adccommunitymod (AutomationDirect) asked a question.
Created Date: March 15,2009
Created By: wrumbaugh
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I have an application that uses a load cell attached to flange of a jack screw; which is moved up and down via a motor. The load cell has a platen that weighs 5LBS on it. The motor has an encoder on it for the purpose of monitoring position of platen. When moving the platen into a test sample, I am capturing position and load via a C-More trend graph; in addition to numeric displays of position and load. I am using a D0-0DD2 PLC with the following cards: HO-CTRIO,HO-ECOM, and a F0-04THM which is configured to 0-39.0625mVDC for 2mv/V 500 pound Load Cell. The Load Cell is capable of measuring tension or compression simply by reversing the + and - Sense out leads. The Load Cell is being excited by an external 10 VDC power supply. The problem is when moving the platen down into the product I do not see the load cell output until I have moved approximately two inches into the sample. I suspect what is happening here is the compression counts are not being seen until I have overcome the 5 pounds of tension on the Load Cell caused by the weight (tenstion) of the platen. I am looking for help or suggessiion in overcoming this? Any help would be greatly appreciated.
Created Date: March 15,2009
Created by: bfitz
Could you set up the card to read your millivolt signal in a bi-polar range? This would allow you to read tension and compression in one input. You will have to work with negative values, but this would allow you to "tare " the tension of the platen from the system. The drawback would be lower resolution in the reading. If this is a problem, you might also be able to connect the sensor to two inputs at the same time (one connected in compression, one in tension.) You would have to create your own logic to handle assembling this "bi-polar " signal into something usable to your program, but it would give you your original resolution back.
Brian
Created Date: March 16,2009
Created by: allukes
Is it possible mechanically support the platen and load cell to the flange? Pins or a bracket that allow movement of the platen in only one direction but would limit the travel and support the weight in the other? Just an idea.
Created Date: March 18,2009
Created by: AZRoger
This sounds a lot like your prior post http://forum.automationdirect.com/showthread.php?t=5489
How about adding springs to pull up on the platten while it's not in contact with anything. You can mechanically force the load cell into compression mode and create a "Zero " output reflecting just the force from the springs less the weight of the platten. Then your real load will just be increasing the compression on the load cell.
Created Date: March 18,2009
Created by: Bob S BN
Yes, as AZ Roger mentioned, looks like the same problem you had before. And I think you'll always have issues with this change from tension to compression.
Are you the one designing/building the test stands?? Have you thought about putting the moving platen with load cell assembly on the bottom of the unit and moving it up to the fixed platen?? Problem solved-always in compression this way.
Flip her over, I'd rather have a puppy.
just a thought,
Created Date: March 15,2009
Created by: wrumbaugh
I have an application that uses a load cell attached to flange of a jack screw; which is moved up and down via a motor. The load cell has a platen that weighs 5LBS on it. The motor has an encoder on it for the purpose of monitoring position of platen. When moving the platen into a test sample, I am capturing position and load via a C-More trend graph; in addition to numeric displays of position and load.
I am using a D0-0DD2 PLC with the following cards: HO-CTRIO,HO-ECOM, and a F0-04THM which is configured to 0-39.0625mVDC for 2mv/V 500 pound Load Cell. The Load Cell is capable of measuring tension or compression simply by reversing the + and - Sense out leads. The Load Cell is being excited by an external 10 VDC power supply.
The problem is when moving the platen down into the product I do not see the load cell output until I have moved approximately two inches into the sample. I suspect what is happening here is the compression counts are not being seen until I have overcome the 5 pounds of tension on the Load Cell caused by the weight (tenstion) of the platen.
I am looking for help or suggessiion in overcoming this? Any help would be greatly appreciated.