Rockwell Automation 8520 9/Series CNC Integration Maintenance Manual Documentation Set - Manual (Page 664)

Section 13A
Connecting 8520 Digital Drive Systems
13A-30
Absolute Encoders
Absolute encoders are an “intelligent” feedback device that provide
absolute axis position feedback to the servo module. These encoders
incorporate a turns counter register and an absolute single turn encoder.
The turns counter register holds the number of turns the motor has made
since the motor was last homed. The absolute single turn encoder holds
the number of pulses encountered, since the marker was last detected to the
present position.
The servo module requests position data from the absolute encoder after
power up. The values contained in the turns counter register and the single
turn encoder are sent to the servo module on the A channel of the encoder
signal cable. These values are used by the servo module to calculate the
absolute axis position.
Once the absolute axis position is determined, the absolute encoder
transmits axis position feedback just like an incremental encoder. Precise
commutation is possible immediately after power up when using the
absolute encoder. Motor winding position is determined using the data
provided by the absolute single turn encoder.
The absolute encoder is supplied with battery backup. This allows the
absolute encoder to retain the absolute position of an axis even after power
to the control is turned off. This eliminates the need to home axes every
time the control is turned on, and also provides a means for limited
recovery from a power failure during machining operations.
The battery power originates from the batteries plugged into the servo
module. Even if battery backup fails, or the encoder cable is temporarily
disconnected, the encoder will still maintain position data for up to 24
hours. This also allows for battery replacement without loss of data.
When an encoder other than the standard incremental or absolute encoder
provides feedback to the servo module, typically the spindle feedback
device, the maximum encoder channel frequency will have to be
determined.
The following formula is used to calculate the maximum encoder channel
frequency:
Maximum Encoder Channel Frequency =
5 x 10
6
360
x 1.15
90-Eq
Where:
5 x 10 6 = Control’s Feedback Clock Frequency
E
Q
= Encoder Quadrature Error in Degrees
1.15 = Feedback Clock Safety Factor