JPS62182674A - Servo circuit character measuring circuit - Google Patents

Abstract

PURPOSE:To measure the transfer characteristics of a servo circuit as a closed loop by inserting an injection resistor for measuring signals in a part of the closed loop of the servo circuit and measuring the frequency characteristics of the closed loop of the servo circuit from the voltage ratio across the injection resistor. CONSTITUTION:An injection resistor R2 for measuring signals is inserted in advance in a part of the closed loop of a servo circuit. In a measurement, a measuring signal source Eg in a floating condition is connected to both ends of the resistor R2 in an adapter fashion. The frequency characteristics of the closed loop of the servo circuit are measured from the voltage ratio across the resistor R2. Thus, the transfer characteristics of the servo circuit is measured as the closed loop.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a circuit that measures the characteristics of a servo circuit in a closed loop.

[Conventional technology]

In CD (compact disc) players, a servo circuit is used to minutely move the lens of the optical pickup, but the performance (stability, etc.) of this type of servo circuit depends on the gain margin, phase margin, and transient response of the open-loop characteristic. characteristics, closed loop frequency characteristics, etc. However, when the loop gain of the open loop is high, saturation and S/N degradation occur, making it difficult to obtain satisfactory measurement results. Therefore, measurements must be performed in a closed loop.

FIGS. 3 to 5 show different types of conventional I11 constant circuits.

The servo circuit is a closed loop with input as X and output as y.
Human is the gain of a circuit that converts a minute mechanical displacement X into an electrical quantity, cB is a gain of a circuit that converts an electrical quantity into a mechanical displacement y, and AI and A2 are adders.

In the example shown in Fig. 3, adder A2 is connected between cA and cra, and the signal from the loop is added to the measurement signal C from the frequency characteristic analyzer FRA, resulting in the sum signal a. =
Measure b+c while injecting it into the loop. For example, the open loop gain GOL is measured by varying the signal C in the range of several kHz to several tens of kHz.

The inherent closed-loop gain of this servo circuit is y/X, but in a CD player, both x and y are mechanical displacements on the order of several μm, so it is difficult to directly determine the ratio. Therefore, as described above, the loop is transformed into a loop in which a is the input and b is the output, and the measurement is performed.

In FIG. 4, the adder A2 is connected in advance in a closed loop, and the operating principle is the same as in FIG. (a) is an overall diagram, and (bl is a detailed diagram of the adder A2).

Adder A2 is a resistor R11゜R12 that adds signals S and c.
It consists of an operational amplifier OP, and a feedback resistor R13, and it is sufficient to leave terminals a and c open when not measuring.
A2 has a gain of 1).

Figure 5 shows a method in which the adder for measurement is connected to both ends of the resistor R1, which is connected in advance to the loop, without cutting the loop or connecting an adder in the loop. If this resistor R1 is selected to a value that is sufficiently larger than the output impedance to G and sufficiently smaller than the input impedance of GB, the connected state at the time of measurement is equivalent to the R1 part being disconnected. Moreover, when not measuring, it does not affect the closed loop.

[Problem that the invention seeks to solve]

However, since all of the above-mentioned methods use adders, the circuits on the adapter side in measurement circuits 1 and 3 become complicated, and in measurement circuit 2, the circuit to be measured becomes large and expensive. Individually, the measurement circuit 1 requires loop disconnection and connection, which takes measurement time and causes a decrease in reliability. Furthermore, the measurement circuit 1.3 affects the circuit under test and causes a reduction in measurement accuracy.

The present invention attempts to eliminate the above-mentioned drawbacks by making the measurement signal source floating to form an adder.

Means for Solving Problem c] The present invention provides a method in which an injection resistance for a measurement signal is inserted in advance into a part of the closed loop of a servo circuit, and a measurement signal is placed in a floating state at both ends of the injection resistance at the time of measurement. The servo circuit is characterized in that the open-loop frequency characteristic of the servo circuit is measured from the voltage ratio across the resistor by connecting the power source using an adapter.

[Effect]

As shown in Figure 1(a), a resistor R is connected in series with a part of the loop.
Even if i is inserted, if the impedance Zi seen from the insertion point to the back of the loop is sufficiently larger than the inserted resistor Ri (Ri<Zi), the influence of the characteristics on the circuit under test can be ignored. That is, when Ri=0, the current (2) is, but if Ri<Zi even with Ri+o, the flowing current I' is E/Zi Zi+Z.

Therefore, there is no influence of Ri. This is during non-measurement, but
If an external signal source Eg is connected during measurement and its output impedance Rs is connected in parallel with Ri, the influence becomes even smaller because Ri 7 Rs < Ri. Note that Zi is the input impedance of Ca, and ZO is the output impedance of GA. This figure shows that the voltage generated on Zi is C
This shows that A and C are multiplied by 8 and appear as voltage E on Zo.

Next, we will explain the reason why the signal Eg can be injected in the figure (bl).If there is no signal source Eg, the voltages Ei and E across the resistor Ri
o are equal, but when Eg is connected, Eo-Ei=E-I
(Zo+Zi), and under the condition of Zi)Ri. If this is shown isometrically, as shown in the same figure (C1), Eg is injected into the loop.

However, the current relationship is Ig=Ix+Iy Ri・Ix= (Zi+Zo)Iy when E=0, and the current I when Eg=O (E:8:O) is, so E≠O, Eg ”=: The current I when O is I
=I++Iy.

The injection resistor Ri also functions as a terminal for the output impedance Rs of the measurement signal source Eg. In this case, Ri(Zi
When Ri is set smaller than Rs to satisfy
If it is necessary to match the output impedance of the signal source, the insufficient amount Rs' may be inserted into the adapter side or the circuit under test.

At this time, the attenuation caused by the voltage division between Ri and Rs' can be solved by increasing the output of the signal source Eg.

Generally, each input of the frequency analyzer FRA has high input impedance and low capacitance, but the oscillator Eg output terminal has a relatively large capacitance C between it and the ground as shown in Figure 1(d), so it is KW may be applied to the measurement circuit. In this case, if the equivalent circuit is as shown in (e), it will be affected by C, but if Rs' is placed between the circuit under test and the oscillator as shown in (f), the effect can be reduced.

〔Example〕

FIG. 2 is a block diagram showing an embodiment of the present invention, and when compared with FIG. 1, Ro = Z o r RL = Z t
The relationship is r R2=Rt, R]=RS'. Therefore, R2<RL Rs-R2+Rz. Of these, R2 is a resistor inserted into the loop, but R3 is on the adapter side, so the configuration on the loop side is simple, and the configuration on the adapter side is also simple (no adder is required in either case). .

FIG. 7 shows the measurement results according to the present invention (the upper row shows the frequency characteristics, and the lower row shows the phase characteristics), which is the same as FIG. 6 (the measurement results according to FIG. 4).

〔Effect of the invention〕

As described above, according to the present invention, the transfer characteristics of a servo circuit can be measured in a closed loop with a simple configuration (basically only an injection resistor), and there is little influence on the side to be measured ( Further, the adapter structure has the advantage that it can be easily attached and detached.

[Brief explanation of drawings]

Fig. 1 is a diagram explaining the principle of the present invention, Fig. 2 is a block diagram showing an embodiment of the invention, Figs. 3 to 5 are block diagrams showing different examples of conventional measurement circuits, and Fig. 6 is a block diagram showing an embodiment of the present invention. FIG. 7 is a characteristic diagram of the measurement results obtained by the conventional measuring circuit 2, and FIG. 7 is a characteristic diagram of the measurement results obtained by the circuit of the present invention. In the figure, CA and CB are gains of the servo circuit, FRA is a frequency characteristic analyzer, Eg is a measurement signal source, Rs is its output impedance, Ri, R2 are signal injection resistors, R3', R
3 is a correction resistance. Measurement of the end of the Honjuku Udan Kutaken 2 drawings 11 each 1 Fig. 3, ΔKΔ(,., Parts

Claims (2)

[Claims] (1) Insert a measurement signal injection resistor into a part of the closed loop of the servo circuit in advance, and at the time of measurement, connect a floating measurement signal source to both ends of the injection resistor in an adapter type, and 1. A characteristic measuring circuit for a servo circuit, characterized in that the open loop frequency characteristic of the servo circuit is measured from the voltage ratio between both ends of the servo circuit. (2) A characteristic measuring circuit for a servo circuit according to claim 1, wherein the injection resistor is also used for terminating a measurement signal source.