Field of the invention The present invention relates to a device for controlling a magnetic door holder including an electromagnet for holding a normally closed door in an open position, wherein the controller comprises a detector module adapted to detect when the door is engaged to the electromagnet, and a release module adapted to release the door a certain period of time after the door was engaged to the electromagnet. The present invention also relates to a magnetic door holder comprising an electromagnet for holding a normally closed door in an open position and a device for releasing the door a period of time after the door was engaged to the electromagnet.

The invention further relates to a method for controlling a magnetic door holder.

Prior Art A magnetic door holder includes a holding device mounted on a stationary support close to a door. The holding device includes an electromagnet having a coil supplied with current. The magnetic door holder may also include a plate made of a magnetic material, such as iron, mounted on the door. If the door is made of a magnetic material, the plate is not necessary. The magnetic door holder holds the door open while the coil is energized. When the coil is de-energized, for example, by a relay controlled by a fire alarm system and/or a manually operated release switch, the door is released to a closed position. Thus, if power fails, the doors are released automatically.

Magnetic door holders is designed to allow doors, such as fire and smoke doors, to be held open under normal conditions, but to automatically close when a fire or smoke alarm system is activated. However, in some places there is a desire, for example, for safety reasons, to have the doors closed during normal conditions. The doors can be opened and closed manually at any time. To close the door, a release button may be manually activated. For example, during transportation of goods through the door, the door must be kept open for a certain time period. However, it is often forgotten to close the doors after the transportation of goods has been finished. Thus, the doors are often left is open position in contrary to safety instructions. In such cases, the magnetic holder is often disconnected, which may lead to the use of wedges to keep the door open during transportation of goods. The wedges are often forgotten to be removed, which leads to that the door is left in open position, even though they are fire doors, which should be kept closed in case the magnetic holder is disconnected.

A solution to this problem is to detect when the door is engaged to the holding device, which means that the door has been opened, and releasing the door by de-energizing the electromagnet after a predetermined period of time.

US4, 440,428 discloses a magnetic door holder including an actuating switch so disposed as to be engaged by the door when moved to its open position. Circuitry is provided between the switch and the electromagnet such that upon engagement of the door with the switch, the electromagnet is energized. The circuitry further includes a time delay means for de-energizing the electromagnet and for releasing the door after a predetermined period of time.

DE202006007204 discloses a unit for closing a door after a specific time, comprising a contacting probe for detecting when the door is open and engaged to the electromagnet.

A disadvantage with the prior art devices for controlling door holders is that they are complicated and requires expensive and bulky components for detecting when the door is engaged to the door holder.

Object and summary of the invention

An object of the present invention is to provide an improved device for controlling a magnetic door holder including an electromagnet for holding a normally closed door in an open position and the electromagnet is supplied with DC current,.

This object is achieved by a device as defined in claim 1.

The device comprises a detector module adapted to detect when the door is engaged to the electromagnet, and a release module adapted to release the door a period of time after the door was engaged to the electromagnet. The device is characterized in that the detector module is adapted to supervise the current supplied to the electromagnet and to detect a transient in the current due to a change in inductance when to the door is engaged to the electromagnet, and the release module is adapted to release the door when a certain time period has lapsed after the transient was detected. When the time period has lapsed, the power supply to the door holder may be interrupted, and the door will be closed. Preferably, the power supply to the door holder is only interrupted for a short period to allow the door to be closed. Then the door holder will regain the power supply to enable to hold the door the next time it is opened.

The invention is based on basic knowledge of resistive/inductive circuits. It may be un- derstood that the current through the magnetic door holder is determined by the resistance, since the power supply is DC, but also the inductance will have an influence on the current. When the door is engaged to the holding part, the inductance will be changed, and that change in inductance causes a transient in the current. When the transient in the current is detected, the time elapsed since the transient was detected is counted.

The device according to the invention retrieves information about the current supplied to the magnetic door holder, and detects the point in time when the door is being engaged to the electromagnet based on the retrieved information from the current sup- plied to the magnetic door holder. Current is easy to supervise, and does not require any expensive and complicated measuring equipment. Thus, no extra components, such as contacting probes or actuating switches, are needed for detecting when the door is engaged to the electromagnet. A device according to the present invention is advantageous since the construction of the device is simple and the components re- quired are cheap and small sized. Accordingly, costs and space are reduced. A further advantage with the present invention is that the device can be positioned at a distance from the door holder, as long as the device is mounted close to the power supply to the door holder. Thus, the device can be a separate unit, i.e. physically separated from the door holder. The device according to the invention can be used for controlling ex- isting door holders, which are already mounted in a building.

A further advantage with the present invention is that it does not affect the basic function of the door holder, i.e. the door is automatically close when the power supply to the door holder is turned off.

The invention is particularly suitable for fire doors, which are normally closed and must be closed once the power supply to the door closer is interrupted. With normally closed in meant that the doors are automatically closed when the door holder releases the door. It is assumed that the doors are equipped with door closers, for example hydrau- lie door closers, to ensure that the door is closed when the door holder releases the door.

According to an embodiment of the invention, the device comprises a switching unit adapted to turn on and off the current supply to the magnetic door holder, and the release module is adapted to instruct the switching unit to turn off the power supply to the magnetic door holder when the time period has elapsed since the transient was detected. Suitably, the switching unit is a relay. According to an embodiment of the invention, the device comprises a measuring device for measuring the power supplied to the electromagnet, and the detector module is adapted to supervise the current supplied to the electromagnet based on measurements from the measuring device. The measuring unit can be adapted to measure current or voltage. The device for measuring the power supplied to the electromagnet can be very simple since there requirement on the accuracy of the measurements is rather low. For example, the measuring device comprises a measuring resistor connected in series with the door holder, and the measuring device is adapted to measure the voltage over the resistor. According to an embodiment of the invention, the device comprises a processing unit adapted to receive measured values from the measuring unit, to detect a transient in the measured values, and to start the timer upon detecting the transient, and to instruct the switching unit to turn off the power/current supply. Thus, the detecting module and the release module can be implemented as software modules running on the processing unit.

According to an embodiment of the invention, the detector module is adapted to detect a transient by detecting when the current supplied to the electromagnet suddenly drops from a steady state level to a level below the steady state level. The transient in the current due to a change in inductance when to the door is engaged to the electromagnet is characterized in that the current supplied to the electromagnet suddenly drops from a steady level to a level considerably below the steady level, and the current returns to the steady level within a certain time period. With steady state level is meant the level of the current before and after the door is engaged to the electromagnet, i.e. before and after the transient.

According to an embodiment of the invention, the detector module is adapted to detect when the current supplied to the electromagnet drops more than a defined value. For example, a transient is detected when the current supplied to the electromagnet drops more than 25 % of the steady state current.

According to an embodiment of the invention, the device comprises a timer and the releasing module is adapted to start the timer upon detecting the transient. The time period can be predetermined. Alternatively, the device is arranged so that the time period is adjustable. Thus, the length of the time period can be defined by a user. The time period may be adjustable within a defined range. For example, the time period is adjustable in a range of 30 to 150 sec.

Another object of the present invention is to provide an improved magnetic door holder comprising an electromagnet for holding a normally closed door in an open position and a device for releasing the door a period of time after the door was engaged to the electromagnet. The door holder is characterized in that is comprises a device for releasing the door according to the present invention.

Another object of the present invention is to provide an improved method for controlling a magnetic door holder including an electromagnet for holding a normally closed door in an open position, wherein the door is released from the electromagnet a period of time after the door was engaged to the electromagnet.

This object is achieved by a device as defined in claim 11.

The method is characterized in that it comprises:

- supplying the electromagnet with DC current,

- supervising the current supplied to the electromagnet,

- detecting a transient in the current due to a change in inductance when to the door is engaged to the electromagnet,

- counting the time elapsed since the transient was detected, and

- releasing the door from the electromagnet when a defined period of time has elapsed since the transient was detected.

According to an embodiment of the invention, the method comprises turning off the current supply to the electromagnet when said time period has elapsed since the tran- sient was detected and by that releasing the door from the electromagnet. The electromagnet is deenergized and by that the door is released from the electromagnet.

According to an embodiment of the invention, the method comprises measuring the power supplied to the electromagnet, and supervising the current supplied to the elec- tromagnet based on measured current values.

According to an embodiment of the invention, detecting the transient comprises detecting when the current supplied to the electromagnet suddenly drops from a steady state level to a level below the steady state level. According to an embodiment of the invention, the method comprises detecting when the current supplied to the electromagnet drops more than a defined value. Brief description of the drawings

The invention will now be explained more closely by the description of different embodiments of the invention and with reference to the appended figures. Fig. 1 shows a part of a building including a closed door, a magnetic door holder, and a device according to an embodiment of the invention for controlling the door holder.

Fig. 2 shows the same view as figure 1, but with the door opened. Fig. 3 shows a circuit diagram of an example of a device for controlling a magnetic door holder according to the invention.

Fig. 4 shows the current changes when the door is engaged to the door holder. Detailed description of preferred embodiments of the invention

Figures 1 and 2 show a portion of a building including a wall provided with a door 1 and a magnetic door holder system for holding the door in an open position. The door 1 is closed in figure 1, and the door is opened in figure 2.The door 1 is normally closed, which means that the door is arranged so that it automatically closes when it is released from the door holder. The door is, for example, equipped with door closers including a spring return mechanism, such as a hydraulic door closer. The door is, for example, a fire or smoke door. The magnetic door holder system includes a holding part 3 mounted on the wall and comprising an electromagnet having a coil supplied with DC current, and a contact part 4 made of a magnetic material, for example iron, and mounted on the door 1. The contact part 4 is, for example, an iron plate. If the door 1 is made of a magnetic material, the contact part 4 can be omitted. An electromagnet is a type of magnet in which a magnetic field is produced by an electric current. The magnetic field disappears when the current is turned off. Figure 2 shows the door 1 when the contact part 4 is engaged to the holding part 3.

When the door 1 is moved to its open position, the contact part 4 on the door makes contact with the holding part 3 on the wall, and the door becomes engaged to the holding part 3 due to the magnetic interaction between the electromagnet and the magnetic material in the contact part 4.

The coil of the holding part 3 is supplied with DC power from a DC power source 6. The power source 6 can be positioned at a distance from the door holder, for example, in another room in the building. Optionally, the magnetic door holder system comprises a switch in the form of a release button 9 to enable a user to manually switch off the power supply to the holding part 3.

To close the door, the release button 9 has to be manually activated. The release button 9 enables a human to close the door by switching off the power to the electromag- net, and by that releasing the door from the door holder. Optionally, the system may include a further switch (not shown in the figure) controlled by a fire or smoke alarm system in order to enable the fire or smoke alarm system to automatically switch off the power supply to the holding part 3. Thus, the door is automatically closed when the fire or smoke alarm system is activated. The magnetic door holder holds the door open while the coil is energize, i. e. as long as the electromagnet is supplied with power. When the coil is de-energized, for example, by the release button 9 or the switch controlled by the fire alarm system, the door 1 is released and moved to a closed position. Thus, if power fails, the door is released automatically. However, the door may be opened or closed manually at any time. The magnetic door holder system further comprises a device 10 for controlling the magnetic door holder by releasing the door a period of time after the door was engaged to the electromagnet. The device 10 is connected between the power supply 6 and the holding part 3. The device 10 will now be described in more details with reference to figure 3.

Figure 3 shows a circuit diagram of an example of a holding part 3 and a device 10 for controlling a magnetic door holder according to the invention. The holding part 3 and the device 10 form an electrical circuit 11. The holding part 3 comprises an electromagnet including a coil 12 supplied with DC current from a DC power source 6. The coil 12 may include an iron core to improve the magnetic field generated by the electromagnet. The device 10 is arranged in series with the coil 12, the power source 6 and the release button 9. The release button 9, for example, includes a relay. The device 10 comprises a measuring device 13 for measuring the current or voltage supplied to the electromagnet. For example, the measuring device comprises a measuring resistor and the voltage over the resistor is measured. The measured voltage may be converted into current values using Ohms law.

The device 10 further comprises a detector module 14 adapted to detect the moment when the door 1 is engaged to the holding part 3 based on the on measurements from the measuring device 13. The detector module 14 is adapted to supervise the DC current supplied to the coil 12 of the electromagnet based on the measurements from the measuring device 13, and to detect a transient in the current due to a change in inductance when to the contact part 4 is engaged to the electromagnet. The device 10 further comprises a timer 15 and a release module 16 adapted to release the door a certain period of time after the moment the door was engaged to the holding part. The release module 16 is adapted to start the timer 15 upon detecting a transient in the current supplied to the electromagnet, and to release the door when the period time has elapsed since the transient was detected.

When the contact part 4, and accordingly the door 1, is engaged to the holding part 3, the inductance in the circuit 11 will be changed. That change in inductance causes a transient in the current supplied to the coil 12. Figure 4 shows a diagram with an example of how the current changes when the door is engaged to the magnetic holder at a point of time. As shown in the figure, the current stays at a steady state level Li as long as the door is closed and until the door is in the vicinity of the holder part 3. When the contact part 4 on the door is engaged to the holder part 3, the inductance of the magnetic door holder changes, and that change in inductance causes a transient 20 in the current. During the transient 20, the current drops significantly, as shown in the diagram. When the door has been engaged to the door holder, the current returns to returns to the same level as before, i.e. to the steady state level Li. The detector module 14 is adapted to detect the transient 20. For example, the detector module 14 is adapted to detect the transient by detecting when the current supplied to the electromagnet drops more than a defined value. The define value can, for example, be the amount of change in the current (ΔΙ), or a defined limit value (L ₂ ). In the example shown in figure 4, the transient 20 is detected at a point in time ti when the current has dropped to a level below a limit value L ₂ . In another example, a transient is detected when the current supplied to the electromagnet drops more than 25 % of the steady state current Li. Thus, if the steady state current is 0.1 A, a transient is detected when the current supplied to the electromagnet drops more than 0,025 A from the steady state current Li. The transient 20 occurring when the door is engaged to the magnetic holder is detected, and the timer 15 is started upon detecting the transient. After a certain period of time, for example in the range of 30 - 150s, the electromagnet is de-energized, and the door is released from the holding part, and accordingly the door will be closed. For example, the power supply to the magnetic holder is interrupted a certain period of time after the transient was detected.

The time period can be predetermined. Alternatively, the device is arranged so that the time period is variable, and the timer period can be selected by a user. For example, the time period can be adjustable in the range of 30 to 150 sec. When the selected time has lapsed, the power supply to the holding part is interrupted and the door will be closed. Optionally, the release unit is adapted to generate a warning signal before the door is released. The warning signal can be audible or visible.

The device further comprises a switching unit 18 adapted to turn on and off the current supply to the coil 12. The switching unit 18 is, for example, a relay. The release module 16 is adapted to turn off the current supply to the coil 12 when a certain time period has lapsed after the transient was detected and by that release the door from the hold- ing part 3. The door will then be closed. Suitably, the release module 16 is adapted to turn on the current supply to the coil 12 when a certain time period has lapsed after the door was released. An advantage with the present invention is that it does not affect the basic function of the door holder, i.e. the door is automatically close when the power supply to the door holder is turned off.

For example, the device 10 comprises a processing unit 19, and the detector module 14, the release module 16, and the timer 15 are software modules running on the processing unit 19. The processing unit 19 may include a micro-processor and suitable memory. Suitably, the device 10 also includes an A/D converter for converting meas- ured current/voltage values into a digital data format readable to the processing unit.

The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims. In the example disclosed in figures 1 and 2, the holding part 3 and the device 10 for controlling the magnetic door holder are arranged as separate apparatus with separate housings. However, in another embodiment the device 10 can be an integrated part of the holding part 3 and the device 10 can be enclosed in the same housing as the holder part 3.