in place of the thyristor 28 of FIG. 3, then it is possible to operate the vibrating armature drive at twice the frequency of the a.c. power source.

Although particular preferred embodiments of the invention have been disclosed in detail for illustrative purposes, it will be recognized that variations or modifications of the disclosed apparatus, including the rearrangement of parts, lie within the scope of the present invention.