Selecting the right PLC.

Hello,
I'm new to PLC, I picked up all of the Phil Melore's books, they are pretty good. The online simulator is great but I want to get a physical unit and make myself a trainer and practice with that.

I have looked at the CLICK and TECO SG2 units they are both relatively cheap and offer a lot of functionality for the price.

Aside from building myself a trainer I want to try to build a temperature controller with it.

What I want to be able to do is, the unit should take input from a temperature sensor (a thermocouple) and then output to an actuator connected to a damper to open close (0 - 100%)

I understand that thermocouple will be an analog input, and I see that both CLICK and TECO have an analog inputs but I think the actuator will also be analog since its position will be variable. I did not see any analog outputs on either CLICK or TECO, Is there another way to achieve this without the analog output?

Thank you

There are a couple of possibilities. First, you can put a pot on the valve operator and use output contacts to move the operator until the feedback agrees with your set position +/- a deadband. This is essentially what the comparator and analog positioner on an electric valve operator do.

The second choice, if you are going use PID for your temperature control, is to use what is called the heat/cool output. The AutomationDirect.com manual explains how to do this. It essentially has timed open/close outputs, with the time of operation proportional to the PID output.

zer0net,

Automation Direct offers three PLCs with analog input and output. They are: C0-02DD1-D, C0-02DD2-D, and C0-02DR-D. Any of these will work for the task you have outlined. Each of these have two analog inputs and two analog outputs.

The Click will not DIRECTLY accept a thermocouple as an analog input. The signal from a thermocouple is a VERY LOW LEVEL signal and is very picky about what it is connected to. Use a 'signal conditioner' in between designed to accept a thermocouple and output a signal (the Click accepts 4-20 ma or 0-05 VDC type analog inputs) appropriate for the unit. While some PLC's have add-on modules designed to directly accept thermocouples the Click line doesn't.

I think everything has been covered, but I wanted to add a small endorsement for the Click. One our forum members actually gave me a click (yes, he generously GAVE it to me) with power supply and two expansion cards.

I must say, that it is incredible for the price. The software is really very, very good. Communication is simple, the editing tools are different but very functional, the instruction set is well designed to be more flexible and cleaner looking on screen with fewer instructions than say a Micrologix 1000.

Coming from RSLogix, my expectations are pretty high, but the free click software measures up to the expensive stuff in most areas, and exceeds RSLogix500 in a handful of categories. No diresepct to RSLogix, it is designed to work with a wider variety of different controllers and the Click hardware is not nearly a diverse (yet). But I like the interface, and think that it would be just as productive for similar types of hardware.

I think it will be a successful product for a long time, and will grow in popularity, so, (without being fair to the TECO you mentioned since I've no knowledge about it), I think you can't go wrong with the Click.

Paul

Thanks everyone for the reply, there is great information here.
Bernie, you have a good point about not being able to connect thermocouple directly to the unit (as i originally thought) so i was thinking instead of getting a signal conditioner, maybe I should get a sensor which i can connect directly to the sensor, can you recommend any?

I havent come across anyone who has used TECO as yet. Seems like i'll be going with CLICK product.

One question I have is, In the spec when they say it can only handle for example 137 instructions, does that mean i can only program in 137 rungs?

Thank you

zer0net said:

...
Bernie, you have a good point about not being able to connect thermocouple directly to the unit (as i originally thought) so i was thinking instead of getting a signal conditioner, maybe I should get a sensor which i can connect directly to the sensor, can you recommend any?...

Click to expand...

Since you are going with AD, Here is my suggestion.
FC-T1

zer0net said:

One question I have is, In the spec when they say it can only handle for example 137 instructions, does that mean i can only program in 137 rungs?

Thank you

Click to expand...

Not sure where you read that. I thought they all had "8000 steps" of program area and 8000 words of data area. This is actually a lot of memory for a small brick PLC. I believe each rung start, branch, and instruction constitutes a "step". Please correct me if I am mistaken.

http://www.automationdirect.com/static/specs/c000dd1d.pdf

I have used an RTD temp transmitter from A/D with excellent results. I bought this one for the compact trasmitter and QD connectors:

http://www.automationdirect.com/adc..._(with_M12_Cable_Connection)/RTD0100-06-010-H
and
http://www.automationdirect.com/adc...ers/Temperature_Transmitters/TTD-20-N40160F-H

Pick a transmitter that fits your application and doesn't go way past the high and low temps you need. This will improve the resolution of the data once you get it connected to the Click.