GB2483631A

GB2483631A - Auto-Resonant Control Circuits

Info

Publication number: GB2483631A
Application number: GB201013415A
Authority: GB
Inventors: Miles Warwick Ashcroft; Andrew John Bowyer; Andrew Dixon
Original assignee: MAGNA PARVA Ltd
Current assignee: MAGNA PARVA Ltd
Priority date: 2010-08-10
Filing date: 2010-08-10
Publication date: 2012-03-21
Anticipated expiration: 2030-08-10
Also published as: GB2483631B; GB201013415D0

Abstract

A circuit for controlling an oscillating system 1, e.g. a piezoelectric device, includes an oscillator which is maintained at the resonant frequency of the system, regardless of non-linear effects and changes in resonant frequency caused by variable loading conditions. The circuit alters the phase of the control signal, rather than its frequency, to ensure that the driving voltage applied to the system remains in phase with the current that it consumes. The circuit generates a control waveform 7 synchronized with the input signal 2 and generates a control event each time the control waveform crosses a threshold value 10, 11, 12. An output waveform generator 13, 14, 15, 16 generates an output waveform synchronized with the control events. The control waveform 7 may comprise a sawtooth wave, which triggers the generation of a rising or falling edge of an output square wave when the slope of the sawtooth crosses the threshold value.

Description

Auto-Resonant Control Circuits

Field of the Invention

S The present invention relates to an electronic circuit for controlling a resonant system close to its resonant frequency. The invention has particular application in AC power supplies for physical systems that may be subjected to variable and unpredictable loads but it may be applied in any situation where it is desirable to excite an oscillatory system that operates either at resonance or at anti-resonance.

io Traditional control circuits for resonant systems monitor the amplitude of a feedback signal and alter the excitation frequency to maximize the system's response. However, this method of control proves to be unstable when employed in high quality factor (minimally damped) systems in which the system's amplitude-frequency characteristic is sharply peaked and therefore very sensitive to changes is in excitation frequency. Ill-defined and variable system loading can further distort the amplitude-frequency characteristic so that it becomes multi-valued and a mplitude-dependent.

Statement of the Invention

The invention provides a method whereby the aforesaid processes may be enacted automatically, without the requirement for adjustment during or prior to operation.

A control circuit in accordance with the invention includes an oscillator which is maintained at the resonant frequency of the system, regardless of non-linear effects and changes in resonant frequency caused by variable loading conditions.

Furthermore, the system is self-exciting and does not require a fixed frequency signal to operate, oscillation being caused entirely as a result of feedback from the system. The control circuit may be employed in a power supply.

The method of the invention comprises: setting a threshold value; generating a control waveform synchronized with a periodic input signal, wherein each cycle of the control waveform contains a slope portion in which the value of the signal changes monotonically with time; triggering a control event each time the slope portion of the control waveform crosses the threshold value; and generating an output waveform synchronized with the control events.

The control waveform may be a voltage signal. The slope portion of the control s waveform preferably has a gradient proportional to the frequency of the signal.

The control waveform may be a sawtooth signal. The rising or falling edges of the sawtooth signal may be synchronized with zero crossings of the input signal. First and second control waveforms may be generated synchronized respectively with positive-going and negative-going zero crossings of the input signal. The control io events triggered when the slope portion of the first control waveform crosses the threshold may trigger the generation of rising edges of an output square wave and the control events triggered when the slope portion of the second control waveform crosses the threshold may trigger the generation of falling edges of the output square wave. The output square wave may then be shaped and amplified. The is control events may be signalled by voltage pulses.

A control circuit in accordance with the invention comprises: means for setting a threshold value; a control waveform generator for receiving a periodic input signal and generating a control waveform synchronized with the input signal; a comparator for comparing the control waveform with the threshold value; an event generator for generating a control event in response to the comparator each time the control waveform crosses the threshold value; and an output waveform generator for generating an output waveform synchronized with the control events.

The control circuit may comprise parallel channels, each including its own control waveform generator, comparator and pulse generator.

The present invention provides an advantage over prior art control circuits for oscillating systems as it may be configured to operate over a great range of frequencies with ease, and is suited to the driving of both low and high power systems. The circuit is such that it reacts swiftly to changes in the resonance frequency of the system that results from changing load conditions and environmental conditions.

In one embodiment of the invention, this is accomplished by employing a transducer as a filter element within the control system feedback loop. It will be appreciated by those of ordinary skill in the art that a piezo electric transducer, for example, may be considered to be a capacitor in parallel with an inductor and resistor, the effective values of which are variable depending on the temperature of, and the force applied to, the transducer. Similar effects are present in other types of transducer and may be employed to good effect in a similar manner. This invention employs the aforementioned effect in order to provide a continuously variable input signal which is derived from the actual electromechanical response of the transducer system, thereby doing away with the need to characterise the oscillating system.

An auto-resonant control system in accordance with the invention alters the phase of the control signal, rather than its frequency, to ensure that the driving voltage io applied to the system remains in phase with the current that it consumes, i.e. that it maintains a resonant condition. The amplitude-phase characteristic of a high quality factor system at optimal loading is stable and flat-topped, resulting in a control methodology that is stable, dependable and unperturbed by ill-defined and variable system loading.

Brief Description of the Drawings

Exemplary embodiments of the invention will now be described in greater detail with reference to the drawings; Fig. 1 Is a representation of the electrical waveforms at various points in an auto resonant control system according to the invention; Fig. 2 is a block diagram showing the principle of operation of the auto resonant control system; and Fig. 3 is a circuit diagram showing an equivalent representation of a piezoelectric transducer.

Specific Description

Fig. 2 shows a block diagram of the preferred embodiment of the invention. A transducer (1) is supplied with a drive signal of suitable voltage by the power amplifier (17). The admittance of the transducer (1) is monitored to provide an input signal (2), corresponding to waveform A in Fig. 1, which is fed into the auto resonant controller (3). Although the waveform A is shown as an ideal sine wave, in practice the waveform may be significantly distorted but the control circuit will act on the fundamental frequency of the waveform. A band-pass filter (4) serves to condition the input signal (2) prior to it being fed into a comparator (5), which compares the signal with the ground reference (6) to generate a train of square wave pulses, shown in waveform B, Fig. 1, the edges of which coincide with the zero-crossing points of the original input. The signal is divided into two channels and the positive-and negative-going edges of the square wave B are used to generate a pair of sawtooth waveforms by means of sawtooth generators (8) and (9), the rise slopes of which are controlled to be proportional to the operating frequency and the falling edges of which correspond respectively to the positive-going and negative-going transitions of the square wave signal. This is represented by C and D in Fig. 1.

The sawtooth generators in the present example comprise frequency to voltage is converters which charge a capacitor via a resistor and NPN transistor. The falling edge is achieved by discharging the capacitor through a MOSFET which is triggered by the signal from an astable multivibrator.

Voltage source V-er (10) is compared with the two sawtooth signals by comparators (11) and (12), generating two square wave signals, represented by E and F in Fig. 1, corresponding to the time that the sawtooth signals are higher than the voltage reference V-er. The rising edges of these square waves cause a pair of monostable multivibrators (13) and (14) to trigger, generating a pair of phase-shifted pulse trains (G and H in Fig. 1) which are used to alternately set and reset a bistable multivibrator (Flip-Flop) (15) thereby outputting a square wave (Waveform I). Said square-wave is passed through a waveform generator (16) thus generating a pseudo-sine wave signal, represented by waveform 3 in Fig. 1, which is fed into the power amplifier (17).

The signal (2) used to feed back into the control system is subject to complex impedance filtering caused by the effective capacitance, inductance and resistance of the piezoelectric transducer (1), the effective circuit of which is shown in Fig. 3.

This represents a form of band-pass filter, the resonant frequency of which may be found using: 11 1? oJ= 4Lc L where w is the resonant frequency, L is the inductance of the inductor (20), C is the capacitance of the capacitor (18) and R is the resistance of the resistor (19).

It should be noted that the effective values of the components within the aforementioned filter are dependent on the force applied to the transducer and will s therefore change according to the load applied to the tool, said load being dependent on the composition of the tool, the workpiece and the inertia afforded to both by virtue of their masses and means of mounting.

The complex impedance of the transducer (1) introduces a phase shift into the system, which must be accommodated in the feedback loop. It may be seen in Fig. 1 that the phase of the output of the power amplifier (17) is shifted relative to the phase of the input signal (2). The phase shift arises because the rise slope of each sawtooth wave (C and D) crosses the threshold Vrer set by the voltage reference (12) a certain time interval ahead of the falling edge. That time interval, and thus the phase shift, may therefore easily be controlled by adjusting the is voltage Vre. The adjustment may be done dynamically during operation but in a typical case it is sufficient to set Vrer prior to operation to a preferred value that is tailored to the system in question.

When powered on, the system will not be in a state of oscillation. A small signal consisting of random white' noise will be present at the output of the power amplifier (17), which will excite the transducer (1) randomly. The feedback signal (2) will then comprise a white noise signal, which will have the effect of triggering monostable multivibrators (13) and (14) randomly, generating a broadband sinusoid signal of random phases and frequencies to be inputted into the power amplifier (17). The signals will cause the piezoelectric transducer (1) and its coupled mechanical system to respond in a transient fashion. However due to the filtering caused by the mechanical and electrical response of the transducer system, the feedback signal (2) will now comprise a pseudo-sine wave and the control system (3) will operate as described hereinbefore.

The above description of the preferred embodiments has been given by way of an example. From the disclosures given, those skilled in the art will understand the invention and its advantages, and will also find apparent changes and modifications to the structures and processes taught herein. It is sought therefore to cover all such changes that lie within the scope of the invention, as defined in the appended claims and equivalents thereof.

Claims

Claims 1. A method of controlling an oscillating system comprising: setting a threshold value; generating a control waveform synchronized with a periodic input signal from the oscillating system, wherein each cycle of the control waveform contains a slope portion in which the value of the signal changes monotonically with time; triggering a control event each time the slope portion of the control waveform crosses the threshold value; and generating an output waveform synchronized with the control events.
2. A method according to claim 1, wherein the control waveform is a voltage signal.
3. A method according to claim 1 or claim 2, wherein the slope portion of the control waveform has a gradient proportional to the frequency of the signal.
4. A method according to any preceding claim, wherein the control waveform is a sawtooth signal.
5. A method according to claim 4, wherein rising edges or falling edges of the sawtooth signal are synchronized with zero crossings of the input signal.
6. A method according to claim 5, wherein first and second control waveforms are generated, which are synchronized respectively with positive-going and negative-going zero crossings of the input signal.
7. A method according to claim 6, wherein the control events triggered when the slope portion of the first control waveform crosses the threshold trigger the generation of rising edges of an output square wave and the control events triggered when the slope portion of the second control waveform crosses the threshold trigger the generation of falling edges of the output square wave.
8. A method according to any preceding claim, further comprising the step of amplifying the output wave and outputting it to the oscillating system.
9. A method according to any preceding claim, wherein the control events are signalled by voltage pulses.
10. A circuit for controlling an oscillating system comprising: means for setting a threshold value; a control waveform generator for receiving a periodic input signal from the oscillating system and generating a control waveform synchronized with the input signal; a comparator for comparing the control waveform with the threshold value; an event generator for generating a control event in response to the comparator each time the control waveform crosses the threshold value; and an output waveform generator for generating an output waveform synchronized with the control events. r
11. A control circuit according to claim 10, further comprising parallel channels, C wherein each channel includes its own control waveform generator, comparator and event generator.
12. A control circuit according to claim 10 or claim 11, wherein the control waveform is a voltage signal.
13. A control circuit according to any of claims 10 to 12, further comprising an amplifier for amplifying the output waveform and outputting it to the oscillating system.
14. A control circuit for controlling an oscillating system, the circuit being substantially as described herein with reference to the drawings.AMENDMENTS TO CLAIMS HAVE BEEN FILED AS FOLLOWS1. A method of controlling an oscillating system comprising: setting a threshold value; generating a control waveform synchronized with a periodic input signal from the oscillating system, wherein each cycle of the control waveform contains a slope portion in which the value of the signal changes monotonically with time; triggering a control event each time the slope portion of the control waveform crosses the threshold value; and generating an output waveform synchronized with the control events.2. A method according to claim 1, wherein the control waveform is a voltage signal.3. A method according to claim 1 or claim 2, wherein the slope portion of the control waveform has a gradient proportional to the frequency of the signal.0. . . 4. A method according to any preceding claim, wherein the control waveform is a sawtooth signal.5. A method according to claim 4, wherein rising edges or falling edges of the sawtooth signal are synchronized with zero crossings of the input signal.6. A method according to claim 5, wherein first and second control waveforms are generated, which are synchronized respectively with positive-going and negative-going zero crossings of the input signal.7. A method according to claim 6, wherein the control events triggered when the slope portion of the first control waveform crosses the threshold trigger the generation of rising edges of an output square wave and the control events triggered when the slope portion of the second control waveform crosses the threshold trigger the generation of falling edges of the output square wave.8. A method according to any preceding claim, further comprising the step of amplifying the output wave and outputting it to the oscillating system.9. A method according to any preceding claim, wherein the control events are signalled by voltage pulses.10. A circuit for controlling an oscillating system comprising: means for setting a threshold value; a control waveform generator for receiving a periodic input signal from the oscillating system and generating a control waveform synchronized with the input signal, wherein each cycle of the control waveform contains a slope portion in which the value of the signal changes monotonically with time; a comparator for comparing the control waveform with the threshold value; an event generator for generating a control event in response to the *. comparator each time the slope portion of the control waveform crosses the * * threshold value; and * .1 s * * an output waveform generator for generating an output waveform *.et 1* synchronized with the control events.11. A control circuit according to claim 10, further comprising parallel channels, ** S. : * wherein each channel includes its own control waveform generator, comparator and * . * *.: event generator.12. A control circuit according to claim 10 or claim 11, wherein the control waveform is a voltage signal.13. A control circuit according to any of claims 10 to 12, further comprising an amplifier for amplifying the output waveform and outputting it to the oscillating system.14. A control circuit for controlling an oscillating system, the circuit being substantially as described herein with reference to the drawings.