In this tutorial, the different kinds of devices which are tagged input devices are discussed. For the purpose of this text, they are specifically termed “Sensors”. However, we will focus our attention on sensors that helps us keep track of position and displacement. These types of sensors are called position sensors.
As indicated by their name, position sensors identify the position of things, which means they are referenced either from or to a fixed position or point. Such kinds of sensors deliver a “Positional” response.
A way of ascertaining a position is to use either “rotation” (angular movement) or “distance”, which could be the distance between two positions like the distance moved or travelled from a particular spot. For instance, the rotation of a robot’s wheel to ascertain the distance it has covered on the ground. All the same, Position Sensors can identify an object’s movement in a straight line through the use of Linear Sensors or by its angular movement via Rotational Sensors.
The Potentiometer is the most widely used of all “Position Sensors” due to the fact that it is inexpensive and easy to use. It features a wiper contact connected to a mechanical shaft that can either be linear (slider type) or angular (rotational) in its movement, and which alters the resistance value between the slider/wiper and the double end connections, sending an electrical signal output that has a proportional relationship between the real wiper position on the resistive track and its resistance value. Essentially, resistance is proportional to position.
Potentiometers are available in a vast array of sizes and designs like the generally available round rotational kind or the flat and lengthier linear slide kinds. When applied as a position sensor, the mobile object is linked directly to the slider or rotational shaft of the Potentiometer.
A DC reference voltage is applied across the two outer fixed connections making the resistive element. The sliding contact’s wiper terminal produces the signal.
The result of this configuration is a potentiometer circuit output that is proportional to the position of the shaft. Subsequently, for instance, if you apply a voltage of about 10v across the resistive element of the potentiometer, the highest output voltage would be the same as the supply voltage at 10 volts, with the least output voltage being 0volts. Thus, the potentiometer wiper will adjust the output signal from 0 to 10 volts, with 5 volts showing that the slider or wiper is at its mid-way or centre point.
The output signal (Volt) from the potentiometer is gotten from the centre wiper connection as it progresses along the resistive track, and is proportional to the angular position of the shaft.
Although resistive potentiometers have several advantages – simple to use, inexpensive, low tech etc., they also have numerous downsides such as poor repeatability, poor accuracy rating, quick wear as a result of moving parts and restricted frequency feedback.
However, there is a major downside to using the potentiometer for sensing position. The scope of movement of its slider or wiper (and consequently the output signal gotten) is restricted to the physical size of the potentiometer being utilised.
For instance, a single turn rotational potentiometer usually just has a fixed mechanical rotation of between 0o and about 240 to 330o maximum. Still, one can get multi-turn pots of up to 3600o (10 x 360o) of mechanical rotation.
Most kinds of potentiometers utilise carbon film for their resistive track but these kinds are electrically raucous (the hiss on a radio volume control), and have a brief mechanical life as well.
Rheostats or Wire-wound pots (as they are also called) in the form of a wound coil or straight resistive wire can be utilised as well. However, wire wound pots experience resolution issues as their wiper moves from one wire segment to the next, making a logarithmic (LOG) output, causing errors in the output signal. These also experience electrical noise.
For high accuracy low noise applications, conducive plastic resistance element type polymer film or cermet type potentiometers can now be gotten. They are smooth and they do not produce much noise. They are also highly durable and have high resolution values. They can also be gotten as both single turn and multi-turn devices. Steering wheels, computer game joysticks, industrial and robot applications are some applications utilising this sort of high precision position sensor.
Inductive Position Sensors
Linear Variable Differential Transformer
This type of position sensor does not experience mechanical wear issues. This inductive type position sensor operates on the same principle like the AC transformer that is utilised in determining movement. It is an incredibly device for gauging linear displacement and whose output is proportional to the position of its transferrable core.
It essentially comprises of three coils looped on a hollow tube former, one making the primary coil and the other two coils making matching secondaries electrically linked in series but 180º out of phase either part of the primary coil.
A moveable soft iron ferromagnetic core (occasionally referred to as an “armature”) which is linked to the object being evaluated, glides, or moves up and down within the tubular body of the LVDT.
A little Alternate Current voltage is applied to the main winding. This AC voltage is called excitation signal. The application of this voltage causes an electromotive force signal in the next two secondary windings (transformer principles).
The armature made from soft iron is precisely placed in a null position between the windings and tube so that they are 180 degrees out of phase. By this arrangement, the positions of the secondary windings nullify each other. Consequently, the core is moved a bit to one side or the other from this zero or null point, the caused voltage in one of the secondary’s will get higher than the other secondary and an output will be generated.
The direction of the moving core and its displacement determines how polar the output signal is. The output signal increases with increase in motion of the soft iron core as it moves away from its null position at the centre. The output is a differential voltage which has a linear variation from the position of the core. Hence, this kind of position sensor has an output signal with these dual characteristics; a polarity showing the movements direction and an amplitude which is as a result of the displacement of the core.
The output signal phase can be likened to the phase in which the primary coil is excited. This helps electronic circuits like the AD592 Linear Variable differential transformer sensor amplifier to determine the part of the coil housing the magnetic core and thus determine the travel direction.
When there is movement of the armature via the centre position from an end to another, there is a change in the voltage of the output from the highest to zero and right back to the highest again.
However, the phase angle is changed by 180º in the process. For this reason, the LVDT is enabled to give out an output AC signal with a magnitude that is representative of how much movement there is from the centre position. Also, its phase angle is representative of the core’s direction of motion.
A good example of where a linear variable differential transformer (LVDT) sensor can be used is as a pressure transducer. Here, a force is produced by the pushing of the measured pressure against a diaphragm. The sensor then converts the force into a voltage signal that can be read.
In comparison with a resistive potentiometer, some of the advantages of the linear variable differential transformer (LVDT) are that it has excellent voltage output to displacement or linearity, highly accurate, has a great resolution, is highly sensitive and operates without friction. In addition, they can be used harsh environment.
Inductive proximity sensors
The inductive proximity sensors which may also be referred to as an Eddy Current Sensor is another kind of inductive position sensor that is commonly used. Although, these sensors do not measure angular rotation or displacement, their main use is to detect an object in close range with them. This is why they are called proximity sensors. These sensors are position sensors that do not require contact but make use of the magnetic field to detect objects using the reed switch which is the most basic magnetic sensor.
An inductive sensor has a coil wound round an iron core inside a field of electromagnetism to give rise to an inductive loop.
Placing a ferromagnetic object inside an Eddy current field that is produced around the inductive sensor like a ferromagnetic metal plate or screw significantly changes the coil’s inductance. The change generating an output voltage is detected by the detection circuit of the proximity sensors. Hence, the electrical principle of Faraday’s law of Inductance is what fuels the inductive proximity sensors.
There are 4 main components of an inductive proximity sensor namely
- The oscillator producing the electromagnetic field.
- The coil producing the magnetic field
- The detection circuit that picks any field changes when there is encroachment by an object.
- The output circuit generating the output signal using normally open (NO) contacts or normally closed (NC) contacts.
With inductive proximity sensors, metallic objects in front of the head of the sensor can be detected while making contact with the detected object physically. Therefore, they are excellent for usage in wet or dirty environments. Proximity sensors have very small “sensing” range of about 0.1mm to 12mm.
In addition to industrial use, inductive proximity sensors are used normally to control traffic flow by changing traffic lights at cross roads and junctions. Inductive wire loops that are rectangular in shape are sunken deep into the road surface of the tarmac.
On passing over the inductive loop, cars or other road vehicles have their loop inductance changed by the vehicle’s metallic body and the sensor is activated. This signals the traffic light controller that there is a waiting vehicle.
A major disadvantage of these position sensor types is that they sense metallic objects from all directions otherwise said to be “Omni-directional”.
In addition, non-metallic objects are not detected by these sensors. But there are ultrasonic and capacitive sensors. Other magnetic positional sensors available are hall effect sensors, variable reluctance sensors and reed switches.
Rotary Encoders are a kind of position sensor that look like the potentiometers discussed before. However, these encoders are non-contact optical devices utilised in changing the angular point of a rotating shaft into digital or analogue data code. That is to say, they change mechanical movement into electrical signal (digital preferability)
Every optical encoder operates on the same fundamental principle. Light from an infrared of LED light source is moved through a rotating high resolution encoded disk that has the needed code patterns, either grey code, BCD or binary. The disk is scanned by photo detectors as it rotates and the information is processed by an electronic circuit into a digital form as released binary output pulses are emptied into controllers or counters which ascertain the correct angular point of the shaft.
The two fundamental kinds of rotary optical encoder are Incremental Encoders and Absolute Position Encoders.
It is also called quadrature encoder. Some people also refer to it as a relative rotary encoder. Of all position sensors, it is the easiest to manipulate. The output they generate is a series of square wave pulses made by a photocell arrangement like the codes disk, with properly spaced dark and transparent lines termed segments in its surface, rotate or moves beyond the light source. The encoder induces a release of square wave pulses, which when numbered, shows the angular point of the rotating shaft
Incremental encoders possess two individual outputs known as “quadrature outputs”. Both outputs are displaced at 90 degrees out of phase from each other and the direction of the shaft’s rotation is measured from the sequence of the output.
The amount of dark and transparent sections on the disk shows the device’s resolution and raising the amount of lines in the pattern raises the resolution per degree rotation. The usual encoded discs have a resolution of close to 256 pulses or 8-bits per rotation.
The easiest incremental encoder is known as a tachometer. It possesses a solitary square wave output and is typically utilised in unidirectional applications where just basic speed or position information is needed. The “Sine Wave” or “Quadrature” encoder is generally used and comes with two output square waves. These waves are usually known as channel A and channel B. The device makes use of double photo detectors, a bit displaced from themselves by 90o thus making two individual cosine and sine output signals.
Simple Incremental Encoder
Via the Arc Tangent mathematical function, the angle of the shaft can be measured in radians. Typically, the optical disk utilised in rotary position encoders is circular in nature. Thus, the output’s resolution will be stated as θ = 360/n, where n is the amount of sections on coded disk.
Consequently, for instance, the amount of sections needed to provide an incremental encoder with a resolution of 1o will be 1o = 360/n, thus n = 360 windows, etc. In addition, the rotation’s direction is ascertained by taking note of which channel first delivers output, either channel B or A providing two rotation directions. B leads A or A leads B.
A major downside to using incremental encoders as a position sensor, is that they need external counters to ascertain the complete angle of the shaft inside a particular rotation. If the encoder skips a pulse because of a dirty disc or nose or if power temporarily goes off, the subsequent angular information will give an error. A way of surmounting this downside is to utilise absolute position encoders.
Absolute Position Encoder
Absolute Position Encoders are more complicated than quadrature encoders. They offer an inimitable output code for each position of rotation showing both direction and position. Their coded disk comprises of numerous concentric “tracks” of dark and light sections. Each track is self-sufficient with its individual photo detector to concurrently read an exclusive coded position value for every movement angle. The amount of tracks on the disk matches the binary “bit” resolution of the encoder so a 12-bit absolute encoder would possess 12 tracks and the same coded value shows up just once in each revolution.
4-bit Binary Coded Disc
A major benefit of an absolute encoder is its non-volatile memory which recalls the precise position of the encoder without having to go back to a “home” point if power crashes. Majority of rotary encoders are identified as “single-turn” devices. However, multi-turn devices that get responses over multiple revolutions by including extra code disks, can be gotten.
Usual application of absolute position encoders is in computer hard drives and CD/DVD drives where the absolute position of the drives write/read heads are observed or in plotters/printers to correctly place the printing heads on top of the paper. We have discussed multiple examples of sensors that can be utilised in ascertaining the presence or position of objects in this Position Sensors tutorial. In the next one, we will be dealing with sensors used to gauge temperature like thermostats, thermocouples, thermistors, which are typically termed Temperature Sensors.