Can PLC's be used for my application?

I have a small business (ROV's(Remotely Operated Vehicles))which are small unmanned submarines that are controlled from the surface by means of a antique telemetry system which sends commands to the ROV down 4000' of cable via 2 x Shielded Twisted pair wires in a RS422 format.
The PCB's that we use are no longer available and cannot be repaired, therefore I need to find an alternate way of controlling the ROV's.
I need at least 8 x Analoque Channels sent down to the ROV for Thruster control and 16 x Digital On / off for various functions (Down Traffic).
Up Traffic requires at least 4 x Analoque Channels and 8 Digital on / off's.
So, this brings me back to my original question....Can PLC's be used for my application? I obviously have no experience with PLC's..so please pardon my ignorance.

Thanks in advance.
Wolfgang

I want to say "Yes", since one can use a PLC to control just about anything including replacing embedded system control but...

the devil is in the details. Some of the questions that immediately came to mind are:

Where is the control located, in the ROV or on the surface?

How much room you have in the ROV? Because it might be easier to have the IO rack remotely mounted there.

Analog = Standard 4-20mA?

These commands, are they simple commands like on/off, % ouput, or more higher level command like 'go straight', 'dive', 'surface'?

I would say talk to an experienced local System Integrator and see what can be done.

Thx for the reply.
The Rov is controlled from the Surface by means of a 3 axis Joystick which gives the forward, reverse, turns and lateral movement commands with a single potentiometer for vertical up/down command. The 'Pilot' also has an array of on/off switches on the Surface console for various functions on the ROV such as Camera's, Lights, Pan and Tilt, Manipulator Arms (on/off functions used to operate Hydraulic solenoids).
Normally the Joystick and other potentiometers are used simply as voltage dividers whereby -5dc to +5dc (O Volts is neutral) is sent to a 'comparator board' then on to a Analoque to Digital board and finally the Processor (12 bit resolution) to be sent to the ROV.
The Rov has basically the same setup and interprets the incoming commands (at 9600 baud) and processes the info sends it to the D/A and finally to the Motor Drives and relays etc.
The Rov then sends the Info to the Pilots Surface Processor (Depth, Heading, Pan /Tilt angles etc (12 bit res).
There's quite a lot of room in the Underwater housings on the ROV to install a new Telemetry system, Stable DC power should not be a problem either.

I would say yes, a PLC sounds like it would work for your setup.
I would also think that a PC with some DDE/OPC aplication would make a good surface control setup.

This application could be developed once, than run on any number of cheap PCs.

You probably want two PLCs - one on top reading the joystick and so on, and one on the bottom reading the sensors and executing the commands.

A good PLC programmer / controls engineer can easily do what you want. Because of the publicity factor for such a unique applications, you may even be able to find a PLC vendor that will give you a break on the hardware. I would not suggest, however, that you program this yourself. This is a little beyond the scope of a learning experience, since the system is above average in complexity, the consequences of a bug are significant, and access to part of the system may be a little difficult.

Writing the program would be challenging, but not the worst part. Obviously you would test it in a controlled location before sending it to the Marrianas Trench, but part of the experience factor is knowing what to test and how to stress the programming and hardware to verify results in your pool before sending it to look for Captain Nemo.

Thx for the info so far, you have all been extremely helpful.
How do the PLC's communicate with eachother? RS 232? if so then will I still require a Modem (rs422 / rs485) to drive the Telemetry commands down the 4000' umbilical?
What does the PLC system actually consist of? i.e. A/D Module, Processor Module, Digital on/ off module that are all wired together....or is it just one unit that does it all?
Us old Electronic guy's are just sooooo far behind the times!!!
Thanks again for your patience people!!

The first place to start is the tutorial at the top of the page, and I strongly suggest you buy the book too.

You also can get general info on PLCs at these sites:

http://claymore.engineer.gvsu.edu/~jackh/books/plcs/

http://www.freestudy.co.uk/plc/

You can see vendor specific info at:

http://web1.automationdirect.com/adc/Shopping/Catalog/PLC_Hardware

http://www.ab.com/plclogic/

http://www.squared.com/us/squared/corporate_info.nsf/unid/ECA90110AB7098CA85256A3A007091D7/$file/productsa2zFrameset.htm

http://www.sea.siemens.com/automat/product/plc/auov.html

My favorite is the AutomationDirect.com but for international use you may be better off with the Siemens, since they are largest outside the US.

The answer to your question is "it depends". The brands I recommended above can all use serial communications to connect two PLCs. Whether it is through RS232 or RS485 depends on specific models. At worst case you would need an RS232/485 converter at each end.

Some PLCs have everything in one brick sized block, others allow you to add I/O to the end of the brick, and some have slots in a rack that you add CPU and I/O cards to. Again, it depends. All the brands I listed have a variety of analog and discrete I/O available.

This sounded familliar, so I did a search and found this thread: http://www.plctalk.net/qanda/showthread.php?s=&threadid=9075&highlight=pond

Wonder what became of that??

I'm not quite sure runnin an analog signal on a 4000' cord is a really good idea.

My first inclination would be, as Tom said, have two PLCs. One on top and the other on the bottom that would be network via a two wire RS 485 network. The PLC on the ROV would be stuffed with the Analog outputs to drive motors etc.. (closer to home).

As I see it you'll need 4 analog outputs to control pitch, yaw, roll, and thrust. The DL06 from AutomationDirect has 4 expansion slots that can take up to 4 analog input or output modules. Port 2 of the DL06 can provide the networking between the topside and ROV PLCs. If you need 4 axis control then there's not much left to get real time feedback with.

In the event you lose your nework you can have some code in the ROV PLC that will shut down all outputs. Additional code in the topside PLC will also alarm you to a lost network. I'm assumeing that the cord has a steel core that will allow you then to hoist the ROV back topside.

Another issue maybe a comm delay. Your driver will have to be pretty carefull about this, but you may be able to manage with it. How much delay? I'm not sure, but it's something to consider.

Another important question is with how is the cord attached or interfaced topside? I'm thinking that you may have wiping contacts between the cord spool and the topside controls. If so that'll play he11 on the network connection. Not sure how to get aroung that issue.

Regards and good luck, Mike.

Submarines

INTRIGUING
Only drove one submarine SSN 596, Thresher class.

My first thought is the sub able to come to surface all on its own??

Is it tethered and you winch it up (or down) to change depth?

If not how do you change depth (swim up / down and or change ballast ??)

With an onboard PLC you could have auto salvage or "return to surface" if you lost communications with surface controller. You would need a separate battery just for the PLC so that control would always be there AND maybe any required power for the auto salvage feature.

Dan Bentler

Sub PLC

This would be a good aplication for most PLCs. You could use two Allen Bradley PLCs and set one up to act as a remote rack, say on the sub, and us DH+ for communications. The distance is not a problem on DH+. There are PLCs that can be configured as master and slave. This would also work.
This is a good project, you will have a lot of fun with this one!

Take care, Have a good week end
Bruce.

Dan,

That's an exelecent idea! Auto Salvage. Lost com = blow ballast for controlled assent, (dont want it to come up like a cork though).

Re: Sub PLC

Bruce99 said:

The distance is not a problem on DH+.

Click to expand...

Plus, at the 4000' length, the DH+ speed could be over 100 Kb. If going this route I'd want to check with A-B for their recommendation on a cable for marine application.

Will 4000' RS232/485 work?

This sounds like an outstanding project.

I concur with 2 plc's. I have been involved with something similar, for salvage, but not 4000'. Also, several similar projects in underground and strip mining, and also industrial shuttles. In the mining applcations and some others, the cable is the weak link. Having an extensive background in RF, I tried to use it whenever possible. If 12 volt, or space (height) contraints are a concern, then I would look into Entertron Elite 2000 "LINK" in the sub. Actually one at each end would work satisfactorily. They are available with 12 or 24 volt supplies, 16-64 digital inputs, 16-64 digital outputs, up to 16 analog inputs, and up to 16 analog outputs. Software is FREE.

I would have some sort of setup so that if communication were lost, the sub would either stop, or surface. Is there a strobe on it?

If you weren't familiar with Allen Bradley, I would get someone to do the engineering for you, specially since the software is kind of pricey.

Also, 4000 foot of RS232/RS485 will TAX some systems! So, has someone familiar with the equipment verify what will or won't worrk, before investing very much $$$.

Any thoughts from the group? I like RF, but we would be looking at several hundred to several thousand $. I used RF in many applications becuase RS232 wouldn't go the distance.

regards.....casey

kc9ih said:

Any thoughts from the group?

Click to expand...

Yes. RF through 4000' of sea water?...

beerchug

-Eric