Parker 590 series, Руководство по эксплуатации продукта

Страница: 163 / 242

Control Loops
9-1
590 Series DC Digital Converter
9
C
ONTROL
L
OOPS
Principle of Operation
Note: Selection between Current Control or Speed Control (default) is
made by the I DMD ISOLATE (current demand isolate) parameter
using Digital I/P3 (Terminal C8). If ENABLED the Converter
operates as a current controller, and if DISABLED (the default) it
operates as a speed controller.
Current Loop
The current loop accepts a demand from either the speed loop, or directly from the plant, and
forms an error signal which is the difference between demand and average value of feedback.
The error signal is fed into a Propor Integral compensator which produces the output of
the current loop, i.e. the firing angle signal.
In the Converter, the error signal is created in two different forms:
1. The average error is computed as the difference between demand and average value of
feedback and fed into the Integral part of the P + I algorithm.
2. The instantaneous error is computed as the difference between demand and instantaneous
value of feedback and is fed into the Proportional part of the P + I algorithm. This gives
higher transient performance since it does not contain any time lag, unlike the average which
has a built-in lag of 1/6 of mains cycle. However, the average is the true measurement of
torque which is the objective of the current control and this is not affected by the small time
lag in achieving zero steady-state error.
The firing angle signal is translated into a certain time delay from the mains zero cross point
(obtained via a Phase-Lock-Loop) and this results in a firing command being issued to the
thyristor stack every 1/6 of a mains cycle in steady-state.
Some special features of the current controller are discussed separately below.
Adaptive Current Control
The gain of a thyristor 6-pulse converter (voltage-time area over firing angle) drops dramatically
at discontinuous values of armature current. Therefore a gain boost is required in the current
controller to compensate for that.
In the Converter, this is handled by an adaptive algorithm which allows the current to follow the
demand in one step (firing) within the discontinuous region of operation.
Back EMF (BEMF) Estimate
With the motor at standstill, the firing angle for zero current is 120 degrees. When the motor is
rotating at different speeds the firing angle for zero current follows a cosine locus.
It is of paramount importance to track this locus as close as possible throughout the speed range
if the current loop bandwidth is to be maintained at its highest possible level during current
reversals from master to slave bridge and visa-versa.
There are two reasons for the loss of bandwidth at current reversals.
Firstly, the loss of converter gain needs to be compensated in an accurate way which is the
objective of the adaptive algorithm.
Secondly, the above algorithm also relies on the right start-up value of firing angle in the
incoming bridge in order to minimise both the "dead-time" (time interval of zero current referred
to below) as well as the rise time to the required current demand.
In order to get the right start-up value of firing angle the knowledge of the operating BEMF is
necessary. In the Converter, this is achieved by a combination of a hardware peak current
detector and appropriate software algorithm.
MMI Menu Map
1 SETUP PARAMETERS
2 CURRENT LOOP
I DMD ISOLATE