The controlled flow may be, e. g., a gaseous fluid such as air. Different types of control devices can be employed for air flow rate measurement and control. A simple control device comprises a rotatable damper placed inside a duct, whereby the rotation of the damper serves to control the cross-sectional flow area of duct and thus the air flow rate through the duct. One drawback of this kind of damper is that the air flow rate control also changes the direction of the air flow.

FI patent publication no. 50,184 discloses a different type of air flow con- trol device known as an iris damper comprising an annular housing formed by two adjacently mounted halves. A rotatable con-trol ring and the control leaves of the iris diaphragm are adapted between said halves. The control leaves are rotatably mounted by their one ends on pivot pins attached to one half of the annular damper housing. The other ends of the control leaves have pegs adapted to slidably move in radially expanding slots made on the control ring. Thus, the rotation of the control ring forces the iris orifice formed by the shaped control leaves to constrict or open as re- quired.

Conventionally, the control of the flow orifice cross sec-tion in an iris damper has been accomplished by manual tech-niques requiring, e. g., use of a wrench for rotating a control shaft end nut placed on the exterior surface of the annu-lar damper housing, whereby the rotation of the shaft by the nut at the shaft end fur-ther turns the toothed control ring that actu- ates control leaves. The toothing is made on the inner circum-ference of the control ring.

Automation of the iris control has been attempted by means of a separate motor connected via a complicated linkage mechanism to the control shaft end. In practice, however, such linkage mechanisms have been found short-lived due to the high torque needed. This is because of safety reg- ula-tions requiring the control shaft to be frictionally preloaded so tight as to prevent manual tampering of the adjustment nut by fingers. Also the control accuracy is com-pro-mised by the slack of the linkage mechanism.

It is an object of the present invention to provide a motor-actuated iris damper with a more compact construction, higher operating reliability, more precise controllability and easier maintenance over the performance of prior-art constructions. The air flow control device according to the in- vention is characterized in that -a portion of the outer circumference of the control ring is provided with toothing; -the outer circumference of the annular damper housing is provided with an opening; -in said opening, so as to extend partially outside the outer circum-ference of said annular damper housing, there is mounted a gear wheel having its teeth adapted to engage with the toothing of the outer circumference of said control ring, and -the shaft of said gear wheel is connected via reduction gear to a control motor.

By virtue of the invention, the accuracy of flow rate control is improved be- cause the prior-art linkage mechanisms can be disposed with. Resulting- ly, mechanical play is reduced and calibration accuracy of flow rate is im- proved. Inasmuch the flow rate adjuster (i. e., the control shaft end nut in prior-art constructions) is now concealed, the retaining friction of the con- trol shaft can be smaller, thus permitting the use of a control motor of lower output rating. Also this feature is one of those imparting the device a longer service life.

A preferred embodiment of the invention in characterized in that there is mounted a potentiometer on the drive shaft of the gear wheel to the end of sensing the position of the gear wheel. Thus, the potentiometer serves to provide a realtime indication of the actual control position of the damper.

Another preferred embodiment of the invention is character-ized in that the control motor, reduction gear, control gear wheel and position-indicating potentiometer are integrated into a compact unit which is adapted into a casing, whereby said unit with its casing can be mounted detachably on the housing of the flow control device. The benefit of the integrated actua- tor construction is a smaller size. Additionally, the serviceability of the de- vice is improved as said integrated actuator motor unit can be readily dis- mounted from the damper housing and replace by a new set.

In the following, the invention will be described in great-er detail with the help of an exemplifying embodiment by making reference to the appended drawings in which Figure 1 shows a side view of a flow duct iris damper equipped with a control motor unit according to the invention; Figure 2 shows a section on lines A-A of Fig. 1; Figure 3 shows a top view of the structure of Fig. 1; and Figure 4 shows a top view of the control ring and control leaves of the iris damper.

In Fig. 1 denoted by reference numeral 1 is shown a portion of a flow duct, particularly an air flow duct. The duct is provided with an iris-diaphragm type flow control device, whose housing is denoted by reference numeral 8 in the diagram. The internal con-struc-tion of the flow control device may be, e. g., similar to the iris damper dis-closed in FI Pat. No. 50,184 with the exception of the damper control technique employed herein. The iris damper according to the in-ven-tion is namely equipped with a control motor unit 2 comprising a motor 3, reduc-tion gear 4, a gear wheel 5 and a position-indicating poten-tiometer 6. The control motor unit 2 is detachably

mounted on the iris damper housing 8 as shown in the sectional view of Fig. 2 taken on lines A-A and is ex-ter-nally protected by a casing 7. A more detailed illustration of the control motor unit is also shown in Fig. 2.

The outer perimeter of the iris damper housing 8 is provided with an opening at the gear wheel 5 thus per-mitting the gear wheel teeth to mesh with toothing 10 made on the outer perim-eter of the iris damper control ring 9 (cf. Fig. 4). The gear wheel 5 is located chiefly outside the damper housing as can be seen from Fig. 2.

The flow rate control by means of the iris damper is implemented accord- ing to the invention so that the output shaft of the motor 3 rotates via the output shaft 11 of the reduction gear 4 the meshing gear wheel 5 that in turn rotates the control ring 9. Resultingly, the control leaves 12 of the iris damper are moved in a conventional manner so as to increase or constrict the orifice 13 of the iris damper, depending on the direction of rotation of the control ring.

The potentiometer mounted on the shaft of the gear wheel (which may be the output shaft of the reduction gear, as well), indicates the actual posi- tion of the iris damper control ring thus facilitating an extremely accurate control of the damper orifice.

To a person versed in the art, it is obvious that the invention is not limited to the above-described exemplifying embodiments, but rather may be varied within the scope and spirit of the appended claims. Detailed de- scription of the construction of the iris damper, which is prior-art technol- ogy, has been omitted herein as such a discussion can be found in FI Pat.

No. 50,184, for instance.