Abstract:
A method and apparatus for detecting interruptions of continuity in a circuit is provided. The method includes transmitting a signal through a brush contacting a conductive band of a slipring, wherein the brush and the slipring have a relative motion. The apparatus is configured to determine a circuit interruption using a plurality of parameters of the signal transmitted through the brush and the slipring, and provide an indication of an interruption when the parameters of the signal transmitted through the brush and slipring are indicative of a circuit interruption.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to methods and apparatus for diagnosing intermittent electrical circuits, and more particularly to methods and apparatus for diagnosing intermittent communications in electrical circuits using brushes and sliprings, including circuits used CT scanning systems using sliprings to send information between stationary and rotating sides of a gantry. 
     In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile. 
     In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display. 
     Data and instructions are communicated bidirectionally between electronics on stationary and rotating sides of the gantry. For example, communication is sent across a copper band on a slipring. A brush block maintains an electrical connection to the copper band. A problem with known systems of this type is that there is no indication of whether a connection is broken or whether the connection is simply inactive, with no data being sent. Moreover, sliprings also are known to have problems with “micro disconnects,” in which a brush temporarily loses electrical contact from the slipring. 
     It would therefore be desirable to provide methods and apparatus to monitor the state of brush contacts while a system, such as a CT imaging system, is in operation, and to provide a diagnosis of disconnection events that occur. 
     BRIEF SUMMARY OF THE INVENTION 
     There is therefore provided, in one embodiment of the present invention a method for detecting interruptions of continuity in a circuit, including steps of transmitting a signal through a brush contacting a conductive band of a slipring, the brush and slipring having a relative motion; determining whether parameters of the signal transmitted through the brush and the slipring are indicative of a circuit interruption; and providing an indication of an interruption when the parameters of the signal transmitted through the brush and slipring are indicative of a circuit interruption. 
     This embodiment provides a method for monitoring the state of brush contacts within a system, such as a CT imaging system, while it is in operation. In addition, information provided by this method embodiment can be used to localize and diagnose disconnections when they occur. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a pictorial view of a CT imaging system. 
     FIG. 2 is a block schematic diagram of the system illustrated in FIG.  1 . 
     FIG. 3 is a block diagram of a transmitter on one side of the gantry of the CT imaging system of FIGS. 1 and 2, showing transmission of a signal to the other side of the gantry via a brush and a slipring. 
     FIG. 4 is a drawing of a portion of a receiver on the other side of the gantry from the transmitter shown in FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1 and 2, a computed tomograph (CT) imaging system  10  is shown as including a gantry  12  representative of a “third generation” CT scanner. Gantry  12  has an x-ray source  14  that projects a beam of x-rays  16  toward a detector array  18  on the opposite side of gantry  12 . Detector array  18  is formed by detector elements  20  which together sense the projected x-rays that pass through an object  22 , for example a medical patient. Detector array  18  may be fabricated in a single slice or multi-slice configuration. Each detector element  20  produces an electrical signal that represents the intensity of an impinging x-ray beam and hence the attenuation of the beam as it passes through patient  22 . During a scan to acquire x-ray projection data, gantry  12  and the components mounted thereon rotate about a center of rotation  24 . 
     Rotation of gantry  12  and the operation of x-ray source  14  are governed by a control mechanism  26  of CT system  10 . Control mechanism  26  includes an x-ray controller  28  that provides power and timing signals to x-ray source  14  and a gantry motor controller  30  that controls the rotational speed and position of gantry  12 . A data acquisition system (DAS)  32  in control mechanism  26  samples analog data from detector elements  20  and converts the data to digital signals for subsequent processing. An image reconstructor  34  receives sampled and digitized x-ray data from DAS  32  and performs high speed image reconstruction. The reconstructed image is applied as an input to a computer  36  which stores the image in a mass storage device  38 . 
     Computer  36  also receives commands and scanning parameters from an operator via console  40  that has a keyboard. An associated cathode ray tube display  42  allows the operator to observe the reconstructed image and other data from computer  36 . The operator supplied commands and parameters are used by computer  36  to provide control signals and information to DAS  32 , x-ray controller  28  and gantry motor controller  30 . In addition, computer  36  operates a table motor controller  44  which controls a motorized table  46  to position patient  22  in gantry  12 . Particularly, table  46  moves portions of patient  22  through gantry opening  48 . 
     Referring to FIGS. 3 and 4, a slipring  50  is used to send information between rotating and stationary sides of gantry  12 . Electrical communication is established via a conductive band  52  of slipring  50  in gantry  12 . Brush  54  maintains physical and electrical contact with conductive band  52 , which is, for example, a copper band. Although only one of each is shown in FIGS. 3 and 4, three conductive bands  52  and three brush blocks  54  are provided in one embodiment, one band  52  and one brush block  54  for transmission in each direction. Another conductive band (not shown in FIGS. 3 and 4) is provided as a reference band having a reference voltage  56 . 
     Because similar circuitry is used for transmission of signals in each direction, only transmission of signals in one direction is shown and described. However, for transmission in both directions, different relative motions of brush  54  and slipring  50  (actually conductive band  52 ) may be required for different sliprings  50 . For convenience, it shall be assumed in this description that slipring  50  is rotating and brush  54  is stationary. 
     Signals  58  representing serial data are amplified by amplifier  60  into bipolar signals  62  and are received by receiver  64 , only an input section of which is shown in FIG. 4. A reference signal  56  is connected to signal line  66  via a resistor R 1  (for example, a 1 kilohm resistor). This connection causes a floating condition appears as zero (0) volts with respect to reference  56 . During operation, input signals on line  66  transition between +5V and −5V, for example, although other embodiments utilize different ranges. In one embodiment, a finite transition time between +5V and −5V is provided to limit radiation across slipring  50 , but the transition time is limited to 50 nanoseconds (ns). The limitation on transition time allows receiver  64  to discriminate between normal transitions and actual circuit disconnections. 
     In one embodiment, two comparators  68  and  70  are used in receiver  64  as input devices. Comparators  68  and  70  are provided with reference voltages V 1  and V 2  that define a voltage band around the floating condition voltage that lies strictly within the maximum and minimum voltages of bipolar signal  62 . (“Strictly within,” as used herein, is intended to mean that the range does not include voltages equal to the maximum and minimum voltages of bipolar signal  62 .) For example, in one embodiment, comparator  68  is provided with a +1V reference and comparator  70  is provided with a −1V reference. Outputs of both comparators  68  and  70  are used to determine when line  66  is floating (i.e., in the voltage band between −1V and +1V, or at about 0V, plus or minus a volt of noise). A table showing a relationship of the outputs of comparators  68  and  70  for various operating conditions is given below. 
     
       
         
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Output of Comparator 68 
                 Output of Comparator 70 
                 State 
               
               
                   
               
             
             
               
                             0 
                 1 
                 Good 
               
               
                 1 
                 0 
                 Good 
               
               
                 0 
                 0 
                 Disconnect 
               
               
                 1 
                 1 
                 Failure 
               
               
                   
               
             
          
         
       
     
     (Note that this embodiment indicates a receiver failure when outputs from both comparators  68  and  70  are high.) Floating disconnects shorter than 50 ns are filtered by filter  72 . In one embodiment, floating disconnects indicated by receiver circuit  64  are logged in accordance with their severity. For example, a disconnect longer than 50 ns, but shorter than 200 ns, is logged as having no impact on communications in a system having bit times of 400 ns. Disconnects between 200 ns and 1.6 μs are logged as a disconnect having a minor impact on communication, and disconnects longer than 1.6 μs are logged as being disruptive of communication. In one embodiment, filter  72  has three separate outputs  74 ,  76 ,  78  for indicating disconnects of three levels of severity, but other embodiments code severity level in different ways. Filter  72  in conjunction with comparators  68  and  70  provide an indication of a circuit interruption when parameters of bipolar signal  62  transmitted through brush  54  and slipring  50  are indicative of a circuit interruption. In the illustrated embodiment, the parameters are voltage and time, and indication of the interruption occurs when the voltage is within a preselected range (e.g., −1V to +1V) for at least a preselected period of time. Also, filter  72  classifies indicated interruptions according to a length of time a voltage transmitted through the brush and the slipring is within the preselected range of voltage. 
     In one embodiment, receiver  64  in conjunction with computer  36  generates a log record of disconnects. This log record allows a field service engineer to monitor degradation of a communication link and to take corrective action before significant problems are observed by a user of a CT imaging system  10 . For example, a large number of short disconnects may provide an early indication of a brush  54  needing replacement, even though no noticeable effect on communication has been observed. A sufficiently large number of short disconnects may indicate a more serious brush  54  or slipring  50  wear problem. Longer disconnects may indicate a brush  54  problem or a broken or intermittent cable connection. Brush  54  or slipring  50  problems can be directly located by correlating the disconnections with a gantry rotation angle. In one embodiment, computer  36 , which also controls rotation of gantry  12 , correlates logged disconnects with a rotation angle of gantry  12  and/or slipring  50 . Gantry  12  can also be rotated under manual control to locate a disconnect. 
     It will thus be observed that the embodiments of the present invention described herein can be used for monitoring the state of brush contacts within a system, such as a CT imaging system, while it is in operation. In addition, information provided by embodiments of the present invention can be used to localize and diagnose disconnections when they occur. 
     Although the invention is described in conjunction with a CT imaging system  10 , the invention is also applicable to other types of systems. For example, the invention is applicable to any other system having communication, whether unidirectional or bidirectional, across a slipring, whether or not bipolar modulation is used. In other systems in which a “floating zone” can be defined, the present invention may be modified to detect input signals within that zone to detect disconnects or other degradation problems. In some embodiments, currents rather than voltages are compared to determine when interruptions occur. (The comparison of currents is considered equivalent to the comparison of voltages for purposes of this invention, because of their relationship under Ohm&#39;s law.) 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.