PLC control programming for two heaters, TCK, s/s relays

[AligatorAmy]

Hi,
I would like to provide the PLC control for a scenario where the temperature in a machine, heated by the two electric heaters, is controlled with use of 1 TCK and 2 s/s relays. Please see draft “PLC4” sent attach. The TCK is fixed to the machine and is not in a physical contact with the heaters. Nominal temperature of machine is 200degC. The machine temperature tolerance is +/-2degC. This means that the heaters should be turned on when machine T drops to 198degC and turned off when T reaches 202degC.
For this moment I have created the start PLC diagram outlined on draft “PLC4”.
I am still learning PLC. Therefore, I would be very grateful for your opinion whether the PLC diagram shown on picture “PLC5”, in terms of the tools used (direct contact, direct coil, etc.) and their layout, is of proper design.
I know that there is much more work needed to complete this task (e.g. set up of the variables, etc.).

Please help.
Thank you.

 

[Tom.G]

Referring to your image PLC4.jpg, the block diagram of the system shows the PLC driving the SSrs which drive the Heaters, as expected.
But the Ladder Logic portion shows the SSRs as inputs and the Heaters as outputs. Was this intended?

--| |-- is a normally open input contact, --( )-- is an output

I hope you realize that ±2°C at 200°C may require some thermal insulation, especially if this is the desired outer surface temperature.

Considering your temperature requirements and the learning curve of PLCs, a separate, dedicated temperature controller may well be cheaper in the long run. That's assuming this is a one-of-a-kind device. If it is a volume product to be sold, then the tradeoff should include ease of field servicing, even by the customer. Standalone controllers can be just swapped out on failure.

Which PLC are you using?

 

[AligatorAmy]

@Tom.G
Thank you for your reply.
The SSR should be the output here. Thank you for pointing at this.
I have corrected the PLC tools layout concept and I send it attached.

Indeed, ±2°C at 200°C is quite tight considering that the heated machine component has bad thermal insulation. This temperature range is the desired target however, it may be that without thermal insulation the range will be broader.
I need to go ahead with the PLC. The component of the machine is heated by 12 heaters. Heaters are paired. Effectively, 1 TCK controls 2 heaters.
There are 4 components in total in machine. Therefore, we have 48 heaters and 24 TCKs.
Using PID controllers would mean requirement for tens of them.
The company has a few machines with same number of heating elements/TCK...
I use Unitronics PLC, UniLogic software.

 

[Tom.G]

Yes, I can see why you need a PLC rather than individual temperature controllers. Your posts so far seem to indicate a simple bang-bang (on-off) control is to be used. Maybe just use thermostats, perhaps embedded? You wouldn't even need all those PLC inputs and outputs.

The company has a few machines with same number of heating elements/TCK...

What kind of control is used for the existing machines, On-Off or PID? And is this new machine similar enough that the same approach will work?

I'm not familiar with the Unitronics PLCs. I've used the larger Allen-Bradley and TI ones and, briefly, a few smaller ones. I watched a Unitronics video and checked one of their Ladder Programming manuals and could not find any PID controller blocks available. If PID is needed, hopefully the PLC model you are working with has PID controllers as a pre-defined functional block; and supports as many as you need. Some smaller PLCs have only a few (or zero) PIDs available.

If the machine parts have stable thermal loading, you could possibly use just a few controllers driving mutliple heaters and average the TC signals. If the relative thermal loads of the heaters in a group are stable, you could perhaps scale the power to the individual heaters; or even scale the heater sizes. .
Depending on the thermal coupling between the temperature controlled machine regions, it may be challenging to avoid sever interaction between them.

Ah well, all engineering trade-offs.

Then again, in a large (read expensive) machine, the cost of a larger PLC becomes just a rounding error in the overall price.

Good Luck, and keep us posted.

 

[AligatorAmy]

@Tom.G
Indeed using embedded thermostats is excellent option. However, the technological process in the machine(s) is very complex, unstable, with large number of critical process parameters. For this moment, most of the controllers and gauges are analog or old digital. There is no data logging and having it would be vital. Etc. etc. Introduction of the PLC with large touch screen with (at some stage in future) all most important process parameters controlable from one panel would be a huge milestone towards process stabilization. Machines are rather old, purchasing new ones is not the option (too expensive).
Presently most of the parameters are controlled through PID controllers.
I think I have might found the PID block in UniLogic. I send attached the new ladder.
The thermal mass of the whole component will hopefully be large enough allowing for uniform heat distribution, what can be advantegous in regards to maintaining tight temperature variation across the component width and heaters not 'fighting' with each other.

Thanks Tom.G

 

[Tom.G]

Great, it has PIDs. Hopefully a sufficient number of them and with auto-tuning capability.

And I see you are doing a retrofit rather than an original build. In other words, if you have a constraint you can't negotiate it away!

As a tip. If the machine will have a significant graphics user interface, the hiring of a graphics-oriented programmer for that part of the project can be a huge asset. The graphics programming and the graphics itself requires a different mindset that most of us engineering types don't have. Sort of like asking an interior decorator to design the machinery that builds cars.

Since you are relatively new to PLC programming, here are a few other tips.

The Ladder Logic program is ideally organized by the workstation on the machine. That is, for a particular process step, all logic that controls/reports movement, temperature control, etc should be grouped together. Then all the logic for the next process step. This makes future maintenance, troubleshooting and changes much easier and quicker. I mention this because their is a tendency by newcomers to group the Ladder Logic so that all PIDs are together, all Timers are together, etc. This make troubleshooting a real nightmare.

This grouping frequently extends to a Graphics Interface, in that it should be considerd a process step. This is not always the case, but there is often some pre-processing required and is an area that requires tweaking during developement.

Out of curiousity, how big is this machine and, if it's not propriety, what does it do or process? I spent several years (decades ago) doing the electrical design of, and PLC programming for, custom machines used in medical manufacturing; along with other larger and smaller projects. That's why I'm curious about the project you are on.

Below are a couple safety suggestions. They are not exact quotes and they are from decades-old work; current requirements may well be more stringent.

Here in the US, the safety standards require Emergency Stop buttons scattered around the machines. There are several pages, but it boils down to a couple things:
1) Emergency Stop push buttons shall be placed so that they are within 3 feet of reach from anywhere around the machine.
2) When an Emergency Stop is pushed, all movement and power is to stop immediately unless such immediate action would cause greater danger. ( For instance, the operator interface and safety devices would be excluded).
(As an example, if something is being held up by an electromagnet, it may be more dangerous to immediately drop it than to let the electromagnet self-destruct several seconds later; thus giving people time to get out of the way.)

Not exactly a safety suggestion, but if the machine is large, it is helpful to show on the user interface which Emergency Stop button was pushed. There may have been more than one.