ELEVATOR SAFETY CIRCUIT

Abstract

An elevator safety circuit for an elevator system in which an output is arranged to selectively provide an electrical current from an input to an electromagnetic brake coil via a current flow path. An actuator transistor is arranged in series along the current flow path between the input and the output, the actuator transistor being arranged to selectively allow passage of the electrical current. A controller is arranged to carry out a test operation when the braking element is in the open position. The test operation comprises operating the actuator transistor in its disabled mode for a time period, monitoring the electrical current through the brake coil, and determining whether the magnitude of the electrical current reduces during said time period, the time period being selected such that the magnitude of the electrical current remains sufficient for keeping the braking element in the open position during the test.

Claims

1. An elevator system comprising: a brake including a braking element and an electromagnetic brake coil, said brake being arranged to pass an electrical current from a supply to the brake coil via a current flow path, said brake being arranged to apply a mechanical bias force to operate the braking element in a closed position when a magnitude of the electrical current is less than a threshold value, wherein the electromagnetic coil produces an electromagnetic force that overcomes said bias force to operate the braking element in an open position when the electrical current is equal to or greater than the threshold value; an actuator transistor arranged in series along the current flow path between the supply and the brake coil, said actuator transistor having an enabled mode in which it allows passage of the electrical current, and a disabled mode in which it interrupts the current flow path thereby preventing passage of the electrical current; and a controller arranged to carry out a test operation when the braking element is in the open position, wherein the test operation comprises operating the actuator transistor in its disabled mode for a time period, monitoring the electrical current through the brake coil, and determining whether the magnitude of the electrical current reduces during said time period; wherein the time period is selected such that the magnitude of the electrical current remains greater than the threshold current during said time period.

2. The elevator system of claim 1, further comprising a safety chain including a plurality of safety chain switches arranged in series, wherein a gate terminal of the actuator transistor is connected to an output of the safety chain, said elevator system being arranged such that when one or more of the safety chain switches is open, the actuator transistor is operated in its disabled mode.

3. The elevator system of claim 1, comprising a plurality of actuator transistors provided in series along the current flow path between the supply and the brake coil, wherein the controller is arranged to carry out the test operation for each of the plurality of actuator transistors sequentially.

4. The elevator system of claim 1, wherein the controller is connected to a current monitor arranged to monitor the electrical current through the brake coil.

5. The elevator system of claim 1, wherein the controller is connected to a current monitor arranged to monitor a current at the output of the actuator transistor(s).

6. The elevator system of claim 1, wherein the controller is connected to a current monitor arranged to monitor a current along a return current path.

7. The elevator system of claim 1, wherein the controller is connected to a voltage monitor arranged to monitor a voltage across the brake coil, wherein the controller is arranged to determine the electrical current through the brake coil from said voltage.

8. The elevator system of claim 7, wherein a fixed resistor is connected in parallel across the brake coil, wherein the voltage monitor monitors the voltage across said fixed resistor.

9. The elevator system of claim 1, comprising a varistor connected in parallel across the brake coil.

10. The elevator system of claim 1, arranged such that the supply is connected to the brake coil via first and second conductors, wherein the actuator transistor(s) are connected in series along the first conductor.

11. The elevator system of claim 10, comprising a freewheel diode connected between the first and second conductors, such that its anode is connected to the second conductor, and its cathode is connected to the first conductor.

12. The elevator system of claim 1, wherein the supply comprises a DC power supply such that the electrical current is a direct current, optionally wherein the DC power supply supplies a DC voltage of at least 10 V, optionally at least 20 V, preferably at least 30 V, and more preferably at least 40 V, optionally wherein the DC power supply supplies a DC voltage of 48 V.

13. The elevator system of claim 1, comprising a drive switch connected between the supply and the actuator transistor(s), optionally wherein the drive switch comprises a MOSFET.

14. An elevator safety circuit for an elevator system, the elevator safety circuit comprising: an input arranged to receive an electrical current from a supply; an output arranged to selectively provide the electrical current to an electromagnetic brake coil via a current flow path; an actuator transistor arranged in series along the current flow path between the input and the output, said actuator transistor having an enabled mode in which it allows passage of the electrical current, and a disabled mode in which it interrupts the current flow path thereby preventing passage of the electrical current; and a controller arranged to carry out a test operation when the braking element is in the open position, wherein the test operation comprises operating the actuator transistor in its disabled mode for a time period, monitoring the electrical current through the brake coil, and determining whether the magnitude of the electrical current reduces during said time period; wherein the time period is selected such that the magnitude of the electrical current remains greater than the threshold current during said time period.

15. A method of testing a brake in an elevator system, wherein: the brake includes a braking element and an electromagnetic brake coil, said brake being arranged to pass an electrical current from a supply to the brake coil via a current flow path, said brake being arranged to apply a mechanical bias force to operate the braking element in a closed position when a magnitude of the electrical current is less than a threshold value, wherein the electromagnetic coil produces an electromagnetic force that overcomes said bias force to operate the braking element in an open position when the electrical current is equal to or greater than the threshold value; and an actuator transistor is arranged in series along the current flow path between the supply and the brake coil, said actuator transistor having an enabled mode in which it allows passage of the electrical current, and a disabled mode in which it interrupts the current flow path thereby preventing passage of the electrical current; the method comprising: when the braking element is in the open position, operating the actuator transistor in its disabled mode for a time period; monitoring the electrical current through the brake coil; and determining whether the magnitude of the electrical current reduces during said time period; wherein the time period is selected such that the magnitude of the electrical current remains greater than the threshold current during said time period.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Certain examples of the present disclosure will now be described with reference to the accompanying drawings, in which:

[0039] FIG. 1 is a schematic diagram of an elevator system;

[0040] FIGS. 2A and 2B are schematic diagrams of a drive including a machine brake for use in an elevator system;

[0041] FIG. 3 is a circuit diagram of a prior art safety circuit;

[0042] FIG. 4 is a circuit diagram of a safety circuit in accordance with an example of the present disclosure;

[0043] FIG. 5 is a timing diagram illustrating partial stroke test operation of the safety circuit of FIG. 4; and

[0044] FIGS. 6A-D are circuit diagrams illustrating possible mechanisms for monitoring the current in the brake coil.

DETAILED DESCRIPTION OF THE INVENTION

[0045] FIG. 1 is a schematic diagram of an elevator system 2, in which an elevator car 4 moves within a hoistway 6. It will be appreciated that the elevator system 2 shown in FIG. 1 is simplified for illustrative purposes and a practical elevator system may include many other parts or be constructed in a different configuration.

[0046] A drive 8 is arranged to drive a belt 10 (or cable or some other suitable means, known in the art per se) which drives motion, e.g. vertical motion, of the elevator car 4 within the hoistway 6. Elements of the drive can be seen in more detail in FIGS. 2A and 2B.

[0047] FIGS. 2A and 2B are schematic diagrams of a drive 8 including a machine brake for use in an elevator system, where FIG. 2A shows the brake in its ‘open’ position and FIG. 2B shows the brake in its ‘closed’ position, as outlined in further detail below.

[0048] The drive 8 includes a motor 12 and a braking element 14, in this case a brake pad, that can come into frictional contact with the motor 12 to slow or stop the motor 12. This braking element 14 is biased to the closed position by resilient members, in this case springs 16. These springs apply a mechanical biasing spring force to the braking element 14 to ‘push’ it into contact with the motor 12, as shown in FIG. 2A.

[0049] This spring force can be overcome by an electromagnetic force that is selectively provided by an electromagnet formed by a brake coil 18, i.e. an electromagnet coil. A supply current 20, e.g. a direct current, can be passed through the brake coil 18, which induces a magnetic field surrounding the coil 18 in a manner well known in the art per se. This current 20 can selectively be supplied by opening or closing a ‘drive’ switch 22 to break or make a complete circuit that provides a current flow path through the brake coil 18.

[0050] As can be seen in FIG. 2B, when the drive switch 22 is closed, the supply current 20 flows through the brake coil 18, inducing the magnetic field. Provided the supply current is sufficiently large, the resulting electromagnetic force overcomes the 30 spring force from the springs 16, pulling the braking element 14 away from the motor 12, allowing the motor 12 to rotate freely.

[0051] The ‘normally closed’ behaviour of the brake ensures that the brake operates if power is interrupted, for safety purposes.

[0052] Generally, the elevator system 2 is provided with a ‘safety chain’ 3, which can cause the brake to engage under certain circumstances. For example, if the elevator car 4 is travelling too quickly within the hoistway 6, or if a fault is detected with one of the components of the elevator system 2, the current flow path through the brake coil 18 can be interrupted, thereby causing the brake to close in response. This brings the elevator car 4 to a safe stop and preventing motion of the elevator car 4 until the issue is dealt with.

[0053] FIG. 3 is a circuit diagram of a prior art safety circuit 24. The current supply is provided by a 48 V DC voltage supply, e.g. from the drive, and is shown as the positive supply rail. A ground rail, GND, is also shown. In this arrangement, the drive switch 22 is provided by a MOSFET, connected in series between the voltage supply and the brake coil 18.

[0054] Also connected in series between the supply and the brake coil 18, downstream of the switch 22, are a pair of relays 26, 28. These relays 26, 28 act as the actuators of the safety circuit, the operation of which depends on the safety chain 3 such that if any of the switches of the safety chain 5 are opened in response to a fault, e.g. of the types outlined above, the relays 26, 28 should open (if working correctly). The reason for having two actuator relays 26, 28 is that they provide redundancy for additional safety. If either (or both) relays 26, 28 are opened, current flow through the brake coil 18 is prevented, thereby ‘dropping’ the braking element 14 to close the brake.

[0055] When current through the brake coil 18 is stopped, the induced magnetic field associated with the brake coil 18 ‘collapses’, which causes a spike of current. This current is dissipated using a flyback arrangement constructed from a flyback diode 30 connected in parallel with the brake coil 18, upstream of the relays 26, 28.

[0056] The diode 30 is arranged such that its anode is connected to ground GND, and its cathode is connected to the output of the switch 22.

[0057] A varistor 32 is also connected in parallel with the brake coil 18 and provides overcurrent protection. During normal operation, the varistor 32 does not conduct.

[0058] However, if there is a large spike in current, the varistor 32 begins to conduct, and dissipates the excess energy.

[0059] In order to check that the safety circuit is able to cause the brake to close, it is generally important to regularly check operation of the actuators, i.e. the relays 26, 28. This is done when the elevator car 4 is at a standstill, i.e. when it is not moving.

[0060] Due to the way relays are constructed, it is possible to use the ‘back contacts’ of the relays 26, 28 to check that they are operating as intended.

[0061] FIG. 4 is a circuit diagram of a safety circuit in accordance with an example of the present disclosure, where elements having like reference numerals correspond in form and function to those described above as appropriate.

[0062] Unlike the prior art system of FIG. 3, the actuators are implemented using a pair of transistors 34, 36, which in this example are a pair of MOSFETs, where the gate terminals of these actuator transistors 34, 36 are coupled to the output of the safety chain 3 via an optocoupler 5 (though some other type of suitable coupler could be used instead as appropriate). However, the operation of these actuator transistors 34, 36 cannot be tested when the elevator car 4 is stopped, i.e. when no current is flowing through the actuator transistors 34, 36 due to the switch 22 being open. It will be appreciated that more than two transistors could be used, however two is generally accepted to be sufficient for redundancy requirements.

[0063] The actuator transistors 34, 36 used in this example conduct when the voltages V.sub.1, V.sub.2 at their respective gate terminals (as determined by the safety chain 3 and controller 38, as outlined below) is high, and do not conduct when the respective voltage V.sub.1, V.sub.2 is low. It will, of course, be appreciated that transistors having the opposite behaviour (i.e. active low) could be used with suitable modifications to the circuit.

[0064] The system in FIG. 4 is advantageously arranged such that operation of the actuator transistors 34, 36 can be tested when the elevator car 4 is in motion, without needing to interrupt motion by fully closing the brake. This is achieved using a ‘partial stroke’ test, as outlined in further detail below.

[0065] The partial stroke test is conducted by a controller 38, which provides control voltages V.sub.1, V.sub.2 to the respective gate terminals of the actuator transistors 34, 36. The controller 38 also monitors (either directly or indirectly) the current I.sub.brake in the brake coil 18. There are several different methods for monitoring this current, some of which are described in further detail below. The controller 38 receives status information from the safety chain 3, e.g. via a controller area network (CAN) bus 39, where the controller 38 uses this status information when determining when to perform the test operation outlined below.

[0066] FIG. 5 is a timing diagram illustrating the partial stroke test operation of the safety circuit of FIG. 4. Initially, the voltages V.sub.1, V.sub.2 applied to the respective gate terminals of the actuator transistors 34, 36 are low, resulting in the actuator transistors 34, 36 being off. As the actuator transistors 34, 36 are off, no current I.sub.brake flows through the brake coil 18, and thus the brake remains closed. Throughout this test, all of the switches of the safety chain 3 are closed, i.e. there are not currently any fault conditions and the elevator otherwise operates normally.

[0067] At an initial time t.sub.1, the voltages V.sub.1, V.sub.2 applied to the respective gate terminals of the actuator transistors 34, 36 are set high, allowing current I.sub.brake to flow through the brake coil 18. The brake current I.sub.brake starts to ramp up.

[0068] At t.sub.2, the brake current I.sub.brake is sufficiently large that it exceeds the current threshold I.sub.threshold required in order to overcome the spring force and thereby open the brake. The brake current I.sub.brake continues to increase for a short time until it reaches its maximum, steady state value.

[0069] During normal operation, i.e. while the elevator car 4 is in motion, a partial stroke test 40 can be carried out. The test 40 is carried out for each of the actuator transistors 34, 36 separately.

[0070] Firstly, at time t.sub.3, the test of the first actuator transistor 34 is started. For the test, the voltage V.sub.1 applied to the gate terminal of the actuator transistor 34 is set low for a very brief period, until t.sub.4. The time between t.sub.3 and t.sub.4 is chosen such that, assuming proper operation of the actuator transistor 34, the brake current I.sub.brake will drop, but not below the threshold I.sub.threshold. This will generally depend on the components and dynamics of the system, but the period may be approximately 50 ms.

[0071] As can be seen in FIG. 5, the brake current I.sub.brake drops between t.sub.3 and t.sub.4, where this drop in current is detected by the current monitor function of the controller 38. This test is deemed a success as it shows that the controller 38 can cause the brake to actuate, using the first actuator transistor 34, if it needs to, i.e. in response to one or more switches within the safety chain 3 opening.

[0072] At t.sub.4, the voltage V.sub.1 applied to the gate terminal of the actuator transistor 34 is set back to high. As the threshold current was not crossed, the brake remains open throughout the test, and thus motion of the elevator car 4 is not interrupted by carrying out the test.

[0073] Subsequently, the other actuator transistor 36 is tested in the same way, with the voltage V2 being ‘pulsed low’ between t.sub.5 and t.sub.6 (where the period between these may, again, be approximately 50 ms). As can be seen in FIG. 5, the brake current I.sub.brake drops between t.sub.5 and t.sub.6, where this drop in current is detected by the controller 38 as before. This test is also deemed a success as it shows that the controller can cause the brake to actuate, using the second actuator transistor 36, if it needs to.

[0074] FIGS. 6A-D are circuit diagrams illustrating possible mechanisms for monitoring the current in the brake coil 18, where like reference numerals indicate like components to those described previously. For ease of illustration, the safety chain 3 and optocoupler 5 are omitted from FIGS. 6A-D for ease of illustration, however these would be included for normal operation, using the same structure and operation as outlined previously.

[0075] FIG. 6A shows an arrangement in which the current is monitored by a current monitor 42 connected in series along the positive supply rail, downstream of the actuator transistors 34, 36.

[0076] FIG. 6B shows an arrangement in which a current monitor 44 is connected in series along the ground rail, downstream of the brake coil 18 and varistor 32.

[0077] FIG. 6C shows an arrangement in which a fixed resistor 46 is connected in series with the varistor 32, and a voltage drop across the fixed resistor 46 is monitored by a voltage monitor 48 connected across the resistor 46.

[0078] FIG. 6D shows an arrangement in which a fixed resistor 50 is connected in series along the ground rail, downstream of the brake coil 18 and varistor 32. A voltage across the fixed resistor 50 is monitored by a voltage monitor 52 connected across the resistor 50.

[0079] One or more of the arrangements shown in FIGS. 6A-D can be used to provide information to the controller 38 regarding the current I.sub.brake flowing through the brake coil 18. As outlined above, the controller 38 then uses this measure of the current I.sub.brake to determine whether the actuator transistor 34, 36 under test is able to cause the current I.sub.brake to drop, i.e. to cause the brake to close and thereby stop motion of the elevator car 4.

[0080] Thus, it will be appreciated by those skilled in the art that examples of the present disclosure provide an improved elevator system in which the elevator safety circuit utilises transistors, the proper operation of which is determined by a partial stroke test. This advantageously allows the use of transistors as the actuators that are coupled to and controlled by the safety chain in the safety circuit. This avoids the noise associated with relays while not requiring normal operation of the elevator system to be interrupted in order to test the safety circuit.

[0081] While specific examples of the disclosure have been described in detail, it will be appreciated by those skilled in the art that the examples described in detail are not limiting on the scope of the disclosure.