# What is a hybrid control system?

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#### Lee Rehorn

I have looked around the internet at information sites, and I understand that a hybrid control system can be thought of as a series of "layers", meeting at an interface. Such a system combines continuous control (for example, a series of repeated motor rotations) and discrete events (for example, a human turning a knob and gettin the aforementioned motor to rotate to a certain position). Is this all it means? I am getting confused by wording such as the following:

"In the bottom layer the plant model is usually described by means of differential and/or difference equations. This layer contains the actual plant and any conventional controllers working at the same level of abstraction. In the top layer the plant description is more abstract. Typical choices of description language at this level are finite state machines, fuzzy logic, Petri nets, etc. Typically the controllers designed at this level are discrete event supervisory controllers. The two levels communicate by means of an interface that plays the role of a translator between signals and symbols."

I'm not sure how this theory can be translated into the ladder logic of a PLC.

Thanks for any help,
Lee Rehorn
[email protected]

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#### Joe Jansen/ENGR/HQ/KEMET/US

Wow. Where are you reading this stuff? <grin>

Ok, Plain english:

Bottom layer. This is what they are calling all the actual control level
stuff. Sensors, PLC's, deviceNet, PC lavel control, touchscreens, buttons,
and the like. This is the stuff that actually does something. Push the
button, something moves.

Top Layer. This is the 'safe' stuff for the people in the offices who
shouldn't be allowed to play with the other stuff, but think they want to
know intimate details of what is happening. This is you top level data
displays, production reports, production scheduling, database viewers, etc.

The interface is the area where all the events of a production day go into
the database, for example. Or the next item on the production schedule
gets loaded into the machine for running.

Hope this helps.

--Joe Jansen

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#### Greg Goodman

Joe,

I read your description and think "plant integration", where control lives on the plant floor and integrates to ERP/management software in
the office. That is, your bottom layer includes embedded and smart device controllers, PLCs and PID loop control, SCADA/HMI and even higher-level online control (S88 batch control, state-based
object-oriented control of batch-wise continuous processing, etc). Your top layer is management's remote view-only access to the process, and
software modules for planning and scheduling, materials/energy reconciliation, etc.

> From: Lee Rehorn <[email protected]>

> "In the bottom layer the plant model is usually described by means of
> differential and/or difference equations. This layer contains the actual
> plant and any conventional controllers working at the same level of
> abstraction. In the top layer the plant description is more abstract.
> Typical choices of description language at this level are finite state
> machines, fuzzy logic, Petri nets, etc. Typically the controllers designed
> at this level are discrete event supervisory controllers. The two levels
> communicate by means of an interface that plays the role of a translator
> between signals and symbols."

I understand the hybrid control Lee is reading about to be various combinations of the different flavors of control in the bottom layer. Plant floor control systems can and do include elements of traditional relay logic, PID loop control, traditional DCS's, PC-based SCADA/HMI packages, and supervisory and hi-level direct control built around expert systems, neural nets, and object-oriented toolsets like ObjecTime and G2.

> I'm not sure how this theory can be translated into the ladder logic of a
> PLC.

If we're talking about what I think we're talking about, you don't implement hybrid control in a PLC, but you can use a PLC and ladder logic in a hybrid system. One example of such a hybrid would be a dry materials batching and delivery system in which:

- a set of PLCs perform low-level control of the equipment (simple weiging operations, delivering material from weighers to conveyor belts to holding hoppers, etc)

- an expert system determines which recipe should be made next based on changing demand and availability of material and idle equipment

- an object-oriented control system coordinates the activities of the PLCs (tells the PLCs what and how much to weigh, initiates discharge when all weigh operations are complete, maintains a log of material usage, weigh accuracy, etc)

Above all this sits an ERP system that takes the material usage reports to perform reconciliation; the event logs to quantify system throughput,
idle time and bottlenecks; and the weigh accuracy reports to identify failing load cells in the weighers.

Regards,

Greg Goodman
Chiron Consulting

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#### Nijssen.Ronald

Simply said a Hybrid Control System combines the advantages of a DCS and a PLC DCS, robust Continuous Control with pre-canned functions, engineering tools equipped to configure control strategies in a comprehensive manner, object
oriented connectivity to field and Operator Stations PLC, High speed logic execution, free-format programming, cheaper hardware than DCS

Hybrid: PLC type Hardware based controller, with openings to do PLC tasks, pre-canned control functions provided by engineering tool or also available in the controller, beside PLC programming environment (Ladder) also continuous function charts, control application generates/integrates with HMI

Examples: Fisher Rosemount DeltaV, Honeywell Plantscape, Siemens PCS7, Moore APACS+ (?), and so on.....

Regards
Ronald

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#### D. C. Pittendrigh

HI All

In my day a hybrid control system was the definition given to a PLC type device which wasn't quite all programmable and still had some cards with diodes and capacitors and other things that had to be set up to deal with the "hi speed stuff" that the processor wasn't fast enough to take care of.

Cheers
D Pittendrigh

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#### Johan Bengtsson

By the description below it sounds like that the bottom layer is a real world description, ie not something you usually want to put in a PLC at all.

The only reason I can see to put this in a PLC is if you want to use a PLC to simulate a machine. That could be useful if you are in the process of writing a PLC-program to a machine you don't have yet, but have a description of. In that case you
could write your program in one PLC and simulate the machine in another PLC in order to do a first test of your program.

Similar reasoning could be done for continous processes and so on even if I really don't think a PLC would be the best way of solving that.

/Johan Bengtsson

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#### Jiri Baum

This sounds like an abstract description of an entire plant, including machines, physical wiring and logic, electric, electronic et cetera.

> I'm not sure how this theory can be translated into the ladder logic of a
> PLC.

It can't - it's not that kind of thing. However, particular *parts* of the description can be translated into ladder logic (the layer with finite state machines, petri nets, etc, and probably the layer immediately below).

To translate a finite state machine, look at the sequential control' thread elsewhere.

To translate a petri net, you'll have to do a bit more work, but again, it's not a problem.

Other parts of the description might be translated' into such things as actual machine parts, motors, sensors, wires, tubes, cylinders or on the other hand enterprise-wide computer systems.

Presumably these layers form a reference model', where you can say that a particular device sits on level 3' (or something), which describes what
general kind of thing it is, what reliability criteria should be used, what devices it's allowed to communicate with (levels 2-4, probably) etc. A way to organize documentation/specification, rather than a programming tool per se.

Jiri
--
Jiri Baum <[email protected]>
http://www.csse.monash.edu.au/~jiribvisit the MAT LinuxPLC project at http://mat.sf.net

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#### Lillie, Dave

This chain from Greg, Joe, and Lee best represents my understanding of a hybrid control system. I think of it as a collection of disparate control and information solutions, ideally selected by design (multi-vender/best
of breed), sometimes selected by personnel/turnover, sometimes selected due to the passing of time.

This chain has so far only addressed a resultant hybrid system runtime. It has not addressed how to manage the design and deployment of a hybrid
system.

My questions are for those who already are, or would think about tackling the design of a hybrid control system:

1) What are the top issues that drain your productivity?
2) What are the major interfaces that you manage by hand, or by custom code?
3) What are the top issues you would need solved in order to consider a hybrid design?

Please feel free to reply to me on the list or offline.

Thanks,
Dave Lillie
Program Manager
Rockwell Software Inc.

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#### Sam Kwok

Look at the Siemens (formerly Texas Instrument) TI505 series (e.g. TI555 ) process controller, this is a real powerful hybrid controller. It comprise of two sections of system, one for fast speed sequential logic control and the other secton is for complex floating point calculation.

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#### ITS SHAHID WAQAS CHAUDHRY

1) The first decision to decide what network you are using.
2) Then get the appropriate communication processor for the PC [consult your local Siemens vendor/distributor].
3) Assuming Plantscape is OPC client capable, use the OPC server that is provided with Siemens.

As simple as 1-2-3.

A

#### Arif Shuja

Hi,

I wish to communicate Honeywell PlantScape with Siemens Simatic S7 PLC. Can anyone suggest options for communication, please?

Thanks,

With Regards
Arif Shuja
Innovative Pvt Ltd
Email: [email protected]
web: http://www.i2cinc.com