Tech Briefs Magazine - December 2022 - MD4

operating points must fall within the
continuous duty curve specified by the
motor manufacturer.
The speed, torque, and voltage relationship
is similar for stepper motors
(Figure 2) although you should be more
conservative in designating operating
points. As a general rule of thumb, the
operating range of a stepper should be
about half of the max of the speed and
torque to ensure that you don't jeopardize
accuracy by missing steps. This is
much different than with servos, which
you can push more closely to the limits
of their operating range. This is because
servos rely on closed feedback loops that
monitor the motor position against its
target position and make constant
course corrections.
With most steppers in use today, there
is no feedback on missed steps, which
presents more of a safety risk. Some of
the newer stepper motors and related
drives, however, have built-in feedback
control or other means of detecting
missed steps. These are gaining popularity
in recent years with more companies
offering matched motors and drives that
deliver excellent positional accuracy
with most of the speed of a brushless
servo motor. (Figure 3)
The All-Critical Inertia Ratio
Once you have established the speed
and torque characteristics, the sizing
process is not complete until you match
the inertia of the load and motor. The
inertia of the load is its weight in kilograms
per meter squared. The inertia of
the motor is a little more complicated in
that it involves both the rotor and the
shaft, but, fortunately, motor manufacturers
will supply that number. If the
ratio of load inertia to the motor is too
large, say more than 10 or 20 to 1, the
load drives the motor instead of the
motor driving the load.
Poor inertia ratio is a common
problem in motor selection. While it
is generally a good practice to seek
the smallest-sized motor possible, focusing
only on motor
speed and
torque without proper attention to
inertia can cause major problems. A
servo system may not have the characteristic
snappy response you would
expect. It could overshoot its position
target and then return with too much
force, wobbling undesirably as it tries
to fix on its final position.
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On the other hand, picking too large a
motor with too much inertia would give
you that snappy response but with great
inefficiency. You would be devoting much
of your energy to spinning up and stopping
the motor itself, rather than applying
that energy to moving the system
load. And selecting a motor that is too
Servo Motor - Torque vs. Speed
Torque
Peak Stall
Torque
Intermittent Duty Zone
Peak Operating Point
Continuous
Stall Torque
Continuous Operating Point
Continuous Duty Zone
Speed
Maximum Speed @ No Load
Figure 1: Servo motor torque vs. speed. The average of the maximum and minimum operating points must fall
within the continuous duty curve specified by the motor manufacturer. (Image: Thomson Industries, Inc.)
Stepper Motor - Torque vs. Speed
Torque
Peak Stall
Torque
Peak Operating Point*
Speed
* 50% of rated torque and speed
Maximum Speed @ No Load
Figure 2: Torque vs speed - stepper motors. The operating range of a stepper should be about half of the max
of the speed and torque. (Image: Thomson Industries, Inc.)
Motion Design, December 2022

Tech Briefs Magazine - December 2022

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