GB2224375A

GB2224375A - Electronic excitation of a spring-mass vibrator

Info

Publication number: GB2224375A
Application number: GB8824794A
Authority: GB
Inventors: Claus-Peter Tzschaschel; Burkhard Dohmeyer
Original assignee: WEINERT E MESSGERAETEWERK
Current assignee: WEINERT E MESSGERAETEWERK
Priority date: 1987-07-08
Filing date: 1988-10-22
Publication date: 1990-05-02
Also published as: DD262084A1; GB8824794D0

Abstract

A circuit arrangement for the electronic excitation of the resonance frequency of a spring-mass vibrator (1) e.g. for opto-mechanical chopper comprises voltage-controlled oscillator (5) whose frequency is automatically adjusted to the resonance frequency of the spring-mass vibrator (1). A phase comparator (3) compares the frequency of a signal from a sensor (3) and the oscillator frequency, which makes it possible to readjust the voltage-controlled oscillator (5) by way of a low pass device (4). A counter (6) can be disposed between the voltage-controlled oscillator (5) and the phase comparator (3) for dividing down a high oscillator frequency. The vibrator (1) is first driven at a frequency which is small than its resonance frequency by at least a factor of 5. <IMAGE>

Description

DESCRIPTION A CIRCUIT ARRANGEMENT FOR THE ELECTRONIC EXCITATION OF THE RESONANCE FREQUENCY OF A SPRING-MASS VIBRATOR The present invention is concerned with a circuit arrangement for the electronic excitation of the resonance frequency of spring-mass vibrators, for use, for example, in opto-mechanical chopping. It is also applicable to cases in which, in addition to sighting an object, a measurement is to be carried out on the same object, for example in pyrometry, spectroscopy, thermography or in cases where light is to be modulated.

Oscillating choppers for the opto-mechanical chopping of radiation from a test object are already known from the field of pyrometry for carrying out flicker-photometer methods. Oscillating choppers are spring-mass vibrators which have to be electrically excited and which are described in Walther/Gerber, "Infrarotmesstechnik" (Infrared Measurement Technology), VEB Verlag Technik, Berlin 1981, p.97/98. As is known, the electrical excitation is effected by free-running oscillators, which require high expenditure on adjustment in order to adjust the operating frequency to be close to the resonance frequency of the spring-mass vibrator.Since the electrical time constant of the oscillator and the mechanical time constant of the spring-mass vibrator are dependent on temperature, their deviations are kept within narrow limits through synchronisation of the oscillator frequency with the resonance frequency of the spring-mass vibrator, which is determined by means of a sensor (e.g. light barrier).

The degree of carefulness with which the oscillator frequency is adjusted is the only way to keep the spring-mass vibrator stable over the long term.

In these arrangements, it is disadvantageous that the free-wheeling oscillator may fail and may no longer vibrate, even in the event of the slight changes in the resonance frequency of the spring-mass vibrator which occur following long-term use.

This disadvantage can only be eliminated by readjusting the operating frequency of the oscillator.

It is also disadvantageous that, in order to achieve a temperature-stable frequency of the oscillator and the spring-mass vibrator, extremely temperature-stable adjusting elements are required. The relatively long period of time which is required during known electrical excitation by means of free-running oscillators until an optimum oscillating state of the spring-mass system is achieved, is also unsuitable. A further disadvantage of the known arrangement is found in the coupling of the spring-mass system to following systems having their own phase displacement, e.g. when impulsing an A-D converter, wherein a direct measurement by means of additional measurement means is required for precise position and time determination of the position of the spring-mass vibrator.

It is an object of the present invention to provide a circuit arrangement for the electronic excitation of the resonance frequency of a spring-mass vibrator, wherein there is automatic adjustment of the oscillator to the resonance frequency of the spring-mass vibrator without temperature-stable adjusting elements, and wherein optimum oscillation build-up behaviour of the spring-mass system is achieved.

In accordance with the present invention, the spring-mass vibrator is electronically excited by a voltage-controlled oscillator, which impulses the spring-mass vibrator at a frequency which is smaller than the resonance frequency by at least a factor of 5.

The sensor is disposed on the spring-mass vibrator in such a way that even very small deviations of the spring-mass vibrator from its rest position will provide pulses. The output of the sensor is connected to a phase comparator, which generates a differential signal from a comparison of the sensor frequency with the oscillator frequency. This differential signal is sent by way of a low pass device to the voltage-controlled oscillator and the voltage-controlled oscillator is retuned until it has reached the resonance frequency of the spring-mass vibrator.

The invention makes use of the fact that given even a small impulse, a spring-mass system will perform autooscillations which can be detected by the sensor and evaluated. Since the time constants of the spring-mass vibrator determines the resonance frequency and the voltage-controlled oscillator can follow the mechanical spring-mass system within wide limits without its own pronounced resonance frequency, an automatic adjustment of the oscillator frequency to the resonance frequency of the spring-mass vibrator is obtained, and no additional adjustment elements are required. If the voltage-controlled oscillator fails or the time constants of the spring-mass vibrator is altered, when the spring-mass vibrator is again impulsed, automatic adjustment of the oscillator frequency to the changed resonance frequency of the spring-mass vibrator again takes place.If a-counter is disposed between the output of the voltage-controlled oscillator and an input of the phase comparator, the voltage-controlled oscillator can operate on an integral multiple of the resonance frequency of the spring-mass vibrator, since it is divided down by the counter to the corresponding value. The resonance frequency of the spring-mass vibrator is then applied to the output of the counter.

Instants which allow a defined position determination of the spring-mass vibrator can be derived from the counter reading through decoding.

The invention can thus solve the problem of providing a circuit arrangement for the electronic excitation of the resonance frequency of a spring-mass vibrator by an oscillator, wherein, in order to ascertain the resonance frequency, a sensor is disposed on the spring-mass vibrator and automatic adjustment of the oscillator frequency to the resonance frequency of the spring-mass vibrator takes place.

The invention also enables the spring-mass system to be coupled to the following systems having their own phase displacement without additional measuring means through an indirect position and time determination of the position of the spring-mass vibrator.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawing which is a flow diagram of one embodiment of a circuit arrangement in accordance with the invention.

A sensor 2, e.g. a light barrier, is disposed on a spring-mass vibrator 1 in a known way. In accordance with the invention, the sensor 2 is disposed on the spring-mass vibrator 1 in such a way that even the smallest deviations of the spring-mass vibrator 1 from its resting position will provide pulses. In the case of a vibrating chopper for a pyrometer, the sensor 2 must be able to detect deviations of 0.2 to 0.5 mm. The output of the sensor 2 is connected to a phase comparator 3, which is connected by way of a low pass filter 4 to a voltage-controlled oscillator 5. The output of the voltage-controlled oscillator 5 is connected by way of a counter 6 to an input A of the phase comparator 3 and the spring-mass vibrator 1.

In accordance with the invention, the spring-mass vibrator 1 is electronically excited by the voltagecontrolled oscillator 5 at a frequency which is smaller than the resonance frequency of the spring-mass vibrator 1 by at least a factor of 5. The differential signal is directed by way of the low pass 4 and the voltagecontrolled oscillator 5 is retuned accordingly. This process is repeated until the voltage-controlled oscillator has reached the resonance frequency of the spring-mass vibrator 1, and the spring-mass vibrator 1 has assumed a stable oscillating state, i.e. it oscillates at its resonance frequency.The important advantage of this circuit arrangement is the automatic adjustment of the frequency of the voltage-controlled oscillator 5 to any resonance frequency of the springmass vibrator 1, so that, in the event of changes in the resonance frequency of the spring-mass vibrator 1, it is not necessary to perform an expensive readjustment; instead, the spring-mass vibrator 1 merely has to be reimpulsed.

The counter 6 advantageously increases the number of possible applications of the circuit arrangement in accordance with the invention. Time-determining resistances and capacitors assume very large values in slowly-oscillating systems (resonance frequencies below 10 Hz) and can no longer be kept temperature-stable.

This disadvantage can only be eliminated if the oscillator operates at a high frequency. Using the counter 6, it is possible to allow the voltagecontrolled oscillator 5 to operate at a frequency which corresponds to an integral multiple of the resonance frequency of the spring-mass vibrator 1. By using the counter 6 to divide down the high frequency of the voltage-controlled oscillator 5 to the resonance frequency of the spring-mass vibrator 1, the circuit arrangement according to the invention can also be used for slowly oscillating systems. A further advantage arising from the use of the counter is that following systems with their own phase displacement can easily be coupled on, e.g. an A-D converter, such as the one used in pyrometers, can be triggered. When coupling such systems, it is necessary to carry out a precise position determination of the spring-mass vibrator 1 for a particular instant of its oscillating cycle. In known spring-mass vibrators, a direct measurement of the position of the spring-mass vibrator 1 must be carried out. When the counter 6 is used in the circuit arrangement according to the invention, each oscillation cycle of the spring-mass vibrator 1 can be divided into as many equal time intervals as the counting capacity of the counter 6 will permit. If the counting capacity of the counter 6 is sufficiently large, a precise position determination of the spring-mass vibrator 1 may be carried out at any point in time without carrying out a direct measurement.

Claims

1. A circuit arrangement for the electronic excitation of the resonance frequency of a spring-mass vibrator by an oscillator, in which in order to ascertain the resonance frequency, a sensor is disposed on the spring-mass vibrator and automatic adjustment of the oscillator frequency to the resonance frequency of the spring-mass vibrator takes place, and wherein the oscillator is a voltage-controlled oscillator which serves to electronically excite the spring-mass vibrator, the spring-mass vibrator being arranged to be first impulsed at a frequency which is smaller than the resonance frequency of the spring-mass vibrator by at least a factor of 5, the output of the sensor being connected to a phase comparator, which is arranged to generate a differential signal from a comparison of the detector frequency with the oscillator frequency, and the output of the phase comparator being connected by way of a low pass means to the voltage-controlled oscillator.

2. A circuit arrangement as claimed in claim 1, wherein a counter, which divides down an oscillator frequency which corresponds to an integral multiple of the resonance frequency of the spring-mass vibrator, is disposed between the output of the voltage-controlled oscillator and an input of the phase comparator, and the resonance frequency of the spring-mass vibrator is applied to the output of the counter.

3. A circuit arrangement for the electronic excitation of the resonance frequency of a spring-mass vibrator by an oscillator, substantially as hereinbefore described with reference to the accompanying drawing.