Need Ideas On How To Close A Water Tap At 1.5 N.m Torque

Hi All, I'm working on designing an automatic machine / tester that will open and close a water tap for about 200,000 cycles (to ensure that the tap complies with BS5412 (1996)Clause 12).
The requirement of the standard is to open the obturator in the tap to 90% of its maximum opening and then to close it with a torque of 1.5 N.m. These are the normal half inch diameter household taps.
Currently I am looking for ideas on how to close the tap with a torque of 1.5 N.m and then to open it automatically. I am familiar with PLCs and electronics with a bit of mechanical knowledge. I am thinking DC motor connected to gearbox conected to a 'breaking torque' device. The DC motor can be reversed for opening / closing and the breaking torque set to 1.5 N.m.
However I am not too sure how accurate and repeatable a breaking torque device can be. I also have this problem where the torque required to reopen the tap (after it has been closed) is higher than the closing torque. Do breaking torque devices work in one direction only or both? The tap would probably not open again if they work in both directions.
Other Questions: 1. Am I on the right path? 2. If you were going to design the system that opens and closes the obturator, how would you do it? 3. Any recommendations on what to use for the torque control? 4.How do you interface the torque device to the PLC so that the PLC can tell that the tap has been closed?
Thanks Manvinder S. Bhullar

On 13 Jul 2004 01:29:55 -0700, snipped-for-privacy@tm.net.my (Manvinder Bhullar) wrote:
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I don't see why this couldn't work. I would dump the constant torque clutch for the reason you are aware of. You could set the stall torque current of the motor seperately in the forward and reverse directions for the required torque value.
The open torque could be set X2 or X3 higher, because it would be transient. A sensor on the shaft to count revolutions might be helpful - to enforce that 90% open condition.
Brian W

(Manvinder Bhullar)
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Some faucets are rising stem. The stem could actuate a switch, or be part of a low voltage detector circuit. Some faucets only open on fractions of a single rotation, (like bathroom faucets) so a modified 4-bar mechanism could be made to service these.
David A. Smith

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David, Thanks for your reply. This faucet does not have a rising stem. It takes less than one turn from closed to fully open. I have no problems detecting the open position (just use an inductive proximity sensor aligned to the actuator and connect that to the PLC).
I am looking for more input on the closing (and subsequent reopening) of the faucet to the specified torque.
BTW, whats a 4-bar mechanism?
Thanks Manvinder

Dear Manvinder Bhullar:
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Someone suggested a rack and pinion, operated by an air cylinder (which could be pretty sweet for your requirements). 1: valve stem 2: planet Earth 3: rack and piston rod 4: cylinder body
The term "bar" can be pretty loose, sometimes...
URL:http://www.mathworks.com/access/helpdesk_r13/help/toolbox/physmod/mech/mech_getting15.html URL:http://www-sop.inria.fr/coprin/logiciels/RP/4bar/notice-html.html URL:http://www.me.polyu.edu.hk/subject/me210/notes/chapt29.pdf
Watch your closing speed on the rack. The mass of the rack will represent a pretty good hammer, so you could end up with a resulting torque you don't intend.
David A. Smith

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Brian, Thaks for the input. I am inclined to follow this path too, where a DC motor's current is limited to control the torque it can output. Then on reverse, the current limit can be set to a higher level.
Except I havent done this before. Do you have any experience with this and can you tell if the system will have good repeatability and reliability? I really can't imagine anything going wrong with this method.
Anybody out there who has more input on this method?
Thanks Manvinder

In article <27c42ac5.0407131810.662a1271
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I've done this with a PM brush motor, current mode servo amp, and servo controller. The reason for servo was there was a requirement for both position and torque control. This was for testing the integrity of the attachment of video tape leader to the cassette hub on a cassette assembly system.
Works pretty well as long as you don't push the motor too hard--the torque/current relationship will change with motor temperature.
Ned Simmons

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"If you were going to design the system that opens and closes the obturator, how would you do it?"
I would either do it myself or hire an experienced mechanical engineer.
Sounds like you need an experienced mechanical engineer.
Am I correct in assuming that these "water taps" a going to be made by the 1000's & are destined for the residential market?

snipped-for-privacy@aol.com (Bob K 207) wrote in message
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Thanks Bob, we do have a mechanical engineer on board the team. Its just a question of deciding on the most viable approach and the mechanical engineer will then go design it. My post is meant to look for input from experienced engineers such as yourself.
These taps will be for the mass market, but the test is only on a sample basis.
Thanks, Manvinder

How about an air ram driving a rack and pinion, regulate closing pressure to limit the close torque, reverse can be at a higher pressure if needed, with a limit switch at each end to reverse direction. You probably need a short delay on the closing switch reverse to ensure the full closing torque is reached. -- Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
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Jonathan, Thanks for the reply. That's one of the options we are considering. This is my worry with the method:-
The spec calls for 1.5 N.m with a tolerance of Minus 0, Plus 0.15 N.m. Thats 10% and to ensure we are in spec we will set the torque on the machine to 1.575 and give that a tolerance of Plus / Minus 0.075 N.m. That works out to +/- 4.76%. That would be the absolute spec. Now to make sure we dont run into trouble with the 'quality police', we intend to tighten that further to +/- 3.5%.
Will the pressure controlled pneumatic system be able to provide consistent, repeatable torque within the +/- 3.5% limits, for up to 200,000 cycles? I need some help in that area.
Thanks Manvinder

Hello Minverder.
From another post in this thread I gather a part turn only is needed by the valve, It's therefor probable that you can use direct drive from a ram... cuts out the rack. Position sensing is easy as most rams have magnetic pistons, and can be fitted with a standard position sensing reed switches.
If you use a low friction ram ( PTFE seals I think ) and a precision regulator for the pressure control I would expect to meet your specification, however being cautious I would add a spring loaded plunger with position sensors to the end of the piston rod to verify the load applied. ( or maybe just use these signals for PLC input and use the PLC to control a standard ram without regulated pressure).
If you go for regulated pressure method it is probable your ram will need to be " run in " for a while as it's output may drift as the seals settle in. unless you use a second hand one :-) ) .
-- Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
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Dear Jonathan Barnes:
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Only if the angle of rotation is close to 90° (and absolutely less than 180°). He does need a specified amount of torque at the end-of-stroke. But it does cut down on the involved mass...
David A. Smith

Valve actuators are available with some reasonably sophisticated controls, see: http://www.kinetrol.com / Std items are good for 95 deg. but 180 deg units are available, you could also use a simple reduction gearset.
Mark.
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Well I did say " probable" , as I was assuming that he is working on a ball or butterfly type valve, both of which use 90 degree opening.
If he wants 180 degrees rotary actuators ranging from 90 to 360 degrees are readily available, but might not give a consistent enough torque... -- Jonathan
Barnes's theorem; for every foolproof device there is a fool greater than the proof.
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snipped-for-privacy@tm.net.my (Manvinder Bhullar) wrote in
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There are systems out there for setting torque on bolts, specifically, look into connecting rod equipment. I think Bosch makes a system. These are very reliable, and are electronic, not breaking torque, so the return should be no issue. The feedback to the PLC is also no issue, since the unit would have outputs available.

Hi All,
My thanks to all of you who took the time to advise and guide us regarding the above matter. The team here appreciates everyone's input to our problem.
We have decided that we will try to use a dc motor with current limit control to control the torque for a start. My own research has led me to believe this is the most viable approach for (see, most of us here except for the mechanical engineer are electronically inclined).
Will try to keep you all updaed on the progress.
Later.
Manvinder