3.2. Digital electronics

Microprocessor electronics is used toprocess the current set-points and actual flow rates and actuate the final control element (proportional valve). The analog sensor signal is filtered by the control electronics and convertedinto a value that corresponds to theactual flow rate using a calibration curve stored in the instrument. Thecontrol deviation x
Gas flow sensor elementHeaterT sensors d between the set-point value w and the actual value x isprocessed by the controller using a PIalgorithm and is used to calculate the manipulated variable y
2 , with whichthe industrial control valve is actuated. The control parameters are set during thecalibration process. To allow for the properties of the controlled system, the controller uses system-dependentamplification factors that are detectedautomatically using a self-optimization routine (autotune). An overshoot (seeFigure 10) must be accepted for highdynamic requirements. The control dynamics can, however, be adjustedretrospectively using the Mass FlowCommunicator communication soft- ware. This also applies to the filter level and the smoothing of the actualvalue signal, which is returned. Figure10 shows the step response of mass flow controller Type 8712 (withCMOS sensor), Figure 11 shows thesame from the mass flow controller Type 8626 (with inline sensor). The figures show the responses of the actual value and controller output signals when the set-point value is increased from 10 %to 95 %. Depending on the design of the in- strument, the set-point and actual value signals can be specified and returned in analog form via the stand-ard signal interface or in digital form via an RS-232 or field bus interface.
Figure 9: Function diagram of the bypass sensor in CMOSens® technology(cross section from the bypass channel) 3.1.3. Bypass CMOSsensor, direct measure-ment in the bypass flow The measurement is taken directly inthe bypass channel. A laminar flowelement in the main channel generates a slight pressure drop, which drives a small proportion of the full flowthrough the bypass channel. The sen-sor located therein records the mass flow rate directly by measuring thetemperature differential. The measure-ment in this case is taken in a specially shaped flow channel whose wallscontain a Si chip in one place with an exposed industrial diaphragm. Using CMO- Sens® technology, a heating resistor is connected to this diaphragm alongwith two temperature sensors up-stream and downstream of it. If the heating resistor is supplied witha constant voltage, the voltage differ-ential of the temperature sensors is a measure of the mass flow of the gas flowing through the flow channelpast the chip. The low thermal mass of the tempera-ture sensors and their direct contactwith the flow (apart from a protective layer) means the sensor signal reacts extremely fast to quick flow-rate chan-ges and the MFC is capable of sett-ling set-point value or actual value changes within a few hundred milli-seconds. In addition, they are extre-mely sensitive, even with very low flow rates, and also offer additionalcorrection and diagnostic facilities viathe signal of a separate temperature sensor located on the chip.