How to spec a PLC

[Volts006]

I am an electrical engineer working for an A-E which designs all sorts of facilities. I’ve had all kinds of experience in lighting systems, fire alarm systems, power distribution, and telecom systems, etc.. My controls experience is some what limited especially in dealing with PLC’s.

I have a project where I have to compare the height of two hangar doors and stop a controls operation if one has moved farther than the other. I have proposed to do this with a rotational encoder on the motor shafts calibrated to measure the heights and encode it into a voltage or current. The existing controls are mechanical relay type. Therefore I have to send the encoded information into a PLC. The PLC must be programmed to compare the two elevations and then send out a stop signal to the existing mechanical relay controls.

Since I don’t deal with PLC’s much, how would something like this typically be spec’ed out.

My assumptions are 1) design a ladder diagram showing all existing relay logic and where the output of the PLC will be input into the existing relay logic to accomplish the desired operations, 2) Also, write a sequence of operations indicating how the entire system needs to be operate together, 3) Get input from a PLC manufacturer or PLC rep. as to a recommendation of a PLC, and 4) Use this recommendation as the guide to write bid specifications.

This seems to be all that needs to be designed on put on drawings as I perceive it.

As far as the PLC issues such as a) Check loss of state on power outage, B) check for the need of a signal delay, etc.; I assume that the specifications above would require that the supplier/assembler/control panel manufacturer would design/check this based on the functions that the system have to operate.

Man that was a mouthful. Any tips, ideas, guidance?

 

[wilheldp_PE]

For a simple application like this, you could probably just use a programmable relay. They make relays that can take 4-8 inputs and put out 2-4 outputs, and can run a simple ladder logic program to control the I/O. Even a small PLC would be overkill for a 2 in / 1 out system.

I think you are on the right track with the encoders. There are two different types of encoder; absolute and pulse. The absolute encoder returns an analog value based on its position at any given time. Pulse encoders simply have a disc with slots cut in it that passes between an LED and a detector. As the disc spins, it sends those pulses to an input. Which type of encoder you use will be based on how accurate your program needs to be, but keep in mind that absolute encoders, and the higher pulse count encoders will require a special type of input that will not be available in lower cost PLCs.

For a very simple ladder logic program, you can expect a program scan time of about 1 to 5 ms, so the pulses from an encoder should be coming less frequenly than that. You should be able to determine the highest pulse count encoder you can use from the scan time of the program and the speed of door.

I work in the controls department of a high-speed conveyor/sortation company, so I can probably help you out with any other questions you might have about this application.

 

[Volts006]

wilheldp,

Ok thanks your post is very informative and reassuring.

Next I'm going to call up a rep. and try to get a guide spec and recommedation on what to use.

As far as accuracy, I need to know when they are about 14" out of alignment in elevation.

In regards to this comment, "For a very simple ladder logic program, you can expect a program scan time of about 1 to 5 ms, so the pulses from an encoder should be coming less frequenly than that. You should be able to determine the highest pulse count encoder you can use from the scan time of the program and the speed of door.", thats where I assume the manufacturer's rep can guide me? Does that sound like the right approach.

If I get lost I may have to ask you for some assistance if possible.

Thanks.

 

[wilheldp_PE]

wilheldp,
Ok thanks your post is very informative and reassuring.

Next I'm going to call up a rep. and try to get a guide spec and recommedation on what to use.

As far as accuracy, I need to know when they are about 14" out of alignment in elevation.

In regards to this comment, "For a very simple ladder logic program, you can expect a program scan time of about 1 to 5 ms, so the pulses from an encoder should be coming less frequenly than that. You should be able to determine the highest pulse count encoder you can use from the scan time of the program and the speed of door.", thats where I assume the manufacturer's rep can guide me? Does that sound like the right approach.

If I get lost I may have to ask you for some assistance if possible.

Thanks.

14" might as well be 100 miles when talking about industrial automation. I don't think you will have any trouble finding a pulse encoder and a cheap PLC/Programmable relay for your application. Any rep with an automation manufacturer should have no problem helping you out.

 

[Sschell]

I have used Automation direct when specing my PLC's: Automation Direct

They have worked wel for me for several projects including water purification systems and liquid oxygen production.

their website is very informative and they have a good selection. For your application, I would look at the DL05 plc, comes with a highspeed counter built in.

I don't know if there is a Quantum Automation in your area, they distribute automation direct products and are VERY helpful. They will even offer you a day long ladder logic training if you buy enough stuff.