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Rotary Encoder

A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to an analog or digital code. The output of incremental encoders provides information about the motion of the shaft which is typically further processed elsewhere into information such as speed, distance, RPM and position. The output of absolute encoders indicates the current position of the shaft, making them angle angle transducers. Rotary encoders are used in many applications that require precise shaft unlimited rotation—including industrial controls, robotics, special purpose photographic lenses[1], computer input devices (such as optomechanical mice and trackballs), and rotating radar platforms. There are two main types: absolute and incremental (relative).

Absolute rotary encoder

Construction

digital type produces a unique digital code for each distinct angle of the shaft. They come in two basic types: optical and mechanical.

Mechanical absolute encoders

A metal disc containing a set of concentric rings of openings is fixed to an insulating disc, which is rigidly fixed to the shaft. A row of sliding contacts is fixed to a stationary object so that each contact wipes against the metal disc at a different distance from the shaft. As the disc rotates with the shaft, some of the contacts touch metal, while others fall in the gaps where the metal has been cut out. The metal sheet is connected to a source of electric current, and each contact is connected to a separate electrical sensor. The metal pattern is designed so that each possible position of the axle creates a unique binary code in which some of the contacts are connected to the current source (i.e. switched on) and others are not (i.e. switched off).

Optical absolute encoders

The optical encoder’s disc is made of glass or plastic with transparent and opaque areas. A light source and photo detector array reads the optical pattern that results from the disc’s position at any one time.

This code can be read by a controlling device, such as a microprocessor or microcontroller to determine the angle of the shaft.

The absolute analog type produces a unique dual analog code that can be translated into an absolute angle of the shaft (by using a special algorithm).

Standard binary encoding

Rotary encoder for angle-measuring devices marked in 3-bit binary. The inner ring corresponds to Contact 1 in the table. Black sectors are “on”. Zero degrees is on the right-hand side, with angle increasing counterclockwise.

An example of a binary code, in an extremely simplified encoder with only three contacts, is shown below.

Standard Binary Encoding
Sector Contact 1 Contact 2 Contact 3 Angle
1 off off off 0° to 45°
2 off off ON 45° to 90°
3 off ON off 90° to 135°
4 off ON ON 135° to 180°
5 ON off off 180° to 225°
6 ON off ON 225° to 270°
7 ON ON off 270° to 315°
8 ON ON ON 315° to 360°

In general, where there are n contacts, the number of distinct positions of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions.

In the above example, the contacts produce a standard binary count as the disc rotates. However, this has the drawback that if the disc stops between two adjacent sectors, or the contacts are not perfectly aligned, it can be impossible to determine the angle of the shaft. To illustrate this problem, consider what happens when the shaft angle changes from 179.9° to 180.1° (from sector 4 to sector 5). At some instant, according to the above table, the contact pattern changes from off-on-on to on-off-off. However, this is not what happens in reality. In a practical device, the contacts are never perfectly aligned, so each switches at a different moment. If contact 1 switches first, followed by contact 3 and then contact 2, for example, the actual sequence of codes is:

off-on-on (starting position)
on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off)
on-off-off (finally, contact 2 switches off)

Now look at the sectors corresponding to these codes in the table. In order, they are 4, 8, 7 and then 5. So, from the sequence of codes produced, the shaft appears to have jumped from sector 4 to sector 8, then gone backwards to sector 7, then backwards again to sector 5, which is where we expected to find it. In many situations, this behaviour is undesirable and could cause the system to fail. For example, if the encoder were used in a robot arm, the controller would think that the arm was in the wrong position, and try to correct the error by turning it through 180°, perhaps causing damage to the arm.

Gray encoding

Rotary encoder for angle-measuring devices marked in 3-bit binary-reflected Gray code (BRGC). The inner ring corresponds to Contact 1 in the table. Black sectors are “on”. Zero degrees is on the right-hand side, with angle increasing anticlockwise.

To avoid the above problem, Gray encoding is used. This is a system of binary counting in which adjacent codes differ in only one position. For the three-contact example given above, the Gray-coded version would be as follows.

Gray Coding
Sector Contact 1 Contact 2 Contact 3 Angle
1 off off off 0° to 45°
2 off off ON 45° to 90°
3 off ON ON 90° to 135°
4 off ON off 135° to 180°
5 ON ON off 180° to 225°
6 ON ON ON 225° to 270°
7 ON off ON 270° to 315°
8 ON off off 315° to 360°

In this example, the transition from sector 4 to sector 5, like all other transitions, involves only one of the contacts changing its state from on to off or vice versa. This means that the sequence of incorrect codes shown in the previous illustration cannot happen.

Single-track Gray encoding

If the designer moves a contact to a different angular position (but at the same distance from the center shaft), then the corresponding “ring pattern” needs to be rotated the same angle to give the same output. If the most significant bit (the inner ring in Figure 1) is rotated enough, it exactly matches the next ring out. Since both rings are then identical, the inner ring can be omitted, and the sensor for that ring moved to the remaining, identical ring (but offset at that angle from the other sensor on that ring). Those two sensors on a single ring make a quadrature encoder.

For many years, Torsten Sillke and other mathematicians believed that it was impossible to encode position on a single track so that consecutive positions differed at only a single sensor, except for the two-sensor, one-track quadrature encoder. However, in 1994 N. B. Spedding registered a patent (NZ Patent 264738) showing it was possible with several examples. See Single-track Gray code for details.

Encoder output formats

In commercial absolute encoders there are several formats for transmission of absolute encoder data, including parallel binary, SSI, “BiSS”, ISI, Profibus, CAN DeviceNet, CANopen, Endat and Hiperface, depending on the manufacturer of the device

Incremental rotary encoder

Encoder ROD 420

An incremental rotary encoder, also known as a quadrature encoder or a relative rotary encoder, has two outputs called quadrature outputs. They can be either mechanical or optical. In the optical type, there are two gray coded tracks, while the mechanical type has two contacts that are actuated by cams on the rotating shaft. The mechanical type requires debouncing and is typically used as digital potentiometers on equipment including consumer devices. Most modern home and car stereos use mechanical rotary encoders for volume. Due to the fact the mechanical switches require debouncing, the mechanical type are limited in the rotational speeds they can handle. The incremental rotary encoder is the most widely used of all rotary encoders due to its low cost: only two sensors are required.

The fact that incremental encoders use only two sensors does not compromise their accuracy. One can find in the market incremental encoders with up to 10,000 counts per revolution, or more.

There can be an optional third output: reference, which happens once every turn. This is used when there is the need of an absolute reference, such as positioning systems.

The optical type is used when higher RPMs are encountered or a higher degree of precision is required.

Incremental encoders are used to track motion and can be used to determine position and velocity. This can be either linear or rotary motion. Because the direction can be determined, very accurate measurements can be made.

They employ two outputs called A & B, which are called quadrature outputs, as they are 90 degrees out of phase.

The state diagram:

clockwise rotation

Gray coding for 
Phase A B
1 0 0
2 0 1
3 1 1
4 1 0
counter-clockwise rotation

Gray coding for 
Phase A B
1 1 0
2 1 1
3 0 1
4 0 0

Two square waves in quadrature (clockwise rotation).

The two output wave forms are 90 degrees out of phase, which is all that the quadrature term means. These signals are decoded to produce a count up pulse or a count down pulse. For decoding in software, the A & B outputs are read by software, either via an interrupt on any edge or polling, and the above table is used to decode the direction. For example, if the last value was 00 and the current value is 01, the device has moved one half step in the clockwise direction. The mechanical types would be debounced first by requiring that the same (valid) value be read a certain number of times before recognizing a state change.

If the encoder is turning too fast, an invalid transition may occur, such as 00->11. There is no way to know which way the encoder turned; if it was 00->01->11, or 00->10->11.

If the encoder is turning even faster, a backward count may occur. Example: consider the 00->01->11->10 transition (3 steps forward). If the encoder is turning too fast, the system might read only the 00 and then the 10, which yields a 00->10 transition (1 step backward).

This same principle is used in ball mice to track whether the mouse is moving to the right/left or forward/backward.
Rotary sensors with a single output are not encoders and cannot sense direction, but can sense RPM. They are thus called tachometer sensors.

Incremental versus absolute encoder terminology

There seem to be some grey areas as to what constitutes an incremental encoder as opposed to an absolute encoder.

Traditional absolute encoders

Traditional absolute encoders have multiple code rings with various binary weightings which provide a data word representing the absolute position of the encoder within one revolution. This type of encoder is often referred to as a parallel absolute encoder. The distinguishing feature of the absolute encoder is that it reports the absolute position of the encoder to the electronics immediately upon power-up with no need for indexing. [2]

Traditional incremental encoders

A traditional incremental encoder works differently by providing an A and a B pulse output that provide no usable count information in their own right. Rather, the counting is done in the external electronics. The point where the counting begins depends on the counter in the external electronics and not on the position of the encoder. To provide useful position information, the encoder position must be referenced to the device to which it is attached, generally using an index pulse. The distinguishing feature of the incremental encoder is that it reports an incremental change in position of the encoder to the counting electronics.[2]

Battery backed incremental encoders

Some encoder manufacturers, such as Fanuc, have taken a different approach to this terminology. These manufacturers use absolute as their terminology for incremental encoders with a battery backed up memory to store count information and provide an absolute count immediately upon power up.[2]

Sine wave encoder

A variation on the Incremental encoder is the Sinewave Encoder. Instead of producing two quadrature square waves, the outputs are quadrature sine waves (a Sine and a Cosine). By performing the arctangent function, arbitrary levels of resolution can be achieved.

Use in industry

Encoders used on servomotors

Rotary encoders are often used to track the position of the motor shaft on permanent magnet brushless motors, which are commonly used on CNC machines, robots, and other industrial equipment. Incremental (Quadrature) encoders are used on Induction Motor type servomotors, but absolute encoders are used in Permanent Magnet Brushless Motors, where applicable. In these applications, the feedback device (encoder) plays a vital role in ensuring that the equipment operates properly. The encoder synchronizes the relative rotor magnet and stator winding positions to the current provided by the drive. Maximum torque results if the current is applied to the windings when the rotor magnets are in a particular position range relative to the stator windings. The motor will perform poorly or not at all if this timing is not adjusted correctly. Improper encoder alignment on the motor can actually cause it to run backwards sometimes resulting in a hazardous run away condition. Correct alignment is absolutely essential to proper operation of these motors.[3]

Encoder technologies

Hall-effect quadrature encoder, sensing gear teeth on the driveshaft of a robot vehicle.

Encoders may be implemented using a variety of technologies:

  • Conductive tracks. A series of copper pads etched onto a PCB is used to encode the information. Contact brushes sense the conductive areas. This form of encoder is now rarely seen.
  • Optical. This uses a light shining onto a photodiode through slits in a metal or glass disc. Reflective versions also exist. This is one of the most common technologies.
  • Magnetic. Strips of magnetised material are placed on the rotating disc and are sensed by a Hall-effect sensor or magnetoresistive sensor. Hall effect sensors are also used to sense gear teeth directly, without the need for a separate encoder disc.

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