GB2250647A - Timed radio frequency heater - Google Patents

Abstract

A radio frequency heater has a timing circuit operable to switch off the radio frequency signal after an elapsed time. The heater may be used for curing resin woodworking adhesives. Closure of a trigger switch 18 energises a relay coil 19 thereby closing contacts 14 and 21. Contacts 14 control application of current to an RC timing circuit 39-42 and Schmidt trigger 35, and contacts 21 control current supply via a delay circuit 44 to a solenoid coil 26 operating a switch controlling application of RF current to heating antennae. Delay circuit 44 ensures that current is not applied to the antennae before valves in an RF generator have warmed up; circuit 44 may have a similar form to the heating period timer. At the end of the timed heating period, trigger circuit 35 energises relay coil 29 which has normally open self-holding contacts 28 and normally closed contacts 22 in the circuit of solenoid 26. The heater is thus latched in an off state until switch 18 is opened then reclosed. The time has RF filter elements 12, 13, 16, 37 and may be screened against RF interference. <IMAGE>

Description

A TIMED RADIO FREOUENCY HEATER The present invention relates generally to a radio frequency heater device, and particularly to a radio frequency heater having means for determining and controlling the operation thereof in timed intervals.

Radio frequency heaters are employed in particular to cure resin adhesives used in the woodworking industry.

Such heaters operate by generating a high energy radio frequency signal which is applied to antennae appropriately shaped to transmit the radio frequency energy in a short lobe encompassing the volume within which the adhesive film forming the bond between adjacent timber elements is located. In operation the electrodes are usually placed in contact with the available faces of the work, orientated in such a way that the transmitted radio frequency lobe is most efficiently orientated to encompass the greatest possible part of the adhesive junction, and the circuit tuned to a resonant frequency which depends in part on the conformation of the workpiece.The length of time for which the radio frequency signal must be transmitted in order to cure the adhesive depends on a number of factors including the nature of the material itself, the attenuation of the signal due to the shape of the workpiece and the average distance from the antennae to the adhesive film. For this reason it is not possible always to predict in advance the precise time interval for which the heater must be operated in order fully to cure the adhesive, although once this has been established for a particular workpiece the same time interval will be required for similar such workpieces in a production run. At present the elapsed time must be monitored by the operator, who must switch on the equipment, observe a timepiece to determine the elapsed time, and then switch off the signal prior to repositioning it to apply the heating action to a new part of the workpiece.

The system has a number of disadvantages. First, it is tedious for the operator to have to monitor the elapsed time continually, and this results in unreliability since it depends on the operator being conscientious in allowing the full required time period to elapse before switching off the machine and moving on to the next part of the workpiece. In such circumstances the adhesive may be inadequately cured, which can result in failure of the workpiece when reliance is placed on the adhesive bonding. The present invention seeks to overcome these disadvantages by providing a timing circuit suitable for use with a radio frequency heater, the operation of which is controlled by the timer, which latter is designed in such a way that the electromagnetic field surrounding the radio frequency heater does not detrimentally influence the timing operation.The present invention also seeks to overcome a disadvantage which arises in certain circumstances, particularly in a factory environment where many different people may from time to time make use of a given piece of equipment, namely that of ensuring that the equipment is allowed to warm up properly before it is put into operation. Radio frequency heaters will not function properly, or may even be damaged, if an attempt is made to operate them before they have had an opportunity fully to warm up, especially where thermionic valves are involved.

According to one aspect of the present invention, therefore, there is provided a radio frequency heater of the type having a generator for generating a high energy radio frequency signal and antennae to which the radio frequency signal is applied for radiation therefrom, in which there is provided an automatic timing circuit operable to de-energise the heater at the end of a predetermined time period from commencement of operation.

Preferably the timing circuit has means for adjusting the said predetermined time period and means for initiating the commencement of a new time period, the said initiation means being separate from the said adjustment means.

The present invention particularly comprehends a radio frequency heater having timing means operable to determine two separate timed intervals namely a first timed interval between switching on the machine and activating it and a second timed interval determining the period of activation. For this purpose there may be provided means for determining a preliminary time period to elapse from initiation of energisation of the said radio frequency generator prior to application of the radio frequency signal to the said antennae, and means for inhibiting application of the said radio frequency signal to the said antennae during the said preliminary time period. The said preliminary time period may be established by a timing circuit substantially similar to that which determines the said predetermined time period.

According to another aspect of the present invention a timing circuit suitable for a radio frequency heater has means for determining an adjustable time period operable to de-energise the heater circuit at the end of the time period, and so arranged that a new time period cannot commence until a heater circuit operating member has been released and then re-engaged to restart the operation of the heater.

An advantage of the radio frequency heater of the present invention, therefore, is that it is not necessary to reset the timing interval after the end of each timed period, but merely to release and re-engage the switch or other trigger means to initiate operation of the heater.

In a preferred embodiment of the invention, the radio frequency heater has an automatic timing circuit in which the predetermined timing period is determined by a capacitor in series with a variable resistor the setting of which determines the capacitor charging period and therefore the timing period of the circuit.

The supply to the said capacitor is preferably delivered via a first relay-operated switch means sensitive to the current through a first relay coil in series with a manually operable control switch of the heater.

The power supply to the heater circuit may be controlled by a second relay operated switch means in series with further switch means controlled by the said first relay, current through the second relay being controlled by a first transistor the switching of which is controlled by trigger means sensitive to the charging state of the said capacitor.

In a preferred embodiment of the invention the said switching transistor is controlled by a second transistor the base current of which is determined by the said trigger, the said second transistor being conductive during the timing interval of the timing circuit and switched off, thereby turning on the first transistor, at the end of the said timing interval. With such an arrangement the said first transistor, when conductive, may inhibit the further operation of the heater. By placing the second relay coil in series with the said first transistor, therefore, the heater remains disabled at the end of the timing interval by the switching of the second relay operated switch mans and this state is maintained until the heater trigger switch is released thereby causing the current through the first transistor to stop flowing. At this point the current through the second relay coil also ceases and this allows the contacts actuated by the second relay coil to return to their initial position so that subsequent engagement of the heater trigger switch finds the timer circuit in its initial state ready to commence a new timing interval.

For this purpose, therefore, the timing capacitor is discharged either immediately upon attainment of the triggering value which causes the said second transistor to turn off, this being suitably controlled by a trigger circuit such as a Schmidt trigger, or alternatively the main charging capacitor may be discharged when the trigger switch is released.

It is an important aspect of the present invention that the timing circuit is screened against radio frequency interference and the power supply thereto includes a radio frequency filter whereby you minimise the disturbance to the operation of the timing circuit by radio frequency electro magnetic radiation.

One embodiment of the present invention will now be more particularly described, by way of example, with reference to the accompanying drawing which is a circuit diagram illustrating the adjustable timing circuit.

With reference to the single Figure of the attached drawing, the timing circuit shown is supplied with alternating current applied between a positive supply terminal 11 and a return supply terminal 20 with an earth terminal 10 connected to an earth line 17. The positive supply terminal 11 is connected via a radio frequency filter circuit comprising a choke 12 and earthed capacitor 13 to a first relay controlled switch contact 14 which is operated by current through a first relay coil 19 end of which is connected to the neutral line 20 and the other end of which is connected via a trigger switch 18 to the positive supply terminal 11.

In series with the first relay coil 19 is a second set of relay contacts 21 which are also operated by the first relay 19. The contacts 21 are joined by a line 23 to a further set of relay contacts 22 which are operated by a second relay coil as will be described hereinbelow. The relay contacts 22 are normally closed and connect the line 23 to a solenoid switch 26 which energises the operation of the heater circuit and to a warning light 25 in parallel with the solenoid switch 26. Power to the solenoid switch 26 is supplied via a delay circuit 44 as will be described in more detail hereinbelow. When current flows through the contacts 22, the line 23, the contacts 21 and the relay coil 19 to the neutral terminal 20 the solenoid switch 26 is energised to close and the heater circuit thus energised.Opening of either or both of the relay contacts 21, 22 will therefore de-energise the solenoid switch 26 deactivating the heater. The warning light 25 is, as will be appreciated, illuminated whenever the solenoid switch 26 is excited so that the light is illuminated at any time that the heater circuit is energised and is extinguished whenever the heater circuit is deactivated.

The first contact set 14 is connected via a full wave rectifier bridge 15 (illustrated as a single rectifier).

A second relay coil 29 is connected in series with the collector-emitter junction of a first transistor 30 between the output of the rectifier 15 and the earth rail 17. A second rectifier 27 shunts the second relay coil 29 whilst a fourth set of relay contacts is connected in parallel with the collector-emitter junction of the first transistor 30 between the second relay coil 29 and the earth rail 17. The cathode of the diode 27 is earthed via a capacitor 16 which serves as an RF filter and smoothes the output signal from rectifier 15.

The base of the first transistor 30 is connected via a resistor 31 to the collector of a second transistor 33 the collector-emitter junction of which is connected between the positive supply rail 50 (with the interposition of a resistor 32) and the earth line 17.

The base of the second transistor 33 is connected via a resistor 34 to the output tine of a trigger circuit 35, which may be a Schmidt trigger one input to which is taken from the junction between a series resistor array comprising two resistors 39, 41 and a variable resistor 40 having a cursor 43, which are connected in a series between the positive supply line 50 and an earthed capacitor 42 which constitutes the main timing capacitor.

A filter composed of a resistor 36 connected in the power line 50 and an earthed capacitor 37 serves to filter RF frequencies in the DC power line 50.

The circuit described above operates as follows: when the device is first switched on the power to the solenoid switch 26 is delayed by the timer circuit 44, which may in fact be identical to that shown in the drawing, the output from which is applied to the solenoid switch 26. This establishes a presentable delay, typically of the order of several minutes, before power is actually applied to the solenoid switch 26. This allows the values in the heater circuit to attain their correct operating temperature before the heater is put into use.

After the heater device has been positioned with the antennae appropriately orientated in relation to the adhesive film to be cured, the trigger switch 18 is closed allowing current to flow through the first relay coil 19. This causes the first relay contacts 14 and the second relay contacts 21 to close allowing current to flow through the solenoid switch 26 turning the heater circuit on. The closure of the first relay contacts 14 allows current to flow through the rectifier bridge 15 to the trigger circuit 35 biasing the transistor 33 to its conductive state.With the transistor 33 conducting the voltage at the base of the first transistor 30 is effectively held close to the ground value so that the transistor 30 is non-conductive and therefore no current flows through the relay coil 29 and the relay contacts 22 and 28 associated with this relay therefore remain in their closed and open positions respectively as illustrated in the drawing.

The capacitor 42 commences to charge at a rate dependent on the current flowing through the resistors 39, 40 and 41, which in turn depends on the position of the wiper 43 on the variable resistor 40. When the charge on the capacitor 42 reaches a critical value this causes the Schmidt trigger 35 to commute (discharging capacitor 42 and) so that the output to the resistor 34 goes low thereby causing the second transistor 33 itself to switch off. This causes the base voltage applied to the first transistor 30 through resistor 31 to rise causing the transistor 30 to turn on allowing current to pass through the relay coil 29. This causes the relay contact 22 to commute to the open position de-energising the solenoid switch 26 and extinguishing the timer light 25. The relay contacts 28 also close thereby latching on the relay coil 29 so that even when the capacitor 42 is fully discharged and the transistor 30 turns back off the relay contacts 22 remain in the open position as long as current continued to flow through the coil 29. Current flows through the coil 29 via the closed relay contacts 14 for as long as the current flows through the first relay coil 19, which in turn is controlled by the trigger switch 18. A new timing interval cannot, therefore, commence, and even though the trigger switch 18 remains closed the heater is deactivated. In order to cause the reactivation of the heater the trigger switch 18 must be released causing current to cease to flow in the first relay coil 19 thereby opening the first relay contacts 14 and the second relay contacts 21.Current now ceases in the second relay coil 29 thereby allowing the fourth relay contacts 28 to open and the third relay contact 22 to close. However, closure of the third relay contact 22 does not result in energisation of the solenoid 26 because now the second relay contacts 21 associated with the first relay coil 19 are open, and activation of the solenoid switch 26 will not take place until the trigger switch 18 is again closed whereupon the cycle of operations just described will be repeated.

The timing circuit of the present invention makes it impossible, therefore, for an operator to overheat the workpiece by inadvertently leaving the trigger switch 18 closed. In prior art heaters having no such timer any lack of attention of the operator to the timepiece resulting in the switch remaining closed for longer than the intended period could result in overheating and even damage to the workpiece.

The values of the resistors 39, 41 are chosen such that the full range of adjustment of the variable resistor 40 allows a time period between two and twenty-four seconds to be determined.

The circuit described hereinabove offers the additional advantage that wider ranges of timing periods can be achieved by replacing the variable resistor 40 with a switched network, offering coarse adjustment, and a further variable resistor for fine adjustment.

Radio frequency heaters can, as described above, be fitted with two substantially similar timer circuits, a circuit 44 controlling the time delay between switch-on and operation, during which closure of the switch 21 will have no effect, and the circuit as illustrated in the drawing (excluding, of course, the delay timer 44) determining the energisation time as described in detail hereinabove.

Claims (10)

1. A radio frequency heater of the type having a generator for generating a high energy radio frequency signal and antennae to which the radio frequency signal is applied for radiation therefrom, in which there is provided an automatic timing circuit operable to deenergise the heater at the end of a predetermined time period from commencement of operation.

2. A radio frequency heater as claimed in Claim 1, in which the timing circuit has means for adjusting the said predetermined time period and means for initiating the commencement of a new time period, the said initiation means being separate from the said adjustment means.

3. A radio frequency heater as claimed in Claim 1 or Claim 2 in which there are further provided means for determining a preliminary time period to elapse from initiation of energisation of the said radio frequency generator prior to application of the radio frequency signal to the said antennae, and means for inhibiting application of the said radio frequency signal to the said antennae during the said preliminary time period.

4. A radio frequency heater having an automatic timing circuit as claimed in any of Claims 1 to 3, in which the predetermined timing period is determined by a capacitor in series with a variable resistor the setting of which determines the capacitor charging period and therefore the timing period of the circuit.

5. A radio frequency heater as claimed in Claim 4, in which the supply to the said capacitor is delivered via first relay operated switch means sensitive to the current through a first relay coil in series with a manually operable control switch.

6. A radio frequency heater as claimed in Claim 5, in which the power supply to the heater circuit is controlled by second relay operated switch means in series with further switch means controlled by the said first relay, current through the second relay being controlled by a first transistor the switching of which is controlled by trigger means sensitive to the charging state of the said capacitor.

7. A radio frequency heater as claimed in Claim 5 or Claim 6, in which said switching transistor is controlled by a second transistor the base current of which is determined by the said trigger means, the said second transmitter being conductive during the timing interval of the timing circuit and switched off thereby turning on the first transistor, at the end of the said timing interval.

8. A radio frequency heater as claimed in Claim 7, in which the said second relay coil is connected in series with the said first transistor such that the heater is disabled at the end of the timing interval by the switching of the second relay operated switch means.

9. A radio frequency heater as claimed in any preceding Claims, in which the timing circuit is screened against radio frequency interference and the power supply thereto includes a radio frequency filter whereby to minimise the disturbance to the operation thereof by radio frequency electromagnetic radiation.

10. A radio frequency heater substantially as hereinbefore described with reference to and as shown in, the accompanying drawings.