how to calculate reliabilty of a plc system

johnny6113 · Apr 23, 2006

Can someone please help me on how to calculate the reliability of a plc system without using the MTTF and MTBF equation?What I mean. is there any other equation that I can use.thanks.

CroCop · Apr 23, 2006

tom_stalcup · Apr 23, 2006

I'm going to expand on the previous post a little bit.....

We have absolutely no idea what you are talking about.

Although a quick google search tells me that it's most likely the "mean time to failure" and "mean time between failure" equations, whatever the heck those are.

jstolaruk · Apr 23, 2006

Some mfg's will give you the MTBF for components, some won't. Beyond the hardware, the programming could be tested with an extensive FMEA (Failure Mode & Effect Analysis) but ultimately one year in continuous service would prove it out.

johnny6113 · Apr 23, 2006

I am sorry for lack of information.Actually I need to find the reliability of a system consist of 4 PLC unit by having four-monthly test.

MTTF means "mean time to failure"
MTBF means "mean time between failure"

Both are use to find the failure rate.

Ken M · Apr 23, 2006

Hi Johnny

The reliability of a system (however you define that system) is the probability of a failure occuring within a defined period of time.

What I mean. is there any other equation that I can use

I'm sorry but the definition of reliability means you have to use the equation that calculates it from MTBF and time. There may be different ways of expressing the equation but at the end of the day it's always going to be the same equation. Why do you want/need to use another equation?



                  -(time/MTBF)

   Reliability = e

Everything fails eventually, so given a long enough 'time' everything's reliability is zero. Equally, if you reduce the 'time' to a small enough period you can achieve a seemingly impossible 100% reliability for anything. It's up to the user to use an appropriate period relevant to their needs e.g. 1 month, 1 year etc.

Regards

Ken

Peter Nachtwey · Apr 23, 2006

Ken, are you sure about that formula. When time=MTBF then the reliability is .37. Where does the mean come in?

We calculate a MTBF for our products. The rating is bogus because adding parts makes the system more unreliable. However, some of these parts are necessary to make the inputs more robust.

johnny6113 · Apr 24, 2006

Ken M said:
Hi Johnny

The reliability of a system (however you define that system) is the probability of a failure occuring within a defined period of time.I'm sorry but the definition of reliability means you have to use the equation that calculates it from MTBF and time. There may be different ways of expressing the equation but at the end of the day it's always going to be the same equation. Why do you want/need to use another equation?

-(time/MTBF)
Reliability = e

Everything fails eventually, so given a long enough 'time' everything's reliability is zero. Equally, if you reduce the 'time' to a small enough period you can achieve a seemingly impossible 100% reliability for anything. It's up to the user to use an appropriate period relevant to their needs e.g. 1 month, 1 year etc.

Regards

Ken

thanks for the explanation.Is there any equation which involve the reliability and the maintainance interval such as the affect on reliability by having a maintenance at 6 monthly interval and 4 monthly interval.

how to calculate reliabilty of a plc system

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