Abstract:
An apparatus is provided for determining the position of a shaft that is rotatably displaceable about a longitudinal axis. The apparatus includes a source of light energy, a light detecting device arranged adjacent the shaft. A rotatably displaceable disk having a triangular opening in the disk is mounted between the source of light energy and the light detecting device. A position-determining circuit connected to the light detecting device measures the magnitude of light that the light detecting device reads from the light emanating from the disk opening and applies the output to a position-determining scheme.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to position determining devices and more particularly to an apparatus for determining the absolute position of a rotating shaft. 
     It is a common problem to want to know the position of a device whose position is being controlled by an actuator or some other means. For example, in the controls industry, devices such as valves, each having a valve stem or valve shaft which is rotatable by an actuator, are typically used to control the flow of liquids or various gasses associated with the industrial process. In these applications, it is a common need to know the precise absolute position of the controlled shaft or stem. This information allows for an improved understanding of the process and, subsequently, a more accurate control of the process by a process control system. 
     A number of prior solutions are known for measuring or determining this aforementioned position. One method is the use of linear variable differential transformers which can provide accurate positional information. However, they require a mechanical linkage to translate the positional information to a sensor and, additionally, consume a relatively high amount of power in their operation. 
     Slidewire, potentiometers, or other rotary transducers again require a mechanical link to the controlled device and also have the disadvantage of a sliding electrical contact which can cause long-term unreliability as well as having a potential for producing arcing and/or sparking, precluding the use of these devices in volatile environments. 
     Hall effect transducers, as they are currently used, generally require a mechanical linkage and are generally susceptible to ambient magnetic fields, electrostatic discharge, and other Electro-Magnetic Interference (EMI). 
     A more efficient form of solution is taught by my U. S. Pat. No. 5,828,060, which issued on Oct. 27, 1998 and is assigned to the same assignee as the present application. 
     Additionally, all of the aforementioned devices and methods require the use of extensive electronic circuitry to convert the predominately analog positional information to the digital signals normally required by modern computer-controlled industrial process control systems. 
     Therefore, it is the object of the present invention to provide a reliable position-determining apparatus that does not require a mechanical linkage between the apparatus and a rotatable shaft. 
     It is a further object of the present invention to provide a position-determining apparatus that has long-term reliability, is immune to EMI, and is cost effective. 
     It is a further object of the invention to provide a position-determining apparatus that exhibits the benefits of a greatly reduced parts count when interfacing the resultant positional signals to a computer-controlled industrial process control system. 
     SUMMARY OF THE INVENTION 
     The apparatus of the present invention contemplates the use of a light detecting device, and a rotatable disk having a single opening through the disk, where the rotatable disk is positionable between the light detecting device and a source of light energy. The center axis of the rotatable disk is aligned along the longitudinal axis of the shaft or valve stem of a suitable rotating device whose position is to be measured. 
     The rotatable disk includes an extended isosceles triangle having two curvilinear sides located on the disk in circumferential relationship to the shaft to be measured. The light detecting device responds to the magnitude of the distance between the two long sides of the triangle which increase in width longitudinally as the disk mounted upon the shaft is rotated. 
     The apparatus of the present invention includes a source of light energy which provides a constant source of light. The light detecting device is positionable in a first position adjacent the rotatable shaft and along a rotational center line of the disk where the aforementioned opening is located. The light detecting device is positioned opposite the light source on the opposite side of the disk arranged to produce a detection signal responsive to the detection of light energy. 
     The disk opening allows light from the light source to fall on the light detecting device, as the disk is rotated in a clockwise direction. The amount of light energy passed by the opening provides a form of modulation to the light detecting device. As the disk is rotated, the opening allows light to fall on the light detecting device, which produces the detection signal. As the disk is further rotated, the light detecting device receives more light. Upon further continued clockwise rotation of the disk, the opening allows more and more light from the light source to fall on the light detecting device, producing a detection signal of increased magnitude. 
     The signals are read by a shaft position-determining scheme whereby the signals are used to calculate the position of the shaft based on the amount of light measured by the light detecting device. 
     Thus, there is provided an apparatus for determining the position of a rotatable shaft and which offers long-term reliability, is immune to EMI, and is cost effective in its operation. The present invention requires sensed analog positional information to be translated into digital information, which is typically required by modern computer-controlled industrial process control systems. 
     It is noted that the present apparatus is a non-contact feedback mechanism with an assumed long-life expectancy and is immune to ambient magnetic fields while inherently self-correcting for drift in the analog parts due to temperature changes or aging of components. 
     As noted previously, the light source and the light detector are both positionable adjacent to the rotating disk. This positioning occurs by virtue of their being attached to a solenoid mechanism which is controllable from a central point to place the light source and the detector adjacent to the disk or withdraw both elements from their position adjacent to the disk until such time as measurement is required. Control of the solenoid would normally be under the control of the computer to which the read out of the light detecting device is applied as taught by the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention may be had by reading the following detailed description in conjunction with the drawings in which: 
     FIG. 1 shows the position detection apparatus of the present invention with the detection apparatus positioned adjacent to a rotatable valve shaft or stem. 
     FIG. 2 shows the rotating disk as used in the present invention. 
     FIG. 3 shows the position detection apparatus of the present invention withdrawn from proximity to the valve having a rotatable valve shaft or stem. 
     FIG. 4 shows a position determining circuit of the present invention used to calculate the position of the rotatable valve shaft or stem. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A typical valve  10 , where the present invention is used to advantage, is shown in FIG.  1 . Valve  10  is comprised of a valve body  15 , suitably connected on one end to a conduit  17  and on an opposite end to a conduit  18 . A valve shaft  11  extends through valve body  15  and includes an opening or orifice  14  extending through the shaft. The valve shaft  11  is disposed to be rotatably displaceable along a longitudinal axis shown as  12 , in the directions shown as  13 . Typically, the valve shaft  11  controls the flow of a controlled medium such as a liquid, or a gas, from one side of the valve body  15  and conduit  17  to the opposite side of the valve body  15  and conduit  18 . While the present invention is described as used with a rotating valve, its use is not so limited. 
     The method in which valve shaft  11  controls the flow of a controlled medium will now be explained. This explanation will be made with the flow of the controlled medium moving in the direction shown by arrow  19 , or from conduit  17  to conduit  18 . As will be understood by those skilled in the art, the controlled medium can also flow in the opposite direction, from conduit  18  to conduit  17 . Valve shaft  11  will operate in the same manner with flow in either direction and is not limited thereto. The flow is controlled by rotating valve shaft  11  in either direction  13 . When valve  10  is in a closed position, a solid surface of valve shaft  11  is presented to the normal direction of medium flow. Rotational displacement of the valve shaft  11  will progressively move the solid surface away from the medium allowing opening  14  to proportionally open from a partially-open position to a fully-open position. 
     It will be understood by those skilled in the art that when the solid surface of valve shaft  11  is presented to the controlled medium, the flow of the medium is effectively stopped from flowing through valve shaft  11  to conduit  18 . Rotational displacement of the valve shaft  11  thereby presenting opening  14  to the medium, allows the medium to flow through the valve shaft  11  and to conduit  18 . The amount of flow across the valve shaft  11  is controlled by the amount of opening  14  that is presented to the medium. A small presentation allows only a small amount of flow to occur, while the full presentation of opening  14  would allow a maximum amount of the controlled medium to flow. Therefore, the amount of flow between conduit  17  and conduit  18  is directly proportional to the rotational displacement of valve shaft  11 . Valve shaft  11  can be either manually rotated by hand, or connected to an actuating device (not shown) which can rotatably displace valve shaft  11  responsive to positioning signals from a process control system. 
     It is desirable within the environment of a process control system to know at any given time the precise position of the valve shaft  11 . Additionally, it is also desirable to be able to monitor the rotational displacement of valve shaft  11  as an actuator moves or displaces the valve shaft  11 , for example, as when the flow of the controlled medium is required to be increased or decreased under control of the process control system. Further, many valve actuating devices require the precise position of the valve shaft be known as it is displaced. This position is normally communicated to the valve actuating device via a feedback signal. This feedback signal is used to gauge the progress of the valve shaft rotation in order to either increase or decrease rotation. This minimizes the over or under displacement of the shaft, or as it is more commonly known in the industry, the overshoot or undershoot, respectively. 
     The valve  10  just explained and shown by FIG. 1 is an example of the environment where the present invention can be used to advantage. It will be appreciated by those skilled in the art that the present invention can also be effectively used in other industrial control functions, such as to control furnace dampers and is not limited thereto. 
     With continued reference to FIG. 1, and also with reference to FIG. 2, the apparatus for determining the position of a rotating shaft of the present invention is shown generally as  30 . The apparatus  30  includes stationary light detecting device  31 . As can be seen and understood, any rotatable displacement of valve shaft  11  will also displace light opening  43 . Light detecting device  31  is any type of the currently-known devices, such as phototransistors, optical detectors, infrared detectors, and the like. These devices typically provide an output signal when light rays of a particular wavelength or spectrum are detected. The selection of the type of light detecting device used is dependent on the light source used, i.e., Light Emitting Diode (LED), infrared, or incandescent light and for purposes of this invention may be light detecting devices that detect light from any of the above-mentioned sources. The light detecting device  31  further includes a signal lead  37 , which is connected to the position-detection circuit of the present invention. 
     A light directing disk  40  is rotatably mounted in front of valve shaft  11  and light detecting device  31 . Light directing disk  40  is constructed of an opaque material and is mounted to a small electric motor  41  via the electric motor&#39;s shaft  42 . Motor  41  is used to rotate shaft  11 . The longitudinal axis of electric motor shaft  42  and the concentric center of disk  40  are aligned along the longitudinal axis  12  of valve shaft  11 . Disk  40  further includes an opening  43 , which traverses through disk  40 . Opening  43  is located on disk  40  adjacent the perimeter edge of disk  40  in a position where, as the disk  40  is rotated, opening  43  will allow different amounts of light from source  51  to fall on light detecting device  31 . 
     As noted above, the cutout  43  is located around the circumference of disk  40 . When the disk is attached to the shaft, it will rotate. With the light source  51  and the light receiver  31  positioned on either side of the disk  40  as shown, light from the LED  51  shines through the triangular slot  43 . The amount of light which reaches the photodetector  31  is directly proportional to the rotation angle of the shaft because the wider part of slot  43  will pass more light than the narrower parts do. The output of the photodetector  31  is then sent to a position detection circuit which includes and analog-to-digital converter. 
     As noted, the light source  51  and the light detected  31  are both connected to solenoid  61 . The solenoid under control of the position detection circuit  70  is periodically actuated whereby the solenoid is retracted periodically moving the light source  51  and the light detector  31  away from disk  40 . Accordingly, when the solenoid is retracted, the LED  51  can shine on the photodetector  31  directly. This reading can be used to indicate “span”. In that manner, drift in the components is automatically compensated for. 
     Y in =Reading of A to D converter when solenoid is extended. 
     V ref =Reading of A to D converter when solenoid is retracted. 
     P=Position of the shaft. 
     P max =Maximum rotation of the shaft. 
     
       
         
           V 
           in 
           /V 
           ref 
           =P/P 
           max 
         
       
     
     The apparatus of the present invention further includes a light source, which in this embodiment is shown to be annular. Light source is comprised of an LED  51 . It should be understood that the form of light source  50  is only shown as an example to help understand the concepts of the present invention. It will be well understood by those skilled in the art that other forms or types of light sources can be substituted for the form and type of light source shown and the present invention is not limited thereto. 
     Light detecting device  31  is mounted adjacent to valve shaft  11 . A rotation of valve shaft  11  along its longitudinal axis  12  would be translated by the width of the beam of light transmitted through opening  43  in disk  42  from light source  51  to light detecting device  31 . Therefore, the location or position of valve shaft  11 , at any one time, can be calculated by measuring the amount of light present at light detecting device  31 . 
     Turning now to FIG. 4, a position-determining circuit used to calculate the position of the rotatable valve shaft  11  is shown. As can be seen, an output signal from the device  31  will cause an output signal. The output of device  31  is connected to the input of an analog to digital (A-D) conversion device  69  that in turn connected to the microprocessor  70 . Device  70  operates in association with analog to digital conversion device  69 . A Read Only Memory (ROM)  71  is associated with microprocessor  70  and is used to store the operating program or processing instructions used by the microprocessor  70 . A Random Access Memory (RAM)  72  is also associated with microprocessor  70  and is used as a memory store for the digital representation of the position of the valve shaft  11 . 
     It is contemplated that the present invention will be used with a process control system (not shown) that includes a Control System BUS  91  that is connected to microprocessor  70  via communications BUS  92 . The process control system will from time to time poll microprocessor  70 , operate the solenoid  61  and request transmission to the process control system of the stored digital data representing the position of the valve shaft  11 . However, it will be understood by those skilled in the art that microprocessor  70  could also be connected to the local controller of a valve actuating system, thereby providing feedback signal representing the valve&#39;s position as the valve is rotated to a desired position. Further, ROM  71  and RAM  72  could also be integral and an internal component of microprocessor  70  as is commonly found in the class of devices called microcontrollers. ROM  71  and RAM  72  are shown here external to device  70  to better explain the way in which the invention is used to advantage. 
     With renewed reference to FIGS. 1,  2  and FIG. 4, an explanation of the operation of the position-determining circuit as used to advantage in the present invention will now be made. 
     As disk  40  rotates, for purposes of this embodiment in a clockwise direction, it will encounter light detecting device  31  first. When the light beam emanating from source  51  (through disk  40  opening  43 ) strikes light detecting device  31 , light detecting device  31  turns on, causing it to  69  send its output signal to the input of microprocessor  70 . Microprocessor  70  then polls its I/O ports for an input signal. Microprocessor  70 , upon detection of this detection device signal, operates an included A-D device  69  to gain a digital output. 
     Under control of an operating routine, the apparatus can measure, calculate, and update shaft travel on a periodic basis replacing the old data with new data. Alternatively, the position of the shaft  11  can be determined on a demand basis or when requested by a process control system. When the process control system issues a request for the latest value of shaft travel, the data is transferred via communication BUS  92  to control system BUS  91  and to a central controller of the process control system. The process control system can then translate the value received into an absolute position of the valve shaft position for display to a human operator or used as process variable in a process controlling routine. It will be appreciated by those skilled in the art that the value of shaft travel measured and calculated by the present invention can also be output to the controller of a valve shaft actuating device where it is used as a feedback signal indicating the present position of the shaft as it is being rotated by the actuator. 
     It must be understood that an alternative way to manufacture the disk is to print a linearly increasing grayscale on a clear disk which would function much the same manner passing minimum and maximum amounts of light depending upon the amount of gray present. While there may be certain economic advantages to this approach, it may be possible it could be affected by the accumulation of dirt or other clouding effects on the disk itself. 
     The present invention has been described with particular reference to the preferred embodiment thereof. It will be obvious that various changes and modifications may be made therein without departing from the spirit and scope of the invention as defined in the appended claims.