The present disclosure is directed in general to drive mechanisms and actuators, and more specifically to bi-stable actuators utilized in unstable systems, including an actuator having a high terminal velocity and significant kinetic energy at end of travel, including but not limited to such actuators used in IR imaging shutters.
Drive mechanisms including actuators are conventionally utilized to control the selective positioning of one or more members of a system. System design requirements dictate, and often limit, the specific design suitable for the application. In some environments, unstable actuators are utilized, such as but not limited to bi-stable actuators.
Rotary solenoid actuators advantageously provide high starting torque, but continually accelerate along the length of travel, resulting in impact at the end of travel. Feedback loops are typically not used with rotary solenoids, so the velocity at the end of travel is typically much higher than needed. Adding position feedback sensors defeats the packaging advantages of using the solenoid.
The problem becomes more pronounced when the actuator is bi-stable, with significant detent forces at the ends of travel holding the actuator into the stops. While this arrangement is highly desirable from a power consumption perspective, it requires higher torque application at the beginning of travel, resulting in high velocities at the end of travel.
Prior applications have tried to limit the impact velocity by limiting the duration that the actuator is powered on. These methods include applying current for only a short duration (timing), and shutting the actuator off after tripping a proximity sensor. These methods work better on statically neutral actuators, where the actuator will coast freely after power is removed. These methods require estimating the amount of time and torque required to meet the travel requirements, particularly when parameters are variable over the operating environment. Furthermore, timing is only partially effective with bi-stable actuators, since the actuator will freely accelerate once it is past the detent position, resulting in high velocity at the end of travel. Furthermore, removing power too soon will result in the actuator not overcoming the detent torque and fail to move to the alternate position as commanded.
What is desired is a means of providing an bi-stable actuator having a high starting torque, but a slower, regulated velocity as the actuator moves through its range of travel, without the added weight and volume of a position feedback transducer.
To address one or more of the above-deficiencies of the prior art, one embodiment described in this disclosure comprises a drive mechanism having a bi-stable motor driving an actuator with a high starting torque, and a slower, regulated velocity as the actuator moves through its range of travel. This advantageously maintains high torque margins at low velocity, and lowers the kinetic energy of the bi-stable actuator at end of travel by limiting the terminal velocity and establishing a softer stop. A solenoid may be used in one embodiment. Actual bi-stable motor values are obtained immediately before the move to maintain accurate control of the motor, such as the resistance and inductance of the motor coil. For instance, the bi-stable motor may be driven into a stop, and the coil resistance may be calculated by sensing current associated with a calibration voltage. Inductance may be measured similarly by applying low level AC currents. Back-emf is sensed through a sense resistor, and an estimated motor rotation rate is sent to a feedback loop to maintain the desired rate.
In one preferred embodiment, a shutter of an IR imaging device is positioned in response to the actuator, which shutter remains thermally isolated from the motor and arm. Other systems including bi-stable actuators may benefit from the present disclosure. Although specific advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated advantages. Additionally, other technical advantages may become readily apparent to one of ordinary skill in the art after review of the following figures and description.
For a more complete understanding of the present disclosure and its advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which like reference numerals represent like parts:
It should be understood at the outset that, although example embodiments are illustrated below, the present invention may be implemented using any number of techniques, whether currently known or not. The present invention should in no way be limited to the example implementations, drawings, and techniques illustrated below. Additionally, the drawings are not necessarily drawn to scale.
Each drive crank 22 further comprises a radially extending arm 46, wherein each of arms 42, and 46 are shorter than the elongated arm 24 as shown in
When the shutter plate 14 is in the full open position, the arm 24 of drive mechanism 16A is in the full open position and the shutter pin 30 of drive mechanism 16A is positioned at a distal end of a slot 60 defined in one end of plate 12 as shown in
Advantageously, as illustrated in
Moreover, the spacing of the arms 24 from rollers 34 provides the motors 18, and thus the respective arms 24, time to accelerate from the respective first rest position or second rest position which advantageously builds momentum in the arms 24 before engaging and driving the respective rollers 34, converting the actuation mechanism from torque transfer to momentum transfer of energy. This additional momentum helps overcome the magnetic detent forces of the magnetic detent latch 32 acting against the shutter pin 30, holding arms 42 or 46 against the stop posts 50 or 52. The impact of the arm 24 engaging the roller 34 during rotation also helps overcome any stiction that may be present. This spacing increases the required force margin from 25% to 900%. The spacing also allows the use of a less precise solenoid motor 18, which has a relatively large amount of play and thus is less suitable for driving the arm 24 directly. Each arm opening 26 provides a loose fitting about the respective shutter pin 30 and roller 34, such that the motor loose play does not impair operation of the shutter aperture. Conversely, the loose tolerances of the arm openings 26 mitigate the risk of an inadvertent rebound. The aperture blades 14 have internal stops, which engage prior to the holding arms 42 or 46 contacting their respective stop. Since the shutter pin 30 is not firmly engaged within the distal slot 26, the aperture blade can rebound before the arm 42 or 46 contacts the stop set screw 54 and rebounds. Additional margin is provided by the fact that the arm has much higher inertia than the aperture blade, and rebounds correspondingly slower. The high level of damping in the actuator bearings in 18 diminishes the magnitude of the arm rebound. These features prevent a situation where the rebounding arm 24 impacts the shutter pin 30 and roller 34 while traveling in the opposite direction. Such impact could exert extremely high forces onto the shutter pin 30 due to the arm's much higher inertia.
As shown in
In one preferred embodiment, a rotary solenoid is used as motor 18 as it provides consistent reliability and an adjustable stroke, such as manufactured by Brandstrom Instruments of Ridgefield Conn. The fine adjustment features of the drive crank 22 using the travel limit screws 54 in the stationary motor mount stop limit members 50 and 52 help establish this stroke. This design is superior to a piezo drive motor that is inherently unreliable, although is functionally acceptable. Alternate rotary motors could comprise DC stepper motors, and limitation to the particular rotary motor is not to be inferred. This invention has advantages over motors and linkages that may allow motor over-travel which may overstress driven parts.
Referring now to
Each motor 18 acts as a tachometer, as motor rotation generates a back-emf proportional to motor rate. In a driven motor, back-emf is masked by the voltage drops across the motor coil impedance. However, the speed of the motor 18 can be computed from a knowledge of the back-emf constant Ke, motor coil resistance Rm, motor input voltage Vd, and motor current Im according to the following equation:
Wm=1/Ke(Vd−ImRm)
The advantages of the back-emf control loop include no need for a transducer as the resistance of the motor coil is used. Further, there is no reduction in motor starting torque. This approach is more effective than energy dissipators. In one embodiment, the impact velocity of limit arm 46 is reduced from 143 rad/sec to 20 rad/sec, which significantly reduces the impact energy by 98%.
Controller 60 obtains the values of bi-stable motor 18 parameters by measurement immediately before driving the motors to advantageously maintain control of the motors in the unstable system. For instance, the resistance Rm and inductance Lm of the respective motor coils are measured. In one embodiment, each actuator motor 18 may drive arm 46 into limit stop 50 or 52 by a small calibration voltage Vd, such as a short 0.1V pulse of 5 or 10 ms, provided by motor control circuit 62. The coil resistance Rm may be calculated by resistance estimator circuit 64 correlating the sensed current Im associated with the calibration voltage Vd. Estimator 64 may be a field programmable gate array (FPGA). Inductance Lm may be measured similarly by control circuit 60 providing low level AC currents to the motors. Advantageously, the motor back-emf is sensed through the coil resistance, and an estimated motor rate circuit 66 determines the motor rate as a function of this back-emf, and sends a feedback signal indicative of the motor rate to a feedback loop junction 68 to maintain the desired motor rate.
The system, shown modeled in Matlab, provides the basis for operation of the back-emf. Upon initialization the system must first identify on which stop it is, achieved by observing the return from the two Hall effect proximity sensors. The system must also determine if the commanded move is in the appropriate direction. If the command to move is consistent with the arm position, then the system initiates the resistance measurement sequence. During this sequence, the arm is commanded to move in the opposite direction, directly into the stop, at a low voltage command level. The current is measured using a sense resistor or other means. Given that sense resistors exhibit far better resistance stability than the copper windings within the motor, the resistance of the motor can be deduced by determining the overall resistance of the system, then subtracting out the sense resistor. If desired, a look-up table can be employed to compensate for the sense resistor thermal changes. While theoretically the system could be operated using only a temperature sensor and a look-up table, the temperature in the motor can change during operation and subsequent moves at a nominal temperature could act against significantly different motor resistance.
Once the resistance is measured, it is sent to the rate estimator to set the gain and the command to move in the proper direction is issued. The motor command is sent into a compensator. In this embodiment, the compensator is described by the transfer function:
The compensated command is then sent to the plant model, described by the transfer function in this embodiment as:
The pole located in the positive domain (s−507.2) is a direct result of the inherent instability of the system. It is also noteworthy that does not attempt to cancel the unstable pole by the addition of an zero in the positive domain (unstable pole cancellation). The closed loop transfer function of the system is described by:
Since unstable pole cancellation was not attempted, one of the closed loop poles (s−0.048) remains unstable. However, the unstable pole is pulled close to the origin and the time constant of the pole is now approximately 21 seconds. Given that the move is completed in less than 100 milliseconds, the response of this pole is sufficiently slow that unstable behavior does not have adequate time to manifest itself before the move is complete. Other actuators and systems may require different compensation. An engineer skilled in the art can be expected to tailor the compensator for a given plant and actuator combination, such that the unstable poles are sufficiently slow so as not to manifest themselves in a deleterious manner. While compensated commands are sent to the motor, the motor rate is estimated by measuring the voltage picked off from a sense resistor. The motor command and the sensed rate is then fed through lead-lag and lag compensation to account for phase shifts generated by the motor inductance. Once a rate estimate is generated, it is fed back to adjust the motor command.
The simulation applied torque disturbances to the actuator arm. These disturbances represented the detent torque acting on the arm from the magnetic latches on the aperture, as well as internal cogging of the motor. These torques acted the most strongly on the arm at the extremes of travel. The disturbances captured the unstable behavior of the actuator. Other torque disturbances, such as friction, viscous damping, and air resistance could be included in other plant embodiment simulations.
In this embodiment, the command to move is terminated when the arm passes the Hall Effect proximity sensor nearest to the end of travel. Iterations of the simulation indicated that the settling time was reduced if the drive current was removed from the actuator prior to hitting the stop. In other embodiments, it may be preferable to apply power to the actuator up to or after initial contact with the stop.
Velocity control of solenoids is not common as they are unstable. Velocity control of bi-stable solenoids is also not common because systems utilizing these devices are inherently unstable, even with closed feedback loops. Accurate measurement of the motor coil resistance is crucial to maintain control, and should be accurate to within +3%/−1% for reliable and stable control of the motors. Errors in motor coil resistance greater than these levels can cause oscillations between stops, and/or settling at a stop position. Unsensed inductance changes of about 20% can be tolerated, and 10% is preferred. Advantageously, the calculation of these motor values is independent of temperature, which is important because the resistance of motor coils, such as copper windings, can vary greatly over operating temperatures. For instance, the resistance of copper over a MIL-SPEC temperature range can vary by over 25%.
Modifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the invention. The components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses may be performed by more, fewer, or other components. The methods may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. As used in this document, “each” refers to each member of a set or each member of a subset of a set.
To aid the Patent Office, and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke paragraph 6 of 35 U.S.C. Section 112 as it exists on the date of filing hereof unless the words “means for” or “step for” are explicitly used in the particular claim.
Number | Name | Date | Kind |
---|---|---|---|
3082674 | Bagby | Mar 1963 | A |
3699863 | Yamamoto | Oct 1972 | A |
3938168 | Lange | Feb 1976 | A |
4121235 | Fujita et al. | Oct 1978 | A |
4592083 | O'Brien | May 1986 | A |
4995700 | Barney et al. | Feb 1991 | A |
5128796 | Barney et al. | Jul 1992 | A |
5402202 | Washisu et al. | Mar 1995 | A |
5689746 | Akada et al. | Nov 1997 | A |
5775276 | Yanai et al. | Jul 1998 | A |
5991143 | Wright et al. | Nov 1999 | A |
6128175 | Wright et al. | Oct 2000 | A |
6133569 | Shoda et al. | Oct 2000 | A |
6285151 | Wright et al. | Sep 2001 | B1 |
6366441 | Ozawa et al. | Apr 2002 | B1 |
6423419 | Teer et al. | Jul 2002 | B1 |
6515285 | Marshall et al. | Feb 2003 | B1 |
6995359 | Hillenbrand et al. | Feb 2006 | B1 |
7410310 | Kihara | Aug 2008 | B2 |
8164813 | Gat et al. | Apr 2012 | B1 |
8911163 | Yanevich et al. | Dec 2014 | B1 |
20020030163 | Zhang | Mar 2002 | A1 |
20040238741 | Gat et al. | Dec 2004 | A1 |
20050035870 | Bauerle et al. | Feb 2005 | A1 |
20060255275 | Garman et al. | Nov 2006 | A1 |
20070046143 | Blandino et al. | Mar 2007 | A1 |
20070090782 | Endo | Apr 2007 | A1 |
20070279793 | Hansen et al. | Dec 2007 | A1 |
20070280679 | Kato et al. | Dec 2007 | A1 |
20080017816 | Willats et al. | Jan 2008 | A1 |
20080030891 | Kim et al. | Feb 2008 | A1 |
20080094728 | Matsumoto et al. | Apr 2008 | A1 |
20080304126 | Powell et al. | Dec 2008 | A1 |
20090293654 | Pintauro | Dec 2009 | A1 |
20100053412 | Sekimoto et al. | Mar 2010 | A1 |
20100220988 | Ohno | Sep 2010 | A1 |
20110174979 | Garman et al. | Jul 2011 | A1 |
20110206362 | Viglione et al. | Aug 2011 | A1 |
20110211823 | Tsai | Sep 2011 | A1 |
20110234892 | Yasuda et al. | Sep 2011 | A1 |
20120019404 | Brosio | Jan 2012 | A1 |
20120063014 | Terahara et al. | Mar 2012 | A1 |
20120257099 | Tsai | Oct 2012 | A1 |
20120260002 | Hildebran et al. | Oct 2012 | A1 |
20140061467 | Buzerak et al. | Mar 2014 | A1 |
Number | Date | Country |
---|---|---|
2416216 | Feb 2012 | EP |
2446606 | Aug 2008 | GB |
2001172766 | Jun 2001 | JP |
2007114672 | May 2007 | JP |
WO 9533226 | Dec 1995 | WO |
Entry |
---|
United States Notice of Allowance dated Aug. 4, 2014 in connection with U.S. Appl. No. 14/170,237; 13 pages. |
U.S. Office Action dated Dec. 4, 2014 in connection with U.S. Appl. No. 14/170,348; 19 pages. |
International Search Report and Written Opinion dated Oct. 27, 2014 in connection with International Patent Application No. PCT/US2014/042009, 8 pages. |
International Search Report and Written Opinion dated Nov. 12, 2014 in connection with International Patent Application No. PCT/US2014/041769, 13 pages. |
International Search Report and Written Opinion dated Oct. 29, 2014 in connection with International Patent Application No. PCT/US2014/041966, 8 pages. |
International Search Report and Written Opinion dated Oct. 29, 2014 in connection with International Patent Application No. PCT/US2014/041963, 8 pages. |
U.S. Office Action dated Mar. 13, 2015 in connection with U.S. Appl. No. 14/088,176; 13 pages. |
U.S. Office Action dated May 21, 2015 in connection with U.S. Appl. No. 14/170,276; 24 pages. |
U.S. Office Action dated Apr. 20, 2015 in connection with U.S. Appl. No. 13/669,996; 13 pages. |
U.S. Office Action dated May 20, 2015 in connection with U.S. Appl. No. 14/071,970; 25 pages. |
Yukio Miyakawa, “Friction and Wear Performance of Gold and Gold Alloy Films”; National Aerospace Laboratory, Tokyo Japan; 1980; pp. 21-30. |
Charles S. Clark; “Resolution for Fretting Wear Contamination on Cryogenic Mechanism”; 41st Aerospce Mechanisms Symposium, Jet Propulsion Laboratory; May 16-18, 2012; pp. 399-410. |
Donald H. Buckley; “Friction, Wear, and Lubrication in Vacuum”; National Aeronautics and Space Administration; 1971; 191 pages. |
Donald F. King, et al., “3rd-generation MW/LWIR sensor engine for advanced tactical systems”, Infrared Technology and Applications XXXIV, Proc. of SPIE, vol. 6940, 2008, 12 pages. |
“Diaphragm (optics)”, Wikipedia, Oct. 7, 2012, 4 pages. |
“Aperture”, Wikipedia, Nov. 4, 2012, 9 pages. |
Kazuhisa Miyoshi, et al., “Durability Evaluation of Selected Solid Lubricating Films”, May 2001, 12 pages. |
M. A. Sherbiney et al., “Friction and Wear of Ion-Plated Soft Metallic Films”, Wear, 45 (1977), p. 211-220. |
S. Jahanmir, et al., “Sliding Wear Resistance of Metallic Coated Surfaces”, Wear, 40 (1976), p. 75-84. |
International Search Report and Written Opinion dated Feb. 6, 2014 in connection with International Patent Application No. PCT/US2013/068649. |
International Search Report and Written Opinion dated Feb. 5, 2014 in connection with International Patent Application No. PCT/US2013/068678. |
International Search Report dated Oct. 27, 2014 in connection with International Patent Application No. PCT/US2014/042010. |
U.S. Office Action dated Jun. 10, 2014 in connection with U.S. Appl. No. 13/669,996; 18 pages. |
U.S. Office Action dated Nov. 6, 2014 in connection with U.S. Appl. No. 13/669,996; 8 pages. |
U.S. Office Action dated Mar. 31, 2014 in connection with U.S. Appl. No. 14/170,348; 10 pages. |
U.S. Office Action dated Aug. 20, 2015 in connection with U.S. Appl. No. 13/669,996; 12 pages. |
U.S. Office Action dated Sep. 18, 2015 in connection with U.S. Appl. No. 14/170,276; 25 pages. |
U.S. Office Action dated Nov. 13, 2015 in connection with U.S. Appl. No. 14/071,970; 19 pages. |
U.S. Office Action issued for U.S. Appl. No. 14/071,970 dated Mar. 22, 2016, 14 pgs. |
Number | Date | Country | |
---|---|---|---|
20140363150 A1 | Dec 2014 | US |
Number | Date | Country | |
---|---|---|---|
61833599 | Jun 2013 | US | |
61833587 | Jun 2013 | US | |
61833592 | Jun 2013 | US |