“The basic concept of position adjustment Position adjustment is also called continuous adjustment, which is named after the existence of position element relays or non-contact switches in the system. The position adjustment system is a nonlinear system, and a stable and continuous oscillation process will appear in the adjustment process (even the unstable oscillation process will diverge to a stable continuous oscillation process), and the three-position adjustment system will also produce Oscillation decay process. The output of the position adjustment system generally appears in the form of relay contact action, and also in the form of non-contact Electronic switch.

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The basic concept of position adjustment Position adjustment is also called continuous adjustment, which is named after the existence of position element relays or non-contact switches in the system. The position adjustment system is a kind of nonlinear system, and a stable and continuous oscillation process will appear in the adjustment process (even the unstable oscillation process will diverge to a stable continuous oscillation process), and the three-position adjustment system will also produce Oscillation decay process. The output of the position adjustment system generally appears in the form of relay contact action, and also in the form of non-contact electronic switch.

The task of position adjustment is to amplify the difference between the measured value and the given value into the action of the relay contact.

Fig. 1a is the adjustment characteristic of the two-position adjustment instrument, and Fig. 1b is the adjustment process of the two-position adjustment system.

Figure 1 Two-position adjustment

When the measured value is 0, higher than Qa, the relay is released, and when the measured value 0x is lower than Qb, the relay is closed. Usually the difference between Qa and 0b ΔQ=Qa-Qb is called the dead zone or dead zone or switching difference, and (Qa+Qb)/2 is called the switching median, and the switching median is the set value 0. The difference is called the control point deviation Qε.

It can be seen from Figure 1 that the two-position adjustment process is a stable and continuous oscillation process. The adjustment deviation fluctuates back and forth within the range of Δ0, and there is no definite value; the insensitive area of ​​the adjuster directly affects the oscillation amplitude of the adjusted parameter. , In order to make the oscillation amplitude not too large, the insensitive area should not be too large; but the insensitive area should not be too small, otherwise the operating frequency of the relay will be too high and its service life will be reduced, and even the relay will not work, so it is necessary to reasonably Adjust the insensitive zone of the regulating instrument. It can also be seen from Figure 1 that if the insensitive area remains unchanged, the time constant of the adjusted object will decrease, and the oscillation frequency will increase, so the bit adjustment is suitable for objects with a larger time constant. operate at a lower frequency in the insensitive area. In addition, if the adjusted object lags the time t, the oscillation amplitude will increase, which is unfavorable, so the position adjustment is only suitable for the occasions where the lag of the object is small. Of course, the lag time t is large, and the oscillation frequency can be reduced; Finally, it can be seen that when the time constant of the adjusted object is constant, the given value Q, or the disturbance changes, the oscillation amplitude and oscillation frequency also change. If the insensitive area remains unchanged, the relay oscillation frequency can be reduced.

From this, it can be concluded that the effect of position adjustment depends not only on the performance of the instrument itself, but also on the characteristics of the adjusted object and the quality and level of the operator and user of the instrument. For a skilled instrument user, even if the adjusted object cannot be changed, the adjustment quality can be improved by adjusting the parameters of the instrument, and vice versa.

Of course, in general, it is always hoped that the oscillation amplitude of the position-type adjustment system is as small as possible, which can reduce the fluctuation range of the adjusted variable and improve the adjustment accuracy. At the same time, it is always hoped that the oscillation frequency is as small as possible. In this way, it can not only reduce the fluctuation degree of the adjusted amount, but also reduce the number of pull-in times of the position element, thereby prolonging the service life of the position element. Instrument, these two indicators are mutually restrictive and cannot be taken into account at the same time. If the three-position adjustment instrument is used, the situation will be greatly improved, as shown in Figure 2.

Figure 2 Three-position adjustment

a) Regulation characteristics b) Transition process

Fig. 2a is the adjustment characteristic of the three-position adjustment instrument, and Fig. 2b is the transition process curve of the two-position adjustment system.

Figure 2a shows the characteristics of a three-position regulator without hysteresis. Q.1 and Q in the figure are the lower and upper limit given values ​​respectively, Q is the average given value, and 2ΔQ is the middle band of the three-position regulator. The so-called wide median and narrow median.

It can be seen from Figure 2: when the input of the regulating instrument is QQ02>0.1, the output of the regulator is 0, and the system stops heating. Due to the inertial lag of the object, the furnace temperature begins to decrease after a period of lag time, and then drops to o