Specifications

120 Control and Automation Solutions Guide
Controllers for BLDC Motors
Since the commutation in a BLDC motor
(Figure 4) is electronic, some means is
required for detecting rotor position
relative to the stationary armature.
Typical solutions for this are Hall-eect
sensors and rotary encoders such as
optical encoders, resolvers, or rotary
variable dierential transformers (RVDTs).
More designs are using sensorless
approaches where stator coil back EMF
variation is sensed, which indicates
rotor position. This information is
typically sent to a microprocessor to
determine power FET drive timing.
Various user interfaces allow soft-
starting, acceleration control, speed
control, and response to locked rotor.
Stepper Motors
Stepper motors are really more like
rotary positioners than motors. They are
usually smaller motors with many poles
used for precise positioning applications
(Figure5). They are often driven “open
loop,” meaning there is no position
detection. Their position is assumed to
follow the step commands exactly. Loss
of step position can occur, however, so
some mechanism must be provided to
indicate slippage and to reset proper
positioning. At low drive rates, they
come to a complete stop between each
step. Many drive waveforms are possible,
the simplest has each winding energized
one at a time. Other variations are
possible where overlap in energization
occurs between adjacent windings to
provide smaller steps. Microstepping is
achieved with sinusoidal, overlapping
current waveforms that give very
smooth and quiet rotation.
DISPLAY
DRIVER
UART
SUPERVISOR
SWITCH
DEBOUNCER
KEYBOARD
SCANNER
KEYBOARD
SWITCHES
DISPLAY
FRONT PANEL
HALF-BRIDGE
DRIVER
ADCs
HALF-BRIDGE
DRIVER
HALF-BRIDGE
DRIVER
ISOLATIONTRANSCEIVER
FIELDBUS
V
CC
RESET
µP
I/0
I/0
V
CC
OP AMP
CURRENT-SENSE
AMP x3
TEMPERATURE
SENSOR
BRUSHLESS DC MOTOR
MOTOR
SPEED,
POSITION
SENSOR
V
CC
BUCK
V
DC
MOSFET H-BRIDGE
SENSOR
INTERFACE
Figure 4. Controller for BLDC motor.
Figure 5. Stepper motor (windings removed) showing multi-
toothed rotor and stator design for fine stepping.