The significance of feedback control for chemical sensors

The conventional way of applying chemical sensors is in an open-loop configuration. A parameter of the chemical domain, such as a gas or ion concentration, is converted into a parameter of the mechanical or electrical domain, often with non-linear transfer characteristics. The paramagnetic oxygen sensor and the ISFET-based urea sensor are treated as typical examples. Such an open-loop procedure is inconceivable in modern electronic and control systems, where the application of feedback is common practice. The question may arise why this principle is hardly adopted in the development of chemical sensors, to stabilize the signal conversion and to linearize the sensor functioning. Based on the analysis of a transistor V-I converter in open-loop and closed-loop configurations, the successful performance of feedback in the case of the paramagnetic oxygen sensor and the ISFET-based urea sensor is analysed. That most other chemical sensors are still used in an open-loop configuration is obviously due to the lack of chemical actuators that can be integrated with the corresponding chemical sensors. It is therefore stated that it will be important to pay more attention to the development of chemical actuators in order to make progress in the field of chemical sensors.