Nira i7x - Page 61

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4.5.2 Feed forward table
Using a feed forward table is a prediction technique that estimates the
output from the proportional-integral-derivative (PID) control algorithm
without waiting for the PID algorithm to respond. If the feed forward
prediction can estimate the control output closely, the PID algorithm, whose
job it is to minimize the error between the current state of the system and the
desired state, will need to do less to correct the error. This reduces the error
faster or keeps the error smaller than relying on the PID algorithm alone.
4.5.3 Proportional factor
The proportional factor is easiest to understand: The output of the
proportional factor is the product of gain and measured error. Hence, larger
proportional gain or error makes for greater output from the proportional
factor. Setting the proportional gain too high causes a controller to
repeatedly overshoot the setpoint, leading to oscillation.
The downside to a proportional-only loop is that when error becomes too
small, loop output becomes negligible. Therefore, even when the
proportional loop reaches steady state, there is still error. The larger the
proportional gain, the smaller the steady state error, but the larger the
proportional gain, the more likely the loop is to become unstable. This
dilemma leads to inevitable steady-state error called offset.
4.5.4 Integral factor
Think of the integral factor as a basket in which the loop stores all measured
error. Remember that error can be positive or negative, so sometimes error
fills the basket (when positive error is added to positive error or negative
error is added to negative) and sometimes it empties the basket, as when
positive error is added to negative, or vice versa.
When the integral factor functions properly in the control loop, the basket is
nearly empty. Even when error is so small that the proportional factor is no
longer effective, the integral is still hard at work, collecting error until it is
large enough to matter, as part of the integral's function is to eliminate
steady-state offset.
In fact, most control loop action at steady state is due to the integral factor.
The downside to the integral factor is that it strongly contributes to controller
output overshoot past the target setpoint. The shorter the integral time, the
more aggressively the integral works.
4.5.5 Derivative factor
The derivative factor is the least understood and used of the three factors. In
fact, a majority of PID loops in the real world are really just PI loops. That
does not negate the fact that there are certain applications in which the