Motor-Amplifier
The motor amplifier may be configured in three modes:
1. Voltage Drive
2. Current Drive
3. Velocity Loop
The operation and modeling in the three modes is as follows:
Voltage Drive
The amplifier is a voltage source with a gain of Kv [V/V]. The transfer function relating the input voltage, V, to the
motor position, P, is
( ) ( )
[ ]
P V K K S ST ST
V t m e
= + +
1 1
where
[s]
and
[s]
and the motor parameters and units are
Kt Torque constant [Nm/A]
R Armature Resistance
J Combined inertia of motor and load [kg.m2]
L Armature Inductance [H]
When the motor parameters are given in English units, it is necessary to convert the quantities to MKS units. For
example, consider a motor with the parameters:
Kt = 14.16 oz - in/A = 0.1 Nm/A
R = 2 Ω
J = 0.0283 oz-in-s2 = 2.10-4 kg . m2
L = 0.004H
Then the corresponding time constants are
Tm = 0.04 sec
and
Te = 0.002 sec
Assuming that the amplifier gain is Kv = 4, the resulting transfer function is
P/V = 40/[s(0.04s+1)(0.002s+1)]
Current Drive
The current drive generates a current I, which is proportional to the input voltage, V, with a gain of Ka. The
resulting transfer function in this case is
P/V = Ka Kt / Js
2
where Kt and J are as defined previously. For example, a current amplifier with Ka = 2 A/V with the motor
described by the previous example will have the transfer function:
Chapter 10 Theory of Operation • 187 DMC-40x0 User Manual
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