Many industrial/instrumentation applications today involve sensory measurement techniques. The function of the sensor is to monitor changes in the system, and then feed this data back to a main control unit. For simple voltage or current measurements the sensor may be resistive. Some sensor systems can be inductive or capacitive, however, which means that the change in impedance is nonlinear across the range of the sensor.
Typical examples of such complex sensors are proximity sensors--used to determine the relative distance to a moving object; and capacitive or inductive sensors--used in the medical industry to measure blood flow or analyze blood pressure or blood quality.
Measuring the impedance of these “complex sensors” requires an AC excitation that sweeps across the frequency range of the sensor. This design idea shows how a single-chip digital waveform generator can be easily used to provide this frequency sweep to greater than 10 MHz. It also describes a complete single-chip impedance converter that integrates the excitation, A/D converter, and signal processing, making it suitable for applications requiring up to approximately 50 kHz excitation frequency.
Sensors: Principle of Operation
Figure 1 shows a model of sensors with inductive or capacitive properties.
Depending on the instantaneous value of the L or C, a frequency that passes through the sensor will exhibit an amplitude-, frequency-, or phase-shift. For example, ultrasonic liquid flow-meters tend to exhibit a phase shift, whereas proximity sensors tend to change in amplitude.
Figure 1. Model of a sensor with complex impedance
The most common way to track this changing impedance is to monitor the resonant frequency of the circuit. The resonant frequency is the point at which the capacitance value equals the inductance value. This will also be the point of highest impedance on the frequency curve. For example, consider the case of the proximity sensor shown in Figure 2. In normal mode, i.e. under static conditions, the L, R and C of the sensor will have one unique value, which will have maximum impedance at the resonant frequency, Fo. As a moving object approaches the sensor, the L and C values of the sensor change, and a new resonant frequency is created. Monitoring the change in resonant frequency (and therefore impedance) allows the distance of the moving object from the sensor to be predicted.
Figure 2. The resonant frequency of a proximity sensor shifts with distance