Technical field

The present invention relates to the field of machine safety, and in particular to reset circuits in machine safety applications.

Background

When operating electrical and mechanical machines, it is of importance to ensure that the risk for accidents is minimized. An important aspect of machine safety is the possibility of manually indicating when it is safe to take a machine into operation. This is often achieved by means of a reset function, also referred to as a restart interlock or a monitored reset function. The reset function uses a reset circuit in determining whether or not it is safe to take a machine into operation. A reset circuit includes a reset switch which has a reset button for manual operation, and the reset function is arranged to distinguish a pressed- down state of the reset button from the released state of the reset button.

A reset circuit is often used together with one or more sensors for monitoring machine circumstances, such as electronic sensors, optical sensors and pressure sensitive sensors. Electronic gates and electronic switches, which can for example be used for monitoring enclosures surrounding dangerous machines or for monitoring the position of different machine parts, are typical examples.

A control system can be employed which stops the operation of the machine, in case the sensor detects a situation calling for reset. Typically, such control system does not allow a restart of the machine until the reset button has been pressed.

One example of a possible use of a reset button is to protect an area wherein a dangerous machine is located. In this implementation, the reset button is used together with an electronic switch which senses whether a door or gate into the area is closed. When staff enters the closed area via a door which is protected by an electronic switch, the operation of the machine will be stopped. When leaving the area, the staff can press the reset button, thus indicating that the machine can be re-started again. Other uses of reset buttons include the protection of machine parts which can be opened, e.g. lids, so that a machine whose lid has been opened cannot be restarted until a reset button has been pressed; the monitoring of whether machine parts are in the correct position, etc. In machine safety, as in many other areas of technology, there is a desire to reduce the physical size of the apparatus.

Summary

A problem to which the present invention relates is how to reduce the physical size of a machine safety system.

One embodiment provides a reset circuit for use in a machine safety system. The reset circuit comprises a reset-switch branch comprising a reset switch having a reset button, and an indicator-light branch comprising an indicator light. A first end of the indicator-light branch is connected to a first end of said reset-switch branch at a connection point, where the connection point is electrically connectable to an I/O terminal of a reset control system. A second end of the indicator-light branch is connected to a first DC voltage source, and the second end of the reset-switch branch is connected to a second DC voltage source.

A reset control system for controlling a reset circuit in a machine safety system is also provided. The reset control system comprises a signal generator and a signal detector, as well as an I/O terminal arranged to be connected to a reset circuit having an indicator light and a reset button. An output of the signal generator is connected to the I/O terminal; and an input of the signal detector is also connected to said I/O terminal, so that the signal detector may receive an input signal representing the potential at said first I/O terminal.

By use of the invention, the physical size of machine safety system can be reduced, in that the interface between a reset control system and a reset circuit according to the invention only requires one connection point. In one embodiment, said I/O terminal arranged to be connected to a reset circuit is the only terminal of the reset control system which is arranged to be connected to a reset circuit. In one embodiment, the signal generator is capable of generating a modulated signal at said first I/O terminal. The signal generator can for example be arranged to generate a modulated signal in response to the reset control system having received a signal indicative a situation calling for reset. Hereby is achieved that the risk of an erroneous detection of a reset operation is reduced: If a fault occurs in the signal generator, the risk of said fault generating a signal corresponding to the modulated signal is slim. Hence, if a modulated signal has been received by the signal detector, followed by a signal representing the signal expected when a reset button is being pressed, the changes are high that the reset button has actually been pressed.

In another implementation, the signal generator is arranged to generate a modulated signal during normal operation when an indicator light of a connected reset circuit is not meant to be lit up. The modulated signal in this implementation could for include voltage peaks of short duration and/or of low magnitude, so that the indicator light will not light up. This implementation could for example be useful if the DC voltage at the second end of the indicator- light branch of a connected reset circuit is the same as the DC voltage at the second end of the reset-switch branch, so that a potential at the I/O terminal at a pressed- down state of a reset button can be distinguished from a signal at the I/O terminal during normal operation.

Further aspects of the invention are set out in the following detailed description and in the accompanying claims.

Brief description of the drawings

Fig. 1 illustrates an embodiment of a machine safety system comprising a reset circuit and a reset control system.

Fig. 2 illustrates an embodiment of a machine safety system comprising a reset circuit and a reset control system.

Fig. 3 illustrates an embodiment of a machine safety system comprising a reset circuit and a reset control system.

Fig. 4 illustrates an embodiment of a machine safety system comprising a reset circuit and a reset control system.

Fig. 5 illustrates an embodiment of a reset control system. Fig. 6 is a flow chart illustrating an example of a method of operating a reset control system.

Detailed description

A reset circuit for machine safety can advantageously include an indicator light as well as a reset button, the reset circuit being arranged so that the indicator light will indicate when a reset is required.

An example of a reset circuit 100 is shown in Fig. 1. The reset circuit 100 of Fig. 1 is connected to a reset control system 103, arranged to monitor and/or control the operation of a reset circuit 100.

The reset circuit 100 of Fig. 1 comprises a reset switch 105a having a reset button 110, mechanically or electronically arranged so that when the reset button 110 is manually pressed, the reset switch 105a opens. Furthermore, the reset circuit 100 of Fig. 1 comprises an indicator light 115. One end of the light 115 of Fig. 1 is connected to a voltage-source terminal 120 of the reset circuit 100, the voltage-source terminal being connectable to a voltage source 121 of voltage U. Oftentimes, a 24 V DC voltage source is used, although other voltage supplies could be used. The other end of the light 115 is connected to an output 125 of the reset control system 103. One end of the reset switch 105 a is also connected to the voltage source, while the other end of the reset switch 105 a is connected to an input 130 of the reset control system 103.

During normal operation, the output 125 and the voltage source terminal 120 of the reset circuit 100 Fig. 1 will typically be at the same potential, so that the light 115 will be turned off. At a stop, when a situation calling for reset has been detected, the reset control system 103 will set the output 125 to a different potential (typically ground), thus causing the indicator light 115 to light up.

When the reset button 110 of the reset circuit 100 of Fig. 1 is pressed, the reset switch 105a will be opened, and the potential at input 130 will go from U to a floating potential. When the reset button 110 is released, the potential at input 130 will go back to U. Typically, the release of the reset button 110, after the reset button 110 has been pressed during a predetermined time duration, is used as an indication to the reset control system 103 that the machine can be returned to its operating state. In the circuit of Fig. 1, this corresponds to a return of the potential at the input 130 to the potential U. The reset control system 103 comprises a signal generator 135 and a signal detector 140, being connected to the output 125 and the input 130 of the reset control system 103, respectively. The signal detector 140 is arranged to detect when the potential at input 130 changes, indicating that the reset button is being pressed or released.

The reset circuit of Fig. 1 can for example be altered in that the reset switch 105 is replaced by a reset switch is open during normal operation and which closes when the reset button 110 is being pressed. The voltage-source terminal 120 of the reset circuit 100 could be connected to any suitable potential, including ground. In order to turn on the light 115 to indicate a need to press the reset button 110 when the voltage-source terminal 120 is at ground potential, the potential at output 125 of the reset control system 103 will have to differ from ground potential (and will often be set to 24 V DC).

A reset switch which is closed during normal operation will hereinafter be referred to as a normally-closed switch, while a reset switch which is open during normal operation will be referred to as a normally-open switch. According to the invention, a reset circuit is provided which comprises a reset switch as well as an indicator light. The reset switch is connected in a reset-switch branch, and the indicator switch is connected in an indicator-light branch. A first end of the reset-switch branch is connected to a first end of the indicator-light branch, the reset switch and the indicator switch thus being electrically connected. The second end of the indicator-light branch is connected to a first voltage source of constant DC voltage, while the second end of the reset-switch branch is connected to a second voltage source of constant DC voltage. The connection point between the two branches is electrically connectable to an I/O terminal of a reset control system. While requiring only one connection point between the reset circuit and a reset control system, this reset circuit provides both an indicator light which can be turned on when the reset button needs to be pressed, and the possibility of detecting when the reset button is actually being pressed. Hence, the interface between a reset control system and a reset circuit having an illuminated reset push-button and can be simplified, in that the number of connection points between the reset circuit and the reset control system is minimized. Furthermore, the size of the reset control system and of the reset circuit can be reduced. An inventive reset control system is also provided.

A machine safety system comprising an example of a reset circuit 200 and a reset control system 203 according to the invention is schematically shown in Fig. 2. The reset circuit 200 of Fig. 2 includes an indicator light 115 and a reset switch 205 having a reset button 110. The reset switch 205 in Fig. 2 is of a normally-open type.

A first end of reset switch 205 of Fig. 2 is electrically connected to a first end of the indicator light 120 at connection point 240, which is arranged to be connected to an I/O terminal 201 of the reset control system 203. The indicator light 115 is further connected to ground via a voltage-source terminal 210, while reset switch 205 is further connected, via a voltage-source terminal 220, to a DC voltage source 221 of voltage U. A 24 V DC voltage source could be used, or any other suitable voltage source.

The branch of reset circuit 200 which includes indicator light 115 and which extends from the connection point 240 to the voltage-connection point 210 will be referred to as indicator- light branch 245. Similarly, the branch of reset circuit 200 which includes reset switch 205 and which extends from the connection point 240 to the voltage-connection point 220 will be referred to as reset-switch branch 250.

In another embodiment of the inventive reset circuit 200, the indicator light 115 is connected between the I/O terminal 201 and a voltage source 221 of voltage U, while the reset switch 205 is connected between the I/O terminal 201 and ground. An example of a reset circuit 200 according to this embodiment is shown in Fig. 3. Also in this

embodiment, one end of the indicator-light branch 245 is connected to one end of the reset- switch branch 250. In this embodiment, the input signal 230 detected by the signal detector 225 will correspond to the ground potential when the reset button 110 is being pressed. A yet further embodiment of the reset circuit 200 is shown in Fig. 4. Also in this embodiment, a first end of the indicator-light branch 245 and a first end of the reset-switch branch 250 are electrically connected to each other. The voltage-source terminal 210 represents the second end of the indicator- light branch 245, and is connected to a first voltage source 221a of voltage Ua. Similarly, the voltage-source terminal 220 represents the second end of the reset-switch branch 250, and is connected to a second voltage source 221b of voltage Ub. Since ground can be considered to be a voltage source of 0 V, the embodiments of Figs. 2 and 3 can be seen as special cases of the embodiment of Fig. 4. In the following description, the resistance in the connection between the I/O terminal 201 and the voltage-source terminal 220 of the reset-switch branch 250 will be considered to be negligible when the reset-switch 205 is closed. The pressing of the reset button 110 will thus be assumed to cause a short circuit between the I/O terminal 201 and the voltage source 221b, and the potential at the I/O terminal 201 will be assumed to correspond to the voltage Ub of the voltage source when the reset-button 110 is being pressed. However, implementations wherein the resistance of the closed reset-switch branch 250 is non- negligible can also be contemplated.

Furthermore, it is assumed that the indicator-light branch 245 has a non-negligible resistance. In some implementations, for example when the indicator light 115 is a light emitting diode, a resistor could be connected in the indicator- light branch 245. The resistance of the indicator-light branch 245 could for example be around one order of magnitude larger than the resistance in the reset-switch branch 250, or more, for facilitating that the pressed and released states of the reset-button can be distinguished. However, even if further resistances are connected between the indicator light 115 and the reset switch 205, the reset switch 205 and the indicator light 115 are electrically connected at connection point 240.

The reset control system 203 of Fig. 4 is arranged to detect when a reset operation has been performed. In many implementations, a reset operation is performed by pressing the reset button during at least a predetermined duration of time, and the detection of a reset occurs when the reset button is released after having been pressed during at least the

predetermined time duration. Hence, a reset control system 203 is often arranged to detect when the reset button is released after having been pressed at least for a predetermined period of time. The predetermined period of time is typically selected to correspond to the duration of a normal pressing of a reset button 110, and could for example be in the order of 100 ms. The value of the predetermined period of time often falls within the range of 10 ms - 3 seconds, but other time durations could alternatively be used. The reset control system 203 of Fig. 4 comprises a signal generator 215, as well as a signal detector 225, both being connected to the I/O terminal 201.

The signal generator 215 is arranged to continuously generate a first output signal 222a at I/O terminal 201 when the reset circuit 200 is in the normal state, and to continuously generate a second output signal 222b at I/O terminal 201 when the reset circuit 200 enters the reset state, i.e. when the reset control system 203 has received a signal indicating that a sensor has detected a situation calling for reset. In the following, the first output signal 222a will be referred to as normal-state output signal 222a, while second output signal 222b will be referred to as reset-state output signal 222b. In the embodiment of Fig. 4, the normal-state output signal 222a is typically a signal corresponding to the potential of voltage source 221a, so that no voltage will appear across the indicator light 210, while the reset-state output signal 222b is a signal which differs from the potential of voltage source 221a, so that the indicator light 115 will be lit up. When referring to output signals from the signal generator 215 at the I/O terminal 201 in general, the reference numeral 222 will be used.

The signal detector 225 of Fig. 4 is arranged to detect the signal 230 at I/O terminal 201, and in particular to detect when the signal 230 at I/O terminal 201 differs from the signal 222 generated by signal generator 215. Signal 230 corresponds to the potential at the I/O terminal, and will hereinafter be referred to as the input signal 230. The reset circuit 100 is typically arranged so that when the reset button 110 in reset circuit 200 is being pressed, the signal 230 detected by the signal detector 225 will differ from the signal detected when the reset switch 205 is open. The signal detector 225 is typically arranged to interpret an input signal 230 which has a potential which lies within a range of the expected potential when the reset button 110 is being pressed, U^ ^ssed , as a signal indicating a pressed-down state of the reset button 110. Similarly, the signal detector 225 is typically arranged to interpret an input signal 230 which has a potential which lies within a range of the expected potential when the reset button 110 is released, indicating a released state of the reset button 110. These ranges could for example be 50 % of

\ ureieased _ u^ ^ssed for both situations; 40 % of \ U [ ^eased - U ^ssed \ for both situations; different ranges for the pressed down state and the released state, e.g. 50 % and 30 %, respectively; etc. Any suitable ranges could be used which give a high probability of a correct reading.

The reset control system 203 can be implemented by a suitable combination of hardware and software. The reset control system 203 could for example include one or more general purpose processors, or one or more processors especially developed for the reset control system 203, in combination with software for performing a method of operating the reset control system, of which an example is illustrated in Fig. 6. Fig. 5 schematically illustrates an embodiment wherein the reset control system 203 comprises a processor 500. Fig. 5 shows the reset control system 203 comprising processing means 500 connected to a computer program product 505 in the form of a memory, as well as to terminals 202 and 510; to the signal generator 215; and to the signal detector 225. The memory comprises computer readable code means that stores a computer program, which, when executed by the processing means 500, causes the reset control system 500 to perform an operating method an example of which is illustrated in Fig. 6. In other words, the reset control system 203 may in this embodiment be implemented with the help of corresponding program modules of the computer program stored in the memory 505.

By execution of the software stored in memory 505, the processing means 500 is arranged to execute the operation of the control system 203: The processing means 500 is arranged to receive a signal 515 from a sensor, the signal 515 being indicative of a situation calling for reset. In response to receiving the signal 515, the processing means 500 of Fig. 5 is arranged to instruct the signal generator 215, via signal 520, to apply a reset-state output signal 222b at the I/O terminal 201. Meanwhile, the signal detector 225 monitors the potential at the I/O terminal 201, to check whether the potential changes in the manner which is expected when a reset operation has been detected. The signal detector 225 is arranged to send a signal 525, indicating that a reset operation has been performed, to the processing means 500 in response to having detected a reset operation. In response to receiving the signal 525 which is indicative of a performed reset operation, the processing means is arranged to output an OK- signal 530. In the embodiment shown in Fig. 5, the OK- signal 530 is applied to an output 510. The output 510 can be an internal output, connected to an input of another part of a machine safety system of which the reset control system 203 forms a part, or an external output. Also in response to receiving the signal 525, the processing means 500 is further arranged to instruct the signal generator 215, via signal 535, to cease applying the reset-state signal 222b at the I/O terminal 201, and to instead apply a normal-state signal 222a. Signal 535 and signal 520 could, if desired, use the same physical connection.

In one implementation, the signal generator 225 is arranged to receive a signal 540 indicating which output signal 222 is currently applied at the I/O terminal 201. In Fig. 5, the signal generator 215 and the signal detector 225 are shown to be entities separate from the processing means 500. However, in an alternative implementation, at least parts of the signal generator 215 and the signal detector 225 form part of the processing means. The processing means 500 could be one or more processors. In one embodiment, two or more processors are employed for redundancy purposes. Similarly, two or more signal detectors 225, and/or two or more signal generators 215, could be used for redundancy purposes. The computer program product 505 could be any type of non- volatile computer readable means, such as a hard drive, a flash memory, an EEPROM (electrically erasable programmable read-only memory) a DVD disc, a CD disc, a USB memory, etc.

The embodiment of reset control system 203 of Fig. 5 is shown to have three input/output terminals: One I/O terminal 201 for the connection to a reset circuit 200, one input 202 for receipt of signals 515 from a sensor and one output 510 for the delivery of an OK-signal 535. A reset control system 203 typically further has two terminals for connection to a power supply. In other implementations, a different number of terminals could be used. However, only one I/O terminal 201 for the connection to a reset circuit 200 will be required. Fig. 6 is a flowchart schematically illustrating an embodiment of a method of operating reset control system 203. The output of signal generator 215 and the input of signal detector 225 of the reset control system 203 of Fig. 6 are connected to the I/O terminal 201, to which connection point 240 of a reset circuit 200 is also connected (cf. Figs. 2-4). A first end of the reset-switch branch 250 as well as a first end of the indicator- light branch 245 of the reset circuit 200 is connected to the connection point 240.

At step 600, the signal generator 215 applies the normal-state output signal 222a to the I/O terminal 201. The normal-state output signal 222a is typically a signal corresponding to the voltage Ua at the second end of the indicator- light branch 245, so that no current will flow through the indicator light 115. At step 605, it is monitored whether a signal 515 has been received which indicates that a situation calling for reset has been detected by a sensor. Such reset-situation signal 515, indicating the presence of a situation calling for reset, could for example be received by the reset control system 203 at an I/O terminal 202 (cf Fig. 5). If a situation calling for reset has been detected, then step 610 is entered.

In step 610, a reset-state output signal 222b is generated by signal generator 215 and applied to the I/O terminal 201, in order to switch on the indicator light 115 to inform staff that a reset operation can be performed. Step 615 is then entered, wherein it is monitored whether a signal 515 has been received which indicates that the sensor has returned to normal. Such back-to-normal signal 515 can for example be generated based on an indication generated by the same sensor which detected the situation calling for reset, and received by the reset control system 203 via the I/O terminal 202.

If it is determined in step 615 that the sensor is back to normal, step 617 is entered, wherein a third type of output signal 222 is generated by signal generator 215 and applied to the I/O terminal 201, this third type of output signal 222 being a signal which goes from a first value to a second value and back, at a frequency which can clearly be detected by the human eye. The first value can for example be the normal- state output signal 222a and the second value can for example be the reset-state signal 222b. This third type of signal will cause the indicator light 115 to flash, and will hereinafter be referred to as a flashing reset-state output signal 222c, or flashing output signal 222c for short. By causing the indicator light 115 to flash in step 617, staff operating the reset button 210 will be informed that the sensor has returned to its normal state and a reset operation can be performed. The frequency of the flashing output signal 222c could for example be in in the range of 0.1-2 Hz. Step 620 is then entered, wherein the potential at the I/O terminal 201 is being monitored by the signal detector 225, the purpose of the monitoring being to detect whether a reset operation has occurred. The signal detector 225 continuously receives an input signal 230 that corresponds to the potential at I/O terminal 201. As explained above, a reset operation can be detected in step 620 by the signal detector 225 detecting a change in the potential at the I/O terminal 201, which lasts for at least a predetermined period of time, and which change corresponds to the change expected if the reset button 110 is being pressed. Assuming that the resistance in the resistor-switch branch 250 is negligible, the potential at the I/O terminal 201 is expected to take the value of the potential at the second end of the reset-switch branch 250 when the reset button 110 is being pressed (i.e. take the value of the potential Ub in the terminology of Fig. 4). Oftentimes, the signal detector 225 is arranged to conclude that a reset operation has been detected when the input signal 230 returns to a signal

corresponding to the reset-state output signal 222b, after a change has been detected which corresponds to the reset-button being pressed for at least a predetermined period of time.

When a reset operation is detected by the signal detector 225, the reset control system 203 typically generates an OK- signal 530 in step 625, which signal is sent from the reset control system 203 to another part of the machine safety system. Step 600 is then reentered, wherein normal- state output signal 222a is again applied to the I/O terminal 201 by signal generator 215, causing the indicator light 1 15 to go out.

Typically, the task of a sensor, which is arranged to detect a situation calling for reset, is to monitor the circumstances of machine equipment. When a situation calling of reset is detected in step 605, a machine affected by the situation calling for reset would typically be stopped. Once the OK- signal 530 has been generated in step 625, the machine can be restarted. In many implementations, the re-start of machines is not performed by the machine safety system, but the re-start of a machine would require the pressing of a start button which is not part of machine safety system. The purpose of the reset operation is typically to permit the restart of the machine if the start button is being pressed. However, an implementation where the pressing of the reset button 110 actually causes the start of machine(s) which have been stopped can also be contemplated. The reset button 110 would in such implementation operate as a start button. The method illustrated in Fig. 6 is an example only, and can be altered in different ways. For example, step 617 could be omitted, so that the indicator light 1 15 shows a steady light also after the sensor has returned to its normal state in step 615. Furthermore, step 615 can be entered before step 610, so that the indicator light 1 15 will not be turned on until the sensor has returned to normal, etc.

The normal-state output signal 222a, applied at I/O terminal 201 by the signal generator 215 during normal operation of the machine(s) which is being monitored by the reset function, will in many implementations be a signal of a voltage corresponding to the voltage Ua of the voltage source 221a to which the indicator light 1 15 is connected.

The reset-state output signal 222b, applied at I/O terminal 201 when a situation calling for reset has been detected, will be a signal which differs both from the voltage Ub of the voltage source 221b to which the reset switch 205 is connected, and from the normal- state output signal 222a, in order to cause the indicator light 1 15 to light up in the reset-state.

The difference between the reset-state output signal 222b and the voltage Ub should preferably be large enough for signal detector 225 to distinguish between the two also when disturbances occur in the reset control system 203.

In one embodiment, the reset-state output signal 222b is a modulated signal, and the signal generator 215 is arranged to generate a modulated output signal 222b at times when the light 1 15 is required to be switched on.

A first voltage value Ui and a second voltage value U ₂ of the modulated signal 222 could take any desired values, as long as Ui≠U ₂ . At least one of voltage values of the modulated signal could advantageously differ from the voltage Ua by an amount which would cause the indicator light 1 15 to light up. In one implementation, one of Ui and U ₂ is the voltage Ua of the voltage source 221a connected at the second end of the indicator-light branch

245. In one implementation, one of the values of the modulated signal is the voltage Ub of voltage source 221b connected at the second end of reset switch 205. When the reset button 210 is in a released state, the input signal 350 will show periodic time slots during which the potential differs from Ub. When the reset button 210 is being pressed, the input signal 350 will instead correspond to the DC potential Ub.

In an embodiment where the voltage Ub of the voltage source 221b differs from the voltage Ua of the voltage source 221a, the modulated signal could, if desired, vary between the values Ui=Ua and U ₂ =Ub. For example, in the embodiments shown in Fig. 2 and 3, a modulated reset-state output signal 222b could oscillate between the potentials U and ground potential. The modulation of a modulated reset-state output signal 222b could be any kind of modulation, such as a square wave signal, another PWM signal or a saw-tooth signal.

In one implementation, the modulation frequency of the modulated signal exceeds the flicker fusion frequency in order for a human observer to experience the indicator light 1 15 as steady, and could for example be a frequency which exceeds 50 Hz.

A modulated signal can for example be beneficial in an embodiment wherein a flashing output signal 222c is generated by the signal generator 215 upon receipt of a signal 515 indicating the return of the sensor to its normal state (cf. step 617 of Fig. 6) and the flashing output signal 222c jumps between the normal-state output reset signal 222a and the reset-state output signal 222b: If the reset-state output signal 222b is a modulated signal with a first voltage Ua and a second voltage Ub, and the normal-state signal is signal of constant potential Ua, the pressed-down state of the reset button 210 can be distinguished from the released state as a continuous signal of potential Ub, regardless of in which part of the flashing period that the reset button 210 is pressed down. In the released state of the reset button 210, the potential at I/O terminal 201 will in this embodiment either correspond to the modulated signal, or be a signal of voltage Ua, depending on where in the flashing cycle that the reset button 210 is pressed down. In an embodiment employing a modulated signal, the sampling frequency of the signal detector 225 should preferably exceed the frequency of the modulated signal 222 in order to ensure adequate signal detection, and typically, the sampling frequency of the signal detector 225 is twice the frequency of the modulated signal, or higher. In one implementation, the sampling frequency is the clock frequency of a processor 500 of the reset control system 203.

A modulated reset-state output signal 222b could also be beneficial in order to reduce the risk that a false signal at the I/O terminal 201 is detected as a reset operation. A false signal could for example be caused by a fault in the signal generator 215. Since the chance is slim that such false signal from the signal generator 215 has the same characteristics as the modulated reset-state output signal 222b, a constant potential at the I/O terminal 201 of approximately the value expected when the reset button 210 is being pushed, can more reliably be assumed to be the result of the pressing-down of the reset button 210. The reliability of the reset function can thus be improved by use of a modulated reset-state output signal 222b rather than a reset-state output signal 222b of constant potential.

In one embodiment of the present technology, the voltage Ua of voltage source 221a to which the indicator light 115 is connected differs from the voltage Ub of voltage source

221b to which the reset switch 205 is connected (cf. Figs. 2 and 3). In this embodiment, the normal-state output signal 222a can advantageously be a signal of voltage Ua.

In another embodiment, the voltage-source terminal 210 of indicator- light branch 245 and the voltage-source terminal 220 of reset-switch 205 are connected to a voltage source of the same potential, this potential being referred to as Uab. In this embodiment, the voltage sources 221a and 221b could be the same, or different voltage sources could be used.

In this embodiment, the reset-state output signal 222b would have be of a different voltage than Uab, thus causing the indicator light 115 to light up. When the reset button 210 would be pressed, the potential at I/O terminal 201 would be Uab. Hence, if a normal-state signal 222a of voltage Uab were to be used, then the signal detector 225 would not be able to differentiate between a situation where the reset button 210 is pressed down and the signal generator 215 generates a reset-state output signal 222b, and a situation where the signal generator 215 generates a normal state output signal 222a. In some applications of the reset function, this will be acceptable. In other applications, the possibility of distinguishing between these two situations may be beneficial. In order to be able to distinguish between these situations in an embodiment where the indicator-light branch 245 and the reset-circuit branch 250 are connected to the same potential, the normal- state output signal 222a generated by the signal generator 215 (cf. step 600 of Fig. 6) could include a modulation. In one implementation, a modulated normal-state output signal 222a is used which includes voltage spikes which would not cause the indicator light 115 to light up, but which would be detectable by the signal detector 225. Such voltage spikes could have a duration short enough to not cause the indicator light 115 to light up, and/or be of a voltage which would not be enough to light up the indicator light 115. .

Oftentimes, the signal generator 215 is a current limited signal generator. This is particularly beneficial when the voltage Ub is lower than the potential of the reset-state output signal 222b .

The voltages of voltage sources 221a and 221b, as well as the potential of reset-state output signals 345a, 345b and 345c, can for example lie within the range of - 50 V to 50 V. In one example, the voltage Ua of voltage source 221a connected to the indicator- light branch is 0 V; while the voltage Ub of voltage source 221b connected to the reset-switch branch is 24 V, and the reset-state output signal 345b is a modulated signal which switches between 0 V and 24 V. However, in other implementations, any other suitable values could be used.

In the above description and appended drawings, the reset button 110 has been illustrated as a push button. However, other implementations of the reset button 110 could be used, such as a lever, a pull-button, etc.

According to the invention, a reset circuit 200 can be connected to a reset control system 203 via a single I/O terminal 201. Connection points are generally space consuming and often take up a considerable part of a reset control system. By ensuring that one connection point is sufficient for connecting a reset circuit 200 to a reset control system 203, the space required for connection points is optimized.

The reset circuit 200 and the reset control system 203 can for example form part of a machine safety system, where the machine safety system typically further comprises a sensor for detecting a situation calling for reset. In one implementation, the reset control system 203 is implemented as part of such sensor, and a reset circuit 200, connected to the reset control system 203, is dedicated to the sensor of which the reset control system 203 forms a part. In another implementation, two or more sensors are monitored by the same reset control system 203 and arranged to be reset by the same reset button 210. In yet another implementation, the reset circuit 200 and reset control system 203 are arranged to monitor a single sensor, while the sensor is separate to the reset control system 203. Reset control system 203 could, if desired, form part of a safety controller including further machine safety functions, or could be a standalone machine safety controller.

Although various aspects of the invention are set out in the accompanying claims, other aspects of the invention include the combination of any features presented in the above description and/or in the accompanying claims, and not solely the combinations explicitly set out in the accompanying claims.

One skilled in the art will appreciate that the technology presented herein is not limited to the embodiments disclosed in the accompanying drawings and the foregoing detailed description, which are presented for purposes of illustration only, but it can be

implemented in a number of different ways, and it is defined by the following claims.