Abstract:
A system and method for the automated measurement of properties related to a stack gas stream, flow velocity for example. A probe assembly ( 24 ) is introduced into a stack through a test port and operated by an automated mechanism mounted on the test port to position a sensing tip ( 28 ) at various locations within the gas stream at proper yaw angle. A framework ( 30, 32, 34, 36 ) of the automated mechanism provides bearing support for the probe assembly while allowing the probe assembly to translate on the framework along an axis but constraining the probe assembly from turning on the framework about the axis. With a gripper assembly ( 44 ) gripping the probe assembly and another gripper assembly ( 56 ) released, a linear actuator ( 40 ) can move the gripper assembly ( 44 ) along the axis to translate the probe assembly on the framework. A motor ( 70 ), also mounted on the framework, can turn a pinion ( 66 ) that is in mesh with a toothed segment ( 64 ) of a ring gear ( 62 ) that attaches to the stack port so that the motor ( 70 ) can turn the framework and probe assembly together about the axis.

Description:
FIELD OF THE INVENTION 
   This invention relates to a system and method for the automated measurement of properties related to a stack gas stream, flow velocity for example, using a probe that is introduced into a stack through a test port and operated to position a sensing tip at various locations within the gas stream at proper yaw angle. 
   BACKGROUND OF THE INVENTION 
   Industrial emissions to the atmosphere, such as those from powerplants that combust fossil fuels to generate electricity, are subject to governmental regulation that is enforced by the United States Environmental Protection Agency (EPA). 
   Pursuant to statutory authority, the EPA has promulgated regulations that are embodied in portions of the Code of Federal Regulations (CFR). Included in portions of 40 CFR are regulations pertaining to measurement of volumetric flow rate of stack gas streams. While the regulations specify acceptable methods of measurement and types of probes that are introduced into stacks through test ports in the stack walls for obtaining those measurements, they leave it to industry to design and develop equipment for use with the probes that will enable the probes to be positioned within a stack for obtaining volumetric flow rate measurements in accordance with regulations. 
   The availability of portable electronic data recording equipment enables stack measurement data to be efficiently recorded on-site in electronic form and then later processed into proper reporting format for demonstrating regulatory compliance. The ability to automate a method for positioning a probe within a stack while electronically recording gas stream data is obviously desirable for increasing the efficiency and accuracy with which a test is performed. 
   Accordingly, it has been proposed to employ a motorized mechanism for positioning a probe within a stack as shown and described in various patents and publications, such as U.S. Pat. No. 5,440,217. 
   EPA regulations specify several test methods (Methods 2, 2F, and 2G) using certain specified probes. For performing Methods 2 or 2G, an S-type (“two dimensional”) probe is specified. For performing Method 2F, a prism head (“three dimensional”) probe is specified. The probe must translate in a direction that is transverse to the direction of the gas stream that is passing upward through the stack and it must also be capable of turning about the axis of translation. Such turning is referred to as yaw nulling. 
   The extent to which the probe needs to be advanced depends on the stack diameter. The larger the stack diameter, the greater the distance that the probe needs to be advanced. In very large diameter stacks, multiple test ports are provided at locations around the stack to allow a probe whose range of translation cannot span the full diameter to be placed at those locations and used for testing. 
   Because the extended probe acts in the manner of a cantilever on whatever structure is supporting it, and because the probe must be able to withstand hostile stack environments, the typical probe will have sufficient mass that will cause the probe to droop to some extent when maximally extended. The EPA test methods specify a maximum allowable droop of 5°. 
   Droop can be minimized by increasing probe stiffness, but increased stiffness is apt to require that probe mass and dimensions be increased, and when that is done, the construction of the mechanism that translates and turns the probe while at the same time supporting the cantilevered weight of the probe needs to be much more substantial, not only from the standpoint of structure but also from the standpoint of more powerful prime movers that are used to translate and turn the probe. 
   The device shown in U.S. Pat. No. 5,440,217 comprises two arrays of roller wheels that are spaced apart along the length of the probe and that bear against the outside of the cylindrical probe wall. Three roller wheels are journaled in roller assemblies that are arranged approximately equiangular about the probe wall in a first array and are forced against the probe wall by spring washers. In the other array, there are two roller assemblies like those of the first array, while the third roller wheel is a drive roller wheel that is coupled to a motor so that by virtue of friction between that roller wheel and the probe wall, rotation in one direction by the motor advances the probe and rotation in the opposite direction retracts the probe. The motor is a stepper motor that operates in increments. 
   The roller assemblies containing the non-driven roller wheels mount on a cylindrical housing within which the probe translates, with the probe increasingly protruding from that housing as the probe increasingly advances, and decreasingly protruding as the probe increasingly retracts. An alternative drive for probe translation is a chain drive as shown in U.S. Pat. No. 5,394,759. 
   Turning of the probe about the probe axis is accomplished by a second motor, also a stepper motor, that acts on the cylindrical housing containing the roller wheels that engage the probe wall. A timing belt is trained around the outside of the cylindrical housing wall and presumably a shaft or sheave of the second motor so that when the second motor operates, it turns the probe by turning the cylindrical housing within which the probe translates. The second motor is housed at one end of an outer housing assembly, whose other end is fit and secured to a mounting ring on the stack at the stack test port. The outer housing assembly surrounds the cylindrical housing containing the roller wheels that engage the probe, for at least some of the length of the cylindrical housing. 
   Encoders track translation and turning of the probe. The motors are controlled by a computer that calculates the points at which the probe is to be positioned for testing and will output signals to the stepper motors for translating the probe to the desired test point and turning the probe to the desired angular orientation. 
   Analysis of the devices shown in the referenced US Patents discloses that a more robust automated probe would be desirable. Some aspects of the patented probes that may compromise robustness include: translational accuracy of the probe; the use of spring washers in one version to force the roller wheels against the probe wall in the apparent interest of providing adequate cantilever support, but at the same time creating additional stresses that must be accommodated by mechanical strengthening, which typically means added mass, and motor size large enough not only to move the probe but in doing so to also overcome the opposing force components of the spring washers; and the use of two housings, one within another, adding complexity and weight. 
   SUMMARY OF THE INVENTION 
   Accordingly, the present invention is directed toward an automated measuring probe having a number of features that contribute to greater robustness, to improved accuracy, ease of use, and the ability to position a larger probe over a greater distance with acceptable droop. 
   One generic aspect of the invention relates to an automated probe for obtaining data at various locations in a stack flow comprising a probe assembly having a sensing tip at one end. The probe further comprises structure comprising a mechanism for imparting translation and rotation to the probe assembly to enable the probe assembly to obtain data about stack flow. 
   The structure comprises a framework providing bearing support for the probe assembly while allowing the probe assembly to translate on the framework along an axis but constraining the probe assembly from turning on the framework about the axis. A translation-imparting mechanism is mounted on the framework and comprises a gripper assembly that can grip and release the probe assembly and a first prime mover for moving the gripper along the axis so that when the gripper assembly is moved while gripping the probe assembly, the probe assembly translates on the framework along the axis. 
   A rotation-imparting mechanism comprises a second prime mover that is mounted on the framework and turns a pinion that is in mesh with a toothed segment that attaches to the stack via a mounting so that operation of the second prime mover rotates the framework and probe assembly about the axis. 
   Another generic aspect relates to a probe assembly for use with a stack probe for obtaining data at various locations in a stack flow. The probe assembly comprises a square tube and a sensing tip for sensing pressure and temperature disposed at one end of the square tube. The square tube comprises individual square tube sections spliced together end-to-end by smaller square tube splices. 
   Still another generic aspect relates to a method for obtaining data at various locations in a stack flow via a probe assembly having a sensing tip at one end. The method comprises imparting translation to the probe assembly by translating the probe assembly on structure that constrains the probe assembly from rotating while allowing translation, and imparting rotation to the probe assembly by rotating the structure on which the probe assembly translates. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side elevation perspective view of the interior of the inventive automated probe for positioning a probe assembly that comprises a square tube and sensing tip, but without the probe assembly being shown, and with some parts of the automated probe being omitted for clearer illustration of the parts that are shown. 
       FIG. 2  is a partial side elevation view in the direction of arrows  2 - 2  in  FIG. 1 , still with some parts omitted. 
       FIG. 3  is a perspective view of one of the parts shown in  FIG. 1 , specifically a gripper assembly. 
       FIG. 4  is a perspective view of the same end of the automated probe shown in  FIG. 2  after the probe assembly has been placed in the automated probe. 
       FIG. 5  is an axial view in the direction of arrows  5 - 5  in  FIG. 4 , but without the square tube of the probe assembly. 
       FIG. 6  is an axial view looking in the opposite direction of  FIG. 5  to show one end wall of the automated probe, with certain parts omitted for clarity of illustration. 
       FIG. 7  is an axial view of the opposite end wall, also with certain parts omitted for clarity. 
       FIG. 8  is a partial side elevation view in the same direction as  FIG. 2 , but showing some of the parts omitted from  FIG. 2 . 
       FIG. 9  is a perspective view of a portion of the square tube. 
       FIG. 10  is a partially exploded view of  FIG. 9 . 
       FIG. 11  is a fully exploded view of  FIG. 9 . 
       FIG. 12  is a diagram useful in explaining how probe assembly translation is measured. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   For automating any of various known types of sensing tips, the invention provides an automated probe  20  comprising a self-contained supporting and positioning mechanism for supporting and positioning a square tube at one end of which a sensing tip is mounted. Probe  20  is adapted for mounting on a stack at a test port of the stack that is designed to allow the probe to be separably fastened to it, thereby supporting the entire probe  20  (including the probe assembly when the probe assembly is in place) on the stack. The probe assembly (to be more fully described later) comprises a square tube and a sensing tip that is compliant with 40 CFR for performing flow velocity measurements, with the sensing tip being fastened to one end of the square tube. The probe  20  can position the probe assembly in two directions, one being translationally across the stack diameter and the other being rotationally about the centerline of the probe assembly. 
   The automated probe  20  is arranged and constructed to allow the probe assembly to be inserted lengthwise, tip end first, into and through the probe interior in preparation for a test, and after the test, to be removed in the opposite manner. 
   Automated probe  20  is shown in the Figures to comprise structure  22  that supports various component parts, and of course a probe assembly  24  when the latter is placed in the automated probe in the manner explained above.  FIG. 4  shows probe assembly  24  to comprise a square tube  26  and a sensing tip  28  at one end. 
   Before a stack test, probe  20  is attached to the stack test port in a manner that will be explained with reference to  FIG. 2 . Probe assembly  24  is inserted, probe end first, into the open proximal (rear) end of structure  22  (right-hand end as viewed in  FIG. 1 , and advanced completely through that structure until the sensing tip end protrudes from the distal (forward) end, as in  FIG. 4 . Pressure lines from sensing tip  28  run through the interior of tube  26  and exit from the proximal end of the tube from whence they continue on to test equipment to communicate pressure sensed by the sensing tip ports to the test equipment. As also specific by EPA regulations a thermocouple mounted in association with the sensing tip is coupled to the test equipment by wires running through the square tube. 
   At its lengthwise opposite ends, structure  22  comprises end walls  30 ,  32  that are joined through two rigid trusses  34 ,  36  that are disposed on opposite sides of the probe assembly. Extending between the end walls is a servo-motor-powered linear actuator  40  having a traveling carrier  42  to which gripper assembly  44  is attached. As actuator  40  operates, it moves carrier  42  to impart travel to gripper assembly  44 . 
   Gripper assembly  44 , shown in more detail in  FIG. 3 , has opposed jaws  46 ,  48  that confront tube  26  from opposite sides. The jaws can be opened and closed by pneumatic or electric actuation. When the jaws are operated open, the gripper assembly can travel freely with carrier  42  relative to tube  26 . When the jaws are operated closed, they forcefully grip the tube sides with sufficient force to enable the gripper assembly to translate the probe assembly as actuator  40  operates, and hence move the probe within the stack. The interior of linear actuator  40  has a screw that when turned by servo motor  50  acting through a coupling to the screw, imparts travel to a nut to which the carrier is attached. 
   When gripper assembly  44  stops, jaws  52 ,  54  of a second gripper assembly  56  that are disposed on opposite sides of tube  26  can be closed to grip the tube. Gripper assembly  56  is stationarily mounted on structure  22 , and thereby prevents tube translation when its jaws are closed on the tube. The jaws must be opened before the tube can be translated by gripper assembly  44 . 
   With these elements, namely the two gripper assemblies and the motor, a sequence for translating tube  26  to position the probe assembly inside the stack can be developed. Gripper assembly  44  has a range of travel along the length of the actuator screw that is limited by suitable limit stops. One limit stop limits rearward travel of the nut and carrier, and the other limits forward travel. The range of travel is marked as  58  in  FIG. 1 . 
   With gripper assembly  44  at the rearward limit stop, tube  26  can be advanced (toward the left in  FIG. 1 ) so as to begin extending the tube and probe into a stack during a test. The sequence comprises operating traveling gripper assembly  44  to clamp tube  26 , releasing stationary gripper assembly  56  from clamping the tube, and operating servo motor  50 , causing actuator  40  to move gripper assembly  44  in the direction from right (broken line position) to left (solid line position) in  FIG. 1 . 
   When gripper assembly  44  reaches the forward limit stop, the stationary gripper assembly  56  is operated to clamp tube  26 , gripper assembly  44  is operated to unclamp the tube, and motor  40  is operated to move gripper assembly  44  back to the opposite limit stop (broken line position). The limit stops define a known increment of length for probe translation (range of travel  58 ). 
   The known increment of length may be chosen to correspond to the distance between consecutive locations where stack measurements are to be taken as the probe tip is positioned within the stack. 
   When the tube is to once again advance further into the stack, the same sequence of operations just described is repeated. 
   Probe  20  is particularly unique because it fastens to a mounting at the stack test port opening via a slewing ring bearing assembly  60  that is itself part of probe  20 . The slewing ring assembly comprises a ring gear  62  having a toothed track  64  which runs along its outer circumference and with which teeth of a pinion  66  on an output shaft  67  of a gear reduction assembly  68  are in mesh. An input shaft  69  of assembly  68  is coupled to an output shaft of a servo motor  70 . Ring gear  62  is disposed inside of a cover  72  that is fastened to the ring gear by screws  74  that thread into holes in the ring gear, with spacers (not shown) between the cover and the ring gear to space the latter from the former. 
   Cover  72  itself fastens to the stack test port mounting via bolts  76  as shown in  FIG. 2 , with bolts  76  passing through holes in ring gear  62  making the ring gear stationary on the test port. The test port mounting comprises a 4 inch NSI pipe flange containing holes through which bolts  76  pass. The bolts are arranged at 90° about the probe assembly centerline. Nuts  80  are threaded onto the ends of the bolts and tightened to draw cover  72  and the ring gear inside the cover tightly against the face of the pipe flange, and consequently align the path of probe assembly travel with the test port opening. 
   The slewing ring bearing assembly comprises an outer race  81  that can turn on an inner race  83  via intervening bearings. It is the outer race that is fastened to cover  72  via bolts  76 . The inner race comprises a ring  86  containing a number of through-holes through which screws  88  pass to fasten the ring to end wall  30 . 
   Gear reduction assembly  68  and motor  70  are mounted on end wall  30  by four fasteners that engage holes  92 . Pinion  66  is disposed on the stack side of end wall  30  where it meshes with track  64 . The shaft on which the pinion is affixed passes through a hole  94  in end wall  30 . End wall  30  has a side wall  96  surrounding the ring gear. Side wall  96  is nominally circular, but protrudes upwardly to partially surround pinion  66 . A small cover  98  completes the enclosure of the pinion and its meshing with the ring gear. 
   When motor  70  operates to turn pinion  66 , the entire probe, except for parts fastened to the stack mounting, cover  72  and ring gear  62  in particular, will turn about the probe assembly centerline, imparting rotary motion to the probe inside the stack. This serves to orient the probe sensing tip in yaw, positioning the probe assembly about its centerline to a position where the tip measures the maximum magnitude of the flow vector at the location to which the probe tip has translated. 
   Holes  101  in end wall  30  are ports for motorized blowers  103  that pressurize the interior to keep stack gases from intruding. 
     FIGS. 7 and 8  show how tube  26  is guided and supported. A guide roller assembly  106  is fastened to the inside of end wall  32  over the opening  108  ( FIG. 1 ) in the end wall through which the tube is introduced. Assembly  106  contains a set of eight rollers  110  for supporting and guiding tube travel within assembly  20 .  FIG. 8  shows a guide roller assembly  115  fastened to the opposite end wall  30 . Assembly  115  is actually spaced a short distance inside that end wall. A series of circular rods  118  extend between the assemblies  115 ,  106  to aid in guiding tube  26  during its introduction into and passage through assembly  20  so that it will align with the rollers of assembly  115  upon reaching them during insertion of the probe assembly into and through assembly  20 . 
   As shown by  FIGS. 9 ,  10 ,  11 , square tube  26  is an assembly comprising a number of individual square tube sections  130  that are connected together by splices  132  and bolts  134 . Each splice  132  is itself a square tube of slightly smaller size than tube sections  130 , allowing each section  130  to telescope over a splice  132  as shown. 
   Metal blocks are applied to the ends and middle of each splice as indicated by reference numerals  136 ,  138 ,  140 . These blocks have thicknesses that make up the dimensional difference between the insides of tube sections  130  and the outsides of splices  132  so that the telescopic fit is close, yet allowing the splices to be freely inserted into the tube sections. 
   The end portion of each tube section  130  has a hole pattern  141  matching a hole pattern  143  in one half of a splice. When a splice has been inserted half way into a tube section, the hole patterns register, allowing shanks of bolts  134  to be passed through the holes in the tube section and the bolts tightened in holes in the splice. Although the bolt heads are exposed, other parts of probe  20  that associate with the square tube are disposed so as not to interfere with the bolt heads. For example, along the sides of the square tube that are gripped by the grippers, the grippers can grip with sufficient force along surface zones that are below the bolt heads. 
   With two sections spliced together, the sections come together essentially end-to-end as marked at  142  in  FIG. 9 . The action of the bolt heads on the each tube section wall slightly deforms the wall along each side as the bolts are fully tightened thereby taking out any looseness in the joint. The use of square probe sections and splices also eliminates difficulties and inaccuracies of having a scribe line or other angular reference mark on sections of a round probe in order to properly align the probe sections. 
   Square tubes of different lengths can be readily fabricated. The use of square tube sections provides a natural passageway for pressure lines and wires from the sensing tip. And because the tube sections and splices are both hollow, a square tube provides greater strength with less weight—an efficient use of material. A square tube probe can be extended a substantial distance without exceeding droop allowed by the EPA regulations mentioned earlier. 
     FIG. 12  illustrates how translation of the probe assembly is measured. A wheel  160  is kept against a surface of square tube  26  to rotate in correspondence with tube translation. Wheel  160  is coupled to an encoder  162  which provides a signal measurement of probe translation based on wheel rotation. The signal provided by encoder  162  is used to control servo motor  50  that operates linear actuator  40  to position carrier  42 . 
   A sensor  164  is associated with linear actuator  40  to measure translation of carrier  42  as it moves over a stroke T. By comparing measurements made by encoder  162  and sensor  164 , it becomes possible to detect slippage between gripper assembly  44  and square tube  26 , allowing the cause to be promptly investigated and appropriate repair made. 
   While a presently preferred embodiment of the invention has been illustrated and described, it should be appreciated that principles of the invention apply to all embodiments falling within the scope of the following claims.