GB2176915A

GB2176915A - Control of magnetic drive system

Info

Publication number: GB2176915A
Application number: GB08612244A
Authority: GB
Inventors: Meirion Jones; Timothy Davies
Original assignee: BIBBY SCIENCE PRODUCTS Ltd
Current assignee: BIBBY SCIENCE PRODUCTS Ltd
Priority date: 1985-05-21
Filing date: 1986-05-20
Publication date: 1987-01-07
Also published as: GB2176915B; GB8612244D0; GB8512857D0

Abstract

A magnetic drive system in which a prime mover eg. an electric motor, is magnetically coupled (but not mechanically coupled) to a magnetic follower eg. a stirrer, is controlled by detecting, at intervals, a variable which gives a quantitative indication of the energy output of the prime mover eg. motor current. The energy output of the prime mover is reduced by a predetermined proportion (such as about 10%) each time the magnitude of the variable decreases by more than a predetermined amount between successive detection instants. The system ensures that magnetic coupling between the motor and stirrer or other magnetic follower is maintained at high speed by keeping the motor current constant. <IMAGE>

Description

SPECIFICATION Magnetic drive system The present invention is concerned with magnetic drive systems, that is, drive systems in which mechanical energy is transmitted from a prime moverto a magneticfollowerwhich is magnetically coupled to the prime mover.

An example of a magnetic drive system is a laboratory stirrer, as illustrated schematically in Figure 1 of the accompanying drawings. Figure 1 shows a motor assembly comprising a motor 1 having a driven shaft 2 connected to a driving magnet3.Themotorassem- bly is disposed beneath a vessel 4containing liquid 5 to be stirred. The vessel 4further contains a magnetic follower 6 (often referred to as a "paddle"); because ofthe magnetic nature ofthefollower6, movement (rotation) ofthe driving magnet 3 wil I bend to cause corresponding movement of the follower 6. The motor may be used to transmit linear motion to the follower, rather than rotary motion.

In a modification of this system, the winding coils of the motor may be used to drive the follower directly (without the intermediary of rotating shaft and magnet). The common feature of all of these driving systems is that there is no direct mechanical coupling between the driving device and the prime mover; the only coupling is of a magnetic nature.

One disadvantage of the use of a magnetic drive system is that there is a possibility of magnetic decoupling at high speeds. We have now devised means of alleviating this problem.

According to the present invention, therefore, there is provided a method of controlling a magnetic drive system in which a prime mover is magnetically coupled, but not mechanically coupled,to a mag netic follower, which comprises: (a) detecting, at a plurality of successive detection instants, a variable which gives a quantitative indication of the energy output of the prime mover; (b) making a comparison between the magnitude ofthe variable at successive detection instants; and (c) reducing the energy output ofthe prime mover bya predetermined proportion each timethe magnitude of the variable decreases by more than a predetermined amount between successive detection instants. Step (c) can be repeated indefinitely until a stable coupling between the driving and driven magnets can be maintained.

Step (c) is preferably effected as follows: The prime mover is slowed down substantially (or caused to stop), typically by substantially reducing the power supply, and braking the prime mover or allowing itto slowdown orstop. The power inputto the prime mover isthen increasedsuchthattheener- gy output is at said predetermined proportion.

Wherethereisa loss of magneticcoupling,the driving magnetand the prime moverexperience a decrease in reactive force, and as a result may increase in speed and/or expend less energy in maintaining the speed. It is forthis reason that a variable which gives a quantitative indication of the energy output is detected in the method according to the invention; thevariable may be, for example, the pow er supply to the prime mover, which is indicative of the energy output. In the case where the prime mover is an electric motor with a substantially constant voltage supply, the power supply may be readily monitored by monitoring the current supply. Thus a substantial decrease in current would indicate decoupling had taken place, so that remedial action would be necessary.That is, step (c) would need to be repeated as many times as necessaryforthe currentto stabilise, which would be associated with recoupling of the magnetic drive to thefollower. Step (c) may be associated with the production of a detectable output (such as a warning lightoragraphictrace)when decoupling hastaken place.

In a preferred embodiment ofthe invention,when the prime mover is an electric motor, after decoupling has been detected, the motor is caused to recouple with rotation at a slightly reduced speed compared with that at which decoupling was detected, because returning to the same speed would tend to cause decoupling to occur again. In this case coupling is more likelyto be maintained, albeit at a reduced speed.

The energy output of the prime mover is reduced by a predetermined proportion each time step (c) is operated; the predetermined proportion is typically at least 5%, for example, about 10%.

The energy output is reduced by the predetermined proportion mentioned eachtimethe magnitude of the variable decreases by more than a predetermined amount between successive detection instants; the predetermined amount preferably corresponds to the maximum variation expected as a result of random fluctuations.

The method according to the invention is preferably controlled in a positive feedback loop - by, for example, a digital computer control system, an analogue electronic system or a mechanical control means. An exemplary sequence of operations in such a feedback loop is illustrated in Figure 2 of the accompanying drawings.

The present invention has been described in terms of a method of controlling a magnetic drive system; the invention further comprises apparatus arranged to control a magnetic drive system by the method described.

Such apparatus comprises: (a) means for mechanically coupling a prime mover to a magnetic follower, (b) means for detecting, at a pl u ral ity of succes- sive detection instants, a variable which gives a quantitative indication of the energy output of the prime mover; (c) means for making a comparison between the magnitude of the variable at successive detection instants and generating a signal each time the magnitude of said variable decreases by more than a predetermined amount between successive detection instants; and (d) means for reducing the energy output of the prime mover by a predetermined proportion each time said signal is detected.

The apparatus preferably comprises a control which operates in a positive feedback loop, and means for producing a detectable output when de coupling has ta ken place.

Claims

1. A method of controlling a mag netic drive sys- tem in which a prime mover is magnetically coupled, but not mechanically' coupled, to a magnetic follower, which comprises: (a) detecting, at a plurality of successive detection instants, a variable which gives a quantitative indication ofthe energy output ofthe prime mover; (b) making a comparison between the magnitude ofthe variable at successive detection instants; and (c) reducing the energy output ofthe prime mover by a predetermined proportion each time the magnitude ofthe variable decreases by more than a predetermined amount between successive detection instants.

2. A method according to claim 1, in which step (c) is effected by reducing the power supply to the prime mover, braking the latterorallowing ittoslowdown orstop, and then increasing the powerinputtothe prime mover such that the energy output is reduced from its initial valve by said predetermined proportion.

3. A method according to claim 2, in which said predetermined proportion is at least 5%.

4. A method according to any of claims 1 to 3, in which step (c) is repeated a plurality oftimes until the variable is substantially constant at successive detection instants.

5. A method according to any of claims 1 to 4, in which said variable is the power supply to the prime mover.

6. A method according to any of claims 1 to 5, in which steps (a) to (c) are all controlled in a positive feedback loop.

7. A method of controlling a magnetic drive sys tem, substantially as herein described with reference to Figure 2 of the accompanying drawings.

8. Apparatus for controlling a magneticdrivesystem,which comprises (a) means for mechanically coupling a prime moverto a magneticfollower, (b) means for detecting, at a plurality of successive detection instants, a variable which gives a quantitative indication ofthe energy output ofthe prime mover; (c) means for making a comparison between the magnitude of the variable at successive detection instants and generating a signal each time the magnitude of said variable decreases by more than a predetermined amount between successive detection instants; and (d) means four reducing the energyoutputofthe prime mover by a predetermined proportion each time said signal is detected.

9. Apparatus according to claim 8, which further comprises positive feedback loop means.