Common challenges and simplified ways of designing

Common challenges in the design of capacitive sensing system and its simplification approach

in many consumer electronics and white goods applications, the emerging capacitive sensing button is replacing mechanical switches as a popular user interface. However, capacitive induction interface design will also bring challenges, and problems may occur in new product development, production and quality control. For example, the parasitic capacitance (CP) of capacitance sensing buttons on different boards may be different, and environmental changes (such as temperature and humidity) may also change the CP. The noise varies with different systems. Another common problem in user interface (UI) design is the portability of design. For example, if the user interface design of the front panel of the TV changes, the design will have to be readjusted to adapt to the changes of layout and wiring, sensor size, etc. the complicated debugging process increases the labor cost and time, and also delays the time of product launch. This paper will focus on the problems faced in the design of capacitive induction interface, and how developers can overcome these problems and design reliable and convenient products

capacitive sensing foundation

Figure 1 shows the appearance of the capacitive sensor circuit board

Figure 1: capacitance sensing circuit board cross section

to sense the presence of fingers, the capacitance sensing system first needs to know the sensor capacitance when there is no finger (see Figure 2a). It is called parasitic capacitance (CP). When the finger approaches (or contacts) the sensor (see Figure 2b), the capacitance value of the sensor will change, which produces another capacitance in parallel with CP, which is called finger capacitance (CF). When there is a finger, the total sensor capacitance (Cx) is shown in equation 1:

CX = CP + CF – equation 1

Figure 2 (a): sensor capacitance when there is no finger

Figure 2 (B): sensor capacitance when there is a finger

the capacitance change caused by CF is used to detect finger touch

capacitance measurement system

the electronic system measures the sensor capacitance by converting the capacitance into a digital value. Figure 3 shows the block diagram of capacitance induction preprocessing circuit. (Note: there are many ways to measure capacitance)

the system adopts a switched capacitor circuit at the front end of the system, which converts the sensor capacitance into resistance, as shown in equation 2 and Figure 4. The sigma delta modulator converts the current measured on the resistance into a digital count. When the finger is on the sensor, the capacitance increases and the equivalent resistance decreases. This results in an increase in the current through the resistor, thereby improving the digital count

this method requires an external component, CMOD. This is an integrated capacitor, which is charged by a constant current source (IDAC) and discharged by an equivalent resistance

req = 1/f Zheng Daqing, senior vice president of BASF Greater China business and market development, added that SCX – equation 2

here FS is the switching frequency of the switched capacitor module

the current measurement is completed by the comparator, and its output code stream (as shown in Figure 3) is input to the counter at a fixed time. This count value (numerical value) gives the magnitude of Cx size. The original count can be used as the measured value. The length of time, the counter counting time determines the response time

counter is called the scanning time. The original count is compared with the reference value to determine whether the sensor is on or off; This reference value is called the baseline. When a finger touches, the original count increases. In order to ensure that the sensor is really on, the increase of the original count must be greater than a threshold, which is called the finger threshold. If the original count change is lower than this value, it is considered as noise, which is called noise threshold (see Figure 5)

Figure 5: diagram of various parameters of capacitive sensor

now that we have understood the basic principle of capacitive induction, let's discuss why and what challenges the realization of capacitive induction will face

comparison between capacitive sensing and mechanical button user interface

capacitive sensing brings a beautiful and easy-to-use touch sensing function to the user interface. Capacitive sensors have replaced billions of mechanical buttons. Capacitive induction not only makes the front panel look smooth and beautiful, but also eliminates the problem of fragile mechanical buttons. Capacitive sensing has been widely adopted in TV/display applications because it reduces processing costs and increases aesthetics

Figure 6: mechanical buttons and capacitive sensing buttons on the front panel of the TV

the following table lists some phase problems of mechanical buttons and the advantages of capacitive sensing buttons over mechanical switches

design process - capacitive sensing user interface

Figure 7 shows a typical design process for capacitive sensing implementation

Figure 7: capacitive induction interface design process

firmware (f/w) development, debugging, production fine-tuning are the key steps in the capacitive induction user interface design cycle

1. F/W development:

in a broader sense, the firmware realizes specific application functions, such as the number of buttons, additional functions (such as PWM, a, binding the extensometer to the sample with a spring or rubber band, DC, DAC, etc.). From the perspective of capacitance sensing, the work of the firmware is to scan the sensor (that is, measure the sensor capacitance) and other related functions, such as processing the sensor on/off status feedback. The system has only capacitive induction and configurable devices. Through serial communication protocol (such as I2C), registers can be configured for specific sensor functions without developing firmware. Flexible programmable devices can be used to realize capacitance sensing to meet the needs of different users and complete sensor scanning and processing

1. Debugging:

debugging is the process of determining the optimal value of capacitance induction parameters, which ensures robust and reliable performance under various environmental conditions and different mechanical structure interfaces. This requires a thorough understanding of the performance of capacitive induction systems under different conditions

key points to be considered during debugging

a. sensor signal-to-noise ratio (SNR)

b. sensor scanning time

c. finger threshold setting

a. sensor signal-to-noise ratio:

one of the main goals of debugging capacitive induction system is to reliably judge whether the sensor is touched. In the calculation of signal-to-noise ratio, signal refers to the change of sensor response when the finger is placed on the sensor

noise refers to the peak to peak change of the sensor response when the finger is not present. For reliable capacitive induction performance, the signal strength needs to be significantly greater than the noise; It is generally recommended that the signal should be at least 5 times the noise, and the minimum signal-to-noise ratio is recommended to be 5:1

figure 8: signal and noise

b. sensor scanning time

sensor scanning time is the time counted by the counter, as described in the capacitance measurement system in the previous chapter. Short scanning time of the sensor will lead to low signal-to-noise ratio. Longer scan times result in response time delays and higher power consumption. Therefore, according to the parasitic capacitance (CP) of the sensor, it is necessary to optimize the scanning time of the sensor, so as to obtain the appropriate signal-to-noise ratio, response time and power consumption

c. finger threshold setting

finger threshold setting is used to indicate finger touch. This finger threshold setting should be very careful to avoid false touch caused by noise and climate change. It is generally recommended that the finger threshold is set to 75% of the signal strength to achieve reliable detection, as shown in Figure 5

Figure 9: typical debugging process of capacitive induction design

it can be seen from Figure 9 that debugging is a time-consuming, complicated and repetitive process, and the operation must be repeated whenever PCB or cover changes during development

1。 Production fine tuning:

the capacitive induction performance depends on the physical characteristics and environmental conditions of the capacitive sensor. The parasitic capacitance of the sensor will change due to supplier changes, process changes, or environmental conditions (such as humidity or temperature). This requires fine-tuning through statistical analysis of samples during mass production, so as to improve the qualified rate. As we can see, there are many steps and problems to be solved before the design is released for mass production

when designing capacitance sensing system for a specific application (such as TV or display), the typical challenge we will encounter is the change of PCB supplier and the impact of noise on capacitance sensing performance, which often leads to the need for re debugging. Some methods can be used to deal with this challenge and reduce the workload of debugging:

1. self tuning test

2. layout consideration

1. automatic debugging

there is an innovative method in this field. By monitoring the system noise and changes in environmental conditions, the equipment dynamically debugs itself (automatic monitoring and setting parameters). This method can also enable the equipment to initialize all capacitance induction related parameters when powered on according to environmental conditions and system mechanical design

a. baseline

the common environmental changes that affect capacitance induction measurement are temperature and humidity. Temperature/humidity drift will cause changes in components/parameters of capacitance measurement circuit, and will also affect CP and CF, resulting in changes in the original count. A typical temperature related raw count change is shown in Figure 10

figure 10: original count changing with temperature

if a constant reference is used to detect the touch of the button, temperature/humidity drift may cause false button press or no button press is detected

baseline compensation is a part of the automatic debugging sequence, which automatically adjusts the reference value (baseline) and noise threshold to keep the low-frequency noise below the threshold to avoid false triggering

b. optimal parameter setting:

according to the physical performance and environmental noise of the sensor, the sensor parameters should be set to the optimal value when powered on, so that the capacitive induction system can work reliably

as mentioned above, in order to work reliably, the key is to keep the signal-to-noise ratio higher than 5:1. First, what the automatic debugging algorithm needs to do is to calculate and optimize the parameter values of specific equipment, and keep the signal-to-noise ratio higher than 5:1. Set the best scanning speed to ensure that power consumption and response time are not increased. Set the best noise threshold and finger threshold when powered on

let's use the most common scenarios to understand the benefits of automatic debugging

1 PCB process changes

a PCB manufacturer may have multiple production sites. Researchers say that the production process will be slightly different, which will change the parasitic capacitance of sensors on different PCB. If the pass rate is not high, it is necessary to re debug, because such changes are too great. For example, in Figure 11, a sensor on # 4 board does not respond to the presence of fingers, which is due to the change of PCB process. This sensor has a high CP

figure 11: performance of circuit boards in different processes

ii. Changes in PCB suppliers:

in order to prevent increased manufacturing costs and insufficient processing capacity, OEMs usually certify many PCB/FPC manufacturing sources. Each PCB manufacturer will use different PCB materials, so the sensor will have different parasitic capacitance, which will lead to a low qualification rate. For example, if the final debugging is done for the circuit board of supplier x, the same parameters may not be fully applicable to the circuit board of supplier y, resulting in a low qualification rate

in Figure 12, the sensors of board 3 and board 4 do not respond to the presence of fingers, because the PCB manufacturers are different, and the sensors have a high CP

the automatic debugging algorithm can automatically determine the parasitic capacitance of the sensor when powered on, which can solve the above two problems

iii. change of dielectric constant/thickness of the cover:

sensitivity is directly proportional to the dielectric constant of the cover and inversely proportional to its thickness. This means that if one is designed and debugged for 2mm glass (dielectric constant () 8.0), it should be changed to 2mm plastic (dielectric constant () 2.8)