Capacitive voltage divider touch sensor

A system for measuring capacitance has a measurement circuit with a first reference capacitor connected to a first node and to a second node. Each of the nodes is connected to a unit operable to apply a reference voltage or ground to one of the nodes. Each node has a first pad connected to the first node and a unit operable to measure voltage between the first node and second node.

TECHNICAL FIELD OF THE INVENTION

This invention relates generally to the fields of touch sensors, proximity sensors and capacitance measurement, which have applications in a wide variety of fields, including automobiles, consumer electronics, and medical technology.

BACKGROUND

Touch sensors and proximity sensors have a myriad of uses. For example, they can function as buttons, sliders, or switches for user interface. These sensors can be found, for example, on kiosks, handheld electronic devices, and home appliances. They can be operated by pressing with a finger, touching a screen with a stylus, or even by the mere presence of a body as a proximity sensor.

The sensors themselves act as a capacitor. As the sensor is pressed, or as a body approaches it, the capacitance qualities of the sensor are changed. By determining the value of the capacitance or comparing it to known or previous quantities, information such as whether a contact was made or for how long contact was made can be determined.

Capacitive sensors are popular alternatives to mechanical buttons and switches, particularly, in microcontroller or microprocessor applications. Consequently, engineers seek to implement them more cheaply with fewer components. Systems and methods for easily measuring and detecting changes in capacitance, while simplifying designs and reducing the requisite number of components are desirable.

SUMMARY

According to an embodiment, a system for measuring capacitance may comprise a measurement circuit with a first reference capacitor connected to a first node and to a second node; each said nodes connected to a unit operable to apply a reference voltage or ground to one of said nodes; a first pad connected to the first node; and a unit operable to measure voltage between the first node and second node.

According to a further embodiment, the system may further comprise a second pad connected to the second node. According to a further embodiment, the units operable for applying reference voltages and ground to nodes may be comprised of a first switch connected to the node and to the reference voltage and a second switch connected to the node and to ground. According to a further embodiment, the system may further comprise a plurality of third nodes, fourth nodes, third pads, second reference capacitors, and units operable to measure voltage, where each said second reference capacitor may be connected to a third node and a fourth node; each said third and fourth node may be connected to a unit operable to apply a reference voltage or ground to the node; each said third pad may be connected to the third node; each said fourth pad may be connected to the fourth node; and a unit operable to measure voltage between the third node and the fourth node. According to a further embodiment, the units operable for applying reference voltages and ground to nodes may be embedded on a chip. According to a further embodiment, the units operable for applying reference voltages and ground to nodes can be enabled through configuration of said chip. According to a further embodiment, the units operable for measuring voltage between the first node and second node may be embedded on a chip. According to a further embodiment, the units operable for measuring voltage between the first node and second node may be enabled through configuration of said chip. According to a further embodiment, the units operable for measuring voltage between the first node and second node may be embodied by an analog-to-digital circuit. According to a further embodiment, the system may comprise a proximity sensor.

According to an embodiment, a method of measuring capacitance of a pad may comprise the steps of applying a voltage to the pad and to a reference capacitor, grounding the reference capacitor to create a circuit comprising the reference capacitor couple in series with the pad, and measuring the resulting voltage across the reference capacitor, said resulting voltage corresponding to the capacitance value of said first pad.

According to a further embodiment, the action of applying voltage across said pad may be automated by a microcontroller. According to a further embodiment, the action of applying voltage across said reference capacitor may be automated by a microcontroller. According to a further embodiment, the action of grounding said reference capacitor to create a circuit comprising the reference capacitor coupled in series with the pad may be automated by a microcontroller. According to a further embodiment, the measurement of capacitance may be accomplished by comparing the voltage of said reference capacitor against a baseline voltage. According to a further embodiment, the method may be applied to the measurement of capacitance of a plurality of pads.

According to an embodiment, a method of calculating the capacitance of a plurality of pads connected across a reference capacitor may comprise applying a voltage across a first pad, applying said voltage across a reference capacitor, grounding said reference capacitor to create a circuit comprising said reference capacitor coupled in series with said first pad, measuring the first resulting voltage across said reference capacitor, said resulting voltage corresponding to the capacitance value of said first pad, applying a voltage across a second pad, applying said voltage across said reference capacitor, grounding said reference capacitor to create a circuit comprising said reference capacitor coupled in series with said second pad, and capturing the second resulting voltage across said reference capacitor, said resulting voltage corresponding to the capacitance value of said second pad.

According to an embodiment, a method of determining the relative capacitance of two or more pads may comprise applying a voltage across a first pad, applying said voltage across a reference capacitor, grounding said reference capacitor to create a circuit comprising said reference capacitor coupled in series with said first pad, measuring the first resulting voltage across said reference capacitor, said resulting voltage corresponding to the capacitance value of said first pad, and for each second pad, applying a voltage across said second pad, applying said voltage across said reference capacitor, grounding said reference capacitor to create a circuit comprising said reference capacitor coupled in series with said second pad, capturing the second resulting voltage across said reference capacitor, said resulting voltage corresponding to the capacitance value of said second pad, and comparing the resulting voltages or capacitance calculations from the two or more pads.

DETAILED DESCRIPTION

FIG. 1shows an embodiment using a microcontroller and a single pad. A programmable microcontroller1may have a plurality of input and output pins4,5. According to various embodiments, the microcontroller may be an ASIC, a programmable processor, a reprogrammable processor, or any other chip which circuitry can be embedded. The microcontroller may be a single unit or comprised of several separate microcontrollers. A pad2which here is implemented as a capacitive sensor is attached to a first pin4. Pad2may be a touch sensor or proximity sensor of any variety. An impedance circuit with known capacitance3is connected between a first pin4and a second pin5. According to one embodiment, the pins,4,5are pins on a microcontroller. Other embodiments of the pins4,5may take the form of a wire, lead, probe, line, or electrode.

InFIG. 1, the microcontroller1is operable to apply a reference voltage9to any of the pins4,5. The microcontroller1is further operable to apply ground to any of the pins4,5. The microcontroller1is further capable of measuring the voltage connected to any of the pins4,5or the voltage between pins4,5.

FIG. 2shows an embodiment whereby the impedance circuit3is implemented as a reference capacitor6. Other embodiments of the impedance circuit3may make use of other passive elements needed for a specific application. The pad2appears to the microcontroller as a unitary element. The pad2is operated by a finger, stylus, or presence of a body, which alters characteristics of the circuit as will be explained in more detail below.

FIG. 3shows the equivalent circuit of an embodiment where the pad2has been operated, such as being touched by a user. After being operated, the net capacitance value of the combination will change. The capacitance of the body operating the pad is represented by CBODY8. The object has a coupling path to the circuit ground through an equivalent capacitance CCOUPLING7. Capacitance CCOUPLING7is in series with the body capacitance CBODY8. The combination of CBODY8and its coupling capacitance to ground CCOUPLING7represent the new capacitance of the pad2, when the pad2is operated.

FIG. 4shows an embodiment with a single pad configuration using switches to apply reference voltages9and ground to pins4,5, and units operable to measure voltage8. According to an embodiment, the reference voltage9is a pull-up direct current voltage source. The reference voltage9may have a value, of, for example, five volts or 3.3 volts. One embodiment of the reference voltage uses a voltage provided externally to the unit. The reference voltage may be embodied by any voltage useful to operate the pad in sensing touch. One embodiment utilizes a first switch10to connect a reference voltage9to a pin4, and uses a second switch11to ground the pin4. Likewise, another first switch12and another second switch13may be used to connect a reference voltage or ground to a different pin5. The switches10,11,12,13may be operable through a circuit or may be operable through firmware.

A unit14operable to measure or compare a voltage is connected to the first pin4and the second pin5. Other embodiments of unit14to measure a voltage across pins4,5may have separate units to measure or compare the voltage of each individual pin. Other embodiments of the unit14operable to measure or compare a voltage may be connected to ground or to a baseline voltage. One embodiment of unit14operable to measure or compare a voltage uses the internal configurable circuitry of a microntroller. The particular mechanism to measure or compare a voltage may be enabled or disabled dynamically according to the configuration of the chip. Embodiments of unit14operable to measure or compare a voltage may be an analog-to-digital converter (ADC), an integrator circuit to make this measurement, or a comparator to make a comparison between the resulting voltage and a baseline voltage without determining the actual value of the resulting voltage. Another embodiment compares a voltage reading to prior resulting voltages to determine whether the capacitance of the pad2has changed.

FIG. 5shows an embodiment of a single pad configuration using units16,16′ capable of applying a reference voltage9or ground to a pin4,5. These units may be embodied by the switch configuration described inFIG. 4. Other embodiments may use a different switch mechanism to apply the reference voltage9or ground the pin, such as a selector or multiplexer. The units16,16′ may be configured and operable through firmware of a microcontroller. Other embodiments may combine units16and16′ to form a unit capable of applying reference voltage9or ground to any number of pins.

Units16,16′ to apply reference voltages and apply ground to pins and units14to measure the voltage across pins may be combined in the embodiments of a microntroller1, or as separate circuits or units working in conjunction with embodiments of a microcontroller.

Another embodiment of a unit capable of applying a reference voltage9to a ground to a pin4,5uses applying an analog voltage output to the pin4,5, instead varying the voltage applied to the pin from the reference voltage value to ground.

FIG. 6demonstrates the method for operating the invention in a single pad configuration. First in step100, a reference voltage9is applied to both the first pin4and the second pin5. The net resulting voltage across the impedance unit3is thus 0 volts, and the resulting voltage on the pad2is the applied reference voltage9.

In the second step101, the reference voltage is removed from both the first pin and the second pin5is grounded. This action causes a circuit to be created with the pad and the impedance unit3in series, as shown inFIG. 3. Charge accumulated on the pad2will be discharged. The impedance unit3previously had no charge, but will acquire charge from the pad2. The third step102is to wait for a steady-state to be reached and the charge on the pad2and the impedance unit3are relatively stable.

The fourth step103is to measure or compare the resulting voltage across the impedance unit3and pad2. In one embodiment, the first pin4can be configured programmatically as an analog input. The analog input can also act in single-ended mode, whereby the voltage is the potential difference between the first pin4and ground.

In one embodiment, once the voltage has been determined, the capacitance of the pad2may be calculated104. In one embodiment, the voltage at the first pin can be compared to another voltage without explicit calculation of capacitance. After the third step102, the current in the equivalent circuit is equal to the capacitance of each component times the change in voltage over time (I=C*dV/dt). The current is the same through both the impedance unit3and the pad2, so the capacitance of the impedance unit3times the change in voltage of the impendence unit3over time is equal to the capacitance of the pad2times the change in voltage of pad2over time:
I=CREF*dVCREF/dt=CPad*dVPad/dt(equation 1)

With the result that
CREF*dVCREF=CPad*dVPad.(equation 2)

Analyzing the equivalent circuit with Kirchoff's Voltage Rule, the sum of the voltages of the impedance unit3and the pad2must be zero. The voltage across the pad2was originally a quantity equal to the reference voltage, and then reduced by a quantity dVPad. The voltage across the impedance unit3was zero, but then acquired a quantity dVCREF. Thus,
dVCREF=VREF−dVPad(equation 3)

Combining equations 2 and 3, the capacitance of the pad2can be expressed by:
CPad=(CREF*dVCREF)/(VREF−dVCREF)   (equation 4)

where CREFis the known capacitance of the impedance unit3, VREFis the reference voltage applied to the pin4, and dVCREFis the voltage measured across the impedance unit3, or alternatively, at the first pin4.

In one embodiment, the calculation of capacitance104is done by comparison of resulting voltage to a baseline voltage, another resulting voltage, or a set value. By determining the capacitance, it can be determined whether a change in capacitance has occurred. This capacitance, or change in capacitance, signals that the pad has been touched by an object, or that the pad acting as a proximity sensor has been approached by an object.

The invention may take further steps depending upon the value of the resulting voltage or capacitance. It may act to turn on a button, activate a control, move a slider, trigger an alarm, communicate data or alerts to another system, or any other activity enabled by a touch and proximity sensor.

FIG. 7illustrates another embodiment of determining resulting voltage and calculating capacitance in differential mode. First, steps100,101,102, and103are conducted, producing a first resulting voltage at the first pin4. Then in step105a reference voltage9is applied across the second pin5and the first pin4. The reference voltage9is removed and the first pin4is grounded106. After a steady-state is reached107, the next step108is to measure the voltage of the second pin5. Alternatively, the voltage at the second pin5may be directly compared to the first resulting voltage. The resulting capacitance of the first pin may be found109according to equation4or by comparison to the voltage at the second pin5.

FIG. 8illustrates another embodiment wherein a second pad17is connected to second pin5, in addition to first pad2connected to first pin4. Second pad15may be used as a touch sensor or proximity sensor with the existing system without any additional components. The various embodiments discussed above are fully applicable to the present embodiment.

FIG. 10illustrates that to determine the capacitance of the second pad17or its equivalent resulting voltage, the steps ofFIG. 6orFIG. 7are repeated, but with the roles of the first pin4and second pin5reversed.

First, steps100,101,102, and103are executed to obtain the resulting voltage from the first pad2. Then in step110a reference voltage is applied across the second pin5and the first pin4. The impedance unit3has no potential, and the second pad17has the potential of the reference voltage. The reference voltage is removed and first pin4is grounded111. The result is that impedance unit3and second pad17are in series together in a closed circuit, the second pad17discharges and the impedance unit3collects the charge. After a steady-state is reached112, the voltage of the second pad17may be measured or compared113and the resulting capacitances of the first pad2and second pad15may be calculated114using equation4. In one embodiment, calculation of the capacitances of the first pad2and the second pad17may happen at different times. In one embodiment, steps100,101,102, and103may be performed after steps110,111,112and113are performed. In one embodiment, the set of steps100,101,102and103or the set of steps110,111,112and113may be skipped to measure the capacitance of a single pad.

FIG. 11illustrates an embodiment wherein the capacitance of a dual-pad configuration is determined in differential mode. With resulting voltages measurements from two pads, the voltages can be compared to each other to determine relative capacitance115. In one embodiment, this is used to implement a toggle switch.

FIG. 9illustrates an embodiment where a series of single or dual-pad configurations are combined to produce a sensor array. The embodiment may contain multiples of a set comprising an impedance unit23connected between a first pin24and a second pin25, a first pad26connected to the first pin24and an optional second pad27connected to the second pin25. Each pad may be operated in the methods previously described. In one embodiment, each set may be connected to a microcontroller1. In one embodiment, each set is connected via its first pin24and second pin25to a unit operable to apply voltages and ground16, or16′ and to a unit operable to measure or compare voltage14.

In one embodiment, units operable to apply voltages and ground16, or16′, and/or units operable to measure or compare voltage14are configurable in a microcontroller1. The internal circuitry of the microcontroller can enable instances of these units at different times during the operation of the invention. In one embodiment, this is accomplished through switches.