Sleep Mode 1 uses the sensor feedback to directly control the drives output frequency.
Sleep mode watches that value and when it reaches a limit you set, it turns the output
frequency to the motor off.
We’ll use this water tank demo that we setup and configured in this video.
These are the basic parameters we used in that video.
For Sleep Mode 1, we have the Ultrasonic sensor set to provide 20mA down here and 4 mA up
here.
So, the senor is telling us how empty the tank is, not how full it is.
The fuller the tank gets the smaller the sensor signal will be.
These are the sleep mode parameters.
This tells sleep mode to watch the frequency command to make its decisions – it could
also look directly at the process variable.
This one is used for PID which we aren’t using here so we can ignore it.
This is the drive frequency where we want to turn the motor off or go to sleep and this
is the drive frequency we want to wake up and start pumping again.
I’m going to set these parameters without explanation, because it will be easier to
understand them AFTER we run the demo.
Let’s enable GSOFTs scope function and adjust the potentiometers to the sleep level and
the wakeup level.
You don’t have to do this in your application – I’m just doing it as a convenience for
this video.
I also added a float in the water tank with some LEDs on it to make it easier for you
to see the water level rise and fall.
Well, given that setup and an empty tank, the sensor is sending a large signal back
to the drive.
So, when I hit run on the drive, the motor starts up fast because the sensor is controlling
the drive speed.
As the water level rises, the sensor signal gets smaller and smaller and the motor runs
slower and slower.
Eventually, it gets to the sleep level we set in Parameter 7.29 and sleep mode disconnects
the frequency command from the output and the motor stops pumping.
Perfect.
I’m going to open this valve to simulate demand on the tank.
As the water level falls away from the sensor, the sensor signal gets larger and larger until
it passes the wake-up level we set in parameter 7.30 where the motor turns back on again and
repeats the cycle endlessly.
That’s sleep mode in a nutshell.
It automatically controls the tank filling process with nothing but an analog input.
We can also delay the wake up and sleep transitions.
I put 2 seconds in for each one.
Now when we run the test – I’ll speed up the video here – we see sleep waits 2
seconds after hitting the wakeup level to turn the motor on and it waits 2 seconds after
hitting the sleep level to turn the motor off.
So, the sensor signal had to be past the threshold AND stay there for 2 seconds before sleep
would activate.
That’s a great way to get rid of intermittent threshold crossing due to noise, for example.
This looks ok, but, there are two big problems with this.
First, since the sensor returns smaller and smaller values as the tank fills, the drive
frequency will get smaller and smaller and the motor will turn slower and slower – all
of which means it will take a long time to fill the tank.
Wouldn’t it be better if the motor could just quickly fill the tank at full speed all
the way up and save this time?
The second issue is, again: Since the motor is going slower and slower as the tank fills,
eventually, the motor speed will get to the point where the centrifugal pump is spinning
too slow to move the water and it will sit there and cavitate, which means it is not
filling the tank, it’s heating up the water, it’s reducing the service life of the motor,
it’ reducing the service life of the pump, it’s wasting your electricity and it’s
wasting your money.
That’s why I set the sleep level at 20 Hz – anything below that and this pump stops
working.
But, since the sensor is controlling the drive speed that means the we can’t fill the tank
past this level.
The solution is simple – we just raise the minimum drive speed to a larger value and
that keeps the motor going fast regardless of command frequency – I’ll set it to
90% of the full 60 Hz which is 54 Hz.
That takes care of speeding up the tank filling operation because we are forcing the motor
to go fast most of the time.
AND, since the new min motor speed is nowhere near cavitation, we can also set the sleep
level to whatever we want.
I’ll set that to 5% which is 3 Hz – and that takes care of issue #2 – filling the
tank to whatever level we want.
Let’s move our sleep level cursor down to 3% so we can see if the motor turns off at
the right time.
So ow as the water level rises and the motor starts to slow down because the sensor value
is getting smaller, when it hits the high minimum speed we just set, the motor just
stays at that minimum speed, even though the frequency set by the sensor is still falling.
When the sensor hits our new the sleep level of 3Hz, the drive shuts off, after the 2 second
delay.
So now the motor is filling the tank quickly independent of the sensor value AND it’s
filling to whatever lever we specify.
Perfect.
The bottom line is, sleep mode watches the frequency command – which in this demo was
being driven by the sensor – and decides when to let it through to the motor.
We added the trick of setting the min drive frequency to a high value and we were good
to go.
Is there a way to do this without tricking the drive’s min frequency?
Yeah, there is.
You just tell the drive to use PID as the frequency reference.
PID’s job is to get a process to whatever level you specify as quickly and efficiently
as possible – right?
Well, that exactly what we want.
We’ll see how to do that in the other videos in this series.
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