Specifications

Motor Control
High-accuracy motor control with encoder data interfaces
106 Maxim Industrial Solutions
Overview
The accuracy with which a motor
needs to be controlled depends on
the system requirements. In some
applications the accuracy require-
ments are very high, as in industrial
robotics or in bottling. A welding
robot, for example, is expected to
operate with high speed and high
precision. Similarly, the motors in a
bottling factory must be controlled
accurately so that bottles are
stopped at the right position for
filling, capping, and labeling. To
control a motor precisely, the rotor’s
speed, direction, and position have
to be determined. These can be
monitored with analog sensors like
resolvers, synchros, RVDTs, or rotary
potentiometers. High accuracy is
obtained with the use of encoders
like optical encoders and Hall-effect
sensors. Encoders provide the
controller with incremental and/or
absolute shaft-angle information.
A motor controller, commonly imple-
mented algorithmically by a digital
signal processor (DSP), calculates the
rotor’s present speed and angle. It
adjusts the actuating power stages to
achieve the desired response efficiently
and optimally. This feedback control
loop requires robust and reliable
information from the sensor, typically
communicated over long cables from
the encoder to the controller.
Incremental information is typically
transmitted to the controller by
quadrature signals, i.e., two signals
phase shifted by 90°. These signals
can be in analog form (sine + cosine)
or in binary form. Absolute position
information, in contrast, is only
communicated by a serialized binary
data stream through RS-482 or RS-422.
As the working environments are
harsh, the data paths need to be
robust and reliable. EMI levels are high,
which explains the use of differential
signaling. High temperatures are
commonly encountered due to the
proximity to the motor.
Maxim’s extensive range of RS-485/
RS-422 and PROFIBUS interface
devices are targeted for these
motor-control applications. Interface
devices like the MAX14840E high-
speed RS-485 transceiver exhibit the
high-signal integrity and robustness
expected for stringent safety control
and for sustaining the up-time of
large capital investments.
High-accuracy motor control with encoder data interfaces