GB2103855A

GB2103855A - A circuit arrangement for monitoring rotational speed

Info

Publication number: GB2103855A
Application number: GB08215328A
Authority: GB
Inventors: Horst Kache
Original assignee: Medizin und Labortechnik Leipzig VEB
Current assignee: Medizin und Labortechnik Leipzig VEB
Priority date: 1981-07-06
Filing date: 1982-05-26
Publication date: 1983-02-23
Also published as: DD200111A1; GB2103855B; ATA110182A; FR2509069A1; FR2509069B1; DE3210184A1; AT376074B

Abstract

A circuit for monitoring the rate of rotation of a high speed centrifuge 1 comprises an optical speed- scanning arrangement 5, 6 and a tachogenerator 14, the output of the optical arrangement being compared with a reference maximum speed setting, and the output of the tachogenerator being used to check that the optical arrangement is functioning correctly. The output of the optical arrangement 5, 6 is compared at 10 with a reference voltage 20 representative of a maximum desired speed n3. When this speed has been reached, comparator 10 supplies an output to logic circuit 17 which causes the power supply to the centrifuge motor 3 to be disconnected. The outputs of the optical arrangement 5, 6 is also supplied, together with the output of tachogenerator 14 to a second logic circuit 16. The circuit 16 causes circuit 17 to disconnect motor 3 if the tachogenerator output indicates that a speed n2 has been reached without optical arrangement 5, 6 indicating that a speed n1 has been reached, where n1<n2<n3. <IMAGE>

Description

SPECIFICATION A circuit arrangement for monitoring rotational speed The present invention relates to a circuit arrangement for monitoring the rotational speed of rotating bodies, e.g. centrifuge rotors. The necessity of having safeguards against excessive rotational speeds applies to any rotation body which is operated up to the limit of it shear resistance or tensile strength, such as the rotors of very high speed centrifuges and also the rotating drive eiements, if the members they drive are to be protected from any damage.

Known centrifuges may be arranged to receive rotors with different speed values and magnitudes in order to comply with the desired special application of the particular centrifuge. Every rotor has its own natural maximum speed value and must be protected from excessive speeds and the damage they cause. In order to detect a magnitude which depends on speed it is known to arrange at the base of the rotors a scanning or sensing disc provided with sectors and to scan the disc with suitable scanning means to produce an electrical signal with a speed-proportional frequency. According to German Offenlegungschrift No. 2415934 this signal is supplied after pulse shaping both to a delay network and a resettable monostable multivibrator with a pulse duration of 3 mS.The outputs of both switching stages are passed to the inputs of an AND gate, the output thereof forming the inputs for a mixer stage and a signal reliability circuit. The outputs from these stages are connected to the inputs of a disconnection circuit of the drive control circuit of the motor. The output from the signal reliability circuit is also connected to the resetting input of the multivibrator. The mixing stage has a second input for the connection of a high stable oscillator.

The output from the mixer stage is fed to a low pass filter of a predetermined frequency. The signal reliability circuit comprises an integrator and a threshold value circuit.

This circuit arrangement produces a disconnection command in the following two cases: 1. The rotor reaches its maximum permitted rotational speed.

2. The pulse width is less than 3 mS and the number of pulses is so few that the threshold value circuit does not respond.

The second case occurs when the scanning means and the pulse shaper fail, when there are interruptions in the line of these switching stages and when the scanning disc is missing.

The circuit arrangement described above has a high cost which is caused in particular by the oscillator circuit with its highly constant frequency, the low pass filter with narrow tolerance inductances and two switching stages preventing any faults in the monitoring circuit.

The arrangement has a relatively long path for signals in the form of pulses so that there is the danger of faulty disconnection due to the reception by radiation of extraneous pulses.

The present invention seeks to lower the component and manufacturing costs of circuits monitoring the revolution speed and/or to ensure that the arrangement is protected from faults due to extraneous pulses.

According to a first aspect of the present invention there is provided a circuit for monitoring the rotational speed of a rotating body comprising first and second means for detecting the rotational speed and supplying respective first and second electrical signals representative thereof, a comparator for comparing the first signal with a reference signal representative of a rotational speed n3 and supplying an output to first logic means, and second logic means having first and second logic inputs connected to the first and second detecting means, a signal supplied to the first logic input being arranged to change when the first detecting means detects that the rotational speed has reached a value n, and a signal applied to the second logic input being arranged to change when the second detecting means detects that the rotational speed has reached a value n2 (where n1, < n2, < n3), the second logic means being arranged to supply an output to said first logic means if said second logic input indicates that speed n2 has been reached but said first logic input does not indicate that speed n, has been reached.

According to a second aspect of the present invention there is provided a circuit arrangement for monitoring the rotational speed of a rotating body by means of a pattern arranged on the rotating body comprising a scanning stage for scanning the pattern and producing an electrical signal, a pulse shaper and a comparator a propagation time generator and an integrator being connected between a first output of the pulse shaper and the comparator, a second output of the pulse shaper being connected to the input of a frequency switch, the output of which is passed to a first input of an OR gate, a tachogenerator being mechanically coupled to the rotating body and electrically coupled to a rotational speed switch which is connected to a second input of the OR gate, the output of the OR gate being connected to a second input of an AND gate, the first input of which is connected to the output of the comparator, the output of the AND gate being connected to means for disconnecting the power supply to drive means for the rotating body, the switching speed (n2) of the rotational speed switch is arranged between the speed (n,) which corresponds to the switching frequency of the frequency switch and the switching speed (n3) of the comparator.

A preferred embodiment of the present invention will now be described with reference to the accompanying drawings of which: Fig. 1 shows a block circuit diagram of a circuit arrangement in accordance with the present invention; and Fig. 2 shows graphs of signals of the circuit arrangement against revolution speed.

Fig. 1 shows a circuit arrangement for monitoring the revolution speed of ultra high speed centrifuges, those parts of the centrifuge which co-operate with the circuit arrangement being shown in schematic view. These are the rotor 1, the rotor shaft 2, the drive motor 3 including gear means, the motor shaft 26 and the drive control circuit 4.

A reflector disc or plate 5 is fixed to the base of the rotor 1 and is provided with a pattern of alternate reflecting and non-reflecting sectors.

Below the reflector disc 5 an optical coupler 6 is arranged as the scanning or sensing stage. Its output is passed to the input of a pulse shaper 7 which has two outputs 18, 1 9. The first output 18 is linked to the input of a propagation time generator 8, the second output 19 to the input of a frequency switch 13. An integrator 9 is connected to the output of the propagation time generator 8 and is connected at the output side to the second input 21 of a comparator 10. The first input 20 of the comparator 10 is applied to a reference voltage source 1 The output from the frequency switch 13 is passed to the first input 22 of an OR gate 1 6. A tachogenerator 14 is mechanically coupled to the motor shaft.It is connected electrically to the input of a revolution speed switch 1 5 which is connected at the output side to the second input 23 of the OR gate 1 6.

The output thereof is applied to the second input 25 of an AND gate 17, its first input 24 being connected to the output of the comparator 1 0.

The output of the AND gate 1 7 is linked to the input of a disconnection circuit 1 2 which is connected at the output side to the drive control circuit 4.

The mode of operation of the circuit arrangement will now be described in conjunction with the signal/revolution speed curves shown in Figure 2.

The optical coupler 6 produces a periodic electrical signal by scanning the rotating reflector disc 5 and its frequency is a function of the revolution speed of the rotor 1. The electrical signal is provided with a rectangular shape in the pulse shaper 7. A d.c. voltage proportional to the frequency of the pulse train is produced in the frequency switch 1 3 and supplied to a threshold value switch, the output level u, of which may only have two values namely L or H in accordance with the logic conditions 0 and 1 (positive logic).

If the rotor 1 reachesaspeedn1, for example 4000R/Min, then the voltage which is being produced exceeds the threshold value and the output level u, jumps from L to H. The level at the input 22 of the OR gate 1 6 then changes from L to H. The tachogenerator 1 4 produces a periodic electrical voltage of which the amplitude and frequency are also proportional to the speed of rotor 1, the latter having a different proportionality factor from the pulse frequency.

The voltage is rectified in the speed switch 1 5 and is supplied to a differential amplifier. At a speed n2 of the rotor 1 , for example 15000R/Min the output level u2 of the speed switch 1 5 jumps from H to L and as a result the level at the input 23 of the OR gate 16 changes from H to L. Tie level u3 at the output of the OR gate 16 remains high (H) because there is H at the first input 22. This would not be the case if the starting level u, of the frequency switch 13 remained at L due to there being no input signal.

The output signal of the pulse shaper 7 is converted in the propagation time generator 8 into pulses of constant width and height. These pulses are integrated in the integrator 9. The integration voltage is supplied to the second input 21 of the comparator 10. A reference voltage of the reference voltage source 11 which is applied to the first input 20 is compared with the input at 21 in the comparator 10. If the rotor 1 exceeds it maximum revolution speed n3, for example 65,000R/Min is then the value of the integration voltage, which is proportional to the revolution speed, exceeds the value of the reference voltage and therefore the comparator 10 is switched. The level of its output u4 jumps from H to L. The level at the first input 24 of the AND gate 17 then changes from H to L, H normally being applied to its second input 25.The output level u6 of AND gate 1 7 changes from H to L. This logic signal initiates a disconnection command in the disconnection circuit 12 and as a result the supply current to the drive motor 3 is interrupted in the drive control circuit 4.

By linking the logic signal of the frequency switch 1 3 to that of the revolution speed switch 15, the motor 3 is prevented from being disconnected in the lower revolution speed range.

If the operation of the scanning device comprising reflector disc 5, optical coupler 6 and the connecting line to the pulse shaper 7 is faulty then the output level u, of the frequency switch 1 3 remains at L (shown in broken lines) and is applied to the first input 22 of the OR gate 16. If the revolution speed switch 1 5 is switched at its switching speed n2 then there is also a L level at the second input 23 of the LR gate 16. The resultant L brought about at the output of the OR gate 1 6 then disconnects the motor 3 via the AND gate 17.

The above-described arrangement for monitoring the speed of rotors of ultra high speed centrifuges has the advantages of being constructed from standard components and stages, not needing inductances, having a short signal path for pulses and being secure with respect to faults due to defective components or assemblies or connecting lines.

Claims

1. A circuit for monitoring the rotational speed of a rotating body comprising first and second means for detecting the rotational speed and supplying respective first and second electrical signals representative thereof, a comparator for comparing the first signal with a reference signal representative of a rotational speed n3 and supplying an output to first logic means, and second logic means having first and second logic inputs connected to the first and second detecting means, a signal supplied to the first logic input being arranged to change when the first detecting means detects that the rotational speed has reached a value n, and a signal applied to the second logic input being arranged to change when the second detecting means detects that the rotational speed has reached a value n2 (where n1 < n2 < n3), the second logic means being arranged to supply an output to said first logic means if said second logic input indicates that speed n2 has been reached but said first logic input does not indicate that speed n, has been reached.

2. A circuit as claimed in claim 1 wherein the output of the first logic means is connected to means for disconnecting the power supply to drive means for the rotating body.

3. A circuit as claimed in claim 2 wherein the first logic means is arranged to disconnect the power supply when the output of comparator indicates that a maximum desired rotational speed n3 has been reached or when the output of the second logic means indicates faulty operation of the first detection means.

4. A circuit as claimed in any preceding claim wherein the first logic means comprises an ANDgate, the outputs of the comparator and the second logic means being respectively connected to first and second inputs of the AND-gate.

5. A circuit according to any preceding claim wherein the first detecting means supplies pulses to means which supplies a low or a high signal to the first logic input of the second logic means depending on whether the speed indicated by the pulses is below or above the value n,.

6. A circuit as claimed in any preceding claim wherein the first detecting means comprises a scanning device for scanning a pattern on the rotating body.

7. A circuit according to any preceding claim wherein the output of the second detecting means is connected to a speed switch which supplies a high or a low signal to the second logic input of the second logic means depending on whether the speed detected by the second detecting means is below or above the value n2.

8. A circuit according to any preceding claim wherein the second detecting means comprises a tachogenerator mechanically coupled to the rotating body.

9. A circuit according to any preceding claim wherein the second logic means comprises an OR-gate.

10. A circuit arrangement for monitoring the rotational speed of a rotating body by means of a pattern arranged on the rotating body comprising a scanning stage for scanning the pattern and producing an electrical signal, a pulse shaper and a comparator, a propagation time generator and an integrator being connected between a first output of the pulse shaper and the comparator, a second output of the pulse shaper being connected to the input of a frequency switch, the output of which is passed to a first input of an OR gate, a tachogenerator being mechanically coupled to the rotating body and electrically coupled to a rotational speed switch which is connected to a second input of the OR gate, the output of the OR gate being connected to a second input of an AND gate, the first input of which is connected to the output of the comparator the output of the AND gate being connected to means for disconnecting the power supply to drive means for rotating body, the switching speed (n2) of the rotational speed switch is arranged between the speed (n) which corresponds to the switching frequency of the frequency switch and the switching speed (n3) of the comparator.

11. A circuit for monitoring the rotational speed of a rotating body substantially as herein described with reference to the accompanying drawings.

12. A high speed centrifuge having means for monitoring its rotational speed comprising a circuit according to any preceding claim.