WO2001086680A1

WO2001086680A1 - Safety switching device for the safe switching on and off of an electrical user

Info

Publication number: WO2001086680A1
Application number: PCT/EP2001/005232
Authority: WO
Inventors: Gerhard Ehrlich; Hubert Strobel
Original assignee: Pilz Gmbh & Co.
Priority date: 2000-05-10
Filing date: 2001-05-09
Publication date: 2001-11-15
Also published as: DE10022722B4; AU7400301A; DE10022722C5; DE10022722A1

Abstract

The invention relates to a safety switching device (10), for the safe switching on and off of an electrical user (24), in particular, an electric motor, with an electromechanical switching element (12), comprising at least one adjustable working contact (14), with a supply unit (26) to produce a working voltage (U'B) for the switch element (12) and with a reducing unit (30), which reduces the operating voltage (U'B) supplied to the switching element (12), from a higher (U1) to a lower (U2) voltage value. The invention is characterised in that the supply unit (26) is a variable unit with adjustable output voltage (UA) and that the reducing unit (30) determines the output voltage (UA) of the supply unit (26), depending upon the operating state.

Description

Safety switching device for the safe switching on and off of an electrical consumer

The present invention relates to a safety switching device for the safe switching on and off of an electrical consumer, in particular an electrical drive, with an electromechanical switching element which has at least one adjustable working contact, with a power supply unit for generating an operating voltage for the switching element, and with a lowering unit which the operating voltage supplied to the switching element is reduced as a function of an operating state of the switching element from a higher to a lower voltage value.

Such a safety switching device is known due to its use.

Generic safety switching devices are used primarily in the industrial sector to switch on and, above all, safely switch off electrically driven machines, such as a press or a milling tool. They are used, for example, in conjunction with a mechanically actuated emergency stop button to switch off the machine quickly and safely in an emergency situation. For this purpose, the power supply to the machine to be switched off is carried out via the adjustable normally open contact of the switching element. Often, for safety reasons, even work contacts of a plurality of redundant switching elements are arranged in series with one another, but this is not absolutely necessary for the present invention. As soon as the work contact (s) are opened, the power supply to the machine is interrupted.

Relays are usually used as switching elements, and so-called contactors are used at high currents to be switched. Such switching elements have an input-side control circuit with a coil in which a control current is generated with the aid of the supplied operating voltage. The control current in turn generates a magnetic field which, with sufficient strength, puts the working contact (s) in their active working position. With a so-called normally open switching element, the normally open contacts are open in the active state, and closed with a so-called normally open switching element. In the passive idle state, the position of the working contacts is reversed. A known problem with generic safety switching devices is that only a certain amount of heat may be introduced into the housing of the switching devices until thermal destruction. Much of this heat is generated in the coils of the switching elements due to ohmic losses. To solve this problem, it is known to lower the operating voltage for the switching element after the activation of the normally open contacts to the lowest possible holding voltage. In contrast, a higher operating voltage is required to activate the normally open contacts, since in this case the normally open contacts have to be moved against a mechanical pretensioning force by using higher energy.

So far, two different methods are known for lowering the operating voltage in generic safety switching devices. In the first case, when the contacts of the switching element have moved into their working position, a series resistor is switched into the circuit of the switching element. The originally applied operating voltage is then divided between the series resistor and the switching element, so that the switching element itself receives a reduced operating voltage. However, this method has the disadvantage that the series resistor also generates heat as a result of ohmic losses, so that overall only a comparatively small reduction in the thermal load can be achieved.

In the second known alternative, a transistor is arranged in the circuit which supplies the operating voltage to the switching element and is clocked via a control circuit. The average operating voltage at the switching element can be determined by clocking with a variable pulse width (pulse width modulation). lower ment. In this alternative, too, a component is required with the transistor that itself produces heat due to its power loss. In addition, the control of the transistor is comparatively complex in this case.

It is therefore an object of the present invention to provide a further alternative of how the thermal load can be reduced in a safety switching device of the type mentioned at the beginning.

This object is achieved in the safety switching device mentioned at the outset in that the power supply unit is a variable power supply unit with a variable output voltage and that the lowering unit determines the output voltage of the power supply unit as a function of the operating state.

In contrast to the previously known measures, the operating voltage is reduced here by the lowering unit directly influencing the output voltage of the power supply. This requires the use of a power supply unit with a variable output voltage, which is unusual in the case of previously known generic safety switching devices. In contrast to the prior art, the "excess" operating voltage is therefore already reduced at its source. In other words, the power supply unit in the safety switching device according to the invention only generates the operating voltage required for the switching element. Deriving or "destroying" a portion of the operating voltage initially provided by the power supply but not required can thus be dispensed with. The measure is simpler in comparison to generic safety switching devices, since switchable series resistors or the measures for clocking and pulse width modulation of the operating voltage can be omitted. In addition, the components previously required for this purpose, which each generate heat as a result of power losses, are also eliminated. Finally, the power supply losses are also reduced in the solution according to the invention, since a lower operating voltage is already generated by the power supply itself.

Overall, the thermal load on the safety switching device according to the invention can therefore be significantly reduced. In addition, the arrangement according to the invention is simpler than the previously known switching devices.

The above task is therefore completely solved.

In one embodiment of the invention, the operating voltage supplied to the switching element is buffered via an additional energy store.

A capacitor is preferably used as the energy store, which additionally, i.e. outside the power supply. The measure has the advantage that the switching element receives a very uniform and stable operating voltage, which reduces the risk of malfunctions and faulty switching operations. This is particularly advantageous with regard to the safety aspect of the switchgear.

In a further embodiment, the switching element requires a maximum switching capacity and for activating the make contact the power supply unit has a nominal electrical power that is less than the maximum switching power.

In this embodiment of the invention, the power supply is undersized with regard to the maximum switching power required. The power supply unit alone is not able to provide the switching power required to activate the make contacts. However, since the maximum switching capacity is only required for a short time when the work contacts are activated, it can be provided by previously charging the additional energy store. In contrast to the power supply unit, the additional energy storage is dimensioned with regard to the maximum switching capacity.

In contrast, when the safety switching device is in operation, only a lower holding power is required for the work contacts. This lower power can also be made available by a power supply that is underdimensioned with regard to the required switching power.

The measure has the advantage that a power supply unit with a smaller transformer can be used due to the lower nominal output, which reduces the size. The safety switching device of this embodiment according to the invention can therefore be implemented in a very small size.

In a further embodiment of the invention, the operating state is the reaching of a working position of the working contact. This measure has the advantage that the operating voltage is reduced as early as possible. As a result, the thermal loads on the safety switching device are reduced at an early stage.

In a further embodiment of the invention, the output voltage of the power supply is regulated as a function of an operating parameter of the switching element.

In this embodiment of the invention, not only the working position of the working contact is taken into account, but further operating parameters, such as an operating temperature, are also taken into account. This makes it possible to lower the operating voltage to an optimal value, which further reduces the thermal load on the safety switching device. At the same time, the control ensures that the safety relay functions correctly.

In one configuration of the measure mentioned above, the operating parameter is an operating temperature of the switching element.

This measure is particularly advantageous since the ohmic resistance of coils increases with increasing temperature. Since the current flow and thus the strength of the magnetic field decrease with increasing resistance, a voltage reserve would have to be taken into account without a temperature-dependent regulation of the operating voltage in order to ensure error-free operation of the safety switching device under all working conditions. This voltage reserve is in view of the thermal load however, a disadvantage that is avoided in this embodiment of the invention.

In a further embodiment of the invention, the operating parameter is a holding current of the switching element.

In this embodiment of the invention, the operating voltage required in each case is determined with the aid of a current sensor which is arranged in the circuit for the switching element. The output voltage of the power supply unit is therefore determined by the lowering unit so that a constant holding current can flow. The holding current for the switching element is impressed as it were. This measure also enables optimum control of the operating voltage to reduce thermal loads.

In a further embodiment of the invention, the safety holding device has a timing element with a defined time constant and the operating voltage is reduced as a function of the defined time constant.

This measure is very simple and inexpensive to implement in terms of circuitry and can therefore be used to reduce costs instead of regulating the operating voltage. However, it can also be used in addition to a regulation to determine the start of the actual regulation process.

In a further embodiment of the measure mentioned above, the timing element contains the switching element. This configuration has the advantage that the timing element can be implemented implicitly, ie without the use of additional components, at very low cost. In this case, the timer is particularly preferably implemented in that an energy store which provides the switching element with the required operating voltage is charged to a voltage value which is still above the higher voltage value in the sense of the present invention. The components involved are then dimensioned such that the initial voltage drops from its maximum value to the higher voltage value mentioned in a time period which corresponds to the defined time constant.

In a further embodiment of the invention, the safety switching device has a start switch with a switch-on fuse which only releases the start switch when the higher voltage value of the operating voltage is present.

This measure ensures that the safety switching device can actually also activate the normally open contacts of the switching element when the start switch is actuated. As a result, the behavior of the safety switching device is more transparent and easier for an operator to understand. The risk of incorrect operation is reduced, which is particularly advantageous with regard to the safety aspect of the switchgear.

It goes without saying that the features mentioned above and those yet to be explained not only in the respectively specified combination but also in other combinations or can be used alone without leaving the scope of the present invention.

Exemplary embodiments of the invention are shown in the drawing and are explained in more detail in the following description. Show it:

Fig. 1 is a schematic representation of a first embodiment of the invention;

Fig. 2 is a schematic representation of a second embodiment of the invention;

3 shows two voltage-time diagrams from which the function of the safety switching device according to FIG. 1 can be seen; and

Fig. 4 shows two voltage-time diagrams of a further embodiment.

1, a safety switching device according to the invention is designated in its entirety by reference number 10.

The safety switching device 10 has an electromechanical switching element 12 in the form of a relay. In the present exemplary embodiment, the switching element 12 has three work contacts 14 arranged parallel to one another, which are arranged serially between an input terminal 18 and an output terminal 20 of the safety switching device 10. A power supply 22 for an electrical load 24 is connected to the input terminals 18 in a manner known per se. The Consumer 24 is in turn connected to the output terminals 20 of the safety switching device 10 in a manner known per se. An electrical drive is shown here as an example of an electrical consumer 24. The electric drive moves a milling head or a press, for example.

In a departure from the illustration shown here, safety switching devices of the type shown often have two mutually redundant switching elements 12, the normally open contacts 14 of which are arranged in series with one another. This redundancy ensures that the electrical consumer is safely switched off even if a switching element 12 fails. This version is sufficiently well known in the art and has not been shown here for reasons of clarity.

The reference number 26 denotes a power supply unit, which according to a preferred embodiment of the invention is a switching power supply unit here. The power supply unit 26 can be connected to a power supply in a manner known per se via two connecting terminals 28. In a preferred exemplary embodiment, this is the power supply 22, which also supplies the electrical consumer 24 (not shown here).

The power supply 26 is a variable power supply with a variable output voltage.

Reference number 30 denotes a schematically illustrated lowering unit, which determines the output voltage of the power supply unit 26 in the manner explained below. However, the lowering unit 30 is not necessary as a separate construction Part included in the safety switching device 10. It can also be spatially integrated in the power supply 26. This is preferably the case when the power supply unit 26 can already process the input signals described below for setting the variable output voltage. In the case of power supply units which cannot process such signals directly, the lowering unit 30 includes signal conditioning and, if appropriate, circuit logic which combines the various input signals described below.

In the exemplary embodiment shown here, the lowering unit 30 receives the output signal of a temperature sensor 32, which determines the operating temperature θ of the switching element 12. The lowering unit 30 determines the output voltage U _{A of} the power pack 26 as a function of the operating temperature θ obtained.

The reference numeral 34 denotes a capacitor which is arranged in parallel with the output of the power supply 26. The capacitor 34 charges up to the output voltage U _{A of} the power supply 26 and forms a capacitive energy store. A voltage sensor 36 is arranged parallel to the capacitor 34 and determines the voltage value present at the capacitor 34. The output signal of the voltage sensor 36 is fed to a start switch 38, which is arranged in series in the voltage supply from the capacitor 34 to the switching element 12. In the present exemplary embodiment, the start switch 38 has a switch-on fuse (not shown here in more detail) which only releases the start switch for an operator when the one determined by the voltage sensor 36 Voltage value is sufficient to move the work contacts 14 into their working position. This required voltage value is preferably the higher voltage value in the sense of the present invention.

The voltage applied to the switching element 12 in this exemplary embodiment is denoted by U _B '. It differs from the output voltage U _{A of} the power pack and thus from the voltage U _{B present} here at the capacitor 34 in that it only jumps to the voltage value present at the capacitor 34 when the start switch 38 is actuated successfully. The temporal relationships are explained in more detail below with reference to FIG. 3.

According to a preferred exemplary embodiment of the invention, the start switch 38 is connected to the lowering unit 30 via a timing element 40. The timer 40 has a defined time constant T. The lowering unit 30 thus receives a signal delayed by the time constant T when the start switch 38 has been actuated successfully. The time constant T is chosen so that the switching element 12 has moved the working contacts 14 into their working position within the resulting time period. The lowering unit 30 can therefore lower the output voltage _{U.sub.A of} the power supply 26 to a value which is sufficient to hold the normally open contacts 14 in their working position when the aforementioned signal is present. This voltage value is generally referred to as the holding voltage of the switching element 12, and it preferably corresponds to the lower voltage value in the sense of the present invention. In a variant of this exemplary embodiment, the timer 40 can be saved if the capacitor 34 is initially charged to a third voltage value which is again higher than the higher voltage value already mentioned. The time span until the capacitor 34 has then discharged to the higher voltage value mentioned corresponds to the time constant T. The mode of operation of this variant is explained in more detail below with reference to FIG. 4.

A further exemplary embodiment of a safety switching device according to the invention is designated in its entirety in FIG. 2 by reference number 50. The same reference numerals designate the same elements as in FIG. 1.

In contrast to the exemplary embodiment according to FIG. 1, the output voltage U _{A of} the power supply 26 in the safety switching device 50 is not regulated as a function of an operating temperature θ of the switching element 12, but rather as a function of the required holding current I _H. The holding current I _H denotes the current that is required to hold the normally open contacts 14 of the switching element 12 in their active working position.

In order to determine the current I _H supplied to the switching element 12, a resistor 52 is arranged in series with the start switch 38 in the present exemplary embodiment, the respective voltage drop of which is measured in a manner known per se with the aid of a voltage sensor 54. The output signal of the voltage sensor 54 is fed to the lowering unit 30, which in dependence thereon applies the output voltage U _{A of} the power supply unit 26 Right. Otherwise, the mode of operation of the safety switching device 50 corresponds to that of the safety switching device 10.

Representing other exemplary embodiments, the mode of operation of the safety switching device 10 is explained in FIG. 3 using two voltage-time diagrams, the timing element 40 being taken into account in these diagrams.

The upper timing diagram shows the profile of the voltage U _B across the capacitor 34. At the same time below, the profile of the operating voltage U ' _B applied to the switching element 12 is shown.

After the safety switching device 10 has been started up, the capacitor 34 is charged to a higher voltage value, which is denoted here by O _x , via the power pack 26. The voltage value Uj is reached at time t ₀ . From this point in time, the capacitor 34 has the required voltage in order to supply the switching element 12 with the power required to activate its normally open contacts 14. At time t _x , the start switch 38 is actuated by an operator. An actuation of the start switch 38 earlier than t ₀ would have no effect, since the switch-on protection has not yet released the start switch due to the capacitor 34 which has not yet been sufficiently charged.

By successfully actuating the start switch 38, the capacitor 34 is connected to the switching element 12. The operating voltage U ' _B therefore jumps to the value of the voltage U _{B present} at the capacitor 34. After the time period T specified by the timer 40 has elapsed, the lowering unit 30 controls the power supply unit 26 in such a way that the voltages U _B and U ' _B drop to a lower voltage value U ₂ . The time period T is chosen so that the working contacts 14 of the switching element 12 have already been moved into their active working position at this time.

The lower voltage value U ₂ corresponds to the required holding voltage of the switching element 12. According to a preferred embodiment of the invention, the voltage value U ₂ also corresponds to the voltage at which the power supply unit 26 outputs its nominal power to an adapted resistor. This value, referred to here as the nominal voltage, is shown in the two time diagrams in FIG. 3 by means of a dotted line 60. As can be seen, the nominal voltage of the power supply unit 26 is below the voltage value U ₁ to which the capacitor 34 must be charged before the start switch 38 is actuated. The power supply unit 26 is therefore undersized with regard to the maximum switching power required.

In the aforementioned variant of the embodiment shown in Fig. 1, the function of the timer 40 is implicitly realized in that the capacitor is charged 34 at the start to a voltage value U _3, which is even higher than the said higher voltage value _Uj. This is shown in FIG. 4. After actuation of the start switch 38 at time _x , the capacitor 34 can discharge via the switching element 12. The voltage U _B across the capacitor 34 drops accordingly. The capacitor 34 and the discharge resistors are dimensioned in such a way that the voltage U _B or U ' _{B is} only required after the time period T to activate the normally open contacts 14. falls below the voltage value _λ . The lowering unit 30 can therefore control the power supply unit 26 from the moment the start switch 38 is actuated.

Claims

claims

1. Safety switching device for the safe switching on and off of an electrical consumer (24), in particular an electrical drive, with an electromechanical switching element (12) which has at least one adjustable working contact (14), with a power supply unit (26) for generating an operating voltage ( U '_B; U _B ) for the switching element (12), and with a lowering unit (30) which the operating voltage (U'_B; U _B ) supplied to the switching element (12) depending on an operating state of the switching element (12) a higher (U _:) to a lower (U ₂ ) voltage value, characterized in that the power supply (26) is a variable power supply with a variable output voltage (U _A ) and that the lowering unit (30), depending on the operating state, the output voltage (U _A ) of the power supply (26) determined.

2. Safety switching device according to claim 1, characterized in that the switching element (12) supplied operating voltage (U '_B; U _B ) is buffered via an additional energy store (34).

3. Safety switching device according to claim 2, characterized in that the switching element (12) for activating the normally open contact (14) has a maximum switching capacity. necessary, and that the power supply (26) has an electrical nominal power that is less than the maximum switching power.

4. Safety switching device according to one of claims 1 to 3, characterized in that the said operating state is the reaching of a working position of the working contact (14).

5. Safety switching device according to one of claims 1 to 4, characterized in that the output voltage (U _A ) of the power supply (26) in dependence on an operating parameter (θ; I _H ) of the switching element (12) is regulated.

6. Safety switching device according to claim 5, characterized in that the operating parameter is an operating temperature (θ) of the switching element (12).

7. Safety switching device according to claim 5 or 6, characterized in that the operating parameter (I _H ) is a holding current of the switching element (12).

8. Safety switching device according to one of claims 1 to 7, characterized in that it has a timer (40) with a defined time constant (T) and that the lowering of the operating voltage (U ' _B ) takes place as a function of the defined time constant (T) ,

9. Safety switching device according to claim 8, characterized in that the timer (40) contains the switching element (12).

0. Safety switching device according to one of claims 1 to 9, characterized in that it has a start switch (38) with a switch-on fuse, the start switch (38) only in the presence of the higher voltage value (Ui) of the operating voltage (U '_B; U _B ) releases.