Control response speed and stable control time of mass flow controller

The control response speed and stable control time of the mass flowmeter controller The physical time constant (Tau) The physical time constant is represented by the Greek letter Tau, which is one of the few standards used to describe the response time today, namely: the controller reaches The time required to set a certain percentage of the value is defined as 1-1/e, where e is an irrational number as the base of the natural logarithm, and its value is 2.72, so 1-1/e is 63.2% of the set value . Although this definition is a physical standard, it may not be telling, because the remaining 36.8% of the set value may take more or less time than the previous 63.2%. From the oscilloscope screenshot below, it can be seen that the ALICAT® mass flow controller reached 63.2% of set point in just 7.4 milliseconds. Arbitrarily specify the percentage change area of ​​the set value Many mass flow controller technical parameters use the change within a certain percentage range of the set value to define the response speed of the equipment, for example: the time required to reach 90% from 10% of the set value . Since the first and last 10% of the control response curve vary the most, the resulting speed of control response may not be equivalent to actual control performance. As shown in the figure below, the ALICAT® mass flow controller took 11.4 milliseconds to reach the set value. It can be clearly observed from the oscilloscope waveform diagram: the 50% segment has the largest span, and the two segments lower than 10% and higher than 90% The span is much smaller. As far as the figure is concerned, if the response speed of the equipment is defined according to the time taken to reach 90% from 10% of the set value, then the conclusion is 6~7 milliseconds. Stabilization time Since the controller may overshoot after reaching the set value, the manufacturer usually uses the time required to reach the set value and stabilize as the response speed of the equipment, and gives a tolerance range for the waveform oscillation amplitude. For example, the allowable range of 2% means that the flow fluctuation is stable within 2% of the set value. Generally, the accuracy of mass flow controllers is between 0.5% and 2%, but some manufacturers have extended the stability tolerance range to 10%. At ALICAT®, we believe that if the user wants to control the flow rate at the desired set point, then its stable allowable range must also fall within the accuracy range of the equipment. As shown in the oscilloscope screenshot below, the controller reached a stable tolerance equivalent to the device's accuracy (1% of full scale) at 27.6 ms.

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