Abstract:
A hydraulic actuator is disclosed having a cylinder with a piston that is moved by hydraulic fluid. A light guide in one end of the cylinder directs a laser beam into the cylinder, and off the piston where the beam is reflected. The beam then exits the cylinder through at least two light guides connected to two corresponding optical fibers. Each of the optical fibers are joined together into one fiber that carries the reflected beam of light to a photo-diode located remote from the cylinder. A control circuit measures the time of flight of the laser beam.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority from U.S. Ser. No. 09/750,866 which was filed in the United States Patent and Trademark Office on Dec. 28, 2000 and is entitled “Laser Based Reflective Beam Cylinder Sensor”. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The invention relates generally to position sensing of hydraulic and pneumatic actuators. More particularly, it relates to sensing using laser light sources and detectors and determining the position of the actuator using time-of-flight algorithms.  
         BACKGROUND OF THE INVENTION  
         [0003]    Position sensing for hydraulic or pneumatic actuators typically uses an external position sensor, such as a rotary rheostat or potentiometer. Alternatively, linear rheostats or variable differential transformers are employed. These systems suffer from poor accuracy, extensive wear, and fragility in many applications, especially demanding applications such as their use on work and agricultural vehicles.  
           [0004]    These sensors are quite susceptible to damage, and suffer from being damaged during vehicle operation, or from the extremes in temperature that work and agricultural vehicles face.  
           [0005]    In an effort to solve these problems, new methods of measuring the position of a hydraulic or pneumatic actuator have been devised that use microwaves. These waves are transmitted from one end of the cylinder, reflect off the piston, and return to a detector. By measuring the time of flight of these waves, the location of the piston can be determined. Such an example is shown in U.S. Pat. No. 6,005,395.  
           [0006]    The microwave transmitter suffers from high cost and difficulties in determining which of the many reflections in the cylinder is the proper one to measure.  
           [0007]    In an alternative system, the pulse generating and timing circuit of U.S. Pat. No. 6,005,395 are used, but drive a laser light source and responds to a reflection of that beam against a laser light detector, such as that shown in co-pending U.S. patent application Ser. No. 09/750,866.  
           [0008]    This arrangement also has drawbacks. When the piston moves toward or away from the source and detector, the reflected light follows multiple paths that, like the microwave transmitter and receiver pair, make the reflected pulses difficult to interpret. It is difficult to extract a good pulse indicative the precise time of flight of the laser beam.  
           [0009]    What is needed is a better arrangement of laser light source and detector that provides a more precise time-of-flight measurement. It is an object of this invention to provide such an arrangement.  
         SUMMARY OF THE INVENTION  
         [0010]    In accordance with a first embodiment of the invention, a fluid actuated cylindrical actuator is provided that includes a cylinder having first and second ends, an end cap fixed to the first end of the cylinder and having a rod opening, a piston disposed in the cylinder, a rod coupled to the piston and extending from inside the cylinder to outside the cylinder and passing through the rod opening, a first light guide extending from inside the cylinder to outside the cylinder and adapted to transmit at least a first beam of laser light at a first frequency from outside the cylinder to inside the cylinder and to bar the passage of the fluid, and a plurality of second light guides having first ends extending from inside the cylinder to outside the cylinder and distal second ends that are coupled to at least one light detector. Each of the second light guides may be adapted to substantially simultaneously transmit at least a first reflected portion of the beam of laser light from inside the cylinder to outside the cylinder.  
           [0011]    The first light guide may be disposed to transmit the first beam of laser light substantially along a longitudinal axis of the cylinder such that the first beam impinges on a reflective portion of the piston over substantially an entire range of piston travel.  
           [0012]    Each of the second light guides may be disposed on opposing sides of the first light guide such that they both receive a reflected portion of the light beam.  
           [0013]    Each of the second light guides may be disposed substantially equidistantly from the first light guide.  
           [0014]    The plurality of second light guides may include at least three light guides that are disposed in a semicircular arc about the first light guide.  
           [0015]    The second ends of the plurality of second light guides may be optically coupled to a single light detector having a single electrical output generated by light carried by at least two of the plurality of second light guides.  
           [0016]    In accordance with a second embodiment of the invention, a hydraulic actuator for an agricultural or construction vehicle is provided, the actuator including a cylinder having a substantially circular internal diameter and a longitudinal cylindrical axis, a piston having a substantially circular outer diameter and configured to be received in and hydraulically sealed against the inner diameter of the cylinder, a piston rod with a substantially circular outer rod diameter that is fixed to the piston and extends from the piston inside the cylinder, through a first end wall of the cylinder to a location outside the cylinder, wherein the first end wall is disposed to enclose and seal a first end of the cylinder and is substantially perpendicular to the longitudinal axis of the cylinder, a second end wall fixed to the cylinder and disposed to seal a second end of the cylinder substantially perpendicular to the longitudinal axis of the cylinder, the second end wall including a first optical path configured to transmit a beam of laser light through the second end wall to a reflective surface fixed to the piston and further including a plurality of second optical paths configured to transmit the reflected beam of laser light back through the end wall, a first optical fiber optically and mechanically coupled to the second end wall to transmit the beam of laser light from a remote laser light source to the first optical path and a plurality of second optical fibers optically and mechanically coupled to the second end wall to transmit the reflected beam of laser light to a remote laser light receiver.  
           [0017]    The first optical path and the plurality of second optical paths may include at least one hermetically sealed fiber optical feed-through or connector extending through the second end wall.  
           [0018]    The first optical fiber and the plurality of second optical fibers may be multi-modal optical fibers.  
           [0019]    The hydraulic actuator may also include a first photo-diode configured to receive light transmitted through at least one of the plurality of second optical fibers. The actuator may include a second photo-diode configured to receive light transmitted through at least another of the plurality of second optical fibers.  
           [0020]    The first photodiode may be disposed to receive light from at least two of the plurality of second optical fibers.  
           [0021]    In accordance with a third embodiment of the invention, a method of determining the position of the piston of the actuator described in the previous paragraph includes the steps of generating the beam of laser light, reflecting the beam of laser light off a surface fixed to move axially with the piston, receiving a first portion of the reflected beam by a first reflected light guide and a second portion of the reflected beam by a second reflected light guide, conducting the first and second portions of the reflected beam through first and second optical fibers to at least one remotely located light detector, and calculating a first time of flight of the beam based at least upon the first and second portions of reflected light.  
           [0022]    The method may include the step measuring a second time of flight of the beam by moving the piston to a second location in the cylinder while simultaneously increasing the optical path length of both the first and second portions of the beam an equal amount.  
           [0023]    The step of moving the piston to the second location may include the step of filling a chamber through which the an optical chamber  
           [0024]    The step of generating the beam may include the step of generating the beam with a wavelength of between 500 and 1700 nanometers, or between 500 and 1400 nanometers, or between 500 and 1150 nanometers, or between 700 and 1150 and within this range (preferably between 700 and 900 or 950 and 1025 or 1030 and 1150 nanometers), 1250 and 1400, or 1450 and 1650.  
           [0025]    The step of generating the beam may include the step of generating the beam with a wavelength in the range of 840 and 980 nanometers.  
           [0026]    The step of generating the beam may include the step of generating a sequence of individual pulses of light, and the step of calculating the first and second times-off-light may include the step of determining the time-of-flight of at least one pulse in the sequence of individual pulses of light. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]    The present invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts, in which:  
         [0028]    [0028]FIG. 1 is a partial cross-sectional view of a hydraulic actuator having the laser-based reflective beam sensor and a control unit for generating the laser beam and calculating the position of the actuator wherein the laser light sources are located remotely from the actuator and cables including three fiber optic light guides couple the control unit to the actuator;  
         [0029]    [0029]FIG. 2 is a partial cross-sectional view of the embodiment of FIG. 1 showing how the light guides are coupled to the cylinder; and  
         [0030]    [0030]FIG. 3 is a plot of transmissivity versus wavelength of laser light through various hydraulic fluids of various types and ages. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    [0031]FIG. 1 is a schematic view of a linear cylindrical actuator  10  in accordance with the present invention. Actuator  10  includes a cylinder  12  having an inner diameter  14  and two end caps  16 ,  18 . Rod end cap  16  encloses one longitudinal end of the cylinder and has an opening  17  through which rod  24  passes. Opening  17  seals against the surface of the rod and prevents actuating fluid from leaking out. End cap  18  encloses the opposing end of the cylindrical potion of the cylinder and prevents actuating fluid from leaking out.  
         [0032]    Actuator  10  also includes a piston assembly  20  which includes a piston  22  having an outside diameter  23  configured to seal against the inner diameter  14  of the cylinder and to slide longitudinally, back and forth, with respect to cylinder  12 . Piston- 22  is coupled to rod  24 , which extends from the inside of the cylinder to the outside of the cylinder through opening  17  and is fixed to piston  22  to move simultaneously with the piston. Surface  26  is a reflective surface fixed to move with piston  22  and is configured to reflect laser light that is introduced into the cylinder. Two ports  28 ,  30  are provided in the cylinder to introduce an operating fluid into the cylinder or remove the operating fluid from the cylinder. Extension cylinder port  28  is disposed in the cylinder such that fluid introduced into the port will cause the piston and piston rod to move in a direction that increases the overall length of the actuator  10 . Retraction cylinder port  30  is disposed in the cylinder such that when a working fluid is introduced into the actuator through this port, it causes the piston assembly to move into the cylinder, or retract, thereby reducing the overall length of actuator  10 . When the working fluid is removed from retraction cylinder port  30 , rod  24  extends farther outside the cylinder, increasing the overall length of actuator  10 .  
         [0033]    The cylinder and piston assembly collectively define two internal cavities separated by the piston into which fluid may be introduced or removed. Extension cavity  32 , when filled (through port  28 ) causes the piston assembly to extend, increasing the overall length of the actuator. At the same time, retraction cavity  34  is emptied. Similarly, when retraction cavity  34  is filled, through retraction cylinder port  30 , retraction cavity  34  fills with fluid, extension cavity  32  empties fluid through extension cylinder port  28 .  
         [0034]    Excluding the effects due to the size of piston rod  24 , actuator  10  has a predetermined internal fluid volume that does not change based upon the position of the piston. This volume (again, discarding the effects due to the size of piston rod  24 ) is equal to the sum of the volumes of extension cavity  32  and retraction cavity  34 .  
         [0035]    An optical coupler  34  is fixed in end cap  18  to communicate laser light into chamber  32  and to communicate laser light from chamber  32  outside the cylinder. The cap itself has a threaded external surface  37  that engages mating threads in end cap  18 . These threads serve to secure the coupler to the end cap and to prevent leakage of hydraulic fluid or air out of the cylinder. The coupler also serves to hold several optical fibers  36 ,  38  in a fixed relationship with respect to cylinder  12 . Coupler  34  is preferably disposed along the centerline of cylinder  12  such that the cylinder and the coupler share a common cylindrical axis  40 . Referring now to FIG. 2, coupler  34  supports eight optical fibers ranged in arcuate, preferably circular, pattern equidistantly spaced from the longitudinal cylindrical axis of the coupler. These fibers gather light that is reflected off surface  26  and conduct it out of the cylinder. Fiber  36  is disposed along axis  40  and conducts light from outside the cylinder into the cylinder. Light that is conducted into the cylinder through fiber  36  is directed towards reflective surface  26  on piston  22 . It reflects off piston  22  and returns in a plurality of paths to each of optical fibers  28 . These fibers receive the light at substantially the same time and conduct the light out of the cylinder. An optical multiplexer (combiner)  42  is optically coupled to fibers  38  and joins their/there individual light beams into a single beam that exits multiplexer (combiner)  42  in optical fiber  44 . Thus, the light carried by optical coupler  44  is the combination of all the individual beams of light carried by optical fibers  38 .  
         [0036]    Referring now to FIG. 1, optical fiber  44  is at its other end connected to optical coupler  46  which directs and focuses the light beam of fiber  44  to photodiode  48 . When the light passes through coupler  46  and falls upon photodiode  48 , it changes the conductivity of the photodiode causing a change in the current flowing through circuit  50 . This change in current, or photodiode signal, is amplified by photodiode amplifier  52 . The output of photodiode amplifier  52  is fed to pulse expansion circuit  54  which increases the width of the photodiode signal. Phase comparison circuit  56  receives two impulses: the expanded pulse from pulse expansion circuit  54  and a trigger pulse from timing circuit  58 . By determining the time difference between the pulse of timing circuit  58  and the expanded pulse from circuit  54 , phase comparison circuit  56  generates a signal indicative of the time delay between these two pulses. This time delay signal is output signal  60 .  
         [0037]    Timing circuit  58  generates periodic pulses on the order of once every tenth of a second. These two pulses are provided on two signal lines: signal line  62  which goes to phase comparison circuit  56  and signal line  64  which goes to laser driver circuit  66 . Laser driver circuit  66 , when it receives this timing signal, generates a pulse that is applied to laser diode  68 . Laser diode  68  turns the signal into a laser light pulse which is transmitted through optical fiber  36  and coupler  34  into cylinder  12 . The laser light pulse traverses cavity  32 , reflects off surface  26  and returns to optical fibers  38  which are held in coupler  34 .  
         [0038]    Referring back to phase comparison circuit  56 , circuit  56  receives a pulse on line  62  generated by timing circuit  58 . It also receives an expanded pulse from pulse expansion circuit  54 . The difference in time of arrival of these two pulses is substantially equal to the amount of time it takes for the laser light pulse to travel from laser diode  68  to photodiode  48 . Whenever piston  20  moves, both the path from laser diode  68  to the piston increases and the path from the piston to photodiode  48  increases. Since this is a linear device, for every inch of movement of piston  20  the path length changes by two inches.  
         [0039]    Pulse expansion circuit  54  is disclosed in more detail in U.S. Pat. No. 6,005,395 as the directional sampler  74 . The output of pulse expansion circuit  54  is an equivalent-time replica of the reflected pulses received by photodiode  48 .  
         [0040]    Phase comparison circuit  56  is described in U.S. Pat. No. 6,005,395 as directional set/reset circuit  100 .  
         [0041]    The output signal  60  is preferably in the form of a square wave having a pulse width indicative of the time required for the light emitted from laser diode  68  to travel through the system. Changes in the pulse width are preferably proportional to the distance the piston has traveled.  
         [0042]    Referring now to FIG. 2, we see a cross-section of the end of actuator  10  taken at section  2 - 2  in FIG. 1. The coupler  34  is fixed to optical fibers  38  that transmit the reflected light beam out of the cylinder. In the embodiment shown, there are eight optical fibers arranged in a circular pattern about optical fiber  36 , which is also supported in coupler  34 . Coupler  34  is preferably disposed within the cylinder, as shown in FIG. 2, such that fiber  38  enters the cylinder substantially coaxial with longitudinal axis  40  of the cylinder. Each of the eight fibers  38  is preferably disposed equidistantly with respect to fiber  38  and together are preferably spaced equidistantly apart. In this manner, each fiber has a corresponding fiber disposed on the opposing side of optical fiber  38  from which they are both equally spaced.  
         [0043]    In addition, the longitudinal axis of each of the optical fibers  38  and optical fiber  36  are preferably parallel such that light transmitted into the cylinder through optical fiber  38  will reflect off surface  26  of piston  20  and return directly to coupler  34 . If surface  26  is disposed in a substantially perpendicular orientation with respect to the longitudinal axes of fibers  38  and  36 , substantially all the light that is emitted into cylinder  12  by optical fiber  38  will arrive back at coupler  34 .  
         [0044]    The benefit of having several optical fibers for receiving reflected light is two fold. First, a smaller diameter optical fiber can be spaced closer to fiber  36 . This closer spacing means that it is in a better position to receive the reflected light that reflects off perpendicular reflective surface  26 . Secondly, by providing several optical fibers, considerably more reflected light can be gathered and provided to photodiode  48 . This provides a substantially larger pulse and reduces any the possibility that stray reflections will trigger photodiode  48 .  
         [0045]    To provide this additive effect, each of optical fibers  38  is preferably the same length as the others of fibers  38 . Thus, when the reflected light pulse is received substantially simultaneously at cylinder ends of optical fibers  38 , each portion of the reflected pulse that travels down each reflected light fiber will arrive at multiplexer  42  at substantially the same time. Thus, any reflected light falling simultaneously on the receiving ends of fibers  38  will be combined and arrive simultaneously at the photodiode.  
         [0046]    The spacing between fiber  36  and each of fiber  38  is preferably small, on the order of one to two centimeters. More preferably it is between five and ten millimeters.  
         [0047]    [0047]FIG. 3 is a plot of transmissivity vs. wavelength. It measures the degree to which laser light is attenuated as it passes through hydraulic fluids of varying types. The types of hydraulic fluid tested include “J” type fluid with entrained air, “J” type fluid, old “E” type fluid, old “F” type fluid, and old “G” type fluid. Each of these types of hydraulic fluid are well known to engineers working with hydraulic fluids, and represent several of the most common fluids used in hydraulic systems today. The “E”, “F” and “G” type fluids are “old” in that the fluids tested have been used in actual hydraulic equipment, and were not new. Three of the four hydraulic fluids that make up the J, E, F and G fluids are Case hydraulic fluids MS 1207 Hi Tran Plus, MS 1209 Hi Tran Ultra, and MS 1230. The reason these fluids were chosen was to see the degree to which aging and use of a hydraulic fluid would cause the optical characteristics of such fluid to degrade. The assumption is that degraded or “old” fluid by its accumulation of moisture, oxygen, and suspended particulates such as metal particles would not transmit laser light as readily as new hydraulic fluids. The chart in FIG. 3 indicates the qualities of each of the aforementioned fluids. Note that the transmission of light is restricted almost entirely in the range of 500-1700 nanometers. Outside this range, there is virtually no transmission of light. Within this range, however, there are three separate sub-ranges in which a significant amount of light is transmitted. These ranges are 700-1150 nanometers, 1250-1400 nanometers, and 1450-1650 nanometers. The broadest of these three ranges is the band between 700 and 1150 nanometers. In this range, there are three significant sub-ranges in which transmissivity is substantial these include the sub-range of 700-900 nanometers, 950-1025 nanometers, and 1030-1150 nanometers. Each of these sub-bands has a local transmissivity maximum at 850, 970, and 1090 nanometers, respectively. The other two major bands have their respective maxima at 1315 nanometers and 1560 nanometers, respectively.  
         [0048]    Note that in comparing each of the hydraulic fluids, that the peak transmissivities in each of the bands and sub-bands does not vary substantially from the peak transmissivities of the other peak transmissivities. Comparing the “G_old” to the “E old” fluids, although the variations in transmissivity at each of their respective maxima varies from 0.1 (at 1090 nanometers) to 0.4 (at 850 nanometers), the wavelengths of these respective maxima are the same.  
         [0049]    Based upon this empirical analysis, it is clear that as hydraulic fluid ages its transmissivity peaks do not shift. An appropriate high power laser diode for transmitting light through the hydraulic fluid is preferably selected to have a wavelength at or near any of local maxima shown in FIG. 3. As that oil ages, and in the absence of any maxima wavelength shift, one would expect the transmissivity to drop, but not to shift radically based upon wavelength. For this reason, a laser diode having a frequency of 850, +80/−125 nanometers, 970+/−30 nanometers, or 1090+/−30 nanometers would be particularly preferred. While the other two major bands also exhibit strong transmissivity at their local maxima, due to the sudden and extreme drop-off on either side of the local maxima there less preferred. Nonetheless, even though they are less preferred, a laser diode having a wavelength of 1325 +/−50 nanometers, or 1560 +/−50 nanometers would also be acceptable.  
         [0050]    While the embodiments illustrated in the FIGURES and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. The invention is not intended to be limited to any particular embodiment, but is intended to extend to various modifications that nevertheless fall within the scope of the appended claims.