Biomedical Engineering Reference
In-Depth Information
FIGURE 3-34
Stepper motor
torque-speed
curves. [Adapted
from (Alciatore et al.,
2003)].
Dynamic characteristics:
The torque-speed characteristics of stepper motors are non-
linear compared with DC motors. In addition, there are two different rotation modes, as
illustrated in Figure 3-34. The normal operational mode is the locked step mode within
which the motor can be stopped, started, and even reversed in the space of a single step.
However, it is possible to continue to accelerate the motor by decreasing the step interval
until the speed is too fast to allow for stopping and starting between steps. In this mode
the motor can continue to travel in only one direction and has to be decelerated into the
lock-step region before it can be stopped and reversed.
At high speeds, the performance of stepper motors can often be analyzed using the
normal phasor analysis model that is used for synchronous AC motors.
Step response:
At low speeds, each step is discernable, and the overall behavior is that
of a series of step input transients. To determine their shape, the motor must be modeled
as a set of nonlinear differential equations. For a variable reluctance motor, these equa-
tions are
R
+
ω
r
i
+
dL
(
θ
)
d
di
dt
=−
1
L
1
L
v
(3.41)
θ
d
ω
r
dt
1
J
τ
−
1
J
τ
load
=
(3.42)
d
dt
=
ω
r
(3.43)
The motor response for a given step change on the input voltage can be obtained by solving
these equations in conjunction with a number of initial conditions for the current, angle,
and angular rate. For a typical motor in isolation, the response is strongly underdamped,
as illustrated in Figure 3-35, and for that reason it is often expedient to introduce some
form of mechanical damping in the load.
FIGURE 3-35
Step response
without and with
viscous inertial
damping.
