Incremental Encoders

Incremental encoders are rotary encoders that provide an output signal in the form of pulses. One pulse corresponds to one period, the increment, after which this type of encoder is named. Incremental encoders are also called rotary pulse encoders because of their characteristics as encoders for rotary motion and their signal form. The use of pulses for measurement is a fundamentally different principle from, for example, potentiometers and absolute encoders.

The most important property for determining the angular accuracy of an incremental encoder is the number of pulses generated per full revolution of the shaft at the output (pulses per revolution, ppr.). This value can be found on any incremental encoder datasheet.

Optical sensor technology has several advantages that make optical encoders the most important incremental encoders. Firstly, the fact that the measuring method of the integrated sensor elements itself generates increments, and therefore the use in incremental encoders is obvious. An overview of optical encoders can be found here.

The optical system of a modern optical incremental encoder consists of at least the following components:

• A light-emitting diode (LED) that produces light
• A collimator that directs the light from the LED in parallel
• A coding wheel having alternating permeable and non-permeable (or reflective and absorptive) regions
• The photodetector, which detects the incident light from the LED and converts it into an electrical signal.

Two methods have become established on the market: The transmissive (imaging) method and the reflective (interferential) method. In the transmissive method, the coding wheel is transilluminated, whereas in the reflective method, the light beam is reflected from the surface of the coding wheel and interference effects are used.

Hall encoders are also available with incremental outputs. As with optical encoders, the measurement technology is contactless and therefore virtually wear-free (apart from the bearing). The main advantages of Hall effect incremental encoders are the virtually unlimited life of the sensor technology (no ageing of the LEDs) and excellent shock resistance. Disadvantages can be the sensitivity to external interference and the fact that the signals are transmitted with a slight time delay (update rate). For an explanation of the measuring principle of Hall-effect encoders, see the guide to absolute encoders guide. For a more detailed analysis of the advantages and disadvantages of different encoder technologies, see the rotary encoder guide.

Incremental encoders usually have several signal outputs. If an incremental encoder outputs several signal packages, the term channel is used in this context. For example "Channel A" and "Channel B". In the literature, the term "track" is also used instead of "channel".

Example:
If the data sheet of an incremental encoder specifies the value 360 ppr and the encoder has the electrical signal outputs "A" and "B" ("Channel A" and "Channel B"), then 360 pulses per one revolution of the shaft (per 360°) are output at output "A" and another 360 ppr 90° ahead or behind the pulses of channel A are output at output "B". In total, the encoder generates 720 ppr per full shaft revolution (360°) for both channels A and B.   

The number of pulses per revolution (ppr) is also referred to as the resolution, the higher the value  ppr), the higher the angular resolution of the encoder.

The 90° signal offset of the square wave signals of channels A and B has an advantage. For each track and signal period, a square-wave signal has one rising and one falling signal edge.
The edge sequence for tracks A and B of a signal period is as follows:

Track A rising edge (1) → after a ¼ period Track B rising edge (2) → after a ½ period Track A falling edge (3) → after a ¾ period Track B falling edge (4)

If an evaluation unit evaluates not only the rising edge of one track, but also the rising and falling edges of both tracks A and B, then the number of pulses can be quadrupled using this method. This is equivalent to a fourfold increase in accuracy without any structural changes to the encoder.

Incremental encoders cannot be operated at any speed. There are mechanical and/or electronic limitations.

The mechanical limitations can be determined from the data sheet and are due to the following causes:

• Max. shaft bearing speed (only applies to encoders with their own shaft bearing, see Shaft encoders). The maximum allowable speed is often less than 10000 rpm.
• The eccentricity (unbalance) of the mechanics. With optical encoders, this is mainly caused by the imbalance of the coding wheel. However, the maximum operating speed can be as high as 60,000 rpm. Magnetic kit encoders, on the other hand, do not normally have this limitation.

The electronic limitation can be calculated: here the result of the calculation is called the "theoretical maximum possible actuating speed".

• The reason for this is the cut-off frequency of the electronics. The electronics cannot process a frequency higher than the cut-off frequency. The higher the cut-off frequency and the lower the resolution of the encoder, the higher is the theoretically possible actuating speed.

The following formula can be used to calculate the theoretical maximum actuating speed from the cut-off frequency:

\(max. rpm =\frac{\text {cut-off frequency} \frac {1} {s} * 60 }{ \text {number of pulses}}\)

Two examples of how to calculate the theoretical maximum actuating speed are given below.

Example 1:
A resolution of 512 ppr is required. The cut-off frequency is given as 100 kHz in the encoder datasheet. You get

\({100000 \cdot 1/s\cdot 60 \text{ s} \over 512} = 11718 \text { rpm} \)

Result: The theoretical maximum permissible actuating speed is 11718 rpm.

Example 2:
The required resolution is 10000 ppr. The cut-off frequency is specified as 100 kHz in the encoder datasheet. Result: The theoretical maximum actuating speed of the actuator is 600 rpm.

\({100000 \cdot 1/s \cdot 60 \text{ s} \over 10000} = 600 \text { rpm} \)

A comparison between the maximum theoretical and mechanically permissible actuation speed shows which one counts for the application: the lower of the two values is relevant!

No incremental encoder provides a perfect signal. For optical incremental encoders, the following describes the uncertainties or tolerances that must be considered for the signals from these encoders. The optical system includes the encoder wheel itself and the encoder module or assembly containing the LED and photodetector. All these elements interact to produce some deviation from the ideal rectangular signal shape and ideal edge position. These tolerances are often described in the datasheet of an optical incremental encoder and help the user to analyse the measurement data more accurately.
In most cases, the signals from channels A, B and, if necessary, Z are shown as a picture. The relationships are then explained using examples in the diagram opposite.

The symbols have the following meanings:
C corresponds to a period
P stands for a ½ signal period
S for ¼ Signal period
Ф is the phase reference between channels A and B

In the ideal case: C = 2 * P = 4 * S = S1 + S2 + S3 + S4.

The more pulses per revolution that are realised with an optical encoder, the smaller the line width of the increments on the code wheel. However, the optical system of an angle encoder can only detect increments up to a certain line width. For example, a 10 mm diameter code wheel cannot have 10,000 lines due to its small size. When incremental encoders with a small housing diameter and high resolution are to be implemented, this is often done on the basis of sine/cosine interpolation.

The open collector circuit is an obvious standard for output circuits for incremental signals, and a major advantage is that it allows the output to be connected to a different voltage level defined by the application. This is possible because there is no pull-up resistor integrated into the encoder and the collector is led out of the housing (open collector). The transistor thus acts as a switch.

The following example applies to a bipolar Si-NPN transistor:

High level at signal output:

• A low level (<0.7V) at the base of the transistor blocks the transistor and the supply voltage (VSUP) is applied to the collector.

Low level at the signal output:

• When the base of the transistor is high (>0.7V), the voltage at the collector (VSUP) is pulled to ground.

The TTL output is often simply referred to as the voltage output. The difference to the open collector output is that the necessary pull-up resistors are already integrated in the encoder housing, so the levels are fixed. Variable level conversion as with the open collector circuit is therefore not possible.

These levels are for standard TTL logic:
< 0.4 V for the low level
> 2.4 V for the high level

The push/pull output circuit is based on a complementary pair of transistors (n-channel and p-channel). It alternately blocks one of the two transistors.

When the output signal is high, it is at the level of VSUP and when it is low, it is approximately at ground. The advantage of a push-pull circuit is that no additional pull-up or pull-down resistors are required. If no level conversion is required, encoders with push-pull output circuitry can be used as a universal replacement for open collector and TTL/voltage outputs.