EP1458002B1 - Keyboard for giving control operations - Google Patents

Description

The invention relates to a keyboard for triggering of tax transactions. Such a keyboard is particular for controlling a technical machine or plant used.

The signals to be given to control a machine or plant comprise a safety-related signal, i. Signals, their proper and instantaneous generation and Transmission required for safety reasons is. These include in particular signals that are immediate the triggering or termination of a process of the machine or Cause plant such. a the movement of a machine part controlling start or stop signal or an emergency stop signal. On the other hand, to control a machine or Facility often deliver signals that are not immediate Have safety relevance. These include, among others Signals that, e.g. in the setting of working parameters, Generate numbers or letters.

The conception and development of industrial machines and equipment is characterized by an increasing use of safety technology. This must especially at a for the generation safety-related signals provided signal generator a malfunction be excluded or, to initiation safety-preserving steps to be detected. at Use of a signal generator in the form of a conventional Push-button with mechanical contacts is a gapless Error detection especially difficult that a Jamming the feeler mechanism by electronic means barely recognizable is. This is problematic, especially as mechanical push-buttons in almost all industrial machines and plants be used on operator panels or control panels and thus are also safety-relevant. application areas lie, for example, in power plant technology, traffic engineering, Chemical plants, generally in mechanical engineering Tool and production machines, passenger lifts, crane systems, Aerospace engineering, etc. Affected application functions are doing u.a. general start and stop functions, Travel keys of drives, the control of valves and general approval and monitoring functions, such as so-called "dead-man keys".

To safely process signals generated by mechanical buttons Therefore, often becomes a redundant signal generation required. A safe but handling technology elaborate method for redundant data generation exists in it, several separate and to trigger a control process to provide simultaneously operated signal generator, so that in case of failure of a signal generator or the signal generator containing signal paths further signal paths for transmission the safety-related information available stand. Such a system is generally two-handed to operate and thus disadvantageous from an economic perspective. Alternatively, it is common for a simplified Handling multiple transducers in a push-button switch to combine so that these are used for redundant signal generation in a single operation can be actuated together. The Realization of such a button with evaluation, as for example DE 199 46 471 A1 is known, but is technically complex and thus costly. On the other hand, this document reveals no signal generator with two signal paths.

From the field of so far mainly for input purposes, e.g. in the keyboard of a computer, used Kurzhubtastschalter Now signalers are known in which a mechanical Malfunction, in particular jamming of the switching mechanism, is excluded by constructive measures. For example, from US 5,990,772 such a key switch known, in which the provision of the switching contact in the resting state under the action of a magnetic force takes place.

By the increasingly space-saving and ergonomic work Construction of industrial machinery and equipment has on the other hand the use of so-called membrane keyboards in Operating area established. Such a membrane keyboard is on their user interface with a comparatively thick, covered with flexible plastic film on which each with an underlying key, i. a signal generator, correspond Keypads are marked.

From DE 100 40 151 C1, for example, such Membrane keyboard known in the frequently used keys in a "crackpot" technology, i. e. one by a, e.g. metallic, snap element mediated Show jump switching behavior. By contrast, less frequently used Keys in an Affordable "Bubble Technology" executed. In the latter case, the front panel is the keyboard in the area of the keypad to form a "switching bubble" outward, ie convex, bulging. The inner surface the front foil is in the area of the switching bubble with a, mostly imprinted, contact surface provided, in rest position by far one or more fixed contacts facing. For operation, the front foil is in the area of the switching bladder pressed. The switching bladder is "dented in," so to speak, wherein the contact surface against the or the fixed contacts pressed, and the switching contact thereby closed becomes. When the actuating pressure is withdrawn, the front foil returns in their bubble-like starting position, the Switch contact is opened again. The well-known keyboard is through the use of simple but susceptible to wear inexpensive to produce bubble-like keys, wherein a quick wear of frequently used keys the local use of the more elaborate but more durable Snap element buttons is avoided. An error security or error detection, as they are safety-related for the generation Signals is required, is in the known Keyboard not given.

The invention has for its object to provide a keyboard, which is inexpensive and safe generation of Signals for triggering control operations allowed.

This object is achieved by the features of claim 1. Thereafter, the keyboard comprises at least two Keypads. A first keypad for triggering safety-related Signals is provided is one with two Assigned signal paths wired first signal generator. Of the Signal state of this first signal transmitter is in both signal paths time resolved and analyzed, with a from the analysis result dependent output signal for driving a technical system is generated. The analysis of the Signal state of the two signal paths takes place according to the following Regulate. If both signal paths are in hibernation have corresponding signal state, a first, the Resting state indicating output signal generated. This signal configuration is hereinafter referred to as the first operating state. In a second operating state is under the Prerequisite that both signal paths one the operating state corresponding signal state, for display of the actuation state generates a second output signal. A regular change between the first and the second operating state is only allowed if within a predetermined switching time a change of the signal state takes place in both signal paths. Otherwise, i. when outside a regular change of operating state none of is present in both operating states, a faulty operating state detected and issued an error message.

Under time-resolved determination of a signal state becomes understood that in addition to the current signal state information via at least one past signal state of the the same signal path is available, so that on the basis of their Comparison a change in the signal state is recognizable. The Determination of the signal state takes place alternatively continuously or discontinuously, with two consecutive Determination times maximum in time by the target switching time are spaced.

For the generation of not immediately safety-related Signals is provided at least a second keypad. This is assigned a second signal generator, the like a bubble is executed or a by a snap element has mediated jump switching behavior.

With the first keypad of the keyboard according to the invention is Advantageously, a secure generation of switching signals and efficient detection of faulty switching operations the associated first signal generator possible. As a result the inventive analysis of the switching state of the first Signalers this can be compared to the conventional standard with the same safety constructively simple, and consequently be very compact. The second keypad associated signal generator, however, is very inexpensive to manufacture. The combination of the first and second keypad as well as the respectively assigned signal generator in a common Keyboard thus allows the inexpensive and compact Realization of an integral keyboard for safety-related and non-safety-related signal generation. Such an integral keyboard is particularly advantageous in terms of simple design, installation and Operation of a technical machine or system. Prefers the keyboard has several corresponding to the first keypad Keypads for safety-related signal generation, and a plurality of keypads corresponding to the second keypad for non-safety-related signal generation.

In a preferred embodiment, the target switching time of the first Signal transmitter about 20 milliseconds. This target switching time is thus a default time, which expediently short towards the human reaction time and thus short in the Compared to typical switching periods. On the other hand this setpoint switching time sufficiently large compared to the actual Switching time of a conventional signal generator, the is on the order of a millisecond.

To have a fast and well reproducible, and therefore good To produce verifiable switching behavior, the first one Signal generator expediently on a snap-action behavior. In other words, the switching of the first signal transmitter takes place regardless of the time course of the actuating force forcibly within a structurally determined switching time.

In an advantageous embodiment of the invention, the two Signal paths in both operating states with regard to their Signal state inversely connected to each other. So, from, Moment of switching apart, the first signal path, for example signal leading, so the second signal path Zero signal on, and vice versa. Thus, in every regular Operating state of one of the signal paths signal leading. To this Way is a line break or another leading to signal loss error even while stationary Operating phases immediately recognizable. Such an inverse circuit the two signal paths is in a particularly simple manner with a first type of changeover switch Signal transmitter possible.

To quickly locate the source of the error in the event of an error can, is expediently a differentiated error message which indicates the type of error that has occurred. Preferably continues to be simultaneously with the error message generates an emergency stop signal as an output signal, which the controlled technical system in a safe state transferred.

In view of its low susceptibility to errors is preferred a Kurzhubtastschalter as the first signal generator for Commitment. A Kurzhubtastschalter is still due to this compact size particularly advantageous in a control device or a control panel of a technical facility integrate.

Preferably, all keypads of the keyboard with a common Covered front foil. This opens on the one hand the Possibility of a uniform design. On the other hand will by the front foil a particularly high dirt resistance achieved and thus the reliability of the keyboard increased.

Advantageously, the at least one safety-relevant first signal transmitter and the at least one non-safety-relevant second signal generator on a common circuit board contacted. This eliminates a costly and difficult to automate wiring of the individual signalers. Furthermore, the keyboard can thereby in a simple Way prefabricated as an integral coherent module which greatly reduces the assembly work on site is. The keyboard can also be particularly special be tested well independent of the device being controlled, which eliminates possible production errors in advance eradicate.

FIG 1

schematisch eine Vorrichtung zum Erzeugen und Auswerten von sicherheitsgerichteten Schaltsignalen mit einem, in seinem Ruhezustand befindlichen ersten Signalgeber und einer diesem nachgeschalteten Auswerteeinheit,

FIG 2

die Vorrichtung gemäß FIG 1 im Betätigungszustand des ersten Signalgebers,

FIG 3

in einem zeitlichen Diagramm Eingangs- und Ausgangssignale der Auswerteeinheit im Normalbetrieb und

FIG 4

in einer Darstellung gemäß FIG 3 Eingangs- und Ausgangssignale der Auswerteeinheit in beispielhaft ausgewählten Fehlerfällen,

FIG 5

in Draufsicht eine Folientastatur mit einer Vorrichtung gemäß FIG 1,

FIG 6

in einem schematischen Schnitt VI-VI die Folientastatur gemäß FIG 5 und

FIG 7

in einer Darstellung gemäß FIG 6 eine alternative Ausführungsform der Folientastatur.

Embodiments of the invention will be explained in more detail with reference to a drawing. Show:

FIG. 1

1 schematically a device for generating and evaluating safety-related switching signals with a first signal transmitter in its idle state and an evaluation unit connected downstream of it,

FIG. 2

the device according to FIG. 1 in the actuation state of the first signal transmitter,

FIG. 3

in a time diagram input and output signals of the evaluation unit in normal operation and

FIG. 4

in an illustration according to FIG. 3, input and output signals of the evaluation unit in error cases selected by way of example,

FIG. 5

in plan view, a membrane keyboard with a device according to FIG 1,

FIG. 6

in a schematic section VI-VI, the membrane keyboard according to FIG 5 and

FIG. 7

in an illustration according to FIG 6, an alternative embodiment of the membrane keyboard.

Corresponding parts and sizes are shown in the figures marked with the same reference numerals.

The device 1 shown in FIG 1 for the production and Evaluation of safety-related switching signals includes a trained as a key switch first signal generator 2, the type of a change-over switch optionally a signal input 3 connects to one of two signal outputs 4 and 5. The signal generator 2 is preferably in a short-stroke technology executed. Such Kurzhubtastschalter are distinguished by extremely compact size and extremely low error rate out. A suitable Kurzhubtechnologie, in which a Malfunction due to jamming of the switching mechanism is excluded by structural measures, for example known from US 5,990,772.

1 shows the signal generator 2 in its idle state. there is the signal input 3 via a switching contact 6 conductive connected to the signal output 5 while the connection between the signal input 3 and the signal output 4 interrupted is. The signal input 3 of the signal generator 2 is On the input side connected to an input line 7, via which the signal input 3, an input signal S0 in the form of a time constant electrical low voltage supplied becomes.

The signal output 5, at which corresponding to the in FIG illustrated switch position of the signal generator 2 voltage is present, is via a first signal path 8 with an evaluation 9 connected. The voltage output 4, at the in Hibernation is no voltage, is about a second Signal path 10 also connected to the evaluation unit 9. At rest, the first signal path 8 thus has an active, i.e. energized, signal state S1 on, during the second signal path 10 is in a passive, i. energized, Signal state S2 is located.

In the evaluation unit 9, the signal state S1, S2 both Signal paths 8 and 10 recorded time-resolved and as required analyzed in more detail below. The evaluation unit 9 generates a corresponding one based on the analysis result Output signal S3 and transmits this via a Output line 11 to a component not shown a technical facility. For example, it is in this component to the drive of a moving part a machine tool.

An ensemble of signal states S1 and S2 and an associated one Output signal S3 at a given time hereinafter referred to as the operating state of the device 1. The case of proper operation of the device 1 to the idle state the signal generator 2 associated operating state referred to as the first operating state. Corresponding sizes S1-S3 are shown uncoated. The at the operating state assigned operating state is in contrast as second operating state and designated by Sizes S1'-S3 'marked. Analogously, two-line mark Signal states S1 ", S2" a faulty operating state and the output signal S3 "in the event of a fault, if necessary output emergency stop signal.

If the evaluation unit detects a faulty ensemble of signal states S1 ", S2", so she gives about a reporting line 12th an error message F to an error indicator 13 off. The error message F preferably includes specific information about the type of detected error. The error indicator 13, e.g. by a suitable sound or light signal, First, to be aware that an error has occurred. On the other hand, he identifies on the basis of the error message F contained information the type of error and thus facilitates fault finding.

The rest state shown in FIG 1 corresponds to a first Operating state in which the signal state S1 in the signal path 8 "active", and the signal state S2 in the signal path 10th is "passive". As belonging to the first operating state the evaluation unit 9 a zero signal as the first output signal S3 off, which for example as a stop command for a Machine drive not shown are used can.

The operation of the signal generator 2 takes place according to FIG 2 by Applying a manual pressure K on an actuator 14th of the signal generator 2. This is by mediating a only indicated switching mechanism 15 of the switching contact 6 in switched its operating state. In this operating state the signal input 3 is connected to the signal output 4, while the connection between the signal input 3 and the signal output 5 is interrupted. In the second operating state is thus the signal state S1 'of the signal path 8 "passive", while the signal state S2 'of the signal path 10th is "active". As belonging to this second operating state the evaluation unit 9 generates a constant, positive output signal S3 ', which in turn is e.g. as start command one not shown machine drive can be fed.

The evaluation unit 9 leaves a change between the first and the second operating state, ie in particular one Change between the first and second output signal S3 or S3 'only then, if a corresponding change of the signal state S1, S1 ', S2, S2' in both signal paths 8 and 10 within a predetermined setpoint switching time ts.

Proper switching processes between the idle state and the operating state, and according to the first and second Operating state, are in FIG 3 in a temporal diagram shown. Here is the time history of the signal states S1, S1 'and S2, S2' are shown above the time axis t. The time course of the output signal S3, S3 'is shown below the time axis t. In that, the diagram according to FIG 3 underlying switching game is at the time t1 of the signal generator 2 is actuated. This is the active Signal state S1 in the passive signal state S1 'via and the passive signal state S2 in the active signal state S2 '. The signal generator 2 has a snap-action behavior. That is, regardless of the timing of the operating pressure K the switching process with proper function of the Signal transmitter 2 always within a predetermined period of time he follows. In particular, at time t1 the signal loss occurs in the signal path 8 due to the mechanical inertia of Signal transmitter 2 time before the signal structure in the signal path 10. Both processes, however, take place within the Time t1 following target switching time ts. Since at the time t1 the state change in time in both signal paths. 8 and 10, the evaluation unit 9 switches from the first Output signal S3 to the second output signal S3 '.

At time t2, the actuator 14 is left free. The switching contact 6 jumps within the switching time ts return to sleep. Due to the mechanical inertia of the signal generator 2, the signal loss occurs in the signal path 10th again in time before the signal structure in the signal path 8. Da the signal change at time t2 again within the Target switching time ts is done leaves the evaluation unit 9 again the change to the first operating state. Another, to the above-described switching game analog actuation phase takes place between the times t3 and t4.

While in FIG 3, a proper switching operation of the device 1, FIG. 4 shows faulty switching processes shown by way of example. By a sloping, parallel Line pair 16 is here in each case an interruption of Time axis t and the respective state curves marked. A faulty operating state occurs, for example then, if when pressing the signal generator 2 is not in both signal paths 8 and 10 within the desired switching time ts a state change takes place. Such a mistake, as he, for example from a defective switching mechanism 15 or a oxidized switching contact 6 can be generated occurs in accordance 4 at time t5. Here, the structure of the Signal state S2 'by more than the desired switching time ts delayed against the decay of the signal state S1. After passing the set switching time ts are both signal states S1 "and S2" "passive". The evaluation unit 9 recognizes this as Irregular operating state and triggers a corresponding error message F off. Even if, as shown in FIG 4, the signal state S2 "becomes" active "at a later date Control unit 9 a change to the second operating state not and continues to output the safe zero signal.

The inverse error case, namely a late return of the Signal generator 2 in the idle state, occurs according to FIG 4 Time t6 on. After waiting for the set switching time ts after Deactivation of the signal state S2 'generates the evaluation unit again an error message F. At the same time it switches output side back to a zero signal and generates it the safe output signal, also referred to as an emergency stop signal S3 ".

In a further error case occurring at time t7 the signal path 10 switches to an active signal state S2 " on, without previously a signal loss in the signal path 8 takes place is. Such an error case can, for example, by a short circuit in the lines 7,8,10 or in the signal generator 2 caused his. The active signal state S1 ", S2" both Signal paths 8 and 10 again become an irregular operating state detected and issued a corresponding error message F.

Activated in the inverse error case occurring at time t8 the signal path 8, without the signal state S2 would have become passive before. Simultaneously with the generation the error message F is then the evaluation unit 9th by generating the output signal S3 "in the safe emergency stop state back.

When using a changeover switch as a signal generator 2 is in the course of the switching process a temporary signal loss in both signal paths 8 and 10 normal. An operating condition, during which both signal paths 8 and 10 are active is however, regardless of its duration implausible. In the event of the error occurring at the times t7 and t8 responds the evaluation unit 9 therefore delay.

There are many alternative embodiments of the invention Method and the associated device conceivable. In particular, the output signals S3, S3 'and S3 " in the form of signal pulses instead of temporally constant Signals are generated. The output line 11 can be multi-channel for the separate output of the output signals S3, S3 'and S3 "Optionally, the signal paths 8 and 10 be designed redundant. Instead of a changeover switch as a signal generator 2 can also be a push-button use in which the signal paths 8 and 10 by parallel switching contacts, in particular an opener-closer pair, connected become.

The device described in connection with the Figures 1 to 4 1 is a membrane keyboard shown in plan view in FIG 19 included. The membrane keyboard 19 includes several Keypads 20a, 20b, which are for non-safety-related Switching processes are provided. With these keypads 20a, 20b are, for example, number or letter keypads 20a and an associated confirmation keypad 20b. Furthermore, the membrane keyboard 19 includes two intended for safety-related switching processes keypads 21. These keypads 21 are for example the direct triggering of a working process of a technical Machine.

The in FIG 6 in a to be understood as a scheme and in particular not to scale cross-section VI-VI shown Membrane keyboard 19 is on all keypads 20a, 20b, 21 common bottom support 22 constructed. To the in assembled state for a user visible outside towards the membrane keyboard 19 of a user interface forming front sheet 23 covered. The bottom support 22 and the Front foil 23 are essentially planar and mutually aligned arranged approximately parallel.

The formed between the bottom support 22 and the front sheet 23 Space is filled with a spacer layer 24, which in the Area of the keypads 20,21 provided with recesses 25 is. The bottom support 22 is preferably a conventional one PCB realized.

To realize the keypads 21 associated safety-related Switching processes is related 1 to 4 described in detail with the device 1 used. The signal generator 2 of the device 1 is in this case in the below each keypad 21 arranged recess 25 places its actuator 14 against the inner surface 26 of the front sheet 23 rests. The front foil 23 is in Field of the keypad 21 convexly curved. By pressure on the flexible front foil 23 in the region of the keypad 21st is the actuator 14 in the manner described and Way acted upon to actuate the signal generator 2. Of the Signal input 3 and the signal outputs 4 and 5 are in not shown manner via corresponding tracks the bottom carrier 22 contacted.

The evaluation unit 9 can - as shown - as a microcontroller executed and also contacted on the bottom support 22 be. Alternatively, the evaluation unit 9 in not arranged closer to the outside of the membrane keyboard 19 be, e.g. as part of a parent Control. In the latter embodiment is also a line break or a short circuit between the membrane keypad 19 and the evaluation unit 9 detected.

For the switching processes to be triggered with the keypads 20a, 20b are not special safety precautions as intended required. The associated signal generator 28a or 28b are therefore advantageously as simple and therefore inexpensive Kurzhubtasten designed. The keypad 20a associated signal generator 28a is like a bubble executed. For this purpose, the front foil 23 is in the area of the keypad 20a with an embossed, with respect to the bottom support 22 convex domed cap and carries on its inner surface 26 an electrically conductive contact surface 29, the one corresponding pair of fixed contacts 30 at a distance faces. By pressure on the front foil 23 in the area of the keypad 20a this is from its convex rest position in a plane or concave with respect to the bottom support 22 Operating position deformed. As a result, the contact surface 29 pressed against the fixed contacts 30, and released by the Bridging the switching process off. When canceling the operating pressure goes the front sheet 23 in its convex rest position back, whereby the switching contact is opened again.

For frequently used keypads without security-related Function, such as Confirmation keypad 20b is preferred a provided with a disc-like snap element 31 Signal generator 28b used. The snap element 31 is while at a distance from the bottom support 22 approximately parallel to this arranged and preferably lies directly on the inner surface 26th the front sheet 23 at. The snap element 31 is in its rest position with respect to the bottom support 22 convexly curved, so that the front sheet 23 in the area of the keypad 20b raised the level of the membrane keyboard 19 is arched out. The Snap element 31 carries a bottom support 22 facing Contact surface 29, which in rest position again one on the bottom support 22 arranged a pair of fixed contacts 30 with Distance is opposite. Under pressure on the button surface 31b the snap element 31 changes abruptly its shape from the convex rest position in respect to the bottom support 22 plan or concave operating position. Here again, the contact surface 29 pressed against the fixed contacts 30 and the the Keypad 20b associated switching contact closed. at decreasing operating pressure jumps the snap element 31st in the starting position, whereupon the electrical contact is interrupted.

In the membrane keyboard 19 shown in FIG 6, the bottom support 22 and the front sheet 23 are substantially parallel aligned with each other, so that the membrane keyboard a in Has substantially flat user interface. Such is preferred primarily for optical reasons. Especially if the installation height of the signal generator 2 and the required Construction height of the signal generator 28a, 28b, each of the minimum Define the distance between the bottom support 22 and the front foil 23, differs significantly, becomes an alternative embodiment the membrane keyboard 19 according to FIG 7 preferred. At this Embodiment is the thickness of the spacer layer 24 in the area the keypads 21 greater than in the field of keypads 20. In a transition region 32, the thickness of the spacer layer increases 24 doing between the keypads 20 and the Keypads 21 continuously closed. Especially the membrane keyboard 19 in this embodiment continue on a common, planar bottom support 22 is constructed, the keypads are 21st thereby against the keypads 20 on the user interface highlighted. The so sublime execution of safety relevant Keypads 21 supported on advantageous Show the tactile orientation of a user on the user interface the membrane keyboard 19, and thus reduces the Probability of incorrect operation. By the continuous Transition region 32 are in particular sharp edges avoided on the user interface, along which the front foil exposed to increased material stress due to buckling would.

The membrane keyboard described in FIGS. 5 to 7 19 associated advantages are in particular that the Membrane keyboard 19 by using the comparatively simple constructed signal generator 28a, 28b inexpensively manufactured is. Because for all safety-relevant switching processes the in 1 to 4 described device 1 is used, The membrane keyboard 19 can also be used for such purposes for which the use of conventional membrane keyboards due to high security requirements so far not or only was conditionally possible. This is particularly advantageous because the membrane keyboard 19 compared to conventional safety-oriented Commands extremely flat and compact is. Another big advantage is that everyone Keypads 20a, 20b, 21 on the executed as a printed circuit board Shelf supports 22 are contacted and thus a heavy Automated wiring of the signalers is eliminated. From Great advantage is that all keypads 20a, 20b, 21 are covered by a common front foil 23. The membrane keyboard 19 is described as integrally connected, preferably even hermetically encapsulated part made. This will in addition to a simple installation and the possibility a uniform design especially a high dirt resistance the membrane keyboard 19 reaches what is in turn has a positive effect on their operational safety.

Claims (10)

1. Keyboard (19) for initiating control operations, said keyboard having at least one first key field (21) assigned a first signal generator (2) that can be switched between an idle condition and an actuated condition and which is connected on the output side via a first signal path (8) and a second signal path (10) to an evaluation unit (9), with said evaluation unit (9) being embodied in such a way that

   (a) in a first operating condition it will generate a first output signal (S3) if the first signal path (8) and the second signal path (10) have a signal status (S1, S2) corresponding to the idle condition,

   (b) in a second operating condition it will generate a second output signal (S3') if the first signal path (8) and the second signal path (10) have a signal status (S1', S2') corresponding to the actuated condition,

   (c) it will only execute a change between the first and the second operating condition if a change in the signal status (S1, S1'; S2, S2') takes place in both signal paths (8, 10) within a pre-defined reference switching time (ts) and

   (d) will otherwise feed out an error message (F),

   and having at least one second key field (20a, 20b) assigned a second signal generator (28a, 28b) embodied as being of switch-dome type or as having a snap-action characteristic conveyed by means of a snap element (31).
2. Keyboard (19) according to claim 1, characterised in that the reference switching time (ts) is approximately 20 ms.
3. Keyboard (19) according to claim 1 or 2,
   characterised in that the first signal generator (2) has a snap-action characteristic.
4. Keyboard (19) according to one of the claims 1 to 3,
   characterised in that the first signal path (8) and the second signal path (10) have a mutually inverse signal status (S1, S2; S1', S2') both in the first and in the second operating condition.
5. Keyboard (19) according to one of the claims 1 to 4,
   characterised in that the first signal generator (2) switches between the first signal path (8) and the second signal path (10) in the manner of a changeover switch.
6. Keyboard (19) according to one of the claims 1 to 5,
   characterised in that the first signal generator (2) is embodied as a short-stroke momentary-contact switch.
7. Keyboard (19) according to one of the claims 1 to 6,
   characterised in that the error message (F) contains an indication of the nature of the fault.
8. Keyboard (19) according to one of the claims 1 to 7,
   characterised in that an emergency stop signal (S3') is generated simultaneously with the error message (F) as an output signal.
9. Keyboard (19) according to one of the claims 1 to 8,
   characterised by a front membrane (23) covering all the key fields (20a, 20b, 21) in common.
10. Keyboard (19) according to one of the claims 1 to 9, characterised by a common printed-circuit board (22) on which both the signal generator (2) assigned to the or to each first key field (21) and the signal generator (28a, 28b) assigned to the or to each second key field (20a, 20b) are contacted.

Images