GB2294336A

GB2294336A - Controlling actuation of switch devices

Info

Publication number: GB2294336A
Application number: GB9520988A
Authority: GB
Inventors: Klaus Bott; Bernd Schukraft
Original assignee: Landis and Gyr Technology Innovation AG
Current assignee: Electrowatt Technology Innovation AG
Priority date: 1994-10-17
Filing date: 1995-10-13
Publication date: 1996-04-24
Anticipated expiration: 2015-10-13
Also published as: ITMI952044A0; DE4441581A1; CH689879A5; GB9520988D0; GB2294336B; FR2725857B1; ITMI952044A1; DE4441581B4; FR2725857A1; IT1275790B1

Description

is 1 CONMOLLING ACTUATION OF SWITCH DEVICES 2294336 The invention relates

generally to deAces of the kind set forth in L- pprecharacterising portion of claim 1.

Devices of that kind are used for example for the purposes of monitoring switches for control nxxrly.--rs such as fuel valves and ventilation flaps, wherein the microprocessor evaluates the items of information supplied by way of rnains voltage-carrying signalling lines, and discontinues corresponding control commands. in particular because of the safety factor required in the switching-on procedure and in operation of oil and gas burners, the switching-off capability of the switch devices which switch loads that are critical from the point of view of safety procedure, such as for example a fuel valve, has to be frequently checked in order to be able to detect a malfunction of the switch device before a dangerous situation occurs.

German patent specification No 30 44 047 C2 and Ge=an patent specification No 30 41 542 C2 which is of earlier priority disclose a control device for oil burners, as set forth in the ptedmrar-terisirg portion of claim 1, in which items of informtion about the switching conditions of relay and sensor contacts are transferred into a microprocessor by means of amplifiers. The switching conditions of the relay contacts are fed in the fo= of mains voltage-carrying signals to a respective amplifier which is connected on its output side to an input of the microprocessor. The microprocessor is programmed to carry out a number of test operations to ascertain whether a system with switched loads is actu&Uy performing a switch- on phase in the correct manner. For that purpose, signals are read in frcm the microprocessor and cempared to reference values. In the event of a defective consumer condition the microprocessor switches the consumers off. In the case of an oil burner which is designed for intermittent operation, the series connection of at least two relay contacts in the actuation branch of the critical loads means that those relays can be checked in regard to their switching-off capability in each switch-on operation or during the stoppage tire of 2 the oil burner, so that it is possible to detect if the relay contacts are welded together.

The control device described in Cerman patent specification No 30 44 047 C2 has a safety relay which can be actuated by way of a semiconductor switch. Because of the nechanical and electrically inductive inertia of the relay, the semiconductor switch can be temporarily switched off and thus checked in regard to its switchingoff capability, without influencing the burning operation. In continuous operation of the burner however it is not possible to detect welding of the working contacts of the safety relay, by way of which the current of the entire installation flows.

Gemian laid-open application (DE-OS) No 29 51 118 Al discloses a circuit arrangement for mnitoring power switches, in which a semiconductor switch and a working contact of a relay are connected in series with a load. The supply voltage of the relay in tkiat arrangement is derived from the control voltage of the semiconductor switch when the semiconductor switch is open and from the voltage drop across the semiconductor switch when the semiconductor switch is closed. In the event of a defect in the semiconductor switch due to a short-circuit, the relay interrupts the current path to the load as soon as the control voltage of the semiconductor switch is switched off. The interruption takes place with a time delay because of the inertia of the relay. Such a circuit arrangement is not suitable for use in a control device for a burner which is designed for continuous operation as checking of the switching-off capability of the semiconductor switch is not possible, without affecting the burning operation.

Checking the switching-off capability of mchanical switch devices in continuous operation of a burner without influencing the burning operation is possible if four switch devices are so disposed in a bridge circuit that the arrangement can be optionally diagonally load, 3 connected through. Such an arrangement however involves a relatively high level of cost if a plurality of loads which are critical from the safety point of view are to be switched.

In accordance with the invention there is provided a device as set forth in claim 1.

Embodiments of the present invention provide a control device for an installation. for example for oil or gas burners, which is reliable in operation, with which checking of the switching-off capability of those switch devices which switch on or off loads that are critical from the safety procedure point of view is possible in continuous operation of the burner, and which is inexpensive.

Embodiirents of the invention are described by way of example hereinafter with reference to the drawings in which:

Figure 1 shows a control device with a current path for each Figure 2 shows a measuring device, and Figure 3 shows a control device with two current paths for each load.

Figure 1 shows a device for controlling an installation which has a microprocessor 1 as a timing and control logic apparatus. It further includes a switch device 2 and a measuring device 3. A load L which is critical frcm the safety procedure point of view can be connected by mans of the switch device 2, by way of a current path K,, to a mains voltage U PG between a phase P and a neutral point G.

The switch device 2 has a switch 4 and a control circuit 5. The measuring device 3 includes a measuring element 6 and a circuit block 7. The measuring element 6 is connected in series with the switch 4 in the current path K 1 and has two outputs 6a and 6b which are taken to inputs 7a and 7b respectively of the circuit block 7. The circuit block 7 has an output 7c which is connected to an input R of the microprocessor 1. The control circuit 5 is connected to an output A of the microprocessor 1. A further input S of the microprocessor 1 is 4 connected to the phase P by way of a galvanic separation m 19. The microprocessor 1, the control circuit 5 and the circuit block 7 are fed in galvanically separated relationship from the mains voltage U PG, the feed circuit (not shown) having the connections V and m.

The control circuit 5 serves as an interface ci = it so that the condition - open or closed - of the switch 4 can be control led by the microprocessor 1. The purpose of the circuit block 7 is to put a signal which is measured by means of the measuring element 6 into a binary form which can be interpreted by the microprocessor 1 as the open or closed condition respectively of the switch 4.

In the exarple shown in Figgire 1, a transformer 8 is provided as the meauring elaTent 6 and the circuit block 7 comprises a bridge rectifier 9 and a ScInitt trigger 10 which are connected in series. In the open condition of the switch 4, flg in the current path K, is an alternating current I which produces in the secondary winding of the transformer 8 an ac voltage U 1 which is rectified by the bridge rectifier 9 and converted into a rectangular voltage U2 by the Schmitt trigger 10 so that both a positive and a negative half-wave of the alternating current I generates a rectangular pulse. During the duration of each rectangular pulse therefore a potential which can be interpreted as logic 111 occurs at the input R of the microprocessor 1. When the switch 4 is closed, no current I flows in the circuit K,. Consectly the voltage U, at the output of the transformer 8 also disappears and the voltage U2 becomes uniform. The closed condition of the switch 4 therefore means that a dc voltage U2 which can be interpreted as logic 101 occurs at the input R of the microprocessor 1. Use of the transformer 8 as the measuring element 6 affords simple galvanic separation between the load circuit K, and the microprocessor 1.

A trilac U is provided as the switch 4. The control circuit 5 has an eptocoupler 12 for the purposes of galvanic separation between the switch 4 and the microprocessor 1. The microprocessor 1 actuates the optocoupler 12 by way of a voltage divider forimed by two resistors 13 and 14, and a transistor 15. The firing connection of the triac 11 is connected to the centre tapping of a second voltage divider which is formed by resistors 16 and 17 and which is connected between the phase P and the neutral point G of the ac voltage mains U PG. The optocoupler 12 is connected on its output side to the terminals of the resistor 17. As long as the microprocessor 1 outputs a low signal voltage at the output A, the optocoupler 12 remains dark, the output transistor 18 of the optocoupler 12 is in a nonconducting condition and the triac 11 is fired and thus switched on in each mains; half-wave. For the purposes of suspending or delaying the firing operation the microprocessor 1 sets the output A to a high potential for a predetermined period of time so that the optocoupler 12 is bright and the resistor 17 is short-circuited by the output transistor 18 which is now conducting. With this circuitry the optocoupler 12 is in operation only precisely during the test cycle, which has an advantageous effect on its service life.

The microprocessor 1 is %.^!d.Lcataiied by a time program to the effect that, during the switch-on phase and in operation of the installation, for example an oil burner, individual loads L such as fuel valves or a fan motor are switched on in a given sequence and individual procedures such as for example the formation of a flame are menitored and if appropriate the entire installation is shut down so that the oil burner is at no time in a for ex"le explosionendangered situation.

In continuous operation of the installation the switch 4 is closed so that the load L is switched on. Therefore rectangular pulses of a predetermined length aptx-= at the output 7c of the circuit block 7. The microprocessor 1 is progra whereby the potential at the input R is continuously interrogated and a check is 6 made to ascertain whether the actual condition of the switch 4, which is derived therefrom, corresponds to the predetermined reference condition. With an 'open' actual condition and a 'closed' refference condition, the microprocessor I switches off the entire installation in accordance with a predetermined subprogram, for example by switching off a main switch 20. That ensures that a component defect in a caT!.mnent in the control circuit 5, which has the result that the triac 11 can no longer be switched off, does not give rise to any danger.

For the purposes of checking the switching-off capability of the switch 4 the microprocessor 1 performs a test cycle which provides that the microprocessor I closes the switch 4 for a period of tire T of the order of magnitude of a few milliseconds, and analyses the s ignal U 2 which occurs during that period at the output of the measuring device 3. The period of time T for which the switch 4 is opened is so short that, in spite of the briefly missing voltage supply, the load L does not change its operating condition, as a result of mechanical or electrical inertia. In that way it is possible to check the switching- off capability of the switch 4 withcut the operating condition of the installation changing. Thus for example the flame of an oi.1 burner is uniformly supplied with fuel even -if the fuel valve is temporarily without a voltage supply.

The signal at the input S is in phase with respect to the mains voltage U PG. Its passages-through-zero define in respect of time the firing points of the triac 11, in the normal operating mcde. Cn the basis of the knowledge of those firing times, it is possible for the microprocessor 1 to delay the mment at which the triac 11 is switched on, by a predetermined period of tire. In order to be able to detect self-firing of the triac: 11, that period of time must last for at least half a mains half-wave so that the mains voltace UPG applied to the triac 11 rea its maximum value. The rising edge of the associated rectangular pulse at the output 7c appears as 7 correspondingly delayed while the switch-off edge of that rectangular pulse appears undelayed. The microprocessor 1 measures the length in respect of time of that and the preceding or the next rectangular pulse. The difference or the ratio of those rectangular pulses contains the information as to whether the triac 11 switched on with a delay as intended. Qxrparing the durations of the two successive rectancular pulses affords the advantage that a phase displacement as between the mains voltage U PG and the current I flowing in the circuit Kl, as is possible in the case of an inductive load, has no influence on the measurement result. Such a test cycle is advantageously performed both with positive and negative balf-waves.

The switches 4 used can also be switching transistors, in particular field effect transistors, or relays. In that respect the ccntrol circuit 5 is to be adapted to the type of switch 4.

Semiconductor switches such as the triac 11 or a transistor afford the advantage that they can be very quickly switched on and off. A Hall element or a resistor can also be used as the measuring element 6. The Hall element supplies a galvanically separated signal, when suitably fed. 20 Figure 2 shows a further example of a measuring device 3. Connected in parallel with the switch 4 in the load circuit K, in series are a resistor 21 and a light emitting diode 22, the light emitting diode 22 being a component of an optocoupler 23. In the closed condition of the switch 4 no current flows through the resistor 21 and the light emitting diode 22 so that the output of the optocoupler 23 carries a uniform signal while in the open condition of the switch 4 a current flows through the resistor 21 and the light emitting diode 22 so that the output of the optocoupler 23 carries an alternating signal. The resistor 21 is of high resistance so that the load L remains switched off in the open condition of the switch 4. When a test cycle is performed the microprocessor 1 establishes by multiple interrogation whether the signal at the input R is uniform 8 or alternating and dete=rdnes therefrcrn the actual condition of the switch 4.

Figt=e 3 shm.m a control device in which the load L is connected to the phase P by way of two separate current paths K, and K 2 respectively. Both current paths K, and K2 each have in series a switch device 2.1 and 2.2 respectively and a measuring element 6.1 and 6.2 respectively. The measuring elermnts 6.1 and 6.2 together with the circuit block 7 fo=n the measuring device 3. The switch devices 2.1 and 2. 2 can be switched separately by the microprocessor 1. Resistors serve as the measuring elements 6.1 and 6.2. For the sake of clarity, Figure 3 shows only one circuit, associated with the resistor 6. 1, of the circuit block 7. Tbat circuit has a Zener diode 30, an amplifier stage 34 ccmprising a series resistor 31, a transistor 32 and a resistor 33, a Scimutt trigger 35 and, if necessary, a galvanic separation me, er 36 whose output is connected to an input R, of the microprocessor 1. When the switch device 2.1 is closed the alternating current flowing in the circuit K, produces across the resistor 6.1 a voltage drop which, by means of the described circuit, generates a sequence of rectangular pulses at the input R 1 In the open condition of the switch device 2.1 on the other hand a uniform voltage at a level corresponding to earth m occurs at the input R 1 The described device operates as follows: in continuous operation of the installation the tyn switch devices 2.1 and 2.2 are closed so that the load L is switched on. For checking the switchingoffE capability of the switch devices 2.1 and 2.2 respectively the microprocessor 1 perform a test cycle whereby the microprocessor 1 alternately opens one of the switch devices 2.1 or 2.2 for a predetermined period of time T and analyses the signall occurring at - R the corresponding input 1 and R2 respectively, determines therefrcm the actual condition of the switch device in the manner already described hereinbefore, and ccnpares it to the reference condition.

9 This device has the advantage that testing of the switching-off capability of the switch devices 2 can be effected irrespective of the switching properties of the switch devices 2.1 and 2.2 and irrespective of the electrical properties of the load L. Mechanical switch ers such as for example relays can be used to advantage as the switch devices 2.1 and 2.2. There is no need for synchronization between the microprocessor 1 and the measuring device 3 as the period of time for which the switch device 2.1 or 2.2 respectively is open can be of any desired length.

The frequency of the test cycles depends on statutory provisions and the working load of the microprocessor 1. A test cycle can be performed for example once every minute or also once per day.

The mode of operation of the control devices has been described hereinbefore 2.n the situation where the mains voltage UPG is an ac voltage. Operation is however readily possible with dc vroltage, utilising the same general idea, if the measuring elements and switch devices are appropriately selected.

It will of course be appreciated that many other changes and modifications can be made to the embodiments described above without departing from the scope of the invention.

Claims

1. A device for controlling an installation which as a timing and control logic apparatus has a microprocessor (1) for the actuation of switch devices (2; 2.1, 2.2) in accordance with a tine program, wherein loads (L) of the installation a connectable to a reins voltage (U PG) by way of a respective current path (K,) provided with at least one switch device (2; 2.1, 2.2), and which has nicasuring devices (3) with measuring elements (6; 6.1, 6.2) for detecting the condition - open or closed - of the switch devices (2; 2.1, 2.2) and for tran-cwnitting same to the microprocessor (1), wherein the microprocessor (1) caTpares the signalled actual condition to the reference condition, characterised in that at least one load (L) which is critical in regard to safety prccedure is connectable to the mains voltage (U PG) by way of ak- least one current path (K,; K,, K 2)' that in each current path (K 1; K,, K2) a reasuring element (6; 6.1, 6.2) is associated with each switch device (2; 2.1, 2.2), and that the mic=processor (1) is prc<;rar.nied to perform a test cycle at predetermined n=ents of time in continuous operation of the installation in order to check the switching-off capability of the switch devices (2; 2.1, 2.2) without 1Che load (L) associated with said switch device (2; 2.1, 2.2) changing its operating condition during the test cycle.

2. A control device according to claim 1 characterised in that there is a single current path (K,) per load (L), that the switch device (2) has a switch (4) and that the test cycle provides that the microprocessor (1) opens the switch (4) for a predetermined period of time T and ccrrpares the signal (U 2) outputted by the measuring device (3) in said period of time T to the signal (U2) outputted prior to or after opening of the switch (4) and determines therefrcm the actual conditien of the switch (4) during the period of tire T.

11

3. A control device according to claim 2 characterised in that the switch (4) is a semiconductor switch.

4. A control device according to claim 3 characterised in that the semiconductor switch (4) is a triac (11) or a field effect transistor.

5. A control device according to claim 1 characterised in that there are two current paths (K,; K2) each having a switch device (2.1 and 2.2 respectively) per load (L), and that the test cycle provides that the nticroprocessor (1) alternately opens one of the tuo switch devices (2.1; 2.2), determines the actual condition of the switch device (2.1 or 2.2 respectively) frm the signal which is ou,;'--pu'kted by the measuring element (6.1 or 6.2 respectively) associated with said switch device (2.1 or 2.2), and closes the switch device (2.1; 2.2) again.

6. A control device according to claim 5 characterised in that the switch devices (2.1; 2.2) are mechanical switch menters.

7. A control device according to one of claim 1 to 6 characterised in that the measuring elements (6; 6.1, 6.2) are a transformer (8), an ohmic resistor or a Hall element.

8. A control device according to one of claims 1 to 6 characterised in that the measuring device (3) has a measuring element with a resistor (21) and an optocoupler (23), wherein the resistor (21) and the light emitting diode (22) of the optocoupler (23) are connected in parallel with the switch device (2; 2.1, 2.2).

9. A control device substantially as hereinbefore described with reference to the accompanying drawings.