Abstract:
A system for detecting an obstruction in the deploy path of a retractable LCD monitor includes a disc that rotates with the monitor as the monitor is deployed, an optical switch operatively coupled to the disc to generate pulses indicating the speed at which the monitor is deploying, and a retriggerable one-shot circuit that receives the pulses from the optical switch. When the pulses fail to arrive at the retriggerable one-shot circuit fast enough, i.e., below a threshold rate, the retriggerable one-shot circuit times out and its output state transitions from a high level to a low level. The high-to-low state transition causes the monitor to retract and re-attempt deployment.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is related to, and being filed concurrently with, an application by Daniel J. Sherlock et al., entitled “Position Detection of Deployable Display Using Optical Encoder.” 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to an obstruction detection system and method and, more particularly, to a system and method of detecting obstructions in the deploy path of a retractable liquid crystal display (LCD) monitor. 
     2. Description of the Related Art 
     A rectractable LCD monitor  10 , illustrated in FIG. 1, is employed in many in-flight entertainment (IFE) systems. It is typically mounted underneath an overhead compartment area  20  and has two standard positions—the stowed (inoperative) position  31  and the deployed (operative) position  32 . While in the stowed position, the monitor is received in a recess provided underneath the overhead compartment area  20  and is held in that position against gravity by a latch  21 . The deployed position of the monitor extends slightly past the vertical axis  41  to provide a more comfortable viewing angle to the passengers, which is indicated by a set of arrows labeled  42 . While in the deployed position, an electromechanical brake (not shown) is typically used to hold the monitor in that position against a rubber bumper  22 . 
     As shown in FIG. 2, the monitor is moved from its stowed position to its deployed position by a motor shaft  14  that is rotatably driven by a motor  15 . The motor  15  is under the control of a motor control circuit  16 . The motor control circuit  16  commands the motor  15  to rotate the motor shaft  14  in the forward direction (FWD) so as to move the monitor from its stowed position to its deployed position at a prescribed deploy velocity ω. The deploy velocity has a prescribed upper limit which is equal to a rotational velocity of the monitor that would impart a maximum force, typically with a value of about of 6.5 pounds. Further, if an obstruction is present in the path of the monitor while the monitor is being moved to its deployed position, i.e. during the deploy path, so that the monitor encounters a force of typically 4.5 to 6.5 pounds, the monitor is required at that time to return to its stowed position, i.e., retract, and re-attempt deployment thereafter. Monitor deployment is attempted typically three times and if still unsuccessful after three attempts, the monitor remains in the stowed position until the unit is commanded on again. 
     In the conventional system, the obstruction is assumed to be present in the path of the monitor in one of two ways. In the first approach, the current through the motor is sensed and if there is an unacceptable increase in the current through the motor, the monitor is returned to its stowed position and monitor deployment is re-attempted. However, this approach is difficult to achieve consistently with temperature changes and over the life of the unit due to changes in the internal friction in the drive mechanism. Further, this approach often requires a separate calibration of the threshold current for each monitor on an aircraft because the current requirements for the motor to move the monitor at the prescribed deploy velocity are likely to be different for each monitor as the internal friction in the drive mechanism invariably differs from unit to unit. 
     In the second approach, the monitor is allotted a fixed period of time to deploy. If it fails to reach the deployed position within the allotted time, the monitor is returned to its stowed position and monitor deployment is re-attempted. However, with this approach, the monitor will continually press against the obstacle that is keeping it from deploying until the allotted time expires. This is undesirable especially when the obstacle is the passenger&#39;s head. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to provide an obstruction detection system and method for a retractable LCD monitor, which exhibits a greater degree of repeatability in setting the monitor to retract when encountering an obstruction. 
     The above and other objects of the invention are achieved with an optical sensor or switch that generate pulses indicating the speed at which the monitor is moving toward the deployed position and a retriggerable monostable multivibrator (also called “a retriggerable one-shot”). The retriggerable one-shot receives the pulses and when the pulses fail to arrive fast enough, i.e., below a threshold rate, the retriggerable one-shot times out and its output state transitions from a high level to a low level. The high-to-low state transition causes the monitor to retract and re-attempt deployment. 
     The invention provides greater repeatability because the variable that is sensed and monitored over time is the deploy speed of the monitor. Based on this data and a time reference, the force applied by an obstruction in the deploy path can be derived and when this force exceeds a certain value, the monitor is commanded to retract. As a result, the conditions under which retraction should occur can be determined from the deploy speed data of the monitor, irregardless of the other variables that the conventional system needed to take into account, in particular the surrounding temperature and the internal friction of the drive mechanism. Further, the invention employs digital circuitry which is more stable over life and environmental changes than analog circuitry that is used to sense motor current. As a result, the invention provides the potential for an adjustment-free design, and reduces the need to readjust units in the field. 
     Additional objects, features and advantages of the invention will be set forth in the description of preferred embodiments which follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in detail herein with reference to the drawings in which: 
     FIG. 1 illustrates the general environment in which a retractable LCD monitor is implemented; 
     FIG. 2 illustrates a conventional retractable LCD monitor; 
     FIG. 3 illustrates a retractable LCD monitor implementing an obstruction detection system according to an embodiment of the invention; 
     FIG. 4 illustrates an encoder disc that is employed in the obstruction detection system according to an embodiment of the invention; 
     FIG. 5 illustrates a dual-section optical switch that is used in conjunction with the encoder disc of FIG. 4 in the obstruction detection system according to an embodiment of the invention; 
     FIG. 6A is a timing diagram of the pulses that are derived from the output of the optical switch of FIG. 5; 
     FIG. 6B is an alternate timing diagram of pulses that are derived from the output of the optical switch of FIG. 5, that are inverted with respect to the timing diagram of FIG. 6A; 
     FIG. 7 is a block diagram of a control unit for a retractable LCD monitor implementing the obstruction detection system according to an embodiment of the invention; and 
     FIG. 8 is a velocity profile of the retractable LCD monitor implementing the obstruction detection system according to an embodiment of the invention. 
    
    
     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred exemplary embodiments of the invention, and, together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An obstruction detection system for a rectractable LCD monitor  10  according to an exemplary embodiment of the invention is illustrated in FIG.  3 . In the exemplary embodiment, the monitor  10  and the obstruction detection system are employed in an IFE system. However, the invention may be employed in other various applications, e.g., passenger entertainment systems for trains, buses, jetfoils, and other mass transportation vehicles, an automobile display system having a retractable monitor, a personal display system where a retractable monitor is mounted under a countertop, cabinet, or other furniture to save space, etc. Further, the invention is not limited to LCD monitors but may be applied to plasma displays, other flat panel displays, and more broadly any panels that deploy and retract. 
     The monitor  10  of FIG. 3 is to be mounted underneath an overhead compartment area  20  of an aircraft, as shown in FIG.  1 . Three positions are shown in FIG.  3 —the stowed (inoperative) position  31 , the deployed (operative) position  32 , and the intermediate position  33 . While in the stowed position, the monitor is received in a recess provided underneath the overhead compartment area and is held in that position against gravity by a latch  21 . The latch  21  is typically a solenoid latch but may be a mechanical latch. If a solenoid latch is used, it is released when deployment of the monitor is commenced. The deployed position of the monitor extends slightly past the vertical axis  41  to provide a more comfortable viewing angle to the passengers, and while in that position, the monitor is held against a rubber bumper  22 . 
     The monitor  10  is moved by a motor shaft  14  that is rotatably driven by a motor  15 . The motor  15  is under the control of a motor control circuit  50 . The motor control circuit  50  commands the motor  15  to rotate the motor shaft  14  in the forward direction (FWD) when the monitor is to be moved from its stowed position to its deployed position. The details of the motor control circuit will be described with respect to FIG.  7 . 
     An encoder disc or wheel  60 , illustrated in FIG. 4, is mounted on the motor shaft  14  to be in rotation with the monitor  10 , so that as the monitor  10  is rotated by the motor shaft  14 , the disc  60  rotates with the same rotational velocity and by the same angle. The disc  60  has a plurality of slits  61 , a beginning slot  62 , and an ending slot  63  formed on its outer periphery. The slits  61  and the slots  62 ,  63  are arranged so that the slits  61  correspond to intermediate positions of the monitor  10 , the beginning slot  62  to the stowed position of the monitor  10 , and the ending slot  63  to the monitor position slightly prior to and at the deployed position. 
     A dual-section optical sensor or switch  70 , illustrated in FIG. 5, has a base and a pair of sidewalls defining a channel  75 . A pair of light sources  71 ,  72 , e.g., light emitting diodes (LEDs), are provided on one sidewall and a matching pair of photodetectors  73 ,  74 , e.g., phototransistors, are provided on the other, opposing sidewall. When the optical switch  70  is operating, the light sources  71 ,  72  shoot corresponding light beams across the channel  75  to a matching photodetector  73 ,  74  located on the opposing sidewall. 
     In operation, the optical switch  70  is held stationary and the outer periphery of the disc  60  is positioned in the channel  75  of the optical switch  70 , so that as the disc  60  rotates, the slits  61  alternately go into and out of alignment with the light beams produced by the light sources  71 ,  72 , and an analog waveform is generated by the photodetectors  73  and  74 . The analog waveforms are then converted to digital or square waveforms using comparators  83 ,  84  (see FIG.  7 ). The comparators  83 ,  84  produce the digital waveforms A and B, illustrated in FIG. 6A, to have a high signal when the light beam is blocked by a non-light transmissive portion of the disc&#39;s outer periphery and a low signal when the light beam is transmitted through one of the slits  71  and received by the corresponding photodetector  73  or  74 . 
     The digital waveforms A and B are then combined using an OR gate  85  to produce a digital waveform C and the digital waveform C is supplied to a retriggerable monostable multivibrator (retriggerable one-shot)  90 . The output of the retriggerable one-shot  90  is configured to be high so long as the pulses of the digital waveform C arrive at its input above a threshold rate, which is adjustable and set in accordance with how much of a decrease in the deploy rate of the monitor  10  is acceptable. Otherwise, the output of the retriggerable one-shot  90  goes low and this event triggers a flip-flop circuit  95  to generate an active, low output to a 3-try circuit  96 . The 3-try circuit  96  in turn issues a command to return the monitor to its stowed position and re-attempt deployment. 
     The pulses of the digital waveforms A and B, which in combination make up the pulses of the digital waveform C, are generated in proportion to the deploy velocity of the monitor  10 . Each pulse of the digital waveforms A and B represents a blockage of the light beam by a non-light transmissive portion of the disc  60 . When the deploy velocity of the monitor  10  is reduced, e.g., by an obstruction or obstacle in the deploy path, the pulses of the digital waveforms A and B stretch out and edge trigger the retriggerable one-shot  90  at a slower rate. If the pulses arrive at the retriggerable one-shot  90  below the threshold rate, the output of the retriggerable one-shot  90  goes low and this event triggers the flip-flop circuit  95  to generate the active, low output to a 3-try circuit  96 . 
     The monitor  10  undergoes three rotational velocity zones in moving from the stowed position to the deploy position. FIG. 8 illustrates the three zones. The first zone is the acceleration zone, in which the monitor  10  accelerates to a deploy velocity. The retriggerable one-shot  90  does not become operative until the monitor  10  nears the end of the acceleration zone. This way, the slow rate of movement at the beginning of deployment is not recognized by the retriggerable one-shot  90  as a condition requiring retraction of the monitor back to its stowed state. The monitor  10  maintains the deploy velocity in the constant velocity zone until it reaches the end of travel and enters the deployed zone. In the deployed zone, the rotational velocity of the monitor drops quickly down to zero. 
     The end of travel of the monitor is sensed by comparing the digital waveforms A and B. The slits in the disc  60  have been arranged such that the high outputs of the digital waveforms A and B do not overlap when the monitor  10  is in the beginning position or any of the intermediate positions. At the beginning position, the digital waveforms A and B are both low, e.g., (0, 0), and at the intermediate positions, the digital waveforms A and B take on one of three states—(0, 0), (0, 1) or (1, 0), where “0” corresponds to low and “1” corresponds to high. The only time the high outputs of the digital waveforms A and B overlap (1, 1) is when the monitor  10  has reached the end of travel—in its deployed position. 
     When the monitor  10  reaches near the end of travel, i.e., when the outputs of the digital waveforms A and B are both high (1, 1), the motor is held at the deployed position using an electromechanical brake, or a solenoid latch, or using a reduced torque to hold the monitor against a mechanical stop. 
     Alternatively, the sense of the light pulses for each digital waveform may be inverted, yielding the digital waveforms A and B, as depicted in FIG.  6 B. The digital waveform (A and B) values are normally high and briefly go low, except at the deploy position where both waveforms (A and B) are simultaneously at the low value. Utilizing the normally “high”, deployed “low” parameters may achieve advantages with respect to disc alignment tolerances and overall system performance. 
     Alternate embodiments to those described above include but are not limited to utilization of microprocessors for implementation of the one-shot functions and the logic associated therewith. 
     In the embodiments of the invention described above, a specially-designed disc and a dual-section optical switch are used to generate the pulse stream to be supplied to the input of the retriggerable one-shot. Any other method known in the art for generating a stream of pulses in proportion to a rotational speed may be used. Further, although an optical switch having two light sources and two photodetectors are preferred for obstruction detection, a single-section optical switch having a single light source and a single photodetector may be used, if it is only necessary to detect the angular velocity of the monitor using the circuit. 
     The output of the optical switch is also used to maintain the deploy velocity below a prescribed upper limit. This may done by any conventional methods, for example, by counting the number of pulses per unit time and if the deploy velocity is above the prescribed range, the current to the motor is decreased. 
     While particular embodiments according to the invention have been illustrated and described above, it will be clear that the invention can take a variety of forms and embodiments within the scope of the appended claims.