Seminar Report

On

CAPACITIVE SENSORS

Submitted by

Student name: Tarun Nekkanti

Roll No: 142

Section: C

In partial fulfillment of the requirements for the award of the degree of

BACHELOR OF ENGINEERING

IN

ELECTRONICS AND COMMUNICATION ENGINEERING

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGINEERING

MANIPAL INSTITUTE OF TECHNOLOGY

(A Constituent College of Manipal University)

MANIPAL – 576104, KARNATAKA, INDIA

13 October 2010

Sensors

Definition

A sensor; is a device that measures a physical quantity and converts it into a signal which can be read by an observer or by an instrument. For example, a mercury-in-glass thermometer converts the measured temperature into expansion and contraction of a liquid which can be read on a calibrated glass tube. A thermocouple converts temperature to an output voltage which can be read by a voltmeter. For accuracy, most sensors are calibrated against known standards.

Description

A sensor is a device which receives and responds to a signal. A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. For instance, if the mercury in a thermometer moves 1 cm when the temperature changes by 1 °C, the sensitivity is 1 cm/°C (it is basically the slope Dy/Dx assuming a linear characteristic). Sensors that measure very small changes must have very high sensitivities. Sensors also have an impact on what they measure; for instance, a room temperature thermometer inserted into a hot cup of liquid cools the liquid while the liquid heats the thermometer. Sensors need to be designed to have a small effect on what is measured; making the sensor smaller often improves this and may introduce other advantages.

Characteristics of a good sensor

A good sensor obeys the following rules:

Is sensitive to the measured property

Is insensitive to any other property likely to be encountered in its application

Does not influence the measured property.

Linearity & Sensitivity

Ideal sensors are designed to be linear or linear to some simple mathematical function of the measurement, typically logarithmic. The output signal of such a sensor is linearly proportional to the value or simple function of the measured property.

The sensitivity is then defined as the ratio between output signal and measured property. For example, if a sensor measures temperature and has a voltage output, the sensitivity is a constant

with the unit [V/K]; this sensor is linear because the ratio is constant at all points of measurement.

Resolution

The resolution of a sensor is the smallest change it can detect in the quantity that it is measuring. Often in a digital display, the least significant digit will fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the precision with which the measurement is made. For example, a scanning tunneling probe (a fine tip near a surface collects an electron tunneling current) can resolve atoms and molecules.

Capacitive Displacement Sensors

An Overview

Capacitive sensors are noncontact devices capable of high-resolution measurement of the position and/or change of position of any conductive target. The nanometer resolution of high-performance sensors makes them indispensible in today's nanotechnology world. Capacitive sensing can also be used to measure the position or other properties of nonconductive targets.

A Capacitive Sensor Measurement System

Capacitive sensor dimensional measurement requires three basic components:

a probe that uses changes in capacitance to sense changes in distance to the target,

driver electronics to convert these changes in capacitance into voltage changes,

a device to indicate and/or record the resulting voltage change.

Each of these components is a critical part in providing reliable, accurate measurements. The probe geometry, sensing area size, and mechanical construction affect range, accuracy, and stability. A probe requires a driver to provide the changing electric field that is used to sense the capacitance. The performance of the driver electronics is a primary factor in determining the resolution of the system; they must be carefully designed for a high-preformance application. The voltage measuring device is the final link in the system. Oscilloscopes, voltmeters and data acquisition systems must be properly selected for the application.

What is Capacitance?

Capacitance describes how the space between two conductors affects an electric field between them. If two metal plates are placed with a gap between them and a voltage is applied to one of

the plates, an electric field will exist between the plates. This electric field is the result of the difference between electric charges that are stored on the surfaces of the plates. Capacitance refers to the “capacity” of the two plates to hold this charge. A large capacitance has the capacity to hold more charge than a small capacitance. The amount of existing charge determines how much current must be used to change the voltage on the plate. It’s like trying to change the water level by one inch in a barrel compared to a coffee cup. It takes a lot of water to move the level one inch in the barrel, but in a coffee cup it takes very little water. The difference is their capacity.

When using a capacitive sensor, the sensing surface of the probe is the electrified plate and what you’re measuring (the target) is the other plate (we’ll talk about measuring non-conductive targets later). The driver electronics continually change the voltage on the sensing surface. This is called the excitation voltage. The amount of current required to change the voltage is measured by the circuit and indicates the amount of capacitance between the probe and the target. Or, conversely, a fixed amount of current is pumped into and out of the probe and the resulting voltage change is measured.

How Capacitance Relates to Distance

Capacitance is determined by Area, Gap, and Dielectric (the material in the gap). Capacitance increases when Area or Dielectric increase, and capacitance decreases when the Gap increases.

The capacitance between two plates is determined by three things:

Size of the plates: capacitance increases as the plate size increases

Gap Size: capacitance decreases as the gap increases

Material between the plates (the dielectric):Dielectric material will cause the capacitance to increase or decrease depending on the material

Area and Dielectric are held constant for ordinary capacitive sensing so only the Gap can change the capacitance.

In ordinary capacitive sensing the size of the sensor, the size of the target, and the dielectric material (air) remain constant. The only variable is the gap size. Based on this assumption, driver electronics assume that all changes in capacitance are a result of a change in gap size. The electronics are calibrated to output specific voltage changes for corresponding changes in capacitance. These voltages are scaled to represent specific changes in gap size. The amount of voltage change for a given amount of gap change is called the sensitivity. A common sensitivity setting is 1.0V/100µm. That means that for every 100µm change in the gap, the output voltage changes exactly 1.0V. With this calibration, a +2V change in the output means that the target has moved 200µm closer to the probe.

Focusing the Electric Field

Probes use a guard to focus the electric field.

When a voltage is applied to a conductor, an electric field is emitted from every surface. For accurate gauging, the electric field from a capacitive sensor needs to be contained within the space between the probe’s sensing area and the target. If the electric field is allowed to spread to other items or other areas on the target, then a change in the position of the other item will be measured as a change in the position of the target. To prevent this from happening, a technique called guarding is used. To create a guarded probe, the back and sides of the sensing area are surrounded by another conductor that is kept at the same voltage as the sensing area itself. When the excitation voltage is applied to the sensing area, a separate circuit applies the exact same voltage to the guard. Because there is no difference in voltage between the sensing area and the guard, there is no electric field between them to cause current flow. Any conductors beside or behind the probe form an electric field with the guard instead of the sensing area. Only the unguarded front of the sensing area is allowed to form an electric field to the target.

Effects of Target Size

The target size is a primary consideration when selecting a probe for a specific application. When the sensor’s electric field is focused by guarding, it creates a field that is a projection of the sensor size and shape. The minimum target diameter for standard calibration is 30% of the diameter of the sensing area. The further the probe is from the target, the larger the minimum target size

Range of Measurement

The range in which a capacitive sensor is useful is a function of the area of the sensing surface. The greater the area, the larger the range. The driver electronics are designed for a certain amount of capacitance at the sensor. Therefore, a smaller sensor must be considerably closer to the target to achieve the desired amount of capacitance. The electronics are adjustable during calibration, but there is a limit to the range of adjustment.In general, the maximum gap at which a probe is useful is approximately 40% of the sensing surface diameter. Standard calibrations usually keep the gap considerably less than that.

Multiple Channel Sensing

Frequently, a target is measured simultaneously by multiple probes. Because the system measures a changing electric field, the excitation voltage for each probe must be synchronized or the probes would interfere with each other. If they were not synchronized, one probe would be trying to increase the electric field while another was trying to decrease it thereby giving a false reading.

Driver electronics can be configured as masters or slaves. The master sets the synchronization for the slaves in multiple channel systems.

Effects of Target Material

The electric field from the probe sensing area is seeking a conductive surface. For this reason, capacitive sensors are not affected by the target material provided that it is a conductor. Because the electric field from the sensor stops at the surface of the conductor, target thickness does not affect the measurement.

Surface finish can affect the measurement. Capacitive sensors will measure the average position of the target surface within the spot size of the sensor.

Measuring Non-Conductors

Nonconductors can be measured by passing the electric field through them to a stationary conductive target behind.

Capacitive sensors are most often used to measure the change in position of a conductive target. But capacitive sensors can be very effective in measuring presence, density, thickness, and location of non-conductors as well. Non-conductive materials like plastic have a different dielectric constant than air. The dielectric constant determines how a non-conductive material affects capacitance between two conductors. By inserting a non-conductive material in the gap between the probe and a stationary reference target, the capacitance will change in relationship to the thickness, density, or location of the material.

Fringing can be used to measure nonconductive targets without a conductive background target.

Sometimes it’s not feasible to have a reference target in front of the probe. If the material has a high dielectric constant and a large sensor is used, measurements can still be made by a technique called fringing. If there is no conductive surface directly in front of the probe, the sensor’s electric field will wrap back to the shell of the probe itself. This is called a fringe field. If a non-conductive material is brought in proximity to the probe, its dielectric will change the fringe field and this can be used to measure the non-conductive material.

Strategies for maximizing effectiveness and minimizing error

Maximizing Accuracy: Target Size

Small targets make measurement accuracy sensitive to small probe position errors.

Unless otherwise specified, factory calibrations are done with a flat conductive target that is considerably larger than the sensor area. A system calibrated in this way will give accurate results when measuring a flat target more than 30% larger than the sensing area. If the target area is too small, the electric field will begin to wrap around the sides of the target. In this case, the electric field extends farther than it did in calibration and will measure the target as farther away.

This means that the probe must be closer to the target for the same zero point. Because this distance differs from the original calibration, error will be introduced. Error is also created because the probe is no longer measuring a flat surface.

An additional problem of an undersized target is that the system becomes sensitive to X and Y location of the probe relative to the target. Without changing the gap, the output will change significantly if the probe is moved left or right because less of the electric field is going to the center of the target and more is going around to the sides.

Maximizing Accuracy: Target Shape

Curved targets change the shape of the electric field, affecting accuracy.

Target shape is also a consideration. Since the capacitive sensors are calibrated to a flat target, measuring a target with a curved surface will cause errors. Because the sensor will measure the average distance to the target, the gap at zero volts will be different than when the system was calibrated. Errors will also be introduced because of the different behavior of the electric field with the curved surface. In cases where a non-flat target must be measured, the system can be factory calibrated to the final target shape. Alternatively, when flat calibrations are used with curved surfaces, multipliers can be provided to correct the measurement value.

Maximizing Accuracy: Surface Finish

Irregular surface finish can cause different measurements as the target moves parallel to the probe face.

When the target surface is not perfectly smooth, the capacitive sensor will average over the area covered by the spot size of the sensor. The measurement value can change as the sensor is moved across the surface due to a change in the average location of the surface. The magnitude of this error depends on the nature and symmetry of the surface irregularities.

Maximizing Accuracy: Parallelism

During calibration, the surface of the sensor is parallel to the target surface. If the probe or target is tilted any significant amount, the shape of the spot where the field hits the target elongates and changes the interaction of the field between the probe and target. Because of the different behavior of the electric field, measurement errors will be introduced. Parallelism must be considered when designing a fixture for the measurement.

Parameters used to determine the quality of the sensor

Sensitivity Error

Sensitivity Error - The slope of the actual measurements deviates from the ideal slope.

A sensor’s sensitivity is set during calibration. When sensitivity deviates from the ideal value this is called sensitivity error, gain error, or scaling error. Since sensitivity is the slope of a line, sensitivity error is usually presented as a percentage of slope; comparing the ideal slope with the actual slope.

Offset Error

Offset Error - A constant value is added to all measurements.

Offset error occurs when a constant value is added to the output voltage of the system. Capacitive sensor systems are usually “zeroed” during setup, eliminating any offset deviations from the original calibration. However, should the offset error change after the system is zeroed, error will be introduced into the measurement. Temperature change is the primary factor in offset error.

Linearity Error

Linearity Error - Measurement data is not on a straight line.

Sensitivity can vary slightly between any two points of data. This variation is called linearity error. The linearity specification is the measurement of how far the output varies from a straight line.

To calculate the linearity error, calibration data is compared to the straight line that would best fit the points. This straight reference line is calculated from the calibration data using a technique called least squares fitting. The amount of error at the point on the calibration curve that is furthest away from this ideal line is the linearity error. Linearity error is usually expressed in terms of percent of full scale. If the error at the worst point was 0.001mm and the full scale range of the calibration was 1mm, the linearity error would be 0.1%.

Error Band

Gap(mm)

ExpectedValue(VDC)

ActualValue(VDC)

ErrorBand(mm)

0.50 -10.000 -9.800 -0.010

0.75 -5.000 -4.900 -0.005

1.00 0.000 0.000 0.000

1.25 5.000 5.000 0.000

1.50 10.000 10.100 0.005

Error Band- the worst case deviation of the measured values from the expected values in a calibration chart. In this case, the total error is -0.010mm.

Error band accounts for the combination of linearity and sensitivity errors. It is the measurement of the worst case absolute error in the calibrated range. The total error is calculated by comparing the output voltages at specific gaps to their expected value. The worst case error from this comparison is listed as the capacitive sensor system’s total error.

High-Performance Capacitive Sensors

It is important to distinguish between "high-performance" sensors and inexpensive sensors. Simple capacitive sensors, such as those used in inexpensive proximity switches or elevator touch switches, are simple devices and in their most basic form could be designed in a high school electronics class. Proximity type sensors are tremendously useful in automation applications and many commercially available models are well made, but they are not suited to precision metrology applications.

In contrast, capacitive sensors for use in precision displacement measurement and metrology applications use complex electronic designs to execute complex mathematical algorithms. Unlike inexpensive sensors, these high-performance sensors have outputs which are very linear, stable with temperature, and able to resolve incredibly small changes in capacitance resulting in high resolution measurements of less than one nanometer.

Advantages

Compared to other noncontact sensing technologies such as optical, laser, eddy-current, and inductive, high-performance capacitive sensors have some distinct advantages.

Higher resolutions including sub nanometer resolutions

Not sensitive to material changes: Capacitive sensors respond equally to all conductors

Inexpensive compared to laser interferometers.

Capacitive sensors are not a good choice in these conditions:

Dirty or wet environment (eddy-current sensors are ideal).

Large gap between sensor and target is required (optical and laser are better).

Applications

Capacitive sensors are useful in any application requiring the measurement or monitoring of the position of a conductive target.

Position Measurement/Sensing

Capacitive sensors are basically position measuring devices. The outputs always indicate the size of the gap between the sensor's sensing surface and the target. When the probe is stationary, any changes in the output are directly interpreted as changes in position of the target. This is useful in:

Automation requiring precise location

Semiconductor processing

Final assembly of precision equipment such as disk drives

Precision stage positioning

Dynamic Motion

Measuring the dynamics of a continuously moving target, such as a rotating spindle or vibrating element, requires some form of noncontact measurement. Capacitive sensors are ideal when the environment is clean and the motions are small, requiring high-resolution.

Disk drive spindles

High-speed drill spindles

Ultrasonic welders

Vibration measurements

Thickness Measurement

Measuring material thickness in a noncontact fashion is a common application for capacitive sensors. The most useful application is a two-channel differential system in which a separate sensor is used for each side of the piece being measured. Details on thickness measurements with capacitive sensors are available in the Conductive Material Thickness Measurement with Capacitive Sensors Application Note. Capacitive sensor technology is used for thickness measurement in these applications:

Silicon wafer thickness

Brake rotor thickness

Disk drive platter thickness

Nonconductive Thickness

Capacitive sensors are sensitive to nonconductive materials which are placed between the probe's sensing area and a grounded back target. If the gap between the sensor and the back target is stable, changes in the sensor output are indicative of changes in thickness, density, or composition of the material in the gap. This is used for measurements in these applications:

Label positioning during application

Label counting

Glue detection

Glue thickness

Assembly testing

Assembly testing

Capacitive sensors have a much higher sensitivity to conductors than to nonconductors. For this reason, they can be used to detect the presence/absence of metallic subassemblies in completed assemblies. An example is a connector assembly requiring an internal metallic snap ring which is not visible in the final assembly. Online capacitive sensing can detect the defective part and signal the system to remove it from the line.

Capacitive Sensing in HID

Capacitance sensors detect a change in capacitance when something or someone approaches or touches the sensor. Capacitive sensing as a human interface device (HID) technology, for example to replace the computer mouse, is becoming increasingly popular. Capacitive sensors are used in devices such as laptop track-pads, MP3 players, computer monitors, cell phones and others. More and more engineers choose capacitive sensors for their flexibility, unique human-

device interface and cost reduction over mechanical switches. Capacitive touch sensors have become a predominant feature in a large number of mobile devices and MP3 players.

Capacitive sensors detect anything which is conductive or having dielectric properties. While capacitive sensing applications can replace mechanical buttons with capacitive alternatives, other technologies such as multi-touch and gesture-based touchscreens are also premised on capacitive sensing.

Working Principle

A basic sensor includes a receiver and a transmitter, each of which consists of metal traces formed on layers of a printed-circuit board (PCB). As shown in Figure 1, the AD714x has an on-chip excitation source, which is connected to the transmitter trace of the sensor. Between the receiver and the transmitter trace, an electric field is formed. Most of the field is concentrated between the two layers of the sensor PCB. However, a fringe electric field extends from the transmitter, out of the PCB, and terminates back at the receiver. The field strength at the receiver is measured by the on-chip sigma-delta capacitance-to-digital converter. The electrical environment changes when a human hand invades the fringe field, with a portion of the electric field being shunted to ground instead of terminating at the receiver. The resultant decrease in capacitance—on the order of femtofarads as compared to picofarads for the bulk of the electric field—is detected by the converter.

In general, there are three parts to the capacitance-sensing solutions:

The driver IC, which provides the excitation, the capacitance-to-digital converter, and compensation circuitry to ensure accurate results in all environments.

The sensor—a PCB with a pattern of traces, such as buttons, scroll bars, scroll wheels, or some combination. The traces can be copper, carbon, or silver, while the PCB can be FR4, flex, PET, or ITO.

Software on the host microcontroller to implement the serial interface and the device setup, as well as the interrupt service routine. For high-resolution sensors such as scroll bars and wheels, the host runs a software algorithm to achieve high resolution output. No software is required for buttons.

AD714x Driver ICs

These capacitance-to-digital converters are designed specifically for capacitance sensing in human-interface applications. The core of the devices is a 16-bit sigma-delta capacitance-to-digital converter (CDC), which converts the capacitive input signals (routed by a switch matrix) into digital values. The result of the conversion is stored in on-chip registers. The on-chip excitation source is a 250-kHz square wave.

These devices interface with up to 14 external capacitance sensors, arranged as buttons, bars, wheels, or a combination of sensor types. The external sensors consist of electrodes on a 2- or 4-layer PCB that interfaces directly with the IC.

The devices can be set up to interface with any set of input sensors by programming the on-chip registers. The registers can also be programmed to control features such as averaging and offset adjustment for each of the external sensors. An on-chip sequencer controls how each of the capacitance inputs is polled.

The AD714x also include on-chip digital logic and 528 words of RAM that are used for environmental compensation. Humidity, temperature, and other environmental factors can affect the operation of capacitance sensors; so, transparently to the user, the devices perform continuous calibration to compensate for these effects, giving error-free results at all times.

One of the key features of the AD714x is sensitivity control, which imparts a different sensitivity setting to each sensor, controlling how soft or hard the user’s touch must be to activate the sensor. These independent settings for activation thresholds, which determine when a sensor is active, are vital when considering the operation of different-size sensors. Take, for example, an application that has a large, 10-mm-diameter button, and a small, 5-mm-diameter button. The user expects both to activate with same touch pressure, but capacitance is related to sensor area, so a smaller sensor needs a harder touch to activate it. The end user should not have to press one button harder than another for the same effect, so having independent sensitivity settings for each sensor solves this problem.

Different Shapes & Sizes in capacitance sensing

As noted earlier, the sensor traces can be any number of different shapes and sizes. Buttons, wheels, scroll-bar, joypad, and touchpad shapes can be laid out as traces on the sensor PCB. Figure below shows a selection of capacitance sensor layouts.

Button

8-Way Switch

Slider

Wheel

Keypad

Touchpad

Selection of capacitance sensors

Many options for implementing the user interface are available to the designer, ranging from simply replacing mechanical buttons with capacitive button sensors to eliminating buttons by using a joypad with eight output positions, or a scroll wheel that gives 128 output positions.

The number of sensors that can be implemented using a single device depends on the type of sensors required. The AD7142 has 14 capacitance input pins and 12 conversion channels. The AD7143 has eight capacitance inputs and eight conversion channels. The table below shows the number of input pins and conversion stages required for each sensor type. Any number of sensors can be combined, up to the limit established by the number of available inputs and channels.

Sensor TypeNumber of CIN inputs required

Number of conversion channels required

Button 1 1 (0.5 for differential operation)

8-Way Switch 4—top, bottom, left, and right 3

Slider 8—1 per segment 8—1 per segment

Wheel 8—1 per segment 8—1 per segment

KeypadTouchpad

1 per row, 1 per column 1 per row, 1 per column

Measurements are taken on all connected sensors sequentially—in a “round-robin” fashion. All sensors can be measured within 36 ms, though, allowing essentially simultaneous detection of each sensor’s status—as it would take a very fast user to activate or deactivate a sensor within 40 ms.

Advantages

Capacitance sensors are more reliable than mechanical sensors—for a number of reasons. There are no moving parts, so there is no wear and tear on the sensor, which is protected by covering material, for example, the plastic cover of an MP3 player. Humans are never in direct contact with the sensor, so it can be sealed away from dirt or spillages. This makes capacitance sensors especially suitable for devices that need to be cleaned regularly—as the sensor will not be damaged by harsh abrasive cleaning agents—and for hand-held devices, where the likelihood of accidental spillages (e.g., coffee) is not negligible.

Conclusion

Capacitance sensors are an emerging technology for human-machine interfaces and are rapidly becoming the preferred technology over a range of different products and devices. Capacitance sensors enable innovative yet easy-to-use interfaces for a wide range of portable and consumer products. Easy to design, they use standard PCB manufacturing techniques and are more reliable than mechanical switches. They give the industrial designer freedom to focus on styling, knowing that capacitance sensors can be relied upon to give a high-performance interface that will fit the design.

References:

• http://www.analog.com/library/analogdialogue/

• http://www.lionprecision.com/tech-library/

• http://electronicdesign.com/Articles/

• http://www.sensorsmag.com/sensors

• http://en.wikipedia.org