Abstract:
System and method for testing a pump. The testing of the pump includes determining a Mechanical Power Index (MPI) for the pump. The MPI is determined by operating the pump at operating speed for a minimum operating interval. When the pump is stopped under normal operation by the application, the pump speed is continuously monitored. The time when the pump speed decreases to an initial speed close to the operating speed is read as an initial time. When the pump speed decreases to a final pump speed close to the critical speed, the end time is read and the elapsed time is determined as Δt=end time−initial time. The MPI is calculated to be the initial pump speed minus the final pump speed divided by Δt. The MPI can be used during the lifetime of the pump to determine when the pump is deteriorating.

Description:
TECHNICAL FIELD 
     The present invention relates generally to systems and methods for testing pumps, and more particularly to systems and methods for predicting pump lifetime and wear. 
     BACKGROUND 
     Pumps are used in a variety of settings for moving liquid matter through a type of conduit made to contain the liquid (or gas). Some types of pumps have evolved into devices used in environments that require high precision and long mission times. For example, turbomolecular pumps are used to create a vacuum in a chamber. Turbomolecular pumps typically operate at high rotational speeds and may be required to operate for long periods of time in which it may move isolated molecules out of the vacuum chamber. Typically, turbomolecular pumps move the gaseous matter from the chamber into a conduit having a backing pump to assist in moving the matter completely out of the chamber. Once the vacuum forms, the pump may be required to remain on in a vacuum state to maintain the vacuum. 
     Like any other device, turbomolecular pumps will fail after some time in operation. The failure of a turbomolecular pump can be costly. Replacement of the pump requires a shutdown of the application in which it was operating. Where the application involves a production line, the shutdown means delays in production and reduction in volume. The failure may also cause a loss of products that would need to be disposed of if the loss of vacuum spoils the product. Similar scenarios involving added costs and inefficiencies may be present in other types of applications. 
     Turbomolecular pumps are currently tested for long-term lifetime ratings during the design and manufacture of the pumps. These tests however involve large sample sizes and may require long periods of time, up to a year or more, to obtain statistically significant sample sizes delaying entry into the market and adding cost to both development and manufacture. Despite all of the testing, it is still impossible to determine when any one individual pump will fail. 
     In view of the foregoing, there is an ongoing need for pump testing methods that predict pump failures and lifetime. 
     SUMMARY 
     In an example implementation, a method is provided for testing a pump. The testing of the pump includes determining a Mechanical Power Index (MN) for the pump. The MPI is determined by operating the pump at full operating speed for a minimum operating interval. When the pump is stopped under normal operation by the application under vacuum, the pump speed is continuously monitored. When the pump speed decreases to an initial speed close to the operating speed, the initial pump speed is stored for time t=0. When the pump speed decreases to a final pump speed close to the critical speed, the elapsed time is read and stored as t=Δt. The MPI is calculated to be the initial pump speed minus the final pump speed divided by Δt. The MPI can be used during the lifetime of the pump to determine when the pump is deteriorating. 
     Other devices, apparatus, systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be better understood by referring to the following figures. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. In the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a schematic diagram of an example application that implements a pump that may be tested during life time using an example implementation of a pump test. 
         FIG. 2  is a schematic diagram illustrating operation of an example system for testing the pump during life time operation. 
         FIG. 3  is an example of a graph illustrating example results from performing the pump test described with reference to  FIG. 2 . 
         FIG. 4  is a flowchart illustrating operation of an example method for testing a pump. 
         FIG. 5  is a flowchart illustrating analysis of data acquired during a pump test. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the term “critical speed” refers to a rotational speed of the pump at which the pump operation becomes unstable. At the critical speed, the instability of the pump has reached a level of unbalance that exceeds a threshold value. 
     As used herein, the term “full speed” refers to the operational speed of the pump where the pump is configured for operation at a single high speed. 
       FIG. 1  is a schematic diagram of an example application  100  that implements a pump  104  that may be tested during life time operation using an example implementation of a pump test. The application  100  in  FIG. 1  creates a vacuum in a chamber  102  using the pump  104 . The pump  104  may be any suitable pump for creating a vacuum. For purposes of illustration, the pump in this description is assumed to be a turbomolecular pump. The pump  104  includes a pump chamber  110  formed by a pump housing  106 . The pump chamber  110  houses a plurality of rotating blades  112  controlled by a motor  107 . The pump  104  includes a bearing (not shown) to stabilize the rotational motion of the rotating blades  112 . 
     In operation, the rotating blades  112  are rotated by the motor  107  to draw gaseous matter  108  from the chamber  102  into the pump chamber  110 . The rotating blades  112  move the gaseous matter  108  into a conduit  130  that extends to a port  140  through which the gaseous matter  108  is expelled from the chamber  102  thereby creating and/or maintaining a vacuum in the chamber  102 . A backing pump  120  on the conduit  130  may be added to further assist in the movement of the gaseous matter  108  through the port  140 . The backing pump  120  may be a second turbomolecular pump, or any other suitable pump operating as a backing pump. 
     Typically, the operation illustrated in  FIG. 1  involves the need to maintain the vacuum in the chamber  102  for a substantial period of time during which the pump  104  is operated at high rotational speeds. In an example implementation, the pump  104  may be controlled to perform pump tests that predict the lifetime of the pump  104  and indicate when the pump  104  is beginning to deteriorate. The pump tests allow for planned replacement of deteriorating pumps and help to avoid situations where the pump  104  fails during operation. 
     The pump tests may be performed while the pump  104  is on-line and in operation. The hardware and software components (described below with reference to  FIG. 2 ) for performing the pump tests may be integrated with hardware and software provided to control the operation of the pump  104 . The pump tests may also be part of a retrofit kit for inclusion into the system controlling the pump  104  after the pump  104  has been activated. 
       FIG. 2  is a schematic diagram illustrating operation of an example system  200  for testing a pump  202  during life time operation. The system  200  in  FIG. 2  includes a pump controller  210  and a data acquisition and control interface  212 . The pump  202  is shown configured to pump matter to a backing pump  204 . The data acquisition and control interface  212  may be implemented as part of a communications interface that provides communication between the pump  202  and the pump controller  210 . The data acquisition and control interface  212  includes a spin sensing signal path connected to a spin sensor mounted in the proximity of the pump  202  to sense the pump speed. In an example implementation, the spin sensor is an accelerometer. 
     The pump controller  210  may be configured to operate as a pump testing module that may be added to a pump already in operation in an upgrade or retrofit. The pump controller  210  may also include general pump control operations for use by a system performing the application  100  (in  FIG. 1 ) for which the pump  202  is being used. The pump testing functions may be integrated with the general pump control operations of the pump controller  210 , which may be further integrated with the pump  202  as a product. The pump testing functions may also be performed by the pump controller  210  in a high-level system control operation that is part of the application  100  requiring the vacuum in the chamber  102  in  FIG. 1 . The high-level system control operation may interface with the pump  202  to control the pump and to obtain information (such as the pump speed) from the pump  202 . 
     The pump controller  210  in  FIG. 2  includes pump testing functions in which a mechanical power index (MPI) is calculated and used to determine the state of the pump, whether or not it is deteriorating, and/or predict a lifetime of the pump. In the context of this description, the MPI is an index that quantifies the state of a bearing used inside the pump  202  in terms of the mechanical power while coming to a stop. The MPI may be calculated by applying a pump braking function under vacuum conditions, measuring the rotational speed of the pump  202  as the pump  202  is coming to a stop, and determining a time interval between pre-determined pump speeds. The rotational speeds measured as the pump slows may be collected and analyzed with the data used to calculate the MPI. An initial time, is identified as the time at which an initial pump speed is attained, where the initial pump speed is a pre-determined rotational speed that is close to the operational speed of the pump  202 . The initial pump speed may be determined and defined for a given pump  202  for use in performing the pump test during the life time of the pump  202 . For implementations in which the pump test is incorporated for operation at the beginning of the life time of the pump  202 , the initial pump speed may be determined as a function of the operational speed determined for the pump  202 . In implementations in which the pump test is added to pumps in operation, the initial pump speed may be determined by monitoring pump speed during operation. 
     During the pump test, once the initial pump speed is determined, the time until the pump speed slows to a final pump speed is measured. An end time, t e , may be identified as the time at which the final pump speed is reached, where the final pump speed is a speed that is close to but greater than the critical speed. The critical speed is the speed at which the unbalance of the rotor blades results in unstable operation. The initial pump speed and the final pump speed may be determined and defined for a given pump  202  for use in performing the pump test during the life time of the pump  202 . For implementations in which the pump test is incorporated for operation at the beginning of the life time of the pump  202 , the initial pump speed may be determined as a function of the operational speed determined for the pump  202 , and the final pump speed may be determined as a function of the critical speed determined for the pump  202 . In implementations in which the pump test is added to pumps in operation, the initial pump speed and the final pump speed may be determined by monitoring pump speed during operation. 
     It is noted that the initial pump speed and the final pump speed are to be speeds that are “close to” the operational speed and critical speed, respectively. The actual value selected for each parameter is not significant. Pumps may be rated or characterized as having an operational speed within an expected range during the lifetime of the pump. The value of the initial pump speed may be determined to be less than the lowest expected operational speed based on the expected range. Similarly, a pump&#39;s critical speed may be expected to vary within an expected range. The value of the final pump speed may be determined to be greater than the highest expected critical speed based on the expected range. 
     The MPI is defined as the difference between the initial speed and the final speed divided by the elapsed time between the time, t i , when the pump  202  has a speed close to the operating speed, and the time, t e , when the pump speed has reached the final pump speed close to the critical speed. The MPI may be determined initially for a baseline measurement when the pump is installed or manufactured or tested prior to shipping. The MPI may be determined at a later point after the pump has been in operation for a predetermined period of time, or duty time. The second MPI measurement may then be compared to the first MPI. If the second MPI is greater than the first MPI, then the mechanical power index has increased indicating that the bearing in the pump is deteriorating. 
       FIG. 3  is an example of a graph illustrating example results from performing the pump test described with reference to  FIG. 2 . The graph  300  in  FIG. 3  is a plot of the rotational speeds against the corresponding times at which the rotational speeds were measured. The time values are indicated on a y-axis  302  and the rotational speeds on an x-axis  304 . Each plotted curve represents a set of measurements of rotational speeds for an individual MPI measurement. The MPI may be determined for an individual pump at selected times during the life time of the pump. The selected times may be regular intervals, or any time the pump is to be stopped. In addition to determining the MPI, the time vs. speed measurements may be stored and used to construct a curve, which may be added to the graph  300  in  FIG. 3 . As a curve is added, the extent to which the slope of each curve continues to increase indicates the extent to which the pump  202  is wearing down at the bearing. The graph  300  in  FIG. 3  may be used to provide an immediate visual indication of how the pump  202  is performing. 
     The measured MPIs may also be stored to preserve a historical archive illustrating the extent to which the pump  202  is deteriorating. The MPI measurements and the graph  300  in  FIG. 3  may be used to predict the expected remaining lifetime of the pump  202 . This data may also be combined with data from other similar pumps (such as pumps of the same model) to provide a more quantitative assessment of the expected lifetime of such pumps. 
       FIG. 4  is a flow chart illustrating operation of an example method for testing a pump. The flowchart  400  is described below as a method performed by the system described with reference to  FIG. 2 . References to components not shown in  FIG. 4  are to components shown in  FIG. 2  unless otherwise noted. 
     The method illustrated by flowchart  400  may be performed after a duty time of on-line operation. At least one MPI has already been determined earlier in the pump lifetime. An initial pump speed and a final pump speed have been determined as described above with reference to  FIG. 2 . The method illustrated by flowchart  400  may be initiated at step  402  for normal operation of the pump  202 . The pump controller  210  may initialize operation of the pump  202  and operate the pump  202  as required by the application. During operation of the pump  202 , a pump braking function under vacuum, or a specific request for testing by determining an MPI may initiate the steps for determining the MPI. At step  404 , the pump  202  is ramped up to full operating speed in a vacuum condition. Step  404  may represent normal operation before the pump braking function, or request for testing, is initiated. The MPI measurement process may include a pre-measurement automatic cycle performed a pre-determined number of times. The pre-measurement automatic cycle may be performed for the pre-determined number of times to obtain a stable and optimal grease distribution inside the bearings. 
     The pre-measurement automatic cycle may involve three phases of pump operation. The first phase is to ramp up the pump  202  to full operating speed. The second phase is to maintain the full operating speed for a pre-defined length of time. The third phase is to decelerate the pump under vacuum conditions. In the method illustrated by the flowchart  400  in  FIG. 4 , step  404  may represent the first phase. At step  406 , the pump is maintained at full operating speed for a predefined period of time thereby implementing the second step. At step  408 , the pump braking function is applied and the pump begins to decelerate for the third phase. 
     The pre-measurement automatic cycles are performed a predefined number of times for a predetermined amount of time. In one example, the pre-measurement automatic cycles may be performed 8 times over a period of 18 hours. The number of cycles and amount of time may vary from pump to pump. 
     At decision block  410 , a check is performed to determine if a sufficient number of pre-measurement automatic cycles have been performed. If not, the ‘NO’ path of decision block  410  transfers control to step  404 . If a sufficient number of cycles have been performed along the ‘YES’ path of decision block  410 , control is transferred to step  412  for monitoring the pump speed while decelerating as the pump braking function is performed. 
     It is noted that the flowchart  400  in  FIG. 4  describes performing the function of determining the MPI by identifying the needed data parameters as the pump  202  is in the process of decelerating. Alternatively, the pump  202  may be permitted to decelerate while the rotational speed of the pump  202  is measured at fixed time intervals, and to collect the rotational speeds for each time interval until the pump  202  stops. The rotational speeds and time intervals may then be analyzed to determine the data parameters to be used for determining the MPI. 
     At step  414  in  FIG. 4 , an initial time at which an initial pump speed is measured is identified to be t i . Step  414  uses the predetermined initial pump speed for all determinations of the MPI to provide reproducibility in the results. 
     At step  416 , as the pump continues to decelerate and an elapsed time is recorded when the pump speed is measured to be a final pump speed. The final pump speed is defined to be a pump speed that is close to the critical speed of the pump  202 . Step  416  uses the predetermined final pump speed for all determinations of the MPI. 
     At step  418 , the MPI is determined by calculating the difference between the initial pump speed and the final pump speed and dividing the difference in pump speeds by the elapsed time between attaining the initial pump speed and the final pump speed. At step  420 , the MPI is compared to a previous MPI to determine if the pump bearings are deteriorating. At decision block  422 , the MPI is checked to see if it is greater than a previous MPI. If the MPI is greater than an earlier MPI, the pump  202  is identified as having a deteriorating bearing at step  424 . Analysis of the measured MPI in step  424  may be performed in view of the history of MPIs for a pump during its lifetime as described with reference to  FIG. 5 . At step  426 , the pump deceleration is complete as the braking function is completed. 
       FIG. 5  is a flowchart illustrating analysis of data acquired during a pump test. At step  502 , a statistically significant baseline may be determined for the family of pumps to which the pump belongs. The baseline may be an MPI value determined from MPI values of a sample of pumps from the same family as the pump under test that are operating well within the performance specifications for the pump family. The pump samples of the same family may be pumps in the same product line, classification, or other suitable category. The MPI of the pump under test, MPI put , is compared with the baseline MPI for the pump family at step  504 . If the MPI for the pump under test is not within the MPI baseline for the pump family as determined at decision block  506 , the pump under test is deemed to have malfunctioned at step  510 . If the pump under test is within the baseline, the measured MPI for the pump under test, MPI put , is compared with a broader MPI for the pump family at step  508 . The MPI for the family, MPI family , may be based on historical data of MPI measurements for all pumps in the family that are deemed to have pump life remaining. Decision block  512  determines if the comparison of the MPI put  with the MPI family  indicates that the pump bearing of the pump under test is not excessively worn, the pump under test is deemed to have operational lifetime remaining at step  516 . If the comparison at decision block  512  determines that the wear of the pump under test is excessive, further testing may be performed. For example, a bearing lube analysis may be performed. The analysis in step  516  may include thermogravimetric analysis (TGA), for example. 
     It will be understood that various aspects or details of the invention may be changed without departing from the scope of the invention. Furthermore, the foregoing description is for the purpose of illustration only, and not for the purpose of limitation—the invention being defined by the claims.