Before implementing PID we need to understand our hardware and more importantly what our
hardware can do.
Let’s fill in the details of this block diagram from the previous video.
We’ll be using this single phase 120-volt 1 horse power drive to control this 3500-rpm
jet pump motor.
The motor is driving an old swimming pool pump I had laying around which forces water
up to some manual butterfly valves over here.
This 15-psi sensor sends a zero to 10-volt signal back to the drive, so it can monitor
the system pressure.
That requires an external 24-volt dc power supply.
I added a process meter here, so we can see what the pressure is while we change the speed
of the motor.
Since it’s just a zero to 10-volt signal we could use a volt meter here, but the process
meter is nice because we can set it up to display psi instead of voltage.
The water is collected in a 55-gallon drum and drains to a reservoir.
The intake to the pump has a check valve to keep the water from draining out when the
pump is not running.
We’ll control run stop with a switch on digital input 1 and we’ll use digital input
2 so we can switch between controlling the drive output automatically by PID or manually
by us.
The drive is powered from a 120 volt single phase input through a disconnect breaker.
This block diagram and this schematic which shows the actual pin numbers can be downloaded
from the link in the description below the video.
I took this block diagram and built a small demo station like this.
This drive controls this motor which drives this pump.
The water goes up here to these valves and drains out the back to a reservoir which is
just a little kiddie pool.
As each valve is opened, the pressure drop is sensed by this zero to 10-volt pressure
sensor which goes back to the drive.
This is the process meter that is displaying the output of that sensor.
These are the two control switches and this is the power supply I’m using to power the
sensor.
The CFW300 drive does have a 10-volt output for powering sensors, but this sensor needed
at least 16 volts.
We have two questions to answer.
First, can this system maintain 6 psi with all the valves open?
And second, how does the process variable – the pressure in our example – react
to changes in the control output or the motor speed.
I’ve already entered these basic parameters for the motor and these for drive control
which says we are only using local mode and it assigns the digital inputs.
We’re not actually using analog input 1 yet but it does default to zero to 10 volts,
so we don’t need to change that.
Great, with one valve open let’s switch to run mode.
Ok, at zero hertz we get basically zero psi – no surprise there.
I’ll increase the drive frequency to 5 hertz we still don’t have any pressure.
10 hertz?
Nope.
15 hertz.
Nothing.
20 Hertz.
Ahh!
Looks like we might finally be getting some pressure.
At 25 hertz ... ok now we are getting something.
And that makes sense.
A centrifugal pump needs some minimum speed to operate – right?
For our system it takes around 20 hertz to pump enough water build up any back pressure.
Let’s keep going.
30 hertz.
35.
40.
45.
50.
OK, it looks like we hit our sensors 15 psi limit so there is no point in continuing.
If we plot that it looks like this.
So, it looks like with one valve open we only need to run the motor at a little under 35
hertz to get the 6 psi we want.
I repeated that exact procedure with 2 valves open three valves open and 4 valves open.
Looks like with four valves open our motor needs to be at a little over 40 hertz to get
the 6 psi we need.
Great!
We have fully characterized our system – that is, we now know exactly what our system is
capable of under all load conditions.
And, more importantly, we know our system can definitely maintain 6 psi regardless.
And looking at these curves, we could probably even maintain 10 or 12 psi.
Cool.
The point is we fundamentally understand how our system works and that is the first key
step in getting PID to work.
The important take away here is this relationship between the control output – the motor speed
– and the process variable – the system pressure in our example.
Join me in part B where we will see how understanding this is key to setting up PID.
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