Galil DMC-13X8 user manual Current Drive, Velocity Loop

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 / Js2

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:

P/V = 1000/s2 [rad/V]

If the motor is a DC brushless motor, it is driven by an amplifier that performs the commutation. The combined transfer function of motor amplifier combination is the same as that of a similar brush motor, as described by the previous equations.

Velocity Loop

The motor driver system may include a velocity loop where the motor velocity is sensed by a tachometer and is fed back to the amplifier. Such a system is illustrated in Fig. 10.5. Note that the transfer function between the input voltage V and the velocity ω is:

ω/V = [Ka Kt/Js]/[1+Ka Kt Kg/Js] = 1/[Kg(sT1+1)]

where the velocity time constant, T1, equals T1 = J/Ka Kt Kg

This leads to the transfer function P/V = 1/[Kg s(sT1+1)]

V Σ Ka

Kg

Figure 10.5 - Elements of velocity loops

Kt/Js

The resulting functions derived above are illustrated by the block diagram of Fig. 10.6.

Chapter 10 Theory of Operation • 162 USER MANUAL