Hello Everyone Welcome to Instrumentation Academy In this video we will discuss what is a Proximity Sensor and How it works. Proximity Sensors are sensors that detect the movement or presence of an object without making physical contact with the object and converts that information captured into an Electrical signal. Devices such as limit switches detect an object by physically contacting it, but proximity sensors detect the presence of an object without touching it.
As these sensors are non-contact sensors they do not cause any damage to the object. proximity sensors do not use any type of physical moving parts, instead they allow signals to transmit through them, when something that is being monitored comes in close proximity of the sensing area. There are many types of proximity sensors and they each sense the presence of an object in their own distinct ways.
The two most commonly used proximity sensors are The capacitive proximity sensor and the Inductive proximity sensor. Now let's discuss about Capacitive Proximity Sensors. Working Principle of Capacitive Proximity Sensor.
Capacitive proximity sensor is based on the principle of a Parallel Plate Capacitor. Parallel Plate Capacitor consists of two parallel plates that are separated by a dielectric material which is a poor conductor of electricity such as plastic, glass or porcelain. The two parallel plates are conductive and they are usually made of Aluminum, Tantalum or other metals.
A metal plate typically has an equal amount of positively and negatively charged particles, which means it is electrically neutral. if we connect a power source or a battery to the capacitor a large number of electrons start moving from the negative terminal of the battery through the conductive wire. when these electrons reach the right side plate of the capacitor, the dielectric material will strongly oppose the movement of electrons from the right side plate to left side plate.
as a result number of electrons that is the negative charge carriers on the right side plate of the capacitor will be higher than the number of protons, that is the positive charge carriers Due to this the right side plate of the capacitor becomes negatively charged at the same time the electrons on the left side plate experience a strong attractive force from the positive terminal of the battery. As a result the electrons leave from left side plate and will be attracted towards the positive terminal of the battery Due to this the number of protons will be higher than the number of electrons in the left side plate and as a result the left side plate of the capacitor becomes positively charged. This is how the charging of capacitor takes place.
The right side plate has now developed a net negative charge and the left side plate has developed an equal net positive charge. this creates an electric field with an attractive force between them which holds the charge of the capacitor. The ability of a capacitor to store electric charge when a voltage is applied is called capacitance and quantity measuring the ability of a dielectric material to store charge is called dielectric constant.
the capacitance of a capacitor is directly proportional to the dielectric constant of the material between the two plates and inversely proportional to the distance between the two plates. There are two types of Capacitive Proximity Sensors The first one is Dielectric type Capacitive Proximity Sensor and the second one is Conductive type Capacitive Proximity Sensor. Dielectric type of capacitive proximity sensor.
Dielectric type of capacitive proximity sensor can detect any target that has a dielectric constant greater than air. It has two parallel plates inside the sensing head which operates like an open capacitor. Here air acts as the dielectric when no target is present the capacitance between the plates will be very less.
These plates are linked to an oscillator and a detector circuit. As the target which has dielectric constant more than air comes near to the sensor the capacitance between the plates increases. Increase in capacitance increases the amplitude of the oscillation of the oscillator.
when the oscillation exceeds a specific value, the detector turns on the output of the sensor. when the target object moves away from the sensor the oscillation amplitude decreases and when it falls below a threshold value the sensor returns to the initial state. Dielectric type capacitive proximity sensors will sense both metallic and non-metallic objects.
Conductive type of Capacitive Proximity Sensor In conductive type capacitive proximity sensor there is only one capacitor plate inside the sensor the target itself becomes the other plate of the parallel plate capacitor. The plates of a parallel plate capacitor should be conductive, so this type of sensor is used if the target is an electrically conductive material. The air gap between the sensor and the target functions as the dielectric.
The plate inside the sensor is connected to an oscillator circuit that is used to generate an electrostatic field. As the conductive target approaches the sensor, the distance between the two plates decreases due to which the capacitance increases and results in increase of oscillation amplitude. once the oscillations exceeds a predetermined value, output signal from the sensor is generated which indicates that the target has reached near the sensor.
Now let's move on to Inductive Proximity Sensor. Inductive proximity sensors are used to detect metal targets. An inductive proximity sensor has four main components.
the first one is the coil. The coil generates necessary electromagnetic field a cup shaped magnetic core holds the coil. This core is necessary to concentrate the coil's magnetic field on the front area of the sensor.
The second part is the oscillator The oscillator is generally an LC oscillator. it produces radio frequency which helps to generate an electromagnetic field. The third part is the trigger circuit.
The trigger circuit senses the change in amplitude of oscillation and gives the signal to the output circuit. The last part is the output circuit. The output circuit has a transistor after receiving the signa,l the transistor switches on and gives an output.
Working principle of Inductive Proximity sensor Inductive proximity sensors operate on the basis of Faraday's law of Inductance. According to Faraday's law of Induction, when an electrically conducting object is placed in a time varying magnetic field, an electric current called a Eddy current will be generated in the object and according to the Len's is law, the Eddy current creates a magnetic field in a conductor and this eddy current magnetic field opposes the magnetic field which created it. Now let's discuss the working of an Inductive Proximity Sensor The proximity sensor is installed in the area where we need to sense the presence of an electrically conductive target object.
when the sensing coil in the oscillation circuit is supplied with an alternating current, a changing magnetic field is created around the coil. when the target object enters this electromagnetic field some of the electromagnetic energy is transferred to the object according to Faraday's law of Induction an electric current called Eddy current will be generated in the object. This eddy current produces another magnetic field called Eddy current magnetic field and it opposes the magnetic field which created it.
As the eddy current magnetic field opposes the magnetic field of the coil, the intensity of the magnetic field of the coil reduces. As the target comes very close to the sensor the eddy current increases, decreasing the amplitude of the electromagnetic field The trigger circuit monitors the amplitude and when the amplitude goes below a predetermined level the output state of the sensor switches from its normal condition. As the target moves away from the sensor the amplitude of the electromagnetic field increases and at a predetermined level the trigger switches the output state of the sensor back to its normal condition.
