Abstract:
The present invention provides a method and apparatus for thermally protecting an electromagnetic actuator used to suppress vibrations in an elevator installation. The apparatus includes a temperature evaluation unit that determines an actual temperature of the actuator on the basis of a signal proportional to a current supplied to the actuator. A limiter restricts the current supplied to the actuator if the actual temperature of the actuator as determined by the temperature evaluation unit is greater than a predetermined temperature.

Description:
The present invention relates to a method and apparatus for preventing overheating of an electromagnetic actuator. 
   BACKGROUND OF THE INVENTION 
   U.S. Pat. No. 5,896,949 describes an elevator installation in which the ride quality is actively controlled using a plurality of electromagnetic linear actuators. Such a system in commonly referred to as an active ride control system. As an elevator car travels along guide rails provided in a hoistway, sensors mounted on the car measure the vibrations occurring transverse to the direction of travel. Signals from the sensors are input to a controller which computes the activation current required for each linear actuator to suppress the sensed vibrations. These activation currents are supplied to the linear actuators which actively dampen the vibrations and thereby the ride quality for passengers traveling within the car is enhanced. 
   In the case where a large asymmetric load is applied to the car or where the car is poorly balanced, it would be necessary for one or more of the linear actuators to be powered continuously to overcome the imbalance. This continual energization would cause the actuator to heat up and, if left unchecked, could potentially lead to the thermal destruction of the actuator itself. It will be appreciated that the foregoing is only an example and that there are other cases where conditions imposed on the elevator car can similarly lead to overheating. 
   A conventional solution to this problem is to incorporate a bimetallic strip into the actuator to control its energization. Accordingly, when the temperature of the actuator rises to the predetermined activation temperature of the bimetallic strip, the bimetallic strip within the actuator would break the energization circuit and the respective actuator would be de-energized until its temperature falls to below the predetermined activation temperature of the bimetallic strip. It will be appreciated that at this switch-off point there would be an instantaneous deterioration in the performance of the active ride control, system since a force would no longer be generated by the effected actuator to stabilize the elevator car. Furthermore, this deterioration in performance would be immediately perceptible to any passengers traveling in the elevator car and would therefore defeat the purpose of, and undermine user confidence in, the active ride control system. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The objective of the present invention is to overcome the problems associated with the prior art electromagnetic actuators by providing an improved apparatus and method for protecting electromagnetic actuator from thermal overload while minimizing the effects of such protective measures upon ride quality. 
   In particular the present invention provides a thermal protection device for an electromagnetic actuator, comprising a temperature evaluation unit that determines an estimated temperature of the actuator from a signal proportional to a current supplied to the actuator, and a limiter that restricts the current supplied to the actuator if the actual temperature of the actuator exceeds a first predetermined temperature. Hence, the actuator is protected from thermal deterioration and destruction. Furthermore, the temperature evaluation unit can be located remote from the actuator in any circuit controlling the current delivered to the actuator. 
   Preferably, the current supplied to the actuator is restricted to a minimal level if the actual temperature of the actuator exceeds a second predetermined temperature. The minimal level can be determined such that energy dissipated in the actuator due to the current is equal to or less than heat lost from the actuator due to conduction and convection. Accordingly, the actuator can be continuously energized, albeit with a limited driving current. 
   The invention is particularly advantageous when applied to actuators used in elevator systems to dampen the vibration of an elevator car as it travels along guide rails in a hoistway. The current to the actuators is gradually limited as the temperature exceeds the first predetermined temperature, as opposed to being switched off completely. Hence, and deterioration in the ride quality is less perceptible to passengers. Furthermore, the thermal protection device and method can be easily incorporated in a controller for the actuators without any additional hardware components. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     By way of example only, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, in which: 
       FIG. 1  is a schematic representation of an elevator car traveling along guide rails, the car incorporating linear actuators to suppress vibration of the car; 
       FIG. 2  is a perspective elevation view illustrating the arrangement of the middle roller and lever together with the associated actuator of one of the guide assemblies of  FIG. 1 ; 
       FIG. 3  is a perspective view of one of the actuators; 
       FIG. 4  is an empirical model of the actuators; 
       FIG. 5  is a graph of the results obtained using the model of  FIG. 4 ; 
       FIG. 6  is a signal flow diagram of the active ride control system for the elevator installation of  FIG. 1  incorporating thermal protection according to a first embodiment of the invention; and 
       FIG. 7  is a signal flow diagram of the active ride control system for the elevator installation of  FIG. 1  incorporating thermal protection according to a second embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a schematic illustration of an elevator installation incorporating an active ride control system according to EP-B-0731051 which further includes a thermal protection unit in accordance with the present invention. An elevator car  1  is guided by roller guide assemblies  5  along rails  15  mounted in a shaft (not shown). Car  1  is carried elastically in a car frame  3  for passive oscillation damping. The passive oscillation damping is performed by several rubber springs  4 , which are designed to be relatively stiff in order to isolate sound or vibrations having a frequency higher the 50 Hz. 
   The roller guide assemblies  5  are laterally mounted above and below car frame  3 . Each assembly  5  includes a mounting bracket and three rollers  6  carried on levers  7  which are pivotally connected to the bracket. Two of the rollers  6  are arranged laterally to engage opposing sides of the guide rail  15 . The levers  7  carrying these two lateral rollers  6  are interconnected by a linkage  9  to ensure synchronous movement. The remaining, middle roller  6  is arranged to engage with a distal end of the guide rail  15 . Each of the levers  7  is biased by a contact pressure spring  8  towards the guide rail  15 . This spring biasing of the levers  7 , and thereby the respective rollers  6 , is a conventional method of passively dampening vibrations. 
   Each roller guide assembly  5  further includes two actuators  10  disposed to actively move the middle lever  7  in the y direction and the two interconnected, lateral levers  7  in the x direction, respectively. 
   Unevenness in rails  15 , lateral components of traction forces originated from the traction cables, positional changes of the load during travel and aerodynamic forces cause oscillations of car frame  3  and car  1 , and thus impair travel comfort. Such oscillations of the car  1  are to be reduced. Two position sensors  11  per roller guide assembly  5  continually monitor the position of the middle lever  7  and the position of the interconnected lateral levers  7 , respectively. Furthermore, accelerometers  12  measure transverse oscillations or accelerations acting on car frame  3 . 
   The signals derived from the positions sensors  11  and accelerometers  12  are fed into a controller and power unit  14  mounted on the car  1 . The controller and power unit  14  processes these signals to produce a current I to operate the actuators  10  in directions such to oppose the sensed oscillations. Thereby, damping of the oscillations acting on frame  3  and car  1  is achieved. Oscillations are reduced to the extent that they are imperceptible to the elevator passenger. 
   Although  FIG. 2  provides a further illustration of the arrangement of the middle roller  6  and lever  7  together with the associated actuator  10 , it will be understood that the following description also applies to the two lateral rollers  6  and interconnected levers  7 . Due to the parallel arrangement of the contact pressure spring  8  and the actuator  10  to the lever  7 , the roller guide assembly  5  remains capable of operating even after a partial or complete failure of the active ride control system because the contact pressure spring  8  urges roller  6  against the guide rail  15  independently of the actuator  10 . Hence, even when no current I is supplied to the actuator  10 , the car frame  3  is passively damped by the contact pressure springs  8 . 
   As shown in  FIG. 3 , the actuator  10  is based on the principle of a moving magnet and comprises a laminated stator  17 , windings  16  and a moving actuator part  18  comprising a permanent magnet  19 . The moving actuator part  18  in connected to the top of the lever  7  so that, as the current I supplied to the windings  16  changes, the magnetic flux changes thus causing the moving actuator part  18 , lever  7  and coupled roller  6  to move towards or away from the guide rail  15 . The actuator  10  has the advantage of simple controllability, low weight and small moving masses, and great dynamic and static force (e.g. 800N) for relatively low energy consumption. 
   The objective of the present invention is to ensure maximum availability of the active ride control system but at the same time preventing thermal destruction of the actuators  10 , particularly when a large asymmetric load is applied to the car  1  or where the car  1  is poorly balanced. In such circumstances it would be necessary for one or more of the actuators  10  to be powered continuously to overcome the imbalance. This continual energization would cause the actuator  10  to heat up and, if left unchecked, could potentially lead to the thermal destruction of the actuator  10  itself. The first step in achieving the objective is to assess the thermal characteristics of the actuators  10 . From first principles, the power dissipated as heat by the electrical circuit (i.e. the windings  16 ) produces an increase in the temperature of the actuator  10 . This can be expressed generally as:
 
Power dissipated→Temperature increase in actuator−(effects of heat conduction &amp; convention)  EQN. 1
 
   This expression gives rise to EQN. 2: 
                     I   2     ⁢   R     =         cM   ⁡     (       T   n     -     T     n   -   1         )         Δ   ⁢           ⁢   t       -       (       T   n     -     T   amb       )     ⁢     (       λ   ⁢           ⁢     A   1       +       h   c     ⁢     A   2         )                 EQN   .           ⁢   2               
where:
         I=average (or RMS) current delivered to actuator during sample period Δt;   R=electrical resistance of coils;   c=specific heat capacity;   M=mass;   T n =actual temperature after sample period Δt;   T n-1 =previous temperature at the start of sample period Δt;   T amb =ambient temperature;   λ=thermal conductivity;   A 1 =conductive surface area;   h c =convective heat transfer coefficient;   A 2 =convective surface area;       
   This equation can be solved for T n  as follows: 
   
     
       
         
           
             
               
                 
                   T 
                   n 
                 
                 = 
                 
                   
                     
                       
                         I 
                         2 
                       
                       ⁢ 
                       R 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       t 
                     
                     + 
                     
                       cMT 
                       
                         n 
                         - 
                         1 
                       
                     
                     - 
                     
                       
                         T 
                         amb 
                       
                       ⁢ 
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         t 
                         ⁡ 
                         
                           ( 
                           
                             
                               λ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 A 
                                 1 
                               
                             
                             - 
                             
                               
                                 h 
                                 c 
                               
                               ⁢ 
                               
                                 A 
                                 2 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                   
                     cM 
                     - 
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         t 
                         ⁡ 
                         
                           ( 
                           
                             
                               λ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               
                                 A 
                                 1 
                               
                             
                             + 
                             
                               
                                 h 
                                 c 
                               
                               ⁢ 
                               
                                 A 
                                 2 
                               
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
             
             
               
                 EQN 
                 . 
                 
                     
                 
                 ⁢ 
                 3 
               
             
           
         
       
     
   
   For a specific type of actuator  10 , the values for c, M, λ, A 1 , h c  and A 2  can easily be determined from experimentation in a climate test chamber. Furthermore, the resistance R of the windings  16  can be set to an average constant value, or for more accurate results the true temperature dependent function for the resistance R can be evaluated and used. 
   In practice, the thermal characteristics of the actuator  10  were modeled using the transfer function shown in  FIG. 4 , which yielded the temperature characteristics shown in  FIG. 5 . In  FIG. 4  transfer function PT 2   s  determines the temperature change (Δt) due to power dissipation of the actuator solenoid windings, while function PT ic  is the corresponding transfer function for the actuator core. The model assumes that energy for solenoid heating does not heat the core. 
     FIG. 6  shows a signal flow scheme of the active ride control system for the elevator installation of  FIG. 1  incorporating thermal protection according to the invention. External disturbances act on the car  1  and frame  3  as they travel along the guide rails  15 . These external disturbances generally comprise high frequency vibrations due mainly to the unevenness of the guide rails  15  and relatively low frequency forces  27  produced by asymmetrical loading of the car  1 , lateral forces from the traction cable and air disturbance or wind forces. The disturbances are sensed by the positions sensors  11  and accelerometers  12  which produce signals that are fed into the controller and power unit  14 . 
   In the controller and power unit  14 , the sensed acceleration signal is inverted at summation point  21  and fed into an acceleration controller  23  as an acceleration error signal e a . The acceleration controller  23  determines the current I a  required by the actuator  10  in order to counteract the vibrations causing the sensed acceleration. Similarly, the sensed position signal is compared with a reference value P ref  at summation point  20  to produce a position error signal e p . The position error signal e p  is then fed into a position controller  22  which determines the current I p  required by the actuator  10  in order to counteract the disturbances causing the sensed position signal to deviate from the reference value P ref . In the prior art, the two derived currents I a  and I p  are simply combined at a summation point  26  and then delivered as a combined current I to the actuator  10 . 
   In the present invention the current I p  from the position controller  22  is further processed by a limiter  25 , producing a current I plim  which is passed to the summation point  26  for combination with the current I a  from the acceleration controller  23  to provide a combined current I to the actuator  10 . 
   The current value I plim  from the limiter  25  is also used as an input to a temperature evaluation unit  24  incorporating a transfer function corresponding to EQN. 3. Since the resistance R of the windings  16  is either a constant or represented as a temperature dependent function and the sampling period Δt can be set to that of the controller  14 , the only variables (inputs) required by the transfer function are current I plim , which as explained above is derived from the limiter  25 , the ambient temperature T amb , which can either be a preset constant or measured using a temperature sensor, and the previously recorded value for the actuator temperature T n-1 , which is stored in a register  24   a  in the temperature evaluation unit  24 . Accordingly, the actual actuator temperature T n  is determined by the temperature evaluation unit  24  and input to the limiter  25 . 
   The limiter  25  determines a maximum permissible current value I pmax  deliverable to the actuator  10  for a given actuator temperature T n  such as not to cause thermal deterioration of the actuator  10 . As modeled by  FIG. 4 , the maximum permissible current value I pmax  is constant for all temperatures up to a lower threshold actuator temperature T nL . This constant current value is purely dependent on the power electronics driving the position controller  22 . As the temperature of the actuator  10  exceeds the lower threshold T nL , the limiter  25  restricts the maximum permissible current value I pmax . If the temperature of the actuator  10  reaches an upper threshold T nH , no current is derived from the limiter  25 . Hence, the actuator  10  is protected from thermal deterioration and destruction. 
   Although the maximum permissible current I pmax , and therefore current I plim , are zero for actuator temperatures above T nH  in the present embodiment, it is clear from EQNs. 1 and 2 that a nonzero current I plim  can still be delivered even in this temperature range without causing a temperature rise in the actuator  10 . In such circumstances, the energy dissipated in the actuator  10  due to the current I plim  flowing in the windings  16  is equal to or less than the heat loss from the actuator  10  due to conduction and convection, and consequently there is no temperature rise in the actuator  10 . Accordingly, it is possible to continuously energize the actuator  10 , albeit with a limited driving current I plim . 
   In the embodiment of  FIG. 6 , the limiter  25  and temperature evaluation unit  24  are applied to the current I p  output from the position controller  22  only. The reason for this is that it is the low frequency disturbances  27 , such as asymmetric loading of the car  1 , which require the continuous energization of the actuator  10  and thereby cause the greatest heating effect on the actuator  10 . These low frequency disturbances  27  manifest themselves primarily in the position error signal e p . An additional limiter  25  and temperature evaluation unit  24  can also be installed on the output of the acceleration controller  23 . Alternatively, a single current limiter  25  and temperature evaluation unit  24  can be applied to the output from summation point  26  to limit the combined current I. 
   It will be appreciated that the temperature evaluation unit  24  and current limiter  25  can be combined as a single unit in the controller. 
   A presently preferred embodiment of the invention is illustrated in  FIG. 7 . In this embodiment, the combined analogue controller and power unit  14  utilizing the modeling of  FIG. 4  have been separated into and replaced by a discrete digital controller  30  and a discrete actuator power unit  31 . This enables the digital processing of signals within the controller  30 , which greatly improves efficiency and accuracy. All components of the controller  30  correspond to those in  FIG. 6 , however it will be understood that digital signals from the position controller  22 , acceleration controller  23 , the limiter  25  and the summation point  26 , referred to as force command signals F in the drawing, are proportional to the currents I in the previous embodiment. It is only after the combined force command signal F from the summation point  26  in the controller  30  is passed to the power unit  31  that the actual driving current I is supplied to the actuator  10 . In contrast to the previous embodiment, the limiter  25  and temperature evaluation unit  24  monitor and limit the combined force command signal (F) derived from the summation of the position force command signal (F p ) and the acceleration force command signal (F a ) at the summation point  26 . 
   Again, the alternatives arrangements discussed in relation to the previous embodiment apply equally to the present embodiment. 
   Furthermore, the guide assemblies  5  may incorporate guide shoes rather then rollers  6  to guide the car  1  along the guide rails  15 . 
   Although the present invention has been specifically illustrated and described for use on d.c. linear actuators in an active ride control system to dampen vibrations of an elevator car  1 , it will be appreciated that the thermal protection described herein can be applied to any electromagnetic actuator.