GB2442021A - Controlling the power supplied to a high frequency agitator - Google Patents

Abstract

The power drawn by a high frequency agitation source such as a piezoelectric crystal 2 from a power supply 10 is indicated by a component 11, such as an opto-isolator, and the resultant signal 58 provides feedback to adjust the power supplied to the source. Power supply 10 may be a switched mode power supply and the feedback may comprise a filter stage, and an amplification stage (fig 2, Amp 1). The agitation source is suited to be used in a nebuliser, (fig 6, 240) that evaporates water collected in a hand-dryer (fig 6, 200). The piezoelectric drive circuitry may further comprise a modulator 6 to provide a pulse train with a variable duty cycle (fig 4) determined by controller 1 and thermistor 7 to detect the temperature rise of the crystal and thereby determine when the nebulisation process has finished.

Description

I

A Circuit The invention relates to a power supply indicating monitoring circuit for a high-frequency agitation source. Particularly, the invention relates to power supply monitoring circuit and a controller for a piezoelectric crystal.

Piezoelectric crystals are well known in the art and are used for a number of purposes.

Piezoelectric motors, transformers and linear drives are common. An important use for a piezoelectnc crystal is in nebulisation. There are many cases where a fine mist of a substance is required without the application of heat. One example of this is a medical nebuliser, wherein a pharmaceutical compound is nebulised by a piezoelectric crystal in order to be inhaled by a patient. Another use for nebulisers is in the field of water S...

dispersal such as garden water features. In order to disperse a dispersal agent S..

: 15 effectively, a high voltage, high frequency drive source is required. Typically, a piezoelectric crystal for use in nebulisation is driven at its resonance frequency. This :.*. frequency varies between piezoelectric crystals, however it is usually in the region of *. 1.6-1.7MHz. * *1 * S.

Drive circuits for piezoelectric crystals are well known in the art. A simple way of generating such a high frequency signal is through the use of a transistor circuit.

However, if this is done, a high voltage amplifier or a transformer is required to generate the peak to peak voltages needed to drive a piezoelectric crystal. Typically, these voltages are in the region of 100-150 V. Transformers are the most commonly used components for this purpose. However, transformers are often bulky and expensive. Other types of drive circuit may be used.

A further requirement for an electronic device that will use a mains power supply is that Electromagnetic Compatibility Standards (EMC) have to be met. These standards define an acceptable level for the harmonic content in the current which electrical equipment draws from a mains AC supply, as well as an acceptable level of voltage distortion. A high-voltage square wave signal may contain an unacceptable level of harmonic content both for efficient driving of a piezoelectric crystal and for meeting the required standards of harmonic content. A common way of solving this problem is to pass the signal through a low-pass filter. If the low-pass filter is tuned to the fundamental driving frequency of the piezoelectric crystal, higher order harmonics can be filtered out, leaving only the fundamental frequency to drive the piezoelectnc crystal. Often, a low-pass filter is also used to give a voltage gain. However, in order to drive a piezoelectric crystal at resonance, a relatively high quality factor is required. In order to achieve this with a low-pass filter such as an LC circuit, the capacitances of the system in which the LC circuit is located needs to be constant. However, the capacitance of wiring and the piezoelectric crystal itself may vary with temperature, age, condition and use.

: * :: :* Therefore, this often makes an LC circuit unsuitable for driving a piezoelectric crystal at * the precise resonant frequency. * . .

When forming part of a nebuliser, a piezoelectric crystal acts on a head of liquid in S..

order to disperse the liquid into a fine mist. During operation of the piezoelectric crystal, the head of liquid absorbs the vibrational energy and sinks some of the thermal * energy of the piezoelectnc crystal. This has the effect of cooling the piezoelectric crystal. In order to operate a piezoelectric crystal efficiently it is important to drive the piezoelectric crystal at a precise frequency providing the greatest resonance and at a frequency corresponding to the greatest rate of dispersal of a head of liquid. Further, it is desirable that unnecessary use or non-optimised use of the piezoelectric crystal is avoided as this can be wasteful of energy. In some cases, for example, if the piezoelectric crystal continues to operate when all of the liquid has been nebulised and without a head of water the temperature of the crystal will rapidly increase and thermal damage will occur. Thermal damage may also occur if the piezoelectric crystal operates in an inefficient way for a long period of time.

US 5,803,362 discloses a device which is capable of varying the power fed to an oscillator circuit depending upon the temperature of a piezoelectric crystal in order to control the temperature of the piezoelectric crystal. This process can prevent the temperature of a piezoelectric crystal from exceeding a maximum temperature-However, varying the power supplied to (and thus the amplitude of oscillation of) a piezoelectric crystal can be an inefficient method of controlling a piezoelectric crystal.

It is an object of the present invention to provide a power supply indication circuit, capable of indicating current supplied to a component, with feedback to the power supply dependent upon a characteristic of the operation of the component, and further to adjust the operation of the component in response to the feedback signal. It is an object of the present invention to provide a power supply indication circuit and controller, :.. suitable for use with a high-frequency agitation source, able to detect the agitation status p...

of the source at the power supply side and adjust performance of the agitation * : :: : 15 accordingly. It is an object of the present invention to drive a piezoelectric crystal at its resonance frequency by monitoring the power requirements of the piezoelectric crystal q.

at the power supply, deducing the resonance status of the piezoelectric crystal from this power information and taking action accordingly to maintain driving the piezoelectric crystal at its resonance frequency.

The invention provides a power supply indication circuit for high frequency agitation source, the circuit comprising a component for generating a signal indicating the power drawn from a power supply, a feedback portion for receiving the signal indicating the power drawn from a said power supply, the feedback portion including a signal generation component for generating a feedback signal, the feedback signal being used to adjust the power supplied to the high-frequency agitation source from a said power supply.

The invention also provides a supply indication circuit adapted for use with a switched mode power supply, the circuit comprising a component for generating a signal indicating the power drawn from the power supply, a feedback portion for receiving the signal indicating the power drawn from the power supply, the feedback portion including a signal generation component for generating a feedback signal, the feedback signal being used to adjust the power supplied to a component drawing current from the power supply.

Preferably the power indication source circuit is adapted for use with a high frequency agitation source, more preferably the high-frequency agitation source is a piezoelectric crystal. By providing an indication of the load drawn from the power supply by the piezoelectric crystal in a feedback loop linked to the piezo controller, the controller including signal generation means for generating a drive signal for the piezoelectric crystal, the piezoelectric crystal can be tuned to its resonant frequency in response to a variation in the load drawn by the agitator. In a nebuliser this allows the piezoelectric * crystal to be driven at the most efficient frequency for nebulisation. * I **.*

Advantageously, the feedback portion comprises a filter stage and an amplification * stage. Preferably the filter stage comprises a low-pass filter. S..

The invention provides a self-contained feedback control system which is able to * * complete a nebulisation process efficiently. The power supply monitoring circuit provides a system that can also minimise unnecessary use of the piezoelectric crystal, and ensure that the agitation source is driven efficiently. The invention is particularly suitable to drive a nebuliser for use in a hand dryer.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Figure 1 is a circuit diagram of part of a power supply indication circuit according to the invention; Figure 2 is a circuit diagram showing further details of a power supply indication circuit according to the invention; Figure 3 is a block diagram of the components and operation scheme of a controller incorporating the indication circuit according to the invention; Figure 4 shows the measurement periods and the occurrences of temperature measurements made by the controller; Figure 5a shows a graph of the expected temperature characteristic of a piezoelectric crystal during a typical nebulisation process; Figure Sb shows a graph of an actual output temperature characteristic of a piezoelectric : crystal during a typical nebulisation process; and * a..

Figure 6 shows a hand dryer incorporating a nebuliser controlled by the controller of Figure 3. * S. S. S

S I..

Figure 1 shows the power supply indication circuit according to the invention. The circuit is powered by a DC power supply originating from an AC/DC converter powered by mains electricity supply. The power supply is an isolating Power Supply Unit (PSU) and the power supply is isolated with an opto-isolator (OPI). Specifically, the embodiment shown in Figure 1 is a voltage controlled flyback opto-isolated Switched Mode Power Supply Unit (SMPSU) such as, for example, SMPSU product numbers NCP1216, NCP12I6A from ON semiconductor, USA (www.onsemi.com).

The values of components of the SMIPSU such as Ri -R6, Cl and C2 are set by the system requirements and available from the application notes and product data sheets for the SMPSU. The opto-isolator (ON) anode side is supplied by a 5V power rail.

The power supply indication circuit is taken from the cathode side of the isolator and is shown on Figure 1 with a dashed line, the signal OPTO_SENSE is converted to a voltage with R3 and processed by the circuit shown in Figure 2.

The feedback circuit of Figure 2 comprises two stages. The first stage comprises an initial filter capacitor, C3, and a first stage amplification with amplifier, Amp 1. A feedback ioop, FB_OPTO is also provided between resistors R7 and R8 and Amp 1.

The second stage of the feedback circuit includes a filter comprising resistor, R9 and capacitor C4 and a second stage of amplification with signal amplifier, Amp 2.

In operation the current through the opto-isolator, OPI, provides an indication of the current load on the power supply in the system i.e. the current drawn by the high frequency agitator, in this embodiment. The SMPSU uses the current flow through the opto-isolator to hold the power supply to the correct voltage for the system using the ratio of resistor, R6 to the sum of resistors, R4 and R5. The voltage signal : :.* OPTO_SENSE from OPI provides an indication of the current load on the power . supply. The indication signal is processed and filtered by C3 to reduce noise in the :::: 15 signal. Amp I amplifies the AC component of the signal and provides a DC offset.

The DC offset component of the signal is then filtered by R9 and C4. The signal is amplified by Amp 2 to produce a voltage signal with a voltage range between 0 and :.: . 2Volts, suitable for use with a microprocessor or controller. S. * * S S * SS

Some typical values of resistors and capacitors suitable for use in this embodiment are shown on Figure 2. The components have units of resistance of Ohms and units of capacitance of Farads.

The operation of a the indication circuit of Figures 1 and 2 will now be described in use in a controller for a high frequency agitation source, such as a piezoelectric crystal.

Figure 3 shows a controller incorporating the indication circuit according to the invention. In the embodiment shown in Figure 3 the high frequency agitation source is a piezoelectnc crystal.

The control system includes a 24 V power supply unit 10 including a power indication unit II. The power supply unit is an opto-isolated SMIPSU. The output signal S7 is connected to the piezo drive 5 and the output from power indication unit Ii is connected to the controller. The controller I includes a signal generator 3. The signal generator 3 generates a synchronisation signal Si at a specified frequency, for example 1.66 kFIz. A phase locked loop (PLL) 4 is connected to the signal generator 3. The PLL multiplies the synchronisation signal SI by a specified amount to give a signal S2 at a higher frequency, for example 1.699 MHz. The synchronisation signal SI has a frequency that is variable in order to drive the piezoelectric crystal 2 at an optimum frequency.

The output S2 from the PLL 4 is connected to the piezo drive 5. The piezo drive 5 comprises switching means such as a Power Metal Oxide Field Effect Transistor (Power * MOSFET). The piezo drive 5 converts the signal S2 to a drive signal S3. The drive signal S3 is a sinusoidal waveform of an appropriate voltage to drive the piezoelectric **.

crystal 2. In this embodiment the signal S3 is a high voltage signal with a drive *:::: 15 frequency of 1.699 MHz. The signal S3 is a sine wave with peak-to-peak voltage of l00-140V. The components and functioning of the piezo drive 5 are not material to the present invention and will not be discussed further. * . * S S SS* S

*..: A modulator 6 is connected to the piezo drive 5 and provides a modulation signal S4 to control the piezo drive 5 as required. The modulator 6 can be used to provide a pulse train with a variable duty cycle.

The optimum frequency of synchronisation signal Si can be determined by measurement of the operational characteristics of the piezoelectric crystal 2 and by transmission of this information to the controller 1. As a high frequency agitator, such as a piezoelectric crystal, approaches resonance the amplitude of modulation or agitation will increase and this requires more electrical energy and power from the drive source. Thus the current drawn from the power supply by the piezoelectric crystal is a measure of the operational state of the piezoelectric crystal and monitoring an indication of the power drawn from the power supply by the piezoelectric crystal can provide information about how close the piezoelectric crystal is to resonance. This information can be used to adjust the frequency of the drive signal supplied to the piezoelectric crystal to tune the frequency towards the optimum (resonance) frequency.

It is to be noted that the use of the feedback and current indication system of the invention, using a signal indicating the power drawn from the power supply at the power supply side with the opto-isolator, OPI and circuit of Figures 1 and 2 provides a low voltage signal. This is particularly advantageous in a system conforming to EMC regulations and where users will have and require access to the controller and other system components.

It is possible to measure the current passing through the piezo directly using a power * resistor sampling signal S3. In the embodiment represented in Figure 3 the signal S3 is *:.. a high voltage signal and a power resistor monitoring the current supplied to the piezo *.

would dissipate a great deal of heat, this could be hazardous and also leads to an :::: 15 inefficient operation of the system. The large voltage (çotential difference) across the power resistor would also give voltage distortion, electrical noise and disturbance in the system and on the signal S3. Furthermore, connecting such a high voltage back to a : : controller in a feedback loop would present a health and safety risk to users. The use of a signal indicating the power drawn from the power supply at the power supply side with the opto-isolator, OPI and circuit of Figures 1 and 2 addresses the risks and disadvantages of a direct current measurement feedback system.

The piezoelectric crystal 2 comprises a ceramic material (which is responsive to an electric field) and electrical contacts. Piezoelectric crystals are well known in the art and any suitable piezoelectnc crystal can be used. A negative temperature coefficient (NTC) thermistor 7 is connected to the piezoelectric crystal 2 by a thermal link 7a. The thermal link 7a is a thermally conductive and malleable material which is in conformal contact with both the NTC thermistor 7 and the piezoelectric crystal 2. The NTC thermistor has a resistance that is dependent upon temperature. A thermistor conditioning block 8 converts a signal S5 from the NTC thermistor 7 into a temperature signal S6 which is suitable for the controller 1. An analogue input 9 forming part of the controller I receives the temperature signal S6 from the thermistor conditioning block 8. The controller I may use the temperature signal S6 to determine the status of the piezoelectric crystal 2 and to control the drive signal S3.

It is con-rn-ion to drive a piezoelectric crystal at a range of frequencies and the magnitude and frequency of the drive source is varied to drive a piezoelectric crystal at, or close to, its resonant frequency. For most piezoelectric crystals this frequency lies in the range between 1.5 to 2 M}Iz. A preferred driving frequency is close to 1.7 MHz.

In operation, the signal generator 3 generates a synchronisation signal SI of a particular frequency. The synchronisation signal Si is then supplied to the PLL 4. The controller * uses the feedback signal S8 from power indicator Sil to determine the status of operation of the piezo and how close the piezo is to its resonance frequency. The **.* controller then adjusts the synchronisation signal Si to optimise the piezo operation * : :: : 15 (maximum current drawn). The PLL 4 multiplies the synchronisation signal by 1024 to generate a signal S2. The piezo drive 5 converts the signal S2 into a drive signal S3. The drive signal S3 has a sinusoidal waveform with a frequency equal to the signal S2. The :: drive signal S3 also has a peak to peak voltage in the region of 100-140 V. The drive signal S3 is supplied to the piezoelectric crystal 2 in order to drive the piezoelectric crystal 2 in the required manner.

The operation of the piezo drive 5 is controlled by the modulator 6. The modulator 6 controls the piezo drive 5 with a modulation signal S4. The modulation signal S4 can take the form of a pulse train having a duty cycle. The duty cycle of the modulation signal S4 is determined by the controller 1. The duty cycle may be determined on the basis of the temperature signal S6. The modulation signal S4 is supplied to the piezo drive 5 and modulates the drive signal S3. Therefore, the modulator 6 is able to control the drive signal S3 by switching it on or off. Under the action of the modulator 6, the drive signal S3 takes the form of a series of wave "packets" or pulses (on state), with a "dead time" (off state) in between. The dead time is determined by the duty cycle which is the ratio of the pulse width to the period.

When the piezoelectric crystal 2 is operating, thermal energy will be generated. This thermal energy will change the resistance of the NTC thermistor 7. This is because the NTC thermistor 7 is in thermal contact with the piezoelectric crystal 2 by means of the thermal link 7a. The change in resistance of the NTC thermistor 7 causes a change in the signal S5. The signal S5 is converted by the thermistor conditioning block 8 into a temperature signal S6 suitable for the analogue input 9 of the controller 1. The temperature signal S6 contains the same information as the signal S5.

When the analogue input 9 receives the signal S6, the controller I evaluates the temperature signal S6. In this embodiment, the temperature signal S6 is sampled at * regular intervals. It is advantageous that the temperature signal S6 is sampled when the piezoelectric crystal 2 is not in operation. This is to reduce the background noise and temperature variations which may be introduced by the operation of the piezoelectric * : : : : 15 crystal 2. Figure 4 shows a schematic diagram illustrating the points at which the :. temperature signal S6 is sampled. The sample points P1, P2, P3, P4 are uniformly spaced and occur in the "dead time" between pulses of the drive signal S3. The pulses :: of the drive signal S3 have a pulse width a and a period b. Therefore, in this case the duty cycle D is equal to a/b. The "dead time" in between pulses is the optimum time for sampling the temperature signal S6. The value of the temperature signal S6 is related to and representative of the actual temperature so that the controller I can determine the actual temperature of the piezoelectric crystal 2.

In a typical nebulisation process without any temperature control, the temperature of the piezoelectric crystal 2 will rise at different rates depending on the state of operation of the piezoelectric crystal 2. If the piezoelectric crystal 2 is broken (line Cl), there will not be any significant temperature rise. However, when the piezoelectnc crystal 2 is operating correctly, the rate of temperature rise can reveal important information about the environment of the piezoelectric crystal 2.

The operation of the piezoelectric crystal 2 through a power cycle will now be described with reference to Figure 5a. Initially, the duty cycle is set to a maximum so that the average power delivered to the piezoelectric crystal 2 is high. Therefore, operation of the piezoelectric crystal 2 will cause the piezoelectric crystal 2 to heat up. It has been shown by experimental analysis that, during a nebulisation process, the temperature of a piezoelectric crystal follows a characteristic profile, initially, the temperature is seen to increase (first stage). Once the system reaches thermal equilibrium, the energy imparted by the piezoelectric crystal is used to nebulise the liquid. Therefore, the rate of change of temperature with time is seen to decrease (second stage). The value of the temperature may remain constant or even decrease in this stage. Finally, when the liquid has been completely nebulised, the rate of change of temperature with time is again seen * to increase (third stage). Figure 5b is a graph of an actual measurement sequence :.. showing the temperature profile described above. *.** * S S...

* : :: : 15 The temperature change can be used to detect when the nebulisation process has :. finished. The controller 1 can vary the power delivered to a piezoelectric crystal acting on a head of liquid in order to prevent the temperature of the piezoelectric crystal : : exceeding a pre-determined maximum value. In this embodiment, the pre-determined maximum value is 45 C. This maximum allowable temperature is chosen to prevent the build up of limescale. By preventing the build- up of limescale, the life of the piezoelectric crystal 2 can be extended.

The above-described embodiment of the invention is a low-cost, safe solution for a power supply indication circuit, capable of indicating current supplied to a component, with feedback to the power supply dependent upon a characteristic of the operation of the component, and further to adjust the operation of the component in response to the feedback signal. The invention may be used in any situation where a source such as a high frequency agitation source is required to be driven reliably and effectively with a feedback loop and where it is not desirable to take the feedback signal directly from the component, or agitator, side. This is of benefit to applications where users have access to the component for example, household appliances or medical devices.

It will be appreciated that the invention is not limited to the embodiment illustrated in the drawings. The drive source may be varied depending upon the required application.

The power supply source may be varied depending upon the required application. For example, the drive source may be a linear power supply, a current controlled SMPSU or another type of isolated power supply unit.

Further, the physical quantities of the described electronic components also may be varied in value. This could be done, for example, to change the resonant point of the filter stages. The number of filter stages could also be varied. * S.

Further, other forms of signal generator could be used. What is important is that the feedback circuit and signal is taken from the power supply side of the driven component system. S..

The invention may be used in any situation where a high frequency agitation source is : required to be driven reliably and effectively, for example in an automatic system *. .: without user control or in a nebulisation system without water level monitoring. This is of benefit to applications such as, for example, household appliances or medical devices.

The above-described embodiment of the invention is particularly suited for use in a hand dryer such as that shown in Figure 6. The hand dryer 200 includes a cavity 210.

The cavity 210 is open at its upper end 220 and the dimensions of the opening are sufficient to allow a user's hands (not shown) to be inserted easily into the cavity 210 for drying. A high-speed airflow is generated by a motor unit having a fan (not shown).

The high-speed airflow is expelled through two slot-like openings 230 disposed at the upper end 220 of the cavity 210 to dry the user's hands. A drain (not shown) for draining the water removed from a user's hands from the cavity 210 is located at the lower end of the cavity 210. A nebuliser 240 is located downstream of the drain. The nebuliser 240 is shown partially removed from the hand dryer 200 in Figure 5. The nebuliser 240 is partially cut away to show the location of the above-described drive circuit 250. The nebuliser 240 includes a collector (not shown) for collecting waste water and a piezoelectric crystal (not shown) for nebulising the waste water. The piezoelectric crystal is driven by a drive circuit 250 which includes, and is controlled by, the controller I. The use of the indiction circuit of the present invention allows the nebulisation system to be more efficient and reliable in operation. This will result in lower operating and maintenance costs for a consumer.

It will be appreciated that the invention is not limited to the embodiment illustrated in the drawings. The above-described embodiment of the invention with a controller 1 for controlling a piezoelectric crystal forming part of a nebulisation system is also suitable for use in other dryers such as laundry dryers. Other forms of drying apparatus could be envisaged by the skilled reader, for example, other forms of domestic or commercial * . 15 drying apparatus such as washer-dryers, ventilationtype laundry dryers or full-length body dryers.

In addition any number of piezoelectric crystals and controllers could be implemented.

For example, a single controller could control several piezoelectric crystals, for example if the volume of liquid to be nebulised is great. Alternatively, several controllers could be present to handle different types of liquid or operate at different times.

Claims (14)

1. A power supply indication circuit for high frequency agitation source, the circuit comprising a component for generating a signal indicating the power drawn from a power supply, a feedback portion for receiving the signal indicating the power drawn from a said power supply, the feedback portion including a signal generation component for generating a feedback signal, the feedback signal being used to adjust the power supplied to the high-frequency agitation source from a said power supply. * .4

2. A power supply indication circuit according to claim 1, wherein the high frequency agitation source is a piezoelectric crystal. 15 * * 4 p. *

3. A power supply indication circuit according to claim I or 2, wherein the component for generating the signal indicating the power drawn from a power supply is an opto-isolator.

4. A power supply indication circuit according to any one of claim 1, 2 or 3, wherein the feedback portion comprises a filter stage.

5. A power supply indication circuit according to claim 4, wherein the filter stage is adapted to filter a dc component from the signal indicating the power drawn from a power supply.

6. A power supply indication circuit according to claim 4 or 5, wherein the filter stage comprises a low pass filter.

7. A power supply indication circuit according to claims 4, 5 or 6, wherein the feedback portion comprises an amplification stage.

8. A power supply indication circuit according to any preceding claim, wherein the circuit forms part of a switched mode power supply.

9. A power supply indication circuit adapted for use with a switched mode power supply, the circuit comprising a component for generating a signal indicating the power drawn from the power supply, a feedback portion for receiving the signal indicating the power drawn from the power supply, the feedback portion including a signal generation component for generating a feedback signal, the feedback signal being used to adjust the power supplied to a component drawing current from the power supply. * ,.

10. A power supply indication circuit according to claim 9, wherein the component drawing current from the power supply is a high frequency agitation source.

* _.). * ** *. *

11. A power supply indication circuit as hereinbefore described with reference to the : -: accompanying drawings.

. .:

12. A controller for a high frequency agitation source incorporating the power supply monitoring circuit of any one of the preceding claims.

13. A riebuliser incorporating the power supply monitoring circuit of any one of the preceding claims.

14. A hand dryer incorporating the nebuliser as claimed in claim 13.