Considerations - Emerson SI-Universal User Manual

3.6

Considerations

When selecting an encoder, there are a number of considerations, these being application, drive
operation, and encoder specification dependant.
3.6.1 Application dependant
1. Operating mode
2. Is the application a positioning application where high resolution is required?
3. Is absolute position required at every power up, for example for operation in servo mode where
a phasing test is not possible at every power-up?
4. What resolution is required (e.g. AB 1024 encoder = 10bit resolution, SC Hiperface 1024 = 19
bit resolution)?
5. What environment is the encoder to be installed in?
6. What cable lengths are to be used?
7. Encoder supply voltage should be selected dependant upon the cable lengths due to voltage
drop
8. Are motor objects to be saved to the encoder?
3.6.2 Drive operation dependant
1. When operating in RFC-S mode the drive requires the absolute position at power-up, be this
from an absolute encoder or through a phasing test at every power-up
2. When operating in RFC-A either an absolute or non-absolute encoder can be used
3. Encoder power supply and loading when operating with long cable lengths
3.6.3 Encoder specification dependant
1. Encoder voltage levels, are these compatible with the drive?
2. Incremental encoder signals are these compatible (SC, AB, FR, FD)?
3. Incremental signals do not exceed maximum input frequency for option module
4. Comms encoder protocol is compatible (Hiperface, EnDat, SSI, BiSS*)
5. Comms encoder baud rate is compatible with drive
6. Application cable lengths do not exceed incremental signals cable length
7. Application cable lengths do not exceed the recommended cable length for comms operation,
this being baud rate specific
8. Encoder loading does not exceed encoder power supply from module (external power supply
should be used if this is the case)
3.6.4 Drive resolution / Feedback accuracy
The following values calculated are not a direct representation of performance at the motor shaft,
with the motors inductance and load inertia smoothing out the shaft value to a much lower level.
The value calculated is the instantaneous change in the internal speed feedback value seen by the
drive between sample periods, and when the number of counts per revolution changes by 1 count.
This change is due to at any given speed it is unlikely that the number of counts per sample period
will always be a whole number e.g. 1 in 10 sample periods may have an extra pulse to ensure the
average speed is as demanded.
* BiSS is not currently supported.
SI-Universal Encoder User Guide
Issue: 2
21