Page 667 - Mechatronics with Experiments

ELECTRIC ACTUATORS: MOTOR AND DRIVE TECHNOLOGY 653

PWM circuit
Triangular
Sinusoidal Current wave gen. DC power
commutation alg feedback loop source
i
cmd + Inverter
sinθ _ K I Driver DC+
Comparator
Driver Tr 3 Tr 5
Tr
1
sin(θ+120) _ K I Driver Tr 4 Tr 6
Comparator Tr 2
Driver
DC–
sin(θ+240) _ K I Driver
Comparator
Driver
A S

+ T
+
Rotor and Stator windings
K
phase position sensor
adv
Enable phase
advance option
FIGURE 8.32: Brushless servo motor drive block diagram.

vector generated by the stator. In order to generate the maximum torque per unit current,
the objective is to keep the stator’s magnetic field perpendicular to that of the rotor. By
controlling the phase currents in the stator phase windings, we control the stator’s magnetic
field (magnitude and direction, a vector quantity). Therefore, the torque direction and
magnitude can be controlled by controlling the stator’s magnetic field relative to that of
the rotor.
There are two types of brushless drives based on the commutation algorithm:

1. sinusiodal commutation, and
2. trapezodial commutation.

If the winding distribution and the effective magnetic circuit of the motor is such that the
back EMF function is a sinusoidal function of rotor angle, such a motor should be controlled
using a drive that uses the sinusoidal commutation algorithm. Similarly, if the motor back
EMF is of trapezoidal type, the drive should be the type which uses the trapezoidal current
commutation. The variation of the back EMF voltage as a function of rotor angle is the
same a function for torque gain as a function of the rotor position under constant current
conditions.
The sinusoidal commutation drive provides the best rotational uniformity at any
speed or torque. The primary difference between the two types of drives is a more complex
control algorithm. For best performance, the commutation method of the drive is matched
to the back EMF type of the motor. The back EMF of the motor is determined primarily by
its winding distribution, lamination profile, and magnets. The feedback sensor and power
electronics components remain the same. In recent years, as the cost of high performance
digital signal processors has fallen, the use of the sinusoidal commutation brushless drive
has exceeded that of all others.