How to Select Incremental Rotary Encoders
Image credit: Dunkermoteren USA | Dynapar Corp. | Renishaw
Incremental rotary encoders are electro-mechanical devices that use pulses to track motion.
Understanding Incremental Rotary Encoders
Rotary encoders convert the angular position of a shaft or axle to an analog or digital code. Whereas absolute encoders can be used to track both motion and position, incremental types can only track motion; this motion data can then be used by external equipment to determine position and speed.
Absolute and incremental encoders each have specific advantages in different situations. Because absolute encoders use a unique code to describe each position, position data is not lost when power is cycled through the device. When power is cycled through an incremental encoder, position data is lost until the "home" position is once again triggered. Some incremental encoders, however, feature an index channel which produces a single pulse per revolution to use a reference marker, eliminating this drawback. Additionally, incremental encoders are typically lower in cost and simpler to use compared to absolute encoders.
This video provides an overview of incremental encoders.
Video credit: DesignWorld
Using Rotary Encoders
Encoders prove valuable in various uses, and these applications often determine the relative importance of certain encoder specifications.
Motor feedback is the most common rotary encoder application. The rotary encoder is mounted directly on the motor, or indirectly using a measuring wheel or chain-and-sprocket. The most important specification in this case is the encoder's speed capability.
Encoders can generally be used in any application requiring feedback concerning speed, direction, or distance, and may be implemented in any of the following products or applications:
Coordinate graphing systems
Performance specifications include resolution, accuracy, and count rate.
Resolution refers to the number of bits the encoder uses to encode the position. Resolution can be calculated using the formula 2n, where n is the number of bits. For example, a 9-bit encoder yields 512 counts per disc revolution, whereas a 10-bit encoder yields 1,024 counts per revolution; the resolution of these two distinct encoders would be 9 and 10, respectively. The 10-bit encoder, with twice as many counts as the 9-bit one, would be capable of much more precise position measurement.
Encoder accuracy is typically measured in arc seconds; one degree contains 3600 arc seconds. The accuracy specification refers to the maximum error of a position reading.
Count rate refers to the rate at which new position readings can be generated and updated, and is specified as a variation of Hertz or cycles (Hz, kHz, MHz, etc.)
Encoder diameter/width, shaft speed, and rotor inertia are important mechanical specifications.
Diameter refers to the the body diameter of a circular encoder. If the encoder's body is square, width refers to the maximum side dimension.
Mechanical shaft speed specifies the maximum speed at which the encoder can rotate without sustaining damage.
Rotor inertia is specified as a product of mass, distance, and/or time, such as oz-in-sec2 or kg-m2.
Incremental rotary encoders may feature one of several different types of signal outputs.
Quadrature encoders employ two binary outputs, A and B, which are 90 degrees out of phase. These signals are decoded to determine a count up pulse or a count down pulse. The following diagram shows the interaction between the A and B signals:
Image credit: Sagsaw
In the above diagram, the lower position of both signals indicates '0', whereas the upper position indicates '1'. Therefore, during the first occurrence of the first phase, both the encoder code would read '00'. By moving one phase to the right, it is now apparent that the code would read '01'; the encoder reads moving from '00' to '01' as a half-turn clockwise.
Single channel encoders are also known as tachometer encoders. They have a single channel which allows for one count per physical line.
Pulse and direction output involves adding a direction channel to a traditional single channel or quadrature encoder.
Buyers of incremental rotary encoders should also consider important electrical or interface data when selecting a product.
Analog signals include voltage (such as 0-10 V) or current (such as 4-20 mA) outputs.
Serial signals include serial synchronous interface (SSI), FOUNDATION Fieldbus, CANbus, INTERBUS, and SUCOnet. SSI in particular represents a general standard for absolute encoders.
Digital output allows for direct interface with a processor. Square wave (as illustrated above) is the most common type of digital output.
When selecting incremental rotary encoders, it is important to understand the different types available. There are four types of rotary encoders: optical, mechanical, fiber optic, and magnetic.
Mechanical rotary encoders consist of a metal disc with a ring of cut-out openings as well as a row of sliding contacts fixed to a stationary object. The metal disc is attached to a rotating shaft, and each stationary contact is connected to an electrical sensor. As the disc rotates, some of the contacts touch the disc and switch on, while others fall in the gaps where the metal has been cut out. The combination of switched-on and switched-off contacts creates a unique binary code for each disc position.
Because contacts are susceptible to wear with heavy use, mechanical encoders are well-suited to low-speed manual applications such as radio tuning knobs.
The image below shows a cross-section of a mechanical rotary encoder. In the image at left, note the two rows of cut-out "teeth" labeled A and B. The top of the encoder, shown at the right image, fits on top of the discs, and the brush contacts, also labeled A and B, make contact with the A and B disc tracks. The two sets of contacts labeled "common" are used to maintain contact with the inside of the discs.
Image credit: Robot Room
An optical encoder's disc is made of glass or plastic and features some transparent and some opaque areas. Using a light source and photo detector array, the encoder can read the disc's optical pattern, determine its position, and translate it into a code. The code is then read by a controller to determine the angle of the shaft.
Optical encoders are fairly inexpensive, but are rarely suitable for operation in extreme temperatures (greater than 70° C) and harsh environments because they use LEDs and glass components.
An optical encoder. Image credit: Newport
Magnetic encoders consist of a plastic rotary drum whose circumference is lined with magnetic poles. As the drum turns against a magnetic sensor, the sensor relays an electric signal indicating the drum's position.
Magnetic encoders are relatively new devices and are expensive compared to optical encoders. They do offer several advantages over the latter type:
Improved temperature and environmental resistance due to the materials used
Smaller space requirements due to shorter axial direction
Lower cost of custom encoders
The image below compares the operation of a magnetic and an optical encoder.
Image credit: Astec Corporation, Ltd.
Fiber optic encoders transmit position information by light using similar sensing technology as optical types. This type of transmission is advantageous in applications where noise or electrical interference could occur with signals.
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Absolute Rotary Encoders
Absolute rotary encoders use optical, mechanical, or magnetic indexing for angular measurement. They do not lose their position after power-down, but instead provide absolute position upon power-up without requiring a home cycle or any shaft rotation.