I recently received a question on PNP and NPN sensors. They wanted an explanation of what a sink is and how to wire one. Confusion over the Normally Open and Normally Closed function of the sensor is also a common question. Several diagrams will show a resistor attached to the blue wire and a load across the others, what does it all mean?
It is quite confusing sometimes the language we use for these devices. Sometimes it is the way we talk about the sensor and sometimes about the device we are connecting. (PLC)
We will break it down and go over the wiring to a PLC input. Let’s get started. Continue Reading!
A proximity sensor (switch) is able to detect object presence without physical contact like a limit switch. No physical contact means that the switch has no parts that will wear out. The life span of the sensor is increased with less maintenance.
A capacitive proximity sensor will detect ferrous and non-ferrous objects. The sensor works by oscillating the charge on the plates in the sensor. When an object is placed in front of the surface, the amount of current flow is detected. (Capacitance) The dielectric of objects will determine the distance that the object can be detected.
We will be wiring a capacitive proximity switch into the input of our Click PLC. The CK1-00-2H is an 18mm diameter, NPN/PNP N.O./N.C. selectable output with a 12mm sensing distance. That means that the sensor can be wired as positive (Sourcing) or negative (Sinking) switch. This unshielded 10 Hz switching frequency sensor also has a 4-pin M12 quick disconnect. Let’s get started. Continue Reading!
Turn a capacitive proximity sensor into a level sensor. This is ideal for tanks and vessels that you cannot drill through the side for mounting.
Wrap the bare wire around the sensing head and extend it to the length you need to detect. Insulate the wire using electrical tape all the way down, but leave the end exposed. This acts as an antenna. Anything touching the end will now be detected. I have used 14 and 12 AWG wire, depending on the application.
Some quick information about capacitive proximity sensors.
A capacitor is defined as two electrically charged plates separated by a dielectric. In the case of a capacitive proximity sensor, the dielectric is the material that you are trying to detect. As the material moves closer to the electrostatic field of the sensor, oscillation begins. It will get past a threshold and trigger the output to switch.
Some common dielectric constants for material:
Glass – 5
Wood – 2.7
Paper – 2.3
Air, Vacuum – 1
Water – 80
Once you know the dielectric constant you can determine the sensing range (Sr) by a chart that usually comes with the sensor or in the manufactures manual.
Example: Water is 80 so the sensing range is 100%
I generally just try the sensor to determine if it will be suitable for the application.
If you have any questions or need further information please contact me.
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