How Capacitive Proximity Sensor Works?

Capacitive Proximity Sensor Control Technology Proximity Sensor
Capacitive Proximity Sensors detect all materials, including liquids, powders and metallic and non-metallic solids. These sensors are often used to control the levels of pellets, liquids and powders in production control. Like their inductive counterparts, they are manufactured in shielded and non-shielded configurations and are available in both AC and DC power formats. The shielded models are used to detect solid products such as cartons, stacks of paper, wood or liquids through the wall of a non-metallic container. The non-shielded models are used to detect liquids or powders where the product flows around the sensor. Shielded models have a shorter detection range since part of the field is lost in the shielding process. All detection ranges given on the following pages are for a steel target which is equal to, or larger than the diameter of the sensor. Non-conductive products, such as wood or plastic, will be detected at a reduced detection range. The range at which a product is detected is directly related to the dielectric constant of the material; the greater the dielectric constant, the greater the detection range
Shielded Configuration: Non-shielded Configuration: Shielded sensors have a straight-line electrical field. They scan for the presence of solids (e.g. wafers, components, PCB’s, hybrids, cartons, bottles, plastic blocks and stacks of paper) at a distance. Shielded capacitive sensors can also detect liquids through a separating wall (glass or plastic up to a maximum of 4mm thick). Non-shielded sensors have a spherical electrical field. They are designed to touch the product with their active surface. They are used to detect mainly bulk goods or liquids (e.g. granulate, sugar, flour, corn, sand, oil, water or pastes).
Sensitivity Adjustment All capacitive models have an adjustable sensitivity that allows the calibration to be made in the field. This allows the sensitivity to be set for the target desired; for example, the sensitivity can be reduced to ignore a glass container but still detect the liquid inside the container. Similarly, the sensitivity can be reduced to ignore a build-up of a viscous product such as honey while still detecting the level when a large amount of the product reaches the sensor. To adjust the sensitivity on a Capacitive Proximity Sensor, mount the sensor in the working position. Allow the target material to reach the position where detection should take place. If the material has not been detected, rotate the sensitivity potentiometer clockwise, with the screwdriver provided, until detection first occurs (LED will illuminate). Continue to rotate the potentiometer for another 1/4 turn. Remove the target material and ensure that the sensor turns OFF (LED will turn off). If the sensor turns OFF, leave the sensitivity at that position. If the sensor remains ON, decrease the sensitivity (counterclockwise rotation) until the sensor turns OFF. For best results, position the sensitivity potentiometer half way between these two points.
Capacitive Sensor Oscillator Circuit Capacitive proximity sensors consist of an RC-oscillator with a special multi-part sensing electrode. The electrode and the oscillator circuit have a tube connected with earth potential for lateral shielding. This enables flush mounting of sensor in metal, since the electrical field is only present in front of the sensing electrode. This field is the active zone of the sensor. When the conductive material is removed from the active zone, the oscillator is undamped and the oscillation amplitude decreases. The amplifier os the oscillator voltage and the sensitivity of the sensor can be altered by the built-in potentiometer. The middle electrode together with the built-in re-coupling gives very effective compensation under conditions of humidity, dust or icing. Special circuitry automatically compensates for these influences. The preset sensing distance remains nearly constant. The electrode design, along with the compensating circuitry of capacitive sensors, is a unique design, and provides performance advantages far superior to other capacitive sensors.
Capacitive Switch Block Diagram
Applications for Capacitive Proximity Sensors
Liquid Level Control		Carton Detection		Bin Level Control		Resist Liquid Level
Capacitive Proximity Sensors reliably detect liquid levels.		The packaging industry relies on Capacitive sensors to detect paper and cardboard cartons.		Capacitive Sensors can detect liquids, powders, plastic pellets and pastes for level control.		Use Capacitive Sensors to monitor resist liquid levels in pipes.

How Inductive Proximity Sensor Works?

OPERATING PRINCIPLES FOR INDUCTIVE PROXIMITY SENSORS Proximity Sensors

Inductive proximity sensors are used for non-contact detection of metallic objects. Their operating principle is based on a coil and oscillator that creates an electromagnetic field in the close surroundings of the sensing surface. The presence of a metallic object (actuator) in the operating area causes a dampening of the oscillation amplitude. The rise or fall of such oscillation is identified by a threshold circuit that changes the output of the sensor. The operating distance of the sensor depends on the actuator's shape and size and is strictly linked to the nature of the material (Table 1).

Table 1. Sensitivity when different metals are present. Sn = operating distance. Fe37 (Iron) 1 x Sn Stainless steel 0.9 x Sn Brass - Bronze 0.5 x Sn Aluminum 0.4 x Sn Copper 0.4 x Sn Outputs: DC Voltage 2 wire DC: These sensors contain an output amplifier with the function N.O. or N.C. that can pilot a load connected in series. In this system a residual current flows through the load even when in the open state and a voltage drop occurs to the sensor when it is in the closed state. Attention must be paid to these restrictions when selecting relays or electronic controls to be used with these sensors. They are compatible with P.L.C. units. 3 & 4 wire DC: These amplified D.C. sensors contain an output amplifier. They are supplied as 3 wire with function N.O. or NC and as 4 wire with complementary outputs (NO + NC) in the types NPN and PNP. Standard version include protected against short circuit, protected against polarity and peaks created by the disconnection of inductive loads. They are compatible with P.L.C. Units Analog & Linear: In these 3 wire amplified sensors a current or voltage output varies in proportion to the distance between the sensor and a metallic object. NAMUR: These are 2 wire non-amplified sensors whose current varies in the presence of a metallic object. The difference between these sensors and traditional sensors is the absence of amplifier trigger stages. Their current and voltage limits allow them to be used in hazardous (explosive) environments when used with approved amplifiers. In standard applications (normal atmospheres) the sensor must be used with amplifier units ALNC, ALN2 or similar. AC Voltage 2 wire AC: These are two-wire sensors that contain a thyristor output amplifier. In this system a residual current flows through the load even when in the open state and a voltage drop occurs to the sensor when it is in the closed state. Attention must be paid to the minimum switching current, residual current and voltage drop when selecting low consumption relays or high impedance electronic controls to be used with these sensors. They are compatible with P.L.C. Units Definitions: NO (normally open): A switch output that is open prohibiting current flow when an actuator is not present and closes allowing current flow when an actuator is present. NC (normally closed): A switch output that is closed allowing current flow when no actuator is present and opens prohibiting current flow when an actuator is present. NPN Output: Transistor output that switches the common or negative voltage to the load. The load is connected between the positive supply and the output. Current flows from the load through the output to ground when the switch output is on. Also known as current sinking or negative switching. PNP Output: Transistor output that switches the positive voltage to the load. The load is connected between output and common. Current flows from the device's output, through the load to ground when the switch output is on. Also known as current sourcing or positive switching. Operating Distance (Sn): The maximum distance from the sensor to a square piece of Iron (Fe 37), 1mm thick with side's = to the diameter of the sensing face, that will trigger a change in the output of the sensor. Distance will decrease for other materials and shapes. Tests are performed at 20ºC with a constant voltage supply. This distance does include a ± 10% manufacturing tolerance. Power Supply: The supply voltage range that sensor will operate at. Max Switching Current: The amount of continuous current allowed to flow through the sensor without causing damage to the sensor. It is given as a maximum value. Min Switching Current: It is the minimum current value, which should flow through the sensor in order to guarantee operation. Max Peak Current: The Max peak current indicates the maximum current value that the sensor can bear in a limited period of time. Residual Current: The current, which flows through the sensor when it is in the open state. Power Drain: The amount of current required to operate a sensor. Voltage Drop: The voltage drop across a sensor when driving the maximum load. Short Circuit Protection: Protection against damage to a sensor if the load becomes shorted. Operating Frequency: The maximum number of on/off cycles that the device is capable of in one second. According to EN 50010, this parameter is measured by the dynamic method shown in fig. 1 with the sensor in position (a) and (b). S is the operating distance and m is the diameter of the sensor. The frequency is given by the formula in fig. 2. Repeatability (%Sn): The variation between any values of operating distance measured in an 8 hour period at a temperature between is 15 to 30ºC and a supply voltage with a <= 5% deviation. Hysteresis (%Sn): The distance between the "switching on" point of the actuator approach and the "switching off" point of the actuator retreat. This distance reduces false triggering. Its value is given as a percent of the operating distance or a distance. See Fig. 3 Flush Mounting: For side by side mounting of flush mount models refer to Fig. 4a. Non-flush mount models can be embedded in metal according to Fig. 4b. for side by side refer to fig. 4c. Sn = operating distance. Protection Degree: Enclosure degree of protection according to IEC (International Electrotechnical Commission) is as follows: IP 65: Dust tight. Protection against water jets. IP 67: Dust tight. Protection against the effects of immersion Fig. 1 Fig. 2 Fig.3