APPARATUS AND METHOD FOR MONITORING AN ELECTROMAGNETIC BRAKE

The present invention is directed to an apparatus for monitoring an electromagnetic brake, an electromagnetic brake comprising such an apparatus for monitoring an electromagnetic brake, an elevator system comprising such an electromagnetic brake and a drive control system, and a method for monitoring the operation of such an electromagnetic brake.

Electromagnetic brakes are known and widely used in a plurality of technical applications, e.g. in people mover applications such as elevator systems. Particularly in these applications there is a need for detecting the proper opening and closing of the brake in order to avoid either running a drive against the brake or to shut off the drive at the end of a run when the brake is not closed.

Currently mechanical switches are commonly used in order to detect the prop. er opening or closing of the brake. However, as the mechanical movement of the brake lever or disk is very small the adjustment of said switches is very sensitive and not considered to be very robust. Accordingly it would be beneficial to provide improved means for detecting the proper opening or closing of an electromagnetic brake which are more reliable and robust.

Exemplary embodiments of the invention include an apparatus for monitoring an electromagnetic brake comprising a brake monitoring means adapted for being coupled to an electrical supply for an electromagnetic brake having braking means, wherein the brake monitoring means is configured to monitor over time the course of electrical current supplied to the electromagnetic brake for actuating the braking means and to detect movement or a status of the braking means using the monitored course of the electrical current.

Exemplary embodiments of the invention further include a method for monitor- ing the operation of an electromagnetic brake comprising the steps of monitor-

ing over time the course of electrical current supplied to the electromagnetic brake for actuating braking means of electromagnetic brake and detecting movement or a status of the braking means using the monitored course of the electrical current.

Embodiments of the invention will be described in greater detail below with reference to the Figures, wherein:

Figure 1 shows a schematic sectional view in axial direction of an exemplary electromagnetic drum brake, which may be monitored according to the invention;

Figure 2 shows a schematic sectional view of an exemplary electromagnetic disk brake , which may be monitored according to the invention;

Figure 3 shows a schematic circuit diagram of an exemplary electromagnetic brake coupled to an apparatus for monitoring the electromagnetic brake according to the invention;

Figure 4 shows a diagram of various functional modules comprised in an embodiment of the apparatus according to the invention;

Figure 5a shows a graph illustrating the course of the electrical current or time slope supplied to the electromagnetic brake over time when opening the brake;

Figure 5b shows the differentiated signal of the signal course shown in Figure 5a;

Figure 6a shows a graph illustrating the course or time slope of the electrical current supplied to the electromagnetic brake over time when closing the brake; and

Figure 6b shows the differentiated current signal of the signal course shown in Figure 6a.

Figure 1 shows a schematic sectional view in axial direction of an exemplary electromagnetic drum brake, which may be used, e.g., in an elevator system for controlling the movement of a car. In the upper part of Figure 1 a drum 12 is shown which is rotatably supported at its center by an axis 26. Two brake shoes 10 are arranged to the left-hand side and to the right-hand side of said drum 12, respectively. Each of said brake shoes 10 can be tilted in order to be either pressed against or released from the drum 12. The axis 26 may be connected to a corresponding driving mechanism (not shown) of the elevator sys- tern.

At their respective lower ends the brake shoes 10 are respectively connected to brake levers 36 which extend downwards from the lower end of each of the brake shoes 10.

A spring member 28 is arranged in a horizontal direction below drum 12. A left end of spring member 28 is connected to the left brake lever 36 and the right end of the spring member 28 is connected to the right brake lever 36. The spring member 28 is arranged to push the brake levers 36 apart from each oth- er in order to press the brake shoes 10 against the drum 12 in order to apply a braking force to drum 12, which is the moving member of the brake to be fixed or released, respectively.

An electromagnet 21 is arranged below the spring member 28 as an actuating element of the brake. The electromagnet 21 comprises an electrical coil 22. The axis of the electrical coil 22 is running horizontally so that a horizontal channel 23 is formed inside the electrical coil 22. Two iron cores 34 are introduced into said channel 23 from the left-hand side and from the right-hand side, respectively, leaving an air gap 24 between each other. The left end of the left iron core 34 is connected to the left brake lever 36 while the right end of the right iron core 34 is connected to the right brake lever 36.

Thus, when the coil 22 is supplied with an electrical current for actuating the brake 8 an electromagnetic field is generated within channel 23 pulling the iron cores 34 further into said channel 23. This movement is transmitted by the

brake levers 36 to the brake shoes 10 releasing the brake shoes 10 from drum 12. As a result the brake 8 is released and the drum 12 can rotate freely around axis 26. It is noted that the inductance of the electrical coil 22 changes if a larger part of the iron cores 34 enters into the channel 23 reducing the air gap 24.

Figure 2 shows a sectional side view of an electromagnetic disk brake 9 as an alternative embodiment of a brake as shown in Figure 1. At the center of Figure 2 an axis 26 is running horizontally from the left-hand side to the right-haπ_d side. A rotating member 12 is rotatably supported by axis 26 by means of a hub 30. A brake disk or sheave 11 is arranged to the right-hand side of said rotating member 12 as a braking means to be fixed or released, respectively. A stator 38 is provided to the right-hand side of said brake disk 11 . Said stator 38 comprises two holes which are open towards the disk 11. Spring members 28 are introduced in each of said holes. The length of said spring members 28 is larger than the depth of said holes so that said spring members 28 protrude out of the respective opening of said holes towards the disk 11 in order to press the disk 11 against the rotating member 12. A flange 32 is arranged to the left- hand side of the rotating member 12 so that the rotating member 12 is clamped between said flange 32 and said brake disk 11 by. the force of said spring members 28 in order to brake the rotating member 12. In an embodiment brake pads (not shown) are fixed to the interface of the rotating member 12 with said flange 32 and with said brake disk 11, respectively.

Stator 38 also comprises an electrical coil 22. The axis of said electrical coil 22 runs parallel to the axis 26 of the rotating member 12.

When the electrical coil 22 is supplied with an electrical current a magnetic field is generated which attracts the brake disk 11 made of a magnetic material. Thus, the brake disk 11 moves toward the electrical coil 22 releasing the rotat- ing member 12. By this movement the air gap 25 formed between the electrical coil 22 and the brake disk 11 is narrowed or even closed which will change the inductance of the electrical coil 22.

In an embodiment the brake disk 11 may comprise a protrusion which extends into an opening which is formed inside the electrical coil 22. In such an embod-

iment a lower electrical current will be needed for actuating and moving brake disk 11.

Figure 3 shows a schematic circuit diagram of an electrical circuit coupled to an electromagnetic brake, such as the brake shown in Figures 1 or 2. An electrical supply 6 is shown in the upper left of Figure 3 which is coupled to an electrical coil 22 of the brake. As discussed above, the breaking means are actuated by the electrical coil 22 for fixing and releasing a corresponding moving member (e.g. drum 12 in Fig. 1 or rotating member 12 in Fig. 2). The inductance L and the resistance R of the electrical coil 22 are symbolized at the right-hand side of Figure 3. The electrical supply 6 and the electrical coil 22 are connected with each other through electrical leads 42. An electrical switch 44 is provided in order to open and close the electrical circuit for activating and deactivating the electromagnetic brake, respectively.

An apparatus 1 for monitoring the electromagnetic brake according to the invention is coupled to the electrical circuit at node 40. The coupling may be galvanic, capacitive or inductive. The apparatus 1 for monitoring the electromagnetic brake comprises a measurement module 13 for measuring the electrical current J flowing through leads 42 providing an analog signal, an A/D-conver- sion module 14 for converting the analog signal into a digital signal and a current analysis module 16 for analyzing the digital signal.

An output of the apparatus 1 for monitoring the electromagnetic brake is coupled to a drive control system 20. The drive control system 20 uses an output signal provided by the apparatus 1 for controlling a mechanical system (not shown) as e.g. an elevator system, a moving escalator system, a train system, or the like.

Figure 4 shows a schematic diagram of the modules comprised in the apparatus 1 for monitoring the electromagnetic brake 8, 9 according to the invention and their respective interaction. The brake current measurement module 13 is shown at the top left of Figure 4. This module measures the electrical current J supplied to the electromagnetic brake for actuating the breaking means and produces an analog signal. Said analog signal is delivered to the A/D conver-

sion module 14. This A/D conversion module 14 filters noise from the analog signal and converts it into a digital signal. The digital signal is delivered to the analysis module 16, which analyzes the signal e.g. by comparing said signal with a predetermined reference signal. The digital signal is indicative of the ac- tual brake current time slope measured at node 40, whereas the reference signal is indicative of a reference brake current time slope provided by module 18.

Said reference signal may be generated by a self-learning module 18 by analyzing the course of the electrical current J for a plurality of switch-on and switch- off operations which are executed in a self-learning mode which is executed before the apparatus 1 is put into practical use.

A result of the analysis executed by the analysis module 16 is delivered to the drive control system 20 which uses said signal for controlling a drive system (not shown), such as the drive system of an elevator installation.

Figure 5a shows the course or time slope of the electrical current J supplied to the coil 22 of the electromagnetic brake 8, 9 over time when the brake is opened, i.e. when the supply 6 is switched on. The time t is plotted on the x- axis while the current J is plotted on the y-axis.

When the switch 44 is closed at time t=0 the current J starts rising according to a predetermined function characterized by a first time constant τi, which is determined by the electrical system characteristics such as inductance Li and re- sistance R of the electrical coil 22 and voltage U of the power supply 6, wherein Li is the inductance L of the electrical coil 22 in a first state where the iron cores 34 are separated (cf. Fig. 1) or where an air gap 25 is present between the brake disk 11 and the electrical coil 22 (cf. Fig. 2), respectively. At some point of time the electrical current has increased to a point where the electromagnetic field induced by said current J becomes large enough to move the iron cores 34 (cf. Fig. 1) or the brake disk 11 (cf. Fig. 2) against the force of the spring member 28. Due to this movement a larger part of the iron cores 34 enters into the channel 23 of the electrical coil 22 (drum brake) or the gap between the brake disk 11 and the electrical coil 22 (disk brake) is reduced. This results in a change of the inductance L of the coil 22 at point ti. Thus, from this point of

time the current J continues to increase according to another predetermined function characterized by a second time constant τ ₂ , which is determined by electrical system characteristics such as such as inductance L ₂ and resistance R of the electrical coil 22 and voltage U of the power supply 6, wherein L ₂ is the inductance of the electrical coil 22 in a second state, wherein a larger part of the iron cores 34 have been introduced into the channel 23 of coil 22 (cf. Fig. 1) or the air gap 25 between the brake disk 11 and the coil 22 has been reduced (cf. Fig. 2).

This change of inductance causes a dip 46 in the course of the electrical current J over time at point ti. The current analysis module 16 is configured to detect this dip 46 in order to provide a clear and reliable indication that the brake 8, 9 has been opened. This detection can be e.g. implemented by comparing the course of the electrical current J over time with a predetermined reference sig- nal.

Figure 5b shows the differentiation J' of the course of the electrical current J shown in Figure 5a over time. The dip 46 at time t, in the current which results from the change of inductance from Li to L ₂ causes an even more prominent dip 47 (negative peak) in the differentiated current J' followed by a discontinuity. Such a prominent dip 47 can be detected easily by comparing the differentiated current signal with a predetermined threshold Ti.

Figure 6a shows the course of the electrical current over time when the brake 8, 9 is closed. At time t=0 the switch 44 is opened, i.e. the electrical supply is separated from the electrical coil 22.

The current J starts decreasing according to another predetermined function characterized by the second time constant τ ₂ which is determined by electrical system characteristics such as inductance L ₂ and resistance R of the electrical coil 22 and voltage U of the power supply 6, wherein L ₂ is the inductance of the electrical coil 22 in the second state, wherein a larger part of the iron cores 34 have entered into the channel 23 of the electrical coil 22 (cf. Fig. 1) or the air gap 25 between the brake disk 11 and the electrical coil 22 has been reduced (cf. Fig. 2).

At the time t ₂ the magnetic field induced by the current J is no longer strong enough to hold the iron cores 34 or the brake disk 11 against the force of the spring members 28. Thus, the spring members 28 press the brake shoes 10 or the brake disk 11 against the rotating member 12 pulling the iron cores 34 out of the channel 23 of the electrical coil 22 or separating the brake disk 11 from electrical coil 22, respectively. Moving out the iron cores 34 or separating the brake disk 11 causes the inductance of the electrical coil 22 to change from L ₂

Thus, the current flowing through the electrical coil 22 decreases further according to another predetermined function characterized by the first time constant T ₁ , which is determined by the electrical system characteristics such as inductance L ₁ and resistance R of the electrical coil 22 and voltage U of the power supply 6, wherein L ₁ is the inductance L of the electrical coil 22 in the first state where the iron cores 34 are separated (cf. Fig. 1) or where an air gap 25 is present between the brake disk 11 and the coil 22 (cf. Fig. 2), respectively.

This change of inductance L causes a bump 48 in the course of electrical current J. This bump 48 is detected by the analysis module 16 in order to detect the movement of the braking means 10, 11.

Figure 6b shows the differentiation J ¹ of the current signal J of Figure 6a with respect to time. The bump 48 at time t ₂ in the course of the electrical current J results in a prominent peak 49 in the differentiated signal J'. Such a peak 49 can be detected easily by comparing the differentiated signal J' with another predetermined threshold T ₂ .

An apparatus and a method for monitoring an electromagnetic brake according to exemplary embodiments of the invention as described above allow to detect the opening and closing of an electromagnetic brake properly and reliably. It avoids the need for any additional mechanical parts as e.g. mechanical switches and setups thereof. As a result it is very robust, easy to assemble and reduces the need for maintenance considerably.

An idea of the invention is to measure the brake current time slope and to detect the movement of the braking means by the typical shape of the time transient. Due to the movement of the braking means the magnetic system characteristics i.e. inductance of the brake coil changes. This correlates with a typical dip in the brake current time slope when the brake lever or disk starts moving. This is shown in the figures and the description. By means of signature analysis, done in the brake control system, i.e. pattern comparison of the time slope with a reference slope, the brake status is derived. In case of a significant dip in the slope a more simpler detection by means of signal differentiation and com- paring with a threshold can be used.

The current sensor could be located in the control system, this allows to get rid of the signal cables as used in the prior art for the connection of mechanical brake switches.

In an exemplary embodiment of the invention as described above the brake monitoring means further comprises a measurement module for measuring the electrical current, an A/D-conversion module for converting an analog signal from the measurement module into a digital signal, and a current analysis mod- ule for analyzing the digital signal. Such a modular layout facilitates the assembly and the maintenance of the brake monitoring means. In particular standard modules can be used which can be obtained easily and economically prized.

In an embodiment the current analysis module is configured to compare the digital signal with a predetermined reference signal. This enables an easy and particular reliable detection of the movement or status of the braking means.

The apparatus can also comprise a self-learning module configured to generate a reference signal in a self-learning process. By such a self-learning module the reference signal can be generated very convenient without the need for any further input from the operator.

In a further embodiment the brake monitoring means is configured to identify a dip or peak in the course of the electrical current. Identifying a dip or peak in

the course of electrical current is a very easy and reliable way to detect a movement or the status of the braking means.

In a further embodiment the brake monitoring means is configured to compare a differentiation of electrical current over time with a predetermined threshold. Such a comparison can be implemented very easily at low cost and provides a reliable detection of the movement and the status of the braking means.

According to an embodiment the apparatus for monitoring the electromagnetic brake according to the invention is attached to an electromagnetic brake comprising braking means for braking a moving member. Thus the opening and closing of such an electromagnetic brake can be monitored very reliably.

In an embodiment the invention comprises an elevator system comprising a drive control system and an electromagnetic brake comprising an apparatus according to the invention, wherein the apparatus for monitoring the electromagnetic brake is coupled to the drive control system. Such an elevator system can be operated very reliably and safely, as the movement and the status of the brake are reported reliably to the drive control system.

A method according to the invention may further comprise detecting a change of at least one electromagnetic system characteristics of the electromagnetic brake in the step of the detecting movement or the status of the breaking means. The change of an electromagnetic system characteristics is a very reli- able measure for detecting the movement or the status of the braking means.

The method according to the invention may further comprise detecting a change of inductance of an electrical brake coil included in the electromagnetic brake for actuating the braking means in order to detect the movement or sta- tus of the braking means. A change of inductance of the electrical brake coil is easy to detect and gives a reliable measure for the movement or the status of the braking means.

The method according to the invention may further comprise comparing a pat- tern of the course of electrical current over time with a predetermined refer-

ence course. Such a pattern comparison gives a very reliable indication of the movement or status of the braking means.

In an embodiment of the invention the method further comprises differentiating the course of electrical current over time and comparing such differentiation with a predetermined threshold. Such a differentiation and comparison can be implemented easily at low cost and gives a reliable indication of the movement or the status of the braking means.

The features, embodiments and advantages as described with respect to the apparatus for monitoring an electromagnetic brake can also be realized in terms of method steps, with a method for fabricating the apparatus according to the invention.

While the invention has been described with reference to exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition many modifications may be made to adopt a particular situation or material to the teachings of the in- vention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed, but that the invention include all embodiments falling within the scope of the dependent claims.