An incremental encoder produces an output digital signal when the shaft turns at the specified angle. The amount of output signals (pulses) every revolution defines the precision that the encoder has. The greater resolution, the lower displacements of angular amplitudes can be recorded which results in higher precision in measurement.
A incremental encoder doesn’t provide an exact position. The position that the shaft takes is calculated by counting electrical impulses. In the event of a lack in power, value of the position is calculated using the zero point, i.e. from the location in the point where it stopped. The internal parts that make up an incremental encoder – in addition to other reasons related to the inability of ability to determine the exact shaft’s position are more simple and less costly to make than absolute encoders.
An incremental encoder is equipped with at the very least only one output sign “A”, but usually there are two output signals, referred to as “A” and “B” signals, separated from each opposite by 90deg. When the shaft is turned clockwise, it will send”A” or the “A” pulse before the “B” pulse. The opposite of turning the shaft clockwise will trigger”B” pulse before the “A. “B” pulse before the “A” pulse. This is the way in which the direction of rotation is determined. There are encoders that have three signals for output. A third output signal “Z”, referred to as “zero” or “reference” only occurs when the disc is completely turned and is used as an example to identify that the point of reference (datum) location of a device or machine.
Alongside the ability to position, these encoders are frequently employed to measure speed. The location in relation to the beginning point is determined by taking how many pulses while speed can be determined by multiplying each pulse by the time interval.
Based on the kind of sensor utilized for their development The incremental encoders are classified into magnetic encoders and optical encoders.
Incremental linear encoder
The general operating principle of linear and rotary encoders is very similar. The code disc, in the case of linear encoders, is converted to a linear form (scale).
The displacement along the length of a machine part is measured by taking note of the scale’s bars that move when moving. An angular encoder in contrast is used to measure angular displacement by taking lines from a rotary disc that has a scale which can will also shift position in motion. Both types of encoders can be read two ways: optically and magnetically.
Algorithm of linear encoder operation in a CNC machine tool
When building CNC machine tools , linear and rotary encoders that have excellent accuracy in measuring are used. They permit the creation of the feedback loop which informs about the position of the spindle as well as the table within the machine’s coordinate system. Figure 1 illustrates the working that a linear encoder can perform absolute.
Algorithm for encoder operation:
A beam of light is reflected through an aperture which holes are created in the right direction and in the correct.
The light source is reflected by the opaque and darkened photoelectric elements that are placed on the scale (tape).
The photoelectric elements recognize the areas through which light travels and send the information about the current position by way of electrical impulses.
The benefit of using an absolutely linear encoder is the fresh method of locating the reference point isn’t needed when the machine is restarted or after an electrical drop within the machine. This means that the machine can remember its location using the encoder. Absolute encoders don’t require any additional reference marks, as do incremental encoders.
In addition to absolute linear encoders linear encoders that are incremental can be utilized for CNC machines. The distinction in their design and operation is comparable to the rotary encoders. One example is one that is an incremental linear encoder, also known as optical. Linear encoders consist of an array of slot (dashes) with the appropriate graduation. The aperture is moved with respect to the scale and behind it is a light source that is released from a lamp or LED. The function of the aperture is to direct and focus the light beam to the slits mentioned above of the scale. Slits’ presence triggers the photoelectric elements to light and consequently an electrical impulse is created. The position is determined by summing the impulses in proportion to the number slits that are displayed (traversed) via the opening. Two signals both A and B which are offset 90deg apart are generated in order in order to identify the displacement direction.
Incremental quadrature encoder
The principal elements of these types of encoders with incremental angles are the circular glass plate that is covered by translucent and transparent stripes, which form the scale, and an optical system. It is comprised of an illumination source (LED) and an focusing lens to one end of the plates, and an optical detector (photodiode) on the opposite side.
The disc is attached to a shaft and as it spins around it, it traverses the light rays, which are block by transparent scale fields, or, when they penetrate the plate, they fall onto the photodetector. Depending on the quantity of light hitting its photosensor, the strength the current it outputs alters. The output signal from the encoder is composed of a sequence of zero pulses, which are analyzed in the electronic circuits that controls the conversion.
When you relate the results obtained by this method with the amount of pulses required for one full rotation the angle of rotation can be calculated. If, for instance, 1000 pulses equal 360deg rotation, then every pulse that is counted corresponds to the equivalent of a 0.36deg turn that the shaft is experiencing. To identify the direction of movement it is complemented by a second sensor, which produces two phase-shifted pulse patterns (quadrature signals).
It is possible to determine the direction by analyzing the delay of each. The primary distinction between incremental and absolute encoders is the kind of output signal that they generate. In the first instance, it’s a pulse however for absolute encoders, the results are in the appearance of an encoded word.
This is because the dial’s graduation is done by using an actual binary code, that is known as a Gray code. Furthermore, these types of encoders as opposed those that use incremental encoders preserve the data about the previous angle of the dial before turning the power supply off again.
Incremental encoder signals
Incremental encoders typically generate two 90° offset rectangular signals, which can be used to determine the position and direction of movement of an object. Due to their physical construction, different types of outputs are used. Thus, the main types of output interfaces include type outputs:
- 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.
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.
The Open Collector interface is cheap and very simple, but it has many limitations. It is usually used in applications with low output frequency.
In the SIN/COS outputs it is transformed into two functions : sine (signal A) and cosine (signal B). This allows for precise positioning as well as high-resolution readout in all times.
Incremental encoders that have SIN/COS output receive the electrical signal similar as the above described – the mechanical component remains the same. They do however, have a an advanced method of digital signal processing that is derived through the mechanical. This process involves the conversion of the impulse signal into sine value (Phase A) and cosine function (Phase B) (see Fig.5. The conversion is performed by studying the time of the change in phases (signals) both A and. In the case of an optical encoder that has 2048 slots and 2048 pulses of the (A) and (B) signals are produced. Each single pulse is transformed – within a time-span of one of the periods of the trigonometric function to 2048 cycles of the periodic sine (cosine) functions. Thus, an encoder has an accuracy of approximately. 4 million impulses per revolution (2048 x 2048 = approx. 4 million).
An encoder’s output could be a pulse signal that is ready to be processed by the control system, or an analog signal that is converted only by the automation system.
The transmission of data in increment encoders is accomplished through digital outputs. Due to the set amount of signals as well as the data transmitted (binary) the wiring usually has the same configuration, based on the quantity of signals.
