West Control Solutions N6600 - Manual (Page 7)

Operator Mode: This is the mode for day-to-day use. The parameters
in this mode are freely available to the operator. The adjustment
facilities available in this mode are dependent upon the settings of
parameters in the Set Up Mode.
1.2 HOW IT WORKS
The Plastics Controller is tailored towards plastics applications. The function of the
Controller is best described in terms of the control it exercises over the process
and the use of its alarms.
1.2.1 Control
The temperature at which the process must operate is called the setpoint (SP). The
actual process temperature which is being measured and controlled is called the
process variable (PV). Thus, if the setpoint is adjusted to 200 ° C, the Controller
endeavours to maintain the process variable at 200 ° C.
PID control (also known as three-term control) is a well-proven and widely-used
method for high accuracy automatic control. Best results are obtained when the
Controller is correctly tuned - easily achieved by just letting the Controller tune
itself to your process, using the automatic tuning features.
Heater current is controlled via Output 1. Although the heater can only be either
fully-on or fully-off, the process reacts only to the average power, assuming heater
on-off periods are sufficiently brief. The PID algorithm can control average power
very accurately, thereby ensuring smooth and accurate control of the process.
This method of controlling output power is very common and is known as
time-proportioning. An adjustable Cycle Time parameter determines the duration
of each on-off output cycle. Longer cycle times prolong mechanical relay
contact life; shorter cycle times (normally using solid state relays) will be needed
on processes which react more quickly. Output 2 can be used as a cooling
output, if required.
The Controller has a special Soft Start feature, used when a gentle start-up phase
is required in order to avoid damage to the process. An adjustable Soft Start
Setpoint is used by the Controller during a Soft Start. The duration of the Soft Start
phase is determined by an adjustable Soft Start Time parameter. During this
phase, Ouput 1 power is kept within an adjustable limit and the Output 1 cycle
time is reduced to a quarter of its normal value (but never less than 0.5 seconds)
to reduce further the risk of thermal shock to the process.
The Controller can be put into Manual Control if selected by the operator. In this
mode, the operator adjusts manually the Controller’s output power. When
switching between automatic control and manual control, the Controller
minimises any sudden power changes; this is known as a “bumpless transfer” and
avoids thermal shocks to the process. Manual Control mode can be configured to
be a non-adjustable zero power value or disabled completely.
O075-4 1-3
PM-0075