CN111817697A

CN111817697A - Capacitive touch key environment compensation circuit and method

Info

Publication number: CN111817697A
Application number: CN201910290005.5A
Authority: CN
Inventors: 张善辉
Original assignee: Shenzhen Feiyi Technology Co ltd
Current assignee: Shenzhen Feiyi Technology Co ltd
Priority date: 2019-04-11
Filing date: 2019-04-11
Publication date: 2020-10-23

Abstract

The invention discloses a capacitive touch key environment compensation circuit and a capacitive touch key environment compensation method, wherein the capacitive touch key environment compensation circuit comprises a switch S1, a switch S2, a switch S3, a reference capacitor C1, a charging voltage source VT, a comparator/AD converter, a voltage arithmetic unit, a voltage following amplifier, a variable resistance unit and a logic control unit, wherein the capacitance on a touch channel consists of a touch capacitance Cx and a stray capacitance Cp; the compensation method comprises the following steps: before starting, the reference capacitor initially discharges, the detected capacitor is switched between a charging voltage source and the reference capacitor by using a high-frequency switch, charges charged by using a charge transfer method are transferred to the reference capacitor, corresponding negative voltages are obtained by carrying out signal operation on the voltage of the reference capacitor, the charging voltage source and reference voltages and amplification of driving capability is carried out, charges discharged from the reference capacitor are discharged by using a variable resistor, the discharged charges are guaranteed to be exactly equal to the charges charged by a stray capacitor in the period, only the charges charged by the touch capacitor are reserved on the reference capacitor, the reference capacitor stops when the reference capacitor voltage is judged to reach a target reference voltage by a comparator or an AD converter, and the size of the touch capacitor is judged by using the switching times or time of the high-frequency switch.

Description

Capacitive touch key environment compensation circuit and method

Technical Field

The present invention relates to a non-capacitive touch key circuit, and more particularly, to a capacitive touch key environment compensation circuit and method.

Background

The capacitive touch key is a common non-contact key, has the advantages of water resistance, long service life, attractiveness, simple structure and the like compared with the traditional mechanical key, and is widely applied to operation panels of various electronic products. The basic principle of the realization is that a touch sensing disc with a certain area is arranged in a product, and the touch sensing disc, a product shell, a circuit board and the like form an inherent capacitance Cx, and the size of the capacitance Cx is about 5P generally. When the finger of the user approaches, a certain capacitance (about 0.2P-1P) is formed with the finger, and is superposed on the original Cx to increase the Cx. The capacitance type key detection circuit is used for detecting the small capacitance variation, thereby realizing the application of the touch key.

However, in practical applications, besides the Cx, the trace capacitance of the key circuit on the PCB is large. In most products, the number of keys is more than one, but generally one integrated circuit chip is used for processing, and for cost consideration and mutual interference prevention, the chips all adopt multi-way switch switching to share one detection unit at different times. However, most products have the keys arranged far apart, so that no matter how chips are placed in the PCB design, the routing distances of the keys are greatly different. The key traces on the PCB may form non-negligible coupling capacitances Cp with adjacent traces, the length of the traces and the length of the traces differing by several times. For example: the coupling capacitance formed between the wiring length of more than 15cm and the adjacent ground wire with the spacing of 0.6mm is up to more than 10p, and the wiring length of the key of the adjacent chip can not be 1 p. In addition to the individual key trace capacitances on the PCB, each key channel in the chip also has a parasitic capacitance of 5-10p, but the difference between the channels is not significant, and we also include it in the Cp capacitance.

The existing capacitive touch key detection technologies mainly include two types:

the first is a touch key detection circuit manufactured by adopting the working principle of an RC oscillator, the change of a capacitor is detected by using the oscillator principle, and the internal circuit structure is simple and low in cost. Since Cp and Cx are both very small, the R resistance value of the corresponding RC needs to be relatively large, and the input impedance of the schmitt trigger approaches infinity, so that the external interference applied to the touch key electrode through radio frequency induction easily triggers the schmitt trigger by mistake or delays to trigger the schmitt trigger, so that the counting result is interfered. Although the prior art adopts many complicated circuit variants and software skills, the problem of radio frequency interference cannot be fundamentally overcome. Thus, the method is generally only used on inexpensive products that do not have high reliability requirements and is not discussed herein.

Another conventional method for detecting a touch key by using the "charge transfer" principle is shown in fig. 2. The circuit mainly comprises 2 high-frequency switching switches SA and SB, 1 reference capacitor C3, 1 comparator, a counter and other logic units. The reference capacitance C3 is typically large, reaching the order of 10nF in number. The logic circuit drives the switches SA and SB to be conducted in turn through the high-frequency signal and starts counting. So that the power supply charges Cp and Cx through SA and then (Cx + Cp) discharges the reference capacitance C3 through SB. The count is stopped when the voltage at C3 reaches the set fixed reference value of the comparator. The value obtained by the counter can directly reflect the size of (Cx + Cp), thereby achieving the purpose of detecting the capacitance. In this manner, since the capacitance of C3 is large, external interference will not trigger the comparator by mistake.

Wherein, we get the counting value CNT (or the corresponding time value) to measure the capacitance. From the circuit model, it is known that CNT is inversely proportional to the total capacitance measured (Cx + Cp) and directly proportional to the reference capacitance C3. Assuming that the difference between the 2 key channels Cp is large, resulting in a difference of one time between the total capacitances, it can be deduced that:

1. the reading CNT values of the two-time difference is one time, and the larger the Cp is, the smaller the CNT value is;

2. rate of change of capacitance due to fixed key actuation: Δ C/(Cx + Cp) will differ by a factor of two, the larger Cp the smaller its rate of change;

3. therefore, the CNT change value Δ CNT (Δ C/(Cx + Cp)) caused by the final key pressing operation reaches a difference of 2 squared and 4 times.

In order to ensure the stable reliability of the system, it is generally required that Δ CNT generated at the time of key pressing can be more than 2 times of the inherent noise of CNT, so in this example, to satisfy the channel performance with Cp large, the reference capacitance C3 is increased by 4 times to increase Δ CNT to satisfy the reliability. At this time, the CNT value of the channel with a small Cp is increased by 4 times, which means that the single detection time of the channel is increased by 4 times. Under some application conditions, the difference between the total capacitance of the long-wiring key and the short-wiring key is more than 5 times, and after the reference capacitance is adjusted to meet Cp maximum key, the shortest key needs to consume more than 25 times of Cp maximum key, so that the whole response time is greatly influenced in the processing of multi-key application, and the problem that the whole response time cannot be overcome is solved.

In summary, in the multi-channel charge transfer type key detection circuit widely used at present, the defect that the parameter difference of each channel is large and the matching is not good generally exists, so that a method is urgently needed, the influence of environment parasitic capacitance with different channel sizes on the detection range, precision and speed of the touch key in practical application can be overcome, and more convenient application can be realized.

Disclosure of Invention

Aiming at the defects of the charge transfer type capacitive key circuit in the application of a multi-key system, the invention provides a capacitive touch key environment compensation circuit, which adopts the compensation circuit to compensate the routing parasitic capacitance of each channel respectively, so that each channel can obtain the approximate reading value and the key variation under the fixed reference capacitance value, namely each channel has the approximate resolution and measurement time.

In order to achieve the purpose, the invention adopts the following technical scheme:

a capacitive touch key environment compensation circuit and method, wherein, the circuit includes detecting element, voltage arithmetic element, voltage follower amplifier, variable resistance unit and logic control unit, the detecting element includes switch S1, switch S2, switch S3, reference capacitance C1, charging voltage source VT and comparator/AD converter, the voltage follower amplifier is the voltage follower amplifier, the variable resistance unit is the variable resistance, the electric capacity on the touch channel is made up of touching electric capacity Cx and stray capacitance Cp;

the compensation method comprises the following steps:

s1, before starting, the reference capacitor C1 is initially discharged,

s2, the detected capacitance (Cx + Cp) is switched between the charging voltage source VT and the reference capacitance C1 by the high frequency switch, the charge charged by the charge transfer method is transferred to the reference capacitance C1,

s3, obtaining corresponding negative voltage by signal operation of the voltage of the reference capacitor C1, the charging voltage source VT and the reference voltage Vref, and amplifying the driving capability,

s4, the reference capacitor C1 is discharged by the variable resistor and the discharged charge is guaranteed to be exactly equal to the charge charged by the stray capacitor Cp in the period, so that only the charge charged by the touch capacitor Cx is reserved on the reference capacitor C1,

s5, when the comparator/AD converter determines that the voltage of the reference capacitor C1 reaches the target reference voltage Vref,

s6, the size of the touch capacitance Cx is determined by the number of times of switching the high frequency switch or the time.

The test results are only related to the touch capacitance Cx and the ambient stray capacitance Cp is compensated.

Preferably, the voltage operation unit is used to operate the voltage of the reference capacitor C1, the charging voltage source VT and the reference voltage Vref, so as to obtain a voltage amount proportional to the amount of charge currently charged in the stray capacitor Cp, which is used as the basis for the compensation amount.

Preferably, the output of the voltage operation unit is converted into a corresponding negative voltage, and is driven and amplified, so that the output has a certain driving capability for charge compensation.

Preferably, the amplified negative voltage, which is proportional to the amount of charge in stray capacitance Cp, is driven to access reference capacitance C1 for charge draining through a variable resistor controlled by software logic to cancel the effect of the charge in stray capacitance Cp on the reference capacitance voltage.

Preferably, when the system is initialized, the control software matches a variable resistance value suitable for each touch channel and used for compensating the stray capacitance Cp, so that the influence of the stray capacitance Cp specific to the channel is exactly offset when the channel is compensated, thereby eliminating the difference between the channels, enabling the parameters of each channel to be matched with the size of the reference capacitance C1, obtaining a data value with small difference, and obtaining a data value with small difference caused by the same key action, namely realizing the consistency of the measurement accuracy. Meanwhile, because the influence of the stray capacitance Cp is eliminated, under the same reference capacitance value, the obtained charging times are increased, which means that the obtained data is increased, and the variation quantity is increased when the key is pressed.

Compared with the prior art, the invention has the beneficial effects that:

the circuit and the method for compensating the touch key environment overcome the problem that the touch key has different key parameters in a multi-key system, improve the sensitivity and the precision of the key, simplify the complexity of judging the key by a main logic system and save microprocessor resources. The circuit has simple structure, strong reliability and easy integration, and can be widely integrated in various touch key detection chips.

Drawings

FIG. 1 is a schematic diagram of a capacitive sensing key circuit using the "RC oscillator" principle;

FIG. 2 is a schematic diagram of a capacitive sensing key circuit using the "capacitive charge transfer" principle;

FIG. 3 is a schematic diagram illustrating a capacitive sensing button according to an embodiment of the present invention;

FIG. 4 is a flowchart illustrating initialization of compensation resistance parameters according to an embodiment of the present invention.

Detailed Description

The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments.

Referring to fig. 3-4, a capacitive touch key environment compensation circuit and method, the circuit includes a detection unit, a voltage operation unit, a voltage follower amplifier, a variable resistance unit and a logic control unit, the detection unit includes a switch S1, a switch S2, a switch S3, a reference capacitor C1, a charging voltage source VT and a comparator/AD converter, the voltage follower amplifier is a voltage follower amplifier, the variable resistance unit is a variable resistance, and a capacitance on a touch channel is composed of a touch capacitance Cx and a stray capacitance Cp; the voltage operation unit is used for operating a charging voltage source VT, a voltage VC on a reference capacitor C1 and a reference point voltage Vref to obtain a certain voltage value, and the voltage difference between the certain voltage value and the VC is in direct proportion to the charge amount charged into the stray capacitor Cp at the current time; the voltage follower amplifier is used for amplifying the power of the calculated voltage value, so that the voltage follower amplifier has certain driving current capacity and can perform compensation discharge on the reference capacitor C1; the variable resistor unit is internally provided with a plurality of resistors with different resistance values, and the resistors with different resistance values are connected to the reference capacitor C1 for charge discharge through software or logic circuit control, so that different compensation quantities can be obtained, namely, the environmental capacitors Cp with different sizes can be compensated.

The compensation method comprises the following steps:

s1, before starting, the reference capacitor C1 is initially discharged,

The voltage operation unit is used for operating the voltage of the reference capacitor C1, the charging voltage source VT and the reference voltage Vref to obtain a voltage amount which is proportional to the charge amount currently charged in the stray capacitor Cp as a basis for compensation.

And converting the output of the voltage operation unit into corresponding negative voltage, and performing driving amplification, so that the voltage operation unit has certain driving capability to perform charge compensation.

The amplified negative voltage, which is proportional to the amount of charge in stray capacitance Cp, is driven through a variable resistor controlled by software logic to access reference capacitance C1 for charge draining to cancel the effect of the charge in stray capacitance Cp on the reference capacitance voltage.

When the system is initialized, the control software matches the variable resistance value which is suitable for each touch channel and used for compensating the stray capacitance Cp, so that the influence of the specific stray capacitance Cp in the channel is just offset when the channel is compensated, the difference among the channels is eliminated, the parameters of each channel can be matched with the size of the reference capacitance C1, the obtained data values have small difference, the data value variation caused by the same key action is also small, and the consistency of the measurement precision is realized. Meanwhile, because the influence of the stray capacitance Cp is eliminated, under the same reference capacitance value, the obtained charging times are increased, which means that the obtained data is increased, and the variation quantity is increased when the key is pressed.

The initialization process specifically comprises the following steps: a target CNT value is set first, so that the measurement speed and the key variation can meet the requirements under the CNT value. And switching the multi-way switch to a channel 1, setting the variable resistor to be the maximum resistance value, measuring, and if the obtained data is larger than the target CNT value, reducing the value of the reference capacitor C1. If the obtained data is less than the target CNT value, the variable resistor is adjusted to a first gear to reduce the resistance value, the measurement is carried out again according to the previous judgment method, and the value of the set resistance gear is recorded until the obtained data exceeds the target CNT value. The same approach matches other channels and stores data. It should be noted that the finer the shift of the resistor, the more precise the CNT value is matched. Generally, 16 grades can meet the requirement.

As shown in FIG. 3, adjusting the series resistance Rn on the touch channel to an appropriate value ensures that the touch capacitance (Cp + Cx) is fully charged by the voltage source VT each time Rn (Cp + Cx) T/2, where T is the switching period and T/2 is the charging period. In this case, the capacitance formula C is Q/U

The total charge on the touch capacitor after the charging cycle is derived as Q1 ═ C ═ U ═ (Cp + Cx) × VT

After the discharge period is completed, the total charge on the touch capacitor is Q2 ═ C ═ U ═ (Cp + Cx) × Vc

(where Vc is the voltage on the reference capacitor at the end of the discharge cycle)

The charge charged to the reference capacitor per cycle is:

Q＝Q1-Q2＝(Cp+Cx)*(VT-Vc)＝Cp*(VT-Vc)+Cx*(VT-Vc)

it can be seen that it contains 2 parts, respectively charged by Cp and Cx. The compensation circuit functions to drain the part Cp (VT-Vc) charged by Cp from the reference capacitor C1.

The voltage operation unit samples VT and Vc and then operates the VT and Vc to generate corresponding negative voltage, namely- (VT-Vc), amplifies the driving capability, and then is connected to C1 through the variable resistor to discharge.

According to the defined formula of the current: the amount of charge bled off to C1 during this period can be found to be:

Q＝I*t＝T*(VT-Vc)/R＝(T/R)*(VT-Vc)

if a proper value of R is selected, let T/R be Cp, the amount of charge discharged is Cp (VT-Vc), which is exactly equal to the charge charged by Cp.

In summary, after compensation, the charge amount charged into the reference capacitor is Cx (VT-Vc) each time, which is independent of Cp, i.e. the finally obtained reading charge and discharge times CNT (or time) is independent of Cp.

Fig. 4 is a flowchart illustrating an initialization process of the compensation resistance parameter according to an embodiment of the present invention. And when the software logic control system is initialized, the discharge R value of each key channel is matched to obtain the R value corresponding to the required compensation amount, and then the data is stored. And when the actual measurement is carried out on each key channel in turn, calling out the corresponding R value, and starting the measurement after setting.

In the main detection program, before channel scanning, the gear value of the corresponding compensation resistor is filled, then scanning is started, the CNT value obtained finally does not differ too much, and the variation of the CNT value is not too different when a key is pressed, so that software processing is convenient, the time of software processing is saved, a particularly high CPU speed is not needed, and resources can be saved to a certain extent. Because the stray capacitance that the button was walked the length and is caused compensates one and no longer causes the design of puzzlement touch performance, user's product can have more design flexibility, and PCB walks and also can suitably reduce the requirement, can walk more densely, and even multiplex touch line and other functions, for example drive LED pilot lamp etc..

In summary, the environment compensation circuit and method for touch key detection of the present invention overcome the problem of inconsistent key parameters of touch keys in a multi-key system, and simultaneously improve the key sensitivity and accuracy, simplify the key judgment complexity of the main logic system, and save microprocessor resources. The circuit has simple structure, strong reliability and easy integration, and can be widely integrated in various touch key detection chips.

The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered to be within the technical scope of the present invention, and the technical solutions and the inventive concepts thereof according to the present invention should be equivalent or changed within the scope of the present invention.

Claims

1. A capacitive touch key environment compensation circuit and method are characterized in that: the circuit comprises a detection unit, a voltage operation unit, a voltage following amplifier, a variable resistance unit and a logic control unit, wherein the detection unit comprises a switch S1, a switch S2, a switch S3, a reference capacitor C1, a charging voltage source VT and a comparator or an AD converter, the voltage following amplifier is a voltage following amplifier, the variable resistance unit is a variable resistor, and a capacitor on a touch channel consists of a touch capacitor Cx and a stray capacitor Cp;

the compensation method comprises the following steps:

s1, before starting, the reference capacitor C1 is initially discharged,

2. The capacitive touch key environment compensation circuit of claim 1, wherein: the voltage operation unit is used for operating the voltage of the reference capacitor C1, the charging voltage source VT and the reference voltage Vref to obtain a voltage amount which is proportional to the charge amount currently charged in the stray capacitor Cp as a basis for compensation.

3. The capacitive touch key environment compensation circuit of claim 1, wherein: and converting the output of the voltage operation unit into corresponding negative voltage, and performing driving amplification, so that the voltage operation unit has certain driving capability to perform charge compensation.

4. The capacitive touch key environment compensation circuit of claim 1 or 3, wherein: the amplified negative voltage, which is proportional to the amount of charge in stray capacitance Cp, is driven through a variable resistor controlled by software logic to access reference capacitance C1 for charge draining to cancel the effect of the charge in stray capacitance Cp on the reference capacitance voltage.

5. The capacitive touch key environment compensation circuit and method according to claim 1, wherein: when the system is initialized, the control software matches the variable resistance value which is suitable for each touch channel and used for compensating the stray capacitance Cp, so that the influence of the specific stray capacitance Cp in the channel is just offset when the channel is compensated, the difference among the channels is eliminated, the parameters of each channel can be matched with the size of the reference capacitance C1, the obtained data values have small difference, the data value variation caused by the same key action is also small, and the consistency of the measurement precision is realized. Meanwhile, because the influence of the stray capacitance Cp is eliminated, under the same reference capacitance value, the obtained charging times are increased, which means that the obtained data is increased, and the variation amount is increased when the key is pressed, namely the final sensitivity is increased.