EP0402625B1 - Drive for gas metering and mixing pump - Google Patents

Description

The invention relates to the use of stepper motors for driving mixed gas pumps instead of electric motors with mechanical gears.

Mixed gas pumps are piston metering pumps of a special design. Two piston pumps are permanently coupled to one another via pairs of wheels or transmission gears. The ratio of the piston strokes per unit of time is fixed via these pairs of wheels. The volume ratio of the mixed gas pump is thus also predetermined. The volume ratio can be changed by changing the pairs of wheels. In order to be able to change the number of piston strokes per unit of time during operation, manual transmissions can be provided instead of the pairs of wheels. These manual transmissions are operated by hand or using magnets. Mixed gas pumps are driven by synchronous or shaded-pole motors. The delivery rate is therefore almost constant.

• In einer Mischgaspumpe können immer nur zwei Komponenten eines Gases gemischt werden,
• sind mehr als zwei Komponenten eines Gases zu mischen, so müssen bei n Gaskomponenten n-1 Mischpumpen in Kaskade verwendet werden,
• die Mischungsverhältnisse sind durch Räderpaare oder Getriebeübersetzung fest vorgegeben,
• Ändern des Mischungsverhältnisses bedingt ein Wechseln der Räder oder ein Umschalten des Getriebes (nur in Stufen möglich),
• die Schalthäufigkeit der Getriebe ist gering und damit ein kontinuierlicher Dauerbetrieb nicht möglich,
• hoher Wartungsaufwand,
• die Dosierleistung ist nicht veränderbar.

This design of mixed gas pumps has several disadvantages:

• Only two components of a gas can be mixed in a mixed gas pump,
• if more than two components of a gas are to be mixed, n-1 mixing pumps must be used in cascade for n gas components,
• the mixing ratios are fixed by pairs of wheels or gear ratio,
• Changing the mixing ratio means changing the wheels or changing the gearbox (only possible in stages),
• the number of starts of the gearbox is low and continuous continuous operation is not possible,
• high maintenance,
• the dosing rate cannot be changed.

It is known to use a peristaltic pump system for metering liquids in medicine and chemistry, which is driven by a permanent magnet stepper motor (cf. Feinwerktechn. U. Meßtechn., F + M, 91 (1983), No. 3, p. 111-112). However, in contrast to gases, liquids are incompressible at low pressure. In addition, it is difficult to achieve the appropriate dosing accuracy when dosing gases. In addition, hose pumps for gases become disproportionately large.

It was also known to use a piston metering pump which, in high performance chromatography, used solvents in the filling, e.g. drive into the substrate forming the stationary phase with a stepper motor (cf. DE-A 29 50 412). This is intended to compensate for pressure fluctuations and / or to keep the delivery rate constant. With mixed gas pumps, however, the delivery rate of the individual gases should be able to be varied continuously.

Finally, a metering device was known in which a piston is moved in a cylinder by means of a threaded spindle through a stepper motor in order to convey the cylinder contents into an analyzer in several portions of the same amount (cf. GB-B 1 202 079). Continuous funding is not possible with such an institution.

The task now was to find a replacement for the mechanical transmission gears or pairs of wheels for mixed gas pumps and at the same time to create the possibility of producing continuously changing mixing ratios of multi-component mixtures in one step.

It has been found that the task can be solved with the help of electric stepper motors.

The invention thus relates to a mixed gas pump as a piston metering pump, consisting of two coupled piston pumps, characterized in that a stepper motor is used for driving, the piston pumps not being connected to one another by mechanical transmission gears or pairs of wheels.

Any commercially available mixed gas pump can be used for the use according to the invention, provided the drive can be removed. Piston pumps are preferred.

A stepper motor is installed on the mixed gas pump instead of the electric motor and the gearbox. If several mixed gas pumps work as a cascade, a stepping motor is preferably only attached to the first pump of the cascade as a drive.

Suitable stepper motors are commercially available and can be selected from the manufacturers' lists. Their size and their technical data depend on the requirements of the mixed gas pumps.

With a stepper motor, the walking speed and the number of steps per unit of time can be adjusted electrically. This adjustment is made via a control and power section. In this way the dosing volume per unit of time can be changed. This control part can in turn be controlled by an analysis computer or a similar device.

• die hohe Genauigkeit der Schrittmotoren,
• kein mechanischer Verschleiß (im Gegensatz zu Schaltgetrieben),
• keine Notwendigkeit einer Rückstellungsmeldung,
• geringe Wartung,
• transparenter Aufbau der Hardware,
• die Möglichkeit einer kontinuierlichen Gasmischung während des laufenden Betriebes.

Using a stepper motor as a drive for a mixed gas pump has the following advantages:

• the high accuracy of the stepper motors,
• no mechanical wear (in contrast to manual transmissions),
• no need to report a deferral,
• low maintenance,
• transparent hardware structure,
• the possibility of a continuous gas mixture during operation.

example

In a cascade of several mixed gas pumps, which produced an H₂ / C₂H₄ reference gas mixture for a gas analyzer, the first pump was equipped with a stepper motor, while the other pumps in the cascade kept their normal drives by means of gear drives.

The control part of the stepper motor was controlled via a V.24 interface of the analysis computer.

The value transmitted by the analysis computer was converted into the corresponding number of steps in the control section and the corresponding number of steps of the stepping motor was set via the power section.

The mixed gas pump driven in this way created the gas mixing ratio H₂ / C₂H₄, which after passing through the pump cascade gave the reference gas mixture for the gas analyzer. Depending on the process situation, the mixing ratio of the gases H₂ / C₂H₄ could be changed continuously and automatically by the analysis computer during operation without any mechanical intervention.

Compared to the old arrangement with a mechanical gearbox, the new arrangement extended the operating time of the system because maintenance was practically no longer necessary.

Claims (1)

1. A gas-mixing pump as a proportioning piston pump, comprising two piston pumps coupled to one another, wherein a stepping motor is used for the drive, the piston pumps not being connected to one another by mechanical transmission gearing or pairs of wheels.