Electricity: motive power systems – Braking
Patent
1999-06-23
2000-09-19
Ro, Bentsu
Electricity: motive power systems
Braking
318365, 318757, H02P 308
Patent
active
061217395
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND
1. Field of the Invention
This invention relates generally to methods and apparatus for dosing liquids, and more particularly to a method and apparatus for increasing the dosing precision of a dosing pump.
2. Discussion of Related Art
Dosing pumps are used for the precision dosing of different liquids. Typically a dosing pump is actuated by external pulses, by a standard signal or by an internal clock. The external pulses can also be generated, for example, by a liquid counter and optionally multiplied or divided. Through use of an eccentric gear, the diaphragm or the piston of the pump is supposed to complete a stroke, the maximum stroke frequency generally amounting to about 125 strokes/minute.
Asynchronous motors are preferably used for dosing for metering pumps with a motor output of about 40 watts or higher. Through their moment of inertia, asynchronous motors are capable of slowing down without braking when switched off. This results in overdosing. With motor outputs of about 40 to 60 watts, the slowing down responsible for this overdosing can be reduced, but not avoided, by stiffening the mounting or by fitting a pressure-retaining valve. A particular problem can arise where the diaphragm is driven through an eccentric gear. If the motor comes to a stop outside its dead center, the starting torque for the next cycle can exceed the permitted value so that the motor does not start.
Mechanical brakes are unsuitable for stroke frequencies of more than 3- strokes/minutes. Hitherto, electrical brakes have only been practicable for motors with an output of more than 1 kilowatt and for braking times of more than 1 second. In the case of electromagnetic braking, the electromagnetic forces are in any case only used to drive mechanical brakes. With eddy-current brakes, additional external parts are necessary which is expensive and makes such brakes impossible to use for small motors.
SUMMARY OF THE INVENTION
If exact dosing is to be achieved, controlled braking of the pump motor is essential. Accordingly, the problem addressed by the present invention is to develop a method for braking the motor of a dosing pump in such a way that the diaphragm or the piston of a dosing pump comes to a stop in the exact dead center position after the suction stroke. The dead center position after the suction stroke is also known as the rear dead center. Another problem addressed by the invention is to develop a dosing pump to carry out this method which would not require additional parts on the motor. Braking times of well below 1 second are desired, and be suitable for a range of motor sizes.
The problems stated have been solved by one embodiment of the invention in which the motor supply voltage to a motor of a dosing pump is switched off about half a cycle time before a required dosage stoppage, the motor then running freely for part of the cycle time before a d.c. voltage is applied to the motor for about one quarter of the cycle time so that an eddy current is induced in the rotor which generates a counter-magnetic field that brakes the rotor to a standstill. Another embodiment of the invention a dosing pump includes a motor driven by a motor supply voltage adapted to be switched off about half the cycle time before a desired stoppage, the self induction voltage in the free-running motor then falls to a value which does not affect a braking circuit, after which a d.c. voltage is applied to the motor for about one quarter of the cycle time so that an eddy current is induced in the rotor and generates a counter-magnetic field which brakes the rotor to a standstill.
The advantage of the method according to the invention is that the motor itself is the brake and no additional parts are needed. A particular advantage is that the d.c. voltage can be applied by simple means. The d.c. voltage required for braking may be taken from a battery or an accumulator. However, it may also be provided directly or by transformation from the operating voltage or, with particular advantage, by rectification and storage in a cap
REFERENCES:
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patent: 4110670 (1978-08-01), Araki et al.
patent: 4195255 (1980-03-01), Guttmann
patent: 4245483 (1981-01-01), Murai
patent: 4354147 (1982-10-01), Klaussner
patent: 4721439 (1988-01-01), Leinweber
patent: 5198734 (1993-03-01), Johnson
patent: 5449992 (1995-09-01), Geiger et al.
J. Murphy Glenn E.
Jaeschke Wayne C.
Lang Apparatebau GmbH
Leykin Rita
Ro Bentsu
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