Patent Application
20120060290
This invention relates generally to examination tables for medical procedures, and more specifically, a motor control for the examination tables.
Examination tables are incorporated in medical offices for supporting or positioning patients undergoing medical procedures or examinations. Conventional examination tables include a base and a support surface mounted on the base. In order to provide a more comforting support arrangement for the patient, the support surface may include a seat portion and a backrest portion that pivots with respect to the seat portion. Thus, the support surface can be moved from a chair position where the support surface resembles a chair to an examination position where the support surface resembles a substantially flat and elevated examination table, depending upon the current needs of the patient and user.
Conventional examination tables also typically include an actuation system for moving the support surface and the backrest portion. The support surface is moved vertically by a scissor lift or another lifting mechanism incorporated into the base of the examination table. The backrest portion of the support surface may be pivoted with respect to the seat portion with a lift cylinder or another similar drive mechanism. The lifting and drive mechanisms of the actuation system may be independently driven by electric motors, hydraulic motors, or other types of motors. Conventional examination tables also include a control system operatively connected to hand-operated and/or foot-operated control panels provided on the examination table. The control system receives input from the control panels and then activates the motors of the actuation system to move the support surface or the backrest portion.
A Brushless DC (BLDC) motor is a rotating electric machine, typically having a 3-phase stator and a rotor employing permanent magnets. BLDC motors are well suited for use in medical examination tables because they have several advantages over other types of motors, including higher torque, higher efficiency, longer operating life, lower maintenance, and quieter operation. BLDC motors may be configured with rotors located inside, outside or stacked next to the stator.
Because the stator field necessary to move the rotor in a particular direction may be dependent on the orientation of the rotor, to accurately control rotor motion, a motor controller should be able to determine the positions of the rotor magnets relative to the stator. This allows the controller to activate the stator windings in a sequence that continually shifts position of the stator magnetic field to keep the field ahead of the rotor. Rotor position may be detected with sensors, or by sensing a back EMF on the stator windings. Because the back-EMF is only produced when the rotor is moving, starting a BLDC motor without position sensors from a dead stop can be challenging. One method to initiate rotation is to assume an arbitrary rotor start up phase and later correct the phase if the startup phase turns out to be wrong. A disadvantage to this method is that incorrect rotor phase assumptions may cause the motor to either not move initially or move backwards until the phase error is corrected. These start up problems can also occur in BLDC motors employing position sensors if the sensors are dirty or misaligned, or may occur due to limitations on position sensor resolution.
Another start up challenge with BLDC motors involves stator driver voltages. For some drive circuits, the controller circuitry may operate at voltages in the range of approximately 3-12 volts, while a BLDC motor generally requires much higher voltages, sometimes in the range of approximately 50-100 volts, depending on the application. Circuits supplying power to a single winding of the stator typically include two switching devices, with one device connecting the stator winding to the motor's positive power supply voltage, and the other connecting it to ground. In this way, the drive circuit may cause current to flow into or out of the stator winding as needed by activating the respective switching device. Many driver circuits use a MOSFET, IGFET, or Bi-Polar transistor as the switching device, with the controller circuitry applying a voltage to a gate driver, which in turn causes the device to turn it on and off. Because the controller circuitry is running at a much lower voltage than the motor, it is incapable of supplying a high enough voltage to keep the high side switching device on when it is applying power to the stator winding. This is typically solved by placing a gate driver between the controller circuitry and connecting a bootstrap capacitor between the input of the stator winding and gate driver. The bootstrap capacitor causes the gate driver supply voltage to rise along with the stator winding input voltage so that it can keep the switching device active. However, the driver device generally cannot activate at initial motor start up until the bootstrap capacitor has built up a charge sufficient to power the gate driver. This can cause an under-voltage lockout condition that prevents motor from turning.
Another challenge with BLDC motors relates to precisely stopping the motor through use of active braking. This problem is exacerbated by the widely varying loads seen on medical examination tables, which may cause the moving parts of the table to drift past the desired stopping point. One way to achieve active braking is to cause the BLDC motor to apply torque in opposition to the forward motion of the examination table. However, because of the aforementioned difficulties in knowing how much torque to apply and for how long, simply reversing the BLDC motor may result in the examination moving backwards away from the desired stopping point.
Because of the challenges associated with consistent starting and stopping of BLDC motors, and the sensitive nature of medical examination tables, there is a need for systems and methods to ensure that BLDC motors both start and stop consistently as well as rotate in the correct direction when used to adjust the position of medical examination tables so as to avoid alarming patients and doctors using the table.
Embodiments of the invention provide an examination table, which includes a base, a support surface moveable with respect to the base, and a motor having a rotor and a stator. The motor may be coupled to the support surface and configured to move the support surface. A motor controller is in electrical communication with the motor. The motor controller may be configured to respond to a motion request for the support surface changing from an active state to an inactive state by braking the movement of the support surface. The braking in some embodiments may be accomplished by terminating power to a plurality of windings of the stator of the motor for a predetermined amount of time, and, selectively activating a subset of windings of the plurality of windings of the stator for a predetermined amount of time.
In some embodiments, the motor controller is further configured to terminate power to stator windings of the plurality of stator windings associated with either of a first drive state or a second drive state of the stator, depending on which is currently active. If the rotor of the motor has not reversed by a predetermined amount, one of the first drive state or second drive state of the stator as activated as above.
Embodiments of the invention also provide a motor control system for the examination table. The motor control system includes a motor drive circuit having a plurality of drive transistors each associated with a respective winding of a plurality of windings on the stator of the motor. The control system further includes a motor controller configured to brake movement of the support surface in response to a motion request for a support surface of the examination table changing from an active state to an inactive state. The braking, in some embodiments, may be accomplished by terminating power to the plurality of windings of the stator of the motor for a predetermined amount of time and selectively activating a subset of windings of the plurality of windings of the stator for a predetermined amount of time.
The motor controller is further configured to terminate power to stator windings of the plurality of stator windings associated with one of the first drive state or second drive state of the stator, depending on which of the drive states is active. If the rotor of the motor is not reversing by a predetermined amount, one of the first drive state or second drive state of the stator is activated.
Embodiments of the invention also provide a method of braking movement of a support surface of an examination table with a motor. In response to a motion request for the support surface changing from an active state to an inactive state, the motor is used for braking movement of the support surface. Braking is accomplished by terminating power to a plurality of windings of a stator of the motor for a predetermined amount of time and selectively activating a subset of windings of the plurality of windings of the stator for a predetermined amount of time.
In some embodiments the method further includes terminating power to stator windings of the plurality of stator windings associated with one of the first drive state or second drive state of the stator, and, in response to the rotor of the motor not reversing by a predetermined amount, activating one of the first drive state or second drive state of the stator. In other embodiments, the method further includes: terminating power to stator windings of the plurality of stator windings associated with one of the first drive state or second drive state of the stator, and in response to the rotor of the motor reversing by a predetermined amount, maintaining the support surface in a fixed position.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description given below, serve to explain the invention.
It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the sequence of operations as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes of various illustrated components, will be determined in part by the particular intended application and use environment. Certain features of the illustrated embodiments have been enlarged or distorted relative to others to facilitate visualization and clear understanding. In particular, thin features may be thickened, for example, for clarity or illustration.
Embodiments of motor start/stop control include software algorithms configured to control BLDC motors, which may be deployed in a medical examination table environment. Such algorithms may include a run-state algorithm, a pre-charge algorithm, a phase-retardation algorithm, and a braking algorithm. Each of these algorithms may be implemented in a BLDC motor controller in some embodiments, or in other embodiments may be implemented in other control systems utilized by the medical examination table. Any of the control circuits used for motor control may be implemented with appropriate logic circuits, microprocessors, FPGAs, ASICs, etc.
One embodiment of an examination table 10 is illustrated in
The table portion 14 further includes a table frame 22 and a support surface 24. The table frame 22 defines a generally planar upper surface 26 for supporting the support surface 24. The table frame 22 may also include a plurality of storage drawers 28 and retractable instrument pans 30 at a front surface 32 of the table frame 22. The storage drawers 28 and retractable instrument pans 30 provide convenient storage areas for a user such as a medical professional during patient examinations and procedures on the examination table 10. The table frame 22 may further include at least one electrical outlet 34 positioned along a side surface 36 of the table frame 22. The electrical outlet 34 is powered by the power supply to the examination table 10 and permits convenient electrical power for accessory devices used with the examination table 10 or during a medical procedure.
The support surface 24 is divided into a seat portion 38 and a backrest portion 40. The support surface 24 is generally padded or cushioned to more comfortably accommodate a patient. The seat portion 38 is rigidly coupled to the upper surface 26 of the table frame 22 adjacent to the front surface 32. The backrest portion 40 extends behind the seat portion 38 and may be pivoted with respect to the seat portion 38. A lift cylinder 42 or similar device is engaged with the backrest portion 40 and the table frame 22 to pivot the backrest portion 40. The lift cylinder 42 and scissor lift 18 combine to form an actuation system for moving the examination table 10 through various positions such as the initial position shown in
The actuation system also includes a first motor 44 operatively coupled to the scissor lift 18 and a control system (such as controller 300 in
The actuation system of the examination table 10 further includes a second motor 48 operatively coupled to the lift cylinder 42 and the control system. The second motor 48 drives the lift cylinder 42 to move the backrest portion 40 of the support surface 24 between a first position adjacent to the table frame 22 and a second position angled upwardly from the table frame 22 and seat portion 38. The second motor 48 is also a brushless direct current (DC) electric motor in the illustrated embodiment. The control system includes a second Hall-effect sensor 50 coupled to or incorporated into the second motor 48. The second Hall-effect sensor 50 operates in an identical manner as the first Hall-effect sensor 46 to detect rotations of the second motor 48. The first and second Hall-effect sensors 46, 50 provide motor rotation position information to the control system, and the control system actuates the first and second motors 44, 48 in accordance with these sensed rotations.
The control system of the examination table 10 further includes a control panel 52 as shown in
BLDC Controller 300, illustrated in
Processor 302 may operate under the control of an operating system, and may execute or otherwise rely upon computer program code embodied in various computer software applications, components, programs, objects, modules, data structures, etc. to read data from and write instructions to the BLDC Motor Driver 310, and BLDC Motor 330 through I/O interface 306, whether implemented as part of the operating system or as a specific application. The computer program code typically comprises one or more instructions that are resident at various times in memory 304, and that, when read and executed by processor 302, causes the BLDC Motor Control Algorithm (BMCA) 308 to perform the steps necessary to execute steps or elements embodying the various aspects of embodiments of the invention. In particular, the resident computer program code executing on BLDC Controller 300 may include operations to collect and store in memory 304, BLDC Motor 330 operational parameters through I/O interface 306. The operational parameters may be collected from a current sensor 324 and one or more Hall-effect sensors 336, though in other embodiments, other types of sensors may also be used. The current sensor 324 and Hall-effect sensors 336 may be electrically isolated from the I/O interface 306 by optical isolators 348, 350, 352, 354, or by some other isolation device or circuit. The current sensor 324 may provide the BLDC Controller 300 with information related to the current being supplied to the BLDC Motor 330 by the BLDC Motor Driver 310, while rotor 334 position information may be supplied by the one or more Hall-effect sensors 336.
The BLDC motor driver 310 provides voltages to the stator 332 windings based on signals from the I/O interface 306, which may be electrically isolated from the BLDC motor driver 310 by optical isolators 342, 344, 346. The voltages provided by the BLDC motor driver 310 have suitable magnitude and current sourcing ability so as to cause the rotor 334 to produce torque and rotation sufficient to provide motion to the examination table actuation systems, such as scissor lift 18 or lift cylinder 42. The BLDC motor driver 310 may include three FET gate drivers 312, 314, 316 which provide switching voltages to power transistor devices 318, 320, 322. The motor driver 310 may also include the current sensor 324 for reporting current levels back to the controller 300. The FET gate drivers 312, 314, 316 and power transistor devices 318, 320, 322 may utilize switching devices such as MOSFETs, IGFETS, bipolar transistors, SCR's, relays or any other suitable switching device.
BLDC Motor 330 includes the stator 332, the rotor 334, and one or more Hall-effect sensors 336. In one embodiment of the BLDC Motor 330, the rotor 334 is positioned inside the stator 332. However, other embodiments of BLDC motors 330 may have varied configurations placing the stator 332 in proximity to the rotor 334, such as, for example, the stator 332 may reside inside the rotor 334, or may be adjacent to the rotor 334. The stator 332 may include a number of magnetic elements arranged in a cylindrical shape. The magnetic elements include windings configured such that a magnetic field is provided in the hollow interior of the stator 332 when a current is passed through a winding. The windings are typically distributed around the periphery of the stator, forming an even number of magnetic poles. The rotor 334 is positioned within the stator 332 and includes one or more permanent magnets forming at least one magnetic pole pair, with poles alternating between north and south along the exterior periphery of the rotor 334. The rotor 334 is configured to move relative to the stator 332 by activating the stator 332 windings sequentially in a controlled manner with signals from the BLDC controller 300 as conditioned by the BLDC Motor driver 310. Hall-effect sensors 336 may be used to detect the position of the rotor 334 and provide this information to the BLDC controller 300 through I/O interface 306. The BMCA algorithm 308 in turn uses this rotor 334 position information to help generate control signals, which are sent from the BLDC motor controller 300, to the BLDC motor driver 310, forming a feedback loop.
When the BLDC motor 330 is running, it is normally desirable to keep the angle between stator 332 and rotor 334 magnetic fluxes at approximately 90°. In a BLDC motor employing a 3-phase stator, the angle between the rotor flux and the stator flux generally varies between approximately 60° and 120°. As the rotor 334 advances, the angle between the rotor 334 and stator 332 fluxes decreases. When the angle reaches approximately 60°, the BMCA algorithm 308 will alter the voltages supplied to the stator 332 windings, causing the stator 332 flux to advance approximately 60° to the next state so that it is now approximately 120° ahead of the rotor 334 flux. To achieve this effect, the angular position of the rotor 334 relative to the stator 332 may be calculated by a commutation table generator 402 based on the Hall-effect sensor 336 signals. Using the rotor 334 angular position, the commutation table generator 402 may calculate which stator 332 windings to energize in order to achieve the desired angle between the stator 332 and rotor 334 fluxes. The desired stator 332 windings state is then supplied by the commutation table generator 402 to the BLDM motor driver signal generator 406.
Speed of the BLDC motor 330 is controlled by the magnitude of the voltages applied to the stator 332. The magnitude of the voltage applied to the stator 332 affects the amount of current flowing through the windings, and thus the intensity of the stator 332 flux, where a stronger stator 332 flux results in more force on the rotor 334. When the force applied by the stator 332 flux causes rotor 334 torque to exceed a load on the BLDC motor 330, the rotor 334 accelerates to a higher speed. The speed calculator 404 determines the speed of the rotor 334 based on its angular position over time as supplied from the Hall-effect sensors 336. This speed information is relayed to the proportional integral calculator 408, which is configured to determine the level of voltage to apply to the stator 332 and relays this information to the BLDM motor driver signal generator 406. In BLDC motors, the voltage level is typically adjusted using pulse-width modulation (PWM) of the voltage pulses sent to the stator 332.
The current sensor 324 is configured to detect an amount current flowing through stator 332 and generates a signal proportional to that stator 332 current. The over current fault detector 410 monitors the signal from the current sensor 324, and if the stator 332 current exceeds a threshold, generates a current fault and relays the fault to the BLDM motor driver signal generator 406.
The BLDM motor driver signal generator 406 then uses the desired winding state information from the commutation table generator 402, the voltage level information from the proportional integral calculator 408, and the stator 332 current information from the over current fault detector 410 to generate a PWM signal that energizes the desired stator 332 windings with the appropriate voltage.
When the BLDC motor 330 has been idle for a period of time, the charge on the boot-strap capacitor 504 may bleed off. When this happens, the boot-strap capacitor may not have sufficient charge to keep the high-side gate driver 506 power supply voltage above VM when the high-side transistor 500 is switched on. The high-side gate driver 506 may then fail to activate; entering a low-voltage lock-out state instead. If this occurs, the BLDC motor 330 may not start, which is not desirable. To assist in preventing a lock-out state from occurring, when the BMCA 308 receives a command to start the BLDC motor 330 after a period of idleness, a pre-charge algorithm may be executed, which first checks the orientation of rotor 334 to determine which stator 332 winding is to be energized. The BMCA 308 then applies a voltage to a low-side gate driver 507, activating a low-side transistor 502 for that stator drive output 510, pulling the stator drive voltage 510 to ground and insuring that the boot-strap capacitor 504 is fully charged before beginning normal operation by switching to the run-state algorithm 308. The period of activation should be long enough so that the boot-strap capacitor 504 obtains sufficient charge. In one particular embodiment of the invention, the period of activation may be approximately 80 milliseconds, depending on the size of the boot-strap capacitor 504. In cases where the BLDC motor 330 is run off both positive and negative voltage supplies, the low-side transistor 502 may be activated in a similar manner to charge boot-strap capacitor 504 by connecting the stator drive voltage 510 to the negative supply.
If the rotor 334 orientation is not at state one (“No” branch of block 604), the BMCA algorithm 308 will check to see if the rotor 334 orientation is in state two (block 610). If so (“Yes” branch of block 610), the BMCA 308 will activate the phase A low-side transistor using the pre-charge duty cycle and duration (block 615). Once the boot-strap capacitor 504 is charged, the BMCA 308 will proceed to start the BLDC motor (block 608).
If the rotor 334 orientation is not at state two (“No” branch of block 610), the BMCA algorithm 308 will check to see if the rotor 334 orientation is in state three (block 611). If so (“Yes” branch of block 611), the BMCA 308 will activate the phase B low-side transistor using the pre-charge duty cycle and duration (block 616). Once the boot-strap capacitor 504 is charged, the BMCA 308 will proceed to start the BLDC motor (block 608).
If the rotor 334 orientation is not at state three (“No” branch of block 611), the BMCA algorithm 308 will check to see if the rotor 334 orientation is in state four (block 612). If so (“Yes” branch of block 612), the BMCA 308 will activate the phase B low-side transistor using the pre-charge duty cycle and duration (block 617). Once the boot-strap capacitor 504 is charged, the BMCA 308 will proceed to start the BLDC motor (block 608).
If the rotor 334 orientation is not at state four (“No” branch of block 612), the BMCA algorithm 308 will check to see if the rotor 334 orientation is in state five (block 613). If so (“Yes” branch of block 613), the BMCA 308 will activate the phase C low-side transistor using the pre-charge duty cycle and duration (block 618). Once the boot-strap capacitor 504 is charged, the BMCA 308 will proceed to start the BLDC motor (block 608).
If the rotor 334 orientation is not at state five (“No” branch of block 613), the BMCA algorithm 308 will check to see if the rotor 334 orientation is in state six (block 614). If so (“Yes” branch of block 614), the BMCA 308 will activate the phase C low-side transistor using the pre-charge duty cycle and duration (block 619). Once the boot-strap capacitor 504 is charged, the BMCA 308 will proceed to start the BLDC motor (block 608).
If the rotor 330 orientation is not at state six (“No” branch of block 614), then checking begins again at block 604, or in other embodiments, an error signal may be produced, or the algorithm may proceed to start the BLDC motor (block 608).
Referring now to
A BLDC motor 330 employing a stator 332 with three windings thus has six phase states, with each state representing the stator 332 magnetic flux orientation generated by windings energized as shown. The BMCA 300 run-state algorithm may also incorporate adjustments to BLDC motor driver signal phase in order to compensate for the effects of rotor 334 motion and to maintain desired flux orientation between the stator 332 and rotor 334 while the BLDC motor 330 is in operation.
The relationship between rotor 334 phase and stator 332 drive currents desired for motor operation while the rotor 334 is in motion may be non-optimal for inducing motion in the rotor 334 when it is stationary. For example, if run-state algorithm phase relationships are used under start-up conditions, the motor 330 may not move initially, or more seriously, move backwards. Because initial retrograde motion of an exam table may be startling during a medical examination, it is highly desirable for the BLDC motor 330 to start moving in the correct direction at start-up. To ensure that initial start-up direction is correct, the phase-retardation algorithm detects that the motor is in start-up mode, and adjusts the BLDC motor driver 310 phase by retarding it one state. In one embodiment of the invention, this may be accomplished by adjusting the Hall-effect sensor 336 signals so that the BLDC Controller 300 generates BLDC Motor Driver 310 driver signals for a rotor phase state 700 one state behind what would be generated while in the run-state. For example, if rotor 334 is in rotor phase state 700 two at start-up, the Hall Effect position sensor 336 signals are adjusted to be 1-0-1 instead of 1-0-0 for purposes of determining desired stator 332 winding currents. The phase-retardation algorithm maintains phase retardation for one or more rotor phase state 700 transactions as required until the BLDC motor 330 is safely rotating in the desired direction. Although for clarity, the exemplary embodiment of the invention represented by FIGS. 9 and 10A-F operates with a counterclockwise rotation, it will be apparent to a person having ordinary skill in the art that the sequence of stator drive currents may be easily altered to achieve clockwise rotation. It will also be apparent that the duration and magnitude of phase-retardation that provides optimal start-up characteristics may vary depending on the specific configuration of the BLDC motor 330, and its relation to the medical examination table 10.
Referring now to
In an alternate embodiment of the invention and referring now to
Although the preceding embodiments disclose toggling between the motor drive states represented by rotor phase states 700 one and four as presented in FIGS. 9 and 10A-F; in alternative embodiments of the invention, the braking algorithm may toggle between other motor drive states represented in FIGS. 9 and 10A-F, or between a larger number of motor drive states up to and including one less than the total number of winding drive states available.
Once the motor is stopped, the table may be maintained in a fixed position by friction and/or components in a transmission connecting the motor to the table. Alternative embodiments of the invention may include additional brakes, friction devices, and/or locking mechanisms to assist in maintaining table position.
While the present invention has been illustrated by a description of one or more embodiments thereof and while these embodiments have been described in considerable detail, they are not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative examples shown and described. Accordingly, departures may be made from such details without departing from the scope of the general inventive concept.
This application is a continuation-in-part of U.S. application Ser. No. 12/878,321, entitled “BRUSHLESS DC MOTOR STARTS FOR A BARRIER FREE MEDICAL TABLE,” filed on Sep. 9, 2010, the entirety of which is incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 12878321 | Sep 2010 | US |
| Child | 12906595 | US |
