The effect of an element having a logarithmic exponential characteristic on the performance of a process control system

• Main Author: Rashwan, Abdel-Hamid Ahmed
• Published: University of Surrey 1956
• Subjects: 620
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.751493

Regulating units is process control are mostly control valves, these usually have either linear or exponential characteristic. The effect of these control valves in comparison with linear valves is investigated. The loop chosen for investigation consist a of a 3-stage RC network representing the process, a 3-term controller and a regulating unit which is considered either linear or or exponential. The characteristic of the exponential regulating unit being a non-linear one results in closed loop equations that are non-linaear differential equations. To solve these equations a numerical method which is a modification of that used by Tustin is used. The closed loop response to step changes in the desired value and load for both regulating units and under various control modes is calculated. The results obtained from the numerical solutions of the linear equations is checked by solutions using the Laplace Transform method. All results obtained from the solution of the linear and equations are checked experimentally by a simulator built especially for this purpose. Then the closed loop response to a step change in the desired value is calculated in the presence of a distance- velocity lag in the loop for both regulating units using the proportional control mode. Also the closed loop response considering saturation of the regulating units is cases of a large change in the desired value and when derivative control action is present is calculated. The results proved that an exponential regulating unit is inferior to a linear one in the case of desired value changes and is not markedly superior in case of load changes. The work is concluded by commenting on the numerical method used in the solution of the differential equations involved. It proved to be a powerful tool in dealing with non-linear control system in particular and with non-linear differential equations in general.