Help a non-PLC guy with this control application 2

[KGisME]

[Cross-posted in the PLC forum. Mods, please merge these or close one if cross-posting is not allowed.]

Hi all. Please be gentle, I’m an ME and I know very little about PLCs / Process control. I think I have a bit of an unusual application here.

We have a VERY expensive (as in, in the same ballpark as the price of a nice house) avionics computer in a ground based simulator that we cool with ambient air supplied by a small fan (originally a residential furnace draft inducer fan).

We currently use a basic, fixed setpoint pressure switch on the outlet of the fan to monitor the fan pressure. If the fan fails and is no longer supplying 1”W.C. of pressure, the pressure switch opens, which tells our system to shut off power to the computer.

But unfortunately we recently discovered a failure mode that this system did not catch, where our fan failed partially. It was still spinning and delivering enough pressure to keep the pressure switch closed, but not enough to adequately cool the computer. The computer overheated and failed.

The first thought then was that we need our pressure switch set to a higher setpoint. We’ve spoken with the people who involved with the design and everyday test and application of these computers, and still are not able to figure exactly what pressure we'd want to set our switch to (or flow rate if we were to use a flow switch instead of a pressure switch).

But if we take a step back, the parameter we’re really concerned with is the temperature of the computer itself.

So we’re wanting to find a solution that allows us to monitor both the temperature of the computer (with a surface mount thermocouple or possibly an IR pyrometer) and the fan status (likely with a pressure transducer/switch) and control based on those 2 inputs.

We’d like to program it such that if the fan fails to produce a given pressure, or the computer gets too hot, a relay opens which will tell our PDU to shut off power to the computer. We’d also like to have an intermediate alarm (relay?) output, that will notify us if the computer is getting a little warm, so that we have some advanced warning that it’s getting a little hotter than we prefer before it gets to the critical temperature that will require us to shut it down. At the same time, we'd like to still monitor pressure downstream of our cooling fan, and ideally be able to trigger an alarm signal and power off the computer if the fan pressure drops below a certain setpoint.

Some type of serial communication interface (or maybe Ethernet? Not sure if our system would interface with that as easily as a serial protocol) would be great as well, so that we could view our computer’s temperature and fan pressure from our remote operator station.

To be clear, our cooling fan is a single speed fan, so we’re not trying to modulate or change fan speed for now, just to turn off the computer if it gets too hot (although it would be nice to get a controller that has the capability to modulate fan speed, as I would like to switch to more modern/reliable fans in the future that are speed controllable. BUT that will take some time for me to convince the “powers that be” to spend the money on that, if we can get the money to do it at all.)

I have a little bit of background in industrial/"heavy" commercial HVAC equipment, but as an ME, I don’t know as much about controls & programming as I wish I did.

Although I definitely wouldn’t mind learning the skill, I (and my manager) would prefer I not to have to have to spend the time to learn to program a PLC (or HVAC controller) if possible. If there’s no getting around that, what cheap, reliable options are out there that are relatively easy to learn? (I’m no Computer Science/software guy, but I do know my way around a computer enough to be pretty confident in Solidworks, NX (FKA UniGraphics), and Excel).

I'm thinking an industrial automation controller would be the cheaper & easier way (vs HVAC controls) to do what I’m wanting to do. Something a step down from a PLC in cost & complexity. Maybe one of these? https://www.automationdirect.com/ad...cess_controllers/advanced_process_controllers along with a thermocouple and one of these pressure transducers https://www.automationdirect.com/ad...ensors/pressure_transmitters/ptd25-10-0100wch

Or would it be best to keep the temperature and pressure monitoring separate in 2 devices, and use one of these for pressure https://www.automationdirect.com/ad...re_switches_-z-_transmitters/eps25-100wc-1001

And one of these for temperature https://www.automationdirect.com/ad...controllers/standard_temperature_controllers# ?

Or is there some small low cost but reliable HVAC controller out there that could do what I’m wanting to do without too much of a learning curve and/or time investment?

Or another simpler way that I'm overlooking?

 

[waross]

Shutdown:
A temperature switch or thermostat in series so that if either opens, the power will be cut. The start button on thee fan may be arranged to bypass the pressure switch until the fan is up to speed. (This is similar to many clothes driers wheree thee start switch must bee held until the drum is turning.
Alarm:
A second temperature switch or thermostat. Don't alarm the fan, if it is failing shut down.
You may want to add latching relays so when the power shuts down, you know why.

industrial automation controller

Another name for a PLC.

When PLCs first arrived and older engineer was heard to remark;
"These young engineers would use a PLC to control an outhouse light."
If you use a PLC, don't forget thee outhouse light.

 

[KGisME]

Thank you sir.

alrighty, found an outhouse light

 

[danw2]

Loop controllers differ from PLCs; which is why they're still around and commercially available some decades after PLCs came into being.

The PID loop controllers, called "advanced Process controllers", are configured by keypad/display, picking from a list of options for various parameters, like Input type, input range, control action with a built-in HMI (operator interface) that display the variable you're controlling (temperature or pressure) and its setpoint, the value you want control action to take place at. The 7 segment displays are generally fairly cryptic acronymns for the parameters. It's a read-the-manual situation, even for those, like me, who have dealt with them for decades. But they're great as alarming devices.

A PLC is programmed as a "blank slate" where you program the control algorithm in Windows development software, either structured text or as a graphical ladder logic, and the download the program to the PLC and let the PLC run. The PLC does not display any data, you need a separate HMI for that, which depending on the brand, is a separate development program.

Both loop controllers and PLCs have learning curves. The PLC being more flexible is generally a steeper learning curve.

Dedicated analog switches with relay logic wiring will do the same job, albeit without data communications.

Dwyer has a whole line of low pressure switches. Analog capillary bulb thermostats probably cost more than a loop controller and a thermocouple because of construction costs nowadays.

 

[KGisME]

Shutdown:
A temperature switch or thermostat in series so that if either opens, the power will be cut. The start button on thee fan may be arranged to bypass the pressure switch until the fan is up to speed. (This is similar to many clothes driers wheree thee start switch must bee held until the drum is turning.
Alarm:
A second temperature switch or thermostat. Don't alarm the fan, if it is failing shut down.
You may want to add latching relays so when the power shuts down, you know why.

Any suggestions for the best equipment/hardware to do all you suggest here with?

 

[KGisME]

Loop controllers differ from PLCs; which is why they're still around and commercially available some decades after PLCs came into being.

The PID loop controllers, called "advanced Process controllers", are configured by keypad/display, picking from a list of options for various parameters, like Input type, input range, control action with a built-in HMI (operator interface) that display the variable you're controlling (temperature or pressure) and its setpoint, the value you want control action to take place at. The 7 segment displays are generally fairly cryptic acronymns for the parameters. It's a read-the-manual situation, even for those, like me, who have dealt with them for decades. But they're great as alarming devices.

A PLC is programmed as a "blank slate" where you program the control algorithm in Windows development software, either structured text or as a graphical ladder logic, and the download the program to the PLC and let the PLC run. The PLC does not display any data, you need a separate HMI for that, which depending on the brand, is a separate development program.

Both loop controllers and PLCs have learning curves. The PLC being more flexible is generally a steeper learning curve.

Dedicated analog switches with relay logic wiring will do the same job, albeit without data communications.

Dwyer has a whole line of low pressure switches. Analog capillary bulb thermostats probably cost more than a loop controller and a thermocouple because of construction costs nowadays.

Alrighty. Yeah, I'm very familiar with Dwyer having come from the HVAC world. So I have looked at all their offerings that are relevant for this application, although none seemed as good of a value and as flexible/powerful as the Automation Direct house brand controllers. Do you have any experience with the Automation Direct house branded stuff? Just wondering if there's any concerns about their quality since they seem to be the lowest cost option.

So do you have any suggestions for what control hardware you'd use if you were in my situation?

 

[KGisME]

Hmm I wonder If I could use one of those "Standard" temperature controllers plus a "dumb" pressure switch like this Dwyer, and put the pressure switch in series with the relay on the temp controller that shuts off power to the computer?

 

[waross]

If I had a PLC for some other reason I would consider adding the shutdowns to the PLC.
For shutdown only I would keep it simple.

 

[danw2]

>Hmm I wonder If I could use one of those "Standard" temperature controllers plus a "dumb" pressure switch like this Dwyer, and put the pressure switch in series with the relay on the temp controller that shuts off power to the computer?

Yes, when the alarm/output contacts are wired in series like you've suggested, either high temperature or low pressure will drop out the power relay.

I just realized that you don't need the 'Advanced A-D controller'; a Standard A-D controller will do what you need to do. I prefer 1/4 DIN (4"x4") instead of the smaller one because I have fat fingers and the buttons are too close together on the 1/16 (2"x2") or 1/32 DIN (2"x1") sizes.
https://cdn.automationdirect.com/static/specs/solosl9696.pdf

Be aware that the on-off switch points for an On-Off control output from a loop controller is always above or below the setpoint; with an adjustable hystersis band above or below the control setpoint, the setpoint displayed on the digital display. To get the control relay to turn off at the high point either use a small hysteresis band or adjust the setpoint slightly lower to accomodate larger hysteresis.



In comparison, a controller alarm relay trips at its setpoint and an alarm hysteresis is used to provide a reset point, a value below the alarm SP for a high alarm or above the alarm SP for low alarm. . Some controllers, like the one whose hysteresis chart is shown above, use "percentage of input range", not engineering units (degrees) when defining control or alarm hysteresis - gotta read the manual.

The alarm setpoint is not shown on the display, it's the control output setpoint is shown in the digital display.

I haven't used an A-D controller. Love used to make the Dwyer brand, I would avoid Love/Dwyer.



,

 

[KGisME]

Cool cool thanks for your help. I was familiar with the concept of hysteresis (you just reminded me that I once owned a very basic cheap temperature controller for controlling fermentation temps when I used to homebrew). But I certainly didn't not know the nuances of hysteresis behavior in these controllers without this info in your post.

So thank you!

If for some reason I didn't want to put the pressure switch in series with the loop controller output relay, I think I would need the Advanced Controller then (one with the aux input), correct?

 

[MintJulep]

I think you're overcomplicating things.

Thermal switch in series with a differential pressure switch is all that you need.

1NT Series 1/2” Disc Thermostats | Thermtrol Corporation

www.thermtrol.com

Whoops, we couldn't find that.

 

[KGisME]

Hmmm, alrighty...that is true, simple pressure switch and temperature switch in series will do the job.

I just had a meeting with the "higher ups"....apparently equipment cost is not a concern, just my labor cost is...whoops. But they do want the extra features like a second relay to warn when the computer temperature is getting "a little" warm, and also the capability to transmit the temperature & fan pressure to the remote operator station.

But they're also saying we want hardware that's made in the US. It looks like Watlow has some of these types of process controllers that are US made. Any other options? Anyone know if Omega, Honeywell, Rockwell/A-B, or Emerson offers these types of controllers that are made in the US?

 

[System Protection]

Schweitzer Engineering Laboratories (SEL) has automation controllers. (SEL-2411 for example)

They are more utility focused.

But their support is amazing and programming them is quite simple.

 

[danw2]

>I think I would need the Advanced Controller then (one with the aux input), correct?

The 'advanced' controller would have to be a 'dual loop' controller (2 independent loops, typically capable of cascade control) to have each input control a separate output, which it might or might not be, I haven't checked the specs.

It is more common for a second process input to be used as an analog remote setpoint on a 'single' loop controller.

Dual control outputs are typically used on a 'single' loop controller for a heat/cool application (like a home thermostat) using a single setpoint, where the heat control output is used when temperature is below setpoint, and the cool control output is used when the PV is above setpoint.

I heard a number complaints over the years (as a support guy) that an operator made the wrong decision when doing something as simple as changing a setpoint on a 1/4 DIN dual loop controller because it wasn't clear which loop's values were being displayed. My recommendation for 2 loops was always two single controllers, one for each loop, with appropriate labeling to avoid that kind of operator confusion. The single loopers are not that expensive.

 

[KGisME]

>I think I would need the Advanced Controller then (one with the aux input), correct?

The 'advanced' controller would have to be a 'dual loop' controller (2 independent loops, typically capable of cascade control) to have each input control a separate output, which it might or might not be, I haven't checked the specs.

It is more common for a second process input to be used as an analog remote setpoint on a 'single' loop controller.

Dual control outputs are typically used on a 'single' loop controller for a heat/cool application (like a home thermostat) using a single setpoint, where the heat control output is used when temperature is below setpoint, and the cool control output is used when the PV is above setpoint.

I heard a number complaints over the years (as a support guy) that an operator made the wrong decision when doing something as simple as changing a setpoint on a 1/4 DIN dual loop controller because it wasn't clear which loop's values were being displayed. My recommendation for 2 loops was always two single controllers, one for each loop, with appropriate labeling to avoid that kind of operator confusion. The single loopers are not that expensive.

Ahh ok. Yes, I did notice that 2 of the basic controllers would be cheaper than 1 of the advanced controllers.

I think might actually be ok both the pressure and temperature inputs controlling the same output relay (the one that shuts down the computer), as long as I could have separate aux outputs (relays or SSRs ideally) to alarm 1) when the temperature gets a little too high, but not so high that we want to shut down yet, or 2) if the fan pressure is just slightly reduced, but not so low that we have to shut down. We

Or should I just shut down the computer if we sense any measurable decrease in fan outlet pressure, without any intermediate alarm step?

BUT, this is a big department, and we do have a lot of engineers & techs in and out of here and a fair amount of turnover, so the separate, clearly labelled single loop controllers are probably prudent. Rack space is tight, bu it looks like 2 of the smaller single loop controllers would be the same size as one of the bigger advanced ones.

2 separate single loops it is! Thanks for the suggestion danw2.