Capacitive Sensors

 

In electrical engineering, capacitive sensing (sometimes capacitance sensing) is a technology, based on capacitive coupling, that can detect and measure anything that is conductive or has a dielectric different from air.

Capacitance and Distance. Non-contact capacitive sensors work by measuring changes in an electrical property called capacitance. Capacitance describes how two conductive objects with a space between them respond to a voltage difference applied to them.

Unlike resistive touch screens, capacitive screens do not use the pressure of your finger to create a change in the flow of electricity. Instead, they work with anything that holds an electrical charge – including human skin.

In electrical engineering, capacitive sensing (sometimes capacitance sensing) is a technology, based on capacitive coupling, that can detect and measure anything that is conductive or has a dielectric different from air. … Capacitive sensors can also replace mechanical buttons.

The object being sensed is often referred to as the proximity sensor’s target. … For example, a capacitive or photoelectric sensor might be suitable for a plastic target; an inductive proximity sensor always requires a metal target. The maximum distance that this sensor can detect is defined “nominal range”.

In electrical engineering, capacitive sensing (sometimes capacitance sensing) is a technology, based on capacitive coupling, that can detect and measure anything that is conductive or has a dielectric different from air.

Many types of sensors use capacitive sensing, including sensors to detect and measure proximity, position or displacement, humidity, fluid level, and acceleration. Human interface devices based on capacitive sensing, such as trackpads, can replace the computer mouse. Digital audio players, mobile phones, and tablet computers use capacitive sensing touchscreens as input devices. Capacitive sensors can also replace mechanical buttons.

Capacitive sensors for the detectIon of mechamcal quanhtles all rely on a displacement measurement

Description

Applications for Capacitive Sensors

Capacitive Sensors are able to sense both metallic and non-metallic materials and are a important component in industrial automation applications.

Applications:

  • Level Control of Liquids
  • Level Control of Solids
  • Pile-up Control
  • Hazardous Area Environments
  • High Temperature Environments
  • High Pressure Wash-down Environments
  • Feed Hopper Level Monitoring
  • Small Vessel Pump Control
  • Suitable for use in Environments with Inconsistent Power Supplies
  • Grease Level Monitoring
  • Pharmaceuticals Manufacturing
  • Suitable for Use with Chemicals
  • Small Size Allows Use in Confined Spaces
  • Available with Hazardous Area Outputs
  • Pipeline Leak Detection
  • Vessel Leak Detection

Capacitive Sensor Topologies
There are several capacitive sensor topologies that are common depending on the application. The sensor topology depends on:
Sensor-to-target distance
Dielectric constant of target
Desired sensitivity

The basic topologies include:
Parallel plate
Parallel fingers
Single sensor for human recognition

Liquid Level Sensing

Proximity/Gesture Detection

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

Position Measurement and 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 interpretted 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 measurements.

Precision machine tool spindles
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 availabe in the Conductive Material Thickness Measurement with Capacitive Sensors Application

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 indicitive 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 sensors are widely used for a large variety of functions, among them proximity sensing (personnel detection, light switching, vehicle detection), measurement (flow, pressure, liquid level, spacing, scanned multiplate sensor, thickness measurement, ice detection, shaft angle, linear position, and balances), switches (lamp dimmer, keyswitch, limit switch), communications (wireless datacom), and computer graphics input. Capacitive sensing include accelerometer, hygrometer, inclinometer, and microphone.

Capacitive sensing is used for:

Proximity sensing for industrial machine shut-off
Touch sensing / button replacements (isometric buttons on microwave ovens, stovetops etc., rotary input like on the iPod)
Micrometers
Material properties – most famous is probably the Stud Sensor patented in the seventies, that allows you to sense studs, water and mains lines in walls.
Vehicle/Personnel detection
Capacitor/Condenser microphones
Liquid level sensing
Tilt/Rotation sensing
Accelerometers
X-Y Input (Tablets/Laptop computer touch pads, Smart Phones)

Capacitive displacement sensors “are non-contact devices capable of high-resolution measurement of the position and/or change of position of any conductive target”.They are also able to measure the thickness or density of non-conductive materials.Capacitive displacement sensors are used in a wide variety of applications including semiconductor processing, assembly of precision equipment such as disk drives, precision thickness measurements, machine tool metrology and assembly line testing. These types of sensors can be found in machining and manufacturing facilities around the world.

Applications
Precision positioning
Disc drive industry
Precision thickness measurements
Non-conductive targets
Machine tool metrology
Assembly line testing

Testing the moisture content of grain
Soil moisture
Humidity
Detecting water content in fuels
Fuel composition sensors (for flex fuel vehicles)
Capacitive load cells

Capacitive sensors provide high resolution, down to the nanometer level, over short ranges – typically from 10 micrometers up to 2 millimeters.

Capacitive Position Sensors – Applications
Capacitive position sensors offer the following major advantages compared to other types of precision position sensors –

Capacitive sensors provide high resolution, down to the nanometer level, over short ranges – typically from 10 micrometers up to 2 millimeters.
Capacitive sensors are completely non-contact, and work with and conductive, grounded target
Capacitive sensors offer high measurement bandwidth – up to 10 KHz, 20 KHz or 100 KHz, depending upon model
Our sensors are used in many different applications in a variety of industries, links to application examples are provided below. Please do not hesitate to contact us to discuss your specific application requirements.

Positioning
Servo system feedback sensor for nanopositioners
Position sensor for fast tool servo

Distance and Displacement Measurement
Dimensional gaging and inspection
Axial shaft displacement measurement

Run Out Measurement
Precision motor and shaft run out measurement
Spindle metrology and spindle run out measurement
Straightness and flatness measurement of precision stages

Thickness
Magnetic media thickness measurement
Blu-ray disc stamper thickness measurement

Autofocus and Nulling
Semiconductor wafer surface sensing

Vibration
Precision stage vibration measurement
Shaft vibration measurement
Feedback sensor for active vibration isolation systems

Applications
Capacitive sensors have a wide variety of uses. Some are

Flow
Many types of flow meters convert flow to pressure or displacement, using an orifice for volume flow or Coriolis effect force for mass flow. Capacitive sensors
can then measure the displacement.

Pressure
A diaphragm with stable deflection properties can measure pressure with a spacing-sensitive detector.

Liquid level
Capacitive liquid level detectors sense the liquid level in a reservoir by measuring changes in capacitance between conducting plates which are immersed in the liquid, or applied to the outside of a non-conducting tank.

Spacing
If a metal object is near a capacitor electrode, the mutual capacitance is a very sensitive measure of spacing.

Scanned multiplate sensor
The single-plate spacing measurement can be extended to contour measurement by using many plates, each separately addressed. Both conductive and dielectric surfaces can be measured.

Thickness measurement
Two plates in contact with an insulator will measure the insulator thickness if its dielectric constant is known, or the dielectric constant if the thickness is known.
Ice detector Airplane wing icing can be detected using insulated metal strips in wing leading edges.

Shaft angle or linear position
Capacitive sensors can measure angle or position with a multiplate scheme giving high accuracy and digital output, or with an analog output with less absolute accuracy but faster response and simpler circuitry.

Lamp dimmer switch
The common metal-plate soft-touch lamp dimmer uses 60 Hz excitation and senses the capacitance to a human body.

Keyswitch
Capacitive keyswitches use the shielding effect of a nearby finger or a moving conductive plunger to interrupt the coupling between two small plates.
Limit switch Limit switches can detect the proximity of a metal machine component as an increase in capacitance, or the proximity of a plastic component by virtue of its increased dielectric constant over air.

X-Y tablet Capacitive graphic input tablets of different sizes can replace the computer mouse as an x-y coordinate input device. Finger-touch-sensitive, z-axis-sensitive and stylus-activated devices are available.

Accelerometers Analog Devices has introduced integrated accelerometer ICs with a sensitivity of 1.5g. With this sensitivity, the device can be used as a tiltmeter

Capacitance and Distance. Noncontact capacitive sensors work by measuring changes in an electrical property called capacitance. Capacitance describes how two conductive objects with a space between them respond to a voltage difference applied to them.