Patent Application
20100072932
Fail-safe autopilot servo actuators have traditionally used brushed DC motors with dual lane servo amplifiers and tachometers. In such systems, each lane controls the voltage at one motor terminal. Because each lane implements its own closed loop control, the functioning lane automatically adjusts to a failure in the opposite lane without directly detecting the failure in the opposite lane. This smart servo architecture provides self-compensating dual lane control of the motor while allowing local electronics to provide the servo loop closure. Interfacing equipment can control and monitor the servo actuator through a standard avionics bus protocol such as ARINC 429 digital data interfaces. Replicating these fail-passive features, where each of the two lanes have equal control of the motor, for a more reliable brushless motor would be beneficial.
Aspects of the invention relates to a fail-passive electro-mechanical actuator utilizing dual controllers and a two-phase brushless motor. In one embodiment, the invention relates to a fail passive system for controlling a two phase brushless motor, the system including the two phase brushless motor including a stator, a rotor, a first winding electromagnetically coupled to the rotor, and a second winding electromagnetically coupled to the rotor, first control circuitry coupled to the first winding, the first control circuitry configured to provide a first current to the first winding, second control circuitry coupled to the second winding, the second control circuitry to provide a second current to the second winding, wherein the first control circuitry is configured to receive a first commanded value from an external control circuit, wherein the second control circuitry is configured to receive a second commanded value from the external control circuit, wherein the first control circuitry is configured to receive a negated form of the second commanded value from the second circuitry, wherein the second control circuitry is configured to receive a negated form of the first commanded value from the first circuitry, and wherein the first control circuitry and the second control circuitry are configured to concurrently
In another embodiment, the invention relates to a method for controlling a two phase brushless motor system including a rotor, a stator having a first winding and a second winding, a first control circuitry for controlling a movement of the rotor by providing a first current to the first winding, and a second control circuitry for controlling the movement of the rotor by providing a second current to the second winding, the method including receiving, at the first circuitry, a first commanded value, receiving, at the second circuitry, a second commanded value, receiving, at the first circuitry, a negated form of the second commanded value, receiving, at the second circuitry, a negated form of the first commanded value, selecting, at the first circuitry, a first value, based on a first preselected criteria, from a group consisting of the first commanded value, the negated second commanded value and a first default value, selecting, at the second circuitry, a second value, based on a second preselected criteria, from a group consisting of the negated first commanded value, the second commanded value and a second default value, generating the first current, based on the first value, for the first winding, generating the second current, based on the second value, for the second winding.
Embodiments of a brushless motor control system control a two phase brushless motor. In many embodiments, the control system includes first control circuitry for controlling one stator winding of the motor and second control circuitry for controlling a second stator winding. The first circuitry and second circuitry operate substantially independent of one another. However, they do share commanded value information in a negated form so that the information cannot be misused by either control circuitry. In several embodiments, the first circuitry and the second circuitry each use a mid-value select function to avoid transients, faults, or lags associated with certain input values. The brushless motor can include a four pole rotor and stator windings that each include four coils wound in opposite directions to minimize uncommanded motion, provide maximum efficiency and motor control.
Embodiments of brushless motor systems can provide two-phase construction for simple vector control which often translates to smooth operation with minimum poles, provide sinusoidal torque production, and provide a two winding motor with substantially isolated control drive electronics to eliminate runaways for drive circuit failures.
Embodiments of the brushless motor systems, or smart servo actuator, can be used as part of a digital autopilot system to be used on Part 25 commercial aircraft. The smart servo actuator can be fail-passive such that no single fault results in uncommanded motion greater than 1.5 degrees at the capstan. This can be accomplished using a unique, two phase brushless DC (BLDC) motor, and independent dual-lane DSP controllers with associated redundant sensors, monitors and software as described below. The actuator safety features also include an electromagnetic engage clutch which can be disengaged independently by either controller or by remotely removing the clutch power input.
The dual lane control system 12 includes first lane circuitry 22 and a second lane circuitry 24. The dual control lanes (22, 24) can operate substantially independent from one another, where each lane has individual control over the two phase motor 14. The first lane 22 includes a ARINC 429 universal asynchronous receiver/transmitter (UART) 26 for receiving information such as commands from a flight computer (not shown). The UART 26 is coupled to a digital signal processor (DSP), or lane one control circuitry, 28. The UART 26 can include registers for particular UART modes controlled by logic signals generated by the lane one DSP 28. The UART 26 can generally include FIFO buffers to prevent overrun errors. The lane one DSP 28 is coupled to an H-Bridge driver 29 for amplifying a pulse width modulated (PWM) output of DSP 28. The H-Bridge driver 29 is coupled to H-Bridge circuitry 30 that provides bi-directional control of the current provided to a first winding 32. The first winding 32 can include a number of coils that are in electromagnetic communication with the rotor 42 of the motor. In a number of embodiments, the first winding 32 is a component of a stator 43 for the motor.
The second lane 24 includes a ARINC 429 UART 34 for receiving information such as commands from a flight computer (not shown). The UART 34 is coupled to a digital signal processor (DSP), or lane two control circuitry, 36. The UART 34 can include registers for particular UART modes controlled by logic signals generated by the lane two DSP 36. The UART 34 can generally include FIFO buffers to prevent overrun errors. The lane two DSP 36 is coupled to an H-Bridge driver 37 for amplifying a PWM output of DSP 36. The H-Bridge driver 37 is coupled to H-Bridge circuitry 38 that provides bi-directional control of the current provided to a second winding 40. The second winding 40 can include a number of coils that are in electromagnetic communication with the rotor 42 of the motor. In a number of embodiments, the second winding 40 is a component of the stator 43 for the motor.
Four position sensors (44a, 44b, 44c, 44d) are positioned at points surrounding the rotor/motor shaft. At an end of the rotor/motor shaft, the motor system includes a planetary gear train 46 followed by the output pinion 48. Engagement of the planetary gear train 46 can be controlled by a clutch 50. The clutch 50 can be independently controlled by the lane one DSP 28 and lane two DSP 36.
In some embodiments, the brushless motor system is referred to as a smart servo actuator. The smart servo actuator can contain all of the components and software necessary to complete motor control loops and monitor servo integrity. Servo commands and instructions are provided to each lane of the servo in using the ARINC 429 high speed format. Position, rate, or torque commands can be accepted by the controller depending on instructions provided to the servo. Message structure can be coordinated with an autopilot system to ensure the integrity of the data received by each lane. Smart servo position, speed, torque data and status can also be transmitted to interfacing equipment in the ARINC 429 high speed format. Hardware interface to the controllers is accomplished using the ARINC 429 UARTs in each lane. Addressing for each lane of the smart servo can be achieved using discretes (52, 54) that are hard-wired during the installation. In other embodiments, other means for addressing the smart servo can be used.
The motor 14 is a two-phase, four-pole BLDC motor specifically designed for fail passive autopilot applications. The rotor assembly includes a number of rare earth magnets attached to a rotor, output gear, and sensor magnet assembly. The motor stator includes two independent windings (32, 40). The current in each winding is independently controlled by either of the controllers. The motor torque is provided by the vector sum of the currents in the two windings. The amplitude of the current vector is proportional to the output torque. The current vector leads the pole position of the rotor by 90 degrees in the direction of the desired torque or rotation. Each winding can have two current sensors (33, 41) provided at each end of the stator windings. One current sensor can be used for independent control and the other for independent monitoring by the opposite lane's controller.
Rotor angular position is sensed by using a pair of linear ratiometric Hall-effect sensors (44a, 44b, 44c, 44d) located radially around a four-pole sensor magnet attached directly to the motor shaft. Sensor output voltage pairs are provided to the DSP controller's analog to digital converters, where the rotor angle is calculated as the arctangent of the voltage ratio. Each lane has dedicated position sensors and independently calculates rotor position from its sensor inputs. Rotor position is used to determine the correct level of current in a particular lane's motor coil, calculate delta position, and compute shaft angular rate of rotation.
In the embodiment illustrated in
Command message structure, reasonableness, and freshness are examined to ensure integrity. Reasonableness can refer to checks to confirm that the incoming messages are uncorrupted before they reach the servo and that data fields of those messages are within a preselected range. This is accomplished by performing a cyclical redundant check (CRC) over the messages. Freshness can refer to the servo receiving messages at a particular rate (e.g., 100 Hz). If messages are not received within this time frame, this can trigger flag indicative of a fault condition forcing the clutch to disengage. Processing of command and sensor inputs to produce a motor coil current command is performed at high speed with current modulation being achieved utilizing a pulse width modulation (PWM) feature integral to the DSPs. Each controller includes a number of sensor inputs, including inputs for current in the controller's windings, current in the other controller's windings, clutch current, supply voltage, internal power supply voltage and position of the rotor. The PWM signal is used to switch MOSFET pairs in a conventional H-bridge (30, 38).
The operation of the motor to produce a desired torque and speed generally requires the proper current in each stator winding (33, 40) as determined independently by its lane's command signal, sensor inputs, and computed current.
A failure in either lane due to a shorted or open stator coil, shorted or opened power MOSFETs, or erroneous switching commands will generally result in a maximum movement of the motor rotor of 90 degrees. A BLDC motor is inherently safe to the extent that the coils must be actively controlled for continued motion. Such a motor generally cannot move more than 90 degrees if the current through one of the coils is constant.
The output of the motor 18 drives a two-stage, 20 to 1 planetary gear reducer which further drives through a 5.85 to 1 spur gear reducer in the servo mount. This planetary gear reducer can provide the required output torque and speed. A description of a planetary gear that can be used in conjunction with the motor 14 is included in U.S. application Ser. No. 12/326,074, filed Dec. 1, 2008, entitled “HIGH RATIO GEAR REDUCER”, the entire content of which is expressly incorporated herein by reference. In the event of a worst case 90 degree hardover in motor rotor position, the upset at the capstan can be limited to less than 1.5 degrees.
The ring gear of the planetary gear system 18 is allowed to freewheel when the servo actuator clutch is disengaged. The ring gear is generally locked in radial position when the clutch is engaged. This is accomplished with a spur gear machined on the outside of the ring gear, which mates with a corresponding spur gear on the clutch pole assembly's outside diameter. When the clutch coil is energized, annular face gears on the pole faces are locked together to ground the ring gear and mechanically engage the servo actuator. The current signal to the clutch is a separate signal provided from the autopilot's controlling computer. MOSFET switches at each end of the clutch coil allow either lane of the actuator to independently disengage the clutch when a failure is detected by a controller via the monitors. Independent clutch current sensors are also provided at each end of the coil. These current signals are monitored by DSP controller and can be used for both closed loop PWM control of the clutch current by an individual lane and for monitoring by the opposite lane. These functions are alternated periodically to detect latent failures in the components that allow the clutch to be internally disengaged.
In addition to dual current and position sensors, each lane also includes a watchdog circuit to monitor DSP performance. Both DSPs independently monitor the power supply voltages. The Serial Peripheral Interface (SPI) bus 56 resident to each DSP is used for cross-checking of input commands, sensor data, monitor inputs, actuator output performance and other functions checked during built-in-testing. Disagreements in the cross-check process can result in the reporting of a fault condition via the ARINC 429 bus. Such disagreements or other faults can also initiate the disengagement of the clutch by the cross side DSP.
The controllers (28, 36) communicate with each other using the SPI bus. In several embodiments, each controller receives a negated form of the other controller's commanded value (e.g., cross-side command value) such as position. For example, if the lane one controller 28 has received a commanded value for a position of 10 degrees and the lane two controller 36 has received a commanded value for a position of 15 degrees, then the lane one controller 28 would receive a value −15 degrees and the lane two controller 36 would receive a value of −10 degrees. By sharing information in this way, the dual lane control system avoids use of true control information from either of the controllers, thereby preventing either controller from accepting the information of the other controller as being its own. Such considerations can be important in flight applications.
The brushless motor can be a slotless, brushless type motor. Most existing slotless brushless motors are 3-phase with three windings (6 coils). In a number of embodiments, the motor of the present invention is a combination of a four-pole rotor and two stator windings with four coils where each is positioned forty five degrees from each other. In other embodiments, a two-pole rotor can be used. In other embodiments, the stator contains two phases with two coils (one coil per phase) positioned 90 degrees from each other.
Depending on the design requirements for a specific application there can be any combination of stator coils and rotor poles. As the number of stator coils or rotor poles are increased, the cost of manufacturing increases while the output torque ripple decreases.
Thus, the contribution of each turn on each coil adds to the contribution of every other coil in controlling the four pole rotor 42′. In other embodiments, a rotor having a different number of poles can be used. Proper orientation of the coils provides a sinusoidal output with low ripple torque.
To understand how the adjacent coils of
Returning now to
Returning to
When two stators windings are assembled together, a complete stator assembly is formed, as illustrated in
In the event of a failure in any of the components in either of the lanes controlled by the respective DSP controllers, the result can have a hardover effect on the motor. The control system described above, however, is designed to minimize any hardover effects by actively monitoring and responding to any such failures. In addition, the physical design of the stator windings and four pole rotor limit the worst case effect of a fault in the system. For example, in the event of a worst case 90 degree hardover in motor rotor position, the upset at the capstan can be limited to less than 1.5 degrees. A failure in either lane due to a shorted or open stator coil, shorted or opened power MOSFETs, or erroneous switching commands will generally result in a maximum movement of the motor rotor of 90 degrees. A BLDC motor is inherently safe to the extent that the coils must be actively controlled for continued motion. Such a motor generally cannot move more than 90 degrees if the current through one of the coils is constant.
For power up 82 (e.g., upon applying power to the brushless motor system), the processor can check that it is functional by testing the RAM and ARINC receive UART. These tests are generally performed only during the power-up built-in-test (PBIT) because they would otherwise interfere with normal processing. After the self-verification test for functionality, the processor generally performs a number of initialization tasks and then enters the executive loop. These tasks can include initialization of all of the necessary variables and hardware including the watchdog timer, ARINC UARTs, SPI bus, control loops, and PWM sub-systems.
The executive loop 84 generally executes those tasks that are to be performed in an orderly, consistent rate that are not critical to the motor control functions. These can include verification of valid message blocks received via ARINC 429 and the SPI bus, coordination and setting of proper commanded modes and values, transmission of ARINC labels and continuous built-in-test (BIT). The loop can execute once every 0.25 milliseconds, or at a rate of 4 kHz. Because ARINC 429 labels are to be sent at 100 times per second in some embodiments, a particular label generally needs to be processed at every 40th execution of the executive loop. The executive loop logical switch statement can have 40 cases. This ensures the data transmitted has as low a latency as possible. After each case is performed, the watchdog timer is serviced. Then for the remainder of available time until the 0.25 millisecond timer expires, the validity of the program in flash is checked by generating and checking a cyclic redundancy check (CRC).
The third major execution path, power fail interrupt 86, can be initiated upon removal of power from the actuator. An interrupt to detect the loss of input power can stop execution of the executive loop 84, the PWM interrupt 88, and then disengage the clutch. Finally, to notify associated equipment of the loss of power, an ARINC 429 Maintenance Label is sent with a power fail indication. From this point the execution of the program will generally halt. If the power interrupt was temporary, the watchdog timer will reset the processor and proceed through the power-up sequence again.
The primary motor control tasks or interrupts 88 performed by the process generally have consistent timing and take priority over other tasks. The control tasks can be structured to operate in a 40 kHz PWM interrupt. The control loops generally complete calculations for re-loading the PWM duty cycle before the end of each period. During each PWM period, the position can be calculated and the current control loop can be processed to set the commutation state and duty cycle. The output mode of the actuator (e.g., Torque, Position, or Rate) is also regulated in a control loop, but generally runs at a 10 kHz rate. Two other tasks are generally performed within the PWM 40 kHz interrupt to maintain particular timing margins. These tasks can include incrementing the 40 kHz timer for the executive loop, and checking the loading and unloading of the SPI registers.
In block 104, the process builds an SPI message block. For the SPI slave, the process can also put the message on the transmit FIFO and reset the transmitter and receiver FIFOs. In block 106, the process performs the mid-value select based on the received commanded mode, a filtered onside commanded value, a cross side commanded value and a compensated position.
In some embodiments, blocks 102, 104 and 106 can be run consecutively. For example, the first time through the executive loop Case 0, or block 108, is executed, The next time through the executive loop Case 1, block 110, is executed. The tenth time through the executive loop the “Process CBIT” block is executed. On the eleventh time through the executive loop, block 108 can be repeated and the cycle can continue until each block is executed a total of four times.
After each of the case blocks has been executed, the process executes block 116 which services the watchdog timer. In block 118, the process calculates and checks the CRC. In block 120, the process determines if the total executive loop time elapsed is equal, or greater than, to 0.25 milliseconds (ms). If it is shorter then 0.25 ms, a wait loop is executed until 0.25 ms is reached. Once the 0.25 ms time has elapsed, the process loops back to the start of the executive loop.
In some embodiments, not all of the actions described in the process are executed. In other embodiments, the actions are performed in a different order than illustrated in the flow chart. In some embodiments, some actions are performed simultaneously. In some embodiments, the process performs additional actions providing other functionality.
In some embodiments, not all of the actions described in the process are executed. In other embodiments, the actions are performed in a different order than illustrated in the flow chart. In some embodiments, some actions are performed simultaneously. In some embodiments, the process performs additional actions providing other functionality.
In some embodiments, not all of the actions described in the process are executed. In other embodiments, the actions are performed in a different order than illustrated in the flow chart. In some embodiments, some actions are performed simultaneously. In some embodiments, the process performs additional actions providing other functionality.
The inputs needed from the flight control system prior to operation can be received or calculated. These inputs can include the Command Mode, Command Value, Torque Limit and Engage Command which are communicated to the servo via ARINC 429 messages. The Command Value is the onside input into the Mid-Value Select function. The Mid-Value Select output, Desired Command Value, becomes an input into an error calculation used to determine the difference between the Desired Command Value and the actual motor state. For example, if the Command Mode is Position and Desired Command Value is 100 degrees, and the current state of the motor is at 0 degrees, the absolute value of the position error becomes 100 degrees. The motor is driven in a manner to achieve a position error of 0. A Torque Limit is used in conjunction with the Desired Command Value to determine the PWM duty cycle used to control the H-bridge. The Torque Limit is used in the control loop to limit the amount of torque needed to achieve the Desired Command Value.
In some embodiments, not all of the actions described in the process are executed. In other embodiments, the actions are performed in a different order than illustrated in the flow chart. In some embodiments, some actions are performed simultaneously. In some embodiments, the process performs additional actions providing other functionality.
After a preselected time period, the lane two control circuitry activates lane two's portion of the clutch control circuit. This allows current to flow through the clutch coil, which produces a magnetic field overcoming the spring force of the clutch armature. As such, the clutch armature is pulled in and engages the clutch mechanically to the motor. The pulling in of the armature takes a specified amount of time and during this period current is allowed to exceed the steady state value. This pull in current value is higher due to the fact that the spring force and associated inertia must be overcome in order for movement of the armature to occur.
The process then determines (C5) the clutch current error as a function of the difference between a desired clutch current and the actual clutch current. The process then determines (C6) a desired clutch voltage as a function of the clutch current error. The desired voltage is used to determine a new PWM value needed to keep the clutch engaged. The PWM control of the clutch is performed by the lane one control circuitry. This PWM value is loaded into the PWM register of the controller.
If the onside and cross side command values are different values and different signs, and the difference between these values is not large enough to flag a fault condition, and the absolute value of the onside command value is greater than the cross side command value (D13), then the output of the MVS is set to the cross side command value (CCV). If the sign of the onside command value and the cross side command value are the same (D14), then the output of the MVS is set to the default value. Since the cross side command value is negated before it is transmitted to other controller, if it is the same sign as the onside command value, lane 1 is being commanded to move in the opposite direction of lane 2. To prevent any potential fighting between the controllers, the desired command value is set to the default value (e.g., zero if the command mode is torque) for both controllers. For this example, the onside and cross side command values are equal and opposite in sign, and therefore sum of their values is zero. In such case, the output of the MVS is therefore set to the onside command value.
The MVS system 200 includes at least three input values: a Default Value 201, an Onside Command Value 202 and a Cross Side Command Value 202. The Default Value 201 is determined based on the commanded mode of operation for the smart servo. For example, if the commanded mode is a torque mode, the default value is 0. The Onside Command Value 202 is the command value sent to the servo over the ARINC 429 bus for the onside controller and received by the onside controller. The Cross Side Command Value 203 is the command value sent to the servo over the ARINC 429 bus for the cross side controller and is received by the cross lane controller. The Cross Side Command Value is communicated to the controller on the opposite lane via the SPI bus in a negated form. This functionality is common to each controller.
Referring now to
The process receives (301), at the first circuitry, a first commanded value. In one embodiment, the first circuitry corresponds to the Lane 1 control circuitry. The process receives (302), at the second circuitry, a second commanded value. In one embodiment, the second circuitry corresponds to the Lane 2 control circuitry. The process receives (303), at the first circuitry, a negated form of the second commanded value. The process receives (304), at the second circuitry, a negated form of the first commanded value. The process selects (305), at the first circuitry, a first value, based on a first preselected criteria, from a group consisting of the first commanded value, the negated second commanded value and a first default value. The process selects (306), at the second circuitry, a second value, based on a second preselected criteria, from a group consisting of the negated first commanded value, the second commanded value and a second default value. The process generates (307) the first current, based on the first value, for the first winding. The process generates (308) the second current, based on the second value, for the second winding.
In some embodiments, not all of the actions described in the process are executed. In other embodiments, the actions are performed in a different order than illustrated in the flow chart. In some embodiments, some actions are performed simultaneously. In some embodiments, the process performs additional actions providing other functionality.
Embodiments of the present invention present a number of benefits. For example, there are significant benefits to the present two phase motor design versus a three phase motor design. The two phase motor is well suited for dual lane control, provides slotless construction to eliminate cogging, and provides comparable torque to a three phase slotted motor. The two phase motor also lends itself more easily to dual control, enabling redundant and fail-passive control, than does a three phase motor.
While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as examples of specific embodiments thereof. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.
The present application claims the benefit of Provisional Application No. 61/085,413, filed Jul. 31, 2008, entitled “A FAIL-PASSIVE ELECTRO-MECHANICAL ACTUATOR UTILIZING DUAL CONTROLLERS AND A TWO-PHASE BRUSHLESS MOTOR”, the entire content of which is expressly incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61085413 | Jul 2008 | US |
