Belt driven centrifugal separator with multi-stage, belt deterioration alerting display

In a centrifugal separator of the type in which driving power of a motor is transmitted to a rotor via a power transmission mechanism, such as a belt, a motor-rotation signal frequency fm and a rotor-rotation signal frequency fr are computed, on the basis of which a frequency ratio A (fr/fm) is computed. When the frequency ratio A exceeds the upper limit of a first predetermined range, a warning message is displayed to prompt the user to perform maintenance. When the frequency ratio A exceeds the upper limit of a second predetermined range, an alarm message is displayed and the motor is stopped.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a centrifugal separator, and more particularly to a belt driven centrifugal separator in which driving power of a motor is transmitted to a rotor via a driving power transmission mechanism, such as belt.

2. Description of the Related Art

Rotor driving systems of a centrifugal separator can be classified into a direct driving type in which a rotor is directly coupled to the rotational shaft of the motor, and an indirect driving type in which the rotor and the motor are coupled via a driving power transmitting mechanism including, for example, a belt. The centrifugal separator of the direct driving type is more frequently used in the art than that of the indirect driving type due to simplicity in structure and high driving power transmission efficiency. However, the direct driving type centrifugal separator requires a motor to be disposed in alignment with the rotational shaft of the rotor, so that the position in which the motor is disposed is restricted and the vertical dimension of the centrifugal separator increases.

When a user desires a low height centrifugal separator, such as a tabletop centrifugal separator, for the reasons of easy-to-access to a rotation chamber, the direct driving type centrifugal separator is more suitable than the indirect driving type. The indirect driving type can provide a low height centrifugal separator because a motor can be disposed aside the rotation chamber with the use of a driving power transmission mechanism including a belt or the like to transmit the driving power of the motor to the rotor. The indirect driving type is adopted when a motor designed to use for another purpose is used for the centrifugal separator or when the direct driving type is not available for the reasons of internal arrangement of the components.

For the indirect driving type centrifugal separator, the rotational speed of the motor is controlled so that the rotational speed of the rotor is set to a target value. Typically, the rotational speed of the rotor is detected magnetically or optically. With the magnetic detection, magnets are secured to the rotor or the rotor shaft and a Hall element is disposed to confront the rotating magnets and generate pulses with a frequency proportional to the rotational speed of the rotor. With the optical detection, a photo-interrupter is used in which light emitting and light detecting elements are disposed in opposition with a disk interposed therebetween. The disk is formed with slits and coaxially attached to the rotor shaft. The light detecting element generates pulses with a frequency proportional to the rotational speed of the rotor. The pulses generated from the Hall element or the light detecting elements are applied to a microprocessor for computation of the rotational speed of the rotor. The rotational speed of the motor is controlled to be a desired value based on the rotational speed of the rotor computed by the microcomputer.

Even if the above-described control is carried out, the belt or other components of the driving power transmission mechanism would suffer from damages when slippage of the belt occurs. If the centrifugal separator is used while leaving the damaged belt as it stands, the motor might be damaged due to overload imposed thereupon or the belt might be fatally damaged. As a result, the rotor may not be able to rotate or to reach to a predetermined rotational speed even if the rotational speed of the motor is increased.

In order to prevent the damage of the motor, Japanese Patent Application Publication No. Hei-10-118529 proposes an abnormality detection system in which abnormality of the driving power transmission mechanism is detected by comparing the rotation signals of the motor and the rotor. However, the proposed abnormality detection system produces abnormal signals whenever the comparison results indicate that the rotational relation of the motor and the rotor is offset from the exactly normal status. Normally, a small amount of slippage does not cause any problem, thus can be neglected. The abnormality signals produced from the abnormality detecting system includes not only real abnormality signals but also redundant and unneeded abnormality signals.

Japanese Patent Application Publication No. 2003-10734 proposes a centrifugal separator with an abnormality detecting device in which redundant and unneeded abnormality signals are not generated.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the invention to provide a centrifugal separator that can accurately detect a broad range of malfunction and alert the user of the present malfunction status.

Another object of the invention is to provide a centrifugal separator that can prevent the mechanical wear of driving power transmission components, such as belt, from increasing.

Still another object of the invention is to provide a centrifugal separator that can prevent the motor from being damaged caused by the mechanical wear of the driving power transmission components.

In order to achieve the above and other objects, there is provided a centrifugal separator that includes a motor that has a driving shaft and generates driving power; a rotor that is configured to accommodate a sample subject to centrifuge; a rotational shaft that supports the rotor to be rotatable therewith; a driving power transmission mechanism that is coupled between the driving shaft and the rotational shaft and transmits the driving power of the motor to the rotational shaft on which the rotor is supported; a monitoring unit that monitors an operating status of the driving power transmission mechanism and outputs a status signal indicative of the operating status of the driving power transmission mechanism; a motor control unit that controls the motor; and a multi-stage alerting unit that alerts a user that the driving power transmission mechanism is one of a predetermined number of different stage malfunction statuses based on the status signal output from the monitoring unit.

When the predetermined number of different stage malfunction statuses includes a first stage malfunction status and a second stage malfunction status, the first stage malfunction status is set less serious in degree of malfunction than the second stage malfunction status. In this case, the motor control unit may forcibly stop rotations of the motor when the multi-stage alerting unit alerts the user that the driving power transmission mechanism is in the second stage malfunction status. Further, the motor control unit may control the motor to decrease torque of the motor when the multi-stage alerting unit alerts the user that the driving power transmission mechanism is in the first stage malfunction status.

Alternatively, the motor control unit may control the motor to decrease the torque of the motor on a step-by-step basis when the multi-stage alerting unit alerts the user that the driving power transmission mechanism is in the first stage malfunction status. In this case, the multi-stage alerting unit may alert the user that the driving power transmission mechanism is in the second stage malfunction status when the torque of the motor has decreased to a predetermined level.

The multi-stage alerting unit may be a display device. The display device may selectively display one of a first indication corresponding to the first stage malfunction status, and a second indication corresponding to the second stage malfunction status. The first indication may be a warning message and the second indication may be an alarm message.

The monitoring unit may include a first pulse generator that generates a first pulse signal having a first frequency determined depending upon a rotational frequency of the motor; a second pulse generator that generates a second pulse signal having a second frequency determined depending upon a rotational frequency of the rotor; and a computing unit that computes a frequency ratio of the first frequency to the second frequency. A display device may further be provided for displaying a warning message when the frequency ratio computed by the control unit is out of a first predetermined range. In this case, the motor control unit may control the motor to stop rotations when the frequency ratio computed by the control unit exceeds upper limit of a second predetermined range. It should be noted that the second predetermined range includes the first predetermined range and covers a broader range than the first predetermined range. It is preferable that the motor control unit control torque of the motor so that the frequency ratio falls within the first predetermined range.

The driving power transmission mechanism includes a first pulley provided to the driving shaft of the motor, a second pulley provided to the rotational shaft, and a belt that is supported between the first pulley and the second pulley and transmits the driving power generated by the motor to the rotational shaft.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A centrifugal separator in accordance with a first embodiment of the invention will be described with reference toFIGS. 1 through 12B.

FIG. 1is a cross-sectional view showing a centrifugal separator1in accordance with the first embodiment. The centrifugal separator1has a housing15in which an operation chamber13is housed. A rotor2and its rotational shaft3are disposed inside the operation chamber13. The rotational shaft3is vertically oriented and rotatably supported on the bottom wall of the operation chamber13. The lower end portion of the rotational shaft3penetrates into and extends outwardly of the bottom wall of the operation chamber13. A pulley3ais fixedly attached to the lower end of the rotational shaft3. The rotor2is detachably mounted on the top end portion of the rotational shaft3to be rotatable therewith.

A motor4, a belt5, and a control unit7are disposed outside the operation chamber13but inside the housing15. The motor4has a driving shaft4ato which a pulley4bis fixedly attached. The belt5is supported with tension between the pulleys4band3a. In accordance with the first embodiment, the pulleys4a,3a, and the belt5amake up a driving power transmission mechanism for transmitting driving power generated by the motor4to the rotor2. As the rotor2rotates, a sample held in the rotor2is subject to centrifugal separation.

A door6and a display panel12are provided above the housing15. The door6covers the upper open portions of the operation chamber13and housing15. The display panel12is used to display a set or actual rotational number of the rotor2, time set to execute centrifugal process or expiration time from the start of centrifugal process, warning or alarm message when a malfunction occurs, as will be described later.

The rotor2is replaceable with another one that can be selected from a plurality of different types of rotors. The rotor2has a bottom plate to which two or more magnets2aare secured. The magnets2aserve as a discriminator for discriminating the type of the rotor2. The magnets2aare arranged on the bottom plate of the rotor2along a circle coaxial with the rotational shaft3. The positional relation between the magnets2aand the number of magnets2asecured to the rotor2aare determined in advance depending upon the type of the rotor, and are thus unique information of the rotor. Stated differently, detection of the positional relation between the magnets2aand the number of magnets2asecured to the rotor enables identification of the type of the rotor. Such information is stored in a memory (not shown) of the control unit7in relation with the type of the rotor. When it is necessary to identify the type of rotor2, the information stored in the memory is retrieved.

A rotor-rotation signal generator8is provided beneath the rotor2. A Hall element is used as the rotor-rotation signal generator8and disposed in a position where the magnets2acan confront when moving with the rotor2. The rotor-rotation signal generator8generates rotor-rotation signals11that differ in waveform depending upon the arrangement positions of the magnets2aand the number of the magnets2a. The rotor-rotation signal11is in the form of a pulse train as shown inFIG. 3. The term “rotor-rotation signal frequency” will be used hereinafter to define a number of pulses occurring per unit time. The rotor-rotation signals11axe transmitted to the control unit7.

A motor-rotation signal generator4dis disposed above the motor4for generating motor-rotation signals10indicative of the rotational speed of the motor4. As shown inFIG. 3, the motor-rotation signal10is in the form of a pulse train. The term “motor-rotation signal frequency” will be used hereinafter to define a number of pulses occurring per unit time. The motor-rotation signal frequency is in proportion to the rotational speed of the motor4. The motor-rotation signals10are transmitted to the control unit7.

FIG. 2is a block diagram showing the arrangement of the control unit7. The control unit7includes a central processing unit (CPU)7a. Various signals are input to the CPU7a. Based on the input signals, the CPU7aimplements various processes including a centrifugal control process, a rotor discriminating process, and a motor control process. The CPU7ahas a built-in memory (not shown). As will be described later, the memory of the control unit7has storage regions called memories TM1through TM6and TR1and TR2. Counters7b,7cand a motor control circuit7dare connected to the CPU7a, and a clock7eis connected to both the counters7c,7d. The motor-rotation and rotor-rotation signals10,11are respectively applied from the motor-rotation and rotor-rotation signal generator4d,8to the control unit7. The control unit7computes actual rotational speeds of the motor4and rotor2based on the motor-rotation signal and the rotor-rotation signal, respectively. Based on the actual rotational speeds of the motor4and rotor2thus computed, the control unit7controls the motor4so that the rotor2stably rotate at a target rotational speed. To this end, the CPU7aoutputs a speed instruction signal to the motor control circuit7dto control the rotational speed of the motor4. The rotational speed of the motor4is controlled so that the rotational speed of the rotor2is brought into coincidence with the target rotational speed.

Under the aegis of the CPU7a, the counter7bcounts up in timed relation with the clocks input from the clock7eto measure a pulse-to-pulse time duration of the motor-rotation signal10, i.e., a time duration from one rising (or falling) edge of the pulse to the succeedingly occurring rising (or falling) edge. Similarly, under the aegis of the CPU7a, the counter7ccounts up in timed relation with the clocks input from the clock7eto measure a pulse-to-pulse time duration of the rotor-rotation signal11, i.e., a time duration from one rising (or falling) edge of the pulse to the succeedingly occurring rising (or falling) edge.

FIG. 3is a timing chart illustrating examples of the motor-rotation signal10, rotor-rotation signal11, and timer interrupt signal. In accordance with the first embodiment, the motor-rotation signal10is given in the form of a pulse signal in which six pulses correspond to one rotation of the motor4. The rotor-ration signal11is also given in the form of a pulse signal in which two pulses correspond to one rotation of the rotor2. The number of pulses generated per one rotation of the rotor2from the rotor-rotation signal generator8is equal to the number of magnets2aprovided to the rotor2.

The CPU7aexecutes an interrupt process “a” in response to a trigger signal10aproduced whenever the rising edge of the motor-rotation signal10is detected. In the interrupt process “a”, the CPU7areads the count value of counter7bthat indicates the pulse-to-pulse time duration of the motor-rotation signal10. Similarly, the CPU7aexecutes the interrupt process “b” in response to a trigger signal11aproduced whenever the rising edge of the rotor-rotation signal11is detected. In the interrupt process “b”, the CPU7areads the count value of counter7cthat indicates the pulse-to-pulse time duration of the rotor-rotation signal11.

A method of controlling the rotational speed of the motor to attain the target rotational speed of the rotor2will be described with reference toFIGS. 4 through 12B.FIG. 4is a main flowchart illustrating the motor rotational speed controlling method applied to the centrifugal separator in accordance with the first embodiment.

In the main flowchart shown inFIG. 4, a computation process of a motor-rotation signal frequency fm is initially executed (step31). Details of this process will be described with reference toFIGS. 5 through 7.FIG. 5is a flowchart illustrating the computation process of the motor-rotation signal frequency fm.FIG. 6is a timing chart showing the motor-rotation signal and count value of the counter7b.FIG. 7is a flowchart illustrating an interrupt process “a”.

In the computation process of the motor-rotation signal frequency fm shown in the flowchart ofFIG. 5, the pulse-to-pulse time duration of the motor-rotation signal10is measured by the counter7b. Specifically, the counter7bcounts up the number of clocks generated from the clock7eoscillating at a predetermined frequency of, for example, 20 MHz during a period of time from the rising edge of a pulse of the motor-rotation signal to the succeedingly occurring rising edge (step101). During the count-up operation by the counter7b, the interrupt process “a” is executed. The CPU7aexecutes the interrupt process “a” at timings t11, t12and on shown inFIG. 6when the rising edge of the motor-rotation signal10is detected. The interrupt process “a” is so programmed that the CPU7areads the count value of the counter7b, stores it into the memory of the CPU7a, and then clears the count value of the counter7b.

Specifically, as shown inFIG. 6, the counter7bis cleared at t11in timed relation with the rising edge of the pulse of the motor-rotation signal10. From t11to t12, count-up operation by the counter7bis performed. At t12, the succeedingly occurring rising edge of the motor-rotation signal10is detected, causing the interrupt process “a” to execute again. As shown inFIG. 7, the interrupt process “a” first determines whether it is the first time for the CPU7ato read or retrieve the count value of counter7b(step111). When it is the first time for the CPU7ato read the count value of counter7b(step111: YES), the count value X1of counter7bis stored in the memory TM1(step112). Subsequently, the number of times the count value of counter7bis read by the CPU7ais incremented (step124). Here, this number is “1”. The counter7bis then cleared (step125) and the routine returns to step101.

Similarly, the counter7bcounts up the clocks during a period of time from t12to t13. At t13, the interrupt process “a” is executed. When the interrupt process “a” determines that it is the second time for the CPU7ato read the count value of counter7b(step113: YES), the count value X2of the counter7bis stored in the memory TM2(step114). Subsequently, the number of times the count value of counter7bis read by the CPU7ais incremented (step124). Here, this number is “2”. The counter7bis then cleared (step125) and the routine returns to step101.

In the manner described above, the interrupt processes “a” are subsequently executed at every timing in coincidence with the rising edge of the pulses of the motor-rotation signal10, and the count values X3through X6of the counter7bare read by the CPU7aand stored in the memories TM3through TM6, respectively (steps115through122). After reading the count value of the counter7bfor six times (step121: YES) and storing the count value X6in the memory TM6(S122), “1” is stored in the separate region of the memory to indicate the number of times that a set of count values of the counter7bis read (step123), and then the counter7bis cleared (step125), whereupon the routine returns to step101. As a result of the series of steps described above, the count values X1through X6counted during the time intervals Tm1through Tm6, respectively, have been stored in the relevant storage regions of the memory.

Referring back to the flowchart ofFIG. 5, the count value X1is read out from the memory TM1(step102). Assuming that the counter7bperforms count-up operations at the frequency of fc Hz (equal to the clock frequency), each count-up operation requires 1/fc seconds. Because the count value during the time interval Tm1is X1, the time interval from t11to t12is X1/fc. Accordingly, the motor-rotation signal frequency is fc/X1Hz. This value is stored in the memory of the control unit7as the motor-rotation signal frequency fm (step103). The motor-rotation signal frequency fm may be computed using any one of the count values X2through X6.

When the motor-rotation signal10shows such a waveform that six pulses occur at an equi-pitch per one rotation of the motor4as shown inFIG. 3, the rotational frequency of the motor4is given by fc/(X1+X2+X3+X4+X5+X6) Hz. This rotational frequency of the motor4is used as a basis for controlling the rotations of the motor4.

Referring back to the flowchart ofFIG. 4, after execution of step31, computation process of the rotor-rotation signal frequency fr is executed (step32), which will be described with reference toFIGS. 8 through 10.FIG. 8is a flowchart illustrating a computation process of the rotor-rotation signal frequency.FIG. 9is a timing chart showing the rotor-rotation signal11and a count value in the counter7c.FIG. 10is a flowchart illustrating an interrupt process “b”.

In the computation process of the rotor-rotation signal frequency fr shown in the flowchart ofFIG. 8, the pulse-to-pulse time duration of the rotor-rotation signal11is measured by the counter7c. Specifically, the counter7ccounts up the number of clocks generated from the clock7eduring a period of time from the rising edge of a pulse of the rotor-rotation signal to the succeedingly occurring rising edge (step201). During the count-up operation by the counter7c, the interrupt process “b” is executed. The CPU7aexecutes the interrupt process “b” at timings t21, t22and on as shown inFIG. 9when the rising edge of the rotor-rotation signal11is detected. The interrupt process “b” is so programmed that the CPU7areads the count value of the counter7c, stores it into the memory of the CPU7a, and then clears the count value of the counter7c.

Specifically, as shown inFIG. 9, the counter7cis cleared at t21in timed relation with the rising edge of the pulse of the rotor-rotation signal11. From t21to t22, count-up operation by the counter7cis performed. At t22, the succeedingly occurring rising edge of the rotor-rotation signal11is detected, causing the interrupt process “b” to execute again. As shown inFIG. 10, the interrupt process “b” first determines whether it is the first time for the CPU7ato read or retrieve the count value of counter7c(step211). When it is the first time for the CPU7ato read the count value of counter7c(step211: YES), the count value Y1of counter7cis stored in the memory TR1(step212). Subsequently, the number of times the count value of counter7cis read by the CPU7ais incremented (step213). Here, this number becomes “1”. The counter7cis then cleared (step217) and the routine returns to step211.

Similarly, the counter7ccounts the clocks during a period of time from t22to t23. At t23, the interrupt process “b” is executed. When the interrupt process “b” determines that it is the second time for the CPU7ato read the count value of counter7c(step214: YES), the count value Y2of the counter7cis stored in the memory TR2(step215). Further, “1” is stored in the separate region of the memory to indicate the number of times that a set of count values of the counter7bis read (step216), and then the counter7cis cleared (step217), whereupon the routine returns to step201. As a result of the steps described above, the count values Y1and Y2counted during the time interval Tr1and Tr2have been stored in the relevant storage regions of the memory.

Referring back to the flowchart ofFIG. 8, the count value Y1is read out from the memory TR1(step202) and then the count value Y2is read out from the memory TR2(step203).

As shown inFIG. 3, the rotor-rotation signal11in accordance with the first embodiment shows such a waveform in which pulses are not generated at an equi-pitch. Two pulses are generated per one rotation of the rotor2. The timings at which the two pulses are generated are different depending upon the type of the rotor, so that the type of the rotor2can be discriminated based on the detected timings of the two pulses.

The rotor-rotation signal frequency fr cannot be determined from only the count value Y1counted during the time interval Tr1(from t21to t22) shown inFIG. 9. As is done for the computation of the rotational frequency of the motor2, the rotational frequency of the rotor2is computed using a sum of the count values in the time intervals Tr1and Tr2. That is, the rotational frequency of the rotor2is given by fc/(Y1+Y2) Hz. Because two pulses occur per one rotation of the rotor2, an average rotor-rotation signal frequency fr is given by doubling the rotational frequency of the rotor2thus computed. To summarize, the following equations are obtained.
fr=(rotational frequency of rotor2)×2=2fc/(Y1+Y2)

Referring to the flowchart ofFIG. 8, the rotor-rotation signal frequency fr as give above is stored in the memory (step204). Note that when the pulses of the rotor-rotation signal11occur at an equi-pitch, the frequency of the pulses calculated based on the pulse-to-pulse time duration is equal to the rotor-rotation signal frequency fr.

Referring back toFIG. 4, a frequency ratio A of the motor-rotation signal frequency fm to the rotor-rotation signal frequency fr is computed, i.e., A=fm/fr, (step33). In the centrifugal separator in accordance with the first embodiment, the motor-rotation signal generator4dgenerates six pulses per one rotation of the motor4whereas the rotor-rotation signal generator8generates two pulses per one rotation of the rotor2, as shown inFIG. 3. The pulses thus generated are applied to the CPU7a. In accordance with the first embodiment, with the pulleys4band3aprovided respectively to the driving shaft4aof the motor4and the rotational shaft3of the rotor2, the rotation number of the rotor2is reduced to one second (1/2) with respect to that of the motor4. This means that the rotor2makes one rotation for two rotations of the motor4. That is, the pulleys4band3ahave such a configuration as to achieve a speed reduction ratio of 1/2. Assuming that no belt slippage occurs, the frequency ratio A is 6. It should be noted that with two rotations of the motor4, the rotor2makes one rotation, and six pulses are generated from the motor-rotation signal generator4dper one rotation of the motor4and two pulses from the rotor-rotation signal generator8per one rotation of the rotor2. Thus, the frequency ratio A can be calculated by (6 pulses×2 rotations):(2 pulses×1 rotation)=6:1. Actually, however, the slippage of the belt5occurs to some extent. Accordingly, an operating status of the driving power transmission mechanism or the operating status of the belt5is monitored. The operating status is judged to be acceptable if the frequency ratio A calculated in step33falls into a first predetermined range of, for example, 5≦A≦7 (step34). In this case, driving of the motor4is continued as the slippage of the belt5within this range does not cause a substantial problem.

On the other hand, when the tension of belt5is lowered due to wear of the belt5or loosening of the belt5, the slippage of the belt5will occur. Particularly, during acceleration or deceleration period of the rotor2, it is highly likely that slippage occurs if the rotor's moment of inertia is large or the rotor's air loss is high and so strong resistive force is applied to the rotor2. As a result, the frequency ratio A may exceed the upper limit of the first predetermined range and fall into a second predetermined range of, for example, 4≦A≦8. If so, it can be understood that the degree of slippage has increased as compared with the operating status judged to be acceptable. The operating status falling in the second predetermined range is considered to be a near malfunction status in which continuous driving can be performed and replacement or adjustment of the belt5is not essential for the time being but maintenance needs to be performed as soon as possible. In the near malfunction status, a warning message or warning indication is displayed in the display panel12to alert the user of this fact (step36). As described, when the degree of slippage is not so great, the user is only warned and prompted to perform maintenance.

When the frequency ratio A further exceeds the upper limit of the second predetermined range, an alarm message or alarm indication is displayed in the display panel12(step37) and the motor4is forcibly stopped (step38). This condition is considered to be a malfunction status. If the belt5is not replaced with a new one or tension adjustment is not performed despite the fact that the user is warned, the operating status would get worse and reach the malfunction status.

The warning and alarm displays will be described with reference toFIGS. 11A-11Band12A-12B.FIGS. 11A and 11Bshow an example of a display device300employing liquid crystal display (LCD) The display device300is a part of the display panel12shown inFIG. 1. The display device300includes a status display portion302for displaying the driving status of the centrifugal separator1, and a message display portion304. A warning message is displayed in the message display portion304as shown inFIG. 11Awhen the driving power transmission mechanism or the belt5is in the near malfunction status. An alarm message is displayed in the message display portion304as shown inFIG. 11B.

FIG. 12Ashows another example of a display device320employing light emitting diodes (LEDs). As shown inFIG. 12A, the display device320includes a speed display portion322, time display portion324, alarm lamp326, and warning lamp328. The warning display is performed by lighting the warning lamp328, and the alarm display by lighting the alarm lamp326.

FIG. 12Bshows still another example of the display device330similar to the example shown inFIG. 12A. Unlike the example shown inFIG. 12A, the display device330shown inFIG. 12Bis not provided with the warning and alarm lamps328,326. In the example shown inFIG. 12B, the speed display portion332is used to indicate a relevant error number previously determined corresponding to the error or alarm messages. For example, an error number “E-19” is indicated in the speed display portion332to indicate an alarm that the driving device is in an abnormal or malfunction status.

As described above, the centrifugal separator in accordance with the first embodiment generates the motor-rotation signal10and the rotor-rotation signal11. The former signal is in the form of a pulse train with a pulse frequency in proportion to the frequency of the motor rotations. The latter signal is also in the form of a pulse train with a pulse frequency in proportion to the frequency of the rotor rotations. Based on the motor-rotation signal10and the rotor-rotation signal11, the motor-rotation signal frequency fm and the rotor-rotation signal frequency fr are computed. The frequency ratio A of the rotational speed of the motor4to that the rotor2is used as a parameter to judge the degree of wear of the belt, because in the belt driven centrifugal separator, wear of the belt tends to increase when the slippage of the belt occurs.

Computation of these frequencies fm and fr requires measurements of pulse-to-pulse time duration of each of the motor-rotation signal10and the rotor-rotation signal11using the counters7band7cand also computation of a time duration corresponding to one rotation of the motor4or the rotor2. Through the above computations, the frequencies of the pulses of the motor-rotation signal10and the rotor-rotation signal11can be computed with high accuracy within a short period of time.

Further, it is possible to recognize the degree of malfunction of the driving power transmission mechanism, particularly wear of the belt5, from the computed frequency ratio A. Specifically, when the computed frequency ratio A exceeds the upper limit of the first predetermined range and falls within the second predetermined range, a warning message or indication is displayed on the display device to alert the user that the driving power transmission mechanism or the belt5is in the near malfunction status and to prompt the user to carry out maintenance. When the computed frequency ratio A exceeds the upper limit of the second predetermined range, an alarm message or indication is displayed on the display device to alert the user that the belt5is in the malfunction or abnormal status. At the same time, the motor4is forcibly stopped. In this manner, the centrifugal separator1of the type in which rotations of the motor4are transmitted to the rotor2via the driving power transmission mechanism can be continuously driven if the driving power transmission mechanism is in the near malfunction status, yet warning the user to perform maintenance.

It should be noted that the first predetermined range is set to such a range that a belt is durable according to data ever obtained. When the computed frequency ratio A falls within the first predetermined range, the user is advised of performing maintenance before the wear of the belt increases. When wear of the belt5increases resulting from occurrence of slippage, the frequency ratio A increases. As the frequency ration A increases, the load imposed on the motor4increases. Accordingly, if the frequency ratio A exceeds the upper limit of the first predetermined range and falls into the second predetermined range, the alarm display is performed and also the motor4is forcibly stopped. By doing so, the motor4is prevented from being damaged by the overload and also the driving power transmission mechanism is prevented from being seriously damaged.

Next, a centrifugal separator in accordance with a second embodiment of the invention will be described. In the following description, the same components as those in the first embodiment will be denoted by the same reference numerals and description thereof is omitted to avoid duplicate description.

FIG. 13is a flowchart illustrating operation of the centrifugal separator in accordance with the second embodiment of the invention. As executed for the centrifugal separator of the first embodiment, computation process of the motor-rotation signal frequency fm (step51), computation process of the rotor-rotation signal frequency fr (step52), and computation of the frequency ratio A are executed.

Next, it is determined whether the computed frequency ratio A falls within the first predetermined range (5≦A≦7) (step54). When the frequency ratio A falls within the first predetermined range (step54: YES), the belt is determined to be in an acceptable status. In this case, the routine returns to step51and the motor4is subject to acceleration/deceleration control to be rotated with a normal torque.

On the other hand, when the tension of belt5is lowered due to wear of the belt5or loosening of the belt5, slippage of the belt tends to occur. Particularly, during acceleration or deceleration period of the rotor2, it is highly likely that slippage occurs if the rotor's moment of inertia is large or the rotor's air loss is high and so strong resistive force is applied to the rotor2. As a result, the frequency ratio A increases and exceeds the upper limit of the first predetermined range, particularly when the motor is accelerating or decelerating. In the first embodiment, only a warning message or indication is displayed on the display device. In the second embodiment, torque control of the motor4is performed to prevent occurrence of slippage of the belt5. Specifically, the CPU7aof the control unit7determines that the degree of slippage increases when the computed frequency ratio A exceeds the upper limit of the first predetermined range and the CPU7ainstructs the motor control circuit7dto control the torque of the motor4so that the frequency ratio A falls with the first predetermined range.

The fact that the frequency ratio A exceeds the upper limit of the first predetermined range indicates that rotations of the rotor2are not in full compliance with the torque of the motor4. Accordingly, in order to change the frequency ratio A to fall within the first predetermined range, it is necessary to decrease the torque of the motor4. To this end, it is determined whether or not the torque of the motor4is lowered 10% or more with respect to an initially set torque value (step55). It should be noted that the toque of motor4is computed, for example, by measuring change in the rotational speed of the motor4. It should also be noted that how the motor torque control is carried out is different depending upon the type of the motor used. For example, the CPU7aof the control unit7controls the motor control circuit7dso as to decrease current flowing in the motor4. The current control may be carried out with a PWM inverter. In this case, the CPU7acontrols the width of a switching pulse applied to a transistor or an FET connected in a path for flowing the current in the motor4. It is desirable that a limiter be provided to set an allowable range in which the torque can change.

When the torque of the motor4is not lowered 10% or more with respect to the initially set torque value (step55: NO), the torque of the motor4is lowered 1% (step56) and then the warning display is performed (S57), whereupon the routine returns to step51. When the torque of the motor4is lowered 10% ore more, that is, at the time of eleventh execution of step55, the alarm display is performed (step58) and at the same time the motor is forcibly stopped (step59). The warning and alarm displays are performed by indicating relevant messages, lighting lamps, or indicating predetermined error numbers as is done in the first embodiment.

As described, the second embodiment alerts the user of the first stage of malfunction by not only performing the warning display but also lowering the motor torque if the motor torque has not been lowered 10%. While lowering the motor torque prolongs the acceleration or deceleration period of time and thus lowers the property of the centrifugal separator, it is advantageous in that the rotor can still be accelerated up to a target rotational speed set by the user. As such, the slightly deteriorated belt can still be used without need for immediate replacement of the belt5or immediate tension adjustment. It is further advantageous in that lowering the motor torque lessens the progress of the belt wear.

Although the present invention has been described with respect to specific embodiments, it will be appreciated by one skilled in the art that a variety of changes may be made without departing from the scope of the invention. For example, in the centrifugal separator in accordance with the first and second embodiments, the driving power transmission mechanism for transmission of driving power from the motor4to the rotor2is configured from the pulleys4b,3band the belt5, a different type of the driving power transmission mechanism can be employed in the centrifugal separator shown inFIG. 1.

Such an example is shown inFIGS. 14A and 14B.FIG. 14Ais a front view andFIG. 14Bis a side view showing an alternative driving power transmission mechanism together with the motor4and the rotor2. In the example shown inFIGS. 14A and 14B, a gear box70serves as the driving power transmission mechanism. The gear box70is coupled between the motor4and the rotor2and transmits the driving power of the motor4to the rotor2. The driving shaft4arotates with the motor4and the driving power of the motor4is transmitted via a coupling71to a rotational shaft72. Rotations of the rotational shaft72are transmitted via a gear73to a pinion77. Rotations of the pinion77is further transmitted via a gear75to the rotor's rotational shaft79, thereby rotating the rotor2connected to the rotational shaft79.

With the centrifugal separator100shown inFIGS. 14A and 14B, the gear box70serving as the driving power transmission mechanism is configured from the gears73,75and the pinion77.

Further, in order to obtain the motor-rotation signal frequency fm and the rotor-rotation signal frequency fr, the number of pulses of the motor-rotation signal or the rotor-rotation signal which occur per unit time may be counted. For example, counting the number of pulses Pm and Pr with the respective counters gives the motor-rotation signal frequency fm and the rotor-rotation signal frequency fr, i.e., fm=Pm (Hz), and fr=Pr (Hz).

Further, the number of pulses defining the motor-rotation signal10and the rotor-rotation signal11and the speed reduction ratio between the pulleys4band3aare not limited to those described in the first and second embodiments and may be set to different number or values. The first and second predetermined ranges change depending on the change in those number and/or values because the frequency ratio A changes depending thereupon.

In the first and second embodiments of the invention, the counters7band7care connected to CPU7awithin the control unit7. However, the counters7band7cmay be internally provided within the CPU7a.

In the second embodiment of the invention, the motor torque is lowered on a step-by-step basis, 1% at a time. However, the lowering degree of the motor torque in each step is not limited to 1% but may be set to another value, or can be changed depending upon the type of the rotor2.