mbuis
Member
I had posted an earlier thread about a scale w/parameters issue. Turns out it may not be the issue. Since, I do not have much PLC experience, nor do any of my colleagues, we contracted a local expert. He was extremely helpful and very knowledgeable.
However, he ran into the same problem we were originally having. He did simplify what we had and made some improvements, though.
Here is the layout:
We are trying to cycle a load from a low load (1000 lbs, for instance) to a higher load (15,000 lbs, for example.) We need to stay within plus/minus 2% of these upper and lower values in order for the test to be valid. We have a load cell which has a load rating of 0 to 50,000 lbs. It has a voltage rating of plus/minus 5 volts. Positive volts meaning it is pushing from 0 to 50,000 lbs and negative pulling. We have a MicroLogix 1100 which will read plus/minus 10 volts. Range from -32603 to 32603.
Here is the problem:
We would vary the input for the higher load trying "to dial it in" and we would reach a point where all of a sudden we are well over (or even under) the 2% variance we are allowed. The load was not stable. If we slowed the pump down, it worked great. But, this is way too slow for this to be a practical application. The guy even speed up the scan rate to the maximum rate (by selecting unfiltered) and it worked better, but still had too much variance.
He was convinced that since the hydraulic pressure is nearly instantaneous, the PLC was not able to keep up with the pressure changes. The original controller had a scan rate of 8 to 10 times faster than the PLC is capable of. Unfortunately, this controller is outdated and cannot be replaced.
After, seeing the responses we were getting today I believe the guy is correct. However, I was wondering if anyone had any thoughts on this?
I even found this article which seems to support this hypothesis:
http://www.controldesign.com/articles/2008/105.html
However, he ran into the same problem we were originally having. He did simplify what we had and made some improvements, though.
Here is the layout:
We are trying to cycle a load from a low load (1000 lbs, for instance) to a higher load (15,000 lbs, for example.) We need to stay within plus/minus 2% of these upper and lower values in order for the test to be valid. We have a load cell which has a load rating of 0 to 50,000 lbs. It has a voltage rating of plus/minus 5 volts. Positive volts meaning it is pushing from 0 to 50,000 lbs and negative pulling. We have a MicroLogix 1100 which will read plus/minus 10 volts. Range from -32603 to 32603.
Here is the problem:
We would vary the input for the higher load trying "to dial it in" and we would reach a point where all of a sudden we are well over (or even under) the 2% variance we are allowed. The load was not stable. If we slowed the pump down, it worked great. But, this is way too slow for this to be a practical application. The guy even speed up the scan rate to the maximum rate (by selecting unfiltered) and it worked better, but still had too much variance.
He was convinced that since the hydraulic pressure is nearly instantaneous, the PLC was not able to keep up with the pressure changes. The original controller had a scan rate of 8 to 10 times faster than the PLC is capable of. Unfortunately, this controller is outdated and cannot be replaced.
After, seeing the responses we were getting today I believe the guy is correct. However, I was wondering if anyone had any thoughts on this?
I even found this article which seems to support this hypothesis:
http://www.controldesign.com/articles/2008/105.html