Strain Gauge

I've been asked to do a project that involves fitting a strain gauge to a vertical pressing machine, and then using the signals from it, to interlock the machine.

My question is, how do i work out where to mount the strain gauge, or can i mount it any where on the backplate of the press.

I can do the controls side of the project no problem, but haven't got a clue as to where i should mount the strain gauge.

Thanks for any help

Sparky

If this is a press, you may need to look as some machine safety guarding. One supplier is:

http://www.sti.com/

When you say strain guage are you talking about an actual resistive starain guage that you will have to glue to the surface and wire up in a bridge circuit or do you have a force transducer with a conditioned output (i.e. 0-10VDC)?

When you say you will use the strain gauge to 'interlock' the machine, what functions to you wish to prohibit or enable? What conditions of strain gauge output will signal the enabling or prohibiting of the press?

These are all very important questions and if human safetey is involved then you really need to look at devices made specifically for press guarding like the ones I linked to above.

Is this a hydraulic press? If so, just put a pressure transmitter in the hydraulic line at the main cylinder.

If it is a strictly mechanical press, you have to put the strain guage or load cell in a location that changes stress or load significantly when the press operates. The edge of the platen, for example probably wouldn't work because its stress level doesn't change during operation. A connection point on the frame might work, but needs to be examined carefully. This is actually a mechanical engineering problem!

I used to work for a mechanical press company designing and programing their control systems. We used to use a strain gage/tonnage package from a supplier, but for the mounting of the strain gages they were mounted on either 2 or all 4 columns of the straightside presses and either side of the frame on gap or "C" frame presses. In any case on most mechanical presses you will want at least 2 strain gages to read the tonnage right/left or front/back. As Tom said it is a mechanical issue and the strain gages should be mounted where they will see some direct flexing on the frame. Another thing you will have to do is calibrate the strain gages with tonnage cells. Otherwise it will be hard to figure out just how hard you are hitting the material.

As Tom has said, if it hydraulic definately abort the strain guage and go to the pressure transducer. Will be much more reliable and robust than a resistive transducer. On a recent project I did we had 200 ton presses which had a 8' x 16' platen and 8 hydraulic cylinders to generate the tonnage. Any good mechanical engineer can tell you the pressure relation to the calculated surface area of the hydraulic cylinders. A little math with the pressure feedback and presto there is your "applied force". Also there is a side benefit to this that you can then detect a blown line condition based on the pressure feedback and potentially prevent unsafe press operation conditions.

Thanks for all of the replies,

ndzied1, when i talked about interlocking, i didn't mean for safety purposes, i meant to actually stop the machine continuing its auto cycle, maybe interlocking isn't the right word to use in this case.

Having read all of your posts, i realise that the mounting of the strain gauge is a mechanical engineers issue, not a controls issue.

I've discussed the reservations that i have, and that you guys have mentioned, with the person who requested that i be responsible for the siting of the strain gauge, and basically he is going to get a mechanical engineer involved, and then i can do the controls side when he is ready.

The strain gauge that has been bought is a kistler unit, that is controlled by a black box, that interfaces with the plc using digital i/o, so i can't see any problems with the controls side.

Thanks for the help

Sparky

We use the TQM sensors in most of our presses. I think that they are great.
Toledo
Good Luck!