WO1987000770A1

WO1987000770A1 - Overspeed protection signal override system for a centrifuge apparatus

Info

Publication number: WO1987000770A1
Application number: PCT/US1986/001488
Authority: WO
Inventors: Robert J. Ehret
Original assignee: Beckman Instruments, Inc.
Priority date: 1985-08-09
Filing date: 1986-07-16
Publication date: 1987-02-12
Also published as: EP0232324A1; JPS63500437A

Abstract

An overspeed protection signal override system disables the tach signal from a centrifuge rotor tachometer, so that the inverter-drive control circuit is unable to send a multi-phase motor drive signal to an A.C. induction motor, and the centrifuge rotor and motor are shut down, in response to an alarm signal from the overspeed protection circuit. The override signal is provided to assure that once the overspeed protection circuit of the centrifuge detects an alarm condition characteristic of rotor overspeed, the tach signal which drives the inverter is disabled and withdrawn from motor control, so that an A.C. induction motor will coast to rest even if the current control of the induction motor is not functioning properly.

Description

OVERSPEED PROTECTION SIGNAL OVERRIDE SYSTEM FOR A CENTRIFUGE APPARATUS

Field of the Invention

This invention relates to an overspeed protection and shutdown system for centrifuge apparatus and, more particularly, to an improved overspeed shutdown system for centrifuge apparatus which is driven by an A.C. induction motor to shut, down the drive signal to the induction motor in the event that the conventional overspeed protection and shutdown circuit fails.

Background of the Invention

Biological research centrifuge apparatus require a wide variety of rotors interchangeably to be inserted on the rotor-receiving motor shaft hub so that a multiplicity of experimentation may be achieved by use .of a single centrifuge machine.

Each interchangeable rotor is of a different size and weight and may otherwise cause rotor mishap at differing maximum speeds depending on the strength and weight of the interchangeable rotor attached to the induction motor hub. Therefore, each rotor requires a separate identification and rating in order to assure safe usage of the centrifuge as the different rotors are interchanged during experimentation. It has been determined that the laboratory technician using the centrifuge rotor cannot always be relied upon to place the proper rotor into operation and run that rotor at no more than its rated speed.

It has therefore become necessary to provide automatic overspeed protection. In the past some protection devices have been mechanical and have had the inherent defects of mechanical apparatus such as susceptibility to metal fatigue and wear. In U.S. Patent 3,436,637, there is disclosed one manner of overspeed shutdown protection. U.S. Patent 3,436,637 describes an overspeed protection system having a transducer which detects through a phototransistor an optical signal whose frequency was a function of rotor speed of the centrifuge rotor. The detected frequency was mixed with a standard reference frequency to provide a difference frequency. The difference frequency was passed through a low pass filter to an alarm circuit for disconnecting the drive power to the centrifuge motor. Each rotor had a maximum s.psed rating encoded as an integral part of its base in the farm of an optically detectable disc defining a plurality of sectors corresponding to the speed rating. The greater the revolutions per minute (R.P.M.), the fewer sectors dividing the circumference of the rotor base. For example, at 66,429 R.P.M.'s, 14 sectors were sufficient to compare rotor speed with a local oscillator frequency of 15.5 kilocycles. At 19,787 R.P.M., 47 sectors were required to achieve the same comparison frequency. As the rotor spins up to the speed of the reference frequency, its difference from that frequency is diminished to within the range of the low pass filter. The low pass filer, for example, might be set at 40 cycles per second (Hz) so that when the frequency of. the rotor signal came within 40 Hz of the reference frequency, the system would shut down. The low pass filter would activate a gate trigger signal, turning on a silicon-controlled rectifier (SCR) and diverting the main power current through a pathway other than the drive control circuit, shutting down the drive motor.

The above design was further improved upon in U.S. Patent 4,284,931, which is herein incorporated by reference. The overspeed protection system described in U.S. Patent 3,436,637 depends upon a beat frequency which' approaches zero as the overspeed signal frequency became equal to that of the reference signal. The overspeed demodulator 19 of U.S. Patent 3,436,637 transmitted a frequency difference signal to the low pass filter 23. The low pass filter was designed to allow only very low frequencies of 40 Hz or lower to pass. When such a low frequency signal was passed on to the rest of the system, the overspeed shutdown circuit was turned on. However, experimentation had indicated that the low pass filter was not always accurate in responding to the system parameters. For example, light weight rotors which accelerate rapidly were able to accelerate past their maximum speed rating to a point where the frequency difference between the rotational speed signal and the reference signal was greater than the predetermined frequency. It was found that the low pass filter acting alone could not distinguish between those cases when rotational speed signal had a frequency greater than the frequency of the reference signal and when the signal had a frequency below the reference signal. Therefore, U.S. Patent 4,284,931 added circuit elements to the overspeed protection system which provided additional pathways by means of a digital phase detector which compared the reference oscillator to the tach signal from the amplifier. A exclusive OR gate mixer or demodulator passed signals on to a low pass filter as before. Now, however, the signal output of the low pass filter was added to the output signal from a digital phase detector so -that, by one pathway or another, the alarm conditions would be detected.

An additional method of motor control is disclosed in U.S. Patent 4,286,203. In this 4,286,203 patent, a closed loop drive system for variable speed induction motors is disclosed which maintains a constant slip frequency regardless of motor speed. This system makes use of a phase locked loop to develop a constant frequency for driving an inverter for controlling an A.C. induction motor. This system can also be used to shut down the system by providing a zero slip frequency to the inverter drive.

Induction- motors have been used in centrifuges because they are brushless and not as likely to wear as D.C. motors. An induction motor has current flow in its stator winding creating a rotating magnetic field for driving the motor and centrifuge rotor. The frequency of the current signal imparted to the stator defines the synchronous speed of the motor. An induction motor produces a driving torque at a speed below the synchronous speed. If the rotor speed is greater than the stator frequency, braking torque is produced. In either event,, the induction motor must have slip. The slip frequency is def-ined as the difference between the stator frequency and the actual rotational speed of the motor. During ordinary motor operation, it is desirable to maintain a constant slip frequency regardless of motor speed. Since the slip frequency is the difference between the motor drive frequency and the actual rotational speed (generally detected by a tachometer which generates a tach frequency signal), by creating a zero tach frequency or interrupting the tach frequency signal, the system may cause the centrifuge motor to coast to a stop, thereby avoiding rotor mishap due to rotor overspeed.

In the conventional art, when the rotor protection system detected an overspeed alarm condition, circumstances have arisen where the system was unable to respond to such alarm signal. As disclosed in Patent 4,284,931, the system responds to rotor overspeed by disconnecting power from the drive motor. Should the disconnecting means fail, there exists the possibility that power to the rotor drive might not be interrupted. Such a failure could arise if a relay failed or a computer driven circuit had a logic signal failure. Additionally there might be failure in the system wherein the silicon controlled rectifier (SCR) which is used to divert and shut off the power is not triggered. A loss of control of the current would then occur and the system would not be responsive to an overspeed shutdown condition. To alleviate this problem, it is necessary to further modify the overspeed shutdown system for a centrifuge apparatus as described in U.S. Patent 4,284,931 to further include a method and structure whereby the drive motor would be shut down in the event that the overspeed protection circuitry loses control of the power system.

What is required is a system which will respond to an unsafe rotor speed condition regardless of whether that system has the ability to shut down the current and power to the drive motor.

Summary of the Invention This invention relates to an improved overspeed shutdown system for centrifuge apparatus wherein the improved system responds to a rotor overspeed alarm condition regardless of current and power control of the drive motor.

Induction motors require a tach signal which differs in frequency from the motor drive frequency signal in order to develop torque and be driven by an inverter. If one were to override and remove the tach signal and disconnect that signal from the inverter input, one could achieve motor shutdown even though the current command fails to control current and power to the drive motor. A preferred embodiment which accomplishes this tach signal override configuration is disclosed in this application.

When a rotor overspeed condition is indicated which requires shutdown of the induction drive motor of the- centrifuge, this alarm condition is signalled to a flip-flop or latch which responds to the alarm condition signal by changing state at its output from, for example, a normally logic "high" condition to a logic "low" condition. This output signal is imparted, in the preferred embodiment, to one input of a logic gate, which preferably is a NOR or NAND logic gate. Also input to this logic gate is the tach signal from the drive motor. The tach signal is necessary for normal motor operation and is not usually interrupted by the logic gate. The improved alarm system places the tach signal at a second input to the logic gate. When the signal from the latch indicates alarm, the tach signal is inhibited and the inverter-motor controller no longer receives the tach signal necessary to drive the motor. In this manner, the induction drive motor may be shut down, regardless of the alarm system's ability to control current and power to the motor.

Brief Description of the Drawings The drawing is a sole figure including a schematic diagram, partly in block form, of an improved overspeed shutdown system for a centrifuge apparatus constructed in accordance with the teachings of the present invention.

Detailed Description of the Preferred Embodiment

With reference to the drawing, there is shown an overspeed shutdown system, generally, having a number of elements in common with the overspeed shutdown system of prior U.S. Patent Nos. 3,436,637 and 4,284,931.

More specifically, the circuitry of the invention is generally designed for use with centrifuge apparatus including a rotor 11 mounted for rotation on the drive shaft 12. A full description of the alarm circuitry is provided by U. S. Patent 4,284,931, heretofore incorporated by reference. A reflective disc 15 may be mounted on the underside of the rotor 11 which has alternating reflective and non-reflective sectors for the determination of the maximum safe rotor speed that a particular rotor is rated to operate at. The tranducer means 13 detects a modulated light beam pattern eminating from the disc 15 when light, produced by the optical means 16, is reflected off the disc 15. The transducer produces an alternating periodic real-time rotor signal corresponding in frequency to the modulated 'light beam. This rotor signal is transmitted along conductive pathway 14 for amplification by the amplifier 17.

From the amplifier 17, the rotor signal is routed to frequency comparison circuitry comprising an exclusive OR gate 19 and a digital phase detector 35. The rotor signal is compared to a synthesized reference signal generate by the oscillator 21, said oscillator 21 controlled by the crystal 22 in the preferred embodiment. The comparison of the frequency of the rotor signal from the amplifier 17 with the reference signal from the oscillator 21 at the exclusive OR gate 19 results in an error output signal representing the difference between the rotor and reference signals. This error output signal is transmitted along conductive pathway 20 to the input of low pass filter 24."

As the speed of the rotor is increased by the drive motor 28, the frequency of the rotor signal through amplifier 17 approaches the frequency of the reference signal. As these two frequencies approach equivalence, the error signal is reduced to a very low frequency. The pass band of the low pass filter 24 may be made extremely narrow. Only a very low frequency error signal may be passed through the filter 24 to the summing circuit 37, for further processing and transmission to the overspeed shutdown circuit 26 along pathway 27, for signaling an alarm condition.

This overspeed alarm system inherently has some delay due to the alarm system's use of the low pass filter 24. The delay originates from the time that error signal falls below the threshold of the low pass filter 24 and continues until the time that power is disconnected from the drive motor by an alarm signal emanating from circuit 26. This time delay is of great concern in the case of lightweight rotors which accelerate rapidly. In such a case, the rotor can be accelerated past its maximum rating, to a point where the frequency difference between the rotor and reference signals is greater than the bandpass of the low pass filter 24, so that this lightweight rotor may accelerate into overspeed avoiding detection by the alarm circuity. This is because low pass filter 24 cannot distinguish between the circumstances when the frequency of oscillator 21 exceeds the frequency of amplifier 17 by an amount greater than the pass band of filter 24 and the circumstances when the frequency of oscillator 21 is less than the frequency of amplifier 17 by an amount greater than the pass band of filter 24. To prevent an undetected overspeed incident from occurring, the digital, phase detector 35 produces an output signal along pathway 36 which signifies an unambiguous response to the existence of an overspeed condition. The output of digital phase detector 35 will have one polarity if the frequency of the signal from oscillator 21 exceeds the frequency of the signal from amplifier 17 and will have the opposite polarity if the frequency of the signal from amplifier 17 exceeds the frequency of the signal from oscillator 21. The output signals from the amplifier 17 may be a symmetrical square wave; but, the output signals from the oscillator 21 are usually symmetrical square waves and therefore the edge of the square waves are more easily detected by the digital phase detector 35 than a low pass filter 24. The output of this detector 35 is not as sharp as the output of the low pass filter 24, so the optimum overspeed detection system provides the output signals of the low pass filter 24 or the digital phase detector 35, whichever senses true alarm conditions, to the summing circuit 37. By providing the output signals of the low pass filter 24 or the digital phase detector 35, to the summing circuit- 37, the improved alarm signal system develops an input signal to the protection circuit 26 which is the reversed polarity of the normal negative polarity signal generated by the detector 35. This reversed polarity signal is detected by the overspeed shutdown circuit 26, and is conveyed to circuit 26 along pathway 36, through summing circuit 37, and along pathway 27.

In the preferred embodiment, the overspeed protection circuit 26 may be a silicon controlled rectifier (SCR) which is turned "on" when an alarm signal is generated at the output of the summing circuit 37. Turning "on" the SCR by tripping its gate diverts all current through the rectifier to ground and away from the power supply of the converter 29.

In the event that the overspeed protection and shutdown circuit is unable to respond to the alarm J U

conditions indicated when a rotor approaches its maximum rate- of speed, the improvement of this invention provides the structure for controlling the drive motor and shutting down the rotor even if control over the drive motor current and power fails. The structure for providing an improved rotor overspeed safeguard system includes the circuitry shown generally as 100 within the dotted lines at the lower center of the Figure.

The improved rotor overspeed safeguard system includes a flip-flop or latch 102, which receives an alarm signal along pathway 30 at its set "S" input. This alarm signal to the input of latch 102 creates a logic change of state in the output "Q" of the latch 102. Once the output of latch 102 at "Q" changes state, it may be restored to its original setting only by a signal input to the reset "R" terminal over pathway 110. In the preferred embodiment, the latch 102 may only be reset by power transferred along pathway 70 through a manual control switch 115 or other switching device. The intent of this design is to require human intervention to disable an alarm signal once the alarm is sounded.

A motor tach signal is generated by a transducer 50 which detects the actual motor drive speed from drive motor 28 to produce a signal which is indicative of the true speed of the induction drive motor 28. The motor tach signal generated by the output of the transducer 50 is conveyed to the input of the logic NOR gate 104 via conductive pathway 120. During normal operation of the rotor, the logic NOR gate 104 allows the motor tach signal to be transmitted along conductive pathway 60 for driving the inverter 29 which drives the motor 28 via pathways 40. During normal operation, the motor tach signal must be transmitted on to the inverter 29 along pathway 60 to run the motor. Imparting an alarm logic signal from latch 102 to logic gate 104 inhibits the tach signal 120 and prevents that signal from being imparted to the inverter 29. (The logic gate 104, in the preferred embodiment, is a NOR gate. It may also be a HAND gate or an analog switch, such as a CD4051B, 4052B, or CD4053B COS/MOS analog multiplexer/demultiplexer manufactured by RCA. Any data selector component, such as a digitally controlled analog switch, may be substituted for the NOR gate as logic gate 104.) Inhibiting the tach signal shuts down the induction drive motor, even if the power supply remains- on, since without a tach signal, the induction drive motor 28 has insufficient torque and cannot function.

In this manner, an improved system is simply provided which is responsive to the existence of an alarm condition even where the command of the current and power fail. - -

It should be noted that the preferred embodiment is illustrative of an improved rotor overspeed protection system. The scope of the invention is not necessarily limited to the preferred embodiment. Many structural changes are possible and those changes are intended to be within the scope of this disclosure. For example, the tach signal is inhibited or diverted from the inverter 29 by means of a logic hardware system. In the alternative, a computer which runs the motor drive during normal operation could be programmed for a subroutine which inhibits the tach signal from transducer 50 in the event of an overspeed alarm condition. Consequently, the specific structural and functional details of the preferred embodiment of the overspeed protection and shutdown system are merely representative, yet they are deemed to afford the best embodiment for purposes of the disclosure and for providing support for «2 the claims which define the scope of the present invention. This structure in no way is intended to limit the method of drive motor shutdown disclosed herein in the event of a power failure or a failure to control power to the specific structure disclosed herein.

Claims

What is claimed is:

1. An overspeed protection system for a centrifuge apparatus including a rotor driven by a drive motor, comprising: means for detecting rotor overspeed and generating an output alarm signal responsive to rotor overspeed; means for generating a motor tach signal indicating actual motor speed; and means responsive to the alarm signal for inhibiting the motor tach signal whereby the drive motor may be controlled regardless of whether current to the drive motor is shut down.

2. The overspeed protection system according to claim 1, wherein the signal inhibiting means further comprises: a signal latching means, said latching.means responsive to the output alarm signal generated by the rotor overspeed detection means; the electronic latching means generating a latch output signal, the latch output signal coupled to a logic gate; the motor tach signal also coupled to the logic gate; the logic gate enabling the tach signal and conveying said tach signal to the drive motor when operating conditions are normal; and the logic -gate inhibiting the tach signal, preventing said signal from driving the motor, when the logic gate receives a latch output signal indicative of alarm conditions.

3. The overspeed protection system as in claim 2, wherein the logic gate is a NOR gate.

4. The overspeed protection system of claim 2 wherein the logic gate is a NAND gate.

5. The overspeed protection system of claim 2 wherein the logic gate is a digitally controlled analog switch.

6. The overspeed protection system of claims 3, 4 or 5 wherein the latching means is an RS flip-flop circuit.

7. The overspeed protection system of claim 6 wherein the RS flip-flop circuit is reset by operator intervention only, so that once the flip-flop is set

.to indicate alarm conditions, it cannot be reset except by operator intervention.

8. In a centrifuge, an overspeed indicating system, comprising: means for generating a real-time signal corresponding to centrifuge rotor speed; means for generating an alarm signal which is indicative of conditions when the real-time signal extends beyond a predetermined range; and, means for generating a second real-time signal corresponding to the centrifuge drive shaft speed; means for inhibiting further transmission of the second real-time signal when the alarm signal is generated.

9. In a centrifuge rotor system which has interchangeable rotors for operation in conjunction with a centrifuge motor, a method for preventing said motor from operation beyond the maximum safe speed rated for a selected interchangeable rotor, comprising the steps of

developing a motor timing signal for driving the centrifuge motor;

developing a motor tach signal indicative of actual centrifuge motor speed, said motor timing signal being a dependent function of said motor tach signal;

developing an alarm signal indicative of centrifuge rotor overspeed conditions; and

inhibiting said motor tach signal for correspondingly inhibiting said motor timing signal, thereby terminating the motor drive so that the centrifuge motor will coast to rest preventing rotor overspeed and rotor mishap.