1. How do capacitive proximity sensors work?
Capacitive proximity sensors are non-contact sensors that measure the capacitance between the sensor and a target object to determine the position of the target. A capacitor is an electrical storage device consisting of two conductive plates separated by an insulating layer called a dielectric. If a DC voltage is applied to the capacitor, electrical energy (electrical charge) builds up across the capacitor until it is fully charged at which point current stops flowing through the circuit. When the DC voltage is removed, the capacitor releases the electrical charge back into the circuit in the form of an electrical current. If an AC voltage is applied to the capacitor, its polarity changes at the same frequency as the AC waveform causing an alternating current to flow through the circuit. Thinner dialectic layers result in higher values of capacitance and therefore more current.
Capacitive proximity sensors detect the proximity of a metal or non-metal target to the sensors face by measuring the buildup of electrical charge (i.e. the capacitance) between the sensors face and the target. In case you haven’t guessed by now, the target acts as the 2nd capacitor plate and the gap between the sensor’s face and the target acts as the dielectric (the insulating layer). The capacitive proximity sensor consists of a capacitor plate, an LC oscillator circuit and a current meter. The LC oscillator circuit generates an AC voltage. As discussed in the previous paragraph, when an AC voltage is applied to a capacitor, it causes a current to flow, the magnitude of which is dependent on the capacitance value of the capacitor. Since the capacitance increases as the dielectric becomes thinner, the current increases as the target approaches the sensor face. The current is measured by the current meter, built into the sensor.
Due to their low accuracy, capacitive proximity sensors are rarely available with an analog outputs*. Instead, the capacitive proximity sensor makes use of a comparator to convert the reduction in current to a logic 0 or logic 1 depending on whether a threshold value of current has been crossed. The comparator used in proximity sensors is called a Schmitt trigger, which differs from a regular comparator in that it has built in hysteresis. The practical effect of the hysteresis is illustrated below; as the target moves to within 5 mm of the sensor, the sensor is triggered high. However, the sensor will only be triggered low once the target moves to at least 6 mm away. This prevents an oscillation of the sensors output state about a particular value.
Note: The target object must be electrically grounded for it to be detected by the capacitive proximity sensor. Since the current is so low many machines will be naturally grounded. However, it may sometimes be necessary to connect the target to the electrical ground of a power supply.
*There are capacitive sensors with analog outputs. They are temperature compensated and extremely accurate but also extremely expensive and so are not used in the same applications as traditional proximity sensors. Typical applications include measurement of silicon wafer thickness in the semi-conductor industry and measurement of coating thicknesses within high accuracy manufacturing processes. They are not reviewed within the current discussion.
2. General characteristics of capacitive proximity sensors
Capacitive proximity sensors are low to medium cost sensors, intended for measuring the proximity of metal and non-metal objects at distances up to 40 mm and can detect objects through non-metal walls. They are predominantly available with Boolean outputs (analog outputs are possible but uncommon). The sensors have poor output linearity and high temperature sensitivity, resulting in poor accuracy and a modest operating temperature range.
Capacitive proximity sensors are non-contact and solid state meaning that they are easy to assemble, do not experience mechanical wear and have an extremely long lifetime. They are limited to sampling rates of only 10-100 Hz because of the time they take to charge and discharge (see capacitor time constant). They have excellent environmental protection ratings, but their readings can be affected by contamination of the sensor surface or target object with dirt or liquid.
3. Input and output signals
Capacitive proximity sensors operate on a 10-36 Vdc supply voltage (though a minority of sensors operate on an AC supply voltage). The AC voltage is generated using an internal LC oscillator circuit. The sensor output is either 0 V (for a logic 0) or the supply voltage (less the voltage drop across the transistor) for a logic 1. If the sensor output is normally open (NO) then movement of the target to within sensing range will cause the sensor output to switch from logic 0 to logic 1. If the sensor output is normally open (NC) then movement of the target to within sensing range will cause the sensor output to switch from logic 1 to logic 0. Most capacitive proximity sensors have either a NO or an NC output, with the exception or 4 wire sensors which have both a NO and a NC output.
4. Applications of capacitive proximity sensors
Capacitive proximity sensors are used in a wide range of detection tasks in industry. Since inductive proximity sensors are similarly priced but considered superior for detecting metal objects, capacitive proximity sensors are most often used for detecting non-metal objects (which inductive sensors are not capable of doing). These applications include measuring liquid levels in containers, measuring powder and granule levels in hoppers and counting parts on conveyor belts.
5. Typical specification
|Cost||Low to medium|
|Measurement range||2-40 mm|
|Sampling rate||10-100 Hz|
|Velocity||Limited only by sampling rate|
|Ambient temperature||-25 to +70°C typical (up to 125 °C available)|
|Supply voltage||10-36 Vdc (Vac also available)|
|Output signal||Boolean or analog|
|Vibration resistance||10 g|
|Shock resistance||30 g|
|Ingress protection||IP67 typical (IP69 available)|
6. Purchasing tips
- Flush or non-flush housings: Proximity sensors are available in either flush or non-flush housings. Flush housings extend the entire length of the sensor, enabling the sensor face to be mounted ‘flush’ to a wall (hence the name). Non-flush housings do not extend the full length of the sensor. A flush housing also acts as an electromagnetic shield and so flush housed sensors are shielded whereas non-flush housed sensors are not. The shielding reduces the sensors measurement range and field of view.
- AC or DC: Most capacitive proximity sensors operate on a DC supply voltage. There are models that operate on an AC supply voltage. However, they have sampling frequencies of only 10 Hz. In comparison, those operating on DC supply voltages have sample rates up to 100 Hz.
- NO/NC: Capacitive proximity sensors are available with either normally open (NO) outputs, normally closed (NC) outputs, or both.
- Adjustable threshold: Many Boolean output sensors have adjustable threshold distances. The ability to adjust the threshold sensitivity is particularly important in applications where the sensor must detect an object through a wall.
- PNP/NPN: The transistor used to switch output states (in response to the Schmitt trigger) can be of either PNP or NPN type (also known as current sourcing and current sinking types respectively). When interfacing the sensor to a PLC, it is vital that the sensor is of the opposite type to the PLC port e.g. a current sourcing sensor will only work with a current sinking PLC port.
7. Advantages of capacitive sensors
Capacitive proximity sensors:
- Are of low cost and capable of withstanding harsh environments. They typically have ingress protection ratings of IP67, with some having IP69K.
- Can detect all metals and most non-metals including plastics and wood. They are the only electromagnetic proximity sensor capable of detecting non-metals.
- Are less sensitive to external electromagnetic fields than inductive and hall effect proximity sensors.
- Can detect objects located on the opposite side of a non-metal wall (e.g. a plastic casing).
8. Disadvantages of capacitive sensors
Capacitive proximity sensors:
- Are sensitive to the size of the target. Using a target smaller than the face of the sensor will reduce the sensing range.
- Have a sampling rate of only 10-100 Hz, considerably lower than that of most other proximity sensors on the market.
- Are highly sensitive to changes in temperature (more so than other types of proximity sensors). As a result, capacitive sensors are not able to operate at temperatures above 70 °C.
- Require their target to be electrically grounded, otherwise it will not act as a capacitor plate and will therefore not be detected.
9. Application tips
- Dielectric constant: Capacitive proximity sensors can detect most non-metals, but their ability to do so depends on the target material having a dielectric constant significantly larger than air.
- Through wall detection: Capacitive proximity sensors can be used to detect objects through a non-metallic wall, e.g. liquid level through a plastic container. Such applications require a sensor with an adjustable threshold so that their sensitivity can be calibrated to ignore the wall but detect the object behind the wall.
- Installation: DO NOT countersink a non-flush sensor into a metal casing as this could result in a false signal as the sensor detects the metal casing.
- Multiple sensors: Multiple Boolean output sensors can be connected in series or parallel to a single process controller input port to achieve logic functions. Two sensors connected in series act as an AND gate. Two sensors connected in parallel act as an OR gate. The circuits used to implement parallel and series connections can be found here
- Leakage current: The leakage current (i.e. the residual current flowing when the output is at logic 0) of a 2 wire Boolean output sensor may result in a sufficiently high voltage to cause a reset error. This can be avoided by placing a bleeder resistor in parallel with the controller.