Automatic setting of gain in control loops

Details Automatic setting of gain in control loops Automatic setting of gain in control loops

1998-12-09

2001-07-17

Teska, Kevin J. (Department: 2121)

Data processing: generic control systems or specific application

Generic control system, apparatus or process

Optimization or adaptive control

C369S044360

Reexamination Certificate

active

06263251

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a method and arrangement for automatically setting the gain in control loops, as used in CD or DVD players for example, in order to focus the scanning or recording light or laser beam on the optical recording medium, or to follow the information track.
BACKGROUND OF THE INVENTION
In order to set the gain in a control loop, it is already known practice to superimpose a sinusoidal signal on the reference variable and to compare the controlled variable and the control difference with one another in order to obtain, from this comparison, a criterion for setting the control gain, cf. DE-A 36 35 859. For this purpose, it is preferably the amplitudes of the controlled variable and of the control difference that are compared with one another. In addition, mention is made of measuring a phase margin and using this measurement to derive a criterion for setting the control gain. In comparing the amplitudes, synchronous rectifiers are used to filter out those oscillation components of the control difference and of the controlled variable whose frequency is equal to the input frequency. Using a control unit, the gain of the controller is then changed until a comparator establishes that the filtered-out oscillation components of the controlled variable and of the control difference are of equal magnitude. The known method is therefore an iterative method which takes up a lot of time or is slow on account of the step-by-step tuning. Evaluating the phase information requires a high degree of complexity and is likewise slow, as a result of step-by-step tuning. Reducing the number of comparison steps inevitably leads to greater inaccuracy. When comparing the amplitudes of the controlled variable and of the control difference, care should also be taken to ensure that the control gain is not set exactly, since oscillation components are not in phase with the input frequency of the controlled variable and the control difference. Furthermore, using a frequency which is different from the critical frequency presupposes that the loop gain at this frequency is known.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method and arrangement for automatically setting the gain in control loops, which are characterized by a rapid yet reliable setting and/or tuning procedure.
This object is achieved by the features specified in main claims. Advantageous designs and developments are specified in subclaims.
One aspect of the invention is the acceleration of the tuning procedure when automatically setting the gain. To this end, the final tuning value is set by a single measurement and subsequent calculation. There is no longer any need for a successive approximation to the tuning value, which means that tuning is considerably faster. In only one method step, an existing gain factor is measured whilst a tuning signal is input, a change which is to be carried out to obtain the desired value is calculated and the desired value of the gain is set. The tuning signal or tuning frequency used is preferably a sinusoidal noise frequency which is equal to the frequency at which the absolute-value curve for the gain of the open loop intersects the 0-dB line. The sinusoidal noise frequency is added, for example, to the controlled variable of the control loop. The value of the existing gain which is formed or the existing gain factor which is formed is, for example, the quotient of the amplitudes of controlled variable and control difference. To avoid the disadvantages arising from the phase margin between the controlled variable and the control difference, the amplitudes of controlled variable and control difference are formed from a signal for the controlled variable and the control difference, respectively, which signal has been subjected to band-pass filtering, squaring, low-pass filtering and square-root extraction. Using a band-pass filter, the tuning frequency is filtered out of the control or control difference signal. Squaring the signals is provided in order to produce a signal which is independent of the sign, and a low-pass filter is used to form an average of the squared tuning frequency, from which the amplitude is determined by means of multiplication and root function calculation. The amplitude signals formed in this way, for example, of controlled variable and control difference represent an existing gain factor stated as a quotient of the amplitudes of controlled variable and control difference. The desired gain equal, for example, to 1 or 0 dB is then set using the reciprocal value of the measured gain factor. For this purpose, a gain setting means formed by a multiplier, for example, is provided in the control loop. This is used to set the gain to a desired value in only one method step. As the absolute-value curve for the gain is approximately linear in the range of the tuning frequency, gain factors which differ from the value 1 may also be set with a high degree of accuracy. The reason for this is that the frequency-dependent rise in the gain is determined from the quotient of the amplitudes of the controlled variable and control difference. As this is a measured existing gain, the tuning procedure has a high level of reliability in comparison with solutions which alter the gain using, for example, tables which are determined with the aid of a model.
However, in addition to the above embodiment, which uses the controlled variable and the control difference to measure the gain, it is also possible to implement other embodiments, which use other connection points in the control loop to measure the gain. In this case, it is then also necessary to take into consideration the particular features arising from the selection of connection points for gain measurements. If the input noise signal and the control difference are used for measuring gain, for example, such a measurement will have an offset of 6 dB, but will not differ, in principle, from the solution indicated here. The offset of 6 dB should then be taken into account in assessing the measured gain value. From this aspect, the inventive method for automatically setting a gain in a control loop is relatively independent of the choice of connection points for gain measurement.
In order to ensure that the existing gain factor was determined in the linear range of the gain characteristic curve, an additional check is made to establish whether the quotient of the amplitudes of controlled variable and control difference has a value of less than 1. If this does not apply, a further aspect of the invention provides for the gain to be set at a lower value, before the measurement is even carried out, using a factor of less than 1, preferably 0.75.


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