Patent Publication Number: US-6213713-B1

Title: Apparatus for indicating pitch angle of a propeller blade

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention is generally directed to the detecting and measuring of a higher order degree of rotation of an articulated member undergoing multiple degrees of rotation. More particularly, the present invention is directed to the detecting and measuring of the pitch angle of a propeller blade. The first degree of rotation is defined as the blade rotation about the hub axis and is commonly known as propeller revolutions per minute (rpm). The second degree of rotation is defined as blade rotation about the longitudinal axis of the blade and is commonly known as pitch angle. 
     2. Prior Art 
     Rotating blade assemblies have long been used to convert rotating shaft power into useful work. One specialized and well known blade assembly is a propeller. A propeller normally comprises a hub rotated by a shaft. The hub usually has at least two propeller blades protruding from the hub such that the blades are orthogonal to the shaft. Each blade has an aerodynamic profile and is rotated through a fluid medium such that each blade generates a ‘lifting’ force based on the Bernoulli principle. An important parameter in determining the amount of thrust generated by the propeller is the angle of attack of the blade. The angle of attack is critical in application of the Bernoulli principal because it determines the difference between useful work being performed or a stalled blade where no useful work is performed. It is well know in the art that without the proper angle of attack, increasing propeller revolutions per minute will not increase the amount of useful work output. Therefore the angle of attack is a critical parameter to be known in an operating propeller. The pitch angle of a propeller blade is that angle between the chord of the blade and the plane it rotates in about the hub. It is a very useful parameter in determining the angle of attack. 
     Early propellers were designed with each blade having a fixed pitch angle. Although the blade rotated about the hub at varying propeller RPM, its pitch angle remained constant. It soon became clear to the early inventors that due to the constantly changing environment a propeller operated in, a fixed pitch propeller was not a very efficient solution. Thus arose the impetus for inventors to discover ways to vary the pitch of each blade while the propeller was spinning. Once variable pitch propellers were discovered, it greatly changed the way propellers are used and controlled. Pitch angle is now varied dynamically for a variety of reasons. For example, when a power plant fails and shaft power is reduced significantly or eliminated, a fixed pitch propeller will create a large drag or negative thrust. However a variable pitch propeller can be ‘feathered’ so that each blade is rotated until its chord is parallel to the shaft axis and therefore the blades generate only a small parasitic drag. Blade angle can also be varied to increase or reduce thrust without varying propeller rpm, i.e., a constant speed propeller. In some propeller assemblies pitch angle can be varied so that the blades generate reverse thrust, also known as the ‘beta’ range. 
     An operator or control system is normally used to command the pitch angle of a propeller blade in an operating environment. Simultaneously, the operator or control system usually requires some feedback as to what the blade pitch angle is. Initially, feedback was a position indication of a control arm driving the mechanical linkage to alter pitch angle. This was actually an ‘open loop’ system in that the operator adjusted a power setting and a propeller rpm setting without truly measuring a pitch angle. The control system would increase or decrease blade pitch to achieve the commanded propeller rpm setting. Thus the true blade pitch angle was unknown. 
     Variable pitch propeller systems typically incorporate a hub which encloses a chamber within its interior wherein a pitch angle change actuation system is disposed in operative association with the propeller blades. The actuation system functions to selectively change the pitch angle of the blades thereby altering the lift and drag characteristics of the propeller blades. 
     In most modern aircraft, the pitch change actuator is of the hydromechanical type wherein an output member, typically a piston, is driven in response to adjustments in the pressure of the hydraulic fluid which drives the actuator. The adjustments in fluid pressure are typically affected by either a hydromechanical or electronic control system which monitors engine speed and causes, by way of collateral apparatus, a change in fluid pressure whenever the monitored engine speed departs from the desired engine speed setting. 
     Such hydromechanical pitch change actuation systems are well known in the art. For example, commonly assigned U.S. Pat. No. 4,523,891 to Schwartz and Duchesneau discloses a conventional pitch actuation system wherein each propeller blade is operatively connected to a piston which is driven by the pressure of a fluid which is selectively directed in response to a departure from desired engine speed against the opposite faces of the piston thereby causing a linear displacement of the piston and a resultant change in pitch of the blades operatively connected to the piston. The piston is reciprocally moveable within a cylinder disposed within the hub about a torque tube which extends from the fluid supply to the piston. As shown in U.S. Pat. No. 4,523,891, the pressure fluid is conveyed through a conduit within the torque tube from the fluid supply to a valve associated with the piston which, depending upon its position, selectively directs the fluid against either the front or the rear face of the piston thereby causing the piston to move linearly thereby rotating the blade or blades associated therewith to effectuate the change in pitch. 
     In the foregoing systems, however, there is no measurement of the actual blade pitch angle. The present invention provides a novel method and apparatus to solve this problem. 
     SUMMARY OF THE INVENTION 
     In accordance with a preferred embodiment of the present invention, an apparatus for detecting the pitch angle of a propeller blade comprises a system of a magnetic sensor having a centered magnetic pole piece and two remote magnetic transmitters. The transmitters are strategically located so as to identify a first fixed reference point on the propeller hub and a second movable point on the propeller blade. The magnetic sensor is mounted on a non-moving reference such as the engine housing. The magnetic sensor detects a magnetic pulse from each magnetic transmitter as they rotate on the propeller assembly and pass by the magnetic sensor. The fixed magnetic transmitter, although spinning with the propeller hub, does not move relative to the hub itself because it is mounted on a bulkhead plate which is fixed to and rotates with the hub. Each time the fixed magnetic transmitter passes by the sensor it generates a reference signal which is relayed to a signal processor from the sensor. After the fixed transmitter passes by the sensor, the movable transmitter passes by the sensor generating a second signal which is also relayed to the processor. The movable transmitter moves with the propeller blade as it rotates about the pitch axis in such a manner so as to change the angular distance between the two transmitters. This change, either an increase or decrease, in angular distance between the two transmitters, causes a corresponding change in the relative arrival time of the movable transmitter magnetic pulse at the sensor given a propeller spinning at a constant rate. The signal processor receives the pair of signals and determines the pitch angle of the propeller blade from the difference in the time of arrival of the two signals. 
     The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
     FIG. 1 is a cutaway view of a propeller hub assembly illustrating the propeller blade pitch angle detector; 
     FIG. 2 is a side elevation view of a propeller blade illustrating the grooved ring; 
     FIG. 3A is a cross-sectional view of FIG. 1 taken at line C—C illustrating a propeller blade, a sliding magnetic target, and a magnetic sensor having a centered magnetic pole piece; 
     FIG. 3B is a top plan view of a sliding magnetic target illustrating a head and tail; 
     FIG. 4A is a front elevation view of the bulkhead plate illustrating various positions of the sliding magnetic target; and 
     FIG. 4B is a timing diagram illustrating the relative positions of the fixed magnetic target and the sliding magnetic target during various propeller blade pitch angle positions. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a cutaway view of a propeller hub assembly  10  illustrating some of the components of the propeller blade pitch angle detector is shown. A propeller blade  1  is captured and rotatably held by a propeller hub assembly  10  in a manner well known in the art. As the propeller hub assembly  10  is spinning about the propeller axis  80 , the propeller blade  1  is caused to rotate about a pitch axis  82  through a range of various pitch angles, from feather to reverse, also well known in the art. A magnetic sensor  30  having a centered magnetic pole piece  33  is mounted at a stationary position on the engine frame  31  so as to be in close proximity to a bulkhead plate  20 . The bulkhead plate  20  which is attached to the propeller hub assembly  10  spins with it about propeller axis  80 . A sliding magnetic target  40  has an overall T-shape with it&#39;s head  41  towards the magnetic sensor  30  and its tail  42  slidably captured in a grooved ring  60 . A through-slot  21  is cut in the bulkhead plate  20  to allow the sliding magnetic target  40  to move in an up and down direction with the grooved ring  60  as described in greater detail below. 
     Referring to FIG. 2, a side elevation view of a propeller blade  1  illustrating a grooved ring  60  is shown. The grooved ring  60  is spirally mounted on the propeller blade  1  such that the left end  61  is approximately one inch higher than the right end  62 . The grooved ring  60  is fixed to the propeller blade  1  so that when the propeller blade  1  rotates about the pitch axis  82 , the tail  42  of the sliding magnetic target  40  is driven either up or down depending on the direction of rotation by the propeller blade  1 . Thus the rotational motion of the propeller blade  1  as it varies the pitch angle  90  is converted to a translational motion in the sliding magnetic target  40  as described in greater detail below. 
     Referring to FIG. 3A, a cross-sectional view of FIG. 1 taken at line C—C illustrating a propeller blade  1 , a sliding magnetic target  40 , a magnetic sensor  30  with a centered magnetic pole piece  33 , and various blade pitch angles is shown. The grooved ring  60  is mounted on the circumference of the propeller root  2  (see FIG. 2) and extends approximately half way around the propeller blade  1 . When the propeller blade  1  rotates either clockwise or counterclockwise as viewed in FIG. 3, the grooved ring  60  drives the tail  42  of the sliding magnetic target  40  into or out of FIG.  3 . Rotation of the grooved ring about the second axis  82  causes translation of the tail  42  along the second axis. It should be noted that the pitch axis  82  is perpendicular out of FIG. 3 at point A. 
     Referring to FIG. 3B, a top plan view of a sliding magnetic target  40  illustrating a head  41  and tail  42  is shown. The head  41  is composed of two parts: a ferrous metal bar  45  and an H-shaped section  46  which has two symmetrical and opposing head slots  43 . Each head slot  43  receives and slides on the edge of a target retainer plate  50  as described in greater detail below. The ferrous metal bar  45  is made from a suitable magnetic material so that it has uniform magnetic properties distributed from a first end  48  to a second end  49 . As can be seen by close inspection, the side edge of the first end  48  is visible while the side edge of the second end  49  is not. This is because the ferrous metal bar  45  is not co-planar with the rest of the sliding magnetic target  40 . In fact it is this angle which generates the difference in the magnetic signal detected by the magnetic sensor  30  when the ferrous metal bar  45  passes by. This offset angle is more self evident in FIGS. 4A and 4B. 
     Referring to FIG. 4A, a front elevation view of the bulkhead plate  20  illustrating various positions of the sliding magnetic target  40  is shown. A through-slot  21  has been stamped or cut in the bulkhead plate  20 . A U-shaped target retainer plate  50  is held in place in line with the through-slot  21  by four fasteners  51  on a retainer support member  54  (see FIG.  1 ). The target retainer plate  50  has a retainer plate slot  52  in which the sliding magnetic target  40  is slidably retained. Each head slot  43  of the sliding magnetic target  40  captures and slides up and down on an inside edge  53  of the U-shaped target retainer plate  50 . Three positions of the sliding magnetic target  40  are shown. When the pitch angle  90  of the propeller blade  1  is in reverse, the sliding magnetic target  40  will be in the upper-most position  71 . When the pitch angle  90  of the propeller blade  1  is in mid-range, the sliding magnetic target  40  will be in the middle position  72 . When the pitch angle  90  of the propeller blade  1  is in feather, the sliding magnetic target  40  will be in the lower-most position  73 . 
     Referring to FIG. 4B, a timing diagram illustrating the relative positions of the fixed magnetic target  22  and the sliding magnetic target  40  during various propeller blade pitch angle  90  positions is shown. The sensor arc M is defined as the spatial path which the fixed magnetic target  22  follows as it rotates on the bulkhead plate  20 . The sensor arc M is also required to continually pass by the magnetic sensor  30  (not shown) and therefore for purposes of FIG. 4B, it may be considered that the magnetic sensor  30  lies on the sensor arc M. 
     Pitch angle  90  of the propeller blade  1  is calculated by the time interval detected between the output signal pulses which are generated as the fixed magnetic target  22  and the sliding magnetic target  40  pass by the magnetic sensor  30 . As described above the magnetic sensor  30  has a centrally located magnetic pole piece  33 . The sensitivity of the magnetic pole piece  33  is such that it only detects that portion of the magnetic bar  45  which lies on the sensor arc M. A reference radius R 0  indicates the origin of each of the timing arcs T 1 , T 2 , and T 3 , which correspond respectively to the sliding magnetic target  40  in its upper-most position  71 , middle position  72 , and lower-most position  73 . It is important to note that the magnetic bar  45  of the sliding magnetic target  40  is angled, approximately 45°, with respect to a tangent (not shown) of the sensor arc M at the various points where the sliding magnetic target  40  crosses the sensor arc M. In other words, the tail  42  has a longitudinal axis generally perpendicular to the axis  82  and the head  45  has a longitudinal axis that is at an oblique angle relative to the axis  82 . Thus, as the sliding magnetic target  40  moves radially in and out from the center of the bulkhead plate  20 , that portion of the magnetic bar  45  lying on the sensor arc M changes in radial position to R 1 , R 2 , and R 3 . As long as the propeller is spinning at a relatively constant speed, then a signal processor can perform a calculation that will map the time interval detected T 1 , T 2 , or T 3  into a pitch angle. 
     The present invention provides an effective system for determining propeller pitch angle. Although the system has been described as using a magnetic sensor and magnetic transmitters, it is understood that other sensors and transmitters may be used and the invention is not limited to magnetic devices. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustrations and not limitation.