Abstract:
An elevator system includes a battery; a machine having a motor for imparting motion to an elevator car; an inverter for converting DC power from the battery to AC power for the machine in motoring mode and converting AC power from the machine to DC power for the battery in regenerative mode; and a controller to control the inverter, the controller implementing at least one of: detecting an overload at the battery in motoring mode and reducing car speed in response to the overload; detecting an overcharge at the battery in regenerative mode and reducing car speed in response to the overcharge; detecting motor direct current in a motor field weakening mode and reducing car speed in response to the motor direct current; and detecting car load and adjusting car speed in response to car load.

Description:
FIELD OF INVENTION 
       [0001]    The subject matter disclosed herein relates generally to the field of elevator systems, and more particularly, to elevator car speed control in a battery powered elevator system. 
       BACKGROUND 
       [0002]    Battery powered elevator systems employ a battery as a power source to an elevator machine that imparts motion to the elevator car. A drive unit containing an inverter is typically connected between the battery and the machine. In motoring mode, the inverter converts DC power from the battery to AC drive signals for the machine. In regenerative mode, the inverter converts AC power from the machine to DC power for charging the battery. 
         [0003]    In a battery powered elevator system, the battery may experience overloading when in motoring mode or overcharging in regenerative mode. Overloading negatively affects state of charge/usability of the battery as a voltage/power source. Overcharging negatively affects the health of the battery. Overcharging is normally controlled using a dynamic braking resistor, and overloading is normally controlled with profile modifications. 
       BRIEF SUMMARY 
       [0004]    According to an exemplary embodiment, an elevator system includes a battery; a machine having a motor for imparting motion to an elevator car; an inverter for converting DC power from the battery to AC power for the machine in motoring mode and converting AC power from the machine to DC power for the battery in regenerative mode; and a controller to control the inverter, the controller implementing at least one of: detecting an overload at the battery in motoring mode and reducing car speed in response to the overload; detecting an overcharge at the battery in regenerative mode and reducing car speed in response to the overcharge; detecting motor direct current in a motor field weakening mode and reducing car speed in response to the motor direct current; and detecting car load and adjusting car speed in response to car load. 
         [0005]    Other aspects, features, and techniques of embodiments of the invention will become more apparent from the following description taken in conjunction with the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Referring now to the drawings wherein like elements are numbered alike in the FIGURES: 
           [0007]      FIG. 1  is a block diagram of components of elevator system in an exemplary embodiment; 
           [0008]      FIG. 2  depicts components of elevator system in an exemplary embodiment; 
           [0009]      FIG. 3  depicts plots of battery voltage and elevator car speed in an exemplary embodiment for controlling battery overloading; 
           [0010]      FIG. 4  depicts plots of battery voltage and elevator car speed in an exemplary embodiment for controlling battery overcharging; 
           [0011]      FIG. 5  depicts plots of battery voltage, machine direct current and elevator car speed in an exemplary embodiment for controlling a battery voltage deficiency in field weakening mode; 
           [0012]      FIG. 6  is a plot of car load versus car speed in an exemplary embodiment for controlling car speed in response to car load; 
           [0013]      FIG. 7  is a plot of car load versus car speed in an exemplary embodiment for controlling car speed in response to car load; and 
           [0014]      FIG. 8  is flowchart of a process for controlling car speed in an exemplary embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1  is a block diagram of components of an elevator system  10  in an exemplary embodiment. Elevator system  10  includes a source of AC power  12 , such as an electrical main line (e.g., 230 volt, single phase). The AC power  12  is provided to a switch panel  14 , which may include circuit breakers, meters, etc. From the switch panel  14 , AC power is provided to a battery charger  16 , which converts the AC power to DC power to charge battery  18 . Battery  18  may be a lead-acid battery or other type of battery. Battery  18  powers drive unit  20 , which inverts DC power from battery  18  to AC drive signals, which drive machine  22  to impart motion to elevator car  23 . The AC drive signals may be multiphase (e.g., three-phase) drive signals for a three-phase motor in machine  22 . It is noted that battery  18  is the sole power source to the drive unit  20 , and the AC power  12  is not directly coupled to the drive unit  20 . 
         [0016]      FIG. 2  depicts components of elevator system  10  in an exemplary embodiment. Drive unit  20  includes a first DC link  30  coupled to battery  18  (e.g., a positive DC voltage) and a second DC link  32  coupled to battery  18  (e.g., a negative DC voltage or ground). One or more DC link capacitors  34  are connected between the first DC link  30  and second DC link  32 . An inverter section uses switches  40  to generate drive signals for the motor of machine  22 . Switches  40  may be MOSFET transistors, but it is understood other types of switches may be used. Switches  40  are arranged in phase legs, each phase leg connected between the first DC link  30  and the second DC link  32 . An AC terminal  42  is provided at a junction (e.g., source-drain junction) of the switches  40  in each phase leg. AC terminals  42  are coupled to motor windings of machine  22 . In an exemplary embodiment, machine  22  includes a three-phase, permanent magnet synchronous motor.  FIG. 2  depicts a three-phase inverter and three-phase motor, but embodiments are not limited to a particular number of phases. 
         [0017]    The inverter converts DC power from battery  18  to AC power for driving machine  22  in motoring mode. The inverter also converts AC power from machine  22  to DC power for charging battery  18  when operating in regenerative mode. Regenerative mode may occur when an empty elevator car is traveling upwards or when a loaded elevator car is traveling downwards. Regenerative mode may include a regenerative brake of machine  22  providing AC power. The AC power received at AC terminals  42  is converted to DC power to charge battery  18 . 
         [0018]    During motoring mode, controller  50  provides control signals to turn switches  40  on and off to generate an AC drive signal at each AC terminal  42 . The AC drive signal may be a variable frequency signal. During regenerative mode, controller  50  provides control signals to turn switches  40  on and off to convert AC power from machine  22  to DC power for charging battery  18 . Current sensors  44  are provided at each AC terminal  42  to allow controller  50  to detect current at each AC terminal  42 , in both motoring mode and regenerative mode. A voltage sensor  51  is provided at battery  18  to detect battery voltage and provide a sensed voltage to controller  50 . Controller  50  may be implemented using a general-purpose microprocessor executing a computer program stored on a storage medium to perform the operations described herein. Alternatively, controller  50  may be implemented in hardware (e.g., ASIC, FPGA) or in a combination of hardware/software. Controller  50  may also be part of an elevator control system. 
         [0019]    Drive unit  20  also includes a dynamic braking resistor  60  and a dynamic braking switch  62 . Dynamic braking switch  62  may be a MOSFET transistor, but it is understood other types of switches may be used. In regenerative mode, if the current produced at machine  22  is excessive, the dynamic braking switch  62  is turned on (e.g., pulsed on and off with a duty cycle) and current flows through dynamic braking resistor  60 . Excess energy is dissipated through the dynamic braking resistor  60 . It is understood that multiple dynamic braking resistors  60  and a dynamic braking switches  62  may be employed in drive unit  20 . 
         [0020]    In exemplary embodiments, controller  50  controls the speed of elevator car  23  in response to operating parameters of the elevator system including battery voltage, motor direct current, car load, etc. An exemplary embodiment protects battery  18  from overloading (i.e., overdrawing current) when the machine  22  is operating in motoring mode. If the machine  22  overloads battery  18 , the battery voltage will drop. Controller  50  monitors the sensed battery voltage from voltage sensor  51  and adjusts the car speed in response to the sensed battery voltage. In motoring mode, controller  50  may compare the sensed battery voltage to a threshold and if the sensed battery voltage is less than the threshold (optionally, for a period of time), controller  50  reduces the car speed by some predetermined amount (e.g., a set m/sec or a percentage of current speed). Further, multiple thresholds may be used to provide finer control of the speed reduction. In other embodiments, the car speed is derived based on a function relating battery voltage to car speed, so that continuous speed adjustment is performed by controller  50  in response to the sensed battery voltage. The threshold(s) used or the function relating battery voltage to car speed may also be dependent upon the type of battery (e.g., lead-acid, Li-ion, etc.). 
         [0021]      FIG. 3  depicts plots of battery voltage and elevator car speed in an exemplary embodiment for controlling battery overloading. When the sensed battery voltage drops below 48V (as an example threshold), controller  50  adjusts the car speed from a first speed (e.g., 630 mm/s) to a lower, second speed (e.g., 580 mm/s). If the sensed battery voltage remains below the threshold for a certain period time (e.g., t), controller  50  may again reduce the car speed. As noted above, multiple voltage thresholds may be used or the car speed may be related to the sensed battery voltage through a function to provide continuous speed adjustment. In alternate embodiments, the rate of change of the sensed battery voltage may be used to control car speed, such that if the sensed battery voltage has stabilized (e.g., rate of change of sensed battery voltage less than a threshold), then no further speed adjustment is made. 
         [0022]    An exemplary embodiment protects battery  18  from overcharging when the machine  22  is operating in regenerative mode. In existing systems, the dynamic braking resistor  60  is used to dissipate excess current in regenerative mode. The dynamic braking resistor  60  may be pulsed on-off with a duty cycle to regulate the current dissipated. When a large regenerative current is present, even at a 100% duty cycle, the dynamic braking resistor  60  may not be able to dissipate all of the energy associated with the excess current. This could result in battery  18  being overcharged and damaged. 
         [0023]    To address this issue, controller  50  monitors the sensed battery voltage from voltage sensor  51  and adjusts the car speed in response to the sensed battery voltage. In regenerative mode, controller  50  may compare the sensed battery voltage to a threshold and if the battery sensed battery voltage is greater than the threshold (optionally, for a period of time), controller  50  may increasingly turn on the dynamic braking switch  62  to dissipate regenerative current through dynamic braking resistor  60 . If the dynamic braking resistor  60  is at full capacity (e.g., dynamic braking switch  62  is on with 100% duty cycle) and the sensed battery voltage from voltage sensor  51  is still above the threshold (which threshold may be determined to ensure battery health), controller  50  reduces the car speed by some predetermined amount (e.g., a set m/sec or a percentage of current speed). Further, multiple thresholds may be used to provide finer control of the speed reduction. In other embodiments, the car speed is derived based on a function relating battery voltage to car speed, so that continuous speed adjustment is performed by controller  50  in response to the sensed battery voltage. The threshold(s) used or the function relating battery voltage to car speed may also be dependent upon the type of battery (e.g., lead-acid, Li-ion). 
         [0024]      FIG. 4  depicts plots of battery voltage and elevator car speed in an exemplary embodiment for controlling battery overcharging. When the sensed battery voltage exceeds threshold (e.g., 64 volts), controller  50  activates the dynamic braking switch  62 , which stabilizes the increasing battery voltage as shown at section  200 . Eventually, the dynamic braking resistor can dissipate no further energy and the sensed battery voltage increases. At this point, controller  50  reduces the speed of the car  23  to reduce the regenerative current from machine  22 . If the sensed battery voltage exceeds the threshold for a certain period time, controller  50  may again reduce the car speed as shown at  400 . As noted above, multiple voltage thresholds may be used or the car speed may be related to the sensed battery voltage through a function to provide continuous speed adjustment. In alternate embodiments, the rate of change of the sensed battery voltage may be used to control car speed, such that if the sensed battery voltage has stabilized, then no further speed adjustment is made. 
         [0025]    Another exemplary embodiment protects battery  18  from a voltage deficiency when the controller  50  is operating the motor of machine  22  in field weakening mode. Field weakening mode is a known operational mode for motors, and involves increased winding current (this current is called d-axis current, field weakening current, or voltage regulating current in motor control terminology) to achieve higher speeds at the torques demanded by the motor due to elevator motion. Field weakening is an acceptable mode of operation, as long as the current to the motor is not significant and the battery is not overloaded. 
         [0026]    To protect the battery from a voltage deficiency (due to increased losses in the motor and/or increased power from motor) and also to protect the motor from excessive current, in field weakening mode, controller  50  monitors motor direct current (d-axis current) through processing of current sensors  44  signals (known as 3/2-DQ transformations to the control field). In field weakening mode, controller  50  may compare the sensed motor direct current to a threshold and if the sensed motor direct current is greater than the threshold (optionally, for a period of time), controller  50  reduces the car speed by some predetermined amount (e.g., a set m/sec or a percentage of current speed). Further, multiple thresholds may be used to provide finer control of the speed reduction. In other embodiments, the car speed is derived based on a function of the sensed motor direct current, so that continuous speed adjustment is performed by controller  50  in response to the sensed motor direct current. The threshold(s) used or the function relating sensed motor direct current to car speed may also be dependent upon the type of battery (e.g., lead-acid, Li-ion, etc.). 
         [0027]      FIG. 5  depicts plots of battery voltage, motor direct current and elevator car speed in an exemplary embodiment for controlling a battery voltage deficiency. When the sensed motor direct current exceeds a threshold (e.g., 100 amps), controller  50  reduces the speed of the car  23  to reduce the current draw of the motor. If the sensed motor direct current exceeds the threshold for a certain period time, controller  50  may again reduce the car speed. As noted above, multiple thresholds may be used or the car speed may be related to the sensed motor direct current through a function to provide continuous speed adjustment. In alternate embodiments, the rate of change of the sensed motor direct current may be used to control car speed, such that if the sensed motor direct current has stabilized, then no further speed adjustment is made. Controlling car speed in response to sensed motor direct current in field weakening mode may also be used to allow the car to travel at high speeds for limited time periods until the battery voltage drops, at which point the car speed is reduced to accommodate the battery deficiency. 
         [0028]    Another exemplary embodiment controls car speed in response to car travel direction and car load. When car  23  is traveling upwards and the load is low, the car speed may be set at an upper speed value (e.g., 1 m/s). This is due to the fact that machine  22  does not require a large amount of power to raise car  23  under low loads, which imposes a lower draw of power from battery  18 . As the car load increases, the controller  50  reduces the speed of car  23  to a lower speed value (e.g., 630 mm/s) to reduce power needed at machine  22  and thus drain of battery  18 .  FIG. 6  is a plot of car load versus car speed for an upward traveling car in an exemplary embodiment for controlling car speed in response to car load. As shown in  FIG. 6 , when the load is below a load threshold, the car speed is set at an upper speed value (e.g., 1 m/s). Once the car load crosses a load threshold (e.g., 50% of maximum load) the speed is reduced linearly until a lower speed value (e.g., 630 mm/s) is reached at a load limit (e.g., 80% of maximum load). It is understood that car load may be represented in formats other than a percentage of maximum load. 
         [0029]    The upper speed value, lower speed value, load threshold and load limit of  FIG. 6  are exemplary values. It is understood that other values may be used for these parameters. Further, multiple load thresholds may be used to provide finer control of the car speed. In other embodiments, the car speed is derived based on a function of the car load, so that continuous speed adjustment is performed by controller  50  in response to the car load. 
         [0030]    The opposite direction of travel is shown in  FIG. 7 . When car  23  is traveling down and the load is low, the car speed may be set at a low speed value (e.g., 630 mm/s). This is due to the fact that machine  22  requires a larger amount of power to lower car  23  under low loads, which imposes a larger draw of power from battery  18 . As the car load increases, controller increases the speed of car  23  to an upper speed value (e.g., 1 m/s), as machine  23  requires less power from battery  18  to lower a more loaded car.  FIG. 7  is a plot of car load versus car speed in in an exemplary embodiment for controlling car speed in response to car load. As shown in  FIG. 7 , when the load is below a load threshold, the car speed is set at a lower speed value (e.g., 630 mm/s). Once the car load crosses a load threshold (e.g., 20% of maximum load) the speed is increased linearly until an upper speed value (e.g., 1 m/s) is reached at a load limit (e.g., 50% of maximum load). It is understood that car load may be represented in formats other than a percentage of maximum load. 
         [0031]    The upper speed value, lower speed value, load threshold and load limit of  FIG. 7  are exemplary values. It is understood that other values may be used for these parameters. Further, multiple load thresholds may be used to provide finer control of the car speed. In other embodiments, the car speed is derived based on a function of the car load, so that continuous speed adjustment is performed by controller  50  in response to the car load. 
         [0032]    In the embodiments of  FIG. 6  and  FIG. 7 , the car load may be obtained in a variety of manners. In one embodiment, car  23  is equipped with a load measurement system that measures load of car  23 . In another embodiment, car load is derived from a velocity control output produced by controller  50 . As known in the art, a velocity measurement of car  23 , prior to car acceleration, may be used as an indicator of car load by comparing a speed command value to measured speed. In other words, the output of a speed controller (which may be speed proportional-integral (PI) regulator or PI regulator followed by a P regulator), which is the torque command to the motor, can be latched after the machine brake is lifted, and preferably after a certain amount of time has passed to allow for signal filtering. Preferably, latching the torque command takes place immediately prior to actual elevator motion. This latched torque command can be converted to load in a car estimate via a linear relationship. 
         [0033]    It is noted that the elevator speed control in field weakening mode ( FIG. 5 ) may be used in conjunction with speed control based on car load ( FIG. 6  and  FIG. 7 ). For example, if the elevator speed control routine commands an increase in car speed, this may result in motor of machine  22  entering field weakening mode. In such a situation, controller  50  would reduce the car speed in response to the motor direct current to prevent battery overload as described with reference to  FIG. 5 . In other embodiments, some level of field weakening current injection may be performed to compensate for variations in machine voltage due to temperature or material variation. Field weakening may be used in conjunction with load based speed scheduling to ensure that a commanded speed is achieved. However, if the motor direct current in field weakening mode is above the threshold, then the speed reduction method of  FIG. 5  may be employed as described above. Thus, the elevator speed control in field weakening mode may augment the speed control based on car load. 
         [0034]      FIG. 8  is a flowchart of a process performed by controller  50  in an exemplary embodiment. It is understood that the order of steps in  FIG. 8  is exemplary, and more than one control block may implemented simultaneously, as part of a continuous control process. At  300 , controller  50  determines if battery  18  is overloaded in motoring mode by monitoring battery voltage sensed at voltage sensor  51 . If the sensed battery voltage is too low, flow proceeds to  302 , where controller  50  reduces the car speed by an amount (e.g., a set m/sec or a percentage of current speed). Flow proceeds back to  300 , where the controller  50  continues to monitor battery voltage until the battery voltage is at a suitable level. Further speed reductions at  302  may be employed in a stepwise manner until the battery voltage is below the threshold. 
         [0035]    If battery  18  is not overloaded at  300 , flow proceeds to  304  where controller  50  determines if battery  18  is overcharged in regenerative mode by monitoring battery voltage sensed at voltage sensor  51 . If the sensed battery voltage is too high at  304 , flow proceeds to  306 , where controller  50  attempts to reduce battery voltage through the dynamic braking resistor  60 . If the dynamic braking resistor  60  reduces the battery voltage to an acceptable level, flow returns to  304 . If not, flow proceeds to  308  where controller  50  reduces the car speed by an amount (e.g., a set m/sec or a percentage of current speed). Flow proceeds back to  304 , where the controller  50  continues to monitor battery voltage until the battery voltage is at a suitable level. Further speed reductions at  308  may be employed in a stepwise manner until the battery voltage is below the threshold. 
         [0036]    If battery  18  is not overcharged at  304 , flow proceeds to  310  where controller  50  determines if motor direct current in field weakening mode is too high. If so, flow proceeds to  312  where controller  50  reduces the car speed by an amount (e.g., a set m/sec or a percentage of current speed). Flow proceeds back to  310 , where the controller  50  continues to monitor battery voltage until the battery voltage is at a suitable level. Further speed reductions at  312  may be employed in a stepwise manner until the battery voltage is below the threshold. 
         [0037]    If there is no battery deficiency at  310 , flow proceeds to  314  where controller  50  determines if car load is available. If so, flow proceeds to  316  where controller  50  controls car speed in response to direction of car travel and car load, as shown in  FIG. 6  and  FIG. 7 . As noted above, the car speed control using direct motor current in field weakening mode may also be used in conjunction with adjusting car speed in response to car load, as represented in  FIG. 8   
         [0038]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. While the description of the present invention has been presented for purposes of illustration and description, it is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications, variations, alterations, substitutions, or equivalent arrangement not hereto described will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. Additionally, while the various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as being limited by the foregoing description, but is only limited by the scope of the appended claims.