The latest opportunity for capacitive sensors for

New opportunities for automotive capacitive sensors

in the past, capacitive sensors were rarely used in cars because they were difficult to control, difficult to read data, easy to aging, and dependent on temperature. However, its low production cost, simple shape adaptability and low power consumption characteristics are conducive to its attractive applications. The rise of a new capacitance measurement technology has led to a sharp increase in the number of automotive capacitance sensors

generally speaking, capacitive sensors analyze by converting capacitance into other physical quantities, such as voltage, time or frequency. Microcosmically speaking, capacitive sensors have been used in automobiles for a long time; Micro electro mechanical (MEMS) acceleration sensor is based on the principle of capacitive sensor. These sensors are usually used to detect charge transfer. American Analog Devices Corporation (ADI) has developed a new method to detect capacitance, which uses the input stage of the improved S-D analog-to-digital converter (ADC) to detect unknown capacitance and convert it into digital quantity. ADI company calls it capacitance digital converter or CDC. This article first explains the CDC conversion method. Then it introduces the working principle of several capacitive sensors that can be used in cars. Finally, the alternating transformation method is briefly introduced

capacitance digital converter has many advantages. Because it is closely related to s-dadc, some well-known features of s-dadc can be adopted and modified. These characteristics include high noise suppression ability, high resolution at very low frequencies, high accuracy and low cost, and robustness against external influences such as electromagnetic interference. S-dadc has similar input structure almost without exception, so that different structures can be used for specific measurement tasks, such as extremely low current input, maximum accuracy or high cut-off frequency. Looking further at Figure 1, we will find more advantages. Parasitic capacitance does not work in the initial measured value. The parasitic capacitance approaches 0 at node A and has zero potential. It is not 0 at node B, but it is provided with a certain low resistance potential, so the parasitic capacitance of the node will be charged to the average value without affecting the measurement results. The parasitic capacitance from node a to node B is always connected in parallel with the measuring element and always appears as an offset voltage

ad7745 is the first CDC, which provides 24bit resolution and 16bit accuracy. It has a resolution of about 50af and a precision of 2ff (calibrated). The reference capacitance has a strictly specified temperature coefficient, so it can be used as a built-in temperature sensor, which can be used not only to improve the measurement accuracy, but also as a reference capacitance

capacitance sensor

previous capacitance analysis systems require high measurement capacitance and have large capacitance changes when touched. This requirement for large enough changes often causes problems for sensor manufacturers, while there should be no problems for small capacitive sensors. For example, typical 150pF humidity sensors are not only much more expensive (because they are larger), but also more prone to errors, and their long-term stability is reduced

the capacity of the capacitor is calculated according to the formula c=eoera/d according to its structure, where EO is the dielectric constant in vacuum, Er is the dielectric constant in the medium, a is the effective area of the electrode plate, which is constantly challenging the limit, and D is the distance between the two electrodes. With a few exceptions, such as pressure sensors, all capacitive sensors use the change of flat surface or dielectric to measure the change of capacitor. Most sensors can be classified into two categories: according to the change of electrode area (geometric dimension) (such as liquid level sensor or displacement sensor) and according to the change of Er (such as proximity sensor or humidity sensor)

dielectric sensor

an example of a traditional dielectric sensor is a humidity sensor using a humidity sensitive polymer layer as the medium. With the increase of humidity, water molecules accumulate more and more, so Er increases. The sensor for determining the purity of a liquid (such as oil or fuel) is essentially composed of two fixed plates, and the liquid itself constitutes a dielectric. The nature of the liquid required is mainly determined by experience (such as the increased moisture content in oil or fuel). Temperature plays a decisive role in it, so the decision must be reliable. A simple proximity sensor, which determines the transformation of the medium, usually requires the most complex measuring electronic circuit

in most cases, the proximity sensor is composed of two conductors on the printed circuit board, and the dielectric value between the two conductors has a very low dielectric constant (close to 1, and constantly provide customers with perfect pre-sales and after-sales services through itself and authorized channels). If an object, such as a human hand, moves close to the electric field of the capacitor, the capacitance will change. The moisture content of human body exceeds 90%. Our company specializes in the production, manufacture and sale of experimental machines, so the dielectric constant is very high (about 50). The manufacture of contactless switch is very simple, so it is used in applications such as keyless entry and clamping protection of electronic windows. The core requirement of keyless cars is that the current input is as low as possible - the standard value is less than 100mA. S-dadc has been optimized in the industry for many years, so it can provide a reasonable architecture. The rain sensor can be realized in the same way. These sensors are easy to produce, low cost, and can calculate the torsional strength of materials; It has the disadvantage of small size. The traditional rain sensor based on the light refraction of water droplets must be very small, which reduces the area of the windshield and leads to the problem of repeatable rain hours

geometric size sensor

examples of sensors that vary according to geometric size are pressure sensors, liquid level sensors, and displacement sensors - simply changing the dielectric between two fixed plates. The pressure sensor uses two fixed size plates as the diaphragm; Because the diaphragm is elastic, the distance between the two plates will change when the pressure acts on the sensor

the need for a temperature sensor is due to the geometric dimension change caused by thermal expansion. Imagine that one of the two electrodes is connected to the chip and the other is connected to the shell of metal or ceramic structure. Therefore, the housing itself acts as a sensor. For example, ceramic shell, ceramics can withstand great pressure and corrosive media. Compared with the traditional Wheatstone bridge, the main advantage of the capacitive pressure sensor is its low input current, which makes it particularly suitable for applications such as tire pressure control

in the liquid level sensor, measure a pair of fixed plates immersed in liquid. Manufacturers can use extremely low price printed wires. Another pair of plates are placed at the bottom, allowing the detection medium to change due to temperature or other factors, as shown in Figure 4

of all methods, s-dadc has proved to be the most popular. In many cases, digital filters can always be used to achieve the required dynamic behavior. For example, the liquid level sensor requires constant for a very long time, while the proximity sensor must adapt to the changes of the surrounding environment (such as the humidity sensor used in rain and snow)

alternate conversion method

let's briefly discuss the alternate conversion method. This way of working follows a completely different and even more complex method. On the other hand, this method can be used to measure complex impedance packets, including inductive reactance, impedance and capacitive reactance, or resistive and inductive sensors. In this case, a very precise known frequency is used to excite the sensor. ADI uses direct digital frequency synthesis (DDC) technology for this purpose. The response of the sensor was recorded by fast ADC and fast Fourier analysis. DDS method can accurately know the original phase position at any time. In the same way, the response to other frequencies can be measured. Thus, the real and imaginary parts of the impedance can be calculated and output on the data bus. A complete scan takes only a few hundred milliseconds (MS)

ADI company calls this circuit network analyzer. In addition to capacitive sensors and inductive sensors, appropriate sensors can also record the change of viscosity in the liquid to be measured (such as engine oil or lubricating oil)

conclusion

capacitive sensors are recovering in the automotive industry. A new method has shown the initial success of humidity sensor, rain sensor and proximity sensor. Adopted Σ-Δ ADC technology can provide flexible solutions to meet different dynamic and accuracy requirements, and enable sensor systems to meet extremely low power consumption requirements. Capacitive sensors have been used in a variety of applications, and Adi is developing solutions for tire pressure sensors and keyless vehicles. Then we will develop alternative solutions