Maybe you dont need to totally understand the operation of your home forced air heating
system as we explained in Part 1 of this video series, but it helps to have something to
compare to when trying to understand the operation and benefits of a PID Loop Controller as used
in an industrial process controlled application.
Another application that may be thought of as making use of a PID Loop Control would
be the cruise control in your automobile. Turn the cruise control ON, press SET
when you are at the speed you want to maintain. A sensor measures the vehicles current speed,
while a small computer running a PID program compares the SET speed to the current speed
and adjusts the vehicles throttle valve through a vacuum actuator, with the result being your
speed is maintained. Up and down hills pose no problem with the vehicle automatically
accelerating and decelerating. Press the ACCEL or COAST button to adjust your speed in
1 mile per hour increments. Press OFF, or use the brake, and you are back in control.
It is only fair that we give you a short explanation of whats behind the working of a PID. To
better understand how a process is controlled with the use of a PID Loop Controller, such
as the feature available in the Productivity 3000, take a look at the key elements as shown
here in a typical PID loop diagram. We enter the end result we are looking for as a Set
Point value. We have the ability to measure were the current or real time process stands.
This measurement becomes our feedback, which we reference as our Process Variable. The
Set Point value is compared with the Process Variable and the differences between the two
values becomes our Error Term. The Error Term value is used with the Proportional gain value
to produce how much correction we need from the PID Output to force the Process Variable
closer to our Set Point.
The question that comes up now is how much Proportional gain do we need? Take a look
at just the Proportional term in our PID formula. If we have too little Gain, it will take a
long time for the Process Variable to get to our desired target when a change is made
to the Set Point. If we are too aggressive with the Gain, our Process Variable will overshoot
our Set Point and possibly start oscillating back and forth around the Set Point. Ideally
we want the Gain set to a value that will produce the shortest response time getting
to the Set Point without a large overshoot or oscillating as seen in Diagram three.
Here we see the typical formula that is used to determine the Process Output based on the
calculated Proportional gain, the Integral time and the Derivative rate. It helps to
keep in mind that the Proportional gain acts on the present error. The Output correction
is further modified by the Integral rate value, which is calculated from the accumulation
of past errors. The Derivative rate, when used, is included in the final Output correction
based on a prediction of future errors, with future errors being determined by the current
rate of change.
Our example PID Loop Control application that follows will also include a method to set
the desired result, or the Set Point, monitor the effective result, or the Process Variable,
but instead of an on/off condition of the Output as in the home heating control system,
our example PID Loop Controller will be able to vary the Process Output anywhere from off
to fully on as required during the operation. Any Error between our Set Point and the current
Process Variable will be handled by our PID Loop Calculations. Our Set Point will be the
desired volume in gallons of liquid we require in the Process Tank, and our Process Variable
will indicate current volume based on the liquid level measured by an Ultrasonic Sensor
and converted to gallons.
Join me for the third video in this series in which I cover the PID Loop application
example, and also explain the hardware that was used to create our working demo.