Flow Control by Pulso Width Modulation

Abstract

A novel method of controlling the flow rates of fluids has been devised which is particularly suitable for highly erosive fluids and slurries. The width of pulses in a pulse train actuating an on-off valve is modulated according to the error between the desired and measured flow rates and thus the ratio of the total times the valve spends in its ‘on’ and ‘off’ states is varied. The method has been tested with an on-line computer which was used to measure the flow rate of water by a cross-correlation method to determine the transit time of temperature fluctuations between two thermocouples in the pipe, and to implement the pulse width modulation control. Satisfactory results have been obtained for start-up, set-point changes and load disturbances for flow rates in the turbulent region (Re > 3 600). Future development includes the use of a fluidic diverter as the on-off device.