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Motors and Generators | Abb AC Servo motors
5
In selecting a motion control package, one of the areas
requiring identification is the mechanics of the load which will
be moved. Once this physical data is obtained, the proper
matching of motor and control can easily begin.
The mechanics of the load involve both friction (which is easy
to understand) and inertia (which is an unknown, since we
have difficulty in recalling the physics we had in school).
The first part of the equation, determining friction of the load,
can be accomplished by either estimating, or measuring by
simply using a torque wrench.
The second part is determining the inertia. Inertia is the
resistance of an object to be accelerated, or decelerated.
In motion control, inertia is an important parameter since it
defines the torque required to accelerate the load and get it
into position.
Calculating servo motor requirements
If no one has told you what the inertia is, then to
answer this question, you will have to do a calculation.
However, once sufficient information is obtained, the
task is relatively simple.
To determine the inertia, the mechanical linkage
system which will be moved will be analyzed. These
mechanical systems can be divided into four basic
categories: direct drive, gear drive, tangential drive,
and ballscrew drive.
In the following, each of these mechanical linkage
categories and relevant formulas for calculating the
load parameters will be presented. In all instances,
the formulas reflect the load parameters as “seen” by
the motor. Reflecting all these parameters back to the
motor shaft make the calculation easier for selecting the
motor and control for your motion control application.
Direct drive
The simplest of packages is the first basic category, the direct drive. This would not require the load parameters to be reflected
back, since there are no mechanical linkages involved.
The equations for the direct drive are presented in Figure 1. The speed of the load is the same as the motor, the friction of the
load is the friction which the motor must overcome, and the load inertia is directly what the motor would “see”.
where
S
m
=
motor speed (rpm)
S
1
=
load speed (rpm)
T
m
= motor torque
T
1
= load torque
J
t
= total inertia
J
1
= load inertia
J
m
= motor inertia
speed (motor)
=
speed (load)
S
m
=
S
1
torque at motor
=
torque at load
T
m
=
T
1
total inertia
=
inertia (load) + inertia (motor)
J
t
=
J
1
+ J
m
Direct drive
Figure 1
Motor
Load
Servo motor selection
Abb AC Servo motors