Chapter 4
Signal Connections
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National Instruments Corporation
4-9
Axis <1..4> Encoder Phase A and Phase B
The encoder inputs provide position and velocity feedback for absolute
and relative positioning of axes in any motion system configuration.
If an encoder resource is not needed for axis control, it is available for other
functions, including position or velocity monitoring, digital potentiometer
encoder inputs, or as a master encoder input for master/slave (electronic
gearing) applications.
The encoder channels (Axis <1..4> Encoder) are implemented in an FPGA
and provide advanced features, such as high-speed trigger (position
capture) inputs and breakpoint (position compare) outputs. The encoders
have a maximum count frequency of 4 MHz.
An encoder input channel converts quadrature signals on Phase A and
Phase B into 32-bit up/down counter values. Quadrature signals are
generated by optical, magnetic, laser, or electronic devices that provide
two signals, Phase A and Phase B, that are 90° out of phase. The leading
phase, A or B, determines the direction of motion. The four transition states
of the relative signal phases provide distinct pulse edges that cause count
up or count down pulses in the direction determined by the leading phase.
A typical encoder with a specification of
N
(
N
= number) lines or periods
per unit of measure, which can be revolutions or linear distance, produces
4
×
N
quadrature counts per unit of measure.
The count is the basic
increment of position in NI-Motion systems.
Tip
If your encoder does not supply resolution in quadrature counts per revolution,
determine quadrature counts per revolution by multiplying the encoder resolution in
encoder lines or periods by four. Encoder counts per revolution is also referred to as counts
per index. For example, a 500 line encoder has 2,000 quadrature counts per revolution.
Axis <1..4> Encoder Index (Phase Z)
The Index (Phase Z) input is primarily used to establish a reference
position. This function uses the number of counts per revolution or the
linear distance to initiate a search move that locates the index position.
When a valid Index signal transition occurs during a Find Reference
routine, the position of the Index signal is captured accurately. Use this
captured position to establish a reference zero position for absolute position
control or any other motion system position reference required.
