GTK SMA7215, Руководство по эксплуатации и обслуживанию

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CHAPTER 2: THEORY OF OPERATION
Now that we have a good understanding of how the current is provided from an
“
H TYPE
”
pulse-width-modulated (PWM) bridge, let’s analyze the operation of the current loop.
2.8 Current Loop Operation:
Please refer to figure 2.2A for a diagram of the current loop. In control electronics the
symbol Sigma (with the circle around it) is referred to as a ‘summing junction’. The manner
in which this summing junction operates is as follows:
The current-command signal (also referred to as the velocity error signal when received
from the output of the velocity loop, as shown in figure 2.2A) is added to the current feedback
signal. The signal resulting from this addition, is referred to as the
“
current error
”
signal. This
current-error signal is fed into the current amplifier, which in turn produces a current in the
motor. A voltage which is proportional to the motor current is developed across Rs (shunt
resistor). This voltage is referred to as the
“
current feedback
”
signal. The current in the
motor increases until the current-command signal. At this point the current error signal drops
to zero. and the actual current is equal to the commanded current. If anything happens to
disturb either the current command signal, or the current feedback signal, the same process
occurs again until the current feedback signal is equal in magnitude to the current command
signal, but opposite in polarity.
The type of loop described above is referred to as a
“
servo loop
”
because the current
servos about a commanded value.
We are surrounded in our everyday lives by a multitude of servo loops. For example,
many of today’s luxury cars have what is called ‘automatic climate control’. To operate this
servo loop, you set the climate control to the temperature that you wish to be maintained in
the interior of the car (current command signal). The selected temperature is then summed
with the actual temperature from a thermometer (current feedback), and the output (current
error signal) activates either the heater or the air conditioner until the actual temperature as
measured by the thermometer (current feedback signal) is equal in magnitude, but opposite
in polarity, to the set temperature.
2.9 Velocity Loop Operation:
Please refer to figure2.2A for a diagram of a typical velocity loop. The velocity loop’s
operational description is analogous to the current loop description, except for the fact that
the input signal is called the Velocity Command and the feedback signal from the DC
tachometer is called the Velocity Feedback.
2.10 Protection Circuits:
The High and Low Speed Electronic Circuit Breakers(HS/ECB and LS/ECB) protect the
amplifier and motor from being damaged by high motor current(specified max. peak and rms
current values). The Over Temperature and Over Voltage detection circuits will shut off the
amplifier when the temperature of the amplifier or the buss(B+) voltage exceeds a specified
limit. Also, there are circuits which limit the motor from running in either or both directions.