Incremental encoders are position verification devices that indicate rotary or linear motion and the direction of movement. An encoder plus a counting device make up an incremental system. The counting device will sum the positive and negative increments allowing the user to detect the net amount of movement at any point in time. Linear incremental encoders should be applied to traverse operations. Rotary encoders are the best choice for angular applications.
Basic Operation of a Rotary Encoder
A rotary incremental optical encoder consists of five basic components. The light source (LED or incandescent), the encoder disc, grid assembly, photo-detector, and amplification electronics. These are shown below.
The disc is one of the key elements of the encoder. A simplified disc is shown below.
It is typically a glass material with imprinted marks of metal with slots precisely located. The quantity of marks or slots is equivalent to the number of pulses per turn. A typical resolution is 500 or 1000 pulses per revolution. For example, a glass disc with 1000 marks imprinted would have moved 180° after 500 pulses. The maximum resolution of the encoder is limited by the number of marks or windows that can be physically located on the disk, which determines the number of pulses per revolution.
Most rotary encoders also provide a single mark on the disk called the Z channel or marker. The pulse from this channel provides a reference once per revolution to detect errors within a given revolution.
The LED or incandescent light source is constantly enabled. As the disk rotates, light reaches the photo-detector as each slot or window passes as shown below.
Rotary Incremental Optical Encoder
As the photo-detector senses light it allows current to flow and produces a current sine wave. The frequency of the sine wave is controlled by the alternations of light and dark from the disk's movement. The outputs from the photo-detector are then converted into a square waveform and supplied to a counting device. All incremental encoders rely on a counter to determine position and a stable clock to determine velocity. These systems use a variety of techniques to separate "up' counts from "down" counts, and maximize the incoming pulse rate. Correct position information is dependent on accurate counting and proper transmission of pulses. The use of complementary outputs will prevent false counting due to electrical noise developing stray electrical pulses.
Quadrature Encoder with Complementary Outputs
The majority of all incremental encoders have a second light source and sensor (Channel B) located over the windows on the disk. Channel B is mechanically offset one half window from Channel A's light source and photo-detector.
Channels A and B will produce two square waves that are 90 electrical degrees apart. By monitoring the phase shift of both channels A and B one can determine direction. For example, if channel B leads channel A, that could indicate counter clockwise rotation. If channel A was to lead channel B, the opposite would be true. The waveform shown below is considered standard for forward rotation.
An additional benefit of a quadrature encoder is the ability to multiply the encoder disk resolution by four. By counting the leading and trailing edge of each window on both channels, each slot of window now gives four counts. Using this procedure, a 1000 window disk can have a resolution of 4000 counts per revolution.
Even with the appropriate package, shaft, bearings and disc, the user must exercise care to avoid undue shock and abuse. In particular, the glass code disc can be damaged if the encoder is dropped or if a pulley is hammered onto the shaft. The typical shock and vibration specification for an industrial encoder is 50 grams of shock of 11 msec, as well as a vibration
of 20 grams from 2-2000 Hz. In applications utilizing gears or drive belts, excessive radial (side) loading on the shaft can shorten bearing life. Therefore great care should be taken when tensioning an encoder's drive belt.
Incremental Encoder charateristic
Type of Signal:
- Digital Signal (Binary)
- Pulse train (Squarewave)
Type of Information:
- Speed = Freq.
- Direction = Phase Relationship
- Angular Position = Count and Marker
- Windows / turn or Pulses / turn
- Maximum frequency
- Replace as needed
- Critical alignment
- Encoder must be clean
- Properly align
- No preload on the bearing