EducationWhat is Encoder - Types of Encoders

What is Encoder – Types of Encoders

Category articles

In basic terms, an encoder is a kind of motion control sensor that supplies feedback to a control system. An encoder transforms the rotary or linear motion of a device part into an electrical signal, which is then read out to the control system, for instance making use of a counter or PLC on the machine. By utilizing an encoder, the exact position of device components, the angle as well as number of transformations of a motor shaft, the rotational rate or its direction can be identified.

Types of encoders

As a result of the dimension technique and also type of output signal, the complying with are distinguished
incremental encoders (rotary-pulse transducers).
– absolute encoders (rotating encoders).
Encoders are likewise divided right into rotating as well as linear. The previous are used to measure the angular position of an item, while the last (linear) determine its longitudinal motion, i.e. variation. Both rotating and also linear encoders can be incremental or absolute encoders.

Rotary encoders principle of operation

Incremental encoder

An incremental encoder produces a digital output signal as the shaft rotates through a defined angle. The number of signals (pulses) per revolution figures out the resolution of the encoder. The higher the resolution, the smaller angular displacements can be gauged, which converts right into greater dimension accuracy.

An incremental encoder does not produce an absolute placement. This implies that the placement of the shaft is figured out by counting electrical pulses. When it comes to lack of power supply, the position worth is calculated from zero, i.e. from the location in which it was stopped. The inner elements of an incremental encoder – to name a few factors because of the absence of resolution of the absolute shaft position – are simpler as well as much more affordable to make compared to absolute encoders.

An incremental encoder has at least one “A” output signal, yet typically there are 2 output signals, the supposed “A” and also “B” signals, offset from each various other by 90 °. Turning the shaft clockwise sends out the “A” pulse prior to the “B” pulse. Transforming the shaft counterclockwise will send the “B” pulse before the “A” pulse. This is just how the direction of rotation is figured out. There are also encoders with 3 output signals. The 3rd signal “Z”, described as “absolutely no” or “referral”, is just produced once when the disc is fully revolved as well as offers, for example, to establish the referral (datum) point of a machine or device.

Along with positioning, incremental encoders are typically used to determine speed. The placement relative to the beginning factor can be determined by counting the variety of pulses, while the rate can be calculated by dividing the number of pulses by the gauged time interval.
Taking into account the type of sensor used in their building and construction, incremental encoders are separated right into optical encoders and also magnetic encoders.

Optical encoder

In an optical incremental encoder, its major part is a code disc connected to a shaft, on which there are clear and also opaque areas. Light created by an LED illuminates the disc as well as goes through its transparent areas or is quit – in the case of nontransparent fields. A receiver (optoelectronic aspect) mounted on the back of the disc transforms the obtained light into an electrical signal. The kind of the encoder output signal relies on the adopted signal standard, usually it is an electronic (rectangular) signal.

Magnetical encoder

In a magnetic incremental encoder, each angular setting is identified by an electromagnetic field vector. The encoder has a magnetic disc attached to the shaft to generate the needed electromagnetic field. Furthermore, the encoder is equipped with a Hall sensor. Such a sensor is typically in the form of an integrated circuit. It includes a plate made from semiconductor material which is positioned in an electromagnetic field. A voltage is then put on it, forcing electrons to flow in the right direction, that is, vertical to the electromagnetic field lines. By measuring the voltage at the electrodes, which are placed vertical to the field lines and the direction of the electrons, it is very easy to determine the toughness of the electromagnetic field in which the sensor is located. The electromagnetic field leads to a Hall voltage, which is converted to an electronic output signal.
Magnetic encoders do not require direct call between the sensor and revolving axis. Therefore, they have a longer service life because of less relocating parts entering into contact with each other.
Depending on the construction there are built-in as well as open encoders. The former are outfitted with a dust- as well as water resistant casing, while the last do not have such a casing – the Hall sensor itself is in the type of an integrated circuit over which a magnet attached to the axis of a revolving object rotates.

Absolute encoder

The designs of absolute encoders and incremental encoders are virtually the exact same, the difference is just in the approach of measurement. With absolute encoders, the output signal is created in analogue kind by furnishing its disc with an one-of-a-kind code for each and every position. It guarantees that each angular setting of the shaft is assigned its own code value at the output. This is the so-called code signal.

Along with the added coding for one mechanical transformation, the disc of the absolute encoder likewise has an incremental track (transparent and also opaque dashboards). This suggests that both analog and electronic signals can be generated at the output.

Absolute encoders have the ability to give characteristic position values when they are activated and additionally instantly after a power failing. This is done by reviewing the current placement from the previously mentioned code signal placed on the dial. In absolute encoders, this can be a binary code or a Gray code. The advantage of Gray’s code is altering just one bit for nearby settings.

There are single and multi-turn encoders on the marketplace. The very first of them distinguish the angular placement just within one full transformation, i.e. from 0 ° to 360 °. A multiturn encoder has to be used in order to get information about a greater number of turnings. In its mechanical part, generally a system of gears is set up, which develop a relation in between multiple code discs, to ensure that it is possible to store the number of done turnings.

Measurement value retention

Making use of mechanical components (equipments) in an encoder permits the current position value to be stored, no matter whether the encoder is powered or otherwise. This remedy results in a rise in the dimension of the encoder, including a rise in moving components which put on out or can end up being harmed. This is why side remedies are also utilized, such as using an incremental encoder with battery back-up or integrated generator. Battery back-up ways that in case of a power failure, a lithium battery in the cable not only supports the current position however additionally sustains the operation of the entire encoder. This indicates that the incremental encoder can also track any type of adjustment in the setting of an axis when the power supply is switched off.

Linear encoders principle of operation

The general operating principle of linear and also rotary encoders is extremely comparable. Coded disk, in case of linear encoders is changed to linear kind (range).
Figuring out the longitudinal displacement of a machine element is feasible by checking out the lines positioned on a range with a graduation scale, which alter their position throughout motion. An angular encoder, on the other hand, gauges angular position by reading lines on a rotating disc with a range that likewise turn throughout motion. Both linear and also angular encoders can be read in 2 ways – optically or magnetically.

Algorithm of linear encoder operation in a CNC machine tool

In the construction of CNC machine devices linear and also rotary encoders with high determining precision are used. They permit the execution of a feedback loop, educating concerning the placement of the pin and the table, in the coordinate system of the machine.

Formula of encoder procedure:

1. The light beam goes through an aperture in which holes are made in the correct position and also shape.
2. The source of light experiences the darkened and clear photoelectric aspects used on the range (tape).

The photoelectric elements detect the fields whereby the light passes and also send the current placement information in the form of electrical impulses.

The advantage of the absolute linear encoder is that a brand-new strategy to the reference factor is not required when reactivating or throughout a voltage decrease in the machine. This means that the machine memorizes its placement with the encoder. Absolute encoders do not call for additional reference marks as with incremental encoders.
In CNC machine devices, along with absolute linear encoders, incremental linear encoders are likewise utilized. The distinction in their construction as well as operation is analogous to rotating encoders. An instance can be an incremental linear encoder – optical. Linear encoder includes a series of slots (dashes) with proper graduation. An aperture relocates relationship to the range, as well as behind it there is a source of light emitted from a light bulb or a LED. The aperture’s function is to focus and also direct the diffused light beam onto the previously mentioned slits of the range. The existence of the slits causes the photoelectric aspects to light up, and therefore an electrical impulse is created. The position is read by summing the pulses, equal to the number of slits showed (traversed) by the aperture. 2 signals, An and B, offset from each other by 90 °, are produced to establish the instructions of the offset.

How to wire an encoder

Mechanical connections

All encoders operate the basis of mechanical motion, which is exchanged an electrical output signal. Flanges, grooves or holes allow the encoder to be swiftly as well as easily linked to a rotating machine element.

While touching upon the topic of mechanical links of encoders, it deserves stating that in case of rotary encoders we distinguish between shaft encoders which can be attached to the drive shaft e.g. using a combining, and hollow encoders which in turn can be placed straight on the shaft. In a few of the birthed models, the bore and also shaft sizes can quickly be adapted by using reduction bushings, for instance.

Incremental encoders – types of output signals

Incremental encoders usually produce two 90 ° offset rectangle-shaped signals that can be made use of to determine the position as well as direction of a things. As a result of their physical construction, different kinds of results are made use of. Therefore, the primary kinds of output interfaces consist of type results:

  • TTL (Line Driver),
  • HTL (PushPull),
  • Open Collector,
  • SIN/COS.

For incremental encoders, the most common outputs are TTL and HTL. The TTL (Line Driver) outputs provide a high response frequency and very good noise immunity.

Fig.2 Schematic of TTL (Line Driver) output

HTL (PushPull) outputs, on the other hand, are also characterized by very high noise immunity, but they lose information exchange speed and also increase power consumption.

Fig.3 Scheme of HTL (PushPull) type output

The Open Collector interface is cheap and very simple, but has many limitations. It is usually used in applications with low output frequency.

Fig.4 Open collector output schematic

In the SIN/COS outputs, the pulse signal is converted into 2 features – sine (signal A) and cosine (signal B). This permits exact placing as well as high-resolution readout, in any way times.

Incremental encoders with SIN/ COS output obtain the electrical signal likewise to those described above – the mechanical component remains unmodified. Nonetheless, they have advanced technique of electronic signal handling, gotten from the mechanical system. This technique consists in converting the impulse signal into sine (Phase A) as well as cosine (Phase B) operates – see Fig. 5. The conversion is done by examining the time of change of the stage states (signals) An and also B. For instance, having an optical incremental encoder with a disc having 2048 ports, 2048 pulses of the A (B) signal are produced. Then each single pulse is transformed – within the variety of one duration of the trigonometric function – right into 2048 stages of the periodic sine (cosine) function. As a result, such an encoder can attain a precision of approx. 4 million impulses per transformation (2048 x 2048 = approx. 4 million).

The output of a SIN/COS encoder can be either a pulse signal prepared for handling in the control system or an analog SIN/COS signal that is only converted on the automation system side.
Data transmission in incremental encoders happens through electronic outputs. As a result of the set number of signal lines and the transmitted data (binary), the circuitry will normally have a comparable configuration, depending on the number of signals.
Some incremental encoder designs, in addition to the 3 rectangle-shaped signals A, B, Z, are also furnished with signals marked/ A,/ B,/ Z. These signals are the negation of the A, B and Z outcomes. Their analysis by the electronic circuit enables to verify the accuracy of interaction. To reduce electrical disturbance, the signals are normally transmitted in sets (A+/ A, B+/ B, Z+/ Z) by means of twisted set cord. To put it simply, including the negated signal permits to get a differential signal, which is much more resistant to interference. In such a circuit, for that reason, a maximum of 6 cords are required, one for transmission of each signal. Below is a diagram showing the waveform of the signals pointed out (Fig. 5):.

Fig.5 Output signals of incremental encoder

Absolute encoders – types of output signals

Absolute encoders provide the exact position in real time by using binary or Gray code. Information about the current position can be sent by the encoder electronics:

– as a parallel binary signal

– As serial binary signal

– Via analog output

– Via the communication bus

The transmission of a parallel signal is faster than a serial signal. This results from the truth that each transmitted bit has its own electrical interface, which at the same time makes it essential to make use of a separate cable television – for every bit of the placement code. For encoders with huge resolutions, multi-core transmission cables should be used, which at longer distances generates a big price and also makes complex installment. For this function, identical links are abandoned in favor of serial links whenever possible. Signal transmission in series is slower than parallel, but also minimizes the variety of cables, which in turn equates right into price decrease as well as convenience of setup.
Nowadays absolute encoders with serial electronic user interfaces such as SSI, ISI, EnDat, BiSS, Hiperface are one of the most common.
There are likewise encoders with network outcomes on the market which function straight with an industrial network like PROFIBUS or PROFINET. In this case the code signal – identifying the position – is sent out to the controller as a whole through the communication protocol.

Where are encoders used?

Encoders are now commonly utilized devices, both in industry as well as typical home appliances. Below are some instances of how encoders are used:
– in printing applications – feedback from the encoder triggers the printhead to produce a mark at a specific area
– in a lift, encoders inform the controller of the appropriate setting when the elevator reaches the right flooring. Encoder feedback likewise ensures that the elevator doors are open when the correct flooring is gotten to. Without the encoder, you would just be able to go into or exit the lift instead of merely relocating to another level.
– In precision servo systems, the encoder signal is used by the PLC to manage the speed as well as setting of machine components
– on automated assembly lines, encoders supply feedback to robotics concerning motion and also setting. For an automotive production line, for instance, encoders offer automatic welding arms with details concerning the placement in which they are to bond
– In cranes – such as a crane – encoders placed on the electric motor shaft provide placement feedback so you understand when to raise or launch the tons
– In applications where containers or jars are filled, the encoder offers the filling equipments the position of the containers
In any application outfitted with an encoder, the control process follows the exact same principles: the encoder sends electrical pulses to the control system, which then sends out signals to the machine or application to carry out a set feature. Control occurs in a shut circuit, by means of feedback.

Michal Pukala
Electronics and Telecommunications engineer with Electro-energetics Master degree graduation. Lightning designer experienced engineer. Currently working in IT industry.