Rotary encoder#Incremental rotary encoder

As an incremental sensor for detecting location changes ( linear) or angle changes referred to (rotating), able to detect the distance and direction of travel and angular displacement and direction of rotation. Other names are incremental encoder, shaft encoder or general encoder. Compared to continuous measurement systems such as servo potentiometers have an incremental scale with repetitive, periodic ticks. The measurement is based on a determination of direction and a count ( Digital Instruments). The most frequently used rotary optical encoder.

Incremental encoder must be referenced (as opposed to absolute encoders ) after turning if necessary, as changes not the position in the off state can be detected.

Typical applications include determining the position in automation and controls of electronic devices ( " endless digital potentiometer" ).

Operation

Incremental encoders with sliding contacts, work photoelectrically, or magnetic. They provide the output is always two 90 degree mutually phase-shifted signals ( see section Signal processing ), can be determined based on the direction of rotation and angle.

Sliding contacts

Incremental encoder with sliding contacts work in principle like a rotary switch. Common resolutions are about 32 positions per revolution. Disadvantage of the economic solution with sliding contacts is the mechanical wear. Mechanical incremental encoders are therefore used only for occasional operation, such as for digital knobs. If necessary, the contacts must be debounced electronically or in software. Often on axial actuation also a button Contact realized about to switch on / off, confirms an entry, etc ...

Of advantage is the low quiescent current consumption, which may be zero with a suitable design of the locking positions and thus is suitable for remote controls.

Photoelectric scanning

In the photoelectric scanning a distinction between the imaging scanning principle, which is suitable for grating periods down to 20 microns and the interferential scanning principle allows the grating periods down to 4 microns.

To Be or fluoroscopy of the grid a supply current in the milliampere range is required, so that optical methods are less in battery-powered appliances.

Imaging scanning principle

There is a slotted plate between the light emitting diode and two slightly staggered photodetectors. Now this disc rotates, the two photodetectors are alternately illuminated. From these two output signals of the receiver is the rotational direction, and counts the pulses. Quality measuring systems use four sensors, each connected to two anti-parallel in order to obtain a defined zero crossing, and to compensate for drift and aging.

In the multi- field scanning a light beam (usually an infra-red light emitting diode ) is generated by a light source (usually a phototransistor ) is passed through a condenser, a like primed scanning and a screen ( measuring scale ) on a photo- optical device. The scanning grating bears a slightly different pitch, so that a magnified image of the division by the moire effect in the photodetectors. Simple designs, as used for example in computer mice, waive condenser and scanning.

Another measurement principle is the single-field scanning the incident light. In this case, the light beam by the condenser, and a scanning plate, which is provided with two nested phase gratings, directed to a measuring scale. The light beam is then reflected on the scale and the back through the diffraction grating. Once again the four phase-shifted images of the scanning are designed and evaluated as in the multi-field scanning. The method is applicable to linear scales, as it tolerates a slight waviness of the scale and also to a slight local pollution is insensitive.

Interferential scanning principle

Using this principle, the phenomenon of diffraction is utilized in a grid in order to generate a measuring signal. This method is applied to high-precision incremental linear measuring systems. To the light beam is passed through a condenser to a measuring scale, reflected and returned through the condenser to the photoelectric sensors. The supporting a transparent phase grating scanning ensures that three diffracted beam components ( diffraction order -1, 0, 1 ) are generated. After reflection from the measuring scale also a phase grating carrying the scanning plate is passed again through the beam, the zeroth diffraction order is canceled. Subsequently, the ± 1 Diffraction order as shown on three photo-elements that these produce a shifted by 120 ° signal there. These three signals are then converted into a series electronics in industrial grade 2 signal shape.

Magnetic scanning

In an incremental measuring system with magnetic scanning the material measure consists of a hard magnetic carrier, was enrolled in the magnetization by a pitch ( a Flywheel or magnetic tape ). Typical magnetic graduation periods are between 0.5 mm to 5 mm, but larger divisions may be realized and used as incremental. The reading of the magnetic coding can be done by Hall elements or magneto-resistive sensors. With the two different technologies can be a contact-free readout of the magnetization realize. The magnetic sensing is used when the measuring system can not be encapsulated with acceptable effort. It can be made insensitive to liquids and dirt. Ferromagnetic foreign bodies in the gap between the sensor and measuring scale can interfere with the measurement principle. The negative influence of external magnetic interference is dependent on the selected sensor technology, sensor design, and magnetic field strength of the measuring scale.

For the operation of the sense amplifier, only a small quiescent current in the microampere range is required, so that this method for battery-powered devices with Auto Shutdown is well suited for example for digital calipers.

Gear wheel encoder

A further possibility is the scanning of a toothing of unmagnetised ferromagnetic material (such as gears or toothed racks of iron ) using one or more charged with the DC inductor. Either a special measuring gear mounted or used already existing structural elements as measuring. The scanning head has a right to the toothing iron core. Two other coils ( or field plates, or GMR ) sensors register the change in magnetic field, which is evaluated according to polarity and phase. The accuracy is dependent on the pitch and the diameter of the gear, but generally lower than with optical encoders. At higher speeds and dirt passive inductance transmitters are used.

Signal evaluation

With a movement of the two sensors emit two with 90 ° phase-shifted signals. Moves the measuring scale to the right, the signal of channel A over channel B by 90 ° mates first. In the other direction, the signal of the channel A on the channel B by 90 ° lagging. This waveform is repeated for each line of the measuring scale. This section - a bright, dark patterns, 0 ... 360 ° - is also called pitch period.

Special DOWN counter determined from these two signals the direction and count the pulses. This clears the way or angles are measured directly. From the difference between two counts is speed and direction can be determined. In the simple analysis, a dash represents a counter - increment. This is often used in digital knobs, in order to get a hysteresis. In the 4-way analysis of the counter is changed at every edge. This corresponds to a 4 stroke counter increments ( see paragraphs on the right). If the incremental encoder is exactly on the border of an edge, but can constantly through the smallest vibrations counter changes occur. These four times finer resolution is used in many positioning or angle measurements.

Continuous output signal levels are TTL, HTL (both square waves ), and 1 Vpp and 11 uA ( analog sin / cos signals).

For sensors that produce a sinusoidal signal, and an interpolation of the signals may take place, which allows an increase in resolution. A sinusoidal signal can be up to 1024 -fold interpolated. Thus, the quantization error is reduced, and the calculation of the current speed is more accurate. Depending on the version found this interpolation takes place within the sensor unit. Depending on the measuring system precisions to 0.1 microns can be achieved.

If the encoders are used in a servo motor, it still usually features additional tracks. These usually provide tracks for commutation Represent may from analog to 90 ° ( a quarter turn of the motor shaft ) staggered sine / cosine signals (so-called C / D track or Z1 track ) or consist of partial circles in the block commutation.

Referencing

The incremental measures after connecting the power supply only changes with respect to the power-on position. In many applications, but the knowledge of the absolute position is required. Therefore, give the most angular encoders per revolution on a third output from a reference pulse ( zero pulse, reference mark ) once. After turning the encoder must be rotated so long been recognized to the reference pulse. At the latest after one revolution then is the absolute angle available.

Positioning systems with incremental encoders lead after turning so-called homing to an external position sensor (eg limit switches ) from. From this point of the next reference pulse of the incremental encoder is used as a precise reference point.

For incremental length measuring systems, there are several variants:

• A reference mark in the middle
• Two reference marks 25 ... 45 mm from the beginning and end of each measuring length
• Several reference marks every 50 mm ( sometimes one of them with interchangeable magnetic stripe selectable)
• Distance-coded reference marks

If the reference markers distance coded, the absolute position after traversing two successive reference marks. The distance between two reference marks is different over the entire movement. For example, at a pitch period of 20 microns: | ← 10,02 → | ← 9.98 → | ← 10,04 → | ← 9.96 → | ... mm. From this the absolute position can be calculated and an external reference switch is not necessary.

Applications

• The opto- mechanical computer mouse, the movements of the track ball are transferred to two incremental encoder for the X- and Y -axis. At the wheel, there is another incremental encoder.
• For devices in consumer electronics, machine tools and measuring instruments knobs take with incremental encoders often the digitally controlled functions of a potentiometer ( " Jog Dial" ), setting parameters or menu selection.
• Among other things, in ink jet printers measure linear encoder, the position of the print carriage. For this purpose, one attached to this sensor moves along a provided with strokes, tightened band
• In industrial environments, incremental encoders are used to measure distances, speeds or angles of rotation on machine tools, in the handling and automation technology and measuring and testing equipment.
• In surveying incremental encoders are used since the 90s in electronic theodolites, where they replace the previous pitch circles of glass. The technique of the relative angle determination where it is known Inkrementalverfahren, in contrast to the procedure code when using absolute encoders.