GB2037456A - Automatic control - Google Patents

Abstract

A control circuit adapted to monitor the signals from a speed sensor comprises a first comparator circuit having positive feed back, being adapted to actuate a relay or other switch when the signal from the speed sensor falls below a given value; and a second comparator circuit having positive feed back the output of which is connected to a time delay means which in turn is adapted to actuate another relay or other switch when a given time has elapsed. The description refers to control of a centrifuge, spin-dryer etc or of the doors of a vehicle wherein opening of a lid, door etc is enabled on speed being sufficiently reduced, and brakes are released. <IMAGE>

Description

SPECIFICATION Means for controlling braking The present invention relates to a means for controlling the braking of a rotating body.

In many forms of centrifuge it is necessary to sense when the centrifuge rotor ceases to rotate so that the lid of the chamber housing the rotor can be opened safely. Conventionally the speed of rotaton of the rotor or its drive shaft is measured using a moving magnet and coil tachometer. However, it has been thought that the signals from such tachometers are not sufficient at speeds below about 100 rpm to be used to measure speed. It has therefore been the practice, when using a reverse current flowing through the drive motor to induce a magnetic braking effect, to allow the rotor or shaft to pass through the rest point and to begin reverse rotation. The reversal of rotation trips a sensor which switches off the power to the motor and releases any lid lock mechanism.

However, in many cases it is desirable that the rotor should coast to rest with no braking effort applied, since the braking causes disturbance of samples in the rotor. With a reverse rotation sensor the lid lock cannot be released until the sensor is tripped. It is therefore necessary to switch on the reverse current and cause the rotor to rotate momentarily in the reverse direction to trip the sensor. This requires that an operator watch the centrifuge and switch on the current at the appropriate point. Also the sample will be disturbed when tripping the reverse rotation sensor.

We have now devised a means which reduces the above problems.

The present invention provides a control circuit which is adapted to monitor the signals from a speed sensor, which circuit comprises a first comparitor circuit adapted to receive signals from the speed sensor and having positive feed back, the first comparitor circuit being adapted to actuate a relay or other switch when the signal from the speed sensor falls below a given value; and a second comparitor circuit having positive feed back and also adapted to receive signals from the speed sensor, the output of this second comparitor circuit being connected to a time delay means which in turn is adapted to actuate another relay or other switch when a given time has elapsed.

Whilst the speed sensor may be one which gives rise to a pulsed signal or a direct current signal, e.g. by using an optical sensor to detect reflected transmitted light, it is preferred to generate the signal using a moving coil and magnet. Thus, a suitable sensor for present use is a conventional coil and magnet tachometer. In this case the signal output from the sensor is an alternating current signal. It is preferred that this signal be rectified, notably using a half wave peak precision rectification circuit having a time constant of less than 2 seconds.

The circuit of the invention finds use in a wide range of fields where it is desired to actuate some effect or action at a given rotational speed. For example the circuit of the invention can be used to release the brake and the door lock on a washing machine or spin drier and to control and interlock the brakes and door locks on vehicles, e.g. motor cars and trains, to prevent wheel lock during braking and opening of doors whilst the vehicle is in motion. However, the invention is of especial use in sensing when a centrifuge rotor or its drive shaft have nearly come to rest so that the rotor chamber lid lock can be released and/or the braking current to the motor switched off.

The invention thus also provides a centrifuge having: (a) A rotor to be driven by an electric motor and provided with means for braking the rotation of the rotor which means are adapted to be deactivated at a given speed of rotation of the rotor; (b) the rotor or a component which is rotatable with the rotor (e.g. a drive shaft) being provided with a speed sensing means for emitting a signal which varies in proportion to the speed of rotation of the rotor; (c) a chamber within which the rotor is mounted and having a lockable means for access to the rotor; and (d) a control circuit of the invention in which the output from the speed sensing means is connected to the input to the first comparitor circuit, the output from the first comparitor circuit being adapted to deactivate the braking means when the speed of rotation of the rotor falls below the given value and the output from the second comparitor circuit being adapted to release the lock mechanism on the lockable access means when the speed of rotation of the rotor falls below the given value.

For convenience, the present invention will be described with particular reference to a preferred form thereof as shown diagrammatically in the accompanying drawings in which: Figure 1 is a diagrammatic cross section of a centrifuge using a control circuit of the invention; and Figure 2 is a circuit diagram of a preferred form of the circuit of the invention.

A centrifuge is provided with a chamber 1 in which a rotor 2 is to be rotated by a drive shaft 3. Chamber 1 had a lid 4 having a solenoid operated lock 5. Shaft 3 is driven directly or indirectly via suitable gear or belt drive 6 as shown by an electrical motor 7.

Motor 7 is provided with a reversed supply of current to induce magnetic braking in the motor when it is desired to slow the rotor down to rest.

The shaft 3, or some other rotating componerit, e.g. the rotor 2 or motor 7, is provided with means for sensing the rotation of the rotor, which means provides a signal to the brake cut out control circuit. Preferably, the sensing means operates to produce a magnetically induced alternating current. Thus, shaft 3 can carry a multi-pole magnet or ring magnet 10 which rotates within a static coil 11 or vice versa. The coil 11 will generate an alternating electric voltage proportional to the speed of rotation of the shaft. Conveniently magnet 10 and coil 11 take the form of a conventional moving magnet and coil tachometer assembly of the type usually used in a centrifuge.

The signal from coil 11 is fed to a rectification circuit 20 where a direct current signal is derived. In order that the rectification circuit should follow the changes in values of the peaks in the generally sinusoidal waveform of the signal from coil 11 closely it is desirable that circuit 20 have a low time constant. The optimum time constant will depend upon a number of factors, e.g. the inertia of the rotor 2 and the speed at which the brake effort is to be switched off. The output from circuit 20 must not lag behind the signal from coil 11 to such an extent that circuit 20 deactivates the reverse current braking after the rotor has passed through the stationary point and commenced reverse rotation.On the other hand the time constant must not be so small that extraneous noise in the signal from coil 11 causes the relay or switch in the brake deactivation circuit to chatter. In general the circuit 20 should have a time constant of from 0.25 to 1.5 seconds.

A preferred form of rectification circuit is shown as A in Fig. 2 and the value of the time constant is determined by the values of the capacitor X and the resistor Y in known manner.

The output from the rectification circuit 20 is fed to a comparitor circuit 21. This circuit compares the signal from circuit 20 with a reference signal which has a value corresponding to that which circuit 20 would emit at the speed at which braking is to be switched off. The reference signal can be generated by any suitable means. Preferably, the reference signal can be varied so that the brake cut off speed can be selected to suit the particular operation being carried out. It is preferred that the brake cut off speed lie within the range 10 to 1500 rpm.

The comparitor circuit is one which incorporates positive feed back to ensure a sharp actuation point for the relay or other switch controlling the braking effort. Preferably the comparitor circuit is a Schmitt Trigger, an example of which is shown as item B in Fig.

2. However, other comparitor circuits, e.g.

incorporating a zener diode or a 2 transistor circuit, may be used. If desired a microprocessor can be used to achieve the comparison.

The output from comparitor circuit 21 is fed to a relay or other switch 22 which switches on or switches off the braking effort. Preferably the braking effort is achieved by feeding a reversing current to the motor. In this case switch 22 merely switches off the reverse current once the output from circuit 20 drops below the level required in comparitor circuit 21. A suitable relay switching circuit is shown as item C in Fig. 2.

The signal from coil 11 is also fed to a second circuit which activates the lid lock release mechanism. This second circuit comprises a second comparitor circuit 30 which is conveniently of the same type as circuit 21, preferably a non-adjustable Schmitt Trigger operating with a signal input in the range 10 to 100 mV. It is preferred that the signal from coil 11 be filtered to reduce noise therein before it is fed to comparitor 30 using conventional means. It is also preferred to incorporate a voltage limiter between coil 11 and comparitor 30 to prevent overloading comparitor 30 when rotor 2 is spinning at high speeds.

The output from comparitor circuit 30 activates a timer means when the signal from coil 11 falls to a level corresponding to rotation of rotor 2 at only a few, e.g. less than 50, rpm.

The precise level at which circuit 30 activates the timer means will depend upon, inter alia, the time taken for the rotor to come to rest from a given speed and the time lag which can be achieved in the timer means. Usually, it will not be desirable to activate the timer before the rotor has slowed to 10 rpm or less and the timer means should provide a time lag of typically 1 to 20 seconds before activating the lid lock release.

The timer means can be of any suitable form but conveniently comprises a capacitor which is charged when circuit 30 is satisfied, the time taken for the capacitor to be charged providing a time delay before the lid lock release circuit 31 is actuated. The lid lock release circuit 31 preferably incorporates a positive feed back comparitor, e.g. a Schmitt Trigger, to reduce dither in the circuit as the capacitor in the timer means approaches its fully charged state. A convenient timer and lid lock release circuit is shown as item D in Fig.

2.

The circuits described above and shown in Fig. 2 are either commercially available or can be readily made using conventional components.

In operation, the rotor is driven by the motor for the desired time at the desired speed. After this the rotor is brought to rest, either by allowing it to coast down to rest without applying any braking effort or by applying a braking effort. Where no braking is required the comparitor circuit 21 is switched out. However, it will usually be desired to brake the rotor at least part of the way to rest so as to reduce the operating time. Comparitor circuit 21 is then set to operate at the desired speed where braking is to cease. The rotor is braked down to that speed and the comparitor circuit then causes the power to the braking circuit to be switched off allowing the rotor to coast to rest. Circuit 21 can be set to operate over a range of speeds, say 10 to 1500 rpm, and thus give flexibility in operation. Since circuit 21 operates automatically, there is no need for an operator to monitor the progress of the deceleration cycle. Also, since circuits 30 and 31 release the lid lock without the need to reverse the sense of rotation of the rotor, disturbance of samples in the rotor is minimised.

Claims (9)

1. A control circuit which is adapted to monitor the signals from a speed sensor which circuit comprises a first comparitor circuit adapted to receive signals from the speed sensor and having positive feed back, the first comparitor circuit being adapted to actuate a relay or other switch when the signal from the speed sensor falls below a given value; and a second comparitor circuit having positive feed back and also adapted to receive signals from the speed sensor, the output of this second comparitor circuit being connected to a time delay means which in turn is adapted to actuate another relay or other switch when a given time has elapsed.

2. A circuit as claimed in claim 1 adapted for use with a speed sensor which emits an alternating current signal and which comprises a half wave peak precision rectification circuit having a time constant of less than 2 seconds adapted to receive the signal from the sensor and to feed a rectified signal to the first comparitor circuit.

3. A circuit as claimed in either of claims 1 or 2 wherein the rotation sensor comprises a magnet and a relatively moveable coil.

4. A circuit as claimed in any one of the preceding claims wherein the comparitor circuits comprise a Schmitt trigger.

5. A circuit as claimed in claim 1 substantially as hereinbefore described.

6. A circuit substantially as hereinbefore described with respect to and as shown in Fig. 2 of the accompanying drawings.

7. A centrifuge incorporating a circuit as claimed in any one of the preceding claims.

8. A centrifuge having: (a) a rotor to be driven by an electric motor and provided with means for braking the rotation of the rotor which means are adapted to be deactivated at a given speed of rotation of the rotor; (b) the rotor or a component which is rotatable with the rotor being provided with a speed sensing means for emitting a signal which varies in proportion to the speed of rotation of the motor; (c) a chamber within which the rotor is mounted and having a lockable means for access to the rotor; and (d) a control circuit which comprises a first comparitor circuit adapted to receive the signals from the speed sensing means and a second comparitor circuit having positive feed back and also adapted to receive signals from the speed sensing means, the output from the first comparitor circuit being adapted to deactivate the braking means when the speed of rotation of the rotor falls below the given value and the output from the second comparitor circuit being adapted to release the lock mechanism on the lockable access means when the speed of rotation of the rotor falls below the given value.

9. A centrifuge as claimed in claim 8 wherein the speed sensing means comprises a moving coil and magnet tachometer adapted to emit an alternating current signal and the control circuit comprises a half wave peak precision rectification circuit having a time constant of less than 2 seconds and adapted to rectify the signals from the tachometer and to pass the rectified signals to the first and second comparitor circuits.