GB1574236A

GB1574236A - Solenoid actuators

Info

Publication number: GB1574236A
Application number: GB854977A
Authority: GB
Original assignee: ITT Consumer Products UK Ltd
Current assignee: ITT Consumer Products UK Ltd
Priority date: 1977-03-01
Filing date: 1977-03-01
Publication date: 1980-09-03
Also published as: DE2807451A1

Description

(54) IMPROVEMENTS IN AND RELATING TO SOLENOID ACTUATORS (71) We, ITT CONSUMER PRODUCTS (UK) Limited, a British Company, of 190 Strand, London, W.C.2., England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a solenoid actuator having more than two stable states.

In our co-pending patent application No.

8548/77 there is disclosed a tape cassette deck in which all the operations of the deck, such as PLAY, FAST FORWARD, REWIND, etc. are accomplished without the need for mechanical linkages between the manual controls and the various mechanical parts of the deck. In the main this has been possible by the use of a separate drive motor for each tape spool and a third motor for the tape drive. However to bring the tape heads and pinch roller and tape into pressure contact requires a solenoid actuator. Further in order to realise a PAUSE facility it is necessary to be able to move the tape and the heads and the pinch wheel apart just sufficiently to stop the tape.

It would be possible to use two different solenoid actuators operating in parallel, one of them being sufficiently powerful to move the pinch wheel to a position just clear of the tape so that the equipment is effectively in the PAUSE position and the two solenoid actuators together being powerful enough to engage the heads and pinch wheel for the PLAY position.

In order to achieve the PAUSE position from the PLAY position all that is necessary is to deenergize one of the solenoid actuators.

This is an expensive and space consuming solution and it is an object of the present invention to provide a single solenoid actuator with three stable positions.

According to the present invention there is provided a solenoid actuator comprising an electric winding, a magnetic yoke associated with the winding, and a sliding armature sliding in the winding, the armature comprising first and second parts magnetically in series and a spring tending to bias the parts away from one another, there being stop means limiting the separation possible between the first and second parts, the solenoid actuator being actuable into three stable states corresponding to the three possible positions of the first and second armature parts relative to the winding, which positions are assumed in dependence on the magnitude of the current in the winding.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings, in which: Figure 1 shows in cross section a solenoid actuator coupled to move the tape heads and pinch wheel of a tape cassette deck, according to an embodiment of the invention, and Figure 2 is a diagram of a circuit suitable for operating the solenoid actuator of Figure 1.

Referring to Figure 1 the solenoid actuator comprises a soft iron magnetic yoke formed of a U-shaped portion 1 and an end portion 2 having an aperture 3. A winding 4 surrounds a non-magnetic tube 5, preferably of brass.

At the closed end of the tube 5 is situated a soft iron portion 6 forming part of the magnetic yoke and secured to the U-shaped member 1.

An armature assembly denoted generally by the reference numeral 7 comprises a first part 8 of soft iron screwed on the end of a screw threaded stem 9 of non-magnetic material, for example brass, and a second soft iron member 10 which can slide on the brass stem 9. A compressive coil spring 11 acts between the parts 8 and 10 to bias them apart. A stop member 12, comprising two nuts prevents the part 10 coming off the stem 9.

The part 10 has a circular recess 13 which accommodates part of the spring 11. At its other end the part 10 carries a hook 14 which is connected via a spring 15 to a sliding chassis 16 carrying tape heads 17 and 18 and a resilient, preferably rubber, pinch wheel 19. A further spring 20 is connected from the sliding chassis 16 to the main chassis 21 of the cassette deck. The actuator has a stop member 27 preventing the armature coming out of the yoke.

A tape cassette is shown diagrammatically by the dot-dash line 22.

In operation energisation of the winding with a current pulse of about 500 mA will cause the armature part 8 to move up against the portion 6 and the armature part 10 to move up against the rear of the part 8 so that the actuator brings the sliding chassis 16 and the heads 17 and 18 and pinch wheel 19 to the level indicated by the arrow 23. This is the PLAY or START position. If, immediately, the current in the winding 4 is reduced to say 250 mA this position will still be maintained.

The larger part 10 of the armature assembly will be released if the current is reduced to around 150 mA as the spring force of (in the main) spring 11 and the influence of springs 20 and 15 overcomes the magnetic attraction of the winding so that the part 10 moves back causing the sliding chassis 16 and heads 17 and 18 and pinch wheel 19 to then occupy the PAUSE position indicated by the arrow 24.

When the energisation of winding 4 is reduced to zero the springs 20 and 15 cause the smaller part 8 of the armature assembly to be withdrawn and the heads and pinch wheel occupy the STOP position 25.

The PAUSE and PLAY positions would be spaced by about 1 to 2mm preferably l'Emm and the PAUSE and STOP positions by between 5 and 15mm preferably about 8mm, and these preferred spacings correspond generally to the spacings of the parts 8 and 10 and 10 and 6.

The spring 15 is stiffer than spring 20 or spring 11. Spring 11 is stiffer than spring 20. Spring 15 forms a resilient link between the hook 14 and the sliding chassis 16. In the STOP position it is virtually unstressed and in the PLAY position it provides a tensile force on the sliding chassis of about 450 grams. This could be caried to between 300 and 700 grams but, in this embodiment, must be such as to provide a suitable pinch pressure between the roller 19 and the capstan 26 effective to transport the tape without slippage but without squeezing the tape so tight that it becomes damaged or seriously increases frictional forces in the transfer of the tape. This depends on the diameter X of the capstan and to a lesser extent on the pinch wheel. We use a capstan of about 2.5mm diameter.

In our experiments we have found that the preferred angle a of the conical facing ends of the parts 8 and 10 is 300, but it could be between 200 and 600. The preferred angled is 200. In the embodiment described the overall length of part 8 of the armature assembly is about half the overall length of the free space in the tube 5. Part 10 is about twice the overall length of part 8.The relative length of the armature parts for a given yoke can be varied depending on the function the armature has to perform but for the use with a tape deck we believe the smaller part should not have an effective length Ql i.e. measured midway along the conical surfaces, greater than about half the effective length Q2 of the free space in the tube although it could be shorter, say up to a quarter of the effective length of the free space.

In one example the overall lengths of the smaller part was 16mm, the larger part 35mm and the free space about 31mm.

Referring now to Figure 2 the circuit comprises three basic parts, the main part being the solenoid power control part SP in the centre between the two broken lines, and a left hand flip flop with PLAY and STOP control switches and a right hand flip flop with a PAUSE control switch. The solenoid actuator of Figure 1 is designated by the symbol S in Figure 2.

In the quiescent state with the HT rail on in the left hand flip flop LF transistor T1 would be on owing to the different collector loads (resistor R1 larger than R2). In the particular embodiment R1 is 2.2K11 and R2 equals 1KQ.

Transistor T2 would be off. Likewise in the right hand flip flop RF transistor T6 will be off of T2 and transistor T7 will be on.

Assuming we wish to go into the PLAY mode the switch PLAY is closed taking the base of transistor T 1 to ground and therefore switching transistor T2 on. The line L2 causes the negative electrode of capacitor C7 to be taken towards ground potential, effectively charging capacitor C7 up, causing an initially negative potential to appear on the base of transistor T5. This switches transistor T5 on for a short period while the capacitor C7 charges up, mainly through diode D1 which effectively protects the base emitter junction of transistor T5. Transistor T5 is turned on for approximately one second depending on the time constant of the capacitor C7 and its associated circuitry.

As T5 is directly connected to the solenoid actuator S a pulse current passes through the solenoid actuator and causes both armature parts shown in Figure 1 to be drawn hard into the field winding.

When the RH flip flop changed to switch transistor T5 on, at the same time transistor T1 switched from its on state to its off state and a potential approaching HT potential appears at the collector of transistor T I and is passed via line L3 to the base of transistor T4, thus switching this transistor on. The emitter of trans istor T4 is connected via resistor R4 also to solenoid actuator S and resistor R4 acts as an additional load to limit the current to approximately 250 mA. This therefore continues to hold the solenoid in after transistor T5 switches off.

The casette deck sliding chassis of Figure 1 is now effectively in its PLAY mode.

If it is now required to put the deck into the PAUSE mode the PAUSE switch is operated.

The flip flop circuit is operated via a steering circuit comprising resistors R7 and R8, diodes D3, D4, and D5 and capacitors C2, C3, C4 and C5. This part of the circuit is effective each time the PAUSE switch is operated, to transfer a negative potential at the junction of capacitors C3 and C2 to switch off whichever transistor T6 or T7 is currently in its on state. Therefore operation of the PAUSE switch will cause the negative pulse at the junction of C2 and C3 to be transferred, in this instance, to the base of transistor T7 thus switching it from its ON state to its OFF state. Therefore transistor T6 is switched ON. Therefore a positive potential at the collector of transistor T7 is fed via line 14 to the base of transistor T3, thus switching T3 on.

The collector of T6 is almost at earth potential and this, via line L5, draws the base of transistor T4 towards earth potential thus switching transistor T4 off. At this stage the only current through the solenoid actuator is that determined by resistor R9 and the current through transistor T3. In this embodiment that current is arranged to be about 150 mA.

This causes the rear part 10 of the armature to release but the front part of the armature remains held in the solenoid. Therefore the tape heads and pinch wheel move about 1 mum.

from position 23 to position 24, i.e. the PAUSE position.

In order to revert to the PLAY position it is, in this circuit arrangement, only necessary to touch the PAUSE button again which causes the transistors T6 and T7 to revert to their OFF and ON states, respectively. Therefore an earthy potential at the collector of transistor T7 causes transistor T8 to turn hard on and this, in turn, connects the negative electrode of capacitor C6 to ground, initially causing a negative pulse to be applied to the base of transistor T5, therefore turning transistor T5 on as capacitor C6 charges up. Thus once again a pulse of current of about 500 mA is caused to flow, via transistor T5, through the solenoid winding S, thereby causing the rear part 10 of the armature to engage the front part and pull the heads and pinch wheel into the PLAY position.

When the right hand flip flop changed its state the earthy potential at the collector of transistor T7 was transferred, via line 14, to the base of transistor T3 thus switching T3 off and transistor T4 turned on because the control potential via line L3 becomes predominant over the potential on line L5.

In order to revert the stop mode the STOP button is pressed and this causes an earth potential to appear on the base of transistor T2 thereby switching transistor T2 off and transistor T 1 on. An earthy potential appears at the collector of transistor T1 which, via line L3, switches transistor T4 off. Thus the holding current through the solenoid actuator S ceases and both parts of the armature are released and the head and pinch wheel revert to the stop position indicated by the arrow 25 in Figure 1.

It would be possible to replace the flip flop circuits with integrated circuits. It would also be possible to rearrange the circuits so as to use only one electrolytic capacitor in place of the two capacitors C7 and C6.

WHAT WE CLAIM IS: 1. A solenoid actuator comprising an electric winding, a magnetic yoke associated with the winding, and a sliding armature sliding axially in the winding, the armature comprising first and second parts magnetically in series and a spring tending to bias the parts away from one another, there being stop means limiting the separation possible between the firsi and second parts, the solenoid actuator being actuable into three stable states corresponding to the three possible positions of the first and second armature parts relative to the winding, which positions are assumed in dependence on the magnitude of the current in the winding.

2. A solenoid actuator as claimed in claim 1, wherein the armature slides in a non-magnetic tubular member axial of the winding and through one limb of the yoke.

3. A solenoid actuator as claimed in claim 2, wherein the parts are mounted on a common stem which is fixed to the first part and on which the second part slides, there being a compression spring between the two parts acting to bias the parts away from one another.

4. A solenoid actuator as claimed in any preceding claim, wherein respective ends of the first and second parts are arranged to mate with each other in one stable state, the mating end surfaces being tapered.

5. A solenoid actuator as claimed in claim 4, wherein the armature parts have frusto conical mating surfaces, the angle of the cone surface with respect to the armature axis being in the range 20C to 60 .

6. A solenoid actuator as claimed in claim 3, 4 or 5 wherein the effective length of the first part is less than half the effective length of the free space in the winding and is shorter than the second half.

7. A solenoid actuator substantially as hereinbefore described with reference to and as illustrated in Figure 1 of the accompanying drawing.

8. Apparatus comprising two relatively movable parts and a solenoid actuator, as claimed in any preceding claim, whose yoke and winding are fixed to one of the parts and whose armature coupled to the other part, there being means urging the armature out of the winding, wherein the two relatively movable parts can be controlled by the solenoid actuator to assume any one of three stable relative positions dependent on the current in the winding.

9. Apparatus according to claim 8, wherein the spring tending to bias apart the parts of the armature is significantly stronger than the means urging the armature out of the winding.

10. Apparatus according to claim 8 or claim 9, in which the two relatively movable

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. D3, D4, and D5 and capacitors C2, C3, C4 and C5. This part of the circuit is effective each time the PAUSE switch is operated, to transfer a negative potential at the junction of capacitors C3 and C2 to switch off whichever transistor T6 or T7 is currently in its on state. Therefore operation of the PAUSE switch will cause the negative pulse at the junction of C2 and C3 to be transferred, in this instance, to the base of transistor T7 thus switching it from its ON state to its OFF state. Therefore transistor T6 is switched ON. Therefore a positive potential at the collector of transistor T7 is fed via line 14 to the base of transistor T3, thus switching T3 on. The collector of T6 is almost at earth potential and this, via line L5, draws the base of transistor T4 towards earth potential thus switching transistor T4 off. At this stage the only current through the solenoid actuator is that determined by resistor R9 and the current through transistor T3. In this embodiment that current is arranged to be about 150 mA. This causes the rear part 10 of the armature to release but the front part of the armature remains held in the solenoid. Therefore the tape heads and pinch wheel move about 1 mum. from position 23 to position 24, i.e. the PAUSE position. In order to revert to the PLAY position it is, in this circuit arrangement, only necessary to touch the PAUSE button again which causes the transistors T6 and T7 to revert to their OFF and ON states, respectively. Therefore an earthy potential at the collector of transistor T7 causes transistor T8 to turn hard on and this, in turn, connects the negative electrode of capacitor C6 to ground, initially causing a negative pulse to be applied to the base of transistor T5, therefore turning transistor T5 on as capacitor C6 charges up. Thus once again a pulse of current of about 500 mA is caused to flow, via transistor T5, through the solenoid winding S, thereby causing the rear part 10 of the armature to engage the front part and pull the heads and pinch wheel into the PLAY position. When the right hand flip flop changed its state the earthy potential at the collector of transistor T7 was transferred, via line 14, to the base of transistor T3 thus switching T3 off and transistor T4 turned on because the control potential via line L3 becomes predominant over the potential on line L5. In order to revert the stop mode the STOP button is pressed and this causes an earth potential to appear on the base of transistor T2 thereby switching transistor T2 off and transistor T 1 on. An earthy potential appears at the collector of transistor T1 which, via line L3, switches transistor T4 off. Thus the holding current through the solenoid actuator S ceases and both parts of the armature are released and the head and pinch wheel revert to the stop position indicated by the arrow 25 in Figure 1. It would be possible to replace the flip flop circuits with integrated circuits. It would also be possible to rearrange the circuits so as to use only one electrolytic capacitor in place of the two capacitors C7 and C6. WHAT WE CLAIM IS:

1. A solenoid actuator comprising an electric winding, a magnetic yoke associated with the winding, and a sliding armature sliding axially in the winding, the armature comprising first and second parts magnetically in series and a spring tending to bias the parts away from one another, there being stop means limiting the separation possible between the firsi and second parts, the solenoid actuator being actuable into three stable states corresponding to the three possible positions of the first and second armature parts relative to the winding, which positions are assumed in dependence on the magnitude of the current in the winding.

parts comprise the main chassis of a cassette tape deck and a sliding chassis supporting a tape head, the solenoid actuator being operable to move the head and a tape drive mechanism into a first or PLAY, a second or PAUSE and a third or STOP mode of operation corresponding to the first second and third stable states of the solenoid actuator.

11. Apparatus as claimed in claim 8,9 or 10, in combination with an electric circuit connected to the winding to control the current through it, the circuit having first and second current paths arranged to determine first and second different holding currents through the winding to hold the solenoid armature in respective first and second stable positions, and means for providing a current pulse larger than either holding current to urge the armature into the first position from the second or third stable position.

12. Apparatus as claimed in claim 11, wherein the first and second current paths are in parallel and each contains a series-connected semi-conductor switch.

13. Apparatus as claimed in claim 11 or claim 12 comprising a third current path including a semi-conductor switch and arranged to provide the current pulse, through the solenoid winding, the switch having a control electrode coupled to a capacitive circuit whose time constant controls the "on" period of the switch and thus the duration of the current pulse.

14. Apparatus as claimed in claim 8, in combination with an electric circuit connected to the winding to control the current through it, the circuit having a first switch and comprising a first current path adapted to provide a pulse current to urge the solenoid actuator into a first stable position on actuation of the switch, a second current path adapted automatically to provide a first holding current less than the current pulse before the current pulse ceases, and a third current path adapted to provide a second holding current less than the first holding current on actuation of a second switch whereby to release the solenoid actuator and hold it in its second stable state, there being a third switch operable to switch off any current path and thus put the solenoid actuator into its third stable state.

15. Apparatus as claimed in claim 14, wherein a subsequent operation of the second switch is effective to return the solenoid actuator to its first stable state.

16. A tape deck having a slidable chassis operated by a solenoid actuator and control circuit, substantially as hereinbefore described with reference to the accompanying drawings.