WO2019086356A1

WO2019086356A1 - Improvements to the operation of electromagnetic actuators

Info

Publication number: WO2019086356A1
Application number: PCT/EP2018/079501
Authority: WO
Inventors: Hugh-Peter Granville Kelly
Original assignee: Elaut Nv
Priority date: 2017-10-31
Filing date: 2018-10-26
Publication date: 2019-05-09
Also published as: BE1025915A1; GB2567894A; GB201717983D0; BE1025915B1

Abstract

An actuator comprising a coil and a permanent magnet plunger characterised by means for preventing, or reducing to the extent desired, the circulation of currents caused by motion of the plunger within the coil, once power applied to the coil has been switched off.

Description

IMPROVEMENTS TO THE OPERATION OF ELECTROMAGNETIC ACTUATORS

The following invention relates to improvements to the method of operation of permanent magnet electromagnetic actuators. In particular, it relates to such improvements as applied to the types of actuator described in my granted patent no. EP1,305,807 and my co-pending patent, application no. GB 1708753.7.

All permanent magnet electromagnetic actuators incorporate for their operation a coil, which may be in one or several windings, and a magnetic plunger. The coil, when energised, provides a magnetic field for interacting against that of the magnetic plunger so resulting in thrust and movement in the desired direction of motion. As soon as the coil is de-energised, the plunger is freed from the effect of the magnetic field acting thereupon and can return to its original starting position, under the influence for example of gravity or a spring.

It is common practice to switch on and off the current to the coil of an actuator, whether a permanent magnet type plunger or a conventional mild steel plunger solenoid, using drive circuitry incorporating a transistor or some such similar electronic device. However, in either case, on arresting current flow to the coil, a "back emf ' (back electromagnetic force) spike occurs. (The spike arises -in well known fashion- from the collapse on switch-off of the electromagnetic field created by and surrounding the coil.)

The switching junctions of electronic devices are however vulnerable to such "back-emf spikes" which can be considerable, for example many tens of volts, or even thousands of volts for substantially sized actuators. To address this difficulty, it is common practice to connect in reverse polarity a "free wheeling diode" across the coil to allow the aforesaid back-emf to circulate within the winding of the actuator's coil and so self-dissipate. This thereby avoids damage to the drive circuitry. However in terms of the free motion to their start position of permanent magnet plungers, once applied power has been switched off, the situation becomes more complex. (Permanent magnets are used in such plungers to improve performance. Interaction of the magnetic fields provided by the plunger magnets with the fields provided by the coil of the actuator results in enhanced thrust.) A disadvantage arises in this case when a free-wheeling diode is used in this case, as aforesaid, to dissipate the back emf spike. It is well known that any permanent magnet configuration (eg a plunger embodying permanent magnets) having magnetic fields emanating therefrom and moving adjacent to and relative to a coil, will upon being presented with a completed electrical path generate currents in the said coil. In the case of the aforementioned free-wheeling diode, this provides precisely such a completed electrical path. This thus permits these generated currents to flow within the coil of the actuator. In accordance with electrical theory, the magnetic field arising thereby from these generated currents will be in a direction to oppose the direction of motion of the magnetic configuration (the plunger) creating them in the first place. The result can be a sluggish and/or restrained motion of the configuration to its start position. For certain applications, when a frisky (i.e. quick) return is desirable, this is disadvantageous.

According to a first aspect of the invention, an actuator comprising a coil and a permanent magnet plunger is provided with means for preventing, or reducing to the extent desired, the circulation of currents caused by motion of the plunger within the coil, once the applied power to it has been switched off.

By this means, the generation of magnetic fields acting against the free motion of the plunger is rendered impossible, and the plunger can return eg under gravity or spring action unimpeded to its start position.

In practice, it remains desirable still to utilise a free-wheeling diode or the like to absorb the initial back emf spike occurring the instant the applied power is discontinued. Such spikes occur instantaneously, and typically their rise times can have a duration of only a fraction of or a few milliseconds. This is in contrast to the typical duration of currents induced in the coil of the actuator during the time taken for the magnetic plunger to return to its start position, which may be measured in tens of milliseconds.

According to a second aspect of the invention, the means for preventing, or reducing to the extent desired, is a Zener diode which has a conduction voltage sufficiently low to permit higher potentially damaging voltages associated with a back emf spike to be conducted therethrough, but also being sufficiently high to prevent, or reduce to the extent desired, the circulation of currents caused by motion of the plunger within the coil.

By way of explanation, the high back emf spike voltages occurring on switching off power to the coil of the actuator, having exceeded the voltage cut-off point of the Zener diode, are able desirably to dissipate within the coil thereof, and thereby avoid damage to the drive transistor circuitry, while the comparatively lower voltages generated by the lazier motion of the magnetic plunger to its start position fall below, or in the greater part fall below, the cut-off point of the Zener diode and conduction therethrough is prevented. By this means, sluggish motion of the plunger to its start position is avoided.

For many applications, rapid re -powering of an actuator is unrequired, several seconds intervening between one actuation and the next.

According to a third aspect of the invention, the means for preventing, or reducing to the extent desired, is a capacitor having a of sufficient conductance sufficiently high to permit higher potentially damaging voltages associated with a back emf spike to be conducted therethrough, but also being sufficiently low to prevent, or reduce to the extent desired, the circulation of currents caused by motion of the plunger within the coil. Thus currents arising from the very high rate of change voltage associated with the back emf spike are conducted away harmlessly, but currents arising from the lower rate of change voltage associated with motion of the magnetic plunger after switch off cannot to any extent be conducted therethrough. Selection of the actual capacitance value is calculated carefully to ensure that little damping occurs in terms of the rate of rise of the drive voltage to the coil of the actuator upon applying initial power.

In my co-pending patent application no. GB 1708753.7 an actuator is disclosed having a coil and a coaxial permanent magnet plunger, the coil being housed with a metallic housing for the convenient dissipation of heat. Slits in the housing are provided to prevent the circulation of unwanted eddy currents therein, which otherwise would impede the motion of the magnetic rod therethrough.

According to a fifth aspect of the invention, the arrangement of the present invention is used in combination with the actuator of the co-pending application, in which the actuator comprises a means to prevent or substantially limit the circulation of eddy currents within the housing of the actuator during motion of the plunger.

The invention will now be described with reference to the accompanying drawings in which: Fig 1 shows a permanent magnet actuator connected to a drive circuitry of the invention

Figs 2a to 2e show various circuit options for connection to the actuator Fig 3 shows an actuator having a slit housing and powered by the circuitry of the invention

Referring to Fig 1, a permanent magnet electromagnetic actuator is shown at 10. The actuator comprises a housing 11, a coil 12, and a permanent magnet plunger 12a. Bearings 13 and 14 guide the plunger concentrically through the coil. The plunger incorporates magnets Ml and M2 to provide a permanent magnetic field, as shown at the inset at 15. In this example, energisation of the coil raises the plunger from its rest position, 16, and elevates it to a stop position at 17. The plunger falls under gravity once power to the coil is cut. A drive circuit 18 is connected to the coil. This comprises a drive transistor 19 and other components (not shown) for supplying power to the coil of the actuator. A device 20 is provided which further controls power flow to and from the coil.

The action of the circuit is as follows. To energise the coil of the actuator, and thereby to raise its plunger from its rest position to its upper stop position, an activate signal current is fed to the base 21 of the transistor. This permits power flow to the coil. At this point, the device 20 permits this power flow. As soon as it is required to de-energise the coil, the transistor is switched off, and at the same instant, the device 20 de-activates any possibility of reverse power flow from the coil of the actuator back into the drive circuit. Thus the device 20 is a means for preventing, or reducing to the extent desired, the circulation of currents caused by motion of the plunger within the coil, once power applied to the coil has been switched off.

By way of explanation, any permanent magnet type plunger passing through a coil, in such manner that the lines of force emanating therefrom cut the turns of the coil, results in the generation within the coil of an emf (electromagnetic force). In the case of this arrangement, it is desirable that such an emf cannot be permitted to circulate within the coil once power has been cut, and the plunger begins to fall. If not, the field created by the currents circulating in the coil -as generated by the falling rod- will be in a direction, in accordance with electrical theory, such as to resist the motion of the falling rod and thus result in a lazy/restrained motion. This is especially pertinent when a snappy (i.e. quick or substantially instantaneous) motion is required under the influence of gravity to return the rod to its start position. Thus the provision within the drive circuit of the device 20 ensures that no such current flow can occur within the coil after switch off, enabling the rod to fall freely to its start position.

Although the arrangement of the circuit of Figure 1 and use of the device 20 fulfils the objectives of the present invention, in practice it is desirable to dissipate harmlessly any "back emf spike" arising at the moment the coil is switched off. It is common practice to cope with this by reverse connecting a free-wheeling diode across the coil terminals, as shown at 22 in Figure 2a. The supplied current to operate the actuator cannot flow through the diode, but as soon as current flow is switched off, the back emf spike can freely circulate through the diode, as shown by the arrow, and self -dissipate within the coil. This thereby protects the drive transistor from unwanted high voltage back emf spikes. However, this arrangement cannot be used in the case disclosed herein. The very presence of the diode permits, after switch off, the free circulation of the reverse - direction currents which would be generated in the coil during downwards motion of the plunger. A compromise is illustrated with reference to Figure 2b. In this, the means for preventing, or reducing to the extent desired, the circulation of currents within the annular field coil, once power applied to the coil has been switched off is a Zener (or avalanche) diode 23 which is used in place of the conventional diode 22 of Figure 2a. The conduction voltage of the Zener diode is selected to be above, or close to, the maximum voltage that would be generated within the coil resulting from free fall of the plunger. By this means, no current flow can occur upon power switch-off and the plunger is able to fall freely. However, the conduction voltage is also selected having

consideration to the maximum permissible inverse voltage (PIV) of the drive transistor. By ensuring the conduction voltage is well below the PIV figure, the drive transistor is protected. Thus the diode has a conduction voltage sufficiently low to permit higher potentially damaging voltages associated with a back emf spike to be conducted therethrough, but also being sufficiently high to block or substantially block the passage of currents associated with lower voltage generation. (Note, the aforementioned diode 22 remains, being connected as shown in series with the Zener diode, to prevent forward conduction of the Zener diode during the energisation phase of the coil 12.) Examples of voltages and their periodicity generated by the falling plunger, and a typical back emf spike, are shown at 24 & 25 in Figure 2c.

To obviate against the flow of any current arising from the emf generated during motion of the plunger after power to the coil thereto has been cut off, an optional diode 22a can be connected in the power rail feeding the assembly, as shown. This acts to prevent any possible power flow back into the power supply. It is not mandatory for there to be any path to "drain away" the lazy emf arising after power cut-off. In the event no current path is present, experiment proves the emf simply rises and falls away harmlessly.

Typically the former lies in the region of 10 to 20 volts, whereas the latter, the spike voltage, can reach 40 to 100 volts. Thus the use of a zener diode rated at 20 volts will "catch" the high back emf spike while blocking the lazier and slower voltage currents generated during free fall. Such a conduction voltage of 20 volts is typically well within the safe PIV of the types of bi-polar or field effect transistors that would be used to supply power to a 24 volt rail supplied actuator. Thus the conduction voltage is suitably in the range of 25-40V. An additional aspect concerns the timing of the back emf spike and the generated voltages. A back emf spike for small to moderate size actuators can have a duration of less than a millisecond, or up to several milliseconds, whereas generated currents, being consequent upon the rate of movement of a mechanical plunger, are typically an order of magnitude, or more, of these periods. Thus the use of a Zener diode is able to cope with both eventualities, the immediate conduction of high emf spike voltages but the blocking of the subsequent generated emf currents.

The drive transistor of Figure 2a has been shown as a standard bi-polar transistor. In the event an FET transistor is utilised, as shown at 26 in Figure 2d, an additional diode 27 may be placed in the conduction path as shown. By way of explanation, many FET field effect transistors frequently embody for safety their own reverse connected diodes. The presence of the additional diode 27 thwarts the possibility of the said embodied diode from providing a conduction path for the back emf spikes.

As an alternative to the use of Zener diodes, a simple capacitor may be employed, as shown at 28 in figure 2e. Connected across the coil directly, or with the option of a series resistor as shown at 29, the capacitor can serve the purpose of providing a conduction path for a high rate -of -rise voltage back emf spike, but an inadequate conduction path for the much lower rate -of -rise voltage generated within the coil by the falling plunger. That is, the capacitor is of a conductance sufficiently high to permit higher potentially damaging voltages associated with a back emf spike to be conducted therethrough, but also being sufficiently low to prevent, or reduce to the extent desired, the circulation of currents caused by motion of the plunger within the coil. The presence of the series resistor can, for specific cases, also assist in limiting conduction of the generated voltages. An additional diode 27 like illustrated in and described with reference to figure 2d may optionally be provided. Many other circuit configurations are possible for isolating a conduction path for the coil after the cessation of power fed thereto, for example analogue gates.

Referring to Fig 3, a plan view of the actuator 10 of Fig 1 is shown at 30. The housing 30 is slit longitudinally as shown at 31 and 32. The purpose being to prevent the circulation of eddy currents around the housings, as would otherwise be caused by the magnetic plunger 33 travelling therethrough, the consequence of which would be further damping of its free motion. The coil 34 of the actuator is connected as aforesaid to the drive circuitry 35. The combination of the slit housing and the method of operation of the drive circuitry as disclosed herein, provides the actuator with a substantially free motion plunger once power thereto has been switched off.

Numerous variations will be apparent to those skilled in the art.

Claims

1. An actuator comprising a coil and a permanent magnet plunger characterised by means for preventing, or reducing to the extent desired, the circulation of currents caused by motion of the plunger within the coil, once power applied to the coil has been switched off.

2. An actuator according to claim 1 , wherein the means for preventing, or reducing to the extent desired, is a Zener diode which has a conduction voltage sufficiently low to permit higher potentially damaging voltages associated with a back emf spike to be conducted therethrough, but also being sufficiently high to prevent, or reduce to the extent desired, the circulation of currents caused by motion of the plunger within the coil.

3. An actuator according to claim 2 wherein the conduction voltage is in the range of 25 to 40V.

4. An actuator according to claim 2, in which a further diode is connected, reverse biased, in series with the Zener diode to prevent forward conduction through the Zener diode of drive currents intended for the coil of the actuator.

5. An actuator according to claim 1, wherein the means for preventing, or reducing to the extent desired, is a capacitor having a conductance sufficiently high to permit higher potentially damaging voltages associated with a back emf spike to be conducted therethrough, but also being sufficiently low to prevent, or reduce to the extent desired, the circulation of currents caused by motion of the plunger within the coil.

6. An actuator according to claim 4, further comprising a resistor in series with the capacitor.

7. An actuator according to any preceding claim, further comprising a FET transistor for applying power to the coil and a protecting diode in the conduction path between the FET transistor and the coil.

8. An actuator according to any preceding claim, further comprising a drive circuit in which any form of electrical and/or electronic circuit present comprises the means for preventing, or reducing to the extent desired, which means is capable of dissipating and or handling any back emf spike, while blocking or substantially blocking the circulation of the currents generated in the coil by the motion of the composite plunger after the cessation of power fed thereto.

9. An actuator according to any preceding claim, further comprising a housing which houses the coil and means to prevent or substantially limit the circulation of eddy currents within the housing of the actuator during motion of the plunger.

10. An actuator according to claim 8, wherein the means to prevent the circulation of eddy currents comprises one or more slits situated within the housing.

11. An amusement arcade machine comprising an actuator of any of the preceding claims.

12. The amusement arcade machine of claim 9 further wherein the actuator is an actuator of a grabber.