Watlow Electric Revision 5 - Manual (Page 60)

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8.2.3 OUTPUT FILTER
There is no tuning step for the Output Filter. Adjusting the PID without the
Filter gives the most accurate response of the control function of the PID
modes. The Output Filter may be turn on anytime so desired. The number is
increased as necessary to reduce cycling of the control output signal, thus
reducing the cycling of the PV. Some typical settings of the Output Filter is
2, 4, 8, 12, and 15. Use of the Output Filter is not reflected in the PV except
to make the PV more stable, the value of the PV is the actual response time
of the Primary Element and it's value.
8.3 ANALOGY OF PID CONTROL TERMINOLOGY
The terminology of PID may be confusing to technical, as well as non-technical
individuals, who have a need to have some understanding of PID control, due to
work requirements. The comparing of an unknown to the known has been a relative
easy way to explain a difficult subject for many years. The following analogy has
been used for many years and very successfully. The PID terms have been equated
to that of driving a car.
The little ole lady from Pasadena, the grandmother type, was out for a Sunday
drive. As she was waiting at a stop light for the light to turn green, a young man
who shall remain nameless, pulled up along side her. This young man had just
received his driver's license and had Daddy's car out for the first time by himself.
Pumping the gas pedal, he was gunning the engine and looking over at the little ole
lady. Needless to say, that when the light turn green, he stepped on the gas hard.
With burning tires, he squealed away, leaving the little ole lady behind. She in her
turn, gradually stepped on the gas, gently bringing the car up to the speed of 30
mph. The young man in the meantime had reached the next stop light and it was
red. He slammed on the brakes and came to a very quick stop.
While, waiting for the light to turn green, the young man was gunning the engine
and watching in his mirror, as the little ole lady gradually came up behind him. As
she approached the light, it turned green. She went through the light without
needing to change the car speed, while the young man once again, stepped on the
gas hard. They continued to repeat the same action over and over again. She
proportioned her speed to reach each light as it turned green, while the young man
was cycling between stepping on the gas or the brake. His gain was too high, as he
reacted too fast to changing conditions. This caused cycling of his car speed to a on-
off state, not to say anything about his Dad state, if he had known. The little ole
lady had proportional control over her car speed by reacting gradually to changing
conditions. This is known as the PROPORTIONAL FUNCTION.
Now, the little ole lady with proportional control was trying very hard to maintain
the 30 mph. This was the speed that the traffic lights were set for, thus allowing the
smooth flow of traffic. As she approached a fairly steep hill, her speed started to fall
off. Her initial response was a proportional push on the gas pedal. This was not
enough to hold the speed to the 30 mph and the speed was very slowly decreasing
from the 30 mph she wanted. She very gently increased the pressure on the gas
pedal, raising the speed back up to 30 mph. As she started to go down the hill, the