Oscillators – Automatic frequency stabilization using a phase or frequency... – Search sweep of oscillator
Reexamination Certificate
1999-12-21
2001-05-22
Mis, David (Department: 2817)
Oscillators
Automatic frequency stabilization using a phase or frequency...
Search sweep of oscillator
C324S652000, C324S727000
Reexamination Certificate
active
06236276
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for seeking and setting a resonance frequency of the type wherein a predetermined frequency interval is swept to locate a resonant frequency in the intervals, as well as a tuner operating according to the method.
2. Description of the Prior Art
Ultrasound is often used e.g. for nebulizing (atomizing) medication to be delivered to a patient connected to a ventilator. Ultrasound can be appropriately generated with a piezoelectric crystal. The crystal is then a frequency-determining component in an oscillator. The frequency of the ultrasound then depends on the resonant frequency. The acousto-mechanical model is complex for this type of oscillator, especially when electrical impedances in the signal transmission etc. are taken into account. There is then the risk that more than one resonant peak could lie with a specific frequency range. Only one of these peaks coincides with mechanical resonance and elicits the largest output from the crystal. The piezoelectric crystal can be viewed as a load impedance.
In principle, “load impedance” in this description refers to all kinds of loads, i.e. a load which can be wholly resistive, wholly inductive, wholly capacitive or a mixture of two or more of these. The character of the load also can change during operation and, e.g., vary from inductive to capacitive.
One way to find the resonant frequency is to sweep across a predetermined frequency interval and then lock modulation to the frequency yielding the highest peak.
As a result of signal drift, temperature drift etc., problems will develop when the modulation fails to produce the right frequency over time.
One way to solve this problem has been to use a phase locked loop (PLL). This means that an error signal, corresponding to drift in the system, is determined, and modulation is reset to the preset frequency.
Neither of these methods takes into account the fact that the mechanical resonance frequency can change for some reason. One such change can be caused by changes in the mechanical load on the crystal or changes in temperature.
Another problem is also that there are usually several variables involved during the searching for a resonant frequency, all of which influence the searching. Overshooting also is a problem that mainly occurs when a sweep is made at a relatively high speed in relation to time constants of components etc. An overshoot may lead to less accuracy in identifying the proper resonant frequency.
SUMMARY OF THE INVENTION
An object of the present invention is to achieve a method of the type initially described that solves the aforementioned problems.
Another object of the present invention to achieve a tuner capable of performing the inventive method.
The above object is achieved is achieved in accordance with the principles of the present invention in a method for seeking and setting a resonant frequency for a load impedance wherein a first frequency sweep is made over a predetermined frequency interval so as to identify the resonant frequency in this predetermined frequency interval, and wherein repeated sweeps of the predetermined frequency interval are subsequently made in order to repeated identify (update) the resonant frequency.
When repeated sweeps are performed, the modulation can always be related to a relatively recent frequency sweep and identification of the “updated” resonant frequency. Irrespective of whether drift occurs in the control electronics or changes occur in the oscillator which the capacitive load constitutes, the identified resonant frequency will come as close as possible to the true resonant frequency.
The resonant frequency is preferably and advantageously identified as the frequency resulting in the lowest output from the tuner in order to maintain a constant current consumption.
The repeated sweeps are advantageously performed at specific intervals. These intervals can be related to anticipated deviations due to, e.g., signal drift.
As an alternative or a complement, the repeated sweeps can be performed within restricted frequency ranges within the pre-set or predetermined frequency interval. A restricted sweep makes it possible to use a slower sweep without increasing the impact on modulation during the time the sweep is being performed. The biggest advantage of a slower sweep is that signal overshoot is avoided in determinations of the resonant frequency.
One way to arrive at a minimal restricted interval is to select a frequency range around the most recently identified resonance frequency in relation to anticipated signal drift. Anticipated signal drift can, in turn, be assessed dependent on the electrical components being used etc.
Another way to achieve a restricted frequency range is to start with a sweep at the most recently identified resonant frequency and then proceed to a lower end frequency. If a better resonant frequency is encountered within this range, the output signal is changed to this resonant frequency. Otherwise, the output signal is retained unchanged. At the next sweep, the sweep is started at the most recently identified resonant signal (i.e. the prevailing output signal) and then proceeds to a higher end frequency. In the same way as in the preceding sweep, the output signal is changed to a new resonant frequency if a better one is found.
With this procedure, the output signal and searches for the resonant frequency are continuously adapted to any changes in the resonance frequency (irrespective of whether the changes are caused by signal drift in the control electronics to actual changes in the resonant frequency of the load impedance (i.e. the piezocrystal).
Alternating semi-sweeps of this kind also make it possible to sweep a larger total frequency range (than if the same frequency range were swept every time) without any -increased interference with the operation in question (e.g. nebulization) and without risking any particularly large signal drifting. In addition, the risk of overshoot is reduced, since the “true” resonant frequency should be relatively close to the most recently identified resonant frequency.
The lower and upper end frequencies respectively, can be existing frequency limits for a predetermined frequency interval, but other limits are obviously conceivable. The most extreme interval is achieved by alternatively sweeping half the frequency range for the aforementioned minimal restricted interval (based on possible signal drift in system components)
The above object is also achieved is achieved in accordance with the principles of the present invention in a tuner connected to a load impedance, wherein the tuner has a control unit and sweep electronics for performing sweeps of a predetermined frequency interval in order to identify the resonant frequency of the load impedance, the sweep electronics generating an output signal representing the identified resonant frequency, and wherein the control unit operates the sweep electronics so as to perform repeated sweeps within the predetermined frequency interval so as to repeatedly identify (update) the resonant frequency.
In principle, the tuner is devised to carry out the aforementioned method.
One way of identifying the resonant frequency is achieved in an embodiment wherein the tuner has a current measuring unit that measures feedback current from an output stage. The output stage is connected between the tuner and the load impedance and constitutes, in principle, an amplifier for the output'signal from the tuner. The output stage is devised so constant current consumption prevails. In that manner, the resonant frequency can be defined as the frequency which results in the lowest output signal from the tuner in maintaining constant current consumption in the output stage.
When constant current consumption is maintained, impedance or admittance can be used as the actual measurement parameter to which the frequency sweep is related. If impedance is measured, it must be at a minimum at the resonant frequency. If admittance is measured, it must
Mis David
Schiff & Hardin & Waite
Siemens-Elema AB
LandOfFree
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