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INTERFACE OF AN INCREMENTAL ENCODER: OPERATION AND CHARACTERISTICS

incremental_interfaces_encoders
INTERFACE OF AN INCREMENTAL ENCODER

Incremental encoders generate a square wave output whose state changes each time the shaft rotates by a certain angle. The number of peaks per revolution defines the resolution of the sensor. Unlike the absolute encoder, the incremental encoder does not provide an absolute value of the position (only relative). This has the advantage of making the sensor less expensive because it is less technologically complicated.

The first application of incremental encoders is position tracking. This is done by counting the number of peaks related to the initial position.

But they can just as easily be used to measure the speed of a component. This is done by calculating the ratio between the number of peaks and the total operating time.

Operating principle of an incremental encoder :

incremental_pulse_diagram
Operating principle of an incremental encoder

Characteristics of the incremental encoder:

PPR (Pulses per revolution) :

An incremental encoder has an output signal composed of a certain number of peaks which can be assimilated to 1s, the valleys being assimilated to 0s, the peak-trough assembly forming a period. The greater the number of periods, the smaller the physical angle between each of them. This number, called PPR, is usually fixed for an incremental encoder. However, the PPR of programmable encoders can be adjusted to the desired value through the use of programming software.

Output driver circuit:

The majority of incremental encoders today have a Push-Pull (HTL) or RS422 (TTL) output driver circuit. These have replaced most of the old circuits such as NPN collector, PNP collector and voltage output.

A) Push-Pull (HTL)

Push-Pull (HTL) circuits, also called Totem Pole, generate a voltage level corresponding to the supply voltage. This is generally between 8 and 30 VDC.

push_pull_1
Output driver circuit

By using the appropriate connections, it is possible to replace an outlet manifold with a Push-Pull. This is done by connecting an extern diode so as to limit the direction of the current.

B) RS422 (TTL)

RS422 (TTL) circuits provide a constant voltage of 5 V independent of the supply voltage. It is possible to choose a different supply voltage between 4.75 and 5.5 VDC (which can be used to replace the output drivers) or between 8 and 30 VDC. By using differential signals, the output then fully complies with the RS422 standard.

rs422__ttl
RS422 (TTL)

A differential output has better high frequency response and exhibits better noise reduction. However, it should be ensured that the receiver is also differential.

Replace obsolete output circuits

1) Replacement of a PNP collector (current source)

pnp
Replacement of a PNP collector (current source)

2) Replacement of an NPN collector (current sink)

npn
Replacement of an NPN collector (current sink)

Programmable incremental encoder

Non-programmable incremental encoders cannot be configured according to user characteristics only during production. However, for applications requiring changes in characteristics, programmable incremental encoders represent an attractive and easy-to-use solution. It suffices to modify certain software parameters using an external tool (UBIFAST Configuration Tool) to change:

  • The output driver – switch from Push-Pull (HTL) to RS422 (TTL)
  • The PPR (resolution) – programming for a chosen value
  • Direction of the output signal – “A before B“ or “B before A“ (in phase advance)

The programmability of a sensor is a major criterion for resellers, system integrators or machine manufacturers insofar as this allows a reduction in the level of stock. It is now possible to have a relatively small stock consisting of standard encoders, programmable according to the desired use.

Specifications of incremental encoders:

Output voltage:

A logic gate interprets certain output voltages as high (1) or low (0).
TTL (transistor-transistor-logic): A voltage greater than 2 V is interpreted as a logic 1 and a voltage less than 0.8 V will be interpreted as a logic 0. The voltage output range is between 0 and 5 V.
HTL (high-threshold-logic): A voltage greater than 3 V is interpreted as a logic 1 and a voltage less than 1 V will be interpreted as a logic 0. The high output voltage depends on the supply voltage. Due to the greater voltage difference between logic 0 and logic 1, the HTL circuit is less subject to variations due to noise.

Logic UsedVoltage levelSupply voltageOutput voltage
TTL


HTL
Haut
Bas

Haut

Bas
4.75-30 V
4.75-30 V

4.75-9 V
9-30 V
4.75-30 V
min 3 V
max 0.5 V

min 3 V
min Supply Voltage – 3 V
max 0.5 V
Table 1 : Niveau de sortie des codeurs incrémentaux de POSITAL (I=50 mA par voie)

Electrical and mechanical degrees:

Mechanical degree describes the actual rotation of the shaft in degrees. The electrical degree is used for the electrical signal. The time required to perform a complete voltage / current cycle defines the 360 electrical degrees (el °). For incremental encoders, a cycle corresponds to a period. For a given resolution, the electrical degree can be converted to mechanical degree for any sensor.

Quadrature:

Every 90 ° the output signal of the incremental encoder has a rising or falling peak on channel A or B, which is interpreted as an iteration. If a sensor performs 1000 PPR, the counter will count 4000 iterations (4 per period).

Angular phase:

The angular phase defines the distance between two peaks in el °. This parameter is generally defined as constant as is the phase error (square error).

incremental_pulse_diagram_phase_angle

Accuracy (DNL):

The DNL precision translates the absolute value of the quadratic error given in mechanical degrees.

Accuracy (INL):

An incremental encoder has a defined number of periods per revolution. In fact, each pulse is supposed to be at a precise position. The maximum deviation between the ideal position and the real position is called in English integral non linearity (INL). The INL accuracy becomes critical if the sensor is used for positioning measurement.

Duty Cycle:

The duty cycle describes the ratio between the “high” and “low” peaks of the encoder. This is usually 50%, which can translate to 180 el ° highs and 180 ° el low.
The higher the PPR (the resolution) and RPM (the rotational speed), the better the performance of the encoder. The reverse is true for optical encoders. The DNL and INL precision values indicated for our products are calculated for the worst case scenario. Better performance is expected for higher PPR and RPM.

Response frequency:

This is the maximum frequency that the encoder is able to output. For example, an encoder with a resolution of 200 PPR and performing 600 rpm is 2000 Hz (200 x 600/60).

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