Capacitive sensors and related technologies and their applications

Theoretically, a trace, a spacer, and another trace are all that is needed to make a capacitive sensor, as shown in Figure 1(a). Simply covering these traces with an insulating transparent plastic film can make them part of the circuit board. When a finger or an object or a person approaches or touches the sensor, the capacitive sensor detects (or senses) the change in capacitance value, as shown in Figure 1(b). This technology has been used in the industry for many years and can be used to measure liquid level, humidity, and material composition. This technology has many advantages: industrial design differentiation, complete internal sealing and waterproofing, interface life, proximity detection and capacitive touch screen applications. As today's home appliances focus more on product design rather than functional design, it can be expected that capacitive sensing will be used in a wider range of home appliance control fields. Refrigerators, dishwashers, stoves, etc. all have unique application environments that pose challenges to the application of capacitive sensing. As a result, capacitive sensing has gained widespread industrial support and is rapidly becoming the focus of the industry. It has become a cheap and durable sensing technology.

1. Operational characteristics and solutions of capacitance detection

1.1 Operational characteristics

The actual basic capacitive sensor consists of a receiver Tx and a transmitter Rx, each of which has metal traces formed on the printed circuit board (PCB) layer. An electric field is formed between the receiver and transmitter traces, as shown in Figure 2. Most of the electric field is concentrated between the two layers of the sensor PCB. However, there is a fringe electric field generated by the transmitter and extends outside the PCB, then returns to the receiver and terminates. The electric field strength at the receiver is measured using a built-in sigma-dedta capacitance-to-digital converter. Inductive sensors can only detect metal materials, while capacitive sensors can detect conductors and insulators with different characteristics from the sensor electrodes. Coincidentally, this characteristic makes humans very suitable for electric field imaging because most of the human body is water with a large dielectric constant. The human body also contains ions and is a good conductor of electricity. Therefore, when people's hands enter the fringe electric field (see Figure 1(b)), the electronic environment will change, causing part of the electric field to be shunted to the ground instead of returning to the receiver. The capacitance reduction caused by this change is in the range of femtofarads, which is different from the picofarads that can be detected by the converter and are generally used in calculating the electric field.

1.2 Solution

Capacitive sensing chip products designed specifically for human-machine interface applications have been launched in the market today. They provide a trigger for the capacitive sensor, can detect the capacitance change caused by the user's approach, and provide a digital output.

Generally speaking, the solution for capacitance detection includes three parts (see Figure 3):

* Driver IC - provides triggering function, capacitance to digital converter, and compensation circuitry to ensure correct results in all environments.

*Sensor - A PCB with a specific pattern of traces, such as buttons, scroll bars, wheels, or some combination. The traces can be made of copper, carbon, or silver, and the PCB can be made of FR4, flex, PET, or ITO.

* Software running on the main microcontroller - performs serial interface and component setup, as well as interrupt service routines. For high-resolution sensors such as scroll bars and wheels, the main microcontroller executes a software algorithm to achieve high-resolution output. Buttons do not require software.

For example, the AD7142 and AD7143 can trigger and respond to up to 14 and 8 capacitive sensors, respectively. They provide triggering of capacitive sensors, sense the change in capacitance caused by the user's proximity, and provide digital outputs.

2. Application of capacitance detection in human-machine interface of electronic equipment

Capacitive sensing will open up new areas of human-machine interface. Mechanical buttons, switches, and jeg wheels have long been used as interfaces between users and machines. Capacitive sensors are more reliable than mechanical sensors - and there are many reasons for this. Because there are no moving parts, they can be covered with materials. For example, the plastic casing of an MP3 player. The protected sensor will not wear out or break. The human body will never come into direct contact with the sensor, so dirt and spills can be locked out.

2.1 Application of Capacitor Detection in Automobile

Today’s cars have many more switches and buttons. Not only do they have to be numerous, but they must also be easily installed into the increasingly diverse control surfaces. In addition, they must be cost-effective to replace sealed switches. One popular approach is to move to capacitive touch switches (CapSense). With no mechanical parts and the ability to conform to shaped control surfaces, CapSense switches offer the reliability and price point that the automotive industry requires.

*About Capacitive Switch

Components based on capacitive sensing technology are gradually coming out. Here we analyze and introduce the technical characteristics of capacitive switches.

A capacitive switch is basically a capacitor formed by two adjacent traces (see Figure 1(a)); the laws of physics require capacitance to exist between them. If a conductor (such as a finger) is placed close to these plates, a parallel capacitor is coupled to the sensor (see Figure 1(b)). When you place your hand over a capacitive sensor, the capacitance increases. When you remove the finger, the capacitance decreases. By adding circuitry to measure the change in capacitance, you can determine the presence of a finger.

Building a capacitive switch requires: a capacitor and capacitance measurement circuitry, as well as local intelligence to translate the capacitance value into a certain switch state.

Typical capacitive sensors have a capacitance value of 10pF-30pF. The capacitance of a finger coupled to the sensor through a 1mm insulating transparent plastic film is usually 1pF-2pF. When a thicker transparent plastic film is used, the coupling capacitance decreases. In order to detect the presence of a finger, a capacitive touch sensing circuit that can measure the 1% capacitance change caused by the finger needs to be designed.

The relaxation oscillator is an effective and simple capacitance measurement circuit. The typical relaxation oscillator circuit topology is shown in Figure 4. The circuit consists of four components. Initially, the discharge switch is open. When the switch is open, all current enters the sensor. This causes its voltage to change linearly. This charging operation continues. Until the sensor voltage reaches the threshold level of the comparator. The comparator then changes from low to high, causing the discharge switch to close. The capacitive sensor quickly discharges to ground through this low impedance path. This process will cause the output of the comparator to change from high to low. The cycle is then repeated. As shown in the following formula, the output frequency (fout) is proportional to the charging current and inversely proportional to the threshold voltage and sensor capacitance. This frequency is measured to determine the sensor capacitance:

When the charging current is 5μA, the comparator threshold is 1.3V, and the sensor capacitance is 30pf, the output frequency is 128kHz. The longer the time spent measuring the output frequency, the higher the resolution obtained. The improvement in frequency resolution will improve the sensitivity of the capacitance measurement. Increasing the measurement time will increase the capacitance measurement resolution. In each application, the change in measurement time can be adjusted accordingly according to the different sensor sizes and transparent plastic film thicknesses.

*About capacitive sensing technology

Capacitive sensing technology is rapidly becoming a new application technology for panel operation and multimedia interaction. Its durability and advantages in reducing BOM costs make this technology widely used in contactless operation interfaces. For example, the use of PSoC (Programmable System-on-Chip) device series system-on-chip chips has realized the design of contactless, stable and reliable capacitive sensing buttons. The PSoC system-on-chip PSoC microprocessor consists of a processor core, system resources, digital system and analog system.

Capacitive sensing is particularly important when sensing the proximity of an object without actually touching it. This is the concept of "proximity detection". Car door locks and access control are an example of proximity detection. Once an authorized user approaches the door with his hand, the door can be opened or the engine can be started. Of course, there are many other product functions related to proximity events. Other applications of "proximity detection" include fast wake-up of PC peripherals. For example, "proximity detection" can be integrated into a wireless mouse or keyboard to quickly wake up from sleep. Normally, after a wireless device goes into sleep after a long period of inactivity, it requires rebinding between the PC host and the peripheral to wake it up again. With proximity detection technology, wireless peripherals can be woken up before contact is made, thus saving the binding time.

2.2 Application of Capacitive Sensing in Portable Devices

Capacitive sensing uses the capacitance of the human body or a conductive stylus to create a contact interface to replace traditional mechanical control. This technology has many advantages: industrial design differentiation, complete internal sealing and waterproofing, interface life, allowing proximity detection and capacitive touch screen applications.

Capacitive sensing has gained widespread industry support. As today's home appliances focus more on product design than functional design, it is expected that capacitive sensing will be applied to a wider range of home appliance control areas. Refrigerators, dishwashers, stoves, etc., all have unique application environments that present challenges for the application of capacitive sensing. Because the sensing element is completely sealed below the surface with no moving parts, liquids splashed on the control panel will not damage the system and can be easily cleaned up. For automobiles and white goods, since the update speed is not as fast as consumer electronics, higher requirements are placed on the life of the control interface. Capacitive sensing is well suited to the needs of these two types of products. With no mechanical moving parts, there is no wear and tear. Capacitive sensing is perfect from the first touch to every touch.

3. Application of electric field measurement function of capacitive sensor

Today, engineers can use electric fields to detect the presence of other objects without actually making contact. These electric field sensors, or capacitive sensors, are becoming increasingly popular as a cheap and durable sensing technology. Capacitive sensors are used in many industries and consumer products, such as computer peripherals, patient monitoring equipment, refrigerator frost sensors, point of sale terminals, and garage door security sensors. The most popular and immediate application is touch screens and touchpads. When developing a touchpad, there are three things that must be considered: touch electrode design and layout; different insulating materials on the panel surface; and the effects of various environmental conditions on electric field measurements.

Capacitive sensors can also be used for liquid level sensing. For example, a simple design places an electrode rod vertically into water to form a longitudinal capacitor and uses electric field sensing to measure the liquid level. When there is no liquid, there is only one capacitor. When water is added, the capacitor is divided into two parts: one is filled with air (dielectric constant is 1) and the other is filled with water (dielectric constant is 80). Through simple calculations, the liquid level can be determined. However, in applications such as washing machines, when detergents and other impurities are added to the water, this system cannot compensate for the effects of different medium characteristics, affecting accuracy.

A more complex capacitance system uses a slope electrode; it uses two thin sheets with the same thickness variation, placed on top of each other. As the liquid level rises, different areas of the electrode are "in contact" with the water, from which a certain ratio can be extracted. This ratio directly reflects the change in the liquid level, while the absolute value of the area provides information on the dielectric constant, which can be used to estimate other impurities such as soap in the water.

Electric field sensing technology is also commonly used to detect the proximity of an object. The capacitor model equation (Figure 5(a)) shows that the capacitance value is inversely proportional to the distance (1/d) between the capacitor electrodes. In a typical application, one electrode of the capacitor is one end of a conductive electrode, and the other electrode is on the object being measured.

Because the distance and capacitance values ​​correspond to each other, this type of sensor is suitable for high-precision proximity measurement (Figure 5(b)). In a general indoor environment, Freescale's MC34940 can detect a hand 1 to 2 inches away using a 1 square foot electrode. This technology can also be used for touch control, safety and security, and proximity wake-up.

4. How to use capacitive sensors

Sensor traces can be of any size, any shape, and any number. Buttons, wheels, scroll bars, gamepads, and touchpads can all be laid out on the sensor PCB in a trace-based manner.

Design engineers have many options for implementing the user interface, ranging from simply replacing mechanical buttons with capacitive button sensors, to not using buttons at all and replacing them with a gamepad with eight output positions, or a scroll wheel that can provide 128 output positions.

The number of sensors that can be completed by a single sensor component must depend on the type of sensor required. For example, AD7142 has 14 capacitance input pins and 12 conversion channels. AD7143 has 8 capacitance input pins and 8 conversion channels. Any number of sensors can be combined, but the maximum number of available inputs and channels cannot be exceeded.

All connected sensors are measured in a round-robin fashion. All sensors can be measured in 36ms, so the status of each sensor is detected almost simultaneously - so fast that users can trigger or disable a sensor in 40ms.

5. Conclusion

Capacitive sensors, capacitive switches and capacitive sensing are a new emerging technology in human-machine interface applications, and are quickly becoming popular in a wide range of different products and components. Capacitive sensors enable innovative and easy-to-use interfaces for a variety of portable and consumer products. They are easy to design, use standard PCB production techniques, and are more reliable than mechanical switches, making them high-performance interfaces that can meet design needs.