Control.com - Nerds in Control

I want to buy a PID controller. What should be the specifications of such a controller. As of now I only know that it has to input one parameter and output one PID 4-20mA depending on the values of Kp, KI and KD and the error signal. For such a controller what should be the specifications needed.

Does the choice of PID controller also has to depend on the type of final control element. What difference will it have to have if the final control element is a hydraulic power cylinder or pneumatic power cylinder or a pneumatic control valve. Can I not buy a general PID and tune the parameters for the different final control elements and loop responses?

In market PID controllers are available for a whole range of price bands. I want specs that are for just one input/output with a couple of alarm relays. That's all. What fine technical points should I look for while comparing two similar PIDs with a huge price difference?

Any help would be appreciated.

teena

Hi...

In the market PID controllers are available for a price range of $50 to 500. To freeze the specs of PID controller you should be clear about the input signal (if you specify universal type, then the controller input can be set by program parameters or DIP sw. depending upon the model to 4-20mA, 1-5VDC, almost all type of T/Cs, all types of RTDs), output signal 4-20mA for continous operation or contact relay outputs for ON-OFF application with relay ratings, power supply, mounting - panel flush mounted, size - DIN 1/8 (96X48mm) 0r 96X96mm, IP class, transmitter power supply required or not, display specs, accuracy.

The actuator of final control valve is hydraulic or pneumatic does not decide the output of the controller. The instrument and its specs used to drive /stroke the actuator like a I/P converter is the deciding factor.

Hope this helps you to make a right selection.

Regards,

Vinodh
vinodh.gn @ gmail. com

We use a lot of single loopers, so I've seen a fair variety over the years. I could probably schedule salemen controller demos for a continuous 40 hours a week for a couple months, if I wanted to.

The cheaper units generally have poor isolation and little ability to cope with common mode voltage.

Most of the cheapies are designed solely for thermal applications, with an expectation of thermocouple input, tuning constant ranges that are typical for thermal processes, and with auto tune that expects a thermal process reponse.

Many leave you to find your own precision resistor if you want a 4-20ma input.

When used on a flow application, those conditions might be outside the range of what's available.

The 7 segment displays used on cheapies is very annoying, because the characters are not alphabet characters. It's hard to make meaningful words with 4 characters.

The current carrying load ability of the relay outputs varies all over the map.

Sometimes, the relay outputs have limited deadband ajustment, one couldn't use one for sump controller or pump up/down if needed.

Sometimes deviation alarms have to be symmetrical, one can't have a low deviation at
-X and high deviation at 0.5X

If a record of the setup is required, having a Windows program helps keep track. Some hae it, some don't.

Bud

How to judge the performance of a controller (its ability to control a process) Does it matter, what algorithm is being used by the controller? How it can fit to my control requirement? Anybody please clarify.

Pay attention on build-in autotuning possibility - it's one of the very helpful features!