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3.6.5
Available resolution
NOTE
The following Quadrature and SinCos type incremental encoders are available with various lines
per revolution with the drive being compatible with encoders ranging from 1 PPR (4 CPR) to
100,000 PPR (400,000CPR).
The comms only encoders which include both EnDat, BiSS* and SSI are also available with various
comms resolutions with drive being compatible up to 32 bits
AB Quadrature Incremental Encoder
•
A 4096 LPR encoder has 4096 pulses per channel, and 16,384 edges. Available resolution =
16,384 counts / turn.
SC Incremental Encoder
•
An SCS50 SinCos encoder has 1024 sine waves per revolution with the drive interpolating
each sine wave to 9 bits worth of resolution giving a total resolution of 2 x 1024 x 2
1,048,576 counts per revolution
EnDat Comms Only Encoder
•
An EnDat comms only encoder has 25 bits giving a total resolution of = 33554432 counts per
revolution
Comparing a 4096 PPR incremental encoder to a SCS50 SinCos encoder, the SCS50 SinCos
encoder will have a factor of 128 less ripple than the 4096 PPR encoder.
Therefore the encoder selected can influence the digital torque ripple significantly and should be
considered on high resolution / accuracy applications.
3.6.6
Internal digital torque ripple calculation
Following is an example of the internal digital torque ripple calculation
AB Quadrature Encoder
1024 line encoder running at 1500 rpm and drive speed loop sample time = 250 µs
•
1500 rpm / 60 s = 25 rev/s
•
25 rev / s x 1024 ppr = 25600 pulses/s
•
25600 pulses / s x 4 edges = 102400 edges/s
•
102400 edges / s x 250 x 10
Therefore due to the digitisation of the encoder feedback the average number of edges seen will be
25.6, but this must be due to the relevant number of 25 and 26 edges over an infinite length of time.
As such:
25 edges / 250 x 10
100,000 / 4 = 25,000 pulses
25,000 / 1024 = 24.4 rev/s
24.4 x 60 = 1464.8 rpm
26 edges / 250 x 10
104,000 / 4 = 26,000 pulses
26,000 / 1024 = 25.4 rev/s
25.4 x 60 = 1523.4 rpm
1523 - 1464 = 59 rpm
The difference of 1 pulse gives an instantaneous speed change of 59 rpm.
* BiSS is not currently supported.
22
-6
= 25.6 edges per sample period
-6
= 100,000 edges/sec.
-6
= 104,000 edges/sec.
SI-Universal Encoder User Guide
9
=
Issue: 2
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