I am using a small pump, that needs to be manually and automatically varied (using PLCs). I know you can use a potentiometer to control the pump manually. But how do you build a circuit to be able to output a 4 to 20mA signal from the PLC to alter the speed of the pump?

 

Hi,

Unless you have a specific reason for using analogue (4-20mA or 0-10v) then it is a good idea to avoid it. Many PLC / Inverter combinations (s7>micromaster AB>PowerFlex - other automation systems are available)
allow for direct communication, I use the S7-200 to Micromaster using built in USS. You have total control over the drive from the PLC, and can read from the drive, all with two wires! A circuit example is available

http://www.germainesystems.co.uk/drawings.pdf

--
Marc Sinclair
http://www.germainesystems.co.uk

 

You should be able to purchase a DC controller card from the drive manufacturer - typically controlled via 0-5 or 0-10V.

Regards

Brad Stemp
Activ8 Engineering Pty Ltd
[email protected]

 

O and I can also ovbiously use a 0 to 10V control signal too. The main problem is the resolution and fine control required from the pump.

 

Well u can also control the speed using 555 timer. I had a mini project on this problem . I have designed the whole circuit. By varying the duty cycle u can adjust the speed of ur motor.

 

Hi,

I've read your suggestion about using the S7-200 and micromaster setup. Recently i've been asked to do just that. I have got hold of MicroWIN 4.0, a S7-214PLC and a couple of Micromaster 420 now i need to get the code. Would you be able to help me out? Please email me any information or advice you can give me. Thanks a million

Namur Eduardo Herrera
Techna Pty Ltd
[email protected]

 

Hi,

In MicroWin 4.0, all the code is included. When you have the program running, Press F1 (Help) and then Help Topics, Then Type in USS the help is excellent, showing you how to Build your network and even examples of a USS protocol program.


UNFORTUNATELY,
The CPU you have is an old model, the 214 won't support parametrised subroutines, you must use a series 2 (222, 224 or 226) PLC to take advantage of the SIEMENS USS library.

Once you have the USS Library installed (this happens automatically when you reference one of the USS routines - but don't forget to allocate library memory!!) then all you have to do is place a USS control block in your program and tell the drive what to do.

I have many of these systems in the field working perfectly. Two points come to mind though:
1. Use the link timeout on the drives, so that they will stop if the USS goes down, 5000mS works for me.

2. Don't fault immediately on a single drive error. The first few systems I installed kept stopping with drive communication errors, when monitoring I found that one or two telegrams a day were being mangled, probably by electrical noise, I now check to see that the error is stable before shutting down with a fault.

Marc Sinclair
http://www.germainesystems.co.uk