GB2034919A - A control device - Google Patents

Abstract

Two-point controllers (2, 3) are provided for a two compartment heating installation, one having a narrow switching hysteresis (3) and the other having a wide switching hysteresis (2). The controllers receive, in parallel, a control deviation +/- Kw and are linked to two setting members (Y, Y2) via a two-pole changeover switch (4a, 4b). At start-up both controllers operate and in a subsequent control stage the controller 3 cycles. When the disconnecting points of both controllers (2, 3) have been exceeded by the control deviation the switch (4a, 4b) is changed-over and the connections to the setting members interchanged when the control deviation falls below the switching-on points of both controllers (2, 3) the switch is restored. <IMAGE>

Description

SPECIFICATION A control device for a system controlled by two partial setting members of an overall setting value The present invention relates to a control device for a system controlled by means of two partial adjusting members of an overall control output, for example, a two compartment boiler installation.

It is known in controlled systems with variable energy supply for the setting member controlling the energy supply to be divided into two or more partial adjusting members.

For example, in heating systems so-called multi-compartment installations, the required heat output, instead of being provided by one burner of high output, is provided by two or more burners of correspondingly lower individual output, which in accordance with the heat demanddaytime or night setting, weekend reduction, partial separation of whole building complexes and the like, are adapted to be connected individually or in groups to a basic output. According to circumstances, multiple compartment installations are predominantly formed by two compartment installations, in which each of the two compartments provide 50% of the overall output.Thus, upon demand of up to 50% of the overall output one compartment is shut down and controlled, whilst, with a demand of more than 50% of the overall output, one compartment operates continously and the other compartment is controlled.

To control such multiple, and especially also two compartment installations two control systems are known. In one case, control occurs by using a three-point control device acting on a stepping mechanism which rotates the stepping mechanism, provided with a control cam, in accordance with either a positive or negative control deviation in one or the other direction of rotation and thereby connects or disconnects the partial setting values individually.The disadvantages of this control device, on the one hand, reside in the necessity to use a technically involved and therefore costly three-point controller and, on the other hand, in the switching time delay of the stepping mechanism both when connecting and disconnecting, resulting from the need to maintain a certain minimum spacing between the switching cams in combination with the stepwise rotation of the stepping mechanism in dependence upon the control pulses emitted by the three-point controller. The result thereof that there is a relatively large exceeding of or falling below the predetermined nominal value, or, especially in the starting phase, the danger of a considerable exceeding or falling short of the setting value, with a considerable time dependence.These controllers therefore may only be used with satisfactory result in installations with known and unchanging control system parameters.

For controlling such systems, especially for controlling two compartment installations, socalled double two-point switches, which are substantially comprised of two-point controllers acting on one of the two setting values, whereby the necessary effective condition is varying the nominal value to avoid overlapping switching points for a definite control.

The disadvantage of this controller lies particularly in the reference to two different nominal values, the necessary dependence upon the loading of the system on the "true" predetermined nominal value, when connecting or disconnecting the other of the two part setting values over the whole setting region, results in a "false" nominal value or a permanent control deviation. The control deviation is greater the larger a system-conditioned exceeding or falling short and exceeding the nominal value is provided or has to be taken into account, since the spacing between the related nominal values has to exceed the maximum exceeding of or falling short in every case.A further disadvantage also consists in that the control only provides functionally satisfactory results as an electronic circuit, since mechanical two point switches, after a relatively short life, show signs of fatigue which can lead to switch point overlaps and hence to indefinite control conditions.

It is an object of the present invention to provide a simple controller which over the whole system permits control, relative to a common nominal value, within a narrow overshoot range.

According to the present invention there is provided a control device for a system controlled by means of two partial setting members of an overall setting value, for example, a two compartment heating installation using two two-point controllers, wherein a first twopoint controller, having a wide switching hysteresis, and a second two-point controller, having a narrow switching hysteresis, relative to the same nominal value, are provided in parallel with each other and which are linked with the two setting members by means of a two-pole changeover switch, which is controlled by the first two-point controller, having a wide switching hysteresis, such that upon exceeding the disconnecting points of both controllers polarity reversal occurs and after such a polarity reversal, upon falling below the switching-on points of both controllers restoring of the original polarity occurs.

A control device is provided for a control system controlled by two setting members, by means of which, by using two simple twopoint controllers, satisfactory control becomes possible relating to a single nominal value, whereby the controller is readily able to follow every change of nominal value, either by shortening the control system-in heating in stallations by cutting whole building complex es-or by adapting the nominal value in heating installations by connecting or discon necting the night or weekend reduction. A switching point overlap and hence the occur rence of indefinite control conditions is impossible switching delays do no occur, whereby in the starting region a rapid approach to the nominal value without exceeding the overshoot range inherent in the system is attained.

Herewith the two-point controller having the wide switching hysteresis acts as limit value generator and the two-point controller having the narrow switching hysteresis acts over the whole control region to modulate the control system to the nominal value with a narrow bandwidth predetermined by the hysteresis width.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is the block wiring diagram of the control device of the invention; Figure 2 is a schematic view of the controlling of the setting members in the starting phase of the system; and Figure 3 is a diagrammatic view of the actual value course with varying, predetermined, nominal values in the range from below 50%, or more than 50% of the overall setting value.

The numeral 1 in the drawing denotes the system to be controlled, for example, a heating installation, the overall energy requirement of which is covered by the two setting members Y, and Y2, for example, gas burners actuated by magnetic valves. The two setting members are individually or jointly put in circuit or disconnected in dependence upon the predetermined nominal value or the con trol deviation + xw by means of two two-point controllers.

A two-point controller 2 having a wide switching hysteresis and a two-point controller 3 having a narrow switching hysteresis are located in parallel with each other and are linked with the two setting members Y1 and Y2 by means of a twin-pole changeover switch with the switch members 4a, 4b. Both two point controllers are (see especially Fig. 2) related to the same nominal value S, so that the controller 2 has a switching hysteresis E2 A2 enclosing the switching hysteresis E3-A3 of the controller 3.The two pole switch 4a, 4b is controlled by the controller 2, in the manner indicated by dotted lines 5, having a wide switching hysteresis in such a manner that on exceeding the disconnecting points A3, A2 for both setting values Y1, Y2 the polarity is reversed, and after polarity reversal restoring to the original polarity occurs only after dropping below the switching on points E3, E2 for both setting members.

The control device operates as follows: When switching on the installationsee Fig. there is a large negative deviation-xw corresponding to the actual value I in control system 1 (Fig. 1). The actual value hence lies outside the switching hystereses of both con strollers below the switching-on points E3 and E2. Both setting members Y, and Y2 are switched on, whereby the changeover switch 4a, 4b assumes the position indicated by full lines in Fig. 1, in which the controller 2 acts on the setting member Y1 and controller 3 on the setting member Y2.

For the time being assume (see Fig. 3) that the nominal value S, is set to a value which corresponds to a performance of less than 50% of the overall setting value Y1 + Y2 Due to the simultaneous switching on of both setting members, the actual value follows a steep curve determined by the overall output L(Y, + Y2), until the shut-off point A3, of the controller having a narrow hysteresis is reached wereupon the setting member Y2 is shut-off. Consequently, with the assumed conditions, performance slightly less below 50% is demanded repeatedly alternating between connecting and disconnecting of the setting member Y2 between the points E3 and A3, as shown in the graph in Fig. 3.Owing to the consistent output supplied by the setting member Y, (graph L Y1) the actual value drop assumes a shallower course from one switching change to the next switching change.

Upon reaching a satisfactory approximation of the overall system to the nominal value or if the actual value no longer drops below the switch-on point E3 (point Z2), further approach to the nominal value is caused by the setting member Y, alone corresponding to the shallower course of the graph LY,. Upon reaching the disconnecting point A2 at Z3, the second setting member Y, is also disconnected and at the same time the polarity of the changeover switch 4a, 4b (Fig. 1) is reversed into the position shown in dotted lines, in which the controller 2 takes over the control of the setting member Y2 and the controller 3 the control of the setting member Y1.

Owing to the disconnection of both setting members there now occurs a steeper drop of the actual value curve until the switch-on point E3 of the controller 3 is reached, which now cuts in the setting member Y, and controls the control system 1 by means of this setting member within a narrow switching hysteresis by the set nominal value.

If, e.g. when cancelling the night reduction, the nominal value is increased to a value corresponding to more than 50% of the overail output L (Y2 Y1), then the control system has an actual value located below the switchon point E2. Hence, with the immediate switching on of both setting members, there occurs a simultaneous polarity reversal of the switch 4a, 4b again into the position shown in full lines in Fig. 1. The system is accelerated with both setting members in accordance with the graph L (Y2 + Y1) until the disconnecting point A3 at Z5 is reached, at which point the setting member Y2 is disconnected.

Under these nominal value conditions therefore the setting member Y1 for covering the basic load remains permanently switched in, whilst the setting member Y2 serves to control the control system also within the narrow hysteresis E3 to A3 of the controller 3.

Reduction of the nominal value, e.g. when switching on the night reduction again at Z6 corresponds to a considerable positive control deviation + xw, resulting in immediate switching off of both setting members, and subsequent polarity reversal of the switch 4a, 4b into the position shown in broken lines in Fig.

1 The control system drops steeply until reaching the switching on point E3 at Z7. The setting member Y1 in the manner described above again takes over alone with permanently shut off setting member Y2 the control of the control system within the narrow hysteresis of the controller 3 near the new predetermined nominal value.

Claims (2)

1. A control device for a system controlled by means of two partial setting members of an overall setting value, for example, a two compartment heating installation using two two-point controllers, wherein a first two-point controller, having a wide switching hysteresis, and a second two-point controller, having a narrow switching hysteresis, relative to the same nominal value, are provided in parallel with each other and which are linked with the two setting members by means of a two-pole changeover switch, which is controlled by the first two-point controller, having a wide switching hysteresis, such that upon exceeding the disconnecting points of both controllers polarity reversal occurs and after such a polarity reversal, upon falling below the switching-on points of both controllers restoring of the original polarity occurs.

2. A control device substantially as herein described with reference to and as illustrated in the accompanying drawings.