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The PID Control Process:
To get a picture of how the PID process works, first
consider that speed needs to be controlled. To do this,
we send a speed-setting command to the decoder. This
speed-setting command initiates the PID process and is
called the “set point.” In response, our decoder sends
power to the motor, which starts to turn. At the same
time, the decoder reads the Back-EMF signal to
determine how fast the motor is turning. Once the motor
nears the speed of the set point, the PID process begins
to function. The manner in which the individual PID
components interact is briefly described below
.
The Proportional Component:
The “P” part of the process stands for “Proportional,”
which in essence, establishes a “speed band” parameter
around our set point. This “proportional band” tolerates
some speeds above and below the speed entered as the
set point. It is somewhat akin to the way a thermostat
controls furnace temperature in a house. The furnace
continues to produce heat until its “set point” (thermostat
setting) is reached. After this, some heat production
(called overshoot) continues even after the thermostat
turns the furnace off. Most thermostats then wait until the
temperature drops about 2 degrees below the set point
(called undershoot) before the furnace is again turned on.
Its “proportional band” is between the two levels. The
proportional component in PID works in a similar manner.
Integral Component:
The “I” part of the process stands is for “Integral
”
action,
which controls the difference between the set point and the center of the proportional band. This may not be the same due to
system imbalance. The Integral action senses and tries to correct the difference between the set point and the decoder’s “center
point.”
The Derivative Component:
The “D” part of the process is “Derivative” action, which looks for any change in motor
speed. When it senses a change, this function tries to correct it. Such a change could be triggered by a locomotive
starting up a grade with a string of cars, whose increased load, has caused the motor to slow. The derivative
component “sees” this change and applies a correction. Such a slow-down, may also be caused by a bind in the
mechanism or a tight spot in the track. Note that some decoders use only the
P
and
I
(PI) functions and not the
D
. The
Revolution, on the other hand, is weighted toward utilization of the
P
and
D
components. Most of the decoders that use
PID utilize a single set of values for the functions that are used over the total speed range. The Revolution, on the other
hand, has four sets of PID values, between which it alternates depending on speed-step settings.
PID and Locomotive Control (QUANTUM PROGRAMMER REQUIRED):
Note that the only way we have of checking the effects of any PID modifications – for which the Quantum Programmer
is required – is by observing the reaction of the locomotive following each adjustment. There is no “standard” for the
values used in these CVs, nor is there a standard way to set the CVs. For the most part, determining their optimum
values is a matter of trial and error. Fortunately, the Revolution comes with a fairly universal PID structure.
NOTE: We do not list the CV's required to change these parameters because they are absolutely locomotive-
specific. If you are interested in changing these parameters you should purchase a Quantum Programmer and
consult the included CV Manager software manual for more specific information.
