Overspeed Control For Centrifuge

Abstract

The centrifuge overspeed control disclosed herein facilitates the use of different rotors having different maximum speeds and automatically limits the rotational speed of the centrifuge to an appropriate level. As the rotor rotates, an optical pickup derives timing signals from first and second indicia carried by the rotor and deenergizes the centrifuge if the interval between successive timing signals drops below a predetermined value.

Description

BACKGROUND OF THE INVENTION

This invention relates to an overspeed control for centrifuges and more particularly to a control for a centrifuge adapted to employ various rotors having different maximum safe speeds.

While various speed control systems have been devised heretofore for centrifuges, these prior art controls often do not afford complete safety in the case of a centrifuge adapted to employ a variety of rotors having different maximum speeds since the control typically must be set to the appropriate speed. Thus, an incorrect setting could result in the bursting of a rotor when operated at excessive speed.

Among the several objects of the present invention may be noted the provision of apparatus for limiting the rotational speed of a centrifuge; the provision of such apparatus which will automatically limit to an appropriate level the speed of any of various different rotors which may be used with the centrifuge; the provision of such apparatus which is highly reliable; and the provision of such apparatus which is relatively simple and inexpensive.

SUMMARY OF THE INVENTION

Briefly, apparatus of the present invention is operative to limit the rotational speed of a centrifuge which is itself adapted to employ different rotors having respective predetermined maximum speeds. A respective means associated with each rotor carries at least first and second indicia, the angular separation between the first and second being substantially proportional to the predetermined maximum speed of the corresponding rotor. A sensing means provides a pulse signal each time one of the indicia passes a preselected angular position. A timing signal of predetermined duration is generated in response to each of the pulse signals. Further acceleration of the centrifuge is prevented if pulse signals generated by a second of the indicia occur within the durations of timing signals initiated by the first of the indicia.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a centrifuge-drive assembly showing an optical pickup as employed in one embodiment of this invention;

FIG. 2 is a view substantially on the line 2-2 of FIG. 1;

FIG. 3 is a block diagram of an overspeed control circuit according to this invention; and

FIGS. 4 and 5 are graphical representations of signals occuring at various points in the FIG. 3 circuit under different conditions of operation.

Corresponding reference characters indicate corresponding parts throughout the several views of the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there is indicated at 11 a motor drive for a centrifuge. Drive 11 is adapted to spin, by means of a shaft 13, a centrifuge rotor as indicated generally at 15. Rotor 15 may, for example, carry a plurality of sample tubes 17. In order to permit a given centrifuge to be used for different separation processes, it is often desirable that the rotor 15 be readily removable so that various rotors can be substituted. However, as is understood in the art, the different rotors will typically have different maximum safe speeds of rotation, these maximum safe speeds being some preselected portion of their absolute maximum or burst speeds.

According to one aspect of the present invention, there is associated with each rotor 15, a means carrying at least first and second indicia, the angular separation between the first and second being substantially proportional to the predetermined maximum operating speed for that rotor. In the embodiment illustrated, the indicia are in the form of radial slots 21 and 23 cut into the face 25 of a disc 27 on the bottom of the rotor 15.

A photoelectric sensor or pickup 31 is mounted adjacent to the operational position of disc 25 for providing a pulse signal each time one of the indicia passes a preselected angular position. Preferably, the pickup 31 comprises a lamp 33 and a photodetector or photocell 35 arranged so that the light from the lamp 33 is normally reflected off face 25 into the photocell 35, the face 25 being highly polished for this purpose. The slots 21 and 23 are preferably shaped, as illustrated in FIG. 1, so as to deflect the light beam from lamp 33 away from the photocell 35. Thus, as the rotor 15 spins, the photocell 35 will be normally illuminated except for brief periods when the light path is interrupted by the passage of one of the slots 21 or 23. Accordingly, the output signal from photocell 35 will be a relatively high level except for the pulses which are generated as the indicia pass pickup 31. In the embodiment of the control circuit described hereinafter, it is assumed that the normal output of the photocell 35 is a positive voltage with the pulse signals being negative-going.

Referring now to FIG. 3, the motor drive 11 is shown as being selectively energized from a pair of supply leads L1 and L2 through a tach failure relay 37, an overspeed relay 38 and a no rotor relay 39. Relays 37--39 are preferably connected in series in the energization circuit for the motor drive 11 so that all three must be energized before the motor drive 11 can be energized. Thus, if any one of these relays is deenergized, the motor drive 11 cannot be energized or will be deenergized if previously energized. As will be understood by those skilled in the art, the motor drive itself may comprise further control circuit relating to the normal operation of the centrifuge.

The signal obtained from the photocell 35 is applied to a unity-gain amplifier 41 to obtain a low impedance signal source. The signal is then applied to a level-detector circuit 43 and to pulse-shaping circuitry indicated generally at 45. The level-detector circuit 43 operates to energize the no-rotor relay 39 only if the nominal or average DC signal level provided by the photocell 35 is above a preselected level. As may be seen from FIG. 1, light from lamp 33 will not be reflected into the photocell 35 if no rotor is present on shaft 13. Thus, the relay 39 will be deenergized, preventing energization of the motor drive 11, unless a rotor is in place on shaft 13.

In the pulse-shaping circuit 45, the signal from amplifier 41 is AC coupled, through a capacitor C1, to an amplifier of selected gain. The amplifier output is AC coupled through a capacitor C2 to a circuit comprising a pair of diodes D1 and D2 arranged to charge a capacitor C3 to a voltage which is substantially equal to the peak-to-peak voltage of the signal from the photocell. A predetermined proportion of the voltage on capacitor C3 is obtained by means of a divider comprising resistors R1 and R2 and this proportional voltage is applied to one of the inputs of a comparator 47. The AC coupled pulse signal is applied to the other of the inputs of the comparator. Comparator 47 operates to provide an output signal which is a function of the larger of the two input signals applied thereto. Since one of the input signals to comparator 47 is a fixed proportion or percentage of the peak-to-peak amplitude of the photocell pulse signal, this circuit can thus be understood to function as a pulse-shaping circuit which effectively removes or strips off noise or other low-level variations in the nominal level of the signal provided by the photocell, thereby leaving only a relatively well-defined pulse signal. In other words, this circuit provides an operating threshold which varies as a function of the amplitude of the pulse signal provided by the photoelectric pickup. Further squaring of this pulse signal is provided by a Schmitt trigger circuit 49.

The pulse signal provided by the shaping circuitry 45 is applied to trigger a one-shot multivibrator circuit 51. The characteristics or parameters of the multivibrator circuit 51 are selected to provide a relatively short pulse each time the multivibrator is triggered. Thus, the one-shot multivibrator 51 provides a signal comprising a single pulse each time one of the slots 21 or 23 passes the photoelectric pickup 31. However, these pulses are of fixed or predetermined duration as contrasted with the pulses obtained from the pickup itself which have durations related to the speed of the rotor 15.

The pulse output signal from one-shot multivibrator circuit 51 is applied to trigger a second one-shot multivibrator circuit 53, the multivibrator 53 being triggered by the trailing edge of the pulse signal provided by the multivibrator 51. As is explained in greater detail hereinafter, the parameters of multivibrator 53 provide an output pulse having a duration which is a function of the relationship between the angular separation of the indicia on each rotor and its respective maximum speed. The output signals from the two one-shot multivibrators 51 and 53 are combined in an AND gate 55. Thus, gate 55 will pass a pulse only if multivibrator 51 is triggered within the interval of a previously initiated pulse from multivibrator 53.

The output pulses from AND gate 55, if any, are applied to a rate-meter circuit 57 to provide a signal which will trigger a level detector circuit 59 when such pulses are provided repetitively. Rate-meter circuit 57 may, for example, comprise a staircase generator with a predetermined time constant. When triggered, the level detector circuit 59 operates a latch circuit 65 which deenergizes the overspeed relay 38. The operation of the latch circuit 61 is such as to maintain the deenergization of the overspeed relay even through the output signal from the rate-meter circuit 57 subsequently drops below the operating threshold of the level detector circuit 59. The latch circuit 65, however, may be reset to reenergize the overspeed relay by means of a separate reset signal as indicated.

The output signal from the one-shot multivibrator 53 is also applied to a second rate-meter circuit 67 whose output signal is applied to a level detector circuit 69. Level detector 69 in turn controls the tach failure relay 37. The operation of the level detector circuit 69 is such as to deenergize the tach failure relay 37 unless pulses are provided repetitively to the rate-meter circuit 67. The tach failure relay 37 is bridged by contacts K1 for starting purposes.

The output pulses provided by the one-shot multivibrator circuit 53 are further provided to a flip-flop or binary circuit 71 so as to provide at a terminal 73 a square wave signal having a frequency which is half the repetition frequency of the pulse signals from multivibrator 53.

The operation of this apparatus is substantially as follows. As noted previously, the motor drive 11 cannot be energized unless a rotor is present on shaft 13. Otherwise, the nominal output signal from photocell 35 is not high enough to cause energization of the no-rotor relay 39. Assuming that the latch circuit 65 has been reset so that the overspeed relay 38 is energized, the motor drive 11 can then be energized as desired, the tach failure relay 37 being temporarily bypassed by means of contacts K1. Once the rotor 15 is spinning and the contacts K1 are opened, any failure in the system which prevents repetitive triggering of the one-shot multivibrator circuit 53 will cause the tach failure relay to deenergize the motor drive and result in a shutdown. Examples of such possible failures are the burning out of lamp 33 or the breakage of the leads to the photoelectric pickup.

As the speed of the rotor 15 increases, the time interval between each pulse generated by the passage of the slot 23 and that generated by the passage of the slot 21 will decrease.

The angular separation between the slots 21 and 23 for each rotor is selected so that the time interval between the pulses generated by these slots will be equal to the sum of the operating times of the two one-shot multivibrators 51 and 53 when the rotor has just reached its predetermined maximum operating speed. In this regard, the operating times of the multivibrators are chosen so that the range of slot angles available covers a useful range of speeds.

FIG. 4 illustrates the pulse timing which occurs when the rotor 15 is spinning at a normal operating speed while FIG. 5 illustrates the pulse timing which occurs when the speed of the rotor exceeds the predetermined limit speed at which it is desired to initiate a shutdown of the motor drive 11. In FIGS. 4 and 5, the letters adjacent each waveform indicate the correspondingly designated point in the FIG. 3 circuit where the waveform occurs.

When a rotor is spinning at a normal operating speed, e.g., about 80 percent of its preselected maximum as illustrated, the output pulse from the multivibrator 53 initiated by the slot 23 ends before the next pulse from the multivibrator 51 initiated by the slot 21. This can be seen in FIG. 4. However, when the rotor 15 exceeds its preselected maximum speed, an overlap in timing will occur as illustrated in FIG. 5. Thus, the output signal from the AND gate 55 will comprise repetitive pulses as illustrated at C in FIG. 5. As noted previously, a succession of pulses applied to the rate-meter circuit 57 will cause the level detector 59 to trip the latch circuit 65 and thereby deenergize the overspeed relay 38. Accordingly, overspeed operation of the rotor is prevented.

Since the system treats the signals due to the two slots in the same way, there is no need to differentiate between the two slots. Accordingly, it may be seen that the angular separation between slots 21 and 23 may be increased up to a maximum of 180.degree..

As will be apparent from the foregoing description, the maximum speed to which each rotor can be driven is determined by the angle between the slots 21 and 23. Thus, by providing each rotor with slots or other indicia having appropriate angular spacings, different rotors may be substituted without resetting any of the parameters of the control apparatus itself. Accordingly, opportunities for human error are substantially reduced. In addition, the system provides for fail-safe operation in any of a variety of possible human, mechanical or electrical failures.

In view of the foregoing, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

As various changes could be made in the above construction without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

What we claim is:

1. In a centrifuge adapted to employ different rotors having respective predetermined maximum speeds and having a drive means which is energized to rotate a selected one of said rotors, apparatus for limiting the rotational speed of the centrifuge comprising:

a respective means associated with each rotor carrying at least first and second indicia, the angular separation between the first and the second being substantially proportional to the predetermined maximum speed for the corresponding rotor;

sensing means for providing a pulse signal each time one of said indicia passes a preselected angular position as the rotor rotates;

means for generating a timing signal of predetermined duration in response to each of said pulse signals; and

means for deenergizing said drive means to stop the centrifuge rotor is pulse signals generated by the second of said indicia occur within the duration of timing signals initiated by the first of said indicia.

2. Apparatus as set forth in claim 1 wherein said sensing means comprises an optical pickup including a lamp and a photodetector.

3. Apparatus as set forth in claim 2 wherein said first and second indicia are carried by a disc which rotates with the respective rotor and which is adapted to direct light from said lamp to said photodetector except when one of said indicia passes the pickup.

4. Apparatus as set forth in claim 3 wherein said disc has a substantially reflective surface and said indicia are slots which interrupt said surface and break the light path between said lamp and said photodetector.

5. Apparatus as set forth in claim 3 including means for stopping the centrifuge if the average light level sensed by said photodetector falls below a preselected level.

6. Apparatus as set forth in claim 3 wherein said means for providing a pulse signal includes a trigger circuit having an operating threshold which varies in response to variations in the amplitude of the signal provided by said photodetector.

7. Apparatus as set forth in claim 1 wherein said means for generating a timing signal includes a monostable multivibrator which is triggered by said pulse signals.

8. Apparatus as set forth in claim 7 wherein said means for providing a pulse signal includes a monostable multivibrator having an operating period which is relatively short with respect to the predetermined duration of the timing signal provided by the first said multivibrator.

9. Apparatus as set forth in claim 1 including means for stopping the centrifuge if said pulse signal is not provided repetitively.

10. Apparatus as set forth in claim 1 wherein said means for deenergizing said drive means operates to stop the centrifuge.

11. Apparatus as set forth in claim 1 wherein said means for deenergizing said drive means comprises a gate for combining said pulse signals and said timing signals to generate an overspeed pulse signal when one of the first said pulse signals occurs within the duration of a previously initiated timing signal.

12. Apparatus as set forth in claim 11 including means for stopping the centrifuge if said overspeed pulses are generated repetitively.

13. Apparatus as set forth in claim 1 including means for dividing said pulse signal by a number equal to the number of indicia on each rotor thereby to provide a signal having a frequency equal to the rotational speed of the rotor.

14. In a centrifuge adapted to employ different rotors having respective predetermined maximum speeds and having a drive means which is energized to rotate a selected one of said rotors, apparatus for limiting the rotational speed of the centrifuge comprising:

a respective means associated with each rotor carrying at least first and second indicia, the angular separation between the first and the second being substantially proportional to the predetermined maximum speed for the corresponding rotor;

means, including a first one-shot multivibrator, for providing a pulse signal of predetermined duration each time one of said indicia passes a preselected angular position as the rotor rotates;

means, including a second one-shot multivibrator, for generating a timing signal of predetermined duration in response to each of said pulse signals; and

means, including an AND gate for combining said pulse and timing signals, for deenergizing said drive means to stop the centrifuge rotor if pulse signals generated by the second of said indicia occur within the duration of timing signals initiated by the first of said indicia.

15. In a centrifuge adapted to employ different rotors having respective predetermined maximum speeds and having a drive means which is energized to rotate a selected one of said rotors, apparatus for limiting the rotational speed of the centrifuge comprising:

a light source;

a photodetector;

a respective means associated with each rotor carrying at least first and second indicia, the angular separation between the first and the second indicia being substantially proportional to the predetermined maximum speed for the corresponding rotor, said indicia carrying means being adapted to pass light from said source to said photodetector except when one of said indicia passes a predetermined angular position;

means controlled by said photodetector for providing a pulse signal each time one of said indicia passes said predetermined angular position as the rotor rotates;

means for generating a timing signal of predetermined duration in response to each of said pulse signals;

means for preventing operation of the centrifuge if the average amount of light reaching said photodetector is below a predetermined level; and

means for deenergizing said drive means to stop the centrifuge rotor if pulse signals generated by the second of said indicia occur within the duration of timing signals initiated by the first of said indicia.

16. In a centrifuge adapted to employ different rotors having respective predetermined maximum speeds and having a drive means which is energized to rotate a selected one of said rotors, apparatus for limiting the rotational speed of the centrifuge comprising:

a light source;

a photodetector;

a respective means associated with each rotor carrying at least first and second indicia, the angular separation between the first and the second indicia being substantially proportional to the predetermined maximum speed for the corresponding rotor, said indicia carrying means being adapted to pass light from said source to said photodetector except when one of said indicia passes a predetermined angular position;

means controlled by said photodetector for providing a pulse signal of predetermined duration each time one of said indicia passes said predetermined angular position as the rotor rotates;

means for generating a timing signal of predetermined duration in response to the trailing edge of each of said pulse signals; and

means for deenergizing said drive means to stop the centrifuge rotor if pulse signals generated by the second of said indicia occur within the duration of timing signals initiated by the first of said indicia.