AutomationDirects newest Programmable Automation Controller, the Productivity 3000, was designed
to allow the end user the ability to speed up their programming effort, therefore allowing
a quicker turn around in applying automation to a control or process application. In this
video, Ill show you the simple way to program a PID Loop that normally would take longer
in traditional programmable controllers. Lets get started with learning what is a PID Loop
and what it can do for us.
One of the more daunting tasks that typically rises fear in anyone new to programmable
controller programming is dealing with PID Loops. PID Loop Control is typically used
in process control to hold the end result more constant as compared to other control
methods. One example of using a PID Loop Controller would be in maintaining the temperature in
a baking oven. By keeping the temperature constant, the baker can assure better uniformity
in the end result of his product, or in other words, perfect crust in color and texture.
In the course of this video series, I will cover a common application most of you will
be familiar with that includes the basics that can be found in a PID Loop, explain the
operation of Proportional, Integral and Derivative control, describe a simple application used
as our demonstration example, cover the hardware and wiring used in the example, explain set
points and process variables, create and use tagnames to make programming go quicker, show
how to scale the variables used in our loop into understandable engineering units, use
the Productivity 3000?s built-in PID Loop instruction, incorporate a C-more Touch Panel
to make setup, monitoring and operation of the PID Loop a snap, manually and automatically
tune the loop, and finally demonstrate the end result.
Lets start by getting a better understanding of where and how a PID Loop Control would
be used. The simplest way to get a feel for what a PID loop does is to think about the
thermostat in your home. Although the typical home heating system thermostat is not a PID
Loop Controller in the normal definition, all of the basic elements we would find in
a PID controlled loop are there. Set the thermostat to the desired room temperature. We can call
the desired temperature value on the thermostat our Set Point. When the monitored temperature
falls below the desired temperature by a predetermined amount, the thermostat sends a signal to the
Furnaces burner to tell it to turn on and produce heat for the house. Once the desired
temperature is reached, the thermostat turns the heating source off, and waits for the
next cycle. Heres a typical sequence of operation for a home heating system.
Please keep in mind that the heat source in our example is either fully on or fully off.
We will learn that the Output from a PID Loop Controller is varied from no output, to fully
on, to anywhere in between, which allows better control of the end result. The PIDs output,
at any given instance, is based on continuously updated calculations within the PID Loop instruction.
The Thermostat, with the ability to monitor the house temperature and determine when to
signal for heat, is acting as an on-off controller in a Closed Loop system. Closed loop typically
means we have some form of feedback that is used to indicate the desired results are being
produced, so in heating our home we use the thermostat to measure the current room temperature,
which we can also call our Process Variable.
OK, to better understand the operation of the home heating system and get a better feel
for what a PID Loop Controller can do, be aware that there is normally a fan/limit control
switch in the heating system. This allows the air in the plenum, a chamber above the
heat source, to reach a certain temperature before the blower kicks on to force heated
air throughout the home. The use of the fan/limit switch prevents unpleasant cold air from being
felt by the occupants, and also explains the lag between when the furnaces burner turns
on and when the blower kicks on, which you may have noticed in your own home.
Because of this condition, the system stores extra heat in the plenum, which if allowed,
would cause the room temperature to go above, or overshoot the desired temperature. This
is already sounding like a good place to use a PID Loop Control with its ability to do
some predictions based on past history, and prevent this overshoot. Most thermostats do
include an anticipator circuit that will turn the heat off prior to reaching the set point
temperature. On older style mercury switch thermostats, it is a small resistive wire
placed close to the bi-metallic coil that controls the mercury temperature switch. A
slider is used to calibrate how much of the wire is used in the circuit based on the current
draw of the gas burner valve.
Look at what happens now when the thermostat calls for heat. The electrical current that
goes to the gas burner control valve passes through the resistive wire. The wire produces
heat and causes the bi-metallic coil to operate the mercury switch sooner, signaling that
it has reached the Set Point temperature prior to the actual room temperature being satisfied.
Again, the blower will continue to run, which adds additional heated air to the home, and
if all is set properly, the actual temperature felt in the home will be at the Set Point,
or at least close to it.
By the way, the limit part of the fan/limit switch is a safety that will turn off the
burner if the air in the plenum gets too hot, usually an indication that the blower isnt
working. And in case you were curious, sometimes the cooling side of older style thermostats
also includes a form of anticipator, although newer digital set back thermostats, with their
quicker response and accuracy, have done away with wired anticipators, being replaced by
smart programmable electronic set back thermostats.
For the most part, we are comfortable with the end results, although on some days and
under certain conditions, things may not be perfect.
Using the same heating system, but instead of an on/off gas burner, well use an electrical
heater. The electrical heater is controlled with a variable power device that can adjust
the heaters wattage output, or BTUs, from zero to full output, and anywhere in between.
Next replace the thermostat with a PID Loop Controller. We still have our Set Point to
fix our desired temperature and also the ability to measure the actual room temperature, our
Process Variable. But now we can control the amount of heat we need based on the PID loops
calculations. This ability will produce a more constant result and in the process most
likely use less energy.
Join me in Part 2 of this video series as I explain how a PID Loop works.