Stand alone instrumentation versus PLC control

Following on from the recent thread about using a PLC for furnace control, this has me thinking about a potential future project.

I have just completed a B.Eng in mechatronics, in the final year there was a year long project. I am taking a year out now but I will likely return in a year to top this up into B.Eng (Hons). The main difference between the two is the final year project is more substantial (carries 50% more in GPA).

The industry standard (as far as my 10 years in industry have seen) pasteurizer control panel includes;
Honeywell chart recorder
Honeywell UDC for temperature control (sometimes additional UDC for level control of balance tank)
PLC for logic such as back end sterilisation, auto or manual/forward divert
Flow meters, generally PD340's with a PD210 display/interface

The project can be software simulated but it is common to include a hardware model. I plan on making a model heat exchanger (CNC milled from s/s) with the balance tanks etc printed from ABS filament which should withstand the temperatures (72 deg C Milk, 88 deg C soft drinks).

I plan to use a PLC (CPX 5370 L1) to control everything and HMI (Ignition) as paperless chart recorder. This will give me good learning experience of the process control involved. We get taught PID, don't think we go as far as fuzzy logic. The new UDC's have fuzzy, and the charts in well tuned systems look like someone has used a stencil.

I have previously built pas control panels for both dairy and beverage use. Due to the size of instrumentation they generally end up being 1000x800x300 or sometimes 800x600 if the PD210's are mounted elsewhere.

I would like to do a project that is not only good theory and number crunching but useful for my business. So my question is, if I proposed this method of accomplishing control to you, the end user, would you have any issues with it? "Just because you can do it in a PLC, does not mean you should".

Good project. Yes, you can do all of this in a PLC like a S7.
Getting real hardware made can be expensive.
I like the idea of doing a software simulation but doing it right taking into account the log mean temperature difference. If I were a student I would simply buy a Raspberry PI or BeagleBone black. Raspberry PI comes with a home version of Mathematica.

You will be able to learn more faster this way. You don't need PLC robustness until you make a operational machine.

Yes you can buy a Honeywell controller but it would be better if your rolled your own. BTW, there is a HUGE difference between tank level control and temperature control. A temperature system is a non integrating system whereas a tank level control is an integrating system. This would be a good thing to learn.

Thanks Peter for your advice.

Peter Nachtwey said:

Good project. Yes, you can do all of this in a PLC like a S7.

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I've used siemens, not much, enough to know that I'd rather do it in AB. Using Igntion as the HMI the OPC UA server is tag-browse-able, whereas siemens each tag must be defined manually. I am already an ignition certified intergrator. I have been using AB for much longer than S7, and I already have the hardware and software.

Getting real hardware made can be expensive.

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True, but I assembled my own 3D printer and filament is cheap. I would like my hardware model to be realistic looking (everything to scale). I can print up to 200mm^3. Come demo day at the end of the academic year, you can either bring the system down to the uni, or do an online skype conference demo. This year I chose the latter, as I would of needed a van and a couple of men to lift the finished machine. This time I would like to bring it down. That way, if I do pitch this as "the new way of doing things" to factories, I can bring the model to them. I have use of a 3 axis CNC milling machine and lathe in work. If I supply the material and CAD files they will run the job overnight.

I like the idea of doing a software simulation but doing it right taking into account the log mean temperature difference.

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One good thing I can think of as a benefit of software simulation is the time base. My project demo this year was 1 hour in duration (including presentation etc). This one would be longer, so simulating the plant on software at 5x it's hardware equivalent rate could be useful. I not only have to heat the water up, but then exchange that (twice).

If I were a student I would simply buy a Raspberry PI or BeagleBone black. Raspberry PI comes with a home version of Mathematica.

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I didn't know that. I already have 2x Pi's (the latest and greatest model). In SIT we use MatLab with Simulink. I have access to this software via corporate gotomypc to the institute's servers. Also solidworks, including the add on's such as flow simulation/analysis. And any NI LabView software or add on you can think of.

You will be able to learn more faster this way. You don't need PLC robustness until you make a operational machine.

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The time saved now would have to be paid back later when I transpose everything from Pi-> PLC. I think the Pi can run CodeSys, but I've never used that. As I said I already have the PLC (technically PAC). This is your own law.

PLC LAW #11 Spares should not sit of the shelf. They should be used for training and testing.

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Yes you can buy a Honeywell controller but it would be better if your rolled your own.

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I already have a chart recorder, I don't currently have a UDC but they aren't that dear (~£400).

UDC 3500 is a next-generation universal controller in the popular ¼ DIN size. It is available as a single- or dual-loop controller and can be used in wash-down applications. Users benefit from an accuracy of 0.10% and a scanning rate of 166ms.

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Surely a modern PAC can match the accuracy and scan time of the UDC? At the end of the day this is process control, not motion control...

BTW, there is a HUGE difference between tank level control and temperature control. A temperature system is a non integrating system whereas a tank level control is an integrating system. This would be a good thing to learn.[/quote]

Starting from simple to more complex
type 0 ( non integrating, temperature, velocity ) single pole.
type 0 single time constant with dead time, FOPDT
type 0 , two real poles
type 0, two real poles plus dead time. SOPDT
type 0, two complex poles like a mass on a spring.
type 0, 1 real pole and 2 complex poles, not rare but rare to identify as such.
type 1 ( integrating, position, tank level ) and a single pole. Simple motor model and tanks
type 1 2 real poles. Motor models that model inertia and RL time constants.
type 1 2 imaginary poles. hydraulic cylinders and motors.

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We weren't taught about all these different types. Not in L7 anyway, maybe it will come up in the L8 control systems module. We did mason's theroem, routh arrays, laplace transforms etc. I still have all my assignments, notes, lecture recordings (it was an online distance learning course).

One issue I will have when scaling down is resolution. I can overcome this by design, for example instead of going from silo to balance tank to pump to pasteuriser inlet, I can make the silo tall and thin and use a top mounted ultrasonic for £80. I already have analog input and output modules. Another issue is I don't think I will find a small centrifugal pump. I can use a 12V screen washer pump from a car and a DC speed controller. We did a lab day and follow up assignment on proportional flow control with pump voltage as the variable and flow rate as the output. Which of the categories above does it fall in to?


I plan to make some of the pipework using air fittings and clear tubing. And use some thermochromic dye for visual effect.

I have about 3 more years to think about it, should have it well thought out by then.

I would use a Honeywell or equivalent temp controller to handle the PID. It will save tons of headaches.

Make it a slave to the PLC...send the PV and SP from the PLC as 0-20MA / 4-20mA or 0-10V inputs, and also route the CV output thru the PLC to the SCR, for safety or manual functions.

Try an Azbil controller for the PID (Model: SDC36). They used to be called Yamatake. Honeywell even private labels some of their controllers/programmers. Best PID controller in the market (IMHO), and I have used and tested a few. This is mainly due to its Auto-Tuning efficiency It also supports ModBus.

NetNathan said:

I would use a Honeywell or equivalent temp controller to handle the PID. It will save tons of headaches.

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It is a student project. The object is to identify and over come headaches. That is how people learn. A temperature control system is not that hard.

Buying equipment that does all the work is kind of like paying another student to take your tests.

I am a fan of replacing standalone controllers with a single PLC. More devices = more things that can fail = more downtime.