Views: 442 Author: Site Editor Publish Time: 2020-09-30 Origin: Site
Advantages and disadvantages of Capacitive sensors
The capacitive element is mechanically simple and robust.
Capacitive sensors are able to operate over a wide temperature range and are very tolerant of short-term overpressure conditions.
They can be used to measure a wide range of pressure from vacuum (2.5 mbar or 250 Pa) to high pressures up to around 10,000 psi (70 MPa). They’re ideal for both lower-pressure applications and reasonably harsh environments.
Because no DC current flows through the capacitor, they are inherently low power.
Passive devices may not require a power source at all; the excitation signal can be provided by the external reader. This makes them suitable for wearable or implanted medical devices. These applications can be enhanced by new technologies that enable the construction of sensors that are flexible or moulded to shape.
Capacitive sensors exhibit low hysteresis and good repeatability of measurements. They also have low temperature sensitivity.
The response time is in the order of milliseconds, and even faster in the case of MEMS devices.
Because they’re inherently AC devices, capacitive sensors are suitable for wireless applications. They can be used in an oscillator circuit to generate a signal, with a frequency proportional to pressure, that can be received wirelessly.
Alternatively, the reader can use inductive coupling to measure the change in resonant frequency – this is particularly suitable for passive devices that require no power supply.
One of the main disadvantages of capacitive sensors is the non-linearity exhibited because the output is inversely proportional to the gap between the parallel electrodes. This can be improved by using the sensor in touch mode, where the diaphragm is in contact with the insulating layer on the lower electrode. However, this can reduce sensitivity and increase hysteresis. They are also sensitive to vibration.
The interface needs to minimise stray capacitance by having the electronics as close as possible to the sensor. This is another benefit of MEMS technology.