Abstract:
A centrifuge for continuously separating the various constituents of blood or other biological fluids includes a rotating bowl having high-G and low-G walls. An inwardly directed ramped surface on the high-G wall interacts with an interface region between the separated fluid constituents to provide an optically detectable indication of the position of the interface between the high-G and low-G walls. An optical sensor sensing each passage of the ramped surface past a point develops a changing signal indicative of the interface position. Signal processing circuitry responsive to the signal measures such signal parameters as peak amplitude, signal area and signal shape. By so monitoring these signal parameters, a better indication of actual interface position between the high-G and low-G walls can be obtained. This, in turn, results in better control over centrifuge operation and improved separation of the desired fluid components.

Description:
FIELD OF THE INVENTION 
     This invention relates generally to centrifugal processing systems and apparatus. 
     BACKGROUND OF THE INVENTION 
     Various blood processing systems now make it possible to collect particular blood constituents, rather than whole blood, from donors. Typically, in such systems, whole blood is drawn from a donor, the particular blood component or constituent is removed and collected, and the remaining blood constituents are returned to the donor. By thus removing only particular constituents, less time is needed for the donor&#39;s body to return to normal, and donations can be made at more frequent intervals than when whole blood is collected. This increases the overall supply of blood constituents, such as plasma and platelets, made available for health care. 
     Whole blood is typically separated into its constituents through centrifugation. This requires that the whole blood be passed through a centrifuge after it is withdrawn from, and before it is returned to, the donor. To avoid contamination and possible infection of the donor, the blood is preferably contained within a sealed, sterile system during the entire centrifugation process. Typical blood processing systems thus include a permanent, reusable centrifuge assembly containing the hardware that spins and pumps the blood, and a disposable, sealed and sterile fluid processing assembly that actually makes contact with the donor&#39;s blood. The centrifuge assembly engages and spins the fluid processing assembly during a collection procedure. The blood, however, makes actual contact only with the fluid processing assembly, which is used only once and then discarded. 
     As the whole blood is spun by the centrifuge, the heavier components, such as red blood cells, move outwardly away from the center of rotation toward the outer or “high g” wall of a separating chamber included as part of the fluid processing assembly. The lighter components, such as platelet-rich plasma, migrate toward the inner or “low g” wall of the separating chamber. Typically, an intermediate layer of white blood cells forms an interface between the platelet-rich plasma and red blood cell components of the whole blood during centrifugation. Various ones of these components can be selectively removed from the whole blood by forming appropriately located channeling seals and outlet ports in the separating chamber of the fluid processing assembly. Proper separation requires, however, that the interface between the platelet-rich plasma and the red blood cells be located within a particular zone between the high g and low g walls of the separating chamber. Displacement of the interface from the desired location can result in low platelet yield if the interface is too near the high g wall, or can result in a high whole blood cell count in the extracted plasma if the interface is located too near the low g wall. Good platelet yields along with low whole blood cell counts are achieved when the interface is maintained at the proper, desired location. 
     Various known centrifuges, such as those shown and described in U.S. Pat. No. 5,316,667, are operable to automatically keep the interface within the desired zone as the centrifuge operates. Typically, the separating chamber of the fluid processing assembly is loaded between the bowl and spool of a centrifuge. A ramped surface formed on the interior outer wall of the bowl deflects the high g wall of the separating chamber inwardly relative to the axis of rotation. The interface between the generally dark, opaque whole blood cell layer and the generally light, clear plasma layer appears as a line projected onto the ramped surface. Where, exactly, the line appears on the ramped surface is a function of the position of the interface between the high g and low g walls of the separating chamber. Accordingly, the position of the line on the ramped surface can be used to gauge the position of the interface between the high g and low g surfaces. 
     Automatic control over the location of the interface is achieved by sensing the position of the line on the ramped surface and thereafter adjusting the centrifuge operating parameters to place and keep the line within desired limits. In particular, by controlling the rate at which platelet-rich plasma is withdrawn from the separating chamber, the line can be “moved” up or down on the ramped surface. Typically, an optical sensor is used to sense the position of the line on the ramped surface. As the centrifuge spins past the sensor, the sensor develops an electrical pulse having a width related to the position of the line on the ramped surface. As the line moves closer to the high g wall of the separating chamber, the pulse width increases. As the line moves closer to the low g wall, the pulse width narrows. By sensing the width of the pulses developed by the optical sensor and thereafter using the pulse width to increase or decrease the rate at which platelet-rich plasma is withdrawn from the separating chamber, the line can be kept within desired positional limits on the ramped surface. 
     Prior, optically based interface detection and control systems responded only to the width of the pulse developed by the optical sensor and assumed that pulse width alone was a reliable indicator of the position of the interface line on the ramped surface. However, experience has shown that a variety of abnormalities or unusual operating conditions can arise that make pulse width, by itself, an unreliable sole indicator of proper interface positioning. For example, unusually high or low platelet counts in the donor&#39;s blood can change the light transmisitivity of the plasma and thus the apparent width of the detected pulses with no real change in the actual position of the interface line on the ramped surface. Similarly, a temporary accumulation of platelets on the ramped surface can cause a change in pulse width with no real change in the interface line position. Nevertheless, prior systems, which responded only to the width of the pulses developed by the optical sensor, would view either occurrence as requiring corrective action. The result would be to shift the interface away from the optimum position. 
     SUMMARY OF THE INVENTION 
     The invention provides a centrifugal processing system comprising a centrifuge having a rotatable bowl assembly including a high g wall and a ramped surface formed in the high g wall. The centrifugal processing system further comprises a fluid processing assembly including a separating chamber receivable in the bowl assembly and engaging the high g wall. The centrifugal processing system further comprises an optical detector operative to sense the position of an interface between two dissimilar components of a fluid within the separating chamber during centrifugation and to develop a signal indicative of the position of the interface relative to the ramped surface. A signal processor responsive to the wave shape and amplitude of the signal developed by the optical sensor and operable to develop control outputs based on the shape and amplitude of the signal is also included. The centrifugal processing system further includes control circuitry coupled to the centrifuge and responsive to the control outputs for controlling the operation of the centrifuge in accordance with the sensed shape and amplitude of the signal developed by the optical sensor so as to maintain the position of the interface within a desired zone relative to the ramped surface. 
     The invention also provides an interface position control system for controlling the position of an interface between the component layers of a fluid during centrifugation. The system includes an energy source operable to direct energy onto the interface. The energy thus directed is substantially absorbed by one of the component layers and is substantially reflected by the other of the component layers. A sensor operable to sense the energy reflected by the reflective one of the component layers and to develop a signal indicative of the intensity of the energy thus reflected is also provided. The system further includes a signal processor operable to sense the magnitude, duration and wave-shape of the signal developed by the sensor. 
     A comparator is coupled to the signal processor and is operable to compare the wave-shape of the signal to preselected known wave-shapes. A control system responsive to the signal and to the comparator is provided and is operable to control the operating parameters of the centrifugation process in accordance with the sensed magnitude, duration and wave-shape of the signal sensed by the sensor. 
     The invention also provides a method of operating a centrifuge of the type having a ramped surface and an optical detector for developing a signal indicative of the position of a constituent interface relative to a ramped surface carried within the centrifuge. The method includes the steps of sensing predetermined parameters of the signal, comparing the sensed parameters against predetermined standards, and varying operating parameters of the centrifuge in accordance with the sensed predetermined parameters of the signal. 
     The invention also provides an improvement in a centrifuge of the type having a ramped surface and an optical detector for developing a signal indicative of the position of a constituent interface relative to a ramped surface carried within the centrifuge. The improvement includes a sensor for sensing predetermined parameters of the signal, a comparator for comparing the sensed parameters against predetermined standards, and a controller for varying operating parameters of the centrifuge in accordance with the sensed predetermined parameters of the signal. 
     It is an object of the present invention to provide a new and improved interface detection and control system for detecting and controlling the position of an interface between dissimilar materials in a centrifuging process. 
     It is a further object of the invention to provide an interface detection and control system that automatically responds to changing or abnormal conditions to keep an interface between dissimilar materials properly located during a centrifuging process. 
     It is a further object of the invention to provide an interface detection and control system that is flexible and that can be readily adapted to operate under varying or diverse operating conditions. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The invention, together with the further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals identify like elements, and wherein: 
     FIG. 1 is a side elevation view, with portions broken away and in section, of a blood processing system comprising a centrifuge with an interface detection system, which embodies features of the invention, the bowl and spool of the centrifuge being shown in their operating position; 
     FIG. 2 is a side elevation view, with portions broken away and in section, of the centrifuge shown in FIG. 1, with the bowl and spool of the centrifuge shown in their upright position for receiving a blood processing chamber; 
     FIG. 3 is a top perspective view of the spool of the centrifuge shown in FIG. 2, in its upright position and carrying the blood processing chamber; 
     FIG. 4 is a plan view of the blood processing chamber shown in FIG. 3, out of association with the spool; 
     FIG. 5 is an enlarged perspective view of an interface ramp carried by the centrifuge in association with the blood processing chamber, showing the centrifugally separated red blood cell layer, plasma layer, and interface within the chamber when in a desired location on the ramp; 
     FIG. 6 is an enlarged perspective view of the interface ramp shown in FIG. 5, showing the red blood cell layer and interface at an undesired high location on the ramp; 
     FIG. 7 is an enlarged perspective view of the interface ramp shown in FIG. 5, showing the red blood cell layer and interface at an undesired low location on the ramp; 
     FIG. 8 is a side perspective view of the bowl and spool of the centrifuge when in the operating position, showing the viewing head, which forms a part of the interface controller, being carried by the centrifuge to view the interface ramp during rotation of the bowl; 
     FIG. 9 is a perspective view of the viewing head, with portions broken away and in section, showing the light source and light detector, which are carried by the viewing head, in alignment with the interface ramp, as viewed from within the spool and bowl of the centrifuge; 
     FIG. 10 is a side section view of the bowl, spool, and viewing head when the viewing head is aligned with the interface ramp; 
     FIG. 11 is a block diagram of an interface control system that embodies various features of the invention and operates to maintain the interface at the desired location on the ramp. 
     FIG. 12 is a block diagram of the signal verification subsystem included in the control system of FIG.  11 . 
     FIG. 13 is a block diagram of the signal extraction subsystem included in the control system of FIG.  11 . 
     FIG. 14 is block diagram of the pulse width determination subsystem of the control system of FIG.  11 . 
     FIG. 15 is a waveform diagram of a signal appearing at the output of the optical detector, depicting what is considered a “normal” interface signal of the type leading to good platelet yield and low WBC count. 
     FIG. 16 is a waveform diagram similar to FIG. 15 showing a signal displaying a “camel phenomenon” that is sometimes observed during collection procedures. 
     FIG. 17 is a waveform diagram similar to FIG. 15 showing an inflection point that sometimes occurs in the leading edge of the waveform. 
     FIG. 18 is a waveform diagram similar to FIG. 15 showing a signal that frequently results when the donor is lipemic or has a high platelet count. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 show a blood processing system  10 , which incorporates an interface controller  12  that embodies features of the invention. The invention is described in the context of blood processing, because it is well suited for use in this environment. Still, it should be appreciated that use of the invention is not limited to blood processing. The features of the invention can be used in association with any system in which materials that can be optically differentiated are handled. 
     A. The Centrifuge 
     The system  10  includes a centrifuge  14  used to centrifugally separate blood components. In the illustrated embodiment, the centrifuge  14  separates whole blood to harvest red blood cells (RBC), platelet concentrate (PC), and platelet-poor plasma (PPP). The centrifuge  14  can also be used to harvest mononuclear cells and granulocytes from blood. 
     The centrifuge  14  is of the type shown in U.S. Pat. No. 5,316,667, which is incorporated herein by reference. The centrifuge comprises a bowl  16  and a spool  18 . The bowl  16  and spool  18  are pivoted on a yoke  20  between an upright position, as FIG. 2 shows, and a suspended position, as FIG. 1 shows. 
     When upright, the spool  18  can be opened by movement at least partially out of the bowl  16 , as FIG. 2 shows. In this position, the operator wraps a flexible blood processing chamber  22  (see FIG. 3) about the spool  18 . Closure of the spool  18  and bowl  16  encloses the chamber  22  for processing. When closed, the spool  18  and bowl  16  are pivoted into the suspended position for rotation about an axis. 
     B. The Blood Processing Chamber 
     The blood processing chamber  22  can be variously constructed. FIG. 4 shows a representative preferred embodiment. The chamber  22  shown in FIG. 4 provides multi-stage processing. A first stage  24  separates WB into RBC and platelet-rich plasma (PRP). A second stage  26  separates the PRP into PC and PPP. 
     As FIGS. 3 and 4 best show, a port  28  conveys WB into the first stage  24 . Ports  30  and  32 , respectively, convey PRP and RBC from the first stage  24 . RBC is returned to the donor. A port  34  conveys PRP into the second stage  26 . A port  36  conveys PPP from the second stage  26 , leaving PC in the second stage  26  for resuspension and transfer to one or more storage containers. The ports  28 ,  30 ,  32 ,  34 , and  36  are arranged side-by-side along the top transverse edge of the chamber  22 . 
     As FIGS. 1 and 3 best show, a tubing umbilicus  38  is attached to the ports  28 ,  30 ,  32 ,  34 , and  36 . The umbilicus  38  interconnects the first and second stages  24  and  26  with each other and with pumps and other stationary components located outside the rotating components of the centrifuge  14  (not shown). As FIG. 1 shows, a non-rotating (zero omega) holder  40  holds the upper portion of the umbilicus  38  in a non-rotating position above the suspended spool  18  and bowl  16 . A holder  42  on the yoke  20  rotates the mid-portion of the umbilicus  38  at a first (one omega) speed about the suspended spool  18  and bowl  16 . Another holder  44  (see FIG. 2) rotates the lower end of the umbilicus  38  at a second speed twice the one omega speed (the two omega speed), at which the suspended spool  18  and bowl  16  also rotate. This known relative rotation of the umbilicus  38  keeps it untwisted, in this way avoiding the need for rotating seals. 
     As FIG. 4 shows, a first interior seal  46  is located between the PRP collection port  30  and the WB inlet port  28 . A second interior seal  48  is located between the WB inlet port  28  and the RBC collection port  32 . The interior seals  46  and  48  form a WB inlet passage  50  and a PRP collection region  52  in the first stage  24 . The second seal  48  also forms a RBC collection passage  54  in the first stage  24 . 
     The WB inlet passage  50  channels WB directly into the circumferential flow path immediately next to the PRP collection region  52 . As shown in FIG. 5, the WB separates into an optically dense layer  56  of RBC, which forms as RBC move under the influence of centrifugal force toward the high-G wall  62 . The movement of RBC  56  displaces PRP radially toward the low-G wall  64 , forming a second, less optically dense layer  58 . 
     Centrifugation of WB also forms an intermediate layer  60 , also called the interface, which constitutes the transition between the formed cellular blood components and the liquid plasma component. RBC typically occupy this transition region. White blood cells may also occupy this transition region. 
     Platelets, too, can leave the PRP layer  58  and settle on the interface  60 . This settling action occurs when the radial velocity of the plasma near the interface  60  is not enough to keep the platelets suspended in the PRP layer  58 . Lacking sufficient radial flow of plasma, the platelets fall back and settle on the interface  60 . Larger platelets (greater than about 30 femtoliters) are particularly subject to settling on the interface  60 . However, the closeness of the WB inlet region  50  to the PRP collection region  52  in the chamber  22  shown in FIG. 4 creates strong radial flow of plasma into the PRP collection region  52 . The strong radial flow of plasma lifts platelets, large and small, from the interface  60  and into the PRP collection region  52 . 
     Further details of the separation chamber  22  are not material to the invention and can be found in U.S. Pat. No. 5,316,667, previously mentioned. 
     C. The Interface Controller 
     As FIG. 5 shows, a ramp  66  extends from the high-G wall  62  of the bowl  16  at an angle across the PRP collection region  52 . The angle, measured with respect to the axis of the PRP collection port  30  is preferably about 30°. FIG. 5 shows the orientation of the ramp  66  when viewed from the low-G wall  64  of the spool  18 . FIG. 4 shows, in phantom lines, the orientation of the ramp  66  when viewed from the high-G wall  62  of the bowl  16 . 
     Further details of the angled relationship of the ramp  66  and the PRP collection port  30  are not material to the invention. They can be found in U.S. patent application Ser. No. 08/472,561, filed Jun. 7, 1995, now U.S. Pat. No. 5,632,893,and entitled “Enhanced Yield Blood Processing System with Angled Interface Control Surface,” which is incorporated herein by reference. 
     The ramp  66  forms a tapered wedge that restricts the flow of fluid toward the PRP collection port  30 . The top edge of the ramp  66  extends to form a constricted passage  68  along the low-G wall  64 . PRP must flow through the constricted passage  68  to reach the PRP collection port  30 . 
     As FIG. 5 shows, the ramp  66  diverts the fluid flow along the high-G wall  62 . This flow diversion changes the orientation of the interface  60  between the RBC layer  56  and the PRP layer  58  within the PRP collection region  52 . The ramp  66  thereby displays the RBC layer  56 , PRP layer  58 , and interface  60  for viewing through the low-G wall  64  of the chamber  22 . 
     The interface controller  12  includes a viewing head  70  (see FIGS. 1 and 8) carried on the yoke  20 . The viewing head  70  is oriented to optically view the transition in optical density between the RBC layer  56  and the PRP layer  58  on the ramp  66 . The interface controller  12  analyzes the optical data obtained by the viewing head  70  to derive the location of the interface  60  on the ramp  66  relative to the constricted passage  68 . 
     The location of the interface  60  on the ramp  66  can dynamically shift during blood processing, as FIGS. 6 and 7 show. The interface controller  12  varies the rate at which PRP is drawn from the chamber  22  to keep the interface  60  at a prescribed location on the ramp  66  (which FIG. 5 shows). 
     Maintaining the interface  60  at a prescribed location on the ramp  66  is important. If the location of the interface  60  is too high (that is, if it is too close to the constricted passage  68  leading to the PRP collection port  30 , as FIG. 6 shows), RBC, and, if present, white blood cells can spill over and into the constricted passage  68 , adversely affecting the quality of PRP. On the other hand, if the location of the interface  60  is too low (that is, if it resides too far away from the constricted passage  68 , as FIG. 7 shows), the fluid dynamics advantageous to effective platelet separation can be disrupted. Furthermore, as the distance between the interface  60  and the constricted passage  68  increases, the difficulty of drawing larger platelets into the PRP flow increases. As a result, a distant interface location results in collection of only the smallest platelets, and overall platelet yield will, as a consequence, be poor. 
     (1) The Interface Viewing Head 
     Referring to FIGS. 8 to  10 , the viewing head  70 , carried by the yoke  20 , includes a light source  76 , which emits light that is absorbed by RBC. In the illustrated and preferred embodiment, the light source  76  includes a circular array of red light emitting diodes  80 . Of course, other wavelengths absorbed by RBC, like green or infrared, could be used. 
     In the illustrated embodiment, seven light emitting diodes  80  comprise the light source  76 . More diodes  80  may be used, or fewer diodes  80  can be used, depending upon the optical characteristics desired. 
     The viewing head  70  also includes a light detector  78  (see FIGS.  9  and  10 ), which is mounted adjacent to the light source  76 . In the illustrated and preferred embodiment, the light detector  78  comprises a PIN diode detector, which is located generally in the geometric center of the circular array of light emitting diodes  80 . 
     The yoke  20  and viewing head  70  rotate at a one omega speed, as the spool  18  and bowl  16  rotate at a two omega speed. The light source  76  directs light onto the rotating bowl  16 . In the illustrated embodiment (see FIG.  8 ), the bowl  16  is transparent to the light emitted by the source  76  only in the region  82  where the bowl  16  overlies the interface ramp  66 . In the illustrated embodiment, the region  82  comprises a window cut out in the bowl  16 . The remainder of the bowl  16  that lies in the path of the viewing head  70  comprises a light absorbing material. 
     The interface ramp  66  is made of a light transmissive material. The light from the source  76  will thereby pass through the transparent region  82  of the bowl  16  and the ramp  66  every time the rotating bowl  16  and viewing head  70  align. The spool  18  may also carry a light reflective material  84  behind the interface ramp  66  to enhance its reflective properties. The spool  18  reflects incoming light received from the source  76  out through the transparent region  82  of the bowl  16 , where it is sensed by the detector  78 . In the illustrated embodiment, light passing outward from the source  76  and inward toward the detector  78  passes through a focusing lens  120  (shown in FIGS.  9  and  10 ), which forms a part of the viewing head  70 . 
     The arrangement just described optically differentiates the reflective properties of the interface ramp  66  from the remainder of the bowl  16 . This objective can be achieved in other ways. For example, the light source  76  could be gated on and off with the arrival and passage of the ramp  66  relative to its line of sight. As another example, the bowl  16  outside the transparent region  82  could carry a material that reflects light, but at a different intensity than the reflective material  84  behind the interface ramp  66 . 
     As the transparent interface region  82  of the bowl  16  comes into alignment with the viewing head  70 , the detector  78  will first sense light reflected through the plasma layer  58  on the ramp  66 . Eventually, the RBC layer  56  adjacent the interface  60  on the ramp  66  will enter the optical path of the viewing head  70 . The RBC layer  56  absorbs light from the source  76  and thereby reduces the previously sensed intensity of the reflected light. The intensity of the reflected light sensed by the detector  78  represents the amount of light from the source  76  that is not absorbed by the RBC layer  56  adjacent to the interface  60 . 
     (2) The Interface Control System 
     An interface control system  80  incorporated into the interface controller  12  is shown in block diagram form in FIG.  11 . As shown, a pump  82  is provided for drawing PRP from the separation chamber  22 . In general, the location of the interface  60  relative to the ramp  66  can be controlled by controlling the rate at which PRP is withdrawn from the separation chamber  22 . This, in turn, can be controlled by controlling the operating rate of the pump  82 . In general terms, the interface control system functions to monitor the position of the interface  60  on the ramp  66  and to adjust the operating speed of the pump  80  so as to keep the interface  60  within a desired zone on the ramp  66 . 
     As further illustrated in FIG. 11, the output of the light detector  78  is coupled through a low pass filter  84  to an analog to digital converter  86 . As the ramp  66  rotates past the light source  76  and detector  78 , the resulting signal sensed by the sensor  78  is converted to digital form for further processing by the system  80 . In the illustrated embodiment, the output signal from the detector  78  is sampled every 15 μs. It will be appreciated that other sampling rates can be used. 
     For a number of reasons, it is possible that the light detector  78  might provide a signal output or artifact that is not truly indicative of the actual presence of the ramp adjacent the light source  76  and detector  78  or of the actual location of the interface  60  on the ramp  66 . To avoid system response to such false signals or artifacts, the interface control system  80  includes a signal verification subsystem  88  coupled to the output of the analog to digital converter  86 . Generally, and in accordance with one aspect of the invention, the signal verification subsystem  88  functions to assess the output pulses provided by the detector  78  and verify that the pulses are, in fact, valid and truly representative of actual operating conditions within the centrifuge. Details of the construction and operation of the signal verification subsystem  88  are provided below with reference to FIG.  12 . 
     Once the existence of valid output signals from the detector  78  is verified, the system  80  next extracts one or more of the signals for further use and analysis. To this end, the interface control system  80  further includes a signal extraction subsystem  90  coupled to the output of the signal verification subsystem  88 . In the illustrated embodiment, the signal extraction subsystem  90  functions to time average and align five verified, individually acquired signals from the detector  78  and produce a single composite signal based on the time average of the individual signals. The composite signal thus produced is what is then passed along for further analysis and use by the system  80 . Details of the construction and operation of the signal extraction subsystem  88  are provided below with reference to FIG.  13 . 
     Analysis of the composite signal provided by the signal extraction subsystem  90  is accomplished in a pulse width determination subsystem  92  constructed in accordance with various aspects of the invention. The pulse width determination subsystem  92  functions broadly to determine the width of the pulses detected by the detector  78  and to develop a signal indicative of the detected pulse width. The detected pulse width is then fed to a pulse width comparator  94  that compares the detected pulse width against a known, desired interface standard  96 . Depending upon whether the detected pulse width is greater than, less than, or within desired operating limits, the pulse width comparator  94  signals a pump command circuit  98  to increase, decrease or maintain the speed of the pump  82  to keep the interface  60  within desired limits on the ramp  66 . 
     In accordance with one aspect of the invention, the pulse width determination subsystem  92  determines the pulse width of the detected pulses through a sophisticated analysis of several signal parameters beyond simply signal passage above and below preestablished threshold limits. As will be developed more fully below with respect to FIG. 14, the pulse width determination subsystem  92  takes into account such signal features as signal shape, slope, inflection points, etc., in determining the width of the detected pulses. Such sophisticated analysis of the detected pulses results in more accurate determination of the actual interface position on the ramp  66  and more accurate and effective collection of desired blood constituents. 
     The signal verification subsystem  88  is shown in greater detail in FIG.  12 . As noted, the primary function of the signal verification subsystem is to verify that the signals from the detector  78  are valid and are likely to contain truthful information regarding conditions sensed at the ramp  66 . As shown, the signal verification subsystem  88  includes a sample buffer  100 , a sample counter  102 , a buffered sample qualification circuit  104  and a signal detector circuit  106 . A first controllable switch  108  is coupled to the input of the sample buffer  100 , while a second controllable switch  110  is coupled to the output of the sample buffer  100 . 
     System operation begins with a signal to begin sample collection. This signal, which is generated elsewhere in the centrifuge  10  and indicates that fluid processing is beginning, has the effect of closing the first switch  108  and enabling the sample counter  102  and buffered sample qualifier  104 . When the first switch  108  is closed, signals or samples detected by the detector  78  (FIG. 11) are loaded into the sample buffer  100 . The sample buffer  100  generates a “current sample pointer” signal, which indicates the address at which samples entering the buffer  100  will be written. The “current sample pointer” is reset to zero at the start of sampling and is incremented each time a sample is written into the buffer  100 . 
     The samples are also loaded into the sample counter  102 . The sample counter  102  functions to compare each sample against a predetermined amplitude threshold to verify that the magnitude of the sample is greater than the threshold. Preferably, the threshold is above the noise floor of the system but below the peak value of the expected signal. This helps ensure that the system  88  is responding to legitimate signals developed by the detector  78  and not to random noise or artifacts. 
     Samples that exceed the amplitude threshold are applied to the buffered sample qualification subsystem  104 . When enabled by the signal to begin sample collection, the buffered sample qualification subsystem  104  further verifies the validity of the samples by performing a time-based check on when the samples occur. In particular, the buffered sample qualification system  104  functions to determine whether the samples occur when the sensor  78  is adjacent the ramp  66  or at some other time. Only those samples that occur when the sensor  78  is adjacent the ramp  66  are considered valid. To this end, a “count range” signal is developed as the centrifuge operates and is applied to the buffered sample qualification subsystem  104 . The “count range” signal is derived from the relative positions of the sensor  78  and the ramp  66  and defines the beginning and end of the period during each revolution of the centrifuge during which the sensor  78  and ramp  66  are properly aligned. If the sample occurs during the desired “count range,” the buffered sample qualification subsystem  104  generates a “buffer qualified” control signal. The presence of the “buffer qualified” control signal indicates to the remainder of the system that the samples in the sample buffer  100  have been properly qualified by the sample counter  102  and the buffered sample qualification subsystem  104 . 
     Samples that are properly qualified by the sample counter  102  are also applied to the signal detector circuit  106 . The signal detector circuit  106  also receives the “current sample pointer” signal generated by the sample buffer  100  as well as a “trigger count” signal that represents the minimum value of “counted samples” necessary to indicate that the “buffered samples” contain the expected signal. The signal detector circuit  106  compares the “counted samples” against the “trigger count” and develops a “pointer to signal in buffer” that indicates the buffer address at which the “counted samples” equals the “trigger count.” This provides a rough estimate of the location of the interface signal in the buffered samples. 
     The various control and data signals developed and provided by the signal verification subsystem  88  are applied to the signal extraction subsystem  90  shown in FIG.  13 . The signal extraction subsystem  90  functions broadly to time align and average a predetermined number of samples acquired and qualified by the signal verification subsystem  88  to recreate the output signals of the detector  78  acquired as the ramp  66  passes by. In the illustrated embodiment, the signal extraction subsystem time aligns and averages qualified samples representing five such passages. It will be appreciated that a greater or lesser number of qualified samples can be selected. The output of the signal extraction subsystem  90  comprises a time-varying waveform representing the output signal provided by the detector  78  as averaged over five passes of the ramp  66  past the detector  78 . 
     The time aligned and averaged composite sample signal developed by the signal extraction subsystem  90  is then applied to the pulse width determination subsystem  92  shown in detail in FIG.  14 . In accordance with one aspect of the invention, the pulse width determination subsystem  92  functions to analyze such signal parameters as peak amplitude, total pulse area, area enclosed by threshold and slope changes on the leading edge of the signal. These signal parameters can be used to obtain greater knowledge of the actual position of the interface than was provided by prior systems that simply compared signal amplitude against a fixed threshold to sense signal pulse width. 
     As shown in FIG. 14, the pulse width determination system  92  includes an amplitude/area determining circuit  114  that operates to sense both the amplitude of, and the area under, the applied time aligned and averaged signals. The system  92  further includes first and second differentiator circuits  116 ,  118  that determine, respectively, the first and second derivatives of the applied time aligned and averaged waveforms. The system also includes a smoothing circuit  120  that receives the time aligned and averaged signal, and the first derivative thereof, as inputs and generates an output signal that is substantially continuous and a good facsimile of the original output signals obtained from the detector  78 . The outputs of the amplitude/area circuit  114 , the first differentiator circuit  116 , the second differentiator circuit  118 , and the signal smoothing circuit  120  are applied to a control circuit or decision maker  122  that, based on these various inputs, determines and generates an appropriate threshold. This threshold is applied to a software comparator  124  which then compares the output of the signal smoothing circuit  120  against the threshold to establish a pulse width associated with the time aligned and averaged sample signal. It will be appreciated that by raising or lowering the threshold, the apparent pulse width of the sample can be made to change without any actual change in the sample itself. The pulse width determination system  92  functions to determine where best to set the threshold so that the resulting pulse width is most likely to give a true representation or indication of position of the interface  66  relative to the ramp  60 . Unlike in prior systems, the pulse width determination system of the present invention takes into consideration various characteristics of the signal, such as wave shape and slope, is selecting where to set the threshold. 
     The pulse width determination system  92  further includes an LED current adjust circuit  126  that is coupled to the amplitude/area determining circuit  114  and that controls current in the source LEDs  76 . Preferably, the LED current adjust circuit  126  controls the LED current so that the area under the resulting time aligned and averaged signal is between preestablished upper and lower limits. 
     Because the pulse width determination system  92  is capable of sensing and responding to a variety of signal characteristics and parameters, the system is capable of recognizing and compensating for various known discrepancies in the sensed signals. Several of these conditions are shown in FIGS. 15-18. 
     FIG. 15 depicts what is considered a “normal” interface signal that leads to good platelet yield and low WBC count. The signal is characterized by smooth trailing and leading edges free from inflection points or humps. With such a waveform, the threshold level can be set at any point between the absolute maximum and minimum values of the waveform, and the pulse width can be defined as the width between the resulting crossing points. Typically, the threshold can be determined at the start of a procedure and will remain fixed until the end of the procedure, provided the shape of the signal does not change, particularly with regard to amplitude and the shape of the leading edge. 
     FIG. 16 shows a signal displaying a “camel phenomenon” that is sometimes observed during collection procedures. When thresholds are set below the level of the “hump”, platelet yield is usually low. If the threshold is set significantly above the hump, this has been found to cause RBC flow over the ramp. Accordingly, the optimum threshold setting when the “camel phenomenon” occurs is just above the level of the hump. By ascertaining the first and second derivatives of the pulse leading edge, the pulse width determination system  92  can determine where the peak of the hump is and can thereafter place the threshold at that level or some predetermined amount above that level. If the waveform changes as the platelet collection procedure continues, the system  92  will automatically readjust the threshold to maintain it at the desired position. Similarly, if the “camel phenomenon” terminates during the procedure and the signal waveform returns to normal, (FIG.  15 ), the system  92  will automatically reinstate the normal, predetermined threshold. 
     Still another frequently encountered abnormal waveshape is shown in FIG.  17 . In this waveform, an inflection point occurs in the leading edge of the waveform. Although the exact cause of this phenomenon is not fully understood at this time, it is believed to be possibly caused by an accumulation of platelets on the ramp  66 . Signals exhibiting this waveform are also frequently characterized by an oscillating amplitude which decreases with an increase in platelets on the ramp and increases when the cells are washed off. If the threshold is located above the inflection point, a high WBC frequently results. When this phenomenon occurs, the corrective action is to place the threshold below the inflection point. By ascertaining the second derivative of the sample signal, the system  92  can readily determine the inflection point and thereafter set the threshold at, or some predetermined distance below, the inflection point. Again, the system  92  can automatically change the threshold on a substantially continuous basis to adapt to changes in operating conditions and changes in the signal waveform. 
     FIG. 18 shows a signal that frequently results when the donor is lipemic or has a high platelet count. With this condition, light transmissitivity through the blood is low and the signal amplitude drops and remains low throughout the procedure. Platelet collection efficiency can be low. To compensate for the low transmissive property of the blood, the system  92  increases the intensity of the source LEDs  76 . By increasing the LED current, source brightness is increased and leads to a higher amplitude signal. The signal peak amplitude is used as a feedback parameter for controlling the LED current with the aim, in the illustrated embodiment, of keeping the signal amplitude in the 12-14 V range. Maximum LED current is limited to avoid shortening LED life and, in the illustrated embodiment, is limited to 30 mA. It is possible that even at peak LED brightness, the signal peak amplitude cannot be brought into the desired range. In this case, the system  92  sets the threshold the area enclosed by the threshold is 50% of the total area bounded by the signal. It will be appreciated that other operating parameters can also be used. 
     The system  92  herein shown and described offers operating flexibility beyond that provided by prior systems. In particular, the ability to sense and analyze such signal features as shape and area enables the system  92  to recognize abnormal operating conditions and compensate for them on a dynamic real time basis as a collection procedure proceeds. Although various operating examples have been described for illustrative purposes, it will be appreciated that the system  92  can easily be programmed to operate in other modes and that these examples are meant to be illustrative rather than limiting. 
     While a particular embodiment of the invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications can be made without departing from the invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.