Question 1

Advantages of capacitive pressure sensors

Accepted Answer

The capacitive pressure transducer produced by our Sino-Inst has the following advantages:

(1) Good temperature stability

The capacitance value of the sensor generally has nothing to do with the electrode material and only depends on the geometric size of the electrode. And the loss of media such as air is very small, so as long as the mechanical properties such as strength and temperature coefficient are considered, the materials and geometric dimensions can be reasonably selected. Other factors (because the heat itself is minimal) have little impact.

The resistive sensor has resistance and generates heat after power supply; the inductive sensor has copper loss, eddy current loss, etc., which causes its own heating and zero drift.

(2) Simple structure and strong adaptability

Capacitive sensors have a simple structure and are easy to manufacture. It can work in various harsh environmental conditions such as high and low temperatures, strong radiation and strong magnetic fields. It has strong adaptability, especially can withstand large temperature changes, and can work normally under high pressure, high impact, overload and other conditions. It can measure ultra-high pressure and low pressure difference. It can also measure magnetic workpieces. In addition, the sensor can be made very small in order to achieve certain special requirements for measurement.

(3) Small electrostatic attraction

There is an electrostatic field between the two plates of the capacitive sensor. Therefore, electrostatic attraction or electrostatic moment acts on the plate. The size of the electrostatic attraction is related to the working voltage, dielectric constant, and distance between the electrode plates.

Generally speaking, this electrostatic attraction is very small. Therefore, only for elastic sensitive components with very small driving force, the measurement error caused by electrostatic attraction must be considered.

(4) Good dynamic response

Since the electrostatic attraction between the plates is very small (about a few 10-5N), the capacitive sensor requires very little energy. And because its movable part can be made very small and thin, that is, its mass is very light, its natural frequency is very high, its dynamic response time is short, and it can work at frequencies of several MHz, making it particularly suitable for dynamic measurements.

And because its dielectric loss is small, it can be powered at a higher frequency, so the system operating frequency is high. It can be used to measure high-speed changing parameters, such as measuring vibration, instantaneous pressure, etc.

(5) Non-contact measurement can be achieved and has an average effect

When the device under test cannot allow contact measurement, capacitive sensors can complete the measurement task. When using non-contact measurement, the capacitive sensor has an averaging effect, which can reduce the impact of the workpiece surface roughness on the measurement.

In addition to the above advantages of capacitive sensors, the electrostatic attraction between the electrode plates is extremely small, so the input energy required is extremely small. Therefore, it is particularly suitable for measurements with low energy input, such as measuring extremely low pressure, force and very small acceleration, displacement, etc. It can be made very sensitive, with very high resolution, and can sense displacements as small as 0.001µm or less.

Question 2

Disadvantages of capacitive pressure sensors

Accepted Answer

(1) High output impedance and poor load capacity

The capacity of the capacitive sensor is limited by the geometric size of its electrode, which is generally tens to hundreds of pF, making the output impedance of the sensor very high. Especially when using AC power within the audio range, the output impedance is as high as 106~108Ω. Therefore, the sensor has poor load capacity and is easily affected by external interference, causing instability. In severe cases, it may even fail to work. Shielding measures must be taken, which brings inconvenience to design and use.

Large capacitive reactance also requires the resistance value of the insulating part of the sensor to be extremely high (above tens of MΩ), otherwise the insulating part will act as a bypass resistor and affect the performance of the sensor (such as reduced sensitivity). For this reason, special attention should be paid to the impact of the surrounding environment such as temperature, humidity, cleanliness, etc. on the insulation performance.

Although high-frequency power supply can reduce the output impedance of the sensor, the amplification and transmission are far more complicated than at low frequency, and the influence of parasitic capacitance increases, making it difficult to ensure stable operation.

(2) Parasitic capacitance has a great influence

The initial capacitance of the sensor is very small, but the "parasitic capacitance" such as its lead cable capacitance (1~2m wire can reach 800pF), the stray capacitance of the measurement circuit, and the capacitance formed by the sensor plate and its surrounding conductors is relatively large.

Reduced sensor sensitivity;
These capacitances (such as cable capacitance) often change randomly, which will make the sensor work unstable and affect the measurement accuracy. The amount of change may even exceed the change in capacitance caused by the measurement, causing the sensor to fail to work.

Therefore, there are requirements for cable selection, installation, and connection methods.

(3) Output characteristics are nonlinear

The output characteristics of the variable pole pitch capacitive sensor are nonlinear. Although the differential structure can be used to improve it, it cannot be completely eliminated.

The output characteristics of other types of capacitive sensors are linear only when the edge effects of the electric field are ignored. Otherwise, the additional capacitance generated by the edge effect will directly superimpose with the sensor capacitance, making the output characteristics nonlinear.