Abstract:
An electromagnetic braking system and method is provided for selectively braking a motor using an electromagnetic brake having an electromagnet, a permanent magnet, a rotor assembly, and a brake pad. The brake assembly applies when the electromagnet is de-energized and releases when the electromagnet is energized. When applied the permanent magnet moves the brake pad into frictional engagement with a housing, and when released the electromagnet cancels the flux of the permanent magnet to allow a leaf spring to move the brake pad away from the housing. A controller has a DC/DC converter for converting a main bus voltage to a lower braking voltage based on certain parameters. The converter utilizes pulse-width modulation (PWM) to regulate the braking voltage. A calibrated gap is defined between the brake pad and permanent magnet when the brake assembly is released, and may be dynamically modified via the controller.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention described herein was made in the performance of work under NASA Space Act Agreement contract number SAA-AT-07-003, and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958, as amended (42 U.S.C. 2457). The federal government may have certain rights in the invention. 
    
    
     TECHNICAL FIELD 
     The present invention relates to the braking of an electric motor, and in particular to the active control of a power source of an electromagnetic braking system to selectively brake the rotation of a rotor portion of an electric motor. 
     BACKGROUND OF THE INVENTION 
     Electric motors are used in a host of robotic or other automated systems to provide a torque suitable for performing useful work in a system. Electric motors include a stator and a rotor, with either or both of these components having windings or coils for producing a magnetic flux when selectively energized by a power supply. The opposing magnetic fluxes of the stator and rotor ultimately produce the desired rotation of the rotor. The rotational force may be harnessed as needed to produce the desired torque within the driven system. 
     Electric motors come in a variety of alternating current (AC) and direct current (DC) designs. DC motors in particular may be of the brush type, the brushless type, or the stepper motor type, with each design having relative performance advantages. Of these, the brushless DC or BLDC motor eliminates windings from the rotor and thereby provides certain efficiency, durability, and noise-related performance advantages relative to other motor designs. 
     In a fail-safe electromagnetic braking system of the type commonly used with a BLDC motor, electrical power may be selectively applied to an electromagnet to actuate or release the brake depending on the design of the brake assembly. For example, one design applies a voltage to the coils of an electromagnet portion of the brake assembly, with the electromagnetic flux generated by the electromagnet ultimately cancelling a magnetic flux of a permanent magnet portion of the brake assembly. Once the respective fluxes are cancelled in this manner a brake pad disengages from frictional engagement with the rotor. Likewise, interruption of power transmission to the electromagnet allows the magnetic flux of the permanent magnet to move the brake pad into frictional engagement with the rotor, thereby applying the brake. 
     In conventional electromagnetic braking systems the main DC power supply providing electrical power to the motor is usually separate from the power supply used to energize the brake assembly. This is due in large part to the substantial difference between the motor voltage and the required brake release voltage. To optimize performance of a given electromagnetic brake assembly, a controller may apply a constant biasing force using DC power provided by the dedicated brake power supply. However, this practice may result in the generation of excessive heat in the brake assembly, a result that may affect certain heat-sensitive components positioned in proximity to any of the heated surfaces. 
     SUMMARY OF THE INVENTION 
     Accordingly, an electromagnetic brake assembly is provided for use with a BLDC motor as described above. The brake assembly has a low-power braking state that greatly reduces the amount of heat generated therein. Within the scope of the invention, a controller executes an algorithm to control the brake voltage when the brake assembly is released, and when it is subsequently held in the released state. To do so, the controller utilizes a DC/DC buck converter having a pulse width modulating (PWM) circuit adapted for reducing a main bus voltage to a voltage level that is more suitable for control of the brake assembly. That is, the controller generates a suitable PWM voltage level (V PWM ) using the PWM circuit and automatically adjusts the voltage level delivered to the brake assembly as needed based on a set of brake control parameters. 
     The brake control parameters may include values directly or indirectly describing the temperature of the brake assembly, a voltage or current applied to the brake assembly, motor speed, motor torque, etc., with other environmental and/or dynamic parameters also or alternately usable within the intended scope of the invention. The PWM circuit of the controller may optionally include a set of jumpers that allow the brake assembly to be manually released as needed, such as during maintenance of the motor or of the brake assembly. 
     A calibrated gap is defined by and between the brake pad and electromagnet of the brake assembly. The gap size may be sized as needed depending on the particular design of the brake assembly. In one embodiment, the gap size may be dynamically modified and optimized using an actuator. The optimized gap enabled by the present invention may decrease the magnetic effect of the permanent magnet and/or may increase the pull force of a spring, thereby requiring a lower relative level of power transmission to the electromagnet for cancelling the magnetic flux of the permanent magnet. The brake control parameters may be relayed to the controller as continuous feedback values to allow the controller to actively tune the required voltage and/or current supplied to the brake assembly, ultimately increasing the overall efficiency of the brake assembly. 
     In particular, a motor assembly is provided herein that includes a rotor assembly, a stator having windings that are selectively energized by a first voltage from a high voltage bus to cause rotation of the rotor assembly, a brake assembly, and a controller. The brake assembly has an electromagnet, a permanent magnet, and a brake pad, and is adapted for releasing the rotor when the electromagnet is energized using a second voltage, and for braking the rotor when the electromagnet is de-energized. The controller includes a DC-DC converter adapted for converting the first voltage to a second voltage that is lower than the first voltage, and an algorithm adapted for optimizing the second voltage as a function of brake control parameters. 
     A method is also provided herein for optimizing the efficiency of an electromagnetic brake assembly having a housing containing an electromagnet and a permanent magnet, and having a moveable brake pad adapted for selectively braking a rotor of a motor assembly. The method includes detecting the set of brake control parameters, using a DC-DC converter to convert a first voltage from a high-voltage bus to a second voltage that is lower than the first voltage, and applying the second voltage to an electromagnet of the electromagnetic brake assembly to substantially cancel a magnetic flux of the permanent magnet. Flux cancellation allows the brake pad to move out of frictional engagement with a surface of the housing to allow rotation of the rotor. Absent flux cancellation, the rotor is braked via frictional engagement of the brake pad with the housing. The method includes varying the second voltage as a function of the set of brake control parameters to thereby determine an optimal value of the second voltage. 
     The above features and advantages and other features and advantages of the present invention will be readily apparent from the following detailed description of the preferred embodiments and best modes for carrying out the present invention when taken in connection with the accompanying drawings and appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a motor control circuit having an electric motor and an electromagnetic brake assembly that is controllable in accordance with the method of the invention; 
         FIG. 2  is a schematic partial cross-sectional view of the motor and brake of  FIG. 1 ; 
         FIG. 3  is a schematic circuit diagram describing a pulse-width modulating (PWM) circuit usable with the controller of the circuit shown in  FIG. 1 ; and 
         FIG. 4  is a flow chart describing a control algorithm usable with the controller of the circuit shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings wherein like reference numbers refer to like components throughout the several views, and beginning with  FIG. 1 , a motor control circuit  10  includes an electric motor assembly (M)  12 . The motor assembly  12  receives electrical power via a main bus voltage (V M ) from an energy supply (ES)  14  via a high-voltage bus  22 . The motor assembly  12  also receives a brake voltage (V PWM ) from a DC/DC converter  18  resident in or accessible by a controller (C)  16  for continuous voltage control of an electromagnetic brake assembly (B)  30  as set forth in detail below. 
     The motor assembly  12  and the controller  16  may be configured to provide desired levels of motion control of a motorized robot, machine, or any other motor-driven device. The controller  16  receives a set of control inputs, for example from a central robot processor, a user interface, a host machine, etc., and processes the control input as set forth below using a central processing unit (CPU) or processor  20 . A brake control algorithm  100  described below with reference to  FIG. 4  may be executed by the controller  16  using the processor  20  to thereby control the braking action of the motor assembly  12 . 
     Still referring to  FIG. 1 , the controller  16  may be configured as a computer-based control device having a microprocessor or a central processing unit (CPU). i.e., the processor  20  noted above, various electronic and/or magnetic memory locations or areas, one or more network connectors, and input/output (I/O) sections for receiving and transmitting the various I/O control signals being fed to the controller  16  by a process being controlled. The controller  16  may include appropriate control circuitry for executing predetermined motion control sequences in response to one or more input control variables in order to produce a desired control response. The controller  16  may be used in this manner to automatically check the status of various control inputs, usually by reading values determined by any required dynamic or environmental sensors, to execute a function, to update the output status, and then to repeat the cycle as needed. 
     The brake assembly  30  is electromagnetic in design, as explained above, and may be electrically connected to the energy supply  14  and the high-voltage bus  22  through the controller  16 . Using the converter  18  and a predetermined set of brake control parameters, including a temperature signal (T) detected using one or more sensors  15  and ultimately describing a temperature of the brake assembly  30  and, if desired, the motor portion of the motor assembly  12 , the controller  16  provides the brake assembly  30  with a pulse-width modulated (PWM) brake voltage (V PWM ) as described below. In one embodiment the brake assembly  30  may be released when it is energized and engaged when it is de-energized to thereby provide a fail-safe electromagnetic braking design. 
     Referring to  FIG. 2 , the motor assembly  12  according to one embodiment includes a stator assembly  40 , a rotor assembly  42 , and the brake assembly  30 . The stator assembly  40  includes a set of stator windings  41 . As will be understood by those of ordinary skill in the art, when the stator windings  41  are energized by the energy supply  14  shown in  FIG. 1  or by another suitable source of energy, an electromagnetic flux is generated with respect to the windings  41 . The rotor assembly  42  includes a steel hub with magnets  47  attached thereto. The magnetic flux of the rotor magnets  47  opposes the electromagnetic flux of the stator assembly  40 , thereby causing the rotor assembly  42  to rotate with respect to axis  11 . 
     The brake assembly  30  includes a permanent magnet  32  and an electromagnet  34  with a set of coils  33 . The permanent magnet  32  and the electromagnet  34  are housed within a housing  31 . The rotor assembly  42  may include a hub  44 . The hub  44  may include mounting holes  82  suitable for attaching the rotor assembly  42  to a driven member (not shown), e.g., a robotic linkage or other motor-driven component. According to one embodiment the hub  44  may be formed of aluminum or another lightweight and nonmagnetic material, although other materials or designs are also usable without departing from the intended scope of the invention. 
     The brake assembly  30  also includes a brake pad  35  or other suitable friction member, which may be constructed of ferrous or other magnetizable materials. The brake pad  35  may be formed of a unitary or solid/single piece of magnetic material in one embodiment, although multi-piece designs may also be used. The brake pad  35  is connected to the hub  44  using a resilient member, e.g., a set of leaf springs  38 , which move the brake pad  35  in the direction of arrow R (i.e., for “release”) to hold the brake pad  35  flush against the hub  44  whenever the brake assembly  30  is in a released or a disengaged state. The brake pad  35  freely rotates with the hub  44  and the rest of the rotor assembly  42  when the brake assembly  30  is released or disengaged. 
     In other words, the magnetic flux of the electromagnet  34  may be selectively induced or generated to cancel the magnetic flux of the permanent magnet  32 , thereby allowing the leaf springs  38  to release the brake pad  35  and hold the brake assembly  30  in a released state. Likewise, the brake assembly  30  may be applied by selectively de-energizing its coils  33 . When the coils  33  are de-energized, the flux of the permanent magnet  32  overcomes the return force of the leaf springs  38  and attracts the magnetic brake pad  35  in the direction of arrow A (i.e., for “apply”). In this manner, the brake assembly  30  is applied using direct frictional contact between the brake pad  35  and the housing  31 . 
     Referring again to  FIG. 1 , the power supplied to the brake assembly  30  may be controlled via the controller  16  to provide a low-power state without the addition of a dedicated braking power supply. For example, a voltage of approximately 24V may be needed to initially release the brake assembly  30 . A lower voltage of approximately 14V-17V thereafter may be used to hold the released state. The controller  16  may receive a transmitted set of brake control parameters, such as but not necessarily limited to a brake current, time, and brake and/or motor temperature. The controller  16  may then optimize the voltage level of the brake assembly  30  during the release and subsequent release holding states using the values of the brake control parameters. 
     As will be understood by those of ordinary skill in the art, temperature affects the performance and efficiency of the brake assembly  30 . Brake temperature may be determined using the sensors  15 , e.g., via proportional voltage sensing, variable resistance, calculation, etc., and voltage and/or current transmitted to the brake assembly  30  may be controlled to ameliorate any adverse temperature-related effects. That is, the efficiency of the brake assembly  30  may be greatly improved by controlling the holding voltage during release of the brake assembly, which in turn may reduce the heat introduced into the surrounding system. Controlling the brake assembly  30  using a stepped-down or modulated portion of the main bus voltage (V M ) from the HV bus  22  of  FIG. 1 , which is already allocated for delivery of the required motor power, thus eliminates the need for independent brake power to be routed throughout the system, e.g., to a robot or other motor-driven system. 
     The HV bus  22  is connected to the energy supply  14  and powers the motor assembly  12 . To power the brake assembly  30 , the main bus voltage (V M ) conducted via the HV bus  22  may be stepped down via the converter  18  to provide a suitable pulse width modulated (PWM) voltage (V PWM ). That is, the controller  16  uses the algorithm  100  and the converter  18  as described below to selectively reduce the main bus voltage (V M ) to a voltage level that is more suitable for control of the brake assembly  30 , particularly when the brake assembly is holding a released state. In one embodiment, the voltage level for holding the released state is approximately 70% or less of the voltage used for releasing the brake assembly  30 , although other values may be used without departing from the intended scope of the invention. The controller  16  dynamically adapts the value of the voltage (V PWM ) using the values of the brake control parameters to optimize performance of the brake assembly  30 . 
     Referring again to  FIG. 2 , a brake gap (arrow G) is defined by or provided between the brake pad  35  and the permanent magnet  32 . The gap has a calibrated size. In one embodiment, an actuator  80  may be selectively controlled via the controller  16  of  FIG. 1  to vary the size of the gap (arrow G). For example, the actuator  80  may be configured to move the housing  31  along axis  11  as shown in the embodiment of  FIG. 2 . Alternately, the actuator  80  may be configured to move the brake pad  35  and/or the rotor assembly  42 , or any suitable portions thereof, to achieve the same gap sizing effect, thereby optimizing the size of the gap (arrow G) in response to the brake control parameters. For example, the gap size may be increased when the motor assembly  12  is in a standby or low-power state, such as when the brake assembly  30  is applied, and decreased when the motor assembly is in an active state. Actuator  80  may be embodied as an electromechanical, hydraulic, pneumatic, piezoelectric, shape memory alloy (SMA), or other suitable actuator device capable of moving the housing  31  or the permanent magnet  32 , with respect to the brake pad  35 . 
     Referring to  FIG. 3 , the converter  18  of controller  16  (also see  FIG. 1 ) is operable for stepping down the main bus voltage (V M ) using the process of pulse width modulation (PWM), and for applying the resultant voltage (arrow V PWM ) to the low-side of the brake assembly  30 . The term “high-side” as used herein refers to the “BRAKE POWER” line in  FIG. 3 , which is ordinarily at the 96V level in the embodiment of  FIG. 3  unless 24V power is applied by positioning of jumper  70  as described below. The term “low-side” refers to the “BRAKE RETURN” line in  FIG. 3 , which may be disconnected or connected to 96V Ground as explained below. 
     Two bus voltages may be provided: 24V for operation of the various required processors, sensors, etc., and the main bus voltage (V M ) of 96V for powering the motor portion of the motor assembly  12  shown in  FIG. 1 . The 24V bus may be referred to as “logic power”, and is itself regulated to different voltage levels based on the requirements of the various logic chips, e.g., approximately 3V to approximately 4V in the case of a field programmable gate array (FPGA) used as or with the processor  20  of controller  16  (see  FIG. 1 ). The voltage signal (arrow V PWM ) is transmitted to a field effect transistor (FET)  56  via a signal line  52  from the controller  16  (see  FIG. 1 ) through a suitable resistor  54 . 
     Modulation of the voltage signal (arrow V PWM ) ultimately turns the FET  56  on and off. When the FET  56  is turned off, the low-side of the brake assembly  30  is disconnected, and power transmission to the brake assembly  30  is terminated. When the FET  56  is turned on, the low side of the brake assembly  30  is connected to 96V Ground, i.e., the brake assembly  30  is directly connected to the HV bus  22  (see  FIG. 1 ), which is at a maximum of approximately 96V according to this particular embodiment. The controller  16  thereafter applies PWM to modify the duty cycle of the voltage signal (V PWM ) as needed to thereby optimize the voltage level transmitted to the brake assembly  30 . 
     In particular, the controller  16  selectively tunes the 96V of the HV bus  22  to a lower voltage that remains suitable for allowing the brake assembly  30  to disengage. Initially, a voltage of approximately 24V may be sufficient for releasing the brake assembly  30 , i.e., a duty cycle of 24/96=0.25. After the brake assembly  30  is released, the controller  16  may use the brake control parameters to lower the voltage level even further, e.g., approximately 14V to approximately 17 V or lower in one embodiment, using a corresponding duty cycle of 17/96=0.177, although the actual value may vary depending on the size of the gap (arrow G) of  FIG. 2  as well as the values of the brake control parameters. When using the actuator  80  of  FIG. 2 , the gap (arrow G) shown in that Figure may be selectively varied based on the values of the brake control parameters as set forth above. 
     Still referring to  FIG. 3 , the brake assembly  30  may be electrically connected with a decoupling capacitor  58  that filters out or decouples noise. A sensor  60  may be adapted relaying an output voltage (V OUT ) based on the brake current (arrow I B ), whether using proportional voltage sensing or another suitable current sensing device, which may be read using an analog-to-digital chip (not shown) or other device at an electrical lead  65 . The sensor  60  has a power supply  63 . As shown in the embodiment of  FIG. 3 , the power supply  63  may include capacitors  67  and  69 , with capacitor  67  acting as decoupling capacitor and capacitor  69  setting the bandwidth of sensor  60 . A suitable resistor  64  and capacitor  66  may also be used to ultimately provide a filtered voltage value (V OUT ) that may be read at the electrical lead  65 . 
     Optionally, a set of jumpers  70 ,  72  may be used to manually release the brake assembly  30  as needed, for example during maintenance of any portion of the motor assembly  12  of  FIGS. 1 and 2 , or during maintenance of anything connected to the output of the motor assembly. That is, jumper  70  may remain in place at positions  2 - 3  during normal operation, and may be moved to location  1 - 2  for manual brake release, while jumper  72  remains positioned as shown in  FIG. 3  only during such manual brake release. 24V logic power may be used to release the brake assembly  30  if the controller  16  is not communicating or is otherwise down, e.g., during maintenance. Added circuit protection may be provided using a fly-back diode  74  or other suitable device as shown in  FIG. 3 . 
     Referring to  FIG. 4 , and beginning with step  102 , the brake assembly  30  of  FIGS. 1 and 2  may be initially placed in an idle or engaged state. The algorithm  100  then proceeds to step  104 . 
     At step  104  the algorithm  100  determines whether the brake assembly  30  has received a command to disengage. Such a command may be provided by the controller  16  in response to a separate control algorithm (not shown) used to control the motor assembly  12  of  FIG. 1 . Should such a command be detected the algorithm  100  proceeds to step  106 , otherwise repeating step  102 . 
     At step  106  the algorithm  100  detects, measures, or otherwise determines values for a calibrated set of brake control parameters. For example, step  106  may entail using the sensor(s)  15  of  FIG. 1  to determine the temperature (T) of the brake assembly  30  and/or another portion of the motor assembly  12 . Sensor  60  of  FIG. 3  may be used to determine brake current. The brake control parameters may also or alternately include any other useful values, including but not limited to brake voltage, motor speed, motor torque, etc. Step  106  therefore encompasses the measurement of each of the values used for the brake control parameters. At step  106  the controller  16  may also process other known values describing the status of a particular joint of a robot using the motor assembly  12 , if so configured, for example using speed and torque measurements. The algorithm  100  then proceeds to step  108 . 
     At step  108  the values of the various brake control parameters determined at step  106  are transmitted to and received by the controller  16  and temporarily stored in memory therein. The algorithm  100  then proceeds to step  110 . 
     At step  110 , the controller  16  processes the values from step  108  and generate a suitable PWM signal (arrow V PWM  of  FIG. 3 ) in response to the brake control parameters. The brake assembly  30  may be controlled using the PWM signal (V PWM ) from step  108 . At step  110  the FET  56  of  FIG. 3  is driven between on and off states at a predetermined duty cycle on the low side of the brake assembly  30 , with the duty cycle ultimately determining the voltage level delivered to the brake assembly  30 , and particularly during its released state. The algorithm  100  then proceeds to step  112 . 
     At step  112 , the algorithm  100  adjusts the brake assembly  30  as needed based on the changing environment, i.e., changes to the brake control parameters including any parameters ultimately describing the temperature (T), brake current (I B ), brake voltage, motor speed, motor torque, etc. Step  112  may include the automatic adjustment of the gap (arrow G) of  FIG. 2  using the actuator  80  if so configured, with the controller  16  adjusting the gap size based on the values of the brake control parameters. After adjusting the brake assembly  30  as needed, the algorithm  100  returns to step  104  and repeats steps  104 - 112  in a continuous loop. 
     Algorithm  100  automatically continues in a closed loop unless it is forced to terminate, e.g., if logic power is removed at any time. Should this occur, all processors, chips, sensors will automatically shutdown, and the algorithm  100  would be unable to continue, causing the brake to engage. 
     While the best modes for carrying out the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.