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#1
User is offline   archana_sharan 

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Hi
Can anyone please give me a simplified circuit diagram for a zero speed sensor which uses a Namur proximity sensor to count pulses. I want the circuit diagram for the controller part which can drive the namur sensor and count the time between pulses from proximity to indicate fall in speed. It would be further helpful if the speed setting can be adjusted. Output can be a semiconductor switch.

Otherwise can you please suggest a useful site where I can get these details.

I know this is a very simplistic query but I need to start from scratch and develop the same and then get it certified through certification agency for one of our application.
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#2
User is offline   BobLfoot 

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I don't find your question overly simple. I must wonder why a prepackaged option like the Omron-STI Sensor wouldn't work. Especially since it has several key acceptances under it's belt. And no I have no conflict of interest relationship with Omron or STI.
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User is offline   panic mode 

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connect sensor to one of the standard controllers or smart relays and write code
that resets timer every time you detect transition from the sensor. changing timer preset
will give you adjustment you are looking for.

if you want this done in a circuit (without using PLC, microcontroller or smart relay)
it is of course doable and it works exactly the same way (check "monostable multivibrator")
google and wikipedia have plenty of examples.

there are so many ways to achieve what you ask, so what do you want this for?
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User is offline   TConnolly 

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I'm sure you could do this with a 555 chip and a simple circuit, but I have to side with Bob. Its better to use a device that has the aproval of regulatory agencies.

This post has been edited by Alaric: 01 May 2008 - 09:17 PM


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#5
User is offline   Peter Nachtwey 

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View PostAlaric, on May 1 2008, 07:16 PM, said:

I'm sure you could do this with a 555 chip and a simple circuit, but I have to side with Bob. Its better to use a device that has the aproval of regulatory agencies.

Define zero speed.
Infinite resolution is required to determine if the speed is 0.
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User is offline   BobLfoot 

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View PostPeter Nachtwey, on May 2 2008, 09:59 AM, said:

Define zero speed.
Infinite resolution is required to determine if the speed is 0.

Peter I nearly fell out of my chair. A motion question and you answered it in only two lines. You must be into the summer rush already - LOL
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User is offline   TConnolly 

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That reminds me of when my wife got a ticket for running a stop sign from one of the local revenue... er, I mean police officers. She pled not guilty and went to trial. The officer admitted she stopped but didn't stop long enough. My wife then said the law didn't say how long you had to stop, just that you had to stop. The judge agreed so my wife moved for dismissal and the judge granted it.

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User is offline   Peter Nachtwey 

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View PostAlaric, on May 2 2008, 09:25 AM, said:

That reminds me of when my wife got a ticket for running a stop sign from one of the local revenue... er, I mean police officers. She pled not guilty and went to trial. The officer admitted she stopped but didn't stop long enough. My wife then said the law didn't say how long you had to stop, just that you had to stop. The judge agreed so my wife moved for dismissal and the judge granted it.

Define stopped.
Does it just mean velocity=0
or does it mean velocity and all its derivatives =0


As far as a car go it means velocity=0 so I only need to be stopped for a pico second to be stopped for a stop sign. The derivatives don't need to be zero. Try that argument with a cop and a judge.

In motion control I use the second definition even though I can't measure all the derivatives, just the velocity and acceleration.

We get tech support calls were customers want to do something when stopped. These are the question I ask and what is the resolution of the encoder. Then I must explain reality and it is a bitch. Some customer buy the 512,000 count/revolution encoders. I laugh when someone mentions a prox switch and a few detection points.
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User is offline   paulengr 

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View Postarchana_sharan, on May 1 2008, 08:01 AM, said:

Hi
Can anyone please give me a simplified circuit diagram for a zero speed sensor which uses a Namur proximity sensor to count pulses. I want the circuit diagram for the controller part which can drive the namur sensor and count the time between pulses from proximity to indicate fall in speed. It would be further helpful if the speed setting can be adjusted. Output can be a semiconductor switch.

Otherwise can you please suggest a useful site where I can get these details.

I know this is a very simplistic query but I need to start from scratch and develop the same and then get it certified through certification agency for one of our application.


If you are developing your own then you don't need the long winded answer.

The short answer is that there are speed switches that do exactly what you describe already available on the market that are rated as safety devices. Omron/STI is one source. Also, Pilz is another and Allen Bradley is a relative newcomer.

If it does not need a safety rating (as in for instance the certification agency is an environmental one), then you can buy very reliable speed sensors from Electro Sensors. These have been used for years in mining companies and are acceptable to MSHA in the U.S. (Mine Safety and Health Administration) at least in applications where you don't need self-checking or redundancy.

In both cases they all come with lots of documentation where they specify the performance of the speed sensor.

If your "certification agency" is ISO 9000, then you are dealing with idiots. You will never, ever satisfy whatever ridiculous things they dream up. The safest approach is to rewrite your procedures and remove the speed sensor from consideration, often at a very high level. For instance, one company I worked at certified the shipping department. The plant produced internal COA's. Shipping did initial/final inspection (they only inspected once), sent any rejects back to the manufacturer (the "plant", for rework), and shipped the rest. The ISO inspectors effectively couldn't leave the shipping area!
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