DE3045717A1 - Constant rate control for change of process - storing value of start quantity and comparing instantaneous actual data and ramp function data - Google Patents

Abstract

The industrial process control is applicable where the rate of change of a quantity, e.g. wt., during a filling process, must be kept constant. This is achieved by the use of a ramp-generator followed by a differentiator. The problem of the initial delay due to the differentiator is overcome by measuring and storing the starting quantity value. The required value is obtained from the instantaneous stored value and ramp value. The outputs of the amplifier (3), the ramp generator (4) and the actual value (x1) are fed to the regulator (5) that produces the process control signal (y).

Description

"Method and circuit arrangement for keeping constant

the rate of change of a physical quantity 11 The invention relates to a method and a circuit arrangement for keeping the Rate of change of a physical quantity with analog measuring and control devices. In numerous industrial processes it is often desirable or necessary the rate of change of a physical quantity is constant during the process to hold, e.g.

that of the weight during filling processes or the like.

The rate of change of a quantity x, e.g.

of a weight, is described by the differential quotient dx / dt, which is determined with the help of a differentiator and switched to a controller as the actual value will. This known principle is shown in Fig.l.

At first glance, such an arrangement according to Fig.l represents a The optimal solution is to use an "ideal" differentiator in the mathematical sense Could use that already at time t1 (see Fig. 2) when the process started provides the required differential quotient (curve a).

With the real differentiators used in practice However, a correspondingly precise approximation of the final one is only available at time t2 Differential quotient at the output (if the transient process is considered practically completed can be viewed), see curve b in Fig.2.

The response of a real differentiator to a ramp function is described by the following equation: t dx = m. a. T. (1-e) (1) dt where: = output variable of the differentiator m - slope of the ramp function a = gain T = time constant of the differentiator t The transient process is described by the term e, which tends towards "0" for t ->#. The product m a. T corresponds to the final differential quotient.

In FIG. 2, T also denotes the time constant of a * real differentiator, t is the time after which the settling process of a real differentiator is practically is complete, and tp is the process run time.

The prerequisite for the practical application is that t * = t2 - t1 # tp is.

For this reason, the method just described is known can only be used in a few applications.

The object of the present invention is therefore to provide a method and to create an arrangement of the type mentioned above, without restrictions are applicable.

The inventive method consists in that the change the process size is specified by specifying a ramp function that the initial value the process variable is measured and stored that from the instantaneous value of the ramp function and stored value of the setpoint is determined and compared with the actual value will, and in a manner known per se, one influencing the process sequence from the deviations Manipulated variable is derived.

A circuit arrangement suitable for carrying out this method contains a ramp generator, a hold amplifier and a controller to which the output variable of the ramp generator, the value stored in the hold amplifier and the current one The actual value can be supplied and the manipulated variable can be taken from the output.

An exemplary embodiment for a suitable circuit arrangement is shown on the basis of FIG described in more detail.

The output of a transducer 1 is sent to a transducer 2, the electrical output variable of which is fed to an input of a holding amplifier 3 is and at the same time fed to a first input of a controller 5 as actual value x1 will. A ramp generator 4, which is started at the start of the process, and its speed is expediently controllable, provides a ramp function x2 at its output, which reaches a second input of the controller 5. On a third entrance this The controller receives the initial value x3 stored in the hold amplifier 3.

The controller 5 contains an adder-subtracter (I), which the ramp function x2 and the initial value x3 are supplied. The computing device determines from these variables a setpoint w, which reaches a comparator II, which at the same time also has the actual value x1 is fed. The output voltage of the comparator II corresponds to the control deviation xw, from which the manipulated variable y for the process can be derived.

The way it works is as follows. The linear rise or fall a process variable, e.g. the filling or emptying of a container, is indicated with Simulated using the ramp generator 4. The initial value x3 of the process variable is measured and stored in the hold amplifier 3 before starting the process.

The initial value x3 and the instantaneous value of the ramp x2 are stored in the arithmetic unit I are added with the correct sign and thus form the setpoint w of the control loop, which (in this example) falls linearly according to the desired function.

The controller uses the instantaneous value of the actual value X1 and the of the specified setpoint w, the control deviation xw = x1 - w and influences the Process in such a way that in the settled state xw = °, i.e. x1 = w.

When the process starts, the initial value is saved and the Ramp generator started. After the process has finished, the ramp is set back to "zero", so that the process can begin again. Through the hold amplifier the Initial conditions set automatically. The slope of the ramp is both internal as well as externally controllable.

The adder / subtracter I can either be connected to an additional computing device or implemented in a controller.

The signs entered in Fig. 3 apply to a linearly decreasing process variable, e.g. "Emptying a Container".

Claims (4)

Claims 1. A method for keeping the rate of change constant a physical variable, characterized in that the change in the process variable is specified by a ramp function that the initial value. the process size is measured and stored, and that from the instantaneous value of the ramp and stored value the setpoint is determined and compared with the actual value, and in itself known A manipulated variable influencing the process sequence is derived from the deviations will. 2. Circuit arrangement for performing the method according to claim 1, complete with a ramp generator (4), a hold amplifier (3), and a Controller (5) to which the output variable (x2) of the ramp generator (4), which is in the hold amplifier (3) stored value (x3) and the current actual value (x1) can be supplied and at the output of which the manipulated variable (y) can be taken. 3. Circuit arrangement according to claim 2, characterized in that the controller (5) contains a computing device (I). 4. Circuit arrangement according to claim 2, characterized in that Ramp generator (4) and hold amplifier (3) combined in one device (e.g. integrator) are.