Abstract:
A power drive unit having drive roller elements that transport cargo and a sensor that detects motion of the cargo, e.g., the speed at which the cargo is transported, where the sensor operates substantially independently of the motion of the-drive roller elements. Specifically, the power drive unit comprises a motor, an output shaft driven by a motor, at least one drive roller element fixedly mounted on the output shaft, the drive roller element thereby driven by the motor, a sensor frictionally coupled to the output shaft conditionally permitting relative rotation between the sensor and the output shaft, and a motion detector that detects motion of the sensor. Associated electronics are configured to remove power from the motor driven roller elements where cargo is jammed or parked upon them, thereby increasing the useful lifetimes of the motor and the drive roller elements of the power drive unit.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/160,266, filed on Oct. 19, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power drive unit for transporting cargo, and particularly to the transportation of cargo within an aircraft. 
     2. Description of the Prior Art 
     A large variety of motorized systems for moving cargo are known. Motor driven rollers are employed in some such systems. Cargo and passenger planes in particular often employ series of motor driven power drive units (PDUs) to propel cargo containers and pallets, or unit load devices within the aircraft cargo compartment quickly and efficiently. This configuration can allow for the transportation of cargo from the external loader to the interior of the airplane by a single operator controlling the PDUs. 
     Cargo within an airplane cargo deck is typically supported by a system of freely rotating floor-mounted conveyance rollers (see FIGS.  1  and  2 ). Sets or banks of PDUs can be simultaneously elevated from beneath the cargo deck to a level just above the conveyance rollers. Each PDU is a separate electro-mechanical actuator which includes one or more rubber coated wheels or drive rollers. The drive rollers of the elevated PDUs contact and move cargo above the conveyance rollers in the commanded direction upon energization. The movement of cargo depends on the coefficient of friction between the PDU drive rollers and the bottom surface of the cargo, as well as the lifting force generated by the PDU lift mechanism. When the PDUs are deenergized, roller rotation ceases and the cargo stops moving. 
     Several sets of PDUs can be arranged along a common path of conveyance, and each set can be operated separately, thereby allowing for the. transfer of multiple pieces of cargo. An operator supervising the transportation of cargo into the cargo deck area can guide cargo by means of a joys tick and an on/off switch or similar controls. 
     PDUs can be damaged when they continue to operate beneath immobilized cargo, a condition known as scrubbing, which can occur when cargo is too heavy or has come upon an obstruction such as a wall within the cargo compartment. Scrubbing can quickly wear away the rubber coating on the rollers necessitating their replacement and can result in damage to the PDU motor. 
     Cargo container stall sensors integrated within a PDU are used to sense a stalled container and to remove power to the PDU after a predetermined delay to avoid PDU damage. PDUs typically have a manual de-select switch for removing power to the PDUs when a stall condition is sensed. Unfortunately, this de-select switch is often not used properly by operators, who are focused on loading cargo rather than protecting PDUs. Thus, damage to PD Us when scrubbing conditions occur is a common problem. 
     Problems associated with scrubbing are addressed in U.S. Pat. No. 5,661,384. Experiments have shown that when the temperature of the PDU motor surpasses a certain level, it is indicative of a stalled or jammed condition. The &#39;384 patent describes a PDU that determines when cargo is jammed by monitoring the temperature of the motor and removing power to the unit when a temperature limit is exceeded. This arrangement, while a significant advantage, is subject to error during, for example, extreme environmental conditions. 
     Other known stall sensors employ protruding mechanical wheels which are depressed when a payload is positioned over a PDU. This type of system can be a burden to maintain. 
     Moreover, the extended wheels are not protected from oddly shaped cargo and are readily susceptible to damage. 
     U.S. Pat. No. 5,568,858 describes a system for detecting a stall condition that employs a sensor driven by the movement of the cargo itself. When overlying cargo is jammed, the sensor does not move, and this absence of motion is sensed electronically. Power to the PDU is then interrupted. This solution is not entirely satisfactory because of the unevenness of the bottom surface of the cargo. In other words, variations in the cargo surface can cause erroneous stall signals. Complex additional sensors may be required to overcome this problem. 
     In view of the above, it should be appreciated that there is a continuing need for a cargo stall sensor for use with a PDU with improved reliability, and durability. There is also a need for a PDU having a cargo stall sensor that is relatively inexpensive to manufacture and easy to maintain. The present invention satisfies these and other needs and provides further advantages that will be apparent from the description below. 
     SUMMARY OF THE INVENTION 
     The present invention is embodied in a PDU having drive roller elements that transport cargo and a sensor that effectively detects movement or immobilization of the cargo. Thus, the invention is configured to remove power from the motor driven roller elements when cargo is jammed or otherwise immobilized, thereby preventing damage to the motor and the drive roller elements of the PDU. 
     The PDU comprises a motor, an output shaft driven by the motor, at least one drive roller element mounted on the output shaft so that the drive roller element is driven through the shaft by the motor, a sensor also driven through the output shaft by a drag device that conditionally permits relative rotation between the sensor and the output shaft, and a motion detector that detects rotation of the sensor. 
     In one embodiment of this invention, the stall sensor is mounted on the output shaft. The sensor is configured as a wheel, that can carry one or more magnets. When there is no load (cargo) positioned over the PDU, the stall sensor, coupled through the drag device to the drive roller element, rotates at the same speed as the drive roller element. A motion detector, such as a Hall effect sensor, senses the frequency with which the magnet passes and a signal corresponding to that frequency is sent to a logic circuit which causes power to be removed from the PDU when the frequency falls below a predetermined value. This will occur when cargo in contact with the sensor becomes jammed or stopped, causing the frictional forces at the cargo surface to be greater than that at the drag device, thereby stopping sensor rotation. 
     In a second embodiment of the invention, the stall sensor and at least one of the drive roller elements are frictional connected to each other such that the friction at a drive roller element contact face drives the stall sensor when the friction at the stall sensor is overcome by the friction at the cargo surface. When the frictional force at the cargo surface becomes greater than at the drive roller element contact face because the cargo is jammed, a motion detector detects the relative change in motion between the stall sensor and the drive roller element and sends a signal to a logic circuit and, if a predetermined value is exceeded, the power to the motor is removed. 
     The invention can be embodied in an airplane with a cargo transportation system comprising a hull with a floor with an array of conveyance rollers mounted on the floor and cargo power drive units for moving cargo on the conveyance rollers. The cargo power. drive unit comprises a translatable frame for retracting and extending the power drive unit into a cargo plane of conveyance, a motor mounted on the frame, an output shaft driven by the motor, at least one drive roller mounted on the shaft and coupled thereto to be driven by the motor to move cargo positioned above the drive roller in a plane of conveyance, a rotatable generally circular wheel frictionally coupled to the output shaft by a drag device for rotation therewith, a plurality of magnets mounted on the wheel, a Hall effect sensor, which can be mounted on the power drive unit to detect the motion of the magnets, and a logic circuit connected to the Hall effect sensor and the motor that removes power to the motor when a signal below a predetermined level is received from the Hall effect sensor. 
     Other features and advantages of the present invention will become apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The detailed description of the preferred embodiments, set out below to enable one to build and use one particular implementation of the invention, is not intended to limit the claims, but to serve as a particular example thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an illustration of the underside of an airplane; 
     FIG. 2 shows a portion of the cargo bay of the airplane of FIG. 1; 
     FIG. 3A is an isometric view of a PDU according to the present invention for moving cargo in the cargo bay of FIG. 2; 
     FIG. 3B is a top view of a second exemplary PDU construction according to the present invention; 
     FIG. 4 shows an exemplary stall sensor incorporated into the PDU of FIGS. 3A and 3B; and 
     FIG. 5 is a block diagram of the PDU of FIGS. 3A and 3B. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A generally H-shaped conveyance surface  26  forms the lower deck of an aircraft (see FIG.  2 ), adjacent a cargo bay loading door  23  (see FIG.  1 ). It is emphasized, however, that there are many aircraft cargo deck configurations to which the present invention can be applied. For example, some aircraft, particularly those configured primarily for the transportation of cargo without passengers, have the upper passenger deck removed and an additional larger cargo deck installed. Other aircraft may have three or more parallel longitudinal tracks rather than the H-shape shown in FIG.  2 . 
     The cargo surface includes a system of freely rotating conveyance rollers  27  mounted in the cargo deck (see FIGS. 1 and 2) to define the conveyance plane. Cargo loaded onto the aircraft cargo deck can be moved manually throughout the cargo bay upon the freely rotating conveyance rollers. However, it is desirable to electro-mechanically propel the loads with minimal or no manual assistance. To this end, the H-shaped cargo surface includes a number of PDUs  28 , that provide a mechanism upon which cargo is propelled over the conveyance rollers  27 . Each PDU typically includes a drive roller element which can be raised from a lowered position beneath the cargo deck to an elevated position. In the elevated position, the drive roller element contacts and drives the overlying cargo that rides on the conveyance rollers. 
     The exemplary H-shaped conveyance surface  26  includes a left track along which cargo is to be stowed in parallel columns during flight (see FIG.  2 ). As the aircraft fuselage narrows at the aircraft&#39;s tail, the cargo deck is also separated into a tail section  11  and a main section  12 , with the tail section being tilted slightly upwardly. Thus, the left and right tracks are divided into four sections, two forward sections  13  and  15  and two aft sections  17  and  19 . In addition to the four sections just described, there is an additional path  21  between both tracks at the cargo door  23  (see FIG.  1 ), at the junction of the tail and main sections  11  and  12 . This path is used to move cargo into and out of the aircraft, and also to transfer cargo between the left and right storage tracks. 
     A human operator manipulates the controls to selectively energize the PDUs  28  in each of the five aforementioned sections  13 ,  15 ,  17 ,  19  and  21 . Typically, these controls are mounted in an operator interface unit connected to a PDU power relay box by a cable. The control elements may be mounted on a wall or other structure within the cargo bay or may be in a hand held pendant. These controls will typically have an on/off switch and a joy stick which, depending on the direction pushed, will energize a set of PDUs  28 , causing groups of drive roller elements to be elevated and rotated in one of two possible directions. A section of PDUs will remain energized as long as the joy stick is held in a corresponding position. When the joy stick is released, the set of PDUs selected is de-energized and the drive roller elements are returned to their retracted position below the plane of the conveyance rollers  27 , or brakes are applied to hold the cargo containers in place. Control systems of this type as known to those skilled in the art are not described in further detail here. 
     An exemplary PDU  29  of the present invention (shown in FIG. 3A) includes a stationary base, which is mounted to the aircraft  25  below the conveyance plane  40  and a translatable frame  37 , which is pivotable on an axle  38  such that a swinging end  39  can be elevated above the plane of conveyance  40 . PDUs  29  are typically low profile rectangular arrays with a frame that can be easily mounted within and removed from the aircraft cargo deck. The translatable frame supports a motor  41 , reduction gears  43 , a stall sensor  44 , drive roller elements  45  for contacting and propelling cargo, and a cam element  47  for moving the frame toward the plane of conveyance  40 . Also mounted on the translatable frame  37  is a motion detector  73 , which is in electronic communication via a wire  72  with a logic circuit  71 . The logic circuit  71  may be mounted elsewhere, depending on design preference. 
     When energized, the PDU motor  41  applies torque to the reduction gears  43 . The torque provided to the drive roller elements  45  by the reduction gearing  43  is thus applied to a rockable shaft  46  on which cam element  47  rides. A torque control device  42  is used to transmit a portion of the motor torque to the lift mechanism. As a result, a fraction of the torque from the motor is passed almost entirely to the rockable shaft  46 , causing the cam element  47  to rotate, and to translate the frame  37  toward the conveyance plane  40 . This elevating movement of the frame  37  is explained in detail in U.S. Pat. No. 5,661,384, which is incorporated herein by reference. The PDUs  29  are mounted close enough to the plane of conveyance  40  so that only a short stroke is needed to bring the stall sensor  44  and the drive roller element  45  into a position where they can contact the underside of the cargo. 
     Once drive roller element  45  has been elevated to contact heavy cargo, or the swinging end  39  has reached the limit of its travel, the drive roller elements rotate in the commanded direction. As explained below, the torque from the motor, which also drives the drive roller element  45 , is imparted to a stall sensor  44  by a drag device  65  (see FIG. 4) allowing the sensor to rotate with the drive roller element when the drive roller element is not in contact with cargo or when the cargo is moving. Because of the gear configuration, the drive roller element is adapted to rotate either clockwise or counter-clockwise about its central axis, depending upon the direction of motor shaft  48  rotation (see FIG.  5 ). 
     When the motor  41  is in operation and its output direction is reversed, the cam element  47  rotates and translates frame  37  so that the PDU  29  is lowered from the plane of conveyance  40  to a retracted rest position. The reduction gears  43  are then ready for reverse motion and some of the torque from the motor is passed to the rockable shaft  46 , causing the cam element  47  to rotate, and move the frame  37  toward the conveyance plane. 
     The exemplary stall sensor  44  is in the form of a spoked wheel  61 , the diameter of which is slightly larger than the drive roller element  45  (see FIG.  4 ). An outer surface of the spoked wheel is formed by a layer of a material with a relatively high coefficient of friction. Optionally, the outer surface may be formed by an elastomeric O-ring  63  disposed within a groove on the outer circumference of the spoked wheel  61 . A soft deformable component, such as an elastomeric O-ring or a tire, is preferable because it absorbs the impact of cargo placed on the sensor and it compensates for varying drive roller element sizes resulting from manufacturing variances or wear. 
     Forming an inner circumferential surface of the sensor wheel  44  is a drag device. This device may be a second elastomeric O-ring  65  held within an inner groove along the inner circumference of the wheel  61  or other device that is of a shape, material and dimension for generating a predetermined frictional drag between the sensor wheel and the motor output shaft  48 . 
     When the PDU motor  41  is activated, the drive roller element  45  is driven directly by the motor output shaft  48 , while the drag device  65  transmits a predetermined frictional drive force between the stall sensor  44  and the output shaft, resulting in the stall sensor being driven at substantially the same rotational speed as the drive roller element. A motion detector  73  housed on the PDU  29  detects any relative rotational motion between the stall sensor  44  and the drive roller element  45 . A plurality of magnets  67  are equally spaced along a side surface of the sensor wheel  44 . Adjacent to the stall sensor  44  and mounted on the frame  37  is the motion detector  73 , preferably a Hall effect sensor. When the stall sensor is in motion, the magnets rotate about the motor output shaft  48 . Preferably, the motion detector is mounted on the translatable frame aligned with the path of the magnets so as to sense a magnet passing in front of it. A signal corresponding to the rate at which magnets pass in front of the Hall effect sensor is sent to a logic circuit  71 . From this signal, the relative distance traveled, acceleration or velocity may be computed. 
     The PDU motor  41  is attached to an output shaft  48 , which in turn is coupled to two drive roller elements  45 . The motor  41 , when activated, rotates the output shaft  48  which rotates the drive roller elements  45 . Also attached to the shaft  48  is a stall sensor  44  (shown adjacent to a drive roller element for illustration only) A drag device  65  forms part of the stall sensor  44  and couples the sensor to the output shaft  48  to conditionally impart rotational motion to the sensor (see FIG.  5 ). 
     In operating proximity to the sensor  44  is a motion detector  73  that detects the speed of rotation of the sensor. The motion detector in turn is connected to logic circuit  71  that may be programmed to limit the operation of the PDU  29  if one of several predetermined conditions are detected. These conditions may be transmitted to a monitor or otherwise displayed to the operator of the PDU. 
     In operation, the PDU  29  is activated and the output shaft  48  drives the drive roller element  45  and the stall sensor  44 . When the drive roller elements  45  are in contact with cargo, the cargo is driven by the drive roller elements, while the stall sensor  44  also contacts the cargo and continues to be driven by the motor output shaft  48 . The cargo may stop moving in the commanded direction if: (1) the drive command is removed (the PDU is deenergized); (2) the cargo encounters an abutment within the aircraft; (3) the cargo above the PDU contacts other cargo that is stationary; or (4) the cargo stalls due to a jam condition. If any of the latter three conditions occurs, the drive roller elements  45  continue to rotate against the bottom of the cargo (scrub), but the stall sensor stops rotating because the frictional force at the cargo surface, overcomes the torque provided by the drag device  65 . 
     When cargo stops moving and the stall sensor  44  consequently stops rotating, the rotational velocity of the stall sensor falls below a predetermined value which is detected by the motion detector  73 . The system logic circuit  71  then shuts off the power to the motor  41 . This logic circuit  71  can be located in either the PDU  29  itself or in a central system controller. This logic circuit  71  allows the operator to attempt to clear jammed cargo by reversing the drive command while preventing PDUs that have been shut off by the stall sensor from resuming operation under parked cargo. The logic circuit can, if desired, be programmed so that the motor output shaft  48  cannot resume rotation unless it is commanded to operate in the opposite drive direction first. 
     It should be noted that variations in the bottom surface of the cargo, which are common, cannot generate a false jam signal. This is because the sensor is not driven by the cargo, but is only stopped by the cargo. 
     The logic circuit  71  can be programmed to ignore delays associated with powering up the PDUs  29 . A three second delay is generally sufficient to allow the series of PDUs to be elevated from their non-operational position and for the drive roller elements  45  to begin rotating. If it is desired to reverse the direction applied to the drive roller elements, then another delay, representing the amount of time needed for the drive roller elements to stop moving, for the PDU to lower and then raise, and for the PDU to start rotating in the opposite direction, is typically needed. 
     An alternate PDU construction wherein the motion detector  73 , reduction gears  43 , cam element  47 , and motor  41  are obscured by a covering  51  and the logic  71  is remotely connected is shown in FIG.  3 B. It includes a stall sensor  44  coaxially mounted on an output shaft  48  (shown in broken lines in FIG. 3B) adjacent a drive roller element  45 . In this alternative exemplary embodiment, the stall sensor  44  can be mounted adjacent and in frictional contact with a contact face  81  on a drive roller element  45 . Preferably, the stall sensor and the drive roller element create sufficient drag for the stall sensor  44  to rotate with the drive roller element when there is no cargo on the PDU  29 . Such an arrangement requires that at least one of the surfaces in contact with the cargo have a surface capable of creating the necessary friction. This configuration eliminates the need for a drag device that engages or is driven through the output shaft  48 . This particular configuration requires that the friction at the cargo surface when the stall sensor contacts the cargo be greater than the friction supplied by the stall sensor&#39;s contact with the drive roller element when jammed cargo is on the PDU. Such an arrangement would work in the same manner as illustrated in the block diagram of FIG.  5 . 
     It will be understood by those skilled in the art that the stall sensor may be positioned in a variety of ways as long as it is rotatably coupled to the output shaft. Having the stall sensor mounted in close proximity to a drive roller element is advantageous because the drive roller elements can then best protect the stall sensor from damage by the stall cargo. It is preferred to have the diameter of the stall sensor and the drive roller elements substantially equal such that all items contacting the drive roller elements contact the stall sensor. 
     While a particular form of the invention has been illustrated and described, it will be apparent that various modifications can be made without departing from the spirit and scope of the invention. Accordingly, it is not intended that the invention be limited, except as by the appended claims.