ABB H&B Publication 60-0.15 EN

Fundamentals of Control Engineering

Set point control through a computer of any design. With such an arrangement, the computer adjusts the set point according to superordinate criteria. These may include the order book situation at any given time, the breakdown of the orders, the power consumption at a given time or any of numerous other criteria. The aim is in most cases to optimize production. – Direct Digital Control. In normal cases control is a task of a superordinate computer. Compact controllers are subordinate to this computer and assume the control function in a bumpless manner if there is a computer fault. The following operating modes are conceivable: Retention of the last computer correction value in manual operation Automatic operation with safety set point Automatic operation with the last value of the controlled variable adopted as the current controller set point (x-tracking) Cascade control. – Operation and monitoring of the system. Important information for operation of the system is displayed in a suitable manner on one or more screens to enable processes to be monitored and changes made if necessary. – Feedback documentation of the parameters set in the controller and any change made to their configuration.

– Process visualization: A powerful software programme, in conjunction with a master computer or PC, makes it simple for a user to centrally operate, monitor, control and automate a process.

Definitions These definitions are extracts in an abbreviated form from DIN 19 226. Where doubt exists, the original definitions are definitive. Analogue signal A signal with a continuous value range. Limiting control A combination of at least one main controller and an additional controller which ensures that the variable to be limited does not exceed predefined alarm values. Binary signal A digital signal with only two values. Digital signal A signal which can assume any one of a finite range of values. Set value control The reference variable is set to a fixed value (which can be changed). Follow-up control The value of the controlled variable follows the changing value of the reference variable. Response to set point changes The response of a controlled system to changes in the reference variable. Limit signal The binary signal of a limit monitor. Limiting value The value of the input variable of a limit monitor at which its binary output signal changes. Manual control Human control of at least one element of a control loop. Cascade control The output variable of the (master) controller forms the reference variable for one or more slave controllers. Configuration The elaboration of a control concept from preconstructed programme modules. Control station Operating mode switches, adjusters for reference and output variables and the necessary display functions are brought together in the control station. Optimization Establishing a quality criterium. Parameterization The assignment of values for the characteristics of the modules of a system Programming Developing, coding and testing of a computer programme. Switching point The value of the input variable of a limit monitor at which its binary output signal changes. Differential gap The difference between the switching points (hysteresis) for which the binary output signal of a limit monitor changes with rising and falling input variables. Actuating time The time taken for the output variable to run through the entire correcting range at maximum speed.

Disturbance variable Any variable acting on a system which disturbs the intended effect. Feedforward control Integration of the measurement of disturbance variables in the control algorithm. Disturbance response The response of a controlled system to disturbances. Structuring a) Analysis Breakdown of a system so that its relationships become visible. b) Synthesis The assembly of a system from functional units so that the requirements are met. Time-programme control The reference variable is changed according to a time-schedule. Cycle time Time interval between two sequential, identical, cyclical recurring processes.

Symbols The symbols below are taken from DIN 19 226. The symbols used in the controllers may differ in some respects for technical reasons. If so, then please refer to the relevant Operating Manual. e Control deviation e = w - x (see also xw or xd) KD Derivative-action coefficient (KD = Tv ⋅ Kp) KI Integral-action coefficient (KI = Kp / Tn) Kp Proportional-action coefficient (see also Xp) Ks Controlled system gain (transfer coefficient) r Feedback variable (derived from x) S Controlled system t Time (operating) Tg Recovery time Th Half-life Tn Integral-action time (Tn = Kp / KI) Tt Dead time Tu Delay time Tv Derivative-action time (Tv = KD / Kp) Ut Dead zone USd Differential gap w Reference variable (set point) Wh Range of reference variables x Controlled variable (actual value) xA Object variable XAh Object range xd Error signal (replaced by e) Xh Control range Xp Proportional band (Xp = 1 / Kp) XSd Differential gap (hysteresis) xw Control deviation (xw = x - w) replaced by e, corresponds to deviation from set point y Output variable Yh Correcting range yR Controller output variable z Disturbance variable Zh Range of disturbance variables