© National Instruments
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4-7
NI 7330 User Manual and Specifications
Encoder <1..4> Phase A/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 (Encoder <1..4>) are implemented in an FPGA and are high performance
with extended input frequency response and advanced features, such as high-speed position
capture inputs and breakpoint outputs. The encoders have a maximum count frequency
of 20 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 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
Determine quadrature counts by multiplying the encoder resolution in encoder
lines by four. The encoder resolution is the number of encoder lines between
consecutive encoder marker or Z-bit indexes. If the encoder does not have an index
output, the resolution is referred to as lines per revolution, or lines per unit of
measure, such as inch, centimeter, millimeter, and so on.
Encoder <1..4> Index (Phase Z)
The Index 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.
The default MAX settings guarantee that the Find Index routine completes successfully if the
encoder generates a high index pulse when phases A and B are low and the encoder is connected
through an NI UMI or drive accessory. Figure 4-4 shows the default encoder phasing diagram at
the inputs to the controller.
