Rieter

Closed-loop control

Index

Fig. 42 – The principle of closed-loop control; A, measuring sensor; C, amplifier; D, adjusting device; F, set-value input; G, dead-time distance

The measuring sensor is usually arranged in the delivery region, i.e. downstream from the adjusting device (Fig. 42). In contrast to  open-loop control, the measuring point is after the adjusting point. The same measuring, regulating, and adjusting devices can be used, but no storage is needed. Moreover, the actual value does not have to be established as an absolute value but can be derived as negative, positive, or neutral pulses.

If too much material passes through the sensor, the regulating transmission receives a negative signal (i.e. reduce speed) until the actual and set values coincide again. Neither a positive nor a negative signal is produced when there is coincidence – the instantaneous speed is maintained. The principle is substantially simpler than open-loop control. However, this advantage, and the advantage of self-monitoring, must be weighed against a serious disadvantage, namely the dead time inherent in the system. The measured portion has already passed the adjusting point when the adjusting signal arrives.

Compensation cannot be achieved in this measured portion; i.e. some of the long and medium-term errors, and all of the short-term errors, remain in the product. It is therefore clear that closed-loop control is unsuited to compensation of irregularity over short lengths.