CONTROL SYSTEM FOR A CENTRIFUGE INSTRUMENT

Abstract: A control system (50) for a centrifuge instrument (10) samples the velocity of the shaft (34) at first and second measurement times to generate a signal representative of actual acceleration of the shaft. The actual acceleration signal is compared to a reference acceleration to determine whether a rotor (18) is present on the shaft.

Description: CONTROL SYSTEM FOR A CENTRIFUGE INSTRUMENT Background of the Invention Field of the Invention The present invention relates to a centrifuge instrument having a system for automatically verifying whether a rotor is present on the rotor shaft. Description of the Prior Art A centrifuge instrument is a device adapted to expose a liquid sample carried in a rotating member, called a rotor, to a centrifugal force field. The centrifuge instrument has a motive source that includes a drive shaft. The shaft is adapted to receive any one of a predetermined plurality of rotors thereon. The shaft is an elongated member having a cone-shaped adapter, typically termed a "spud", disposed at the upper end thereof. To mount a rotor on the shaft, the spud is inserted into a correspondingly shaped recess in the rotor. For stability purposes the ratio of the moment of inertia of the spud with a rotor mounted thereon about the axis of rotation with respect to the moment of inertia of these members about an axis perpendicular to the axis of rotation should be greater than one. However, when no rotor is present on the spud this ratio of moments of inertia is unfavorable. Instances have been noted wherein the motive source of the instrument has been asserted without a rotor being present on the shaft. In such instances shaft failure has occurred. In view of the foregoing it is believed advantageous to provide a control system for a centrifuge instrument which automatically ascertains whether a rotor is present on the shaft. Summary of the Invention The present invention relates to a control system for a centrifuge instrument which is adapted to determine whether a rotor is present on the shaft. The centrifuge instrument includes a motive source having a shaft adapted to receive a rotor thereon. Means, such as a tachometer, is adapted to generate a signal representative of the actual velocity of the shaft at predetermined measurement time(s) following initiation of the source. in accordance with the preferred embodiment of the invention means responsive to signals representative of the actual shaft velocities ω a ι , ω 3 2 a respective first and second predetermined measurement times t m ι and tm2 is provided for generating a signal representative of the actual acceleration of the shaft. The signal representative of the actual acceleration of the shaft is compared to a reference acceleration value thereby to determine whether a rotor is present on the shaft. The measurement times occur within a predetermined time window immediately following initiation of a power source and the application of power to the motive source of the instrument. In an alternate embodiment the value of the difference in actual shaft velocities, without the computation of the actual acceleration of the shaft, may be compared to a signal representative of a reference velocity difference thereby to determine whether a rotor is present on the shaft. The predetermined measurement times t ι and tm2 may be selected in any convenient fashion. For example, the time at which the source is energized to initiate rotation of the shaft may be selected as the first predetermined measurement time tmi > such that the calculation of a velocity difference actually degenerates into a comparison of the actual velocity at a predetermined measurement time following initiation of a power source and the application of power to the motive source of the instrument to cause rotation with a predetermined reference velocity. In another alternate embodiment the value of the actual acceleration may be otherwise derived, as by direct measurement and this actual acceleration value compared to a reference acceleration to determine the presence of a rotor on the shaft. Brief Description of the Drawing The invention may be more fully understood from the following detailed description thereof, taken in connection with the accompanying drawing, which forms a part of this application and in which: The Figure is a highly stylized pictorial representation of a centrifuge instrument with which a control system in accordance with the present invention may be used. Detailed Description of the Invention Shown in the Figure is a stylized pictorial representation of a centrifuge instrument generally indicated by reference character 10 with which a control system generally indicated by the reference character 50 embodying the teachings of the present invention may be used. The instrument 10 includes a framework schematically indicated at 12. The framework 12 supports a bowl 14. The interior of the bowl 14 defines a generally enclosed chamber 16 in which a rotating element, or rotor, 18 may be received. Access to the chamber 16 is afforded through a door 20. The bowl 14 may be provided with suitable evaporator coils (not shown) in the event that it is desired to refrigerate the bowl 14, the rotor 18 and its contents. One or more energy containment members, or guard rings, 22 is(are) carried by the framework 12. The guard ring 22 is arranged concentrically with respect to the bowl 14 and serves to absorb the kinetic energy of the rotor 18 or fragments thereof should a catastrophic failure of the rotor 18 occur. The guard ring 22 is movably mounted within the framework 12, as schematically indicated by the rollers 24, to permit free rotation of the ring 22 to absorb any rotational component of the energy of the rotor fragments. It is important to absorb the energy of the rotor and to contain the possible fragments which if permitted to exit the instrument may cause injury to an operator. A motive source 30 is mounted within the framework 12. The motive source 30 may be any one of a well-known variety of sources, such a brushless DC electric motor, an induction motor, or an oil turbine drive. A power source 32 is connected to the motive source 30. The motive source 30 is connected to or includes as an element thereof a drive shaft 34. The drive shaft 34 projects into the chamber 16. The upper end of the shaft 34 is provided with a mounting spud 36 which receives the rotor 18. Any one of a predetermined number of rotor elements may be received on the spud 36. A tachometer generally indicated by the reference character 38 is arranged to monitor the rotational speed (i. e., angular velocity) of the shaft 34. Any convenient form of tachometer arrangement 38 may be utilized and remain within the contemplation of the present invention. An electrical signal representative of the actual angular velocity of the shaft 34 (and of a rotor, if the same is mounted thereon) is carried by an output line 38L from the tachometer 38. As mentioned earlier it is vitally important to verify that a rotor 18 is indeed mounted to the shaft 34. The control system 50 serves this end. The control system 50 in accordance with the present invention includes means 52 associated with the tachometer 38. The means 52 may, for example, sample the tachometer signal representative of the actual velocity ω a of the shaft at one or more predetermined measurement timβ(s) following initiation of the power source 32 and the application of a predetermined power level to the motive source 30. The measurement times occur within a predetermined time window immediately following initiation of the source 32 and the application of power to the motive source 30. Typically the time window is on the order of a few seconds (e.g., three seconds) following initiation of the source 32 and the application of power to the motive source 30. During this measurement time window only a predetermined limited power level is applied from the power source 32 to the motive source 30. The measurement time window occurs early in the centrifugation run to insure that the shaft 34 does not reach a potentially destructive velocity before the presence (or absence) of a rotor is verified. In the preferred instance signals representative of the actual velocity co a 1 and the actual velocity ω a 2 of the shaft 34 at at least a respective first predetermined measurement time tmi and a second predetermined measurement time t m 2 following energization of the motive source 30 are used by the means 52 to generate a signal on an output line 52L thereof representative of the actual acceleration of the shaft 34. The actual acceleration signal on the line 52L is applied to a comparator 56. In the comparator 56 the actual acceleration signal is compared to a reference acceleration signal carried on a line 58L. The reference acceleration signal on the line 58L may be derived from a suitable reference source, as a memory 58. The results of the comparison are output on a line 56L and provide an indication as to whether a rotor is mounted to the spud. As an example, if the actual acceleration of the shaft 34 is greater than the reference acceleration an inference may be made that the shaft 34 is not loaded by a rotor. Accordingly, the output on the line 56L may be used to represent this error condition and may be applied to interrupt power applied to the power source 32 to the motive source 30. In an alternate embodiment of the invention the signal on the line 52L may represent merely the difference between the signal representative of the actual velocity ω a ι of the shaft 34 at the first predetermined measurement time tmi and the signal representative of the actual velocity ω 3 2 of the shaft at the second predetermined measurement time tm2 both immediately following initiation of the source 32 and the application' of power to the motive source 30. The velocity difference signal on the line 52L may likewise be applied to the comparator 56 where it is compared to a reference velocity difference signal, again derived from the memory 58 over the line 58L. In a manner similar to that earlier discussed the results of the comparison provide an indication as to whether a rotor is mounted to the spud and the appropriate action taken. The velocity difference signal should be construed to encompass an arrangement which utilizes the difference in the actual velocity ω a ι of the shaft 34 at measurement time t = 0, (i. e., at the measurement time t m ι corresponding to the energization of the source 30, when the actual velocity is equal to zero) and the actual velocity ω a 2 of the shaft 34 at predetermined measurement time thereafter. It should be understood that the invention as herein described is preferably implemented using a microprocessor based programmable device operating in accordance with a suitable program. As an example of a possible microprocessor implementation following is a listing in Motorola M6801 assembly language of a program implementing one embodiment of the present invention. EXAMPLE The actual value of the acceleration may be calculated in any convenient fashion. One technique is to include a calculation of acceleration as part of the instrument speed control program. The acceleration is calculated every 0.10 second, over a preceding one second period. A speed reading is derived from the tachometer and stored in a buffer-the "Alpha" buffer- at 0.1 second intervals. At each interval, when a new speed reading is available, the value of the speed at one second in the past is recalled. Since the speed readings are spaced one second apart, an acceleration value in units of "revolutions/minute-second" is equal to the difference between the two speed readings. The following subroutine uses the acceleration value "Alpha" to determine whether a rotor is disposed on the shaft. The determination is performed following a three second interval (implemented by lines 1 to 13) during which a predetermined constant current (one ampere) is applied to the electric motor drive. At the end of this time interval the acceleration "Alpha" (derived from the speed control routine as discussed above) is loaded to a buffer (line 14) and is compared (line 15) to a reference acceleration value (225 revolution/minute-seconds, line 15). The reference acceleration value is determined empirically and corresponds to one-half of the acceleration of the shaft with no rotor disposed thereon when the shaft is driven by the motor at reduced current (one amp). To nondestructively determine the acceleration of a shaft having no rotor acceleration of the shaft with a rotor of known inertia is first measured. The acceleration of the shaft with no rotor is then computed by dividing the inertia of the rotor and shaft by the inertia of the shaft alone and multiplying this quotient by the measured acceleration of the shaft having the rotor mounted thereon. The reference acceleration is one-half of this value. If the acceleration does not exceed the reference normal current is applied to the motor and continue instrument operation continues (line 18). If the acceleration exceeds the reference acceleration value, an error is noted (line 17) and control branches to the "_err7a" subroutine (lines 21 to 27) and drive current to the motor is interrupted, thus stopping the run. j s r enbl drv ; enable rail and bridge, output setting to PWM chip 7 cl r Pulse timer;Pulse timer=0 initially 8 bra end7a ; Return to main speed control for 0.1 second 9-nxt7a Idaa Pulse_timer ; pulse timer = pulse timer + 0.1 second 1 0 inca 1 1 staa Pulse_timer 1 2 cmpa#30 ; compare to thirty increments (three seconds) of pulse timer 1 3 bne end7a ; if less than three seconds, return 14 Idd #Alpha ; check if the acceleration exceeds reference (225 rpm/sec) 1 5 Idx #_max_accel_at_lamp 1 6 j s r fcmpr 1 7 bcs err7a ; if Alpha > Max accel at one amp, signal error 1 8 Idaa #SC SETSTRT ; else Alpha < Max_accel, set normal current Sstate _end7a Bfaults ; fault, acceleration too great #IMPROB_SPD ; post fault Bfaults ErrTyp .set shutdown bit #DRVERR ErrTyp shutdown ; shut off drive current -o-O-o As yet another alternative, the control system 50 may include means 60 that is directly or indirectly associated with the motive source 30, the power source 32 or the shaft 34 to generate a signal representative of the actual acceleration of the shaft. The actual acceleration signal may be applied to the means 60 on an input line 60I. As an example, an accelerometer may be mechanically coupled to the shaft 34 and used to generate a signal representative of the actual acceleration thereof. The signal representative of the actual acceleration, as otherwise derived, is applied to the comparator 56 on the line 60L. Again a comparison between the actual acceleration signal on the line 60L and a reference acceleration signal on the line 58L is effected. The results of the comparison are again used to provide an indication as to whether a rotor is mounted to the shaft. Those skilled in the art, having the benefits of the teachings of the present invention as hereinabove set forth, may effect numerous modifications thereto. Such modifications are to be construed as lying within the contemplation of the present invention, as defined by the appended claims.

Claims: WHAT IS CLAIMED IS: 1. A centrifuge instrument comprising: a motive source having a shaft adapted to receive a rotor thereon; means for generating a signal representative of the acceleration of the shaft at a predetermined measurement time tm immediately following initiation of the source; means for comparing the signal representative of the acceleration of the shaft with a predetermined reference acceleration thereby to determine whether a rotor is present on the shaft. 2. A centrifuge instrument comprising: a motive source having a shaft adapted to receive a rotor thereon; means for generating a signal representative of the actual velocity ω a ι of the shaft at at least a first predetermined measurement time t m . and a signal representative of the actual velocity ω a 2 of the shaft a second predetermined measurement time t m 2, both the measurement times tmi and t m 2 occurring within a predetermined time window following initiation of the source and the application of a predetermined power level to the shaft; means responsive to the velocity signals ω a ι and ω a 2 for generating a signal representative of the actual acceleration of the shaft; and means for comparing the signal representative of the actual acceleration of the shaft with a reference acceleration thereby to determine whether a rotor is present on the shaft. 3. A centrifuge instrument comprising: a motive source having a shaft adapted to receive a rotor thereon; means for generating a signal representative of the actual velocity co a ι of the shaft at a first predetermined measurement time t m ι and a signal representative of the actual velocity ω a 2 of the shaft a second predetermined measurement time t m 2 » both the measurement times t m ι and tm2 occurring within a predetermined time window following initiation of the source and the application of a predetermined power level to the shaft; means responsive to the velocity signals ω a ι and ω a 2 for generating a signal representative of the actual velocity difference; and means for comparing the signal representative of the actual velocity difference with a reference velocity difference thereby to determine whether a rotor is present on the shaft. 4. The centrifuge instrument of claim 3 wherein the first predetermined measurement time t m ι is the time at which the source is energized to initiate rotation of the rotor. 5. A method for determining the presence of a rotor on a shaft centrifuge instrument comprising: initiating a power source to apply a predetermined power level from the power source to a motive source having a shaft adapted to receive a rotor thereby to initiate rotation of the shaft; generating a signal representative of the acceleration of the shaft at a predetermined measurement time t m immediately following initiation of the source; comparing the signal representative of the acceleration of the shaft with a predetermined reference acceleration thereby to determine whether a rotor is present on the shaft. 6. A method for determining the presence of a rotor on a shaft centrifuge instrument comprising: initiating a power source to apply a predetermined power level from the power source to a motive source having a shaft adapted to receive a rotor thereby to initiate rotation of the shaft; generating a signal representative of the actual velocity ω a ι of the shaft at at least a first predetermined measurement time tmi and a signal representative of the actual velocity ω 3 2 of the shaft a second predetermined measurement time tm2. both the measurement times t m ι and t 2 occurring within a predetermined time window following initiation of the source and the application of a predetermined power level to the shaft; generating a signal representative of the actual acceleration of the shaft in response to the velocity signals ω a ι and co a 2 ; and comparing the signal representative of the actual acceleration of the shaft with a reference acceleration thereby to determine whether a rotor is present on the shaft. 7. A method for determining the presence of a rotor on a shaft centrifuge instrument comprising: initiating a power source to apply a predetermined power level from the power source to a motive source having a shaft adapted to receive a rotor thereby to initiate rotation of the shaft; generating a signal representative of the actual velocity ω a ι of the shaft at a first predetermined measurement time t m i and a signal representative of the actual velocity ω a 2 of the shaft a second predetermined measurement time t 2 , both the measurement times t m ι and t m 2 occurring within a predetermined time window following initiation of the source and the application of a predetermined power level to the shaft; generating a signal representative of the actual velocity difference in response to the velocity signals ω a ι and ω a 2 ; and means for comparing the signal representative of the actual velocity difference with a reference velocity difference thereby to determine whether a rotor is present on the shaft. 8. The method of claim 7 wherein the first predetermined measurement time t m ι is the time at which the source is energized to initiate rotation of the rotor.

Inventors: GOLD MICHAEL SCOTT, MIRANDA VICTOR WILLIAM JR, GOLD, MICHAEL, SCOTT, MIRANDA, VICTOR, WILLIAM, JR.
Applicants: DU PONT, E.I. DU PONT DE NEMOURS AND COMPANY
Application year: 1991
Main classification: G01M19
IPC codes: B04B13, G01M19