Ransburg DynaFlow User Manual - Manual (Page 61)

LN-9400-00.9
DynaFlow TM User Manual - Operation
57
Derivative Action (Kd)
Derivative action is proportional to the rate of
change of the error. The derivative term dampens,
or slows down process overshoot and improves
the response to changes in the process being
controlled. Another way to view this term is that
it "anticipates" or leads what is happening with
the actual flow.
Derivative action provides a sudden shift in the
control output as a result of a quick change in the
actual flow (transient) or set point. If the actual flow
drops quickly, the derivative term will provide a
large change in the output in an attempt to correct
the perturbation before it goes too far.
Derivative action should be associated more with
transient response control and less with overshoot
inhibition such as during start-up, or trigger ON.
Oscillation due to derivative action is typically a
cyclic "wander" away from the set point.
Putting It All Together
The optimum PID controller settings are deter-
mined based on the application. Types of applica-
tions that will effect PID considerations are:
• Dynamic control of flow rate while spraying
parts - robot mounted,
• Short GUN trigger times
• Long trigger times - constant flow rate re-
quested during entire part
• Precise 2K mixing ratio required at all times
• Low flow rate applications
• Applicators mounted on oscillators or re-
ciprocators that may produce cyclic back
pressures within the fluid lines
• Systems with piston pump type supplies
• Long pilot line lengths from fluid regulator
to transducer
General Guidelines
1. The type of fluid regulator represents the
larg-est overall impact on system response. The
following table lists control settings for various
fluid regulators that should result in stable con-
trol. The determination of these parameters was
based upon worst case conditions and therefore
constitutes "conservative" control response. It is
Deadband
This represents a flow range above and below
the set point value in which the PID control is
suspended. This keeps the control output from
continually changing and produces stability when
close to the requested value.
Proportional Action (Kp)
Proportional action simply means that the con-
troller output changes in proportion to the error
between the set point and the actual flow. It is
also commonly referred to as gain, proportional
gain and proportional band to name a few. If the
proportional gain is set too high, the system will
oscillate. If set too low, the fluid flow will "wonder"
due to a lack of responsiveness.
Integral Action (Ki)
The Integral element of the PID controller forces
the actual output (flow) to match the desired by
utilizing the sum of the error in flow rate.
Integral action is proportional to the sum of the
error. This term is needed to remove long term,
or steady-state error that cannot be removed by
the proportional term.
Integral action is the most important factor gov-
erning control near the set point. The integral
term changes the control output as a result of a
continuing error between set point and actual. The
integral term will continue to shift the output until
the actual flow rate falls within the Deadband value.
Integral action will also effect transition response
times. The greater the change in requested flow
rate, the more the integral action will effect the
response time.
The integral gain, Ki, must be chosen such that
oscillations do not occur. Increased integral gain
will cause faster response times, but can lead to
process instability and uncontrolled oscillations.