Heat Manager Pro User Manual V1.5 Page 66
In the early history of automatic process control the PID controller was implemented as a
mechanical device. These mechanical controllers used a lever, spring and a mass and were
often energized by compressed air. These pneumatic controllers were once the industry stand-
ard. Later on the first electronic analog PID controllers were made with tube-amplifiers.
After the invention of the solid-state transistor things were progressing into analog solid state
PID controllers and later, with digital IC technology, the first truly digital controllers were invent-
ed that make very advanced adaptable PID controllers possible under quickly varying process
conditions.
B.2
F
ROM THEORY TO PRACTICE
In order to fully understand a PID controller, an extensive and thorough mathematical
knowledge of Laplace and Z-domain transformations is necessary. That goes too far for this
manual.
We therefore take a more practical approach and try to describe a PID controller in common
language that almost everybody can understand.
To make things easier we will start out with a very simple on/off controller, think of the old
thermostat that was used in the past to control the temperature in a room (the modern types
used nowadays are almost all PID controlled).
B.2.1 ON/OFF control action
ON/OFF temperature control is the simplest and least expensive form of control available. The
output signal from a controller is either FULLY ON or FULLY OFF depending on the direction of
the deviation between the set value and process value.
The figure below shows the characteristics of an ON/OFF control action:
The ON/OFF control action takes place if any deviation from the set value occurs.
This action responds quickly, but is sensitive to input noise which causes chattering (ON/OFF
switching at short intervals). Therefore, in actual use, the ON/OFF temperature control action
has some hysteresis which is named dead band or control sensitivity. This prevents the quick
chattering around the set value when the process value is very near.
