Resistive Sensors (Potentiometers & Strain Gages)
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 applications. The voltage measuring device is the final link in the system. Oscilloscopes, voltmeters and data acquisition systems must be properly selected for the application.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.
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.Fig1:Applying a voltage to conductive objects causes positive and negative charges to collect on each object. This creates an electric field in the space between the objects.Fig2:Applying an alternating voltage causes the charges to move back and forth between the objects, creating an alternating current which is detected by the sensor.
The capacitance between two plates is determined by three things:Size of the plates: capacitance increases as the plate size increasesGap Size: capacitance decreases as the gap increasesMaterial between the plates (the dielectric):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.
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.
The target size is a primary consideration when selecting a probe for a specific application
Capacitive sensors are most often used to measure the change in position of a conductive target.The presence of the nonconductive material changes the dielectric and therefore changes the capacitance.
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.
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.The devices can be set up to interface with any set of input sensors by programming the on-chip registers.
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.
Buttons, wheels, scroll-bar, joypad, and touchpad shapes can be laid out as traces on the sensor PCB.
•A sensor; is a device that measures a physical or abstractquantity and converts it into a signal which can be read byan observer or by an instrument.•A sensors sensitivity indicates how much the sensorsoutput changes when the measured quantity changes.•Sensors need to be designed to have a small effect onwhat is measured, making the sensor smaller often improvesthis and may introduce other advantages.•In most cases, a micro-sensor reaches a significantlyhigher speed and sensitivity compared with the one with amacroscopic approach.
•Sensitive to the measured property•Insensitive to any other property likely to beencountered in its application•Does not influence the measured property•Sensitivity is defined as the ratio between outputsignal and measured property.•Resolution of a sensor is the smallest change it candetect in the quantity that it is measuring.
• Transducer– a device that converts a primary form of energy into acorresponding signal with a different energy form– take form of a sensor or an actuator• Sensor (e.g., thermometer)– a device that detects/measures a signal or stimulus– acquires information from the “real world”• Actuator (e.g., heater)– a device that generates a signal or stimulusrealworldSensorActuatorIntelligentFeedbacksystem
SensorInputsignal(measurand)MicrocontrollerSignal processingcommunicationSensor dataanalog/digitalNetworkdisplayElectronic Sensors are those which convert desiredparameter into electrically measurable signalA General Electronic Sensor consists ofPrimary transducer: changes “real world” parameter intoelectrical signal. They include•Resistive Sensors•Inductive Sensors•Capacitive Sensors•Piezoelectric Sensors
Secondary transducer: converts electrical signal into analog ordigital values. They include• Wheatstone Bridge• AmplifiersRealworldAnalogsignalPrimarytransducerSecondarytransducerSENSORUsableValues
•Capacitive displacement sensors are noncontactdevices capable of high-resolution measurementof the position and/or change of position of anyconductive target.•Used for detecting proximity, position, etc.,based on capacitive coupling effects.•Capacitance sensors detect a change incapacitance when something or someoneapproaches or touches the sensor.•The technique has been used in industrialapplications for many years to measure liquidlevels, humidity, and material composition.
Capacitive sensor dimensional measurement requires three basiccomponents:•a probe that uses changes in capacitance to sense changes indistance to the target•driver electronics to convert these changes in capacitance intovoltage changes•a device to indicate and/or record the resulting voltage change.
•Capacitance is a property that exists between any two conductivesurfaces within some reasonable proximity.•Changes in the distance between the surfaces changes thecapacitance.• It is this change of capacitance that capacitive sensors use toindicate changes in position of a target.
C= Area X DielectricGapIn ordinary capacitive sensing,•the size of the sensor probe,•the size of the target,•and the dielectric material (air)remain constant.• The only variable is the gap size. Based on thisassumption, driver electronics assume that allchanges in capacitance are a result of a changein gap size.C≈ 1Gap
When a voltage is applied to a conductor, an electric field isemitted from every surface.For accurate gauging, the electric field from a capacitive sensorneeds to be contained within the space between the probe’ssensing area and the target.If the electric field is allowed to spread to other items or otherareas on the target, then a change in the position of the otheritem will be measured as a change in the position of the target.To prevent this from happening, a technique called guarding isused.
To create a guarded probe, the back and sides of the sensingarea are surrounded by another conductor that is kept at thesame voltage as the sensing area itself.When the excitation voltage is applied to the sensing area, aseparate circuit applies the exact same voltage to the guard.Because there is no difference in voltage between the sensingarea and the guard, there is no electric field between them tocause current flow.Any conductors beside or behind the probe form an electricfield with the guard instead of the sensing area. Only theunguarded front of the sensing area is allowed to form anelectric field to the target.
Effects of Target SizeWhen the sensing electric field is focused by guarding, itcreates a slightly conical field that is a projection of thesensing area. The minimum target diameter is usually 30% ofthe diameter of the sensing area. The further the probe isfrom the target, the larger the minimum target size.Range of MeasurementThe range in which a probe is useful is a function of the size ofthe sensing area. The greater the area, the larger the range.A smaller probe must be considerably closer to the target toachieve the desired amount of capacitance. In general, themaximum gap at which a probe is useful is approximately 40%of the sensing area diameter
Figure 10.Nonconductors can bemeasuredby passingthe electricfield throughthem to astationaryconductivetargetbehindMultiple Channel SensingFrequently, a target is measured simultaneously by multipleprobes. Because the system measures a changing electricfield, the excitation voltage for each probe must besynchronized or the probes will interfere with each other.Effects of Target MaterialThe sensing electric field is seeking a conductive surface.Provided that the target is a conductor, capacitive sensorsare not affected by the specific target material; they willmeasure all conductors—brass, steel, aluminum, or saltwater—as the same.
Other Factors to be considered for optimization:•Maximizing Accuracy•Target Shape•Surface Finish•Parallelism•Environment
Capacitive sensors can be very effective in measuring• density,• thickness, and• locationof nonconductors as well.The dielectric constant determines how a nonconductivematerial affects the capacitance between two conductors.When a nonconductor is inserted between the probe and astationary reference target, the sensing field passesthrough the material to the grounded target .Capacitance will change in relationship to the thickness ordensity of the non-conducting material.
Simple capacitive sensors:•used in inexpensive proximity switches or elevator touch switches,•simple devices and in their most basic form could be designed in ahigh school electronics class.In contrast, capacitive sensors for use in precision displacementmeasurement and metrology applications use complex electronicdesigns to execute complex mathematical algorithms.Unlike inexpensive sensors, these high-performance sensors haveoutputs which are• very linear,• stable with temperature,• and able to resolve incredibly small changes in capacitanceresulting in high resolution measurements of less than onenanometer.
Compared to other noncontact sensing technologies such asoptical, laser, eddy-current, and inductive, high-performancecapacitive sensors have some distinct advantages:•Higher resolutions including sub-nanometer resolutions•Not sensitive to material changes: Capacitive sensorsrespond equally to all conductors•Less expensive and much smaller than laserinterferometers.Capacitive sensors are not good choice in these conditions:•Dirty or wet environment (eddy-current sensors are ideal)•Large gap between sensor and target is required (opticaland laser are better)
Position Measurement/SensingCapacitive sensors are basically position measuringdevices. The outputs always indicate the size of the gapbetween the sensors sensing surface and the target.When the probe is stationary, any changes in the outputare directly interpreted as changes in position of thetarget. This is useful in:•Automation requiring precise location•Semiconductor processing•Final assembly of precision equipment such as diskdrives•Precision stage positioning
Dynamic MotionMeasuring the dynamics of a continuously movingtarget, such as a rotating spindle or vibrating element,requires some form of noncontact measurement.Capacitive sensors are ideal when the environment isclean and the motions are small, requiring high-resolution measurements:•Precision machine tool spindles•Disk drive spindles•High-speed drill spindles•Ultrasonic welders•Vibration measurements
Thickness MeasurementMeasuring material thickness in a noncontact fashionis a common application for capacitive sensors. Themost useful application is a two-channel differentialsystem in which a separate sensor is used for eachside of the piece being measured.Capacitive sensor technology is used for thicknessmeasurement in these applications:•Silicon wafer thickness•Brake rotor thickness•Disk drive platter thickness
Nonconductive ThicknessCapacitive sensors are sensitive to nonconductive materialswhich are placed between the probes sensing area and agrounded back target. If the gap between the sensor andthe back target is stable, changes in the sensor output areindicative of changes in thickness, density, or composition ofthe material in the gap. This is used for measurements inthese applications:•Label positioning during application•Label counting•Glue detection•Glue thickness•Assembly testing
•Capacitive sensing as a human interface device (HID)technology, for example to replace the computer mouse,is becoming increasingly popular.• Capacitance sensors detect a change in capacitancewhen something or someone approaches or touches thesensor.•Capacitive sensors are used in devices such as laptoptrack-pads, MP3 players, computer monitors, cell phonesand others.•More and more engineers choose capacitive sensors fortheir flexibility, unique human-device interface andcost reduction over mechanical switches.
The three parts to the capacitance-sensing solution:• The driver IC, which provides the excitation, the capacitance-to-digital converter, and compensation circuitry to ensureaccurate 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 canbe copper, carbon, or silver, while the PCB can be FR4, flex, PET,or ITO.• Software on the host microcontroller to implement the serialinterface and the device setup, as well as the interrupt serviceroutine. For high-resolution sensors such as scroll bars andwheels, the host runs a software algorithm to achieve highresolution output. No software is required for buttons.
•These capacitance-to-digital converters are designed specificallyfor 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 on-chip excitation source is a 250-kHz square wave.•The devices can be set up to interface with any set of inputsensors by programming the on-chip registers.•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 activatethe sensor.
•When the sensor is not active, the capacitance value measured isstored as the ambient value.•When a user comes close to or touches the capacitance sensor, themeasured capacitance decreases or increases.•Threshold capacitance levels are stored in on-chip registers. Whenthe measured capacitance value exceeds either upper or lowerthreshold limits, the sensor is considered to be active and aninterrupt output is asserted.
•Decide what types, number and dimension of sensors areneeded in the application.•Place the AD7142 or AD7143 on the same PCB as the sensorsto minimize the chances of system errors due to movingconnectors and changing capacitance.•Other components, LEDs, connectors, and other ICs, forexample, can go on the same PCB as the capacitance sensors•The sensor PCB must be glued or taped to the coveringmaterial to prevent air gaps above the sensors.•For applications where RF noise is a concern, then an RC filtercan be used to minimize any interference with the sensors.•Calibration of capacitance sensing
• Capacitance sensors are more reliable than mechanical sensors.• Humans are never in direct contact with the sensor, so it can besealed away from dirt or spillages.• Capacitive touchscreens are highly responsive• A standard stylus cannot be used for capacitive sensing unless itis tipped with some form of conductive material.• Capacitive touchscreens are more expensive to manufacture.
•Capacitance sensors are an emerging technology forhuman-machine interfaces and are rapidly becoming thepreferred technology over a range of different productsand devices.•Capacitance sensors enable innovative yet easy-to-useinterfaces for a wide range of portable and consumerproducts.•They give the industrial designer freedom to focus onstyling, knowing that capacitance sensors can be reliedupon to give a high-performance interface that will fitthe design.