page_title: Incremental encoder
section_title: Speed measurement
hierarchical_section_title: Incremental encoder / Principal applications / Speed measurement
caption_attribution_description: English: Animated GIF showing a rotating waveguide antenna of a typical X band marine radar on a ship. It radiates a narrow vertical fan-shaped beam of microwaves perpendicular to the antenna bar, along the water surface. The rotating antenna scans the water surface surrounding the ship out to the horizon. The microwaves reflect off any ships present and some return to the antenna, causing a "blip" to be displayed on the radar screen.
context_page_description: An incremental encoder is a linear or rotary electromechanical device that has two output signals, A and B, which issue pulses when the device is moved. Together, the A and B signals indicate both the occurrence of and direction of movement. Many incremental encoders have an additional output signal, typically designated index or Z, which indicates the encoder is located at a particular reference position. Also, some encoders provide a status output that indicates internal fault conditions such as a bearing failure or sensor malfunction.
  Unlike an absolute encoder, an incremental encoder does not indicate absolute position; it only reports changes in position and, for each reported position change, the direction of movement. Consequently, to determine absolute position at any particular moment, it is necessary to send the encoder signals to an incremental encoder interface, which in turn will "track" and report the encoder's absolute position.
  Incremental encoders report position changes nearly instantaneously, which allows them to monitor the movements of high speed mechanisms in near real-time.
context_section_description: Incremental encoders are commonly used to measure the speed of mechanical systems. This may be done for monitoring purposes or to provide feedback for motion control, or both. Widespread applications of this include speed control of radar antenna rotation and material conveyors, and motion control in robotics, CMM and CNC machines.
  Incremental encoder interfaces are primarily concerned with tracking mechanical displacement and usually do not directly measure speed. Consequently, speed must be indirectly measured by taking the derivative of a position signal. The position signal is inherently Quantization_(signal_processing), which poses challenges for taking the derivative due to quantization error, especially at low speeds. If the position signal is Sampling_(signal_processing) (a discrete time signal), then pulses are typically counted by the interface and speed is calculated by a computer which has read access to the interface. To do this, the computer reads the position counts  from the interface at time  and then, at some later time  reads the counts again to obtain . The average speed during the interval  to  is then calculated:
  Alternatively, a speed measurement can be reported at each pulse by measuring the time between pulses. The velocity calculation is the same as before: displacement / time. This technique reduces the error due to (position) quantization error, and introduces errors related due to the quantization of the time reference. Instead of being triggered at specific times (no time quantization error), the measurement is triggered at specific positions (no position quantization error).
  The resulting speed value is expressed as counts per unit time (e.g., counts per second). In practice, however, it is often necessary to express the speed in standardized units such as meters per second, revolutions per minute (RPM), or miles per hour (MPH). In such cases, the software will take into account the relationship between counts and desired distance units, as well as the ratio of the sampling period to desired time units. For example, in the case of a rotary incremental encoder that produces 4096 counts per revolution, which is being read once per second, the software would compute RPM as follows:
  When measuring speed this way, the measurement resolution is proportional to both the encoder resolution and the sampling period (the elapsed time between the two samples); measurement resolution will become higher as the sampling period increases.
caption: Commercial marine radar antenna