Position controllers and two-position control

In control objects that do not have self-leveling, any disturbance effect cannot be localized without the help of an automatic controller and the equilibrium state will not be achieved.

The operation of the automatic regulator is determined by the type of relationship between the deviations of the controlled parameter and the regulating effect of the regulating body, which occurs as a result of its movement. This dependence is called the dynamic characteristic of the controller or the regulatory law of the controller... According to the type of this dependence, regulators are divided into positional, static or proportional, astatic and isodromic.

The regulator in a positioner can have two or more fixed positions, each of which corresponds to certain values ​​of the controlled parameter.

According to the number of positions, regulators can be two-position, three-position and multi-position.

In practice, the largest application is found two-position regulators... They should be discussed in more detail.

In a two-position regulator, when the controlled parameter deviates from the set value (by an amount greater than the insensitivity of the regulator), the regulating body occupies one of the extreme positions corresponding to the maximum or minimum possible flow of the regulating substance. In a particular case, the minimum value may be zero inflow.

The movement of the regulating body from one end position to another with on-off regulation is usually carried out at high speed — theoretically instantaneous in a time instant equal to zero.

Equality between inflow and outflow is not observed for a given value of the controlled parameter. This can only happen at maximum or minimum loads. Therefore, in two-position control, the system is usually in a non-equilibrium state. As a result, the controlled parameter continuously oscillates in both directions from the set value.

The amplitude of these oscillations in the absence of delays, as it is easy to assume, will be a certain insensitivity of the regulator... The zone of possible oscillations of the controlled parameter depends on the regulator dead zone and is determined assuming that there is no delay.

The deadband of the controller is the range of change of the controlled parameter required to initiate motion of the controller in the forward and reverse directions. So, for example, if the room temperature regulator, set to maintain 20 ° C, begins to close the regulator when supplying hot water to the heater, when the internal air temperature rises to 21 °, and opens it at a temperature of 19 °, then the dead zone of this regulator is equal to 2 °.

The accuracy of maintaining the set parameters with on-off is relatively high.

If the control accuracy is high enough, then it appears that on-off controllers can be used in all facilities. The applicability of on-off control is in most cases determined not by the achieved control accuracy, but by the allowable switching frequency. It should be borne in mind that frequent switching leads to rapid wear of parts (very often contacts) of the regulator and, therefore, to a decrease in the reliability of its operation.

The presence of a delay worsens the regulation process, as it increases the amplitude of parameter fluctuations, but on the other hand, the delay reduces the switching frequency and thus expands the scope of on-off regulation.

A schematic diagram of an electric two-position temperature controller in a drying oven is shown in fig. 1.

Rice. 1. Schematic diagram of an electric two-position thermostat in a drying cabinet: 1 — bimetallic sensor; 2 — heating electric element

This regulator consists of a sensor 1 and an electric heating element 2. The sensor consists of two bimetallic contact plates, which under the influence of temperature can, approaching each other, close or, on the contrary, open an electric circuit.

Usually, a temperature of 105 ° C is maintained in the drying cabinet. Then, when the set temperature is reached, the contacts must be closed and part of the heating element is manipulated.The required value of Qpr after maneuvering the heater can be chosen in such a way that it fully compensates the heat losses from the drying oven Qst.

But it can also be adjusted in such a way that when the set temperature is reached, the heater turns off completely. In the first variant, it is possible to achieve that Qpr = Qst, then the regulator will not switch.

In fig. 2 shows the characteristic of the two-position control process. This figure shows the changes in the controlled parameter over time after a single abrupt change in the object load Qpr or Qst. The movement of the regulating body over time is also shown here.

Rice. 2. Characteristics of the two-position control process

It should be noted that in two-position regulation, a change in load causes a change in the average value of the controlled value, i.e. characterized by certain irregularities. The deviation from the average value of the controlled parameter can be calculated by the formula

ΔPcm = (ΔTzap /W) (Qpr/2 — Qct),

where ΔPcm — maximum displacement of the controlled parameter from the average set value; ΔTzap — transfer delay time; W is the capacity factor of the object.

In normal cases, Qpr = Qct and ΔTzap — the value is insignificant. Therefore, the displacement cannot be very significant and does not exceed the dead zone of the regulator.

Areas of application of on and off controllers

A two-position controller can be used in the event that the degree of self-leveling of the controlled object is close to unity and the sensitivity of the object to disturbances does not exceed 0.0005 1 / s, if there are no other reasons forcing you to abandon this controller. These reasons include:

1. Frequent, less than 4 — 5 minutes, turning on and off the regulator, which is usually done in sites with low capacity factors and with frequent changes in the site load.

It should be borne in mind that the permissible switching frequency is determined by the technical sophistication of regulators at this level. These figures are established by the practice of the automatic control system. Perhaps in the future they can be refined, mainly downwards. In addition, it should be borne in mind that it is possible to determine the permissible switching frequency by setting the required life of the regulator, while knowing the minimum standardized number of operations (cycles) of one of the regulatory elements.

2. Inadmissibility of stopping the supply of the heat carrier, for example to the air heaters of the supply ventilation unit or to the air heaters of the first heating of the air conditioning unit. It should be borne in mind that if during the winter season the coolant supply to the heaters is completely or even partially stopped, then when the fan is working, which sucks in cold air at high speed, it can freeze very quickly.

3.Inadmissibility of large deviations of unregulated environmental parameters. Here it is meant that in a number of cases one of the air parameters is regulated, while the other is not regulated, but must be within certain limits.

For example, you can call maintaining a certain temperature in the shops of the textile industry. Here the task is to regulate such a temperature at which the conditions for maintaining the relative humidity within certain limits will be maintained. However, if the temperature is kept within the specified limits, the fluctuations in the relative humidity exceed the permissible zone.

The last circumstance can be explained by the fact that the capacity coefficients of the controlled object in relation to temperature are relatively higher than the same coefficients in relation to relative humidity. Very often in practice it is necessary to abandon the on-off temperature control in such workshops.

4. Inadmissibility of a sharp and significant deviation of the parameters of the control environment in compliance with the requirements for fluctuations in the controlled parameters.

For example, the temperature of the supply air during the on-off adjustment of the heating capacity of the supply chamber air heater can have such significant deviations as to cause unpleasant sensations of blowing in the workplace. In general, fluctuations in internal temperature will not exceed established limits.

This circumstance can also be explained by different values ​​of the capacity coefficients of the air heater as the object of controlling the supply air temperature and the production room as the object of controlling the indoor temperature.

Thus, if there is a suitable feature of the object and there is no reason to abandon the on-off controller, you should always aim to install the latter. This type of regulator turns out to be the simplest and cheapest, the most reliable in operation and does not require qualified maintenance. In addition, such regulators ensure stable regulation quality.

An important fact is that the actuation of a two-position regulator very often requires minimal energy consumption, since it is used only at the moments of closing or opening.

Two-position controllers are very often used for automatic temperature control in electric ovens.