pressure application????

Hey all! I need some advice here:

I have a press that has a general purpose micro/limit switch that is used for "tonnage high limit". I would like to replace this with an analog device as I am upgrading the controls for this unit. My problem is that I am constrained by space and it would be really nice if I could find an analog micro switch that would fit nicely. I have looked around but i am not having luck as of yet. So I am asking you "brighter tools in the shed" to help me out here.

thanks, six...

I'm a little confused. Limit switches generally measure position and you are saying you want to measure tonnage. These are two different types of measurements.

What type of press is it? (Mechanical, Hydraulic, Servo?) Could it be that your current micro switch is part of an assembly that measures pressure on a ram?

Does the press manufacturer have an option for analog tonnage measurement? They would be the best ones to integrate something in taking all operations of the machine into account.

The presses I have worked on with tonnage monitors had fine resolution strain gauges going to a tonnage monitor controller or an aftermarket AB SLC500 card for tonnage monitoring. Then it was treated as any other analog signal - in your case latch a bit if the input is over xxxxx.x

These strain gauges were about 1" x 2 1/2" with one cable coming off them, attached to the side of the press with 4 screws. Could be 1, 2 or 4 per press depending on application needs.

so I currently have an OLD (1968 to be exact, I am upgrading with a plc) machine that uses: (similar)
http://www.hiwtc.com/products/az-7310-limit-switch-micro-switch-202248-53078.htm

this switch opens a circuit when tonnage exceeds the limit. (as the tonnage increases the arm that actuates the micro switch moves). I would actually like to install a device that senses that arms postion (arm travel is only 1/4" from 0 tonnage to tonnage limit) with out modifying the current configuration of the mechanism. My question is: does anyone know of a device with a frame similar to the current micro switch that will give me an analog signal (instead of on/off) based on the plunger position of the device.

Read @Aabeck's answer.
You are looking for an analog device; a strain gauge is an analog device which "translates" force into an analog signal; they could be as small as a micro-switch, however, you will have to provide the expected force min/max values, ensure they are within the functional limits of the strain gauge and then read the gauge generated analog signal within a qualifying SLC analog input module.
Unfortunately, raw strain gauge analog signals are too small for a stock SLC Analog Input module; you will need an aftermarket SLC Strain Gauge Module or a signal amplifier between the gauge and the (presumingly existing) SLC stock Analog Input module.
Other option, since you mentioned a 0.250" "arm travel", will be the external installation of an Analog Position Transducer which will translate the position of the "arm" into a 0-10VDC or 0/4-20mA signal which could be picked up by your stock SLC analog Input module.
1/4" is a "huge" distance to me measured; many manufacturers have lines which could solve your problem; just make sure you pick a device with the closest range to 0.250" thus providing your SLC system with maximum data granularity.

Checkout the M150 model from http://www.celesco.com/stringpots/

It'll give you a 2 Volt resolution for every quarter inch of travel when fed at 10 VDC.

I believe I am smarter now, thanks for the input. I have a direction!!

Here’s what’s concerning me. You can’t measure tonnage by position unless what’s being pressed is a constant (I.E. the same material that reacts the exact same way as far as how far it compresses depending on how much pressure is being applied). Think of it this way, an empty pop can will crush with only a few pounds where as a piece of 1/16th inch walled pipe with the same height and diameter as the pop can will take a lot more pressure to crush so the press will move the same distance to crush both items but the amount of pressure applied will be very different.
If that’s how it’s being done now (a simply micro switch to determine position) then I most certainly would upgrade to an actually pressure switch that translates the rams pressure into an analog signal that then can be read by the PLC. That can be done via a strain gauge however if it is a hydraulic ram then you should be able to tap into the hydraulic line feeding the ram and measure the pressure being applied to the ram. When you know that then you should be able to calculate the rams pressure.

The current system is actually monitoring the stress on the frame of the press (vertical stretch), not the actual cutting pressure between the die and the platen. It is not hydraulic, It is a mechanical platen on a crankshaft (paperboard die cutter). I am currently schooling myself on strain gauges but I am thinking the linear position sensor might work if I can find a good match.

Just keep in mind that deflection of that arm is probably not in direct correlation with tonnage.

Shawn

Until I learn otherwise I am going to assume that sixstringz has confirmed that the arm position is proportional to tonnage. It certainly should be assuming that the frame material or geometry is constant.

Take a look here:

http://www.keyence.com/products/measure/index.jsp

It will give you some ideas on what is out there. Balluff as well as Sick also makes several sensors that would work for you.

And here is yet another option:

http://www.macrosensors.com/position_sensor_selection_guide.html

Keith

shawn_75 said:

Just keep in mind that deflection of that arm is probably not in direct correlation with tonnage.

Shawn

Click to expand...

I assume it is as the tonnage dial indicator is also attached to the arm.