NI cDAQ-9178 - Position Measurement

© National Instruments Corporation 47 NI cDAQ-9178/9174 User Guide and Specifications
Position Measurement
You can use the counters to perform position measurements with quadrature encoders or two-pulse
encoders. You can measure angular position with X1, X2, and X4 angular encoders. Linear position can
be measured with two-pulse encoders. You can choose to do either a single point (on-demand) position
measurement or a buffered (sample clock) position measurement. You must arm a counter to begin
position measurements.
Refer to the following sections for more information about the NI cDAQ-9178/9174 position
measurement options:
• Measurements Using Quadrature Encoders
• Measurements Using Two Pulse Encoders
• Buffered (Sample Clock) Position Measurement
Measurements Using Quadrature Encoders
The counters can perform measurements of quadrature encoders that use X1, X2, or X4 encoding.
A quadrature encoder can have up to three channels—channels A, B, and Z.
• X1 Encoding—When channel A leads channel B in a quadrature cycle, the counter increments.
When channel B leads channel A in a quadrature cycle, the counter decrements. The amount of
increments and decrements per cycle depends on the type of encoding—X1, X2, or X4.
Figure 36 shows a quadrature cycle and the resulting increments and decrements for X1 encoding.
When channel A leads channel B, the increment occurs on the rising edge of channel A. When
channel B leads channel A, the decrement occurs on the falling edge of channel A.
Figure 36. X1 Encoding
• X2 Encoding—The same behavior holds for X2 encoding except the counter increments or
decrements on each edge of channel A, depending on which channel leads the other. Each cycle
results in two increments or decrements, as shown in Figure 37.
Figure 37. X2 Encoding
Ch A
Ch B
Counter Value
7 7 6 5
5
6
Ch A
Ch B
Counter Value
56 8 97 56 8
9 7