DETECTOR FOR BRAKE

Abstract

An electronic safety actuator for an elevator safety brake, includes: a solenoid; a permanent magnet, movable by the solenoid between a first position proximate to the solenoid and a second position distal from the solenoid; and a measurement circuit arranged to measure the inductance of the solenoid and thereby detect the position of the permanent magnet. The inductance of the solenoid is dependent on the magnetic field in which it is situated, which is in turn affected by the permanent magnet. Thus, as the proximity of the permanent magnet to the solenoid changes, the inductance of the solenoid changes (increases or decreases). The change in inductance that occurs in the solenoid of an elevator safety actuator is significant enough to be measurable electronically and this measurement can be used to determine the position of the permanent magnet and thus of the safety actuator.

Claims

1. An electronic safety actuator for an elevator safety brake, comprising: a solenoid; a permanent magnet, movable by the solenoid between a first position proximate to the solenoid and a second position distal from the solenoid; and a measurement circuit arranged to measure the inductance of the solenoid and thereby detect the position of the permanent magnet.

2. An electronic safety actuator as claimed in claim 1, wherein the measurement circuit is arranged to compare the measured inductance with a predefined threshold value selected such that the inductance of the solenoid is below the predefined threshold value when the permanent magnet is in the first position and the inductance of the solenoid is above the predefined threshold value when the permanent magnet is in the second position.

3. An electronic safety actuator as claimed in claim 2, wherein the predefined threshold value is determined through calibration and is set prior to installation of the electronic safety actuator.

4. An electronic safety actuator as claimed in claim 1, wherein the measurement circuit is arranged to measure the inductance of the solenoid by changing a current through the solenoid and integrating the voltage across the solenoid to produce a voltage integral measurement.

5. An electronic safety actuator as claimed in claim 4, wherein the measurement circuit samples a peak value of the voltage integral measurement at a first time after the applied current change.

6. An electronic safety actuator as claimed in claim 5, wherein the first time is selected such that the voltage integral measurement has substantially reached its peak.

7. An electronic safety actuator as claimed in claim 5, wherein the measurement circuit samples the voltage across the solenoid at a second time, later the first time.

8. An electronic safety actuator as claimed in claim 7, wherein the second time is selected such that the voltage across the solenoid has substantially no component due to the inductance of the solenoid.

9. An electronic safety actuator as claimed in claim 7, wherein the measurement circuit is arranged to compare the voltage measurement at the second time with a second predefined threshold and to output an error signal based on the comparison.

10. An electronic safety actuator as claimed in claim 7, wherein the measurement circuit is arranged to measure the voltage integral using an analogue integrator whose output will decay over time and wherein the measurement circuit is arranged to measure the voltage across the solenoid at the second time by measuring the analogue integrator output after it has substantially decayed to a constant.

11. An electronic safety actuator as claimed in claim 4, wherein the current change in the solenoid that is used for measurement is in the same direction as a current change that would cause the actuator to move the permanent magnet from the first position to the second position.

12. An electronic safety actuator as claimed in claim 11, wherein the magnitude of the current change used for measurement is not large enough to move the permanent magnet from the first position to the second position.

13. An electronic safety actuator as claimed in claim 1, wherein the measurement circuit comprises a measurement switch and a resistor in parallel with a trip switch of the actuator.

14. An electronic safety actuator as claimed in claim 13, wherein the trip switch is a thyristor.

15. A method of detecting the position of an electronic safety actuator of an elevator safety brake, the electronic safety actuator comprising a solenoid and a permanent magnet movable by the solenoid between a first position proximate to the solenoid and a second position distal from the solenoid; the method comprising: measuring the inductance of the solenoid.

Description

DRAWING DESCRIPTION

[0028] Certain examples of the present disclosure will now be described, by way of example only, with reference to the accompanying drawings in which:

[0029] FIG. 1a schematically shows a safety actuator with its permanent magnet in a first position;

[0030] FIG. 1b schematically shows a safety actuator with its permanent magnet in a second position;

[0031] FIG. 2 shows a circuit diagram of a safety actuator solenoid with a measurement circuit;

[0032] FIG. 3 shows certain parts of a measurement circuit;

[0033] FIG. 4 is a graph of the voltage across a solenoid in response to a current change, for various inductances;

[0034] FIG. 5 shows graphs of the integrator output and the peak value rectification output on a short timescale; and

[0035] FIG. 6 shows graphs corresponding to the graphs of FIG. 5 but on a longer timescale.

DETAILED DESCRIPTION

[0036] FIG. 1a shows an electronic safety actuator 1 for an elevator car. The safety actuator 1 has a solenoid 2 to which a permanent magnet 3 is selectively attached. In this figure, the permanent magnet 3 is in a first position, proximate to the solenoid 2. In this example the permanent magnet 3 is magnetically attached to the solenoid 2 by virtue of its own magnetic field. The solenoid 2 is not supplied with any electrical current during normal use. In this configuration the permanent magnet 3 is distanced from the guiderail 4 and is not in contact therewith. A mechanical lever 5 attached to the permanent magnet 3 connects to an elevator safety brake (not shown) and when driven parallel to the guide rail 4 causes the safety brake to engage with the guide rail 4 (e.g. via a wedge or roller brake mechanism) so as to bring the elevator car to a stop. In some examples the permanent magnet 3 could be the actual safety brake.

[0037] FIG. 1b shows the same equipment as in FIG. 1b, but with the permanent magnet 3 in a second position, distal from the solenoid 2. In this position the permanent magnet 3 is magnetically adhered to the guide rail 4. In this position, friction between the guide rail 4 and the permanent magnet 3 causes the lever 5 to be driven parallel to the guide rail 4 so as to engage the safety brake and stop the elevator car.

[0038] The permanent magnet 3 is moved from the first position of Figure la into the second position of FIG. 1b by a current being applied to the solenoid 2 so as to create a magnetic field strong enough to repel the permanent magnet 3 away from the solenoid 2 and into magnetic engagement with the guide rail 4. As discussed elsewhere, in other examples the current may be removed from the solenoid to remove or reduce an attractive force holding the permanent magnet 3 in place.

[0039] In use, an elevator car would typically have two safety brakes and two electronic actuators, each electronic actuator being as shown in Figures la and lb. In other examples there may be only one safety brake, or more than two safety brakes (and corresponding numbers of electronic actuators). A control unit (not shown) is capable of actuating both safety brakes. When an event occurs that requires engagement of the safety brakes, the control unit operates switches that cause the solenoid 2 to trip the permanent magnet 3 into the rail-engaged position of FIG. 1b, thereby lifting the lever 5 that connects to the wedges or rollers of the corresponding safety brake.

[0040] In prior art configurations, a mechanical switch is used to detect the movement of the permanent magnet 3 from the first position to the second position (i.e. the movement of the magnet 3 and/or lever 5 causes a physical engagement or disengagement of the mechanical switch), but in the example of FIGS. 1a and 1b no such mechanical switch is needed. Instead, in the example of FIGS. 1a and 1b, the inductance of the solenoid 2 is sensed (as described in more detail below). In this example, as indicated in FIG. 1a, when the permanent magnet 3 is in the first position, proximate to the solenoid 2, the inductance of the solenoid 2 has a lower value (in this case 20 mH), while when the permanent magnet 3 is in the second position, distal from the solenoid 2, the inductance of the solenoid 2 has a higher value (in this case 30 mH). It will be appreciated that these values are provided merely by way of example and are not limiting on the invention as they depend on the particular size, structure and arrangement of the solenoid 2 and the permanent magnet 3.

[0041] When the movement of the permanent magnet 3 and lever 5 from the first position to the second position is detected, the system can take various actions as deemed appropriate. For example the elevator safety chain can be opened leading to an interruption of the power to the drive machine. If only one of two actuators is sensed to have moved from the first position to the second position then the control unit can deploy the other actuator so as to prevent an imbalance in braking by ensuring that both safety brakes are engaged.

[0042] In order to reset the safety actuator, the elevator car can be lifted up to release the safety brakes and then the solenoid 2 can be energized in the opposite direction (with a current in the direction opposite to the actuation direction) so as to create a magnetic field strong enough to attract the permanent magnet 3 away from the guide rail 4 and back to the solenoid 2, i.e. from the second position to the first position. As discussed elsewhere, in other examples the solenoid 2 may be de-energized or re-energized as appropriate for the design.

[0043] While the description above has been given in relation to an elevator car, it will be appreciated that it is equally applicable to safety brakes and actuators for a counterweight.

[0044] FIG. 2 shows a control circuit and measurement circuit for the solenoid 2. The solenoid 2 is located in the middle arm of an H-bridge which is formed from four switches S.sub.1-S.sub.4. The switches S.sub.1-S.sub.4 may optionally be thyristors. The H-bridge arrangement allows the solenoid 2 to be energized in either direction. By closing switches S.sub.1 and S.sub.4 the solenoid 2 is energized in a “trip” direction that generates a magnetic field that repels the permanent magnet 3 so as to move it from the first position to the second position so as to engage the safety brakes. On the other hand, by closing switches S.sub.2 and S.sub.3 the solenoid 2 can be energized in the opposite direction so as to generate a magnetic field that attracts the permanent magnet 3 to draw it away from the guide rail and from the second position back to the first position so as to reset the safety actuator.

[0045] The power supply for the circuit of FIG. 2 may be provided from a main power supply together with a large capacitor which can provide backup power in the event that the main power supply fails. In FIG. 2 the power supply is indicated as 300 V, but again it will be appreciated that this is by way of example only and any desired voltage may be used.

[0046] A measurement circuit 6 comprises a measurement circuit branch in parallel with switch (or thyristor) S.sub.4 which is part of the “trip” circuit for energizing the solenoid 2 so as to move the permanent magnet 3 from the first position to the second position. The measurement circuit branch comprises a transistor T.sub.M (in this example a MOSFET) in series with a relatively large resistor R.sub.M (in this example a 200 kΩ resistor, although again this is by way of example only).

[0047] The measurement circuit 6 also comprises a detecting circuit 7 to detect and measure the voltage V.sub.L across the solenoid 2. The detecting circuit 7 is shown schematically in FIG. 3 and is connected across the solenoid 2 as shown in FIG. 2 at the points A and B. The detecting circuit 7 includes a voltage integrator 8, a peak value rectifier 9 and an analogue to digital converter (ADC) 10. The voltage integrator 8 takes the voltage difference between points A and B and integrates it with time to produce an integrated voltage output V.sub.Int. The peak value rectifier circuit 9 captures and stores the peak value of the output V.sub.Int from the integrator 8 and holds it for sampling by the ADC 10.

[0048] The voltage integrator circuit 8 and the peak value rectifier circuit 9 may be any suitable circuits. However, in some examples the voltage integrator circuit 8 is a simple RC integrator where a capacitor is charged through a series connected resistor and the voltage across the capacitor represents the time integral of the input voltage. In some examples the peak value rectifier circuit 9 may be a simple holding capacitor that is charged by the integrator output and holds the output value long enough for the value to be used (e.g. sampled by ADC 10).

[0049] In operation, a measurement current pulse is applied to the solenoid 2 which causes the solenoid to generate a back voltage opposing the change of current. This back voltage is integrated by the integrator circuit 8 causing the output voltage V.sub.Int to rise until it reaches a peak value once the inductor's back voltage drops substantially to zero. This peak value represents the time integral of the voltage across the solenoid 2 caused by the applied change in current and is representative of the inductance of the solenoid 2.

[0050] After the back voltage of solenoid 2 has decayed down to zero, the integral will correspondingly have reached a peak value and will theoretically be stable (substantially unchanging in time). This output value may be sampled directly by the ADC 10 to give a measurement of the inductance of inductor L (i.e. without the use of the intermediate peak value rectification circuit 9). Therefore by timing the sampling of the ADC 10 so as to read the voltage of integrator circuit 8 after the back voltage has substantially reduced to zero, the ADC 10 can capture the peak of the integral which is representative of the inductance of the solenoid 2.

[0051] However, In the case of a simple integrator circuit, such as an RC integrator, the peak voltage on the capacitor of the integrator will start to fade relatively quickly once the peak has been reached. To make the measurement timing more robust and reliable, the peak value rectifier circuit 9 will hold that peak value for longer until the ADC 10 has sampled the peak value. The peak value rectifier circuit 9 can still be a relatively simple circuit, but can be designed with a decay constant that allows a longer time window in which to use the peak integral voltage, e.g. by acquiring a sample via ADC 10.

[0052] Now, by comparing the peak integral sample taken by the ADC 10 with a calibrated threshold value stored in a memory, the measurement circuit 6 can determine whether the inductance of the solenoid 2 is above the threshold value or below the threshold value. By setting the threshold value to lie between the inductance of the solenoid 2 with the permanent magnet 3 in the first position and the inductance of the solenoid 2 with the permanent magnet 3 in the second position, the measurement circuit 6 can readily distinguish the two inductances and thereby determine whether the permanent magnet 3 is in the first position or the second position.

[0053] In other examples, it will be appreciated that the value of the peak value rectifier 9 can be used via analogue comparisons rather than sampling via ADC 10.

[0054] After the back voltage of the solenoid 2 has decayed to zero, the peak integral value V.sub.Int of a simple RC integrator will gradually decay until eventually it reaches a minimum value that is representative of the steady state voltage across the solenoid 2, which in turn is now dependent only on the resistance of the solenoid 2 and the measurement current applied. It may be noted that in the case of a simple peak value rectifier circuit 9, this is also the case as, while the time constant of that circuit is longer, it will still decay, just over a longer time. As this is also a stable (non time-varying) value, it is also useful for diagnostic purposes as it indicates the presence of a correct measurement current through the inductor 2 and the measurement circuit 6. This voltage can also be captured by ADC 10 (either directly from the voltage integrator 8 or via the peak value rectifier 9) by sampling at a second time that is suitably chosen based on the expected discharge time of the relevant circuit (e.g. the time constant of the capacitor of the relevant circuit). This second sample can also be compared against one or more thresholds to ensure that it lies within an expected range. Any deviation from the expected range is indicative of a failure of the measurement circuit 6 which can in turn be considered a fault in the electronic safety actuator. Appropriate action can then be taken such as indicating a need for repair or replacement and/or taking the elevator car out of service.

[0055] FIGS. 4, 5 and 6 show graphs that are illustrative of the voltages in the above process in one example. Each graph shows the appropriate voltage curve for a number of inductances ranging from 20 mH through to 50 mH so as to show how the curves and values change with inductance. These are exemplary only, and any values may be used.

[0056] FIG. 4 shows the back voltage across the solenoid upon commencement of the measurement current. As can be seen, this is an exponentially decreasing voltage according to the formula:

V.sub.L=V.sub.0e.sup.−Rt/L

[0057] Where V.sub.0 is the initial voltage, R is the resistance of the measurement resistor R.sub.M, and L is the inductance of the solenoid 2. The voltage starts around 300 Volts in this example and decays exponentially down to substantially zero in a time period of from around 60 to 140 microseconds, depending on the inductance.

[0058] FIG. 5 shows two graphs on a timescale of 0 to 1 millisecond after the measurement current pulse. The lower graph shows the output V.sub.Int of the voltage integrator circuit 8, again for each of a number of different inductances. As can be seen, the peak integral value is reached after around 0.5 to 1.5 milliseconds, depending on the inductance, and then starts to decay (the integrator is an RC integrator in this example). The upper graph of FIG. 5 shows the output of the peak value rectifier circuit 9 corresponding to the example of the lower graph. From this it can be seen that the peak of V.sub.Int is held stable for a much longer period of time, providing a stable measurement window.

[0059] FIG. 6 shows the same graphs as those of FIG. 5, but on a longer timescale of 0 to 300 milliseconds so that the different decays can be seen (the peak value rectification circuit here is again a relatively simple, capacitor-based circuit). As shown in the lower graph, the output V.sub.Int of the integrator circuit 8 decays quickly (on this timescale) down to a constant value (but a non-zero value). As shown in the graph, the residual value is reached after no more than about 10 milliseconds. This residual value is non-zero because there is still a voltage drop across the inductor due to its resistance. In this example the measurement pulse ends at 150 milliseconds, at which point the integrator output voltage V.sub.Int quickly returns to zero. This residual voltage can be used to indicate correct functioning of the measurement circuit and to provide an alert in the case of malfunction (e.g. by comparing the voltage to one or more thresholds). For example a zero voltage or a very high voltage here could indicate a fault.

[0060] The upper graph of FIG. 6 is again the output of the peak value rectifier circuit 9. While this is shown to be substantially constant in the upper graph of FIG. 5, it can be seen from FIG. 6 that this stored peak value also decays over time as the capacitor of the peak value rectifier circuit discharges. However, that discharge takes place over a much longer period, taking around 20 to 90 milliseconds to reach the residual value, depending on the inductance. At that point the peak value rectifier circuit also remains stable at that residual value until the end of the measurement pulse at 150 milliseconds. The residual value can therefore be measured from the output of the peak value rectifier 9 at any time in this window (from 20/90 milliseconds up to 150 milliseconds). After the measurement pulse has ended, the peak value rectifier output also decays back to zero by around 240 milliseconds.

[0061] It will be appreciated that the timescales in these graphs are by way of example only and will vary depending on the particular circuit design. However, they clearly demonstrate the timings in which measurements can be made.

[0062] By way of example, in the example shown in FIGS. 5 and 6, if sampling directly from the integrator circuit 8, a sample could be taken from around 0.1 milliseconds, giving a good estimation of the peak integral value (as shown in the lower graph of FIG. 5). However, that measurement would ideally need to be taken within a relatively short time window (e.g. by around 0.5-1 milliseconds) due to the relatively fast decay of the voltage. If sampling from the peak value rectifier circuit 9 instead, then the peak value could again be sampled from around 0.1 milliseconds (as shown in the upper graph of FIG. 5), but could reliably be taken within a period of up to 10-20 milliseconds (and maybe up to 50 milliseconds or more, depending on the inductances involved) while still distinguishing different inductances. Thus the timing of the sampling is easier with the peak value rectifier circuit 9. Likewise, to sample the residual value for evaluating the measurement circuit 7, the output of the integrator circuit could be sampled any time in the window from about 10 milliseconds to 150 milliseconds (as shown in the lower graph of FIG. 6), or, if sampling the output of the peak value rectifier circuit 9, the residual value could be sampled any time in the window from about 90 milliseconds to 150 milliseconds.

[0063] While the present disclosure has been described with reference to certain 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 present disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present disclosure without departing from the essential scope thereof. Therefore, it is intended that the present disclosure not be limited to the particular embodiments disclosed, but that the present disclosure will include all embodiments falling within the scope of the claims.