Encoder Frequently Used Terms Ⅲ！

PPR: Pulses per Revolution (see Resolution).

Gray Code: A common absolute encoder output format based on binary counting, but it avoids the multiple transition problem entirely, It differs from Natural Binary in that only one bit can transition at a time.

Quadrature: Two incremental output channels that are deliberately out of phase by 90 electrical degrees. The relationship between the two channels is set within the encoder. A typical quadrature relationship would be described as: A leads B for counterclockwise (CCW) rotation of the input shaft. Quadrature allows for discrimination between direction of movement (CW versus CCW), error detection in high vibration environments and higher resolution by using edge detection.

Hysteresis: A dead band purposely introduced in the encoder electronics. This helps to prevent transition ambiguities if the system happens to dither on a transition.

Repeatability: The deviation of the actual encoder position produced between subsequent identical code readings. The ability to repeat exact bit placements. Repeatability is unrelated to resolution and is usually 4 to 10 times better than accuracy.

Resolution: The number of bits or words contained in the complete code. For incremental and tachometer encoders, resolution is defined as cycles per revolution. For absolute single turn encoders, it is called counts per turn. Multi turn encoders are specified as positions per turn of the input shaft and the number of turns of the internal gear ratio. Resolution is not the same as accuracy.

Serial Synchronous Interface (SSI):SSI is a means to transmit encoder position information on a single pair of conductors instead of one conductor per bit. A clock pulse train from a controller tells the encoder when to send out data bits: one bit of position data is transmitted to the controller per one clock pulse received by the sensor.

Transition: Change of state of the digital word. This occurs at the bit or word edge.

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