Heat Manager Pro User Manual V1.5 Page 69
B.2.4 Differential action
The D stands for differential (or sometimes called: derivative) action. This takes the rate of
change per time unit of a deviation between set value and process value into account.
Differential action temperature control is also referred to as rate. The degree of differential
action is expressed by the differential time in seconds.
The controller measures the rate of the temperature increase per time unit and moves the
proportional band to minimize overshoot. The output change is directly proportional to the rate
of change in the process value (PV) per time unit.
Theoretically speaking a heating process is a pure integrator and can be controlled by using
only a P and I action (most slow processes are). Practice however shows that a small amount of
D action can improve accuracy and especially responsiveness to changing external circum-
stances just a bit further.
A D-action is often used in positioning systems (servo applications) where quick movements are
performed. A D-action is also important in a tunnel furnace where the product load differs from
time to time (sometimes no product feed), here the D-action is used to prevent over- or under-
shoot under various process load conditions that change more quickly than the integral time of
the process itself.
A too high D action will result in a very unstable process with large over- and undershoots.
A too low D action will result in a system that does not respond quick enough (too late) to
Varying process conditions. This will also result in an unstable system with over- and under-
shoots.
B.2.5 Cycle time
Another important setting in a PID controller is its cycle time. This time is the time span used by
the controller to calculate how long a relay should be on or off.
Here is an example: we have a mechanical relay that is driving a heating element and the cycle
time is set to 15 seconds.
When the controller output is 100% the relay will be continuously activated. When the process
value goes a bit over the set value the controller starts to react to this by
modulating
the
controller output back to a lower value, e.g. 50%.
When the output is 50%, and the cycle time is 15 seconds, the relay will be on for 7.5 seconds,
then switch off for 7.5 seconds, switch back on for 7.5 seconds, etc. etc. etc.
When the controller output is only 10% the relay will be switched on for 0.1 x 15 = 1.5 seconds
and then remain off for 15
– 1.5 = 13.5 seconds.
The problem is that we introduce some kind of
lag time
into the system. Of course it would be a
good idea to set this cycle time as low as possible, because then the controller can react more
quickly to varying process conditions. The major disadvantage is then, that the
mechanical
relay
has to switch very quickly on and off. This will damage the contacts of the mechanical relay too
quickly and wear them out too fast.
As a
compromise
the cycle time has been set to 15 seconds for this purpose.
