Classification of Proximity Sensors

Proximity Switches/Sensors

Sensors are the components just like limit switches whose internal contact will

become ‘NO’ to ‘NC’ when it sense (detect) some object. The major difference between

mechanical switches and proximity sensors are that sensors actuated without any physical

contact with the object.

Digital Sensors Classification

 The schematic symbols for the discrete sensor are drawn as limit switches in a diamond shaped box. In figure 3-16 symbols (a) and (b) are respectively N/O and N/C sensors. 

For discrete sensors, there are two types of outputs, the NPN or sinng output and the

PNP or sourcing output. The NPN or sinking output has an output circuit that functions

similar to a TTL open collector output. It can be regarded as an NPN bipolar transistor with a grounded emitter and an uncommitted collector, as shown in figure. 

Classification of Proximity Sensors according to its working:-

Proximity sensors are discrete sensors that sense when an object has come near to the

sensor face. There are four fundamental types of proximity sensors-the inductive proximity

sensor, the capacitive proximity sensor, the ultrasonic proximity sensor, and the optical

proximity sensor. In order to properly specify and apply proximity sensors, it is important to

understand how they operate and to which applications each is best suited.

Inductive Proximity Sensors

As with all proximity sensors, inductive proximity sensors are available in various

sizes and shapes as shown in figure. As the name implies, inductive proximity sensor operate on the principle that the inductance of a coil and the power losses in the coil vary as a metallic (or conductive) object is passed near it. Because of this operating principle, inductive sensors are only used for sensing metal objects. They will not work with non-metallic materials.

To understand how inductive proximity sensors operate, consider the cutaway block

diagram shown in figure. Mounted just inside the face of the sensor (on the left end) is a coil

that is part of the tuned circuit of an oscillator. When the oscillator operates, there is an alternating magnetic field (called a sensing field) produced by the coil. This magnetic field

radiates through the face of the sensor (which is non-metallic). The oscillator circuit is tuned

such that as long as the sensing field sense non- metallic material (such as air) it will continue to oscillate, it will trigger the trigger circuit, and the output switching device (which inverts the output of the trigger circuit) will be off. The sensor will therefore send an OFF signal through the cable extending from the right side of the sensor in fig

When a metallic object (steel, iron, aluminum, tin, copper, etc.) comes near the face of

the sensor, as shown in fig, the alternating magnetic field in the target produces circulating eddy currents inside the material. To the oscillator, these eddy currents are a power loss. As the target moves nearer, the eddy current loss increases, which loads the output of oscillator. This loading effect causes the output amplitude of the oscillator to decrease.

As long as the oscillator amplitude does not drop below the threshold level of the

trigger circuit, the output of the sensor will remain off. However, as shown in figure 3-29, if

the target object moves closer to the face of the sensor, the eddy current loading will causes the oscillator to stall (cease to oscillate). When this happens, the trigger circuit senses the loss of oscillator output and causes the output switching device to switch ON.

The sensing range of a proximity sensor is the maximum distance the target object

may be from the face of the sensor in order for the sensor to detect it. One parameter

affecting the sensing range is the size (diameter) of the sensing coil. Small diameter sensors (approximately ¼” in diameter) have typical sensing ranges in the area of 1 mm, while large diameter sensors (approximately 3” in diameter) have sensing ranges in the order of 50 mm or more. Additionally, since different metals have different values of resistivity (which limits the eddy currents) and permeability (which channels the magnetic field through the target), the type of metal being sensed will affect the sensing range. Inductive proximity sensor manufactures derate their sensors based on different metals, with steel being the reference (i.e., having a derating factor of 1.0). some other approximate derating factors are stainless steel, 0.85; aluminum, 0.40; and copper, 0.30.

As an example of how to apply the derating factors, assume you are constructing a

machine to automatically count copper object as they travel down to a conveyor, and the

sensing distance will be 5 mm. in order to detect copper object (derating factor 0.30), you would need to purchase a sensor with a sensing range of 5 mm / 0.30 = 16.7 mm. let’s say you found a sensor in stock that has a sensing range of 10 mm. if you use this to sense the copper object, you would need to mount it near the to conveyor so that the object will pass within (10 mm) (0.30) = 3 mm of the face of the sensor.

Inductive proximity sensors are available in both DC and AC powered models. Most

require three electrical connections ground, power, and output. However, there are other

variations that require two wires and four wires. Most sensors are available with a built-in LED that indicates when the sensor output is on. One of the first steps a designer should take when using any proximity sensor is to acquire a manufactures catalog and investigate the various types, shapes and output configurations to determine the best choice for the

application.

To illustrate some of the many possible applications of inductive proximity sensors

(sometimes called inductive proximity switch), consider these uses:

 By placing an inductive proximity switch next to a gear, the proximity switch can

sense the passing gear teeth to give rotating speed information. This application is

currently used as a speed feedback device in automotive cruise control systems where

the proximity switch is mounted in the transmission.

 Inductive proximity switches can be mounted on access doors and panels of

machines. The PLC can be programmed to shut down the machine if any of these

doors and access panels are opened.

 Very large inductive proximity sensors can be mounted in roadbeds to sense passing

automobiles. This technique is currently used to operate traffic lights.

Capacitive Proximity Sensors

Capacitive proximity sensors are available in shapes and size similar to the inductive

proximity sensor (as shown in fig). However, because of the principle upon which the

capacitive proximity sensor operates, applications for the capacitive sensor are somewhat different.

To understand how capacitive proximity sensors operate, consider the cutaway block

diagram shown in fig. The principle of operation of the sensor is that an internal oscillator will not oscillate until a target material is moved close to the sensor face. The target material varies the capacitance of a capacitor in the face of the sensor that is part of the oscillator circuit. There are two types of capacitive sensor, each with a different way that this sensing capacitor is formed. In the dielectric type capacitive proximity sensor, there are two side-byside capacitor plates in the sensor face. For this type of sensor, the external target acts as the dielectric. As the target is moved closer to the sensor face, the change in dielectric increases the capacitance of the internal capacitor, making the oscillator amplitude increases, which in turn causes the sensor to output an ON signal. The conductive type capacitive proximity sensor operates similarly; however, there is only one capacitor plate in the sensor face. The target becomes the other plate. Therefore, for this type of sensor, it is best if the target is an electrically conductive material (usually metal or water-based)

The waveform diagram, as shown in figure the target approaches the face of the sensor, the

oscillator amplitude increases, which cause the sensor, output to switch on.

Dielectric capacitive proximity sensors will sense both metallic and non-metallic

objects. However, in order for the sensor to work properly, it is best if the material being

sensed has a high density. Low-density materials (foam, bubble wrap, paper, etc.) do not cause a detectable change in the dielectric and consequently will not trigger the sensor.

Conductive capacitive proximity sensors require that the material being sensed be an

electric conductor. These are ideally suited for sensing metals and conductive liquids. For

example, since most disposable liquid containers are made of plastic or cardboard, these sensors have the unique ability to “look” through the container and sense the liquid inside.Therefore, they are ideal for liquid-level sensors.

Capacitive proximity sensors will sense metal objects just as inductive sensors will.However, capacitive sensors are much more expensive than the inductive types. Therefore, ifthe material to be sensed is metal, the inductive sensor is the more economical choice.

Some of the potential applications for capacitive proximity sensors include:

 They can be used as a non-contact, liquid-level sensor. They can be placed outside a

container to sense the liquid-level inside. This is ideal for milk, juice, or soda bottling

operations.

 Capacitive proximity sensors can be used as replacements for pushbuttons and palm

switches. They will sense the hand and, because they have no moving parts, they are

more reliable than mechanical switches.

 Since they are hermetically sealed, they can be mounted inside liquid tanks to sense

the tank-fill level.

As with the inductive proximity sensors, capacitive proximity sensors are available with a built-in LED indicator to indicate the output logical state. Also, because capacitive proximity sensors are used to sense materials with a wide range of densities, manufacturers usually provide a sensitivity adjusting screw on the back of the sensor. Then, when the sensor is installed, the sensitivity can be adjusted for best performance in the particular application.

Ultrasonic Proximity Sensors

The ultrasonic proximity sensor operates using the same principle as shipboard sonar.

As shown in fig an ultrasonic “ping” is sent from the face of the sensor. If a target and

returned to the sensor. When an echo is returned, the sensor detects that a target is present, and by measuring the time delay between the transmitted ping and the returned echo, the sensor can calculate the distance between the sensor and the target.

As with any proximity sensor, the ultrasonic proximity switch has limitations. The

sensor is only capable of sensing a target that is within the sensing range. The sensing range

is a funnel-shaped area directly in front of the sensor, as shown in fig.3-34 sound waves

travel from the face of the sensor in a cone-shaped dispersion pattern bounded by the sensor’s beam angle. However, because the sending and receiving transducers are both located in the face of the sensor, the receiving transducer is “blinded” for a short period of time immediately after the ping is transmitted-similar to the way our eyes are temporarily blinded by a flashbulb. This means that any echo that occurs during this “blind” time period will go undetected. These echos will be from targets that are very close to the sensor within what is called the sensor’s deadband. In addition, because of the finite sensitivity of the receiving transducer, there is a distance beyond which the returning echo cannot be detected. This is the maximum range of the sensor. These constraints define the sensor’s useable sensing area.

Ultrasonic proximity sensors that have a discrete output generally have a switch-point

adjustment provided on the sensor that allows the user to set the target distance at which the

sensor output switches on. Note that ultrasonic sensors are also available with the analog outputs that will provide an analog signal proportional to the target distance.

Ultrasonic proximity sensors are useful for sensing targets that are beyond the very

short operating ranges of inductive and capacitive proximity sensors. Off the shelf, ultrasonic proximity sensors are available with sensing ranges of 6 meters or more. They sense dense target materials such as metals and liquids best. They do not work well with soft materials, such as cloth, foam rubber, or any material that is a good absorber of sound waves, and they operate poorly with liquids that have surface ripple or waves. Also, for obvious reasons, these sensors will not operate in vacuum. Since the sound waves must pass through the air, the accuracy of this sensors is subject to the sound propagation time of the air. The most detrimental impact of this is that the sound propagation time of air decreases by 0.17%/0C. This means that as the air temperature increases, a stationary target will appear to move closer to the sensor. They are not affected by ambient audio noise, nor by wind. However, because of their relatively long useful range, the system designer must take care when using more than one ultrasonic sensor in a system because of the potential for crosstalk between

sensors.

One popular use for the ultrasonic proximity sensor is in sensing liquid level. Fig

shows such an application. Note that since ultrasonic sensors do not perform well with liquids with surface turbulence, a stilling tube is used to reduce the potential turbulence on the surface of the liquid.

Optical Proximity Sensors

Optical sensors are an extremely popular method of providing discreate-output

sensing of objects. Since the sensing method uses light, they are capable of sensing any

objects that are opaque, regardless of the colour or reflectivity of the surface. They operate over long distances (as opposed to inductive or capacitive proximity sensors), will sense in a vacuum (as opposed to ultrasonic sensors), and can sense any type of material whether it is metallic, conductive, or porous. Since the optical transmitters and receivers use focused beam (using lenses), they can be operated in close proximity of other optical sensors without crosstalk or interference.

There are fundamentally three types of optical sensors. These are the thru-beam,

diffuse reflective, and retro-reflective. All three types have discrete outputs. These are

generally available in one of three types have discrete outputs. These are generally available

in one of three types of light source-incandescent light, red LED, and infrared LED. The red LED and IR LED sensors generally have a light output that is pulsed at a high frequency and a receiver that is tuned to the frequency of the source. By doing so, these types have a high degree of immunity to other potentially interfering light sources. Therefore, red LED and IR LED sensors function better than incandescent sensors in areas where there is a high level of ambient light (such as sunlight) or light noise (such as welding). In addition to specifying the sensor type and light source type, the designer also needs to specify whether the sensor output will be on or off when no light is received. Generally, this is specified as the logical condition

when there is no specified as dark-on and Dark-off

Thru-Beam (Interrupted)

The thru-beam optical sensor consists of two separate units, each mounted on

opposite sides of the object to be sensed. As shown in fig, one unit (the emitter) is the light

source that provides a lens-focused beam of light that is aimed at the receiver. The other unit,the receiver, also contains a focusing lens and is aimed at the light sources. Assuming this is a dark-on sensor, when there is nothing blocking the light beam, the light from the source is detected by the receiver, and there is no output from the receiver. However, if an object passes between the emitter and receiver, the light beam is blocked and the receiver switches

on its output.

Thru-beam sensors are the most common type known to the general public since they

appear in action movies in which thieves are attempting to thwart a matrix of optical burglar

alarm sensors setup around a valuable museum piece.

Thru-beam opto sensor works well as long as the object to be sensed is not

transparent. They have an excellent (long) maximum operating range. The main disadvantage with this type of sensor is that because the emitter and receiver are separates units, this type of sensor system requires wiring on both sides of the transport system (generally a conveyor) that is moving the target object. In some cases, this may be either inconvenient or impossible.

When this occurs, another type of optical sensor should be considered.

Diffuse Reflective (Proximity)

The diffuse reflective optical sensor, shown in fig 3-37, has the light emitter and

receiver located in the same unit. Assuming it is a dark-off sensor, light from the emitter is

reflected from the target object being sensed and returned to the receiver, which, in turn,

switches on its output. When a target object is not present, no light is reflected to the receiver

and the sensor output switches off (dark-off)

Diffuse reflective optical sensors are more convenient than thru-beam sensors because

the emitter and receiver are located in the same housing, which simplifies wiring. However, this type of sensor does not work well with transparent targets or targets that have a low reflectivity (dull finish, black surface, etc). Care must also be taken with glossy target objects that have multifaceted surfaces (e.g., automobile wheel covers or corrugated roofing material) or objects that have gaps through which light can pass (e.g., toy cars with windows, compact

discs). These types of target objects can cause optical sensors to output multiple pulses for each object.

Retro-Reflective (Reflex)

The retro-reflective optical sensor is the most sophisticated of all the sensors. Like the

diffuse reflective sensor, this type has both the emitter and receiver housed in one unit. As

shown in fig 3-38, the sensor works similar to the thru-beam sensor in that a target object passing in front of the sensor blocks the light being received. However, in this sensor does

not require the additional wiring for the remotely located receiver unit.

Generally, this type of sensor would not work well with glossy target objects because

they would reflect light back to the receiver just as the remote reflector would. However, this problem is avoided by using polarizing filters. This polarizing filter scheme is illustrated in fig.3-39 Notice in our illustration that there is an added polarizing filter that polarizes the exiting light beam. In our illustration, this is a horizontal polarization. In figure 3-39 above picture, notice that, with no target object present, the specially designed reflector twist the polarization angle by 900 and sends the light back in vertical poiazrition to allow the light retiring from the reflector to pass through and be detected by the receiver.

In figure 3-39 notice that when a target objet passes between the sensor and the

reflector, not only is the light beam disrupetd, but if the object has a glossy surface and

reflects the light beam. Since the receiver filter has a vertical polarizaion, the receiver does

not receive the light so it activates its output