Research And Realization Of Capacitive Touch Sensing Technology

The capacitive multi-touch sensing technique,due to its excellent user experiences in the human-machine interaction(HCI),has been widely used in the smart display terminals.On one hand,the capacitive multi-touch sensor system should be low power and low cost in the application of mobile communication and consumer electronics due to the limited battery capacity and the required market competitiveness.On the other hand,fast respond speed and good noise suppression are required in the application of automotive electronics,commercial office,industrial control terminal and so on.This dissertation utilizes the sparsity of the touch information,and proposed a new idea for designing the capacitive multi-touch sensing system based on information selection.The data sampling and processing scale are notably reduced due to the selective sampling of the touch information,and then the hardware and software design complexity is also reduced.Consequently,the system cost and power dissipation are decreased.In addition,the system respond speed is increased and the possibility of reliability optimization is provided.The main innovations and researches in this dissertation are summarized as follows.(1)Considering the issues existed in the traditional capacitive multi-touch sensing systems,such as large amount of sampling data,high system cost and power dissipation,and the limitations in the further performances optimization,the touch prediction and window sensing(TPWS)strategy is proposed which is characterized by multi-dimension observation and selective sampling,and the realization scheme of the TPWS strategy using the combination of point scanning and line scanning is also proposed.Firstly,the pre-sampling data with higher information hit rate and lower spatial resolution are obtained by combined information observation,and then the possible touch regions are roughly estimated with low cost by simple data processing.Secondly,based on the above rough sensing results,the estimated possible touch regions are selected and sampled,and then the sampled information of these local regions,which have higher spatial resolution and higher information hit rate,are acquired.Consequently,the pixel level coordination of the touch positions can be derived by processing the sampled data of these local regions.Since the signal sampling and data processing scale is notably decreased for the TPWS strategy,the sensing efficiency of system is obviously increased comparing with the existed techniques.According to the theoretical analysis,for a 15-inch touch panel with one touch,the sensing efficiency of TPWS system could be increased by 20 times as compared with the traditional multi-touch system.(2)Considering the large and uncertain input signal range required by the TPWS strategy during the measurements with point and line scanning,a new touch signal measurement technique based on the rebalance of Capacitance Bridge is proposed,and a digital front-end readout circuit structure based on the measurement technique is constructed.Traditional front-end readout circuits employ analog circuit including charge integrators for transforming the touch signal to electronic signal linearly,and thus it is very difficult to realize simultaneously a large input range and high precision measurement without distortion.This dissertation adopts a schema of the rebalance of Capacitance Bridge,and a voltage comparator is used for the signal measurement.The measurement precision depends on the minimal variation step of a complementary capacitance and the voltage discrimination of the comparator.The measurement range relies on the value of the complementary capacitance.A high precision and large input measurement system without any distortion can be realized by designing adequate variation step and value of the complementary capacitance and a high precision comparator.A prototype chip of the digital front-end readout circuit is designed and implemented.The quantization resolution is 10 bits,the discrimination of capacitance is 27fF,the maximum input capacitance could be 13.824pF,and the die size per sensing channel is only 0.087 mm~2.In one word,both the large measurement range and high measurement precision are realized with a small die size.A15-inch capacitive multi-touch screen system is realized with the prototype chips.The sensing frame rate of the prototype system is over 83Hz.The dynamic current is less than 6?A per sensing cell.Compared with the existed techniques,the TPWS system is with extremely low power dissipation and low system cost.(3)In order to increase the noise suppression ability and the response speed of the TPWS system,the differential point and line measurements method with fully-driving is proposed.In order to decrease the influences of the display noise and the power switching noise on measurement precision,the differential line measurement and differential point measurement methods with the combination of parallel positive and negative driving are developed,and the low noise signal sampling is realized.Moreover,fully-driving methods are adopted for reducing the RC delay during signal transmission on the sensor electrodes,and thus the signal sampling speed is increased.A low noise and high speed digital front-end readout chip based on the TPWS strategy is designed and implemented.The test results demonstrate that the signal to noise ratio is improved from 26 dB to 35 dB,and the sensing frame rate is increased from 83 Hz to 120 Hz by adopting the above techniques.So,both the signal to noise ratio and the response speed of the TPWS system are improved obviously.The multi-dimension observation and the selective information sampling method proposed in this dissertation can be also used in the signal sampling and processing for the other arrayed sensor with sparse information.And the research and design results in this dissertation have important engineering practical value for developing capacitive multi-touch sensor system.