US3436637A - Overspeed shutdown system for centrifuge apparatus - Google Patents

Description

April 1, 1969 R; .1. EHRET Filed July 29, 1966 OVERSPEED SHUTDOWN SYSTEM FOR CENTRIFUGE APPARATUS ROTOR 22 TIM- CRYSTAL N H FIG l 2| 2 osc.

L I I8 9 Era-25 23 \\\V I f LIMITER OVERSPEED LOW PASS AMPLIFIER DEMODULATOR FILTER T PHOTO 26 DRIVE TRANSISTOR SIGNAL SCR MOTOR DEMOD 24 [4b RPM yg 7? TO DRIVE CONTROL ICIRCUIT |0 IOOPF Q6 'OOPF INVENTOR.

ROBERT J. EHRET ATTORNEYS United States Patent US. Cl. 318-449 3 Claims ABSTRACT OF THE DISCLOSURE An overspeed protection system for a centrifuge apparatus or the like including a transducer for providing an output signal whose frequency is a function of the rotational speed of the centrifuge rotor and a mixer for mixing the output signal with a standard reference frequency signal to provide a difference frequency signal. A low pass filter is coupled to the output of the mixer for passing the difference frequency signal when it falls below a predetermined frequency to means for disconnecting the drive power to the centrifuge rotor. A detector is also provided to monitor the signal output of the transducer and disconnect the drive power in the event of a loss of signal.

The present invention is generally directed to an overspeed shutdown system for centrifuge apparatus and more particularly to a shutdown system for such apparatus having changeable rotors of different maximum speed ratings.

A centrifuge apparatus generally has the capability of accepting rotors of many different speed ratings and sizes to fit the specific centrifuge application desired. Naturally each rotor has its maximum speed rating and must be protected against overspeed and resultant damage.

Overspeed protection has been provided in the past to guard against either operator error or a malfunction of the speed control circuit. Some types of overspeed protection devices have been substantially mechanical and these have sometimes had the inherent defects of mechanical apparatus such as susceptibilit to metal fatigue and wear. Electrical circuits have also been used, but again the circuit itself can malfunction or can be maladjusted by an operator.

It is a general object of the invention to provide an overspeed shutdown system for centrifuge apparatus which is non-mechanical and is not susceptible to maladjustment by an operator.

It is another and more specific object of the invention to provide an overspeed shutdown system in which the system itself is protected against internal failure.

Further objects and features of the invention will appear from the following description in Which the preferred embodiment of the invention has been set forth in detail in conjunction with the accompanying drawings.

Referring to the drawings:

FIGURE 1 is a schematic diagram of an overspeed shutdown system embodying the present invention and also includes a partial elevation view of a centrifuge rotor;

FIGURE 2 is a partial cross-sectional view taken generally along the lines 22 of FIGURE 1; and

FIGURE 3 is a circuit diagram of the schematic block diagram circuit of FIGURE 1.

Referring to FIGURE 1, the centrifuge apparatus includes a rotor 11 having a corresponding maximum speed rating and is mounted for rotation on a shaft 12 within an enclosure 13 (only partially shown). Fixed to the bottom of rotor 11 with its center coincident with the shaft 12 is a reflective disk 14, best shown in FIGURE 2, having alternate reflective and non-reflective sectors 14a and 14b. The total number of sectors is related to the maximum speed 3,436,637 Patented Apr. 1, 1969 of the rotor. Such maximum speed rating of the rotor will be discussed in detail below.

Optical means for sensing the speed of the rotor include light reflective disk 14 and a light source 16 so positioned to direct light toward the reflective sectors 14a of disk 14. Light receiving means responsive to the reflected light include a phototransistor 17 having one terminal grounded and the other terminal coupled to a limiter amplifier 18. When the transistor has light impinging upon its receiver surface, it completes the circuit to ground to energize the limiter amplifier 18. An alternating periodic waveform is thus produced which is related to the actual speed of the disk 14 and the rotor 11 and to the number of sectors in the disk. Limiter 18 is coupled to an overspeed demodulator or mixer 19 and a signal demodulator or detector 20. A stabilized standard signal frequency is also coupled to overspeed demodulator 19 to produce a beat signal or difference frequency signal. Such a standard signal is provided for generation of a very stabilized frequency signal which is then coupled to the overspeed demodulator 19 through a frequency divider 25. The latter eliminates any asymmetr in the oscillator output to produce a relatively symmetrical square wave which has a minimum of harmonics.

A low pass filter 23 is coupled to overspeed demodulator 19 and is constructed to pass a frequency which is relatively low compared to the oscillator frequency. In particular low pass filter 23 is designed to pass the difference frequency output signal provided by overspeed demodulator 20 only when such signal falls below a predetermined minimum frequency. For example, with an oscillator frequency of 31 kilocycles (15.5 kilocycles when divided by 2), the pass band of the filter 23 has been constructed in the preferred embodiment to terminate at approximately 40 cycles per second In this manner, low pass filter 23 passes the difference frequency signal from demodulator 20 only when the speed of rotor 11 exceeds a predetermined maximum speed since as the speed of the rotor 11 increases the frequency of the difference signal, which is the frequency difference between the frequency of the signal proportional to the speed of the rotor and the standard reference frequency signal derived from oscillator 21, derived from mixer 20 decreases.

A silicon controlled rectifier (SCR) 24 has its activating gate coupled to th eoutput of low pass filter 23. When the difference frequency from overspeed demodulator 19 reaches a predetermined minimum, a trigger signal is produced by low pass filter 23 and actuates the SCR 24 to complete a circuit to ground, which in turn causes the centrifuge apparatus to be shut down.

More specifically, the driving system of the centrifuge includes a drive motor 26 coupled to shaft 12 which in turn is controlled by a drive control circuit 27 connected to an external AC line. The operator of the apparatus de termines speed by speed control 28 and in practice adjusts the speed control for the proper revolutions per minute of the particular rotor 11 being used in the centrifuge apparatus. Energization of the drive circuit is determined by a relay 29 having a switch 31 Which is normally open and in series with the energization circuit of the drive system. Switch 31 is closed by energization of a relay coil 32 coupled to the collector of a transistor 33. The emitter of the transistor is connected to a resistor 34 to a plus voltage source, +V, and its base to signal demodulator 20.

Signal demodulator 20 is responsive to any periodic output signal from phototransistor 17 to produce an actuating base signal to transistor 33 to pull in switch 31 and maintain the drive motor in an on condition. An auxiliary drive acceleration circuit (not shown) produces the initial rotation of the rotor 11 so that relay 29 may be closed.

3 The anode of SCR 24 is coupled to the collector of transistor 33 and serves to ground the collector, thus deenergizing relay 29 and opening the drive control circuit when a signal is received from low pass filter 23 closing SCR 24.

OPERATION Since each rotor 11 has a particular maximum speed rating, it includes, as an integral part, a disk 14 which has a number of sectors corresponding to its speed rating. The number of sectors is determined first by the speed rating and secondly by the frequency of the local oscillator which feeds the overspeed demodulator 19.

For example, with a local oscillator frequency of 15.5 kilocycles, the following table relates the maximum speed After the rotor is placed into its cabinet and initially accelerated, a signal from limiter amplifier 18 energizes signal demodulator 20 to cause relay 29 to be closed to energize the main drive control circuit 27. It is apparent that if there is a malfunction in either light source 16, phototransistor 17, or limiter amplifier 18, that the overspeed circuit acts in a fail-safe mode to prevent full energization of drive control circuit 27. Moreover, if an improper rotor 11 is placed in the apparatus which has no disk 14, again the drive system will be prevented from full operation.

Assuming that the system is operating properly, the output of limiter amplifier 18, when coupled into overspeed demodulator 19, will produce a difference frequency which as the rotor is coming up to the speed determined by the speed selector 28, is a relatively high frequency. At this point low pass filter will have no output. However, if due to a malfunction of the drive circuit or an error in the setting of the speed control 28 by the operator the speed exceeds the maximum rated speed, low pass filter will sense a very low difference frequency to actuate SCR 24 grounding the emitter of transistor 33 and shutting down the drive system. This latter function of the overspeed protection system may be best understood with reference to FIG. 3 which is a schematic illustration of the overspeed protection system shown in block diagram form in FIG. 1. The frequency signal proportional to the rotational speed of rotor 11 is effectively coupled to the base electrode of transistor 33 by way of limiting amplifier 18 and transistor Q This signal in turn biases transistor 33 into conduction completing a circuit between the +24 v. power supply voltage and circuit ground. Current then flows from the +24 v. power supply through resistor 34, emitter-collector path of transistor 33, and relay 29 to ground thereby closing switch 31 and coupling power to the drive motor 26. Relay 29 remains energized so long as rotor 11 is operating within its designed speed limits since no signal is passed by low pass filter 23 to the gate terminal of SCR 24.

If rotor 11 exceeds its predetermined rotational speed limit, a signal is passed by low pass filter 23 and impressed upon the gate terminal of SCR 24 switching SCR 24 to its high current conducting state. This connects the bottom side of resistor 34 directly to ground so that current now flows from the +24 v. power supply directly to ground through resistor 34. In other words current is diverted away from coil 32 of relay 29 thereby opening switch 31 and disconnecting the power to the drive motor to shut down the centrifuge apparatus.

Once the SCR 24 is switched to its high current conducting state, it is effectively clamped to the +24 v. power supply. Thus, a constant current whose magnitude is limited only by the resistance of resistor 34 flows from the +24 v. power supply through the anode and cathode terminals of SCR 24 to ground. It is an inherent property of a silicon controlled rectifier that so long as a current of a suflicient magnitude is flowing from its anode to cathode terminals the silicon controlled rectifier will remain in its high current conducting state. Thus, once switched to its high current conducting state SCR 24 remains in this state regardless of the presence or absence of a signal at its gating terminal. This in turn means that once switch 31 is opened it remains open until the power supply is disconnected from the SCR 24 thereby resetting SCR 24 to its low current conducting state. It follows that once power is disconnected from the drive motor due to excessive speed of the rotor the centrifuge system is completely shut down.

As noted above the detailed circuit of FIGURE 3 shows the block electrical components of FIGURE 1 in greater detail, and more specifically includes phototransistor 17, limiter amplifier 18 which includes breakdown diode D and overspeed demodulator 19 which includes a transistor Q and the associated low pass filter 23 which is of the RC type coupled to SCR 24. Signal demodulator 20 includes transistor Q coupled to the base of transistor 33 which in turn is coupled to relay 29. Finally, oscillator 21 comprises transistors Q and Q which are coupled to quartz crystal 22 and the divider circuit 25 includes transistors Q and Q The proper values of resistance, capacitance, voltage and frequency are indicated on the drawing along with the particular transistor types.

The above circuit has been successfully operated and, for example, in the case of a rotor having a maximum nominal rating of 65,000 rpm. with a design rating of 62,000 rpm, the actual shutdown frequency was 61,919 rpm. This is less than a .2% error and is a significant improvement over the prior art which had as much as 5%l0% deviations.

In summary, the present invention provides an overspeed shutdown system which has a relatively high sensitivity coupled with a circuit which is extremely trouble free, stable, and needs almost no adjustment. The crystal controlled oscillator 21, 22 inherently maintains a stable frequency output, and the low pass filter 23 being a passive element cannot be maladjusted. Lastly, fail-safe operation is furnished by the requirement that some sort of periodic alternating signal be detected in the overspeed system by signal demodulator 20.

I claim:

1. An overspeed protection system for a centrifuge apparatus including a rotor driven by a drive motor comprising:

transducer means for providing a signal whose frequency is a function of the rotational speed of the centrifuge rotor;

means for generating a reference signal having a predetermined frequency;

means for mixing the rotational speed signal with the reference signal to provide a difference frequency signal whose frequency is the difference betwen the frequency of the rotational speed signal and the frequency of the reference signal;

a filter coupled to the output of said mixing means for passing signals having frequencies below a predetermined frequency;

means coupled to said filter for disconnecting power to the drive motor in response to an output signal from said filter; and

means connected to said transducer means for disconnecting power to the drive motor in the absence of a signal output from said transducer means.

2. An overspeed protection system as defined in claim 1 wherein said means coupled to said filter for disconnecting power to the drive motor comprises:

a switching means connected in circuit with the drive motor; and

a controlled rectifier having anode, cathode and gate terminals, said anode and cathode terminals being connected in parallel with said switching means and said gate terminal being connected to the output of said filter.

3. An overspeed protection system for a centrifuge apparatus including a rotor driven by a drive motor comprising:

means for providing a signal whose frequency is a function of the rotational speed of the centrifuge rotor;

means for providing a reference signal having a predetermined frequency;

means for mixing the rotational speed signal with the reference signal to provide a dilference frequency signal whose frequency is the difference between the frequency of rotational speed signal and the frequency of the reference signal;

a filter coupled to said mixing means for passing only signals whose frequencies are below a predetermined minimum frequency;

normally closed switching means connected in circuit with the drive motor; and

a controlled rectifier having anode, cathode and gate terminals, said anode and cathode terminals being connected in circuit with said switching means and said gate terminal being connected to the output of said filter so that a signal passed by said filter is impressed upon said gate terminal to drive said controlled rectifier into its high conduction state thereby opening said switching means and disconnecting power from said drive motor.

References Cited UNITED STATES PATENTS 2,809,339 10/1957 Guggi 318327 3,138,357 6/1964 Whitwell et a1. 3l8-480 X 3,258,669 6/1966 Krassoievitch 318318 X 3,281,634 10/1966 Studer 318--318 X ORIS L. RADER, Primary Examiner. B. A. COOPER, Assistant Examiner.

US. Cl. X.R. 317-19; 318-318, 465

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