Servosila-Device-Reference-0xA020192

into the same electric drive. For configuration purposes, the encoders are
identified as a "Motor Encoder" and a "Servo Encoder". The encoders play
different roles as explained below:
• A "Motor Encoder" is an encoder rigidly connected to the motor's rotor or
built into the motor itself. The place of mechanical installation must be
chosen in such a way that no gearbox or a belt could introduce a backlash
in the linkage between the rotor and the encoder. When rigidly attached to
the rotor, such an encoder can be used by the controller to accurately
measure the angular position of the rotor. The angular position of the
rotor is what the controller needs to manage magnetic fields inside the
motor in such a way that the motor generates torque. An added benefit is
that such an encoder can be used for Direct Drive Control, Servo Control,
or Brake functions.
• A "Servo Encoder" on the other hand is meant to be used for Servo
Control only, due to control issues related to backlash of a speed reducer.
An encoder playing this role might be linked to the rotor via backlash-
introducing mechanisms, such as a gearbox or a belt. The backlash
implies that some other means (Hall sensors, a motor encoder, or
sensorless control) is to be used by the controller to sense the angular
position of the rotor. Thus the distinction between the roles of a "Servo
Encoder" and a "Motor Encoder". Note that a "Servo Encoder" can be of
a ROTARY or a LINEAR type.
To summarize, a "Motor Encoder" is a functional substitute for Hall sensors or
sensorless control, with further benefits related to that it can be used for Direct
Drive Control, Servo Control, or Brake functions. Such an encoder is much
better than Hall sensors when it comes to controlling a brushless motor at low or
zero speeds, due to higher resolution of shaft position sensing, but is more
expensive in terms of hardware. Typical Direct Drive motor features such an
encoder employed for both spinning the motor and for servo positioning, thus
killing two birds with one stone.
Besides a zero backlash requirement, the motor encoder's interface to the
controller must be of a low latency. This is especially true for encoders with
digital interfaces such as SPI or SSI. High latency or high backlash of an encoder
makes it impossible for the controller to use the encoder for the purpose of
spinning a brushless motor. In such a case, the controller shall be configured to
use Hall sensors (if present) or sensorless control instead.
Note that some types of encoders share pins and thus cannot be used together at
the same time.
After enabling an encoder here, proceed to configuring an associated encoder
peripheral in a corresponding section.
It usually makes sense to first run a brushless motor in a sensorless mode, and
only later connect an encoder to the controller. This approach helps gradually
10 www.servosila.com