Specifications

Motor Control
Sensing motor speed, position, and movement
www.digikey.com/maxim-industrial 99
Overview
Hall-effect sensors are used to sense
the speed, position, and direction
of motors. With integrated device
logic, the sensors then communicate
that data to the system for real-time
feedback. The sensor also detects
and reports any interruption to
the motor so corrective action can
be taken. Typically, to detect the
direction of movement two Hall-
effect sensors are used.
Commutation can be synchronized
to Hall edges if the system has the
same number of Hall-effect devices as
motor phases, and if the mechanical
geometry of the Hall-effect devices
is correlated with the electrical
geometry of the motor phases.
Maxim’s MAX9641* combines two
Hall-effect sensors and sensor
signal conditioning to provide both
positional and directional outputs.
Hall-effect sensors can also be used
with special Hall-effect sensor
interface products like the MAX9621.
The interface devices provide several
functions: protect against supply
transients, sense and filter the current
drawn by the Hall-effect sensors, and
diagnose and protect against faults.
Hall-effect sensors improve robust-
ness and repeatability, compared to
mechanical photointerrupter-based
systems which are compromised
in environments with dust and
humidity. Since Hall-effect sensors
detect the magnetic field produced
by a magnet or current, they can
operate continuously in such harsh
environmental conditions.
In some applications vibration, dust,
and high temperature cause active
sensors to operate improperly. In
these situations passive elements
can be used to sense the motor’s
operation and feed that data to
the system with an interface IC.
Alternatively, variable-reluctance (VR)
sensors can be used in these extreme
operating conditions.
VR sensors like the MAX9924–
MAX9927 have a coil to sense the
speed and rotation of motors. When
the toothed wheel of the shaft
attached to a motor passes by the
face of the magnet, the amount of
magnetic flux passing through the
magnet and, consequently, the coil
varies. When the tooth is close to
the sensor, the flux is at a maximum.
When the tooth is further away, the
flux drops off. The rotating toothed
wheel results in a time-varying flux
that induces a proportional voltage
in the coil. Subsequent electronics
then process this signal to get a
digital waveform that can be counted
and timed more readily. Integrated
VR-sensor interface solutions
possess many advantages over
other solutions, including enhanced
noise immunity and accurate phase
information.
Sensing motor speed, position, and movement
*Future product—contact factory for availability.